Abstract
A cylindrical constant volume combustion bomb is used to investigate the combustion characteristics and to analyze the simplified heat balance of stratified charge methane-air mixture by using 2-stage injection. To analyze the heatbalance, some terminologies including CHR (cumulative heat release) ratio, UHC (unburned hydrocarbon) ratio and TL (total loss) ratio are defined. The result shows that the effect of the stratification on combustion characteristics is not significant in the case of the overall excess air ratio (λ_<overall>) of 1.1. In the case of the overall excess air ratio of 1.4, as the initial charge pressure (P_<ini>) increases, the CHR ratio has been decreased, while the TL ratio has been increased, as shown in Fig. 1 (here, H1 means homogeneous condition of t_<ig> = 10,000 ms, and H3 means of t_<ig> = 300 ms, respectively). And, the effect of stratification on combustion characteristics of overall excess air ratio of 1.4 is larger than those of λ_<overall> = 1.1 compared to the homogeneous condition of H1. Generally, as the initial charge pressure increases, the amount of injection mixture has been decreased and has resulted in lower mean velocity and turbulent intensity. Also, as shown in Table 1, the excess air ratio of injection mixture (λ_<inj>) is too rich to result in mixing deficiency around the spark electrode gap in the combustion chamber. From these results, it could be possible to acquire some optimal conditions which have higher thermal efficiency and lower total loss than those of homogeneous conditions through the 2-stage injection in CNG direct injection engine.
