Emission characteristics and engine performance of gasoline DICI engine in the transition from HCCI to PPC

Abstract

The injection strategy is commonly manipulated to control the stratification level to simultaneously achieve a low pollutant emission and a high efficiency in internal combustion engines. This paper reports on a joint experimental and numerical study of the emission characteristics and engine performance in a heavy-duty direct-injection compression ignition (DICI) engine operating in the HCCI and PPC regimes, with a primary reference fuel made up of iso-octane (81% by volume) and n-heptane (19%) as a model gasoline fuel. The injection timing was varied to achieve different levels of stratification of the charge in the cylinder while the intake air temperature was adjusted accordingly to keep the same combustion phasing at 3° crank angle (CA) after top dead centre (ATDC). The main results of the present study are: (1) In gasoline DICI engines, the fuel is injected into the cylinder at an earlier SOI. The combustion process can be divided into different regimes, HCCI, PPC, and transition from HCCI to PPC, depending on SOI. Two distinctive classes of in-cylinder combustion temperature distributions could be found from the simulation results for the studied engine: one was for the SOI range from -100 to -48 °CA ATDC, which was the HCCI regime and/or the transition from HCCI to PPC regime, where the mean effective in-cylinder temperature was lower than 1700 K. The second class was for the SOI range from -44 to -20 °CA ATDC, where the combustion temperature was higher than 1850 K. This corresponded to the PPC regime. (2) The NOx emission was not only affected by the mean temperature but also the distribution of temperature in the cylinder. (3) The main source of unburned hydrocarbon (UHC) emission in the HCCI and the transition regimes was the fuel trapped in the crevice region where the oxidation process could not function properly. (4) Carbon monoxide (CO) emissions showed a non-monotonic variation with the injection timing, with one peak forming in the bowl region and one forming in the squish region during the transition from HCCI to PPC. The main source of CO emission was in the low temperature and fuel-lean region in the cylinder. (5) In the present engine operation, the pressure rise rate (PRR) in the PPC regime was higher than that in the HCCI regime, which is contrary to most results reported in the literature. This is a combined effect of low equivalence ratio in the piston bowl and in the squish region, and the stratification of the ignition delay time in the mixture. (6) Highest thermodynamic efficiency was achieved in the PPC regime with SOI from -44 to -31 °CA ATDC. In the transition from HCCI to PPC regime, the thermodynamic efficiency reached its lowest due to the poor combustion efficiency.