一种新型涡轮增压GDI发动机

A new approach to GDI turbo engine: Com- bining new technology to low cost applica- tions Dipl.-Ing. Patricia ANSELMI, Dipl.-Ing. Xavier GAUTROT, Dipl.-Ing. Alex- andre PAGOT, Dipl.-Ing. Pierre LEDUC, Institut Français du Pétrole A new approach to GDI turbo engine: Combining new technology to low cost applications Abstract In a search for lower fuel consumption at high performance, this project combines the latest technologies in gasoline engines while responding to production cost concerns: down-sizing direct injection system and a double valve timing variation, coupled to the reliable and proved technology of a single-scroll turbocharger. The result maintains low end torque targets, maxi- mum power and reduced emissions. The resulting engine has been implemented in a Vel Satis vehicle, the proposal aiming to the same performance offered by a V6 3 l naturally aspired gasoline version but with a 20 % fuel consumption gain. The study leads to a demo car com- pletely optimized and calibrated at IFP, based on simulation results and taking advantage of an adapted in-house control system. The engine is an adapted version of a Renault 2 l direct injection model modified by integra- tion of intake and exhaust variable valve timing. The turbocharger selection is of single-scroll type, proven to be the most reliable system today, at lowest cost and favouring 3-way catalyst heating through a lower thermal inertia. Contrary to the tradition of matching valve overlap- ping to twin-scroll technology, this project has proven that it is also possible to take advantage of the residual gas blow-down effect obtained through double VVT even when coupling it with a single-scroll turbine housing. This way, low end torque characteristic of a single-scroll turbine housing can be improved by means of the maximized filling efficiency obtained by adapted double VVT position. The result is maximum torque of 340 Nm starting from 1500 rpm and a maximum power of 147 kW at 5500 rpm. The integration of the double VVT, down-sizing and the adaptation of the transmission has allowed a CO2 emission equal to that of a 2 l Diesel engine at standard European driving cycle, 195 g/km. Therefore the study yields to an all-winning result of high performance and reduced costs by means of a mono-scroll turbocharger, capable of giving the same performance as a V6 – 3 l naturally aspired engine while a 20 % fuel consumption gain is achieved, bringing it to the low level of a last diesel generation engine. IFP low cost downsizing strategy In comparison to diesel engine, today's conventional gasoline engine suffers from a lack of thermal efficiency but has both advantages of being significantly cheaper to manufacture, and of having a highly efficient, reliable and low energy consumer after-treatment system thanks to three-way catalytic converters (3W cat). IFP considers that one of the major strategies for the gasoline engine is to increase thermal efficiency while preserving these two benefits as far as possible: 3W cat and low cost. Down- sizing is compatible with this mainline. In fact, turbocharging a gasoline engine while reducing its cubic capacity - downsizing - is a major way for reducing CO2 emission. Considering the chain of efficiency leading to the final global efficiency of an engine, this approach reduces pumping and friction losses, in comparison to a conventional naturally aspi- rated gasoline engine (see figure 1). CHAIN OF EFFICIENCY HOW TO IMPROVE DOWNSIZING combustion eff. complete combustion 0 thermodynamic eff. 1-1/r c ( γγγγ -1) => increase r c and γγγγ - cycle eff. => fast combustion at TDC => reduce thermal transfers => reduce pumping - - + mechanical eff. => reduce friction + (reference = conventional naturally aspirated gasoline engine, + : advantage of the downsizing approach - : disadvantage of the downsizing approach : ways of the approach to be improved rc: compression ratio) Figure 1. Improving efficiency by downsizing Even though turbocharging has been used for a long time, it is still subject of extensive re- search. Today, the question lies on maintaining the benefit in terms of fuel consumption and interesting production cost, while gaining a high level of driveability and the time-to-torque typical of naturally aspired engines. In this way, a lot of research and development work deals with improvement in low end torque and reduction of the time-to-torque [1,2,3]. Usually, air-charged spark ignited combustion presents high knock propensity that requires a lower compression ratio and the use of retarded spark timing. This leads respectively to a lack of thermodynamic efficiency and cycle efficiency (marked by in Figure 1). In order to take full advantage of downsizing, the scavenging of residual gases is considered as a way of in- creasing knock resistance. In this way, two