KCFP Annual Report 2012 - LTH...6 KCfp annual report 2012 Efficiency The gross indicated efficiency,...

KCFP Annual Report 2012Centre of CompetenCe Combustion proCessesfaCulty of engineering, ltH | lund university

KCFP ORGANISATION

BOARDTommy Björkqvist, ordförande Annika Mårtensson, LTH

Johan Wallesten, Volvo GTT Ida Truedsson, LTH (PhD

Annika Kristoffersson, Volvo Cars student representative)

Per Lange, Scania

DIRECTORProfessor

Bengt JohanssonSupervisor for:

PPC

AssociateProfessor

Mattias Richter

Supervisor for:PPC and GenDies

Administrator

Elna Andersson

Administrator

Catarina Lindén

ProfessorÖivind Andersson

Supervisor for:GenDies

ProfessorRolf Johansson

Supervisor for:CC

ProfessorMarcus Aldén

Supervisor for:PPC andGenDies

ProfessorXue-Song Bai

Supervisor for:CM

AssociateProfessor

Martin Tunér

Supervisor for:Fuel

ProfessorPer Tunestål

Supervisor for:CC and SIGE

3KCfp annual report 2012

KCFP Centre of Competence Combustion ProcessesThe Centre of Competence Combustion Processes, KCFP, started July 1 1995.The main goal of this centre is to better understand the combustion process in internal combus-tion engines. Of particular interest are the combustion processes with low enough temperature to suppress formation of NOx and particulates, PM, often called Low Temperature Combustion, LTC or Homogeneous Charge Compression Ignition, HCCI.The Centre of Competence Combustion Processes has a budget of 22.25 MSEK per year. This is roughly one third each from the Swedish Energy Agency, STEM, Lund University and the Industry.

CONTENTS

The Partially Premixed Combustion Project 4

PPC - Heavy Duty 4

PPC - Light Duty 7

PPC - System Simulations 11

PPC - Modeling 14

PPC - Fuels 17

PPC - Control 19

The Generic Diesel Project 20

Mixing in Wall Jets of a Heavy-Duty Diesel Engine 20

Influence of Jet-Jet Interactions on the Liquid Spray Length in an Optical Heavy-Duty DI Diesel Engine

22

The Gas Engine Project 23

KCFP PhD Students who Graduated in 2012 26

INDUSTRY PARTNERS

Scania

Volvo GTT

Volvo Cars

Volvo Penta

Toyota

Caterpillar

Chevron

Wärtsilä

Finnveden

Hoerbiger

Cargine

The Swedish Gas Centre

Loge

4 KCfp annual report 2012

The Partially Premixed Combustion ProjectPartially Premixed Combustion, PPC, is a combustion process between Homogeneous Charge Compression Ignition, HCCI and the classical diffusion controlled diesel combustion. With PPC it is possible to moderate the charge stratification and thus control the burn rate better than with HCCI. In comparison to classical diesel combustion the NOx and particulates can be suppres-sed with orders of magnitude. KCFP has five different but linked subprojects on PPC.

PPC - Heavy Duty

Close to Stoichiometric Partially Pre-mixed Combustion - the Benefit of Ethanol in Comparison to Diesel and Gasoline

Mengqin ShenPhD Student

During the year 2012, some time was spent to update the whole system and get it up and running. The engine was boosted by using two-stage compressors by which the inlet pressure could be adjusted. Low pressure EGR, after the back pressure valve, was mixed with air before going into the first compres-

sor. An air valve and an EGR valve were used to control EGR ratio. A heater was placed in the inlet manifold to heat up the inlet charge to the desired inlet tempera-ture .The schematics of the updated system are shown in Figure 1.

Previous results showed that with 50% EGR and lambda 1.5, PPC could meet US10/Euro6 emission levels in a Scania D13 engine at some loads. When running the engine near stoichiometric ratio, NOx, HC and CO emissions are no more an issue since a three way catalyst can be used to reduce them. It is, however, necessary to pay attention to the efficiency and soot penalties which can be substantial. During the past year, the possibility of operating clean PPC all the way from lean to stoichiometric ratio was investigated. Ethanol fuel was used as high octane number fuel in comparison with gasoline fuel and diesel fuel. The penalties in fuel efficiency as well as soot emission were analyzed.

Results

The fuels used in this experiment were Swedish diesel MK1, a gasoline fuel of octane number (RON) 69 and ethanol (99.5% by vol.). Most results discussed here are the cases with 38% EGR and 20.5 bar FuelMEP at constant engine speed 1250 [rpm]. Injection pressure was set to be constant at 1600 bar with a single injection. Inlet temperature was constant at 313K for diesel and gasoline but changed with ethanol to have stable combustion. For each measurement point, optimized combustion phasing was selected based on highest gross indicated efficiency setting.

Figure 1. Updated engine system


Figure 2a. Diesel fuel

-20 -10 0 10 200

50

100

150

200

Crank Angle [CAD ATDC]

RoH

H/1

0 [J

/CA

D] /

Inje

ctor

Cur

rent

=1.69=1.5=1.2=1.06

Figure 2b. Gasoline fuel

-20 -10 0 10 200

50

100

150

200


RoH

H/1

0 [J

/CA

D] /

Inje

ctor

Cur

rent

=1.86=1.56=1.22=1.02

In lean PPC operation, with diesel and gasoline fuel, combustion was less premixed and two peaks are observed when λ is greater than 1.2. As the charge is enriched, premixed burn spike magnitude increase and turns into a more delayed and narrower heat release without any clear distinction between premixed and mixing-controlled combustion. For ethanol, the premixed burn spike remains distinct and dominates the combustion from lean to stoichiometric operation. With less excess air, the rate of heat-release was faster and much narrower which could lead to a higher combustion temperature. Too high heat-release rate together with lower global air- fuel ratio can result in high heat transfer to cylinder wall. At the same time, local rich and stoichiometric air-fuel zone and hot zone existed, which results in high soot emission.

