Potentials and Limits of CO2 Emissions of Gasoline Engines
In MTZ 4 the Technical University of Kaiserslautern and Karlsruhe Institute of Technology (KIT) have presented the mechanical possibilities to further develop homogeneously and stoichiometrically operated gasoline engines. This second part of the article describes new combustion processes to increase engine efficiency. For this purpose stoichiometric and stratified lean operation mode as well as future combustion processes like HCCI and their potentials are presented. The described potentials and limits of both articles are combined and contrasted with each other.
Already in  the possibilities and limits of thermodynamic approaches to optimise process on homogeneously, stoichiometrically operated gasoline engines via valve train variability were evaluated. Therefore the application of cam phasers, discretely staged valve lift shifters and fully variable systems were separately examined. Furthermore the potential to reduce fuel consumption with cylinder deactivation in throttle free engine operation was demonstrated; as well as the technology combination of fully variable compression ratio and mechanically fully variable valve train on inlet and outlet sides. This present article extends these statements by possibilities and limits to further develop combustion process.
FULLY VARIABLE VALVE TRAIN WITH HOMOGENOUS LEANING
During homogenous-lean operation mode, efficient exhaust gas after treatment by three-way catalyst is no longer useable to reduce NOx emissions. Thus, additional extensive exhaust gas after treatment is necessary, e.g. NOx storage catalyst or SCR. From today’s perspective the transfer into mass production is irrelevant due to the small advantages in fuel consumption and the costly emission control.
FULLY VARIABLE VALVE TRAIN AND LAMBDA-SPLIT PROCESS
At higher load points with increased residual gas compatibility, fuel consumption advantages of up to 8 % could be reached . Thus, this technique is particularly suited in combination with throttle-free load control to improve fuel consumption at higher loads (customer fuel consumption). However, after treatment of exhaust emissions also has to be adapted to this engine operation mode.
SPRAY-GUIDED GASOLINE DIRECT INJECTION
Since the average driving speed actually driven on the street is approximately double this amount, a higher average engine performance is required in every day vehicle operation. The engine operating point most applicable to this situation is at n = 2000 rpm, BMEP = 5 bar. In a naturally aspirated engine, the specific fuel consumption at this particular operating point lies at 255 g/kWh which corresponds to a specific efficiency of 33 %. When relating this efficiency to the actual efficiencies between 25 and 30 % achieved today in everyday vehicle operation with homogenous gasoline direct injection and charging, an efficiency increase of 10 up to over 30 % can be attained through the use of spray-guided direct injection with stratified charge and piezo injectors. Through more intensive research and further development of this combustion process, focusing especially on the injection technique used here, but also through improved fuels, efficiencies of more than 40 % may be further attained. To date, spray-guided direct injection opens up the highest potential so far for fuel economy that can be achieved with today’s approved technologies.
It is evident that the overall mass of emitted particles at an injection pressure of 1000 bar is considerably lower than that at an injection pressure of 200 bar. In fact, up to a mean pressure of IMEP = 6 bar, the particle mass remains below 0.05 mg/m3 at an injection pressure of 1000 bar, and only starts to increase at IMEP = 7 bar to 0.5 mg/m3 at IMEP = 8 bar. Given today’s current practice of maximum injection pressures of 200 bar, such a result can only be achieved at an indicated mean pressure of 3 bar.
In so doing a reduction of NOx emissions could likewise be achieved compared to an operation with injection pressures of 200 bar, although further testing is here required. In particular, exhaust gas retention and exhaust gas recirculation strategies are relevant here. In light of this aspect, a combination of direct injection and fully variable valve control on inlet and exhaust side is promising. In fact, tests on stratified exhaust gas recirculation  have shown that NOx emissions can be reduced by far more than 90 %, in some cases even up to 99 %.
HOMOGENEOUS CHARGE COMPRESSION IGNITION
Nevertheless, this combustion technique remains of interest for the further development of gasoline engines. Particularly the extremely low levels of exhaust emissions might make extensive after treatments of exhaust gas (particle filters; NOx catalysts) unnecessary. In this context it is by all means also worth considering combining direct fuel injection with stratified charge and HCCI mode, depending on the operating point in the engine map.
SUMMARY AND FUTURE PROSPECTS
This paper shows that there is still a high potential to reduce the fuel consumption and CO2 emissions in gasoline engines. When examining future combustion technologies it is noticeable that development departs from conventional gasoline combustion. A combination of different combustion methods may allow the lowest possible fuel consumption and exhaust emissions in the overall engine load map.
When comparing the different measures aimed at improving engine efficiency through deliberate development of injection systems, charging, valve trains, friction and auxiliaries as well as factors in basic engine design (geometry of combustion chamber, charge motion, compression), drive train efficiencies of 40 % and beyond are expected to be attained. In relation to the NEDC, according to which today’s new vehicles of the B-segment are consuming on average approximately 6.5 l fuel per 100 km (146 g CO2/km) , 40 % of efficiency results in 4.15 l per 100 km (93 g CO2/km). It is therefore the case that the required 95 g CO2/km by the year 2020 for a vehicle of the B segment (e.g. VW Golf) can be achieved simply through a consequent enhancement of gasoline engines, using additional funds that are still lower than the costs of diesel engines and far below the costs of new driving systems (full hybrid, plug-in hybrid, electric vehicles, fuel cell).
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