MethCar: Perfecting gas engines for future traffic & transport needs.

Methane has significant advantages over gasoline and diesel fuels. Its C/H-ratio is considerably better – which is why CO₂ emissions are 20% lower than those from gasoline given the same energy yield. In addition, assuming stoichiometric operation, methane combustion in an Otto engine is extremely clean and delivers significantly better anti-knock performance than conventional Otto engine fuels. This makes it possible to achieve the high compression ratios needed for improved efficiency and for realizing enhanced downsizing factors without the need for efficiency-reducing, retarded combustion settings. The anti-knock properties offered by pure RE-methane (methane number MN » 98… 100) are even superior to those of natural gas (MN > 65) which are themselves significantly better than those of gasoline.

At low engine speeds, the higher charge-pressure required in comparison with gasoline engines could be realized, for example, by using a smaller turbocharger. However, smaller turbochargers are unsuited for high speeds because they generate so much exhaust backpressure that peak power would have to be significantly reduced. One solution to this problem is the variable-geometry turbocharger (VGT) that can be dynamically adapted to suit the speed-range. Until now, the use of variable-geometry turbochargers has not been possible because they were, in general, not sufficiently heat-resistant for use on Otto engines generating high-temperature exhaust gases. The use of these turbochargers will only become feasible with the development of an optimized engine that delivers the specific power levels expected in today’s market and significantly reduces exhaust temperatures whilst also delivering reliably high operational peak pressure resistance in combination with high anti-knock RE-methane.

Fully variable valve gear

An additional means of reducing temperature involves intermediate cooling of the compressed air before ignition. This can be achieved with the Miller Cycle that requires fully variable valve-gear actuation for air inlet. However, this must first be modified to facilitate the high combustion pressures targeted here. This variable inlet valve gear arrangement also facilitates largely throttle-free partial-load operation that avoids pumping losses and thus enhances efficiency – meeting a further objective of the MethCar program.

MethCar participants are likewise investigating fuel composition requirements because the need for the highest possible levels of efficiency combined with requirements for compliance with stringent future exhaust emission regulations on the one hand, and the robustness of the new components on the other, place certain demands on fuel quality that are not necessarily met by the quality characteristics of today’s natural gas fuels. If hydrogen content is too high, for example, injectors or other components may be damaged and the fuel’s anti-knock properties may be impaired at the same time with a consequent loss of efficiency. Likewise, excessive or inadequate compressor oil content can negatively influence component wear characteristics (particularly in the case of injectors). With particular reference to test stand processes, there is currently still no standard measuring procedure for determining compressor oil content. For this reason, MethQuest project participants are developing suitable new measuring processes as well as recommendations to ensure the best possible operation of optimized engines using RE-methane. Another area of investigation covers the formation of particles during combustion as this can still occur under unfavorable conditions despite combustion characteristics that are fundamentally and significantly superior to those experienced with liquid fuels. The aim is to prove that the optimized methane combustion process is an effective factor inhibiting the creation of particulates.