Figure 2. Stoichiometric PPC in comparison with lean PPC combustion with three fuels (Injector current in low range and rate of heat-released in hugh range, EGR 38%, FUELMEP 20.5 bar)

Figure 2c. Ethanol fuel

-20 -10 0 10 200

50

100

150

200


RoH

H/1

0 [J

/CA

D] /

Inje

ctor

Cur

rent

=1.74=1.58=1.16=1.05

3a. Gross indicated efficiency

1 1.2 1.4 1.6 1.835

40

45

50

55

[-]

Gro

ss In

dica

ted

Effi

cien

cy [%

]

MK1GasolineEthanol

3b. Combustion Efficiency

1 1.2 1.4 1.6 1.897.5

98

98.5

99

99.5

100

[-]

Com

bust

ion

Effi

cien

cy [%

]

MK1GasolineEthanol


Efficiency

The gross indicated efficiency, combustion efficiency and thermodynamic efficiency as a function of λ with 38% EGR, FuelMEP 20.5bar are plotted in Figure 3.a, 3.b and 3.c respectively. Efficiency was much reduced when running PPC from lean to stoichiometric. With diesel fuel, reducing λ from about 1.7 to 1.06, gross indicated efficiency drops from 53.8% to 36.0%. For gasoline, efficiency reduced from 54.9% to 46.7% with λ =1.86 down to λ =1.02. There is less efficiency descent for ethanol fuel, from 53.8% down to 48.9% (λ =1.05). Although λ is down to close to 1, the combustion efficiency is always higher than 98.2%. This means that in general, when λ going down to one, not much energy lost through combustion. Thermodynamic efficiency as a function of λ is presented in Figure 3.c. It follows the general trend with gross indicated efficiency in Figure 9. When λ reduces near to 1, efficiency is very low for diesel fuel (37.1%). Gasoline and ethanol have higher efficiency of 48% and 50.1% respectively.

Soot emission

Engine out soot as a function of λ with 38% EGR for the three fuels is presented in Figure 4. When λ is greater than 1.5, all fuels had smoke less than 0.16 FSN. Soot emission significantly increased when λ was less than 1.2 with diesel and gasoline fuel. As expected, at close to stoichiometric ratio, smoke for diesel fuel increased to an unacceptable level around 9.5FSN. Gasoline also had smoke of 4.3 FSN at close to stoichiometric ratio, which is nearly half of what diesel fuel generated but still is high. In contrast, ethanol fuel, probably because of its molecular structure, had very low smoke in the range of 0-0.04 FSN in this λ sweep. Ethanol shows great advantage over conventional fuels in terms of soot emission at stoichiometric conditions.

Conclusion

It is possible to operate the engine with PPC from lean combustion to close to stoichiometric but with penalty in efficiency and soot emission. Engine efficiency decreased for all three fuels and pronounced soot emission increase for gasoline and diesel fuel. Ethanol showed less efficiency reduction in close to stoichiometric PPC operation as a result of less heat transfer loss. It showed higher efficiency and almost no soot emission (below 0.08 FSN). One conclusion can be drawn that when operating stoichiometric PPC, diesel was a bad choice due to too low efficiency and too high soot emission. Gasoline fuel gave better engine efficiency than diesel fuel, but also much soot. In comparison, ethanol emitted very low soot and gave reasonable efficiency which makes it a viable alternative to produce clean stoichiometric PPC. Gasoline-ethanol blends or gasoline fuels of high octane number could also be interesting to investigate.

Future work

For 2013, data analysis for the current campaign with different combustion chamber and injector umbrella angle is to be finished. Further work is to modify the engine into optical engine and do optical diagnostic for PPC as planned.

Figure 4. Smoke as a function of λ for three fuels (EGR 38%, FuelMEP 20.5bar).

1 1.2 1.4 1.6 1.80

2

4

6

8

10

[-]

Sm

oke

[FS

N]

MK1GasolineEthanol

3c. Thermodynamic efficiency

1 1.2 1.4 1.6 1.835

40

45

50

55

[-]

Ther

mod

ynam

ic E

ffici

ency

[%]

MK1GasolineEthanol

Figure 3. Gross indicated efficiency, combustion ef-ficiency and thermodynamic efficiency as a function of λ for three fuels with FuelMEP 20.5 bar, EGR 38%.


PPC - Light Dutyin collaboration with the Depart-

ment of Combustion Physics

Introduction

Gasoline partially premixed combustion is a promising way to achieve simultaneous high efficiency and low soot and NOx emissions in a diesel engine. Running on gasoline, there is a potential to get adequate premixing of fuel and air to avoid soot formation. With proper fuel injection strategy, the combustion is controlled so that excessive heat-release rates are avoided, which is one of the problems with HCCI combustion at higher loads.

The problem with gasoline partially premixed combustion is the limited operating region running with higher octane number fuels. See Figure 1. The purpose of this project is to increase the attainable operating region and increase the understanding of the combustion process. The approach is to use a variable valve timing system and different fuel injection strategies. Laser diagnostics will be applied to get increased knowledge and detailed information of the combustion.

The effect of negative valve overlap and rebreathing valve strategies and more advanced fuel injections strategies have been investigated in a single cylinder metal engine configuration. The engine was then rebuilt to optical configuration and experiments have been performed using two different measurement techniques.

Zheming LiPhD Student

Rikard WellanderPhD Student

Robert CollinSenior Researcher

Figure 1. Attainable gasoline PPC operating region plotted against fuel octane number. Data was collected by Vittorio Manente on a heavy-duty engine [1].

Patrick BorgqvistPhD Student


Figure 2. Single cylinder metal engine configuration (left) and optical engine configuration (right).

Experimental Setups

The research engine, Figure 2, is based on a Volvo D5 light duty diesel engine. It is run on only one of the five cylinders and is equipped with a fully flexible pneumatic valve train system supplied by Cargine Engineering. The valve open speed is fast compared to conventional systems. In order to have valve clearance around top dead center with a standard Volvo D5 piston, the valves are operated with a minimum negative valve overlap of 60 CAD. The engine control system was developed by Patrick Borgqvist and was made with LabVIEW 2009 software. The control system is run from a separate target PC, NI PXI-8110, which is a dedicated real-time system. The target PC is equipped with an R series multifunctional data acquisition (DAQ) card with re-programmable FPGA hardware, NI PXI-7853R, and an M-series data acquisition card, NI PXI-6251. The user interface is run on a separate Windows based host PC. The host and target PCs communicates over TCP/IP.

