Jae Hyung Lim

In the present study, a chamfered piston crown design was used in order to reduce unburned hydrocarbon (UHC) emissions from the ring-pack crevice. Compared to the conventional piston design, the chamfered piston showed 17–41% reduction in the crevice-borne UHC emissions in homogeneous charge compression ignition (HCCI) combustion. Through parametric sweeps 6 mm was identified to be a suitable chamfer size and the mechanism of the UHC reduction was revealed. Based on the findings in this study, the chamfered piston design was also tested in dual-fuel reactivity controlled compression ignition (RCCI) combustion. In the tested RCCI case using the chamfered piston the UHC and CO emissions were reduced by 79% and 36%, respectively, achieving 99.5% combustion efficiency. This also improved gross indicated thermal efficiency (gITE) from 51.1% to 51.8% in a 9 bar indicated mean effective pressure (IMEP) RCCI combustion case.

Dual-fuel reactivity controlled compression ignition (RCCI) combustion has shown high thermal efficiency and superior controllability with low NO_x and soot emissions. However, as in other low temperature combustion (LTC) strategies, the combustion control using low exhaust gas recirculation (EGR) or a high compression ratio at high load conditions has been a challenge. The objective of this work was to examine the efficacy of using dual direct injectors for combustion phasing control of high load RCCI combustion. The present computational work demonstrates that 21 bar gross indicated mean effective pressure (IMEP) RCCI is achievable using dual direct injection. The simulations were done using the KIVA3V-Release 2 code with a discrete multicomponent fuel evaporation model, coupled with sparse analytical Jacobian solver for describing the chemistry of the two fuels (iso-octane and n-heptane). In order to identify an optimum injection strategy a nondominated sorting genetic algorithm II (NSGA II), which is a multiobjective genetic algorithm, was used. The goal of the optimization was to find injection timings and mass splits among the multiple injections that simultaneously minimize the six objectives: soot, nitrogen oxide (NO_x), carbon monoxide (CO), unburned hydrocarbon (UHC), indicated specific fuel consumption (ISFC), and ringing intensity. The simulations were performed for a 2.44 liter, heavy-duty engine with a 15:1 compression ratio. The speed was 1800 rev/min and the intake valve closure (IVC) conditions were maintained at 3.42 bar, 90 °C, and 46% EGR. The resulting optimum condition has 12.6 bar/deg peak pressure rise rate, 158 bar maximum pressure, and 48.7% gross indicated thermal efficiency. The NO_x, CO, and soot emissions are very low.

Computational studies of heavy-duty diesel engine reactivity controlled compression ignition combustion employing both high pressure common rail diesel and low pressure gasoline direct injection were conducted to examine the benefits of using gasoline direct injection compared to port injected gasoline in improving engine performance. In the current study, multi-dimensional modeling using a discrete multi-component fuel vaporization version of the KIVA-3V code was used to examine gasoline direct injection strategies to improve CO and unburned hydrocarbon emissions while maintaining low indicated specific fuel consumption. For a 9 bar indicated mean effective pressure, 1300 r/min operating condition the engine-out CO and unburned hydrocarbonons were reduced by 27.1% and 7.1%, respectively, while engine-out NOx and soot emissions were maintained low. The predicted indicated specific fuel consumption was 159 g/kW h and the maximum pressure rise rate was 6.1 bar/°, which are comparable to or lower than those of the port injected reactivity controlled compression ignition combustion case. Although further study is needed to find optimum injection strategies to improve the combustion efficiency of reactivity controlled compression ignition combustion using gasoline direct injection, the current results show that direct-injecting both high and low reactivity fuels has the potential to reduce CO and unburned hydrocarbon emissions without sacrificing the thermal efficiency benefit of reactivity controlled compression ignition. Moreover, the reduced maximum pressure and pressure rise rate indicates the possibility of improved brake efficiency in reactivity controlled compression ignition combustion and lowered exhaust gas recirculation requirements. The study also explored the use of a low pressure gasoline direct injection injector for the diesel fuel and found that similar results were obtained to those with the high pressure common rail injector. This indicates that reactivity controlled compression ignition combustion is controlled by the global equivalence ratio and fuel reactivity stratification rather than by the location of the stratification in the combustion chamber. This indicates that the complexity and cost of the injection system can be reduced considerably.