sources of knocking will be optimized: the high temperature of the air-fuel mixture during the combustion process, and the presence of residual burned gases from previous combustion that have a thermal and chemical impact on the fresh mixture [4]. At high load low engine speeds, because of boosted intake air pressure is higher than exhaust back pressure, it is possible to scavenge the residual gases through valve overlapping. The easiest way for performing scavenging is by implementing twin scroll turbocharger (for 4 cylinder or twin-turbo V8 engines with a flat crankshaft configuration) and direct injection of gasoline to avoid fuel presence in the exhaust line. Twin scroll turbocharger is a technology developed for L4 engines that presents a dedicated layout of its turbine housing in order to separate the exhaust gas flows of the different combus- tion chambers and to reduce cylinder to cylinder exhaust flow interaction. With conventional turbo, interaction between cylinders will increase the pressure in the ex- haust manifold at the end of the exhaust stroke due to the beginning of the exhaust stroke of the preceding cylinder. In this case, scavenging of residual burned gases is not effective, and back flow fills the cylinder with exhaust gases if valve overlap is used. With a twin scroll turbo, the pressure difference between intake port and exhaust manifold during overlapping is then higher than with conventional turbocharger and the scavenging is more efficient. The reduction of residual burned gases fraction is significant [5] (see figure 2). Considering all these issues, IFP main strategy regarding downsizing consists in: - combustion process based on high knock resistance thanks to scavenging of residual burned gases for improving thermal efficiency, - keeping stoichiometric combustion with 3W cat after-treatment for an environmental friendly engine, - maintaining interesting engine production costs, using reliable technologies, and es- pecially conventional turbochargers. This strategy offers a high CO2 saving and presents high synergy with mild hybridization for further gain. This paper will deal with the turbo charging technology, proposing single scroll type because of it main benefits: lower cost and reduced thermal inertia (for quicker catalyst light-off). For better results, the technology is combined to double valve timing variation (in- take and exhaust) and direct injection. Taken into account that the main inconvenient of a single scroll housing is a lower low end torque compared to twin scroll turbo, which is negative to the driveability, IFP recent work, presented in this paper, gives a solution to this problem and shows that it is possible to main- tain a high level of low end torque with an engine fitted with a single scroll turbo. Maintaining scavenging process with single scroll turbo on L4 en- gine On a turbocharged engine functioning with large valve overlap and equipped with a single scroll turbo charger, back flow from exhaust to intake at the end of the over lap is awaited. As a matter of fact, with a conventional valve event, we obtain positive scavenging during the first part of the valve overlap, and negative back flow during the second part, as shown in figure 3. With a dedicated valve event, by optimizing opening duration and timings, it is possible to maintain the scavenging process during the whole valve overlap. Maintaining the same valve overlap duration, we reach the same scavenging than with a twin scroll turbo, the volumetric efficiency is then increased and the high knock resistant combustion is reached (see figure 4). 0,7 0,8 0,9 1 1,1 1,2 1,3 1,4 Valve overlap [CAD] Vo lu m e tr ic e ffi c ie n c y [ - ] Twin Scroll Turbo - conventional valve event Mono Scroll Turbo - conventional valve event Mono Scroll Turbo - optimized valve event 8 10 12 14 16 18 20 Valve overlap [CAD] B M EP [b ar ] Twin Scroll Turbo - conventional valve event Mono Scroll Turbo - conventional valve event Mono Scroll Turbo - optimized valve event Figure 2. Effect of valve event optimization: volumetric efficiency and BMEP vs valve overlap at 1250 rpm. With optimized valve timing, and maintaining exhaust lambda value to 1, a high oxygen con- tent in the exhaust gases at low engine speed confirms experimentally that scavenging is really effective despite the use of a single scroll turbo (see figure5). 