Metal Engine ConfigurationThe engine is run without boosted air. The Intake air temperature is constant 40° C, and intake pressure is the same as the ambient pressure, approximately 1 bar. The combustion chamber is a standard Volvo D5 piston with a compression ratio of 16.5.

The fuel that was used in this experiment was gasoline with research octane number (RON) 87. The fuel was supplied by Chevron.

Optical Engine ConfigurationThe optical engine is equipped with a bowditch piston extension. A quartz glass piston and ring was installed to enable optical access. The quartz ring was installed between the cylinder liner and the cylinder head to provide vertical images from the side around top dead center. The glass piston crown design is based on the standard diesel combustion chamber design with a bowl. The quartz glass piston enables horizontal images taken from underneath the combustion chamber. The geometric compression ratio is lower compared to the metal engine configuration and the engine is boosted to get the same pressure at fuel injection timing compared to the naturally aspirated metal engine setup. The intake air was heated to get the same combustion timing as a corresponding metal engine reference case.

In the first measurement campaign a fuel blend of n-heptane and iso-octane was used as fuel and acetone was used as fuel tracer. In the second measurement campaign 69 RON gasoline, supplied by Chevron, was used.


Results

Metal Engine ResultsIn order to extend the low load limit of gasoline PPC two different valve strategies and different fuel injection strategies have been investigated. The idea with the valve strategies is to trap or re-induct hot residual gases to get elevated charge temperature for the subsequent cycle. A rebreathing valve strategy with an additional exhaust valve opening event during the intake stroke has been compared to the NVO valve strategy. The rebreathing valve strategy has higher gas exchange efficiency because recompression of the trapped residuals is avoided. The combustion stability measured as the standard deviation in IMEP for various cases is shown in Figure 1. The different fuel injection strategies are a single injection, a split main injection and a NVO pilot injection strategy. It can be seen that

Figure 3. Standard deviation in IMEP for low load gasoline PPC using different valve and fuel injection strategies.

Figure 4. Unburned hydro-carbon emissions for low load gasoline PPC using different valve and fuel injection strategies.

the combustion stability improves significantly with a pilot injection during the negative valve overlap, the “NVO, NVO inj. case” in Figure 3.

The unburned hydro-carbon (UHC) emissions for the same cases are shown in Figure 4. There is a clear increase of UHC with decreasing load independent of valve and fuel injection strategy. The best strategy in this investigation for minimum unburned hydro-carbon emissions was the rebreathing valve strategy with a split main fuel injection.

Figure 5. Image taken from the side using a vertical sheet with the PLIF high speed laser and camera setup.


Optical Engine ResultsIn the first measurement campaign, attempts were made to investigate the mixing process in an internal combustion engine operated in partially premixed combustion mode (PPC). The idea was to use a high speed laser and camera system to get cycle resolved data of the fuel concentration by means of planar laser induced fluorescence (PLIF), with acetone as a tracer. Several problem have been identified and solved, for instance the interfering fluorescence from the lubrication oil have been removed. However, one major problem persisted. The quartz glass itself fluoresces stronger than the acetone. This can be seen in Figure 6. Unfortunately, the quartz fluorescence overlaps the acetone fluorescence (both are centered at 400 nm) and could therefore not be removed with optical filters. A less fluorescing quartz material should enable applications of this measurement technique in the project.

In the second measurement campaign, chemiluminescence imaging and high speed video was used. Chemiluminescence is defined as the emission of light due to chemical reaction. It is a line of sight method which means that emitted light intensity is that is received by the camera is integrated over the line of sight. The images are taken from below the combustion chamber with the fuel injector in the center. There is a distortion of the images due shapes of the piston crown, as seen in Figure 6. There is more distortion in the center of the piston crown where the diesel bowl is situated. The images still provide qualitative information of the combustion and towards the edges of the image there is less distortion. Different cases with different fuel injection strategies and varying NVO has been investigated and is currently being evaluated. Images were taken both with and without an OH filter. An example image taken with the OH filter a couple of degrees after TDC is shown in Figure 7.

Figure 6. Image of the reference grid inside the cylinder (left) and the reference grid (right).

Figure 7. Chemiluminiscence image with OH filter taken a couple of crank angles after TDC.

Future Work

More experiments with the optical configuration will be performed. One suggestion is to continue to use the high speed video and chemiluminescence setup and investigate effects of also the injection pressure and the glow plug. And also to use the 87 RON gasoline fuel in the optical setup.

References

[1] An Advanced Internal Combustion Engine Concept for Low Emissions and High Efficiency from Idle to Max Load Using Gasoline Partially Premixed Combustion, V. Manente, B. Johansson, P. Tunestal, C. Zander, W. Cannella, SAE 2010-01-2198


PPC - System Simulations

Introduction

Partially Premixed Combustion (PPC) has in single cylinder engine experiments demonstrated remarkably high gross indicated engine efficiencies combined with very low engine out emissions. If the PPC performance from single cylinder experiments could be transferred to practical engine applications, substantial reductions of emissions and operating costs could be achieved in global transportation.

Single cylinder research engines provide an environment that can be precisely controlled and monitored to realize research to develop engine combustion concepts. One limiting factor with single cylinder research engines is that results cannot directly represent production viable multi-cylinder engines since the gas exchange process, friction losses and heat losses will differ. Early experimental attempts to transfer the PPC concept into a production viable engine have not yet been that successful, due to boosting issues and fuel injector problems.

The aims of the PPC System Simulations activity is to support the experimental work by investigating the potential of different PPC strategies and guide the implementation of for instance EGR and boosting strategies for in real applications. This involves the investigation of turbocharger efficiency requirements, turbocharger configurations, EGR routing strategies and other related issues.

The European Stationary Cycle (ESC) is used to provide relevant load and speed operating conditions for the system simulations.

