Experimental investigation on influence of EGR on combustion performance in SI Engine

M. Božić1*, A. Vučetić1, D. Kozarac1, Z. Lulić1 1 University of Zagreb, Faculty of Mechanical Engineering and Naval Architecture

Abstract In this research, the use of stoichiometric air-fuel mixture with exhaust gas recirculation (EGR) technique in spark-ignition engine was experimentally investigated and its influence on main SI combustion parameters is shown. Experimental setup on which this research is performed consists of SI engine, AC Dyno, software for control of engine and dyno, and various measurement devices including the indicating equipment for SI engine. The research is performed by comparing the SI operation at specific load with and without EGR. In this analysis two comparisons are made: one with fixed spark timings and one with optimized spark timings. Introduction

The future development of motor vehicles is oriented to environmental awareness in order to prevent global pollution. Greenhouse gases are proven to be one of the main reasons for global warming [1][2]. Therefore, the internal combustion engines are today designed so that they emit limited pollutants and CO2 emissions, while delivering the required performance. As CO2 emission is proportional to fuel consumption, the efficiency of IC engines is becoming one of the primary development targets [3]. On the other hand, one of the pollutants that is restricted by emission regulations and is produced in large amounts in SI engines is NOX. Although NOX is efficiently cleaned by a three way catalysts in SI engines exploration of pathways toward lower NOx production is still worthy especially if one considers running the engine in lean conditions. In [4] it has been shown that exhaust gas recirculation (EGR) is one of the most effective techniques for reducing engine out NOX emission in SI engines. Suppression of NOX formation is achieved by diluting the in cylinder mixture with EGR, by lowering the in-cylinder temperature through the increase of heat capacity in the in-cylinder mixture and by reducing oxygen concentration [6]. The reduction of fuel consumption of modern SI combustion engines is today obtained by several technologies: direct injection, combustion chamber optimization, turbocharging, etc. Turbocharging increases load and therefore reduces the ratio of friction losses, but at the same time increases the tendency to knock. In [5] it has been shown that cooled EGR can be employed for avoiding abnormal combustion, e.g. knock. This reduction of knock tendency is achieved by the combination of three effects: diluent effect, thermal effect and chemical effect [4]. The use of EGR for lowering fuel consumption was shown in [6] where the improvements from 6% at high speed up to 17% at low speed were presented. Similarly Potteau et al. [9] measured the reduction in BSFC up to 14% by use of EGR. Additionally they showed an exhaust gas temperature decrease up to 100°C. The use of EGR can also be employed for reducing throttling losses at part load as shown in [7].

Although there are some benefits of using EGR in SI there are also some shortcomings and limitations. The use of EGR decreases combustion stability and increases coefficient of variation of IMEP. Even though the use of EGR in SI has been researched [8] and there are some publications regarding its use, there is no comprehensive analysis of the EGR influence on SI combustion, performance and emissions. The purpose of this research is to make a step forward in the understanding of the EGR influence on SI combustion and consequently engine performance and emissions. This will be achieved by a comprehensive set of experimental results showing various influences of EGR and of methods of controlling the engine in conditions with elevated EGR settings for exploiting these benefits. The experimental work will be done at University of Zagreb, Faculty of Mechanical Engineering and Naval Architectures, Laboratory for Engines and Motor Vehicles on the experimental setup shown in Figure 1.

Figure 1. Experimental setup, engine testbed

This paper will show some initial results at single engine speed and load. In the results presented in this paper the external and cooled EGR was set to the levels of 0, 11, 15 and 20% with the goal of obtaining higher engine efficiency through lowering the tendency to knock and enabling earlier spark timings. This could lead to higher indicated mean effective pressure (IMEP) and efficiency. During experiment lots of other performance and emissions data (e.g. coefficient of variation of IMEP (CoVIMEP), in-cylinder temperature, etc.) were measured and evaluated and will be presented in this paper.

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Experimental setup The purpose of this experimental setup is to

enable the acquisition of data from SI engine in various operating conditions so that the influence of these conditions on engine performance and emissions can be studied.

Experimental tests were carried out on a single cylinder diesel Engine HATZ 1D81Z that was converted to SI operation. Engine has two valves per cylinder and combustion chamber of a toroidal type. The main characteristics of the modified experimental engine are listed in Table 1.

Table 1. Testbed SI engine characteristics Manufacturer Hatz, 1D81 Engine type 1 cylinder 4 stroke Bore, mm 100 Stroke, mm 85 Combustion chamber Toroidal Displacement, ccm 667,59 Std. compression ratio 12 Intake Valve Timing Open @ 340°,

