Shijin Shuai

Relationship between super-knock and pre-ignition:

High boost and direct injection are the main tendency of gasoline engine technology. However, pre-ignition/super-knock tends to occur at low-speed high-load conditions, which is the main obstacle for improving power density and fuel economy. This work distinguished the relationship between super-knock and pre-ignition by experimental investigation and numerical simulation. The experiment was conducted on a turbocharged gasoline direct injection engine with compression ratio of 10. The engine was operated at an engine speed of 1750 r/min and the brake mean effective pressure of 2.0 MPa under stoichiometric conditions. Super-knock is the severe engine knock triggered by pre-ignition. Pre-ignition may lead to super-knock, heavy-knock, slight-knock, and non-knock. Significantly advancing spark timing can only simulate pre-ignition, not super-knock. Although knock intensity tends to increase with earlier pre-ignition timing, higher unburned mixture fraction at start of knock, and higher temperature and pressure of the unburned mixture at start of knock, knock intensity cannot be simply correlated to any of the parameters above. A one-dimensional model is set up to numerically simulate the possible combustion process of the end-gas after pre-ignition. Two distinct end-gas combustion modes are identified depending on the pressure and temperature of the mixture: deflagration and detonation. Hot-spot in the mixture at typical near top dead center pressure and temperature condition can only induce deflagration. Hot-spot in the unburned end-gas mixture at temperature and pressure conditions above ’’deto-curve’’ may induce detonation. The mechanism of deto-knock may be described as hot-spot-triggered pre-ignition followed by hotspot- induced deflagration to detonation.

Study of the Effect of Spark Ignition on Gasoline HCCI Combustion

Abstract Homogeneous charge compression ignition (HCCI) has challenges with regard to ignition control. A spark-assisted gasoline HCCI combustion strategy was used to control ignition of HCCI combustion. In this paper, the effects of spark ignition (SI) on HCCI combustion were investigated on a gasoline direct injection (GDI) engine. The experimental results show that SI improves the stability of HCCI combustion at HCCI critical status (HCCI-CS). At the transition from SI to HCCI mode, the transition fluctuations can be smoothed by the SI in SI/HCCI hybrid combustion engine. Local SI can trigger entire HCCI in the cylinder at sub-HCCI-CS. Without SI, misfire occurs at sub-HCCI-CS. The reason is that local high temperature and pressure owing to local heat release from the spark electrode leads to increase of temperature and pressure in the entire cylinder. When the temperature and concentration of mixture are below sub-HCCI-CS, SI has little effect on HCCI ignition.

Mode Switch of SI-HCCI Combustion on a GDI Engine

Multi-mode combustion is an ideal combustion strategy to utilize HCCI for internal combustion engines. It combines HCCI combustion mode for low-middle load and traditional SI mode for high load and high speed. By changing the cam profiles from normal overlap for SI mode to the negative valve overlap (NVO) for HCCI mode, as well as the adjustment of direct injection strategy, the combustion mode transition between SI and HCCI was realized in one engine cycle. By two-step cam switch, the throttle action is separated from the cam action, which ensures the stabilization of mode transition. For validating the feasibility of the stepped switch, the influence of throttle position on HCCI combustion was carefully studied. Based on the research, the combustion mode switch was realized in one engine cycle; the whole switch process including throttle action was realized in 10 cycles. The entire process was smooth, rapid and reliable without any abnormal combustion such as knocking and misfiring. Copyright

Homogeneous charge combustion and emissions of ethanol ignited by pilot diesel on diesel engines

Homogeneous charge combustion and emissions of ethanol ignited by pilot diesel fuel were investigated on a two-cylinder diesel engine. