An investigation of the effects of partial-HCCI application with premixed ethanol on combustion and emissions in a DI diesel engine

In this study, the effects of partial premixed fuel charge on the combustion and exhaust emission characteristics of a single-cylinder, DI diesel engine were investigated. Premixed fuel was injected into the premixed chamber with a low pressure port-type fuel injection system. Ethanol was used as a premixed fuel because of exhibiting single-stage ignition with having high octane number and low boiling point. Homogeneous charge was formed during the intake stroke and ignited by pilot diesel spray before the TDC. This compound combustion mode could be considered as a compromise between the premixed HCCI and conventional DI diesel combustion, and it can be described either as a HCCI-DI or partial HCCI combustion. The experiments were performed with different premixed fuel ratio at 2200 rpm engine speed and four different loads (4, 8, 12 and 16 Nm) under different inlet temperatures (20 and 40 °C) and EGR rates (between 0-30%). In the experiments, premixed fuel ratio was increased up to onset of audible knocking at the high loads and it also was increased up to either the unstable operation limit or misfiring at the light loads. Preliminary engine tests were showed that, the premixed fuel ratio could be reached up to 90% at partial loads. Start of combustion delayed remarkably with the increase of premixed fuel charge, while heat release rate increased gradually in the premixed stage of the combustion and decreased in the diffusion combustion. Consequently, the maximum heat release rate, the maximum cylinder pressure and the maximum rate of pressure rise increased and consequently knocking tendency increased also. Combustion duration was shortened and the center of the heat release tended to move towards the TDC. At the full load, premixed fuel ratio was limited with 30% due to knock occurrence. With the introducing of EGR, suppression of fast heat release rate was achieved at the full load, and engine operational limits expanded up to 50% premixed fuel ratio. At the medium and partial loads, effects of combustion phasing depended on delay were dominated when the premixed fuel ratio exceeded to a certain value and center of the heat release retarded. Thus, ISFC was increased and ITE was deteriorated. At the low loads, effects of the premixed fuel ratio on combustion characteristics were observed moderate. Effects of delay on the start of combustion were eliminated by the increase of inlet temperature. Especially for medium and partial loads, location of the maximum heat release rate stayed almost the same, while the center of the heat release closed to TDC. NOx and smoke emissions decreased simultaneously by 26% and 87% at partial loads. However, NOx emissions increased with the increase of premixed fuel ratio at the full load. This tendency was suppressed by combined use of EGR and premixed fuel. At the full load and 50% premixed fuel ratio, improvements on the NOx and smoke were achieved as 62% and 82% with application of up to 30% EGR rate. Also, these simultaneous improvements were achieved as 81% and 64% for the light loads with the EGR. On the other hand, NOx emissions deteriorated when the inlet temperature increased. In general, CO and HC emissions also increased with the increase of premixed fuel ratio and EGR rate, whereas preheating of the charge caused to decrease.