Optimization of Lambda across the Engine Map for the Purpose of Maximizing Thermal Efficiency of a Jet Ignition Engine 2020-01-0278

Progressively more stringent efficiency and emissions regulations for internal combustion engines have led to growing interest in advanced combustion concepts for spark ignition engines. MAHLE Jet Ignition^® (MJI) is one such concept which enables ultra-lean (λ > ~1.6) combustion via air dilution. This pre-chamber-based combustion system has demonstrated highly efficient lean operation, producing efficiencies competitive with those of advanced compression ignition concepts. Compared to a traditional spark ignition engine, the additional degrees of freedom associated with Jet Ignition introduce further complexity when optimizing the system for peak efficiency throughout the engine map. The relationship between operating condition and the lambda at which peak efficiency occurs for a Jet Ignition engine has been presented in prior work by the authors. This paper further explores this complex relationship, introducing new interdependencies which must be considered in order to improve brake thermal efficiency (BTE).

Experimental data is obtained from a 1.5L 3-cylinder gasoline-fueled Jet Ignition engine. An engine map of BTE is presented. An analysis at several key points throughout the engine map shows how certain parameters are adjusted to optimize BTE, providing a peak value >42%. Insight into the factors affecting efficiency is provided through parameter sweeps of pre-chamber fuel and analysis of high-speed pressure measured in both the pre-chamber and the main combustion chamber. The impact of precise fuel metering on BTE and combustion stability is highlighted. Finally, an analysis of trends in burn duration segments, pre-chamber combustion metrics, and cycle-by-cycle variation in these parameters qualifies the challenges associated with accurate prediction of lean stability limits in a jet ignition engine operating in the ultra-lean region, as well as the opportunities for further efficiency improvement through manipulation of in-pre-chamber conditions.