Investigating the effect of intake temperature on hydrogen HCCI engine combustion and performance characteristics

The homogeneous charge compression ignition (HCCI) engine has historically gained attention due to its potential to provide high efficiency and low emissions simultaneously. Furthermore, utilising hydrogen in an HCCI engine can be considered a promising approach to reach low emissions output. Despite its promise, maintaining stable and efficient combustion over a wide range of operating conditions remains a challenge for hydrogen-fueled HCCI engines. This study aimed to investigate the effect of varying intake temperatures on the performance as well as the combustion characteristics of hydrogen-fueled HCCI engines. The experiments were conducted using a single-cylinder cooperative fuel research engine (CFR) that was modified to operate with port fuel injection of hydrogen. For these tests the compression ratio of the engine was fixed at 17:1. and the relative air-fuel ratio (l) was varied from the HCCI limit to the point of the knocking at each temperature. The results indicate that higher intake temperature significantly extends the HCCI operating range of the engine. At a temperature of 150C, due to the wide flammability limit of hydrogen, the operating range was extended to an ultra-lean condition of l = 5 when running the engine at 600 rpm. However, increasing the intake temperature resulted in a decrease in the overall engine efficiency, as a result of the reduction in the net indicated mean effective pressure (nIMEP) and heat loss. Additionally, it was found that l had to be increased on the lean side to avoid knocking and maintain the power output as the temperature increased. This study also showed that the heat release rate of hydrogen in the HCCI engine was high, and the burn duration became shorter with increasing intake temperature. Also, to maintain the crankshaft angle at which 50% of the air-fuel mixture has burnt (CA50) at a fixed point, the air excess ratio needed to be leaner. Regarding the engine-out emissions, the results showed that emissions of oxides of nitrogen were relatively low under hydrogen HCCI combustion, reaching a minimum of 3 ppm at 600 rpm and 3.1 bar nIMEP. However, due to the characteristics of the CFR engine, CO and HC were present emissions in the exhaust from the evaporation and oxidization of the lubricating oil during the combustion.