Mecca Supercharging Turbocharging

MECCA 3/2003

SIMULATION OF SUPERCHARGED AND TURBOCHARGED SMALL SPARK-IGNITION ENGINE

JI NAVRTIL, MILO POLEK, OLDICH VTEK, JAN MACEK, PAVEL BAUMRUK

VUT v Praze / Vzkumn centrum spal. motor a automobil Josefa Boka, Technick 4, 166 07 Praha 6; Tel. +420224352507, Fax. +420224352500, [email protected]

SHRNUT

Zlepen spoteby paliva je mon doshnout pouitm maloobjemovho spalovacho motoru. Pro udren jzd-nch vkon je poteba zajistit patin nrst vkonu peplovnm.

lnek se zabv porovnnm dvou monch variant peplovn malho zehovho motoru. Prvn varian-tou je tzv. mechanick peplovn. Dmychadlo objemovho typu je spojeno s motorem mechanickou vazbou s konstantnm pevodem. Porovnnm nkolika zpsob regulace s vyuitm krtc klapky a obtoku je vybrn vhodn zpsob zen mechanickho peplovn. Druhou variantou je spojen motoru s turbodmychadlem. Dan turbodmychadlo nen vybaveno promnlivou geometrii. Regulace je provdna odpoutnm vfukovch plyn a krtc klapkou umstnou za kompresorem.

ABSTRACT

Improvements in fuel consumption can be gained through engine downsizing. It is necessary, however, to provide a corresponding increase in power to maintain or improve vehicle performance. This increase can be achieved by supercharging or turbocharging.

This paper deals with comparing two possibilities supercharging and turbocharging of a small SIengine. Supercharging is performed through a mechanical connection between an engine and a supercharger of the Ro-ots type with a constant gear-ratio transmission. A suitable control system was chosen from several concepts of throttling and by-passing intake air. Turbocharging uses a small turbocharger without variable geometry. Boost pressure control is performed by waste-gating exhaust gasses and a throttle valve placed downstream the com-pressor.

KEYWORDS

Supercharger, turbocharger, waste-gate, by-pass, Roots blower, downsized gasoline engine, knocking, simulation, optimization

1. INTRODUCTION

Supercharging or turbocharging may provide engine downsizing, which reduces relative value of losses at low engine loads and vehicle weight, decreasing thus road fuel consumption of a vehicle. By using suitable motor-management it is also possible to modulate the required full-load speed curve.

At the other side, the danger of knocking or even mixture auto-ignition limits the optimum compres-sion ratio set for the best efficiency. The optimum combination of compression ratio and spark advance

must be investigated especially at low engine speed where knocking takes often part. Driving power demand of a supercharger or (to lesser extent) the exhaust back pressure caused by a turbine may com-pensate for the efficiency advantages from engine downsizing in strong dependence on the system of boost pressure control. The importance of it is the bigger the lower is the engine speed at maximum torque. Such multi-parametric optimisation cannot be provided by experiments only because it would be too time-demanding and too expensive. The more suitable way is to combine a virtual-engine

SIMULATION OF SUPERCHARGED AND TURBOCHARGED SMALL SPARK-IGNITION ENGINE 27

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simulation with some basic experiments used for initial calibration of engine model and finally for the confirmation of optimum results checking the vicinity of simulation-predicted optimum matching. The simulation provides moreover a useful tool for the comparison of different boost-pressure control concepts under quasi-steady or rapid change opera-tion parameters.

The aim of the current paper is to present a com-parison of potentials at supercharging or turbocharg-ing of an automotive spark-ignition (SI) engine, us-able in sub-compact cars.

All models presented here were created in GT-Power (GTP) software code. GTP is one of the commercial codes for engine simulation combining a 1-D approach for manifolds, pipes and ports with a 0-D one at engine cylinder. The data presented here were obtained after calibration according to experiments at a real natural aspirated engine. From that reason, they have quantitative, not only qualita-tive significance.

2. SYSTEM OVERVIEW

A 3 cylinder engine was simulated at natural as-piration and then supercharged or turbocharged in a virtual reality. The initial calibration consisted in the use of mechanical efficiency, self-ignition delay and burning rate duration found during ex-periments and generalised according to the authors methodology (to be published later) to the new mixture parameters.

Basic engine parameters are describedin the following:

Swept volume 1.2 dm3 Nominal power 40 kW / 4 750 rpm Maximum torque 105 Nm / 3000 rpm 4 valves for each cylinder head.

Supercharger of the modified (helical gear)Roots type:

Eaton M24. Turbocharger with waste-gate boost control Garrett GT12.

3. PROBLEMS WITH A SUPERCHARGED AND TURBOCHARGED ENGINE

There are some important operation limits caused by the change from a naturally aspirated engine (AE) to a supercharged/turbocharged one (SE). The knoc-king and its influence on the optimum compression ratio has been already mentioned. The best way for the use of octane offer of a specific fuel is to use adaptive spark advance control. It was respected du-ring simulation by maintaining the same safety mar-gin to the knock limit or admitting the same (small) knock intensity.

