Experimental Study of Pre-Chamber Geometry Influence on Performance and Emissions in a Gasoline Spark Ignited Engine

The paper presents the experimental study of an active pre-chamber volume variations on engine performance and emissions. The experiments were performed on a test setup equipped with a single cylinder engine. The modular and custom-made pre-chamber design was used, enabling the variation of pre-chamber volume in the range of 3-5% of clearance volume. During the variation of pre-chamber’s geometrical parameters, the ratio of total nozzle area to the pre-chamber volume was fixed at a value of approximately 0.033 cm^-1. At a given pre-chamber volume the variation of engine load was achieved by the change of excess air ratio in the main chamber from stochiometric mixture to lean limit, while the engine speed was fixed to 1600 rpm. For each pre-chamber variation and on each of the investigated operating points, a spark sweep was performed to obtain the highest indicated efficiency while satisfying the imposed restrictions regarding combustion stability and knock occurrence. The lean limit was studied by calculating the coefficient of variation of indicated mean effective pressure (CoV IMEP) over 300 consecutive cycles. The comparisons of indicated efficiency, combustion phasing, combustion stability and emissions of nitrogen oxides (NO_x), total hydrocarbons (THC) and carbon monoxide (CO) were performed in this study. It was observed that the pre-chamber volume variation mainly affects the engine’s indicated efficiency and lean limit due to the changes in heat losses. The experimental results also show that at very lean mixtures the raw NO_x emissions are below the regulation limit. However, the increase of THC and CO emissions is evident as a result of decreased combustion efficiency (incomplete combustion). On the other hand, the NO_x can be also reduced below the limit at moderately lean conditions (λ = 1.6) by retarding the spark timing at the expense of indicated efficiency. This approach still results with higher efficiencies than those recorded at the very lean mixtures.