Model Development

A Scania D13 PPC engine system model was constructed, based on single cylinder experiments, a validated single cylinder model and a validated Scania D13, Euro 5, production engine model. The system model contains a low pressure EGR system, EGR cooler, charge air cooler and a single stage turbocharger. The system model was applied to cover a typical regime for ESC. For operating points without experimental combustion data the combustion was assumed based on observations from several PPC engines.

Martin TunérAssociate Professor

1. Validation of the developed single cylinder model to accurately represent and reproduce the single cylinder experiments (figure 1).

2. Use of the validated single cylinder model together with assumed heat release rates to extrapolate single cylinder engine performance for the ESC points not covered by experiments.

3. Expansion of the single cylinder model to a six cylinder model, but operated just as the single cylinder model with an external compressor, high pressure EGR loop and a back-pressure valve.

4. EGR circuit changed to a low pressure routing and the compressor added.

5. Inclusion of the variable geometry turbine, VGT, to the model. The compressor and turbine were still separated so that the scaling of both the turbine size and efficiency multiplier could be done with a relatively stable system.

6. A solid link was provided between the compressor and turbine, thus completing the PPC system model (figure 2).

Figure 1. Model of the Scania single cylinder PPC engine

P TP T


Figure 2. The multi-cylinder PPC engine system model.

Figure 3. Measured (red) versus simulated (blue) cylinder pressure from -80 to 80 CAD ATDC for A100.

Figure 4. Turbine efficiency for the operating range covering ESC except idle.

Figure 5. Compressor efficiency for the operating range cove-ring ESC except idle.

Results

Figure 3 shows the A100 point for the validation of the single cylinder model versus the experiments. The agreement is fairly good.

The completed multi-cylinder system model was applied to the ESC. By altering the size of the compressor and turbine and the efficiency of the turbine the conditions required for combustion could be met for all operation points. Compressor and turbine efficiencies are shown in figures 4 and 5..

With an increase of the compressor size with 37% and the turbine size with 30%, plus an increase of turbine efficiency with 8% compared to the standard single stage Holset VGT turbocharger the simulations indicate that a production-like PPC engine can cover the complete ESC with a peak brake efficiency of 47.7% (figure 6). This can be compared with the typical reported peak brake efficiency for production HD diesel engines of around 43%.

Figure 6. Brake efficiency for the operating range covering ESC except idle.


The reported engine peak brake efficiency of 47.7% was reached with an isentropic turbocharger efficiency of 55.3%. This is substantially higher than the highest theoretical efficiency of the standard turbocharger of 52.8%. Thus to realize this engine peak brake efficiency, the boosting system needs to be improved and flow restrictions or pumping work reduced. Possible potential routs involve two-stage turbocharging and reduced EGR and lambda.

As for other low temperature combustion concepts there is a concern, not only that the exhaust energy will not be sufficient for providing the required boosting, but also that the exhaust temperatures are too low for catalytic aftertreatment. For this reason the exhaust temperatures at the entrance of the SCR/muffler are shown in figure 7. For the major part of the operating regime the exhaust temperatures should be sufficient but at A40 and especially B40 the exhaust temperatures are marginal though.

Future work

Future work aims at producing single cylinder engine experimental results for all operating points of the ESC to avoid assumptions of RoHR, and to investigate system model performance with a two-stage turbocharger.

Figure 7. Temperature before SCR for the operating range covering ESC except idle.


PPC - The Modeling Project

scale and turbulent structures, such as vortex formation and shear layer instabilities, that control e.g. fuel and air mixing and fuel/air entrainment. The goal of the modeling project is to improve the knowledge about important underlying fundamental flow and combustion processes in internal combustion engines, and to create synergy effect with the combustion engine and combustion physics research groups in order to expand the overall knowledge of the combustion process in modern PPC engines and diesel combustion engines.

The LES spray and combustion models are evaluated using external experimental facilities, e.g. the Sandia Spray A and n-heptane rigs, and optical diagnostics facilities within the KCFP groups, e.g. the GenDies group. The PPC models have been used to investigate the effect of swirl on fuel/air distribution and combustion behavior in a heavy duty diesel engine and the effect of inter-jet angle in an optical heavy duty diesel engine. Furthermore, validation of the spray model and combustion model has been done in 3-D RANS and LES simulations in engines and constant-volume chamber configurations. Currently, we are focusing on the effect of umbrella angle on fuel distribution and combustion behavior in a light-duty diesel engine operating in PPC mode. This is carried out in order to investigate the potentials of broadening the operating range of low load PPC with gasoline-type fuels.

Rickard SolsjöPhD Student

Modification of Spray Model

Figure 1 shows the effect of the new spray model, employing a stochastic dispersion model and spray induced turbulent energy model for the tracking particles. To the left in figure 1, is the liquid length and fuel mixture fraction with the new model, to the right is the standard model. The results from the new model shows good comparison with the experimental results, whereas the standard model over-predicts the liquid length and under-predict the heat and mass transfer, i.e. the mixing, and thus an erroneous fuel mixture distribution. The results and implications of this model are to be presented in an upcoming paper.

Introduction

The aim of the KCFP modeling project is to perform detailed numerical simulations of Partially Premixed Combustion (PPC) and diesel combustion. Large Eddy Simulation (LES) is used as a tool to simulate the flow and combustion process and provides with information of high temporal and spatial resolution. This type of simulation is needed to resolve the important energetic large-

Evaluation of LES Models

The resolution for LES is usually on the order of the Taylor micro-scale, much larger than the average spray droplet size. To account for the spray breakup process, a Lagrangian Particle Tracking, LPT, method is used to track the motion and dynamics of the liquid fuel. Recently, a more physical representation of the liquid parcels and spray source terms has been implemented into the flow solver OpenFOAM. The result of this can be seen in Fig 1 below. Furthermore, an atomization model has been adopted and is to be compared with the standard approach of neglecting primary breakup. The model is also to be validated for engine LES simulations. Advanced turbulence/chemistry coupling methods will be coupled with Chemistry Coordinate Mapping (CCM) approach for efficient modeling of various combustion modes involved on PPC engines, e.g., ignition and flame propagation. CCM itself has also been improved to include more dimensions in the phase-space. This provides a better representation of the low temperature chemistry process and thus, better prediction of lift-off length. The mesh handling is now capable of managing not only piston motion, but also dynamic adaptive refinement and online cell add/removal. The chemistry/turbulence interaction is modeled using a time-scale model, which essentially requires the direct integration of the chemical kinetic mechanisms into flow transport simulations. The effect of SGS turbulence on the mixing and transport process is modeled using a one-equation Smagorinsky model.