Maximum combustion pressure is boosted by supercharging to values near to the stress limits of a crankshaft, bearings or a piston strength. While an AE has its p

max as a firing pressure well below 6

MPa, the pmax

of a SE goes up to 8 MPa. Therefore, it is necessary to find the best compromise between compression ratio, spark advance, fuel/air ratio and boost pressure to maintain a reasonable firing pres-sure and to avoid knocking. Generally, at the opti-mum compression ratio the spark advance has to be significantly retarded compared to an AE.

Exhaust gas temperature upstream of a turbine or a catalytic converter must be kept below ceratin limit, as well. A rich mixture has to be used at and close to the full-load curve to decrease exhaust tem-peratures. The charge temperature should be as low as possible using intercooling or internal fuel-va-porisation cooling if possible to decrease the knock probability.

4. MECHANICAL SUPERCHARGING

A supercharger represents an apparently cheaper way to increase engine power. A by-pass is used to control the supercharger, whereas the engine torque is controlled by the standard throttle. A mechanical transmission connects the supercharger to the en-gine. It has a constant gear ratio (GR). The first task

Figure 1: Scheme of mechanical superchargingObrzek 1: Schma mechanickho peplovn

28 JI NAVRTIL, MILO POLEK, OLDICH VTEK, JAN MACEK, PAVEL BAUMRUK

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was to find a suitable GR. A low engine speed (2000 rpm) has been used for its determination to obtain high torque at low engine speed. A by-pass or an-other control facility is closed at this low speed and the maximum pressure demand in conjunction with

the reasonable spark advance at good fuel efficiency indicated the best applicable GR. In optimisation procedure the GR was gradually increased until the optimum parameters were reached. The optimal GR was set to 3.2 in this case, giving the supercharger a speed of 6400 rpm at an engine speed of 2000 rpm. If the engine speed is higher than 2000 rpm, it is necessary to use the by-pass or throttling to reduce boost pressure. The scheme of the engine arrange-ment is presented in Figure 1.

Setting gear ratioBy-pass is closed

Mt max = 157 Nm

Pe max = 67 kW

The maximum combustion pressure is shown in Figure 2. Figure 3 shows the full-load characteristics of the supercharged engine. The throttle is opened all the time and the by-pass opens stepwise as engi-ne speed increases.

The increase in engine performance is clearly visible. The torque increase is about 50% and the power increase almost 70% compared to an AE. There is, however, one problem: the exhaust tem-perature increases to over 1200 K with a stoichio-metric mixture. This temperature could lead to clo-se-coupled catalyst damage. One of possible ways to decrease exhaust temperature is to employ a rich mixture, in other words to cool in-cylinder mixture by fuel vaporisation. Thus, the air-fuel ratio changes from 13.8 at low engine speed to 11.9 for the highest engine speeds. Extensive fuel enrichment leads to high specific fuel consumption for all engine speeds but the temperature decreases to 1200 K. The com-puted operating points in the compressor efficiency map are shown in Figure 4. Each point represents an individual engine speed from the full-load characte-ristic. The collocation of operating points is around the maximum compressor efficiency and compres-sor mass flow corresponds to engine mass flow. The compressor matches the engine very well.

4.1 POSSIBILITIES OF SUPERCHARGER CONTROL

The following section compares three possibilities for boost pressure control using a combination of throttle and by-pass. The first alternative uses the throttling upstream the compressor (supercharger) and the bypassing of compressed air from the com-pressor outlet back to the compressor inlet. The

Figure 2: Maximum combustion pressures at full-load curveObrzek 2: Maximln spalovac tlak na vnj charakteristice

Figure 3: Full-load characteristics of supercharged engineObrzek 3: Vnj charakteristika mechanicky peplo-vanho motoru


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scheme is demonstrated in Figure 1.

The next tested variant includes only a throttle. The throttle is placed at the compressor inlet. For this variant see Figure 5.

The third alternative shows throttling down-stream the compressor see Figure 6.

A constant low load performance is used for a comparison of these three boost pressure control options. The intake pressure (right before the inlet valve) is held at a constant value (p=1 bar) by the control mechanism. The low load regime is chosen because there are greater differences between fuel consumption in this mode than on the full-load curve and the control system has to regulate much more to hold the inlet pressure at the required value. See Figure 7 for a comparison of these three models. The operating points are drawn on the compressor efficiency map and they correspond to the chosen

speed regimes. The first alternative (throttling + by-pass) is the most suitable one. The compressor outlet pressure for this variant is around 1 bar. The other variants lie above the first one. In the case of throttling af-ter the supercharger it was necessary to enlarge the compressor map to accommodate the (unrealistic) high--pressure ratio associated with a very poor compressor efficiency, which was in a rather optimistic way

kept at the last measured value for pressure ratio of 3. The curves of specific fuel consumption show the differences between the tested variants Figure 8. A large gap is evident, especially between throttling downstream the supercharger and the other options.