Close @ 590° Exhaust Valve Timing Open @ 154°,

Close @ 380°

In order to enable operation in SI combustion mode with gasoline the engine compression ratio had to be modified. It was lowered from 20.5 to 12 by the modification (machining) of the piston. Additionally the engine was equipped with hall sensors on crankshaft and camshaft in order to acquire crank mechanism position to enable implementation of engine controls. Spark plug was placed centrally on top of the cylinder where it replaced diesel injector. Intake manifold was custom made and so that it enables gasoline port fuel injection. Software for the engine control was programed in-house. Spark timing and injection timing is controlled by the created software and monitored through the signals recorded on indicating equipment. Besides modifications of the engine the additional control and measurement equipment was used inside the testbed. Basic schematic of the used testbed is shown in Figure 2. Emission of CO and CO2 in the exhaust manifold was acquired by Bosch ETT 8.55 EU analyser while for continuous and simultaneous measurement of Total Hydrocarbons (THC) Environnement GRAPHITE 52M heated FID analyser was used. For control of EGR the standard passenger car EGR valve with water cooled heat exchanger was used. The amount of EGR is controlled by opening of electric valve allowing exhaust gas to recirculate from exhaust to the intake manifold. The value of recirculated EGR is determined by measurement of CO2 at the intake and exhaust. This is done by means of two CO2 analysers. First one is already described Bosch analyser, placed on the exhaust manifold. For measurement of the intake CO2 Environnement MIR 2M infra-red analyser was used.

Percentage of the recirculated exhaust gas is calculated by the equation:

where [CO2]Ambient is the concentration of CO2 in ambient air, [CO2]Intake gas is the measured concentration of CO2 in the intake manifold and [CO2]Exhaust gas is the CO2 measured in the exhaust gases [8].

Figure 2. Experimental IC SI Engine testbed

Acquisition and processing of fast crank-angle based signals typical for combustion engines is done by engine indicating equipment. In this experimental work the engine indicating system AVL IndiSmart 612 hardware and Indicom software are used [9,10]. The system was used for acquisition of combustion data from the engine, e.g. In-Cylinder pressure, Intake pressure, IMEP, CoVIMEP. Additionally the system was used for real time monitoring of intake pressure, spark timing, dwell time, injection timing, spray amount, IMEP, CoVIMEP, Knock and Heat Release. The low pressure sensor for measurement of the intake pressure was LP11DA and for high in-cylinder pressure GH14DK as shown in the Figure 3.

Figure 3. Engine indicating system

Resolution of the acquired data was 0.1 degree of crank angle. The fuel flow was measured by OHAUS Explorer mass scale while for the measurement of the intake air mass flow the TSI 2017L laminar mass flow meter was used. The entire setup was equipped with numerous K-type thermocouples for measurement of temperature.

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Experimental work

Experimental data was taken for four different cases of EGR dilution at constant speed of 1600 rpm. All four cases were optimized by multiple measurement points with spark sweep performed from very late towards early ignition up to the point where knock combustion occurs (Figure 4). Initial measurements were made with no EGR with spark timing ranging from -6 to -19 °CA after top dead centre (ATDC). The second case was measurement with 11% EGR with spark timing ranging from -16 to -40 °CA ATDC. The third case was measurement with 15% EGR with spark timing ranging from -24 to -50 °CA ATDC and the fourth case was measurement with 20% EGR with spark timing ranging from -36 to -50 °CA ATDC. The fourth (last) case was not measured up to the point of occurrence of knock because cycle to cycle variations were too high resulting in unstable combustion. Therefore advanced spark timing was not possible.

Figure 4. Spark timing of four measured cases

In order to achieve comparable operating points with the same amount of energy of fuel, intake air flow was set to the average of 20.3 kg/h and fuel flow (Euro super 95) was set to the average of 1.4 kg/h. When dilution with EGR was applied it took a part of the volume inside the cylinder leaving less space for fresh intake mixture. Therefore if all conditions would remain the same less intake charge (energy) would be sucked in the cylinder during intake. To compensate this effect and to ensure comparable operating points with the same amount of energy, the intake pressure was increased together with each addition of EGR. The level of intake pressure was increased in a manner that the amount of fuel and air flow was the same as in the case when there was no EGR dilution. The resulting pressures in the cylinder at the intake valves closure timing are shown in Figure 5.

Figure 5. In-cylinder pressure at intake valve closure

Excess air ratio (λ) was kept at stoichiometric condition (λ =1) by keeping the same air to fuel ratio during all measurements.

Results and Discussion

In the presented results two comparisons are made: one with fixed spark timings and one with optimized spark timings.

For the fixed spark timing the comparison is made with different amounts of applied EGR dilution in order to observe the influence of EGR on the combustion with the same spark timing (ignition) conditions. Measurement points with 0 and 11% EGR dilution are compared at spark timing of -16°CA ATDC, 11 and 15% EGR dilution are compared at spark timing of -30°CA ATDC and 15 and 20% EGR dilution are compared at spark timing of -44°CA ATDC. By looking at the obtained IMEP values shown on Figure 6 the points with significantly different IMEP are chosen to present the influence of EGR dilution on combustion.

Figure 6. Indicated Mean Effective Pressure for fixed

spark timing measurement points

The application of EGR results in reduced IMEP because the combustion temperature is reduced which results in longer combustion duration, lower peak pressures and lower peak of rate of heat release. The results presented in Figure 7 show that the addition of the 11% EGR diluent decreases IMEP from 6.21 to 5.94 bar (4.3%) at spark timing of -16°CA ATDC. Further on, addition of 15% EGR in comparison to 11% EGR diluent decreases IMEP from 6.41 to 6.23 bar (2.8%) at spark timing of -30°CA ATDC, and addition of 20% in comparison to 15% EGR diluent decreases IMEP from 6.55 to 5.52 bar (15.8%) at spark timing of -44°CA ATDC.

