Trapped equivalence ratio determination in two-stroke engines

The chemical composition of the trapped fuel-air-residual gas mixture controls the nature of combustion in internal combustion engines and thus serves as a key determinant of the ensuing emissions and work production processes. A frequently used trapped composition metric for engine control is the trapped equivalence ratio. Unfortunately, in two-stroke engines, it is unfeasible to accurately determine this using traditional intake flow and exhaust emissions measurements because of the simultaneous occurrence of intake and exhaust processes, which causes: (1) exhaust emissions to be diluted by the slippage (short-circuiting) of fresh air through the exhaust ports, i.e., trapping inefficiencies, and (2) high residual combustion product retainment, i.e., scavenging inefficiencies.

The current paper supplements scavenging efficiency data obtained in a previous study for a cross-scavenged, natural-gas, two-stroke engine with experimental trapping efficiency data from the same engine, and characterizes the overall gas-exchange (scavenging + trapping) behavior of the engine at two engine loads, three speeds, and three spark timings. The trapping efficiency experiments use natural gas as a tracer for fresh charge and the total gas-exchange data is used to compute the trapped equivalence ratio. The trapping performance of the engine – which improves with speed and load increase, and spark retardation – along with scavenging, volumetric, thermal, and combustion efficiency changes determine the trapped equivalence ratio of the engine. The relationship between trapped equivalence ratio and NO_x emissions is presented and the importance of accounting for scavenging and trapping inefficiencies in accurately determining equivalence ratio is discussed.