Effect of Inter-Jet Angle in Diesel Engine Combustion

The effect of inter-jet angle on diesel combustion employing two different nozzle-setups was investigated using Large Eddy Simulation with CCM, using 44 species and 174 reactions [1, 2]. The reference case was studied in an optical investigation by the GenDies group at the combustion engine and the combustion physics divisions. The configuration employed three inter-jet angles; 45°, 90° and 135° in a modified Scania D12 engine running� at low load, 5 bar IMPEg. The primary objective was to investigate the mechanism controlling the lift-off length in real diesel engine combustion. The experimental results implied a decreasing lift-off length tendency with decreasing inter-jet angle, believed to be an effect of enhanced mass and heat convection from nearby jets due to the swirling motion. This effect would increase the local temperatures surrounding the auto-ignition zone.

The LES result confirms this hypothesis. As the inter-jet angle decreases, the local temperatures surrounding the auto-ignition zone are higher for the small inter-jet angle case compared with the larger inter-jet angle, Fig 2. Also, the swirling flow generates an asymmetric lift-off length due to transport of hot products from one side of the jet to the other side of the jet, Fig 3. Furthermore, there is a general ongoing discussion on the controlling mechanism for the ignition front propagation in a diesel engine, auto-ignition process

and ignition front propagation. In this study, obvious low-temperature combustion species are formed instantly after leaving the nozzle, indicating the onset of an auto-ignition process further downstream implying that the combustion process is not governed solely by flame propagation, Fig 4.

Figure 2. Comparison between experimental (red), numerical (blue) and theoretical (black) lift–off lengths for the various inter-jet angles.

Figure 1. LES modeling of the ECN spray-A case at 60 bar and 900 K. Left: two-way coupling between discrete phase (droplets) and gas phase by the new model; Right: effects of discrete-phase on the turbulence not included. The liquid length and fuel mixture distribution on the left agrees well with experimental results compared with the results on the right.

Figure 3. Symmetric nozzle-configuration showing the swirl direction and the asymmetric lift-off length comparing the upwind with the downwind side.


Ongoing Work: Low-Load PPC

Currently, LES simulation of an advanced strategy employing three injections at -46 CAD aTDC, -22 CAD aTDC and 3 CAD aTDC is carried out to investigate the effect of umbrella angle on the mixing process and combustion process in an optical light-duty diesel engine. The goal is to gain deeper understanding of the effect of fuel and air mixing and injection timing on the observed engine experiments trend, namely, decreased efficiency and increased engine-out missions when a smaller umbrella angle is utilized. The engine employs a 5-hole standard nozzle with injection pressure of 500 bar. The work is to be presented in the upcoming European Combustion Meeting taking place in Lund, Sweden, Summer 2013.

Figure 4. Key species snapshots at 2 CAD aTDC. There are obvious low-temperature species in the nozzle region, indication an auto-ignition process. The center of the jet contains high levels of unburned fuel due to insufficient fuel and air mixing before onset of combustion.

References

[1] R. Solsjö. M. Jangi, C. Chartier, Ö. Andersson, X-S. Bai, “Liftoff and stabilization of n-heptane combustion in a diesel engine with a multiple-nozzle injection”, PROCI-D-12-00837[2] R. Solsjö. M. Jangi, C. Chartier, Ö. Andersson, X-S. Bai, “Jet-Jet interaction in Diesel Combustion”, COMODIA 2012. [3] M. Jangi, T. Luccini, G. De Erico, X.S. Bai, Effects of EGR on the structure and emissions of diesel combustion. Proc. Combust. Inst., http://dx.doi.org/10.1016/j.proci.2012.06.093.[4] M. Jangi, X.S. Bai, Diesel combustion modeling including finite rate chemistry using chemistry coordinate mapping approach. Combustion theory and modeling, in press, 2012.[5] M. Jangi, R. Yu, X.S. Bai, A multi-zone chemistry mapping approach for direct numerical simulation of auto-ignition and flame propagation in a constant volume enclosure, Combustion theory and modeling, 16: 221-249 (2012).[6] M. Jangi, T. Luccini, G. De Erico, X.S Bai, Numerical Simulation of the ECN Spray A Using Chemistry Coordinate Mapping: n-Dodecane Diesel Combustion, SAE 2012-01-1660


PPC - Fuels

Analysis of Surrogate Fuels Effect on Ignition Delay and Low Temperature Reaction during Partially Premixed Combustion

Fuel effects on ignition delay have been studied previously for diesel, Homogenous Charge Compression Ignition (HCCI) and PPC combustion. The ignition delay plays an important role in partially premixed combustion. Achieving the desired premixed combustion requires increasing the mixing of the fuel and air prior to ignition. The ignition delay depends on two factors; physical and chemical processes. Therefore, understanding the effect of these changes on the early combustion stages is essential.

The work is focused on understanding the connection between fuel properties, in particular content of alcohol, aromatics and alkanes, and combustion process. There were three main goals: First, understand the chemical and physical influence on ignition delay. Second, investigate the influence of chemical and physical parameters on the LTR phase. A previous study showed that ethanol and high level of exhaust gas recirculation suppresses LTR while n-heptane amplifies LTR in HCCI combustion. Hence, the focus of this study is to see if the same trend is present for PPC. Third, investigate if there is a correlation between ignition delay and LTR and if this correlation is influenced by fuel composition or engine operating conditions such as: inlet oxygen concentration, combustion phasing or injection pressure.