Figure 7. Indicated Mean Effective Pressure for fixed

spark timing measurement points

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Significant rise in CoVIMEP is observed when EGR dilution is applied as shown on Figure 8.

Figure 8. Coefficient of Variation of IMEP for fixed

spark timing measurement points

With application of EGR dilution, maximum average in-cylinder temperature decreases, as shown on Figure 9.

Figure 9. Maximum in-cylinder temperature for fixed

spark timing measurement points

The trend of in-cylinder temperature is directly followed by the NOX emission as shown in Figure 10 where a significant decrease in NOX emission is observed.

Figure 10. Nitrous oxides emission for fixed spark

timing measurement points

Combustion duration shows increasing tendency with the addition of EGR dilution. Figure 11 shows that the by the increase in EGR dilution the combustion duration increases.

Figure 11. Combustion Duration for fixed spark

timing measurement points

The influence of EGR dilution on unburned total hydrocarbons is shown in Figure 12. One can observe that there is no significant increase of THC with EGR dilution. Higher emission of hydrocarbons is observed only when unstable combustion occurs like the one with 20% EGR dilution where CoVIMEP was 12.3%.

Figure 12. Total Hydrocarbons emission for fixed

spark timing measurement points

Figure 13 shows the in-cylinder pressure for the presented measurement points. Pressure drop can be observed at fixed spark timings influenced by the EGR dilution.

Figure 13. In-Cylinder pressure for fixed

measurement points

Heat release rate is calculated from the acquired in-cylinder pressure. Heat release on Figure 14 shows retarding of the combustion phasing with the application of EGR dilution. Further on, longer combustion duration and lower peak of rate of heat

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release can be observed in measurement points with higher EGR dilution.

Figure 14. Rate of Heat Release for fixed

measurement points

In order to achieve optimal results from the combustion with EGR dilution spark timing was optimized. Optimized spark timing measurement points are selected from the acquired values of IMEP and MAPO (maximum amplitude pressure oscillation) shown in Figure 15 and Figure 16. The optimized conditions for the selected measurement points were highest achieved IMEP without the occurrence of knock.

Figure 15. Indicated Mean Effective Pressure for

optimum spark timing measurement points

Figure 16. Maximum Amplitude of Pressure

Oscillations for optimum spark timing measurement points

The results show that maximum generated IMEP for the measurement case with no EGR was 6.21 bar.

With EGR dilution applied in the amount of 11% and with spark timing advanced to -36°CA ATDC, IMEP was raised to 6.48 bar. When higher dilution value of 15% was applied and spark timing advanced to -48°CA ATDC, IMEP was raised to 6.57 bar. However, when dilution of 20% was applied IMEP fell to 5.52 because CoVIMEP increased significantly. Therefore with the dilution of 20% the limit on combustion stability was reached. To conclude, IMEP shows almost linear rise with the addition of EGR dilution (Figure 17) up to the limit of combustion stability shown in Figure 18.

Figure 17. IMEP for optimized spark timing

measurement points

The results showed that the contribution of the diluent effect on IMEP rise was 4.4%, 5.8% and -11.2% for the dilutions of 11%, 15% and 20% respectively.

Figure 18. Coefficient of Variation of IMEP for

optimized spark timing measurement points

NOX emissions show significantly lower values with the application of EGR dilution. Even though spark timing was optimized and combustion was advanced to knock limit, NOX emission decreased.

Figure 19. Nitrous oxides emission for optimized

spark timing measurement points

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Increase of cylinder pressure with the input of EGR is influenced by the higher intake pressure and by advancement in spark timing. Heat release shows longer combustion duration and lower peak rate of heat release with the application of EGR dilution as shown on Figure 20.

Figure 20. In-Cylinder pressure and Rate of Heat

Release for optimized measurement points

Conclusions In this research, the use of EGR strategy was experimentally investigated in spark-ignition engine. Engine performance and emissions were studied and following conclusions have been obtained:

• With the application of EGR dilution significant increase in CoVIMEP is observed which implies that there is a limit on how much dilution can be applied in order to cool down the combustion.

• With the use of EGR abnormal combustion was effectively suppressed and the engine performance was improved.

• Spark timing and intake pressure optimization is required in order to achieve higher efficiency when diluting intake mixture with EGR. In general intake pressure has to be increased and spark timing advanced.

• Application of EGR dilution significantly reduces NOX emission.

Acknowledgements

Special thanks to Department for IC Engines and Handling Equipment and Laboratory for Engines and Motor Vehicles.

The study was performed within the FMENA project “Experimental Research, Optimization and Characterization of piston engine operation with DUal-Fuel COmbustion - DUFCOROC” IP-2014-09-1089 funded by the Croatian Science Foundation. This help is gratefully appreciated. http://www.fsb.unizg.hr/miv/dufcoroc

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