Yaopeng Li

Numerical simulation of cyclic variability in reactivity-controlled compression ignition combustion with a focus on the initial temperature at intake valve closing

Cyclic variations in dual-fuel reactivity-controlled compression ignition combustion were investigated using multi-dimensional simulations of a light-duty diesel engine. By comparing results with measured pressure traces from 300 consecutive cycles, it was found that the standard deviation of the 50% burn point in reactivity-controlled compression ignition combustion could be satisfactorily reproduced by monitoring the sensitivity of the 50% burn point to changes in initial in-cylinder temperature at intake valve closing in the simulations. Using this approach, the influences of fuel reactivity, diesel mass fraction, combustion mode, exhaust gas recirculation rate, intake pressure, and injection strategy on combustion stability were investigated. It was found that diesel/methanol reactivity-controlled compression ignition combustion exhibits larger cyclic variations than diesel/gasoline at the same operating conditions due to the lower reactivity of methanol. Compared to gasoline homogeneous charge compression ignition and diesel partially premixed combustion, diesel/gasoline reactivity-controlled compression ignition combustion showed the lowest cyclic variations for a given 50% burn point. When the 50% burn point was kept constant by adjusting the intake temperature, the introduction of exhaust gas recirculation and an increase in intake pressure resulted in decreased cyclic variations. Under the conditions tested in this study, with the employment of retarded injection timing, single injection, and increased injection pressure, the in-cylinder equivalence ratio becomes richer, which is helpful for the reduction in cyclic variations in reactivity-controlled compression ignition combustion. The overall results indicate that the present approach for describing cyclic variability is useful for practical applications.

Effect of combustion regime on in-cylinder heat transfer in internal combustion engines

A multi-dimensional model was applied to investigate the influence of combustion regimes on heat transfer losses in internal combustion engines. By utilizing improved turbulence and heat transfer sub-models, the combustion and heat transfer characteristics of the engine were satisfactorily reproduced for operation under conventional diesel combustion, homogeneous charge compression ignition, and reactivity controlled compression ignition regimes. The results indicated that the total heat transfer losses of conventional diesel combustion are the largest among the three combustion regimes due to the direct interaction of the high-temperature flame with the piston wall, while the heat transfer losses of reactivity controlled compression ignition are the lowest and nearly are independent of combustion phasing because of the avoidance of high-temperature regions adjacent to the cylinder walls. Compared to conventional diesel combustion, homogeneous charge compression ignition shows more potential for the reduction of exhaust energy and improvement of fuel efficiency. In reactivity controlled compression ignition combustion, the reduction of heat transfer and exhaust losses outweigh its increase in combustion losses and offer the opportunity for further improvement of fuel efficiency. Furthermore, by evaluating the widely used Woschni and Chang et al.’s empirical heat transfer correlations, it was found that both correlations considerably overestimate the heat transfer rate for the reactivity controlled compression ignition regime. It is necessary to improve empirical heat transfer models to take account of the flow and combustion characteristics under various combustion modes.

Application of the Optimized Decoupling Methodology for the Construction of a Skeletal Primary Reference Fuel Mechanism Focusing on Engine-Relevant Conditions

For the multi-dimensional simulation of the engines with advanced compression-ignition combustion strategies, a practical and robust chemical kinetic mechanism is highly demanded. Decoupling methodology is effective for the construction of skeletal mechanisms for long-chain alkanes. To improve the performance of the decoupling methodology, further improvements are introduced based on recent theoretical and experimental works. The improvements include: (1) updating the H2/O2 sub-mechanism; (2) refining the rate constants in the HCO/CH3/CH2O sub-mechanism; (3) building a new reduced C2 sub-mechanism; and (4) improving the large-molecule sub-mechanism. With the improved decoupling methodology, a skeletal primary reference fuel (PRF) mechanism is developed. The mechanism is validated against the experimental data in shock tubes, jet-stirred reactors, premixed and counterflow flames for various PRF fuels covering the temperature range of 500–1450 K, the pressure range of 1–55 atm, and the equivalence ratio range of 0.25¬–1.0. Finally, the skeletal mechanism is coupled with a multi-dimensional computational fluid dynamics model to simulate the combustion and emission characteristics of homogeneous charge compression ignition (HCCI) engines fueled with iso-octane and PRF. Overall, the agreements between the experiment and prediction are satisfactory.