From the DoE regression model the predicted ignition delay can be calculated using

together with the regression coefficients. β is a constant term while βN, βT and βE are linear terms for n-heptane, toluene and ethanol respectively. The interaction coefficients are βNT (interaction between n-heptane and toluene), βNE (interaction between n-heptane and ethanol) and βTE (interaction between toluene and ethanol). The quadratic coefficients are βNN (n-heptane quadratic), βTT (toluene quadratic), and βEE (ethanol quadratic). N, T and E are the volume fraction [%] of n-heptane, toluene and ethanol respectively.

The coefficients give an understanding of the response in the ignition delay from each factor. As seen in Figure 1 at CA50=3, the linear parts have more influence on the ignition delay than the interaction and quadratic parts. It is notable that both ethanol and n-heptane affect the ignition delay equally but in opposite directions: ethanol enhances the ignition delay, while n-heptane suppresses it. In the same way the surrogate fuel effect can be seen at other CA50s, inlet oxygen concentrations and injection pressures.

(1)

IntroductionCompression ignited (CI) engines generally have higher efficiency than spark ignition engines. However, the most common combustion concept in CI engines, conventional diesel combustion, struggles with high levels of particulate matter (PM) and NOx emissions. Therefore, new combustion strategies are used in CI engines to reduce engine-out PM and NOx emissions. One such promising strategy,

Hadeel SolakaPhD Student

partially premixed combustion (PPC), is analyzed in this thesis. Independently of it properties, if a combustion concept should be possible to put in production in a near future it is important that it can utilize the available fuel on the market. Hence, it is important to understand how PPC responds to fuel properties, especially RON value and ignition quality.

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Figure 1. Correlation coefficients between ignition delay and surrogate fuel fractions


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Furthermore, the inlet oxygen concentration had impact on ignition delay. The ignition delay increased as the inlet oxygen concentration decreased. A lower inlet oxygen concentration requires more time for the injected fuel to react with the available oxygen. The low oxygen concentration does not affect the physical delay but it has a greater influence on the chemical delay. This is because the low oxygen fraction slows down the chemical reaction rate.

It was found that both ignition delay and LTR had similar features with fuel compositions, inlet oxygen concentrations and injection pressure. Thus, a linear correlation was found between the LTR and ignition delay. This relation was sensitive to both inlet oxygen concentration and combustion phasing. Furthermore the relation was more influenced by inlet oxygen concentration than ethanol fraction in the mixture.

Future Work

• Finnish the analysis of emissions and efficiencies for the data from the TERF campaign.

• Follow up the TERF campaign with kinetics modeling to increase the understanding of the premixing process and combustion chemistry

• Investigate how different ratios of ethanol/diesel mixtures can possibly expand the operating load regime, and which combination that has the largest operating span and lowest emissions.

In Figure 2 the markers represent the experimental data and the curves represent the DoE regression model fit. The regression model for a response is based on the whole data set (17 data points), but few of the surrogate fuel mixtures are plotted. These surrogate mixtures are: PRF, TRF (which consist of 15% toluene and PRF), ERF (which consists of 10% ethanol and PRF), TERFcritical (critical point which consists of �15% toluene, 10% ethanol and PRF) and TERFcp (center point which consists of 7.5% toluene, 5% ethanol and PRF). The rest of the surrogate mixtures captured the same trends. The effect of ethanol and toluene on ignition delay and low temperature reactions are of interest. Ethanol had the strongest effect on both ignition delay and LTR, but the effect was further amplified by adding toluene. This is since for higher RON it is necessary to advance SOI to maintain the same CA50. Thus the cylinder temperature is lower during the fuel injection that slows down the early reactions.


Since many of the parameters in the model are not calibrated it is interesting to evaluate the sensitivity to parameter values for the parameters that were selected a priori. Figure 2 shows the sensitivity to the Arrhenius rate constant which is present both in the ignition model and in the combustion rate model. It can be seen that the pressure modeling is very insensitive to the choice of Arrhenius rate constant. It should be noted here that the calibrated parameters have been recalibrated for each Arrhenius rate constant.

Figure 2. Sensitivity to the Arrhenius rate constant that is present both in the ignition and the combustion models.

References

1. Widd, A., “Physical Modeling and Control of Low Temperature Combustion in Engines”, Doctoral dissertation, Lund University, Department of Automatic Control, April 2012.

PPC - Control

Introduction

The PPC-Control project pursues control and control-oriented modeling of engine PPC combustion with focus on physics-based models. The work on PPC combustion modeling using JModelica continued during 2012 and results concerning parameter sensitivities were produced. Anders Widd received his PhD with the title “Physical Modeling and Control of Low Temperature Combustion in Engines” in April 2012 and the PhD student position in the project has been left vacant for the rest of the year. The plan is to fill the position with a new student in the beginning of 2013.

Model Calibration

The physics-based PPC model described in [1] is automatically calibrated with respect to experiments using a collocation method where the sum of squares of the difference between modeled and measured cylinder pressure were minimized for 360 pressure samples with one crank angle degree separation. The model contains many parameters that could be included in the optimization but in order to keep the optimization problem tractable some parameters were selected a priori. This also reduces the problem with over-parameterization. Figure 1 shows the result of a calibration. The pressure trace using the initial guess parameters as well as the calibrated parameters are shown.

Figure 1. Automatic model calibration using JModelica.

Anders WiddPhD Student

Patrick BorgqvistPhD Student


The Generic Diesel ProjectDue to its relatively high efficiency, the diesel engine is an important power source for road transport. Its importance can be expected to increase as the demands on fuel efficiency increase. However, its exhaust emissions of nitrogen oxides and particulates remain challenging. The Generic Diesel (GenDies) project currently focuses on various in-cylinder mechanisms that explain trends in soot emissions.

After the fuel jets impinge on the walls of the diesel combustion chamber they form wall jets. These are interesting from a soot perspective since the air entrainment into these wall jets is believed to influence the soot oxidation rate. This mixing process was investigated in 2011 using a novel laser diagnostic called SLIPI. During 2012 the results were fully analyzed and published. A summary is presented below.

On a more fundamental level, the liquid spray penetration length was investigated on a multi-hole nozzle. The interest is due to the hot gas reservoirs that form along the jet periphery, which raise the temperature locally and affect the lift-off length. The effect is enhanced when the inter-jet angle decreases, and it is of fundamental interest to see if the inter-jet angle also affects the liquid spray penetration.