0 0.5 1 1.5 2 2.5 3 3.5 4 1000 2000 3000 4000 5000 6000 Engine speed [rpm] ex ha u st 02 co n te n t [ % ] Twin scroll turbo Mono scroll turbo Figure 3. Oxygen content in the exhaust gases Test Bed Results Engine Configuration and Operation Conditions The engine used in this program is an in-line 4 cylinder engine (see table 1). Engine capacity is 1.8 l. Conventional swirl-type direct injectors are implemented. The engine is fitted with two camphasers for conventional variable valve timing (VVT). Ignition coils are pencil type. To enable fast and stable combustion, high air tumble motion is obtained thanks to tumble type air ducts and piston design. A dedicated short duct and low plenum volume intake system is also fitted [6]. Table 1: IFP 1.8 l engine features. engine type in-line 4-cylinder capacity (cm3) 1783 bore x stroke (mm) 82.7 x 83 cylinder head 4 valve / cyl., tumble shape air ducts valve train 2 camphasers (intake and exhaust) injection system swirl type injectors 5-12 MPa injection pressure combustion type homogeneous, stoichiometric compression ratio 10.3 : 1 The engine is operated under the following testing condition: European commercial type RON 95 fuel; lambda value is kept at 1 until a fuel/air mixture enrichment is required to maintain exhaust temperature at the thermal acceptance of the turbine housing (950 °C); exhaust back pressure at maximum power (after turbine) is 40 kPa; spark timing at minimum advance for best torque or KLSA; and full load intake air temperature at intake plenum of 50 °C ± 2 °C, whatever the engine speed. The results presented in this paper with single scroll and twin scroll turbine are obtained with the same engine. In order to perform scavenging process despite the exhaust wave pulses at intake top dead centre in case of single scroll, a well-matched single scroll turbocharger, an adapted exhaust manifold and dedicated valve event durations and timings are adapted to the single scroll Turbine application. Single Scroll Turbo VS Twin Scroll Turbo: Full load comparison Results presented here concern a comparison between a twin scroll and a single scroll applica- tions, with the same performance target: - torque: 24 bar BMEP - maximum output: 90 kW/l, that means 160 kW on this 1.8 l engine. In this first comparison, the characteristics of the turbochargers are quite similar: - the compressors are the same: type, diameter of the wheel and trim of the housing - the turbines are quite similar: type and diameter of the wheel are the same, but the a/r factor of the housing is slightly different because of differences in the housing. Main performance criterion (see table 2) concern low end torque capability and BSFC at ma- ximum power. Table 2: Main performances ( Max temperature upstream turbine: 950 °C for both versions) Performance Criterion Single Scroll Version Twin Scroll Version BMEP @ 1250 rpm (bar) 12.6 14.5 engine speed for 20 bar BMEP (rpm) 1650 1570 BSFC @ max. power (g/kWh) 336 (for 90 kW/l and 950 °C) 332 (for 90 kW/l and 950 °C) lambda value at max. power 0.79 0.80 Both versions have been tested with the same turbine upstream temperature limit, that is 950 °C, since it is the limit of a twin-scroll housing. We can notice that performances obtained on these criterion are very close, especially at high engine speed. The main difference between the single scroll version and the twin scroll version concerns the low end torque, especially for engine speed of 1500 rpm (see figure 6). In fact, the pulsed flow is very important for the effective turbine efficiency, having an influence over the maximum torque at very low engine speed. With a single scroll turbo, the pulses of the exhaust waves are more expanded in the manifold, which is critical for the response time of the turbocharger. The consequence is a lack of BMEP at 1250 rpm of 1.9 bar, despite the fact that the scavenging process is quite the same between the two versions thanks to the optimization of the valve event as explained above. After 1500 rpm, the performances are the same. Figure 4. Single scroll housing compared to twin scroll housing, 24 bar BMEP and 90 kW/l The necessary enrichment to limit the temperature upstream turbine at 950 °C is higher with the single scroll turbine at medium engine speed, while being quite similar at high engine speed and maximum power. This result is due to the temperature sensors upstream turbine. As a matter of fact, the gas flow to the sensor is different between turbine and exhaust mani- folds types: For the twin scroll type, the sensor is under the influence of two cylinder flow, while for a single scroll type all 4 cylinders exhaust flow are seen by the sensor. Therefore, the thermal flow seen by the sensors is not the same for each case and, for the same temperature conditions upstream turbine, the temperatures downstream turbine are different. In the case of the single scroll version this means a 20 °C lower temperature, having an important impact over the enrichment. This phenomena explains that, for the same temperature conditions up- stream turbine, the enrichment necessary is higher with the single scroll housing at medium engine speed. At high engine speed, and especially at maximum power, the enrichment is simi- lar because of the benefit given by the higher flow capacity of the single