Fuel distribution, combustion, and flame propagation characteristics of heavy duty engine with the liquid phase LPG injection(LPLI) were studied in a single cylinder engine. Optically accessible single cylinder engine and laser diagnostics system were built for visualizing flame propagation and quantifying fuel concentration by acetone PLIF(planar laser induced fluorescence) measurements. In case of Otto cycle engine with large bore size, the engine knock and thermal stress of exhaust manifold are so critical that lean burn operation is needed to reduce the problems.
Lean burn strategy makes it possible to reduce emissions and to enhance fuel economy. To the contrary, it may cause unstable engine operation by cyclic combustion variation, misfire or partial burning.
To accomplish successful lean burn operation with LPLI system design parameters such as swirl intensity, piston bowl(combustion camber) shape, and fuel injection timing were investigated. Combustion duration, CBCV(cycle-by-cycle variation), knock tendency, etc. were compared for each parameter.
Swirl ratio were varied between 1.2, 2.3, and 3.4 following Ricardo swirl number(Rs) definition. Rs=2.3 showed the best results with lower CBCV and shorter burning duration in the lean region while strong swirl(Rs=3.4) made these worse for combustion enhancement. Excessive swirl resulted in reverse effects due to high heat transfer and initial flame kernel quenching.
Fuel injection timings were categorized with open valve injection and closed valve injection. Open valve injection showed shorter combustion duration and extended lean limit.
Four different piston shapes, dog-dish and 3 bathtubs, subsequently with different squish flow intensity, were examined. Bathtub shapes(Type B, D) were beneficial for extension of lean misfire limit. It was possible to depress the knock tendency with longer top land length(Type C). For all pistons, MBT(minimum retard for best torque) timing could be achieved without knock occurrence over φ=0.8.
It is generally known that fuel stratification is one of the key technologies to extend the lean misfire limit. The formation of rich mixture in the spark plug vicinity was achieved by open valve injection. With higher swirl strength(Rs=3.4) and open valve injection, the cloud of fuel followed the flow direction and the radial air/fuel mixing was limited by strong swirl flow. It was expected that axial stratification was maintained with open-valve injection if the radial component of the swirling motion was stronger than the axial components.
The axial fuel stratification and concentration were sensitive to fuel injection timing in case of Rs=3.4 while those were relatively independent of the injection timing in case of Rs=2.3. Thus, strong swirl flow could promote desirable axial fuel stratification and, in result, may make flame propagation stable in the early stage of combustion.