Meanwhile, a laser extinction setup has been developed for measuring the late-cycle soot oxidation. It is currently installed on the optical heavy-duty diesel engine and experiments are being started up.

Clément Chartier PhD Student

Johan SjöholmPhD Student

removing the background that arises from scattered laser light outside the sheet.

Figure 1 shows the measured fuel distribution in the combustion chamber. The middle fuel jet intersects the horizontal laser sheet just upstream of the wall (F.J. zone), forms a wall jet that collides with a wall jet from the adjacent spray and thereby forms a recirculation zone (R.Z.). Mixing in the free part of the jet (F.J.) follows the mixing laws that have been established for single jets, whereas mixing at the wall and up to the recirculation zone (R.Z.) is affected by interaction with the wall.

A technique called Structured Laser Illumination Planar Imaging (SLIPI) makes it possible to measure fuel concentrations close to surfaces. This is because it removes background effects from scattered light. This makes the technique exceptionally suited for mixing in wall jets, which occurs on the wall of the combustion chamber. SLIPI uses laser sheets that are spatially modulated with a sinusoidal pattern. By combining images from three such laser sheets it is possible to retain the fluorescence signal from the laser-illuminated plane, while

Mixing in Wall Jets of a Heavy-Duty Diesel Engine


Figure 1: Example image from the SLIPI measurements. White lines indicate the four sprays and a white dot the centrally mounted injector. The free part of the jet (F.J.) refers to the region just upstream of the combustion chamber wall (curved surface on the left), while the recirculation zone (R.Z.) occurs just where two adjacent wall jets collide with each other.

The main conclusions from this experiment is that the wall jet is expected to impede soot oxidation as compared to a free jet, and increased injection pressures do not increase the access to oxygen in the wall jet. Furthermore, parts of the recirculation zone are re-entrained into the free part of the jet, thus decreasing the oxygen concentration in the wall jet region even more. In summary, the conditions for soot oxidation are significantly poorer in wall jets as compared to free jets.

Figure 2: Development of the fuel concentration in the free part of the jet as function of time after start of injection and injection pressure.

Figure 2 illustrates some results from the measurements in the free part of the jet, shown as a function of time after start of injection. The rise in fuel concentration at the start shows the formation of a stagnation point as the jet impinges on the wall. After this, the mixing shows no dependence on the injection pressure, which is consistent with the mixing laws of free jets. Just prior to the end of injection a slight rise in the fuel concentration is seen for the higher injection pressures. The reason can be seen in Figure 1, where the swirling flow is seen to transport fuel from the recirculation zone towards the free part of the jet, where the fuel is re-entrained. This effect should reduce the oxygen concentration by the wall and hence reduce the soot oxidation rate there. Another conspicuous feature in Figure 2 is the rapid leaning out after the end of injection. This occurs before the last fuel from the nozzle reaches the wall and indicates the existence of an entrainment wave (described by Mark Musculus at Sandia).


The main observation in this experiment was that the inter-jet angle does affect the liquid length, but in the opposite direction to what was expected: closer jet spacing actually produced a longer spray length (see Figure 4). We hypothesize that this effect is due to increased evaporative cooling in the volume around the jets when they are closer together. This hypothesis was confirmed by LES calculations by Rickard Solsjö where the evaporative cooling was readily observed. The heating from the hot gas reservoirs is thus concluded to be confined to the flame region, whereas the liquid parts of the jets close to the nozzle predominantly experience entrainment of fresh air.

Figure 4: Boxplot representation of the liquid spray length distribution on the confined (45°) and solitary (135°) jets. The confined jet clearly exhibits a longer mean penetration length.

Figure 3: Bottom-view sketch of the spray laser illumination of the sprays in the combustion chamber (left), and a Mie-scattering image acquired in the engine (right) indicating the inter-jet angles.

Influence of Jet-Jet Interactions on the Liquid Spray Lenght in an Optical Heavy-Duty DI Diesel Engine

Previous results from our laboratory have shown that hot combustion products at the jet periphery increase the temperature locally at the flame base. This shortens the lift-off length. We showed that the inter-jet angle had a strong effect on the lift-off length, as it affects the amount of hot products between the jets.

This experiment investigates whether the hot combustion products also affect the liquid spray length. If so, closer jet-spacing would increase the temperature. This would increase the vaporization rate and thus and shorten the spray length. This assumption was tested in the engine using an asymmetric injector.

Edouard Berrocal PhD Student

Guillaume LequienPhD Student

Figure 3 shows the setup used. The main feature is an asymmetric nozzle which provides two inter-jet angles (45° and 135°). This nozzle was mounted in an optical Scania engine and the sprays were illuminated with a continuous diode laser. Liquid fuel was imaged using Mie scattering during fired conditions.


The Gas Engine Project

IntroductionThe Gas Engine Project at Lund University aims to explore and understand the combustion phenomenon in engine operating on gaseous fuels and develop technologies as an alternative to present day diesel operated heavy duty engines which are facing severe challenges like stringent emissions norms, high technology cost and unsustainable fuel supply. Over the past few years, Natural Gas has emerged as the most promising gaseous fuel due to its benefits in terms of

Experimental WorkAs a first step, the performance and emission characteristics of a 6 cylinder gas engine were compared for operation with conventional and a pre-chamber spark plug which was bought from the market. It was observed that the dilution limit extended slightly with pre-chamber spark plug as seen in Fig. 1. The main observation, however, was the notable reduction in flame development angle with pre-chamber spark plug (Fig. 2) which fortifies the belief that the jets from pre-chamber offer much higher ignition energy than

a single point spark. It was also observed that the pre-chamber spark plug cause charge pre-ignition at load exceeding 10 bar IMEPg mandating the requirement of some form of dilution to restrict the in cylinder temperature which also restricted the maximum load achievable with pre-chamber spark plugs as shown in Fig. 3. Even so, the operation with pre-chamber spark plug was more stable as can be seen in Fig. 4.

