Potentials of Air Path Variabilities and Water Injection in HD Gas Engines

The transportation sector, and commercial vehicles in particular, play an important role in global CO₂ emissions. For this reason, the EU recently decided to reduce CO₂ emissions from commercial vehicles by 30% until 2030. One alternative to conventional diesel propulsion is the usage of stoichiometric natural gas combustion. Due to the lowered C/H ratio and the cost effective exhaust after treatment (EAT) in form of a three way catalyst (TWC), less CO₂ is emitted and it is possible to comply even with most stringent NO_X legislations. However, the stoichiometric combustion of natural gas has also disadvantages. In particular, the throttling and retarded 50 % mass fuel burned (MFB50) positions due to knocking lead to efficiency losses. One way to minimize these is the usage of exhaust gas re-circulation (EGR), Miller cycle and water injection. The reduced knocking tendency allows the geometric compression ratio to be increased further, which leads to an additional efficiency advantage. However, the above-mentioned measures in combination have not yet been investigated holistically on commercial vehicle engines. The aim of this project was therefore to evaluate the above-mentioned measures both experimentally and simulatively for future concepts. The experimental part was already been shown by the author in an earlier publication, in which experimental tests were carried out on a single-cylinder commercial vehicle engine. These results have now been used to calibrate a predictive combustion and knock model. Using a validated air path model of a commercial vehicle engine from MAN Truck & Bus, these three technologies are now transferred to a full engine and the effects with a realistic turbocharging system are investigated. The results with the reference turbocharger show, that Miller valve timing has a higher potential for increasing efficiency compared to EGR, which can be mainly attributed to improved utilization of the exhaust gas enthalpy and lowered unburned gas temperatures which reduces knocking. With a specially designed two-stage turbocharging system for Miller valve timing, the compression ratio could be increased which leads to an additional efficiency increase. Thus, an efficiency improvement of 4.8 % points compared to the reference natural gas engine configuration could be achieved. This corresponds to a CO₂ reduction of 19 % compared to a conventional commercial vehicle diesel engine.