Homogeneous lean spark-ignited combustion is known for its thermodynamic advantages over conventional stoichiometric combustion but remains a challenge due to combustion instability, engine knock and NOx emissions especially at higher engine loads above the naturally aspirated limit. Investigations have shown that lean combustion can partly suppress knock, which is why the concept may be particularly advantageous in high load, boosted operation in downsized engines with high compression ratios.
However, the authors have previously shown that this is not true for all cases due to the appearance of a lean load limit, which is defined by the convergence of the knock limit and combustion stability limit. Therefore, further research has been conducted with the alternative and potentially renewable fuel methane which has higher resistance to autoignition compared to gasoline. Operation with a gaseous fuel on high load was achieved by high pressure direct injection and boosting in a single cylinder research engine. To analyse the combustion further, an endoscope allowing optical access to the combustion chamber was utilized to acquire combustion chamber flame images.
High load lean operation with methane could confirm the hypothesis that without a knock limit, optimal ignition timing could be maintained resulting in high combustion stability, and the lean load limit mitigated. Instead, limitation was reached due to peak cylinder pressure. Direct injected methane resulted in overall higher combustion stability compared to gasoline. However, methane also provided an overall lower fuel conversion efficiency by 1-2 %-units compared to gasoline. Despite higher combustion stability using methane, the maximum air-dilution could only be marginally extended. Flame images using the endoscope revealed that the flame growth post ignition was prohibited, possibly due to flame quenching, at high turbulence conditions.