Figure 1. Comparison of dilution limit with excess air at 1500 rpm and various operating load

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Ashish ShahPhD Student

emission reduction with cheap after treatment devices and increasing fueling station network worldwide. The current phase of this project focuses on exploring alternative ignition techniques as after completion of all previous phases it was observed that the capability of conventional spark plug ignition system was the factor limiting the extent of dilution and hence emission reduction and efficiency improvement.

Two most feasible alternative ignition systems were identified, namely diesel pilot injection and pre-chamber type ignition system but it was soon realized that the former has already received considerable attention as there are products in the market under different names like The Hardstaff OIGI® (Oil Ignition Gas Injection), Westport’s High-Pressure Direct Injection (HPDI) applicable to a wide range of engines. Comparatively, however, the concept of pre-chamber ignition has received limited attention and is mainly applied to stationary or large bore marine engine which do not face as severe speed and load transients as experienced by a heavy duty engine for mobile application. Reasons behind this are believed to be limited knowledge about the mechanism of ignition resulting from a pre-chamber type ignition device and hence gaining deeper insight into this mechanism is currently the objective of the gas engine project.


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Figure 4. Coefficient of Cycle to Cycle variations in IMEPg during ESC-like12 mode test cycle under maximum excess air dilution

The applicability of Ionization current sensing technique when operating with pre-chamber spark plug was also studied. The main observation was the absence of second peak in ion current which is known to coincide with the time of peak cylinder pressure. Also, the strength of first peak was observed to be less affected by dilution and hence contributing to a higher ion current integral value even at dilution limit of operation. Figure 5 and 6 present the ion current signal characteristics under various dilutions for convention and pre-chamber spark plug case respectively.

Figure 5. Ion Current signal for different excess air dilution conditions at 5 bar IMEPg for conventional spark plug

Figure 6. Ion Current signal for different excess air dilution conditions at 5 bar IMEPg for pre-chamber spark plug

Overall, it was observed that the mechanism of ignition resulting from pre-chamber type ignition device was very complex and it was difficult to identify a pattern from the results obtained so far. Several factors like the relative volume of two chambers, design and orientation of connecting nozzle, relative equivalence ratios of two chambers, and location of spark ignition inside the pre-chamber etc. largely affect the resulting ignition in main chamber and hence there is a need to understand the effect of each of these factors by conducting further detailed and controlled experiments.

Literature SurveyThe concept of pre-chamber is not new and has been researched for nearly a century. Hence, it was decided to conduct a survey of literature available on different pre-chamber concepts and related research so as to avoid unnecessary repetition. Following the literature survey, pre-chamber system was identified as one of the divided chamber stratified charge concepts for SI engines which was proposed first by Sir Harry Ricardo in 1918 (US 1,271,942). Since then, several concepts with either similar or slightly different objective have been patented and almost all were based on concept of having a readily combustible (stoichiometric mixture) in vicinity of ignition source in a pre-chamber with a very lean mixture in main chamber with the nozzle connecting these chambers so designed as to have a high velocity jet. Thus, the ignition in main chamber mainly relied on the high temperature of jet coupled with its turbulent mixing with main chamber charge.

A major deviation from this concept occurred in 1968 when a Russian scientist, L. A. Gussak, proposed a concept called Lavinia Aktyvatsia Gorenia (Avalanche activated combustion in English), popularly known as LAG-process which was based on the chain branching theory developed by N. N. Semyonov. This concept proposed the use of very rich mixture in pre-chamber (equivalence ratio around 2) which when ignited will result in incomplete combustion forming active species and ‘chain carriers’ which were then injected into the main chamber rapidly advancing the chain branching reaction. Available literature (SAE 750890, 830592 etc.) offers a very good insight into the mechanism of LAG-process of ignition but also highlights its sensitivity to fuel properties and other design factors. Hence it is necessary to study the behavior of this ignition mechanism with natural gas and at high operating loads as those reported by Gussak are only upto 8 bar IMEPg.

Future WorkIn light of the literature survey and the conclusions thereof, it is planned to conduct single cylinder experiments with specifically designed pre-chambers to study the effect of relative equivalence ratio between two chambers, effect of operating load (inlet pressure) and the effect of pre-chamber volume and nozzle area on the quality of ignition in main chamber.

More results and discussion related to these experiments can be found in our SAE paper number 2012-01-1980 and 2012-01-1632.


Anders Widd, defended his PhD thesis “Physical modeling and control of low temperature combustion in engines” at the Department of Automatic Control on April 10, 2012. Anders Widd presently works at Haldor Topsoe in Denmark.

Elias Kristensson got his Ph.D. in March 2012. The title of his thesis is ”Structured Laser Illumination Planar Im-aging SLIPI Applications for spray diagnostics”. Within KCFP Elias worked with high-speed Planar Laser In-duced Fluorescence Imaging, and with fuel visualization through used of the SLIPI technique.

Johannes Lindén successfully defended his Ph.D. thesis, with the title “Laser-Induced Phosphor Thermometry - Feasibility and precision in combustion applications”, in late September 2012. Within KCFP, Johannes worked mainly with techniques for in-cylinder surface tempera-ture measurements.

Johan Sjöholm got his Ph.D. in June 2012. The title of his thesis is ”High Repetition Rate Laser Diagnostics for Combustion Applications”. Within KCFP Johan was responsible for the high-speed imaging activities us-ing the Multi-YAG system. Among others he performed high-speed measurements of fuel and soot distributions as well as quantitative mixture concentration measure-ments in Diesel sprays. Johan is now employed by MAN Diesel.

Clément Chartier, defended his PhD thesis “Spray Pro-cesses in Optical Diesel Engines: Air-Entrainment and Emissions” at the Department of Energy Sciences on April 16, 2012. Clément Chartier presently works at Sca-nia CV in Södertälje.

KCFP PhD Students who Graduated in 2012

lunds universityKCfpltH box 118221 00 lundtel 046-222 00 00www.lth.se/kcfp

www.lth.se/kcfp

KCFPKompetenscentrum Förbränningsprocesser

Centre of Competence Combustion ProcessesFaculty of Engineering, LTH

P.O. Box 118SE-221 00 Lund

Sweden

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