The combination of throttling with bypassing (Figure 1) is the optimal variant for managing co-operation between engine and supercharger. The choice was based on the comparison of specific fuel consumption and operating point projection.

5. TURBOCHARGED GASOLINE ENGINE

1. Maximum Turbo RPM180 Nm / 74 kW

2. High Performance Engine170 Nm / 74 kW

Figure 4: Compressor efficiency map with engineoperating pointsObrzek 4: Mapa innosti kompresoru s pracovnmi body

Figure 5: Throttling before superchargerObrzek 5: krcen ped dmychadlem

Figure 6: Throttling behind superchargerObrzek 6: krcen za dmychadlem


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3. Economic Engine140 Nm / 55 kW

4. Mechanical Supercharging157 Nm / 67 kW

The GT12 turbocharger has been connected to the engine. The turbine does not have variable geo-

metry. Boost control is ensured by a waste-gate and a throttle that is situated downstream the compres-sor. There is also an intercooler between the compressor and en-gine. Figure 9 shows the resulting scheme of a turbocharged engine. The engine torque and boost pres-sure is controlled using co-ordinati-

on between the throttle and the waste gate position. The waste-gate modulates full-load characteristics and the throttle continues in engine control just after the boost pressure demand approaches to an atmos-pheric one or even goes below it in the low-load en-gine regimes where the waste-gate is fully opened. There is also a problem with the high exhaust tem-perature at the full-load. The enriched mixture

with the spark advance decrease are used to solve this problem, as well in the case of supercharging. Figure 10 shows the possibility of adjusting the full--load curve by using a different waste-gate diame-ters at different maximum torque-speed points. The upper curve shows the absolute maximum values of torque that can be achieved from this engine. The turbocharger RPM is held at the maximum value (around 200 000 rpm). The mixture enrichment to-gether with a very retarded spark advance leads to very high fuel consumption Figure 11. The second line displays the torque curve that could be used for a sports car engine. The fuel consumption is cut down at the lower engine speed but at high engine

RPMs the maximum power is main-tained. The economic engine (third curve) is achieved by waste-gate diameter enlargement. The power is decreased in all engine regimes but the fuel consumption enhancement is clear. See Figure 11 for fuel con-

Figure 7: Compressor efficiency map for choosing applicable type of super-charger regulationObrzek 7: Mapa innosti kompre-soru pro porovnn rznch zpsob regulace

Figure 8: Comparison of fuel consumptions for three typ-es of supercharger regulationObrzek 8: Porovnn mrnch spoteb paliva pro rzn typy regulace

Figure 9: Model of turbocharged en-gineObrzek 9: Model peplovanho mo-toru


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sumption comparison. The last plotted line shows the full-load curves for mechanical supercharging.

Figure 12 presents more information on the waste-gate diameter for the first variant (maximum available performance) of the turbocharged engine. This figure contains a chart of the opening diameter representing the waste-gate free cross-section. The waste-gate also opens at the lowest engine RPMs. This means the loss-making control must switch on too early. Optimally the waste-gate is closed at low engine speed and it opens at high speeds (at full-load curve). Therefore, from it follows that it would be better to use a turbine with larger flow area, which was not available in the time of these simulation calculations. This will provide better efficiency and lower fuel consumption for all variants.hh

6. CONCLUSION

This study was carried out to compare two types of supercharging and to select their optimal means of control. For mechanical supercharging the com-bination of throttle (downstream a supercharger) and a by-pass according to Figure 1 was selected. Mechanical supercharging can effectively increase engine performance but high fuel consumption is its major disadvantage. On the other hand, turbocharg-ing increases the power extensively with lower fuel consumption. In this case, it could probably be bet-ter to use a slightly larger flow-area turbine to avoid loss-making waste-gate control.

The study proved the possibilities of simula-tion for complicated tasks of motor- management at supercharged/turbocharged engines. The unpub-lished results of comparison with some experiments are promising concerning the precision of predic-

tions. The next step is an investigation of transient load response of the engine, which is being carried on just now.

ACKNOWLEGEMENTS

This work has been supported by the Ministry of Education of the Czech Republic, Project No. LN00B073 - Josef Boek Research Centre. This help is gratefully appreciated.

REFERENCES

[1] Morel, T. et al. (2000): GT-Power Users Manu-al, Gamma Technologies Inc. Westmont IL 2000

[2] Vtek O., Navrtil J.: Combination of 0-D Combustion Engine Simulation with 1-D Fluid Dynamics Tuned Manifold Systems and Variable Valve Timing. In: CD-ROM Fisita 2002 Proceedings, Paper Code F02V263

Figure 10: Setting different full-load characteristics using wastegate diameterObrzek 10: Nastaven rznch vnjch charakteristik pomoc vypoutn vfukovch plyn

Figure 11: Engine fuel consumption for different types of full-load characteristicsObrzek 11: Mrn spoteby paliva pro rzn rychlostn charaktirstiky

Figure 12: Waste-gate diameter for full-load curveObrzek 12: Oteven obtokovho ventilu na vnj cha-rakterstice


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Mecca Supercharging Turbocharging

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