Lean combustion by a pre-chamber charge stratification in a stationary spark ignited engine

Abstract

Combustion a lean air-fuel mixture in a spark ignited (SI) engine is one way to reduce nitrogen oxide (NOx) emissions that results in an increase of engine efficiency by decreasing a peak combustion temperature. An effective concept to lean mixture combustion can be a two-stage combustion system of stratified burn mixture in the engine with pre-chamber, in which combustion starts in a pre-chamber (1 stage) and, further, the flame jet from a pre-chamber initiate lean mixture combustion in the engine cylinder (2 stage). The paper presents the results of the laboratory research of the SI stationary engine with two-stage combustion system powered by LPG gas. The results were compared to the results of the dual-fuel engine with two-stage combustion system and the conventional engine with one-stage combustion process. Air-fuel mixture stratification method in the test engine, by using two-stage combustion system with pre-chamber, allowed burning of lean mixture with the overall excess air ratio (») up to 2.0, and thus led to lower emissions of nitrogen oxides in the exhaust gases of the engine. The test engine implementing a conventional, single-stage combustion process has allowed a correct burning of air-fuel mixtures of excess air ratio not exceeding 1.5. Of the value λ > 1.5, an increase of coefficient of variation indicated mean effective pressure (COVIMEP), and it decreased the engine thermal efficiency (ITE), which became virtually impossible to operate. The engine implementing a two-stage combustion process, working with λ = 2.0 allowed to reduce the NOx content in exhaust gases to a level of about 0.02 g/kWh (for gas engine) and 1.15 g/kWh (for dual-fuel engine). These values are significantly less than the values obtained in the conventional engine, which indicated thermal efficiency (34%) characterized by the emission of NOx — 26.25 g/kWh.