Abstract
A natural gas fueled Homogeneous Charge Compression Ignition (HCCI) is attractive combustion technology that could provide high thermal efficiency and low NOx emissions. In general, the natural gas HCCI engine needs high compression ratios and intake air heating because of the high auto-ignition temperature of natural gas.
In the first stage of this study, the basic characteristics of natural gas fueled HCCI were investigated under various intake air temperatures by using an intake air heater. Although HCCI itself with the intake air heater showed high thermal efficiency and low NOx emissions, the actual efficiency of the engine system was lower than that of spark ignition combustion due to power consumption of the intake air heater. Internal exhaust gas recirculation (EGR) by a negative overlap cam profile was investigated as an alternative way of the intake air heater. As a result, HCCI combustion was achieved without the intake air heater and showed higher indicated thermal efficiency compared to a spark ignition combustion engine.
Expanding the operating range of HCCI was attempted in the next step. The effects of combustion chamber were investigated using three types of pistons. As a result, the operating range was expanded compared to the basic combustion chamber. From the CFD simulations, it was indicated that the temperature distribution created by combustion chamber was significantly important for both auto-ignitability and combustion stability. This means that the in-cylinder inhomogeneity such as temperature seems to be effective for improvement of combustion stability, misfire and operating region. To survey the feasibility of this, the effects of internal EGR stratification was also demonstrated by using chemical kinetic calculations.
Because the high load limit of HCCI is fundamentally restricted due to a lack of mixture gas under the high internal EGR ratios condition, one possible approach is turboharging. Indicated mean effective pressure (IMEP) with boosting inlet air pressure and decreasing internal EGR amount increased about 20% compared to natural aspiration (NA). The mixture gas temperature in cylinder also increased due to higher inlet air temperature with charging pressure rise. As a result, the combustion noise (CN) became beyond the acceptable level. External EGR was used to slow down the combustion speed at high load. External cooled EGR remarkably influenced on the combustion characteristics, and IMEP of high load limit of HCCI was increased up to 60% compared to NA.
Keywords:HCCI, Internal EGR, External EGR, Turbocharing, Temperature Distribution