Trapped equivalence ratio determination in two-stroke engines

Abstract

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 NOx emissions is presented and the importance of accounting for scavenging and trapping inefficiencies in accurately determining equivalence ratio is discussed.