scroll housing, when considering a similar a/r factor for the two housings. The BSFC comparison presents the same trend as the enrichment: at high engine speed BSFC is similar between the versions, especially at maximum power. At medium engine speed, there is a gain between 10 and 20 g/kWh for the twin scroll application, fuel consumption being impacted by the different exhaust thermal levels seen by the sensors. Optimized Turbo Matching As it has been presented in the previous chapters, it is possible to re-establish the scavenging process in the case a single scroll turbo used on a in-line 4 cylinder engine and, thanks to that, performances are very close between the two versions, single scroll and twin scroll. In order to improve even more the low end torque, a new turbocharger matching has been defined. The targets are then: - 24 bar BMEP at the lowest engine speed - 82 kW/l, that means 147 kW on this 1.8 l engine For this development, the limit on the exhaust temperature has been changed. Turbine upstream temperature is known to be the technological limit of a twin scroll turbine housing because of thermal stress on the separation strip. In order to reduce production costs it is typically limited to 950 °C. For single scroll turbine housings the temperature limit is higher and can reach up to 1050 °C at a lower production price impact, and such a design has already been tested in mass production [11]. Because of this, and because of the different thermal stress seen by the temperature sensor, as discussed above, tests have been realized with a higher turbine upstream temperature limit for the single scroll turbine. An additional limit has been taken into account in order to respond to actual catalyst limitations, that is a turbine downstream temperature limit of 860 °C. Under these conditions, the following performances (see table 3) have been reached: Table 3: Main performances (Max temperature downstream turbine: 860 °C) Performance Criterion Single Scroll Low End Torque Version BMEP @ 1250 rpm (bar) 20 engine speed for 24 bar BMEP (rpm) 1500 BSFC @ max. power (g/kWh) 289 (for 82 kW/l) lambda value at max. power 0.87 With this turbo matching low end torque is particularly impressive: 20 bar BMEP are available at 1250 rpm, and 24 bar BMEP are available at 1500 rpm. On the other side, consumption remains very low and acceptable at maximum power, 82 kW/l. This is due to the good lambda value, 0.87, thanks to the 860 °C downstream turbine temperature limit. Maximum temperature upstream turbine is then around 1030 °C, which is technologically available at reasonable cost for single scroll turbine housings (see figure 7). Figure 5. Single scroll 24 bar BMEP application (Max temperature downstream turbine: 860 °C). For engine speed higher than 2000 rpm fuel consumption is reduced. For engine speed lower than 2000 rpm, the consumption is higher, with a maximum at 1400 rpm. This is partly a con- sequence of the high scavenging of fresh air in this range of engine speed. A good manner to reduce this is to run the engine lean at the exhaust, keeping cylinder mixture slightly rich for knock resistance. It is then possible to reach the same low end torque with a lower consump- tion level than that presented for a twin-scroll turbine (a 10 to 20 g/kWh reduction) in this range of engine speed. Because of the smaller turbine diameter but same low-end torque, dynamics are improved with the single scroll turbine. At the same time, the lower permeability will help reduce thermal inertia favouring catalyst heating. These advantages will call for better driveability and emis- sions for the IFP optimized engine. Simulation Work: Vehicle fuel consumption Torque curve and fuel consumption map obtained at the test cell have been used in a simula- tion work in order to compute the effect of the use of such an engine in a vehicle. Simulation has been carried out with AMESim simulation environment, using IFP-DRIVE Library [8]. Simulated vehicle is an upper class one, with a total mass of 1900 kg (see table 4). For the computation, the vehicle is fitted with IFP 1.8 l turbo engine, with the configuration corre- sponding to single scroll optimized matching version, 340 N·m (24 bar BMEP) at 1500 rpm and maximum power of 147 kW. Table 4: Simulated vehicle features Mass (kg) 1900 Drag coefficient (SCx, m²) 0.85 Engine features IFP 1.8 l turbo In terms of driving conditions, simulation is made considering: - official European NEDC standard cycle - three other driving cycles, named ARTEMIS urban, road and motorway, more repre- sentative of real life vehicle use, with a lot of transient and, as far as the urban part is concerned, 5 stop and go phases per kilometre [6] - two accelerations: 0 to 100 km/h and 80 to 120 km/h on the 5th gear ratio. Simulation results are compared to the consumption figures obtaine