An Experimental Methodology for the Evaluation of the Trapped Air-Fuel Ratio of a Small 2S LPDI Engine

A typical issue of the two-stroke engine in monitoring the combustion process is to measure the actual burning mixture with a conventional 02-sensor placed in the exhaust duct. In fact, the short circuit of fresh charge affects the correct acquisition of the residual oxygen associated to the completeness of the combustion process, leading to the overestimation of the trapped air-fuel ratio.In a conventional homogenously scavenged two-stroke engine, a possible solution to the aforementioned issue is the direct measurement of the mass flow rate of both the intake fresh air and the fuel delivered by the fuel supply system. This methodology cannot be applied to 2S direct injection engine because air and fuel are not premixed.The paper shows the application of a methodology for the evaluation of the trapped air-fuel ratio of the mixture inside the combustion chamber in a small two-stroke low pressure direct injection (LPDI) engine. More in detail, the air short circuit is estimated by coupling the measurement of intake airflow and O2 raw emissions, while the fuel short circuit is estimated by coupling the measurement of injected fuel quantity with CO, CO2 and HC raw emissions.The results showed the feasibility of this kind of approach and the correlation between the CO emissions and the actual trapped air-fuel ratio in a wide operating range of a 2S LPDI engine.