Effect of engine size, speed, and dilution method on thermal stratification of premixed homogeneous charge compression–ignition engines: A large eddy simulation study

Abstract

High heat release rates limit the operating range of homogeneous charge compression–ignition engines to low and medium loads. Thermal stratification has been shown to stagger autoignition, lower heat release rates, and extend the operating range of homogeneous charge compression–ignition engines. However, the dependence of naturally occurring thermal stratification on the engine size, speed, and internal residual dilution is not fully understood. A three-dimensional computational fluid dynamics model with large eddy simulations and detailed chemical kinetics was developed using CONVERGE. This model was used to simulate two different engines: (1) a light-duty 2.0 GM Ecotec Engine modified for homogeneous charge compression–ignition combustion in one of the cylinders and (2) a medium-duty Cummins B-series engine modified for homogeneous charge compression–ignition combustion in one of the cylinders. For the light-duty engine, five consecutive modeled cycles were compared with experimental data from 300 consecutive cycles using residual gas dilution at 2000 r/min. For the medium-duty engine, five consecutive modeled cycles were compared with experimental data from 100 consecutive cycles using air dilution with intake heating at 1200 r/min. In the light-duty engine, it was found that incomplete mixing between fresh charge and residual gas increased thermal stratification early in the compression stroke for residual dilution compared to air dilution. Residual stratification at the onset of ignition was small and not directly coupled with thermal stratification. Heat losses to the walls were the dominant source of thermal stratification at the onset of ignition. The reduced oxygen concentration due to residual dilution, increased the temperature requirement for autoignition, which increased heat transfer losses and increased the thermal stratification around top dead center. The thermal stratification before ignition reduced when the engine speed increased because of the lower heat transfer losses. The light-duty engine was found to have larger portion of the fuel energy lost to heat transfer than the medium-duty engine, which resulted in larger thermal stratification before ignition.

Enabling robust simulation of polyoxymethylene dimethyl ether 3 (PODE3) combustion in engines

“…Validations in HCCI engines are considered more fundamental than other types of engines as the combustion is solely dependent on autoignition properties of fuels. [116][117][118] Figure 19 illustrates the comparison between the simulations results and HCCI experiment conducted by Wang et al 28 The experiment was carried out at an engine speed of 1600 rounds per minute (rpm) and an ER of 0.34, typical of a HCCI engine. Since much of the experimental details can be found in their paper, 28 they will not be repeated here so as to maintain conciseness.…”

LES investigation of cycle-to-cycle variation in a SI optical access engine using TFM-AMR combustion model

“…Promising and advanced technological solutions have been suggested and explored to reach clean and efficient combustion in spark ignition (SI) engines, such as engine downsizing, 5 direct-injection with charge stratification, 6 lean mixture operation, 7,8 controlled auto-ignition, 9,10 water injection, [11][12][13][14] exhaust gas recirculation. 15,16 Innovative ignition strategies have been also proposed and explored, like plasma assisted ignition systems 17,18 or pre-chamber turbulent jet ignition, [19][20][21] homogeneous charge compression ignition (HCCI) 22,23 and reactivity controlled compression ignition (RCCI). 24,25 These advanced combustion concepts can improve engine efficiency by 10-20% 4,26 and simultaneously reduce pollutant emissions.…”

Potential of reactivity-controlled compression ignition with reverse reactivity stratification (R-RCCI) fueled with gasoline and polyoxymethylene dimethyl ethers (PODEn)

“…As the earliest LTC technique, homogeneous charge compression ignition (HCCI) has been extensively investigated in recent years, since it is capable of maintaining high efficiency and clean combustion. 1−4 However, the application of HCCI to commercial engines has been proven to be very difficult owing to the challenge of combustion phasing control and high peak pressure rise rate (PPRR) at high loads. Several improved strategies, such as fuel stratified combustion, 5−8 partially premixed combustion (PPC), 9−12 and reactivity-controlled compression ignition (RCCI), 13−16 have been developed to regulate the combustion process by applying the direct injection (DI) of fuel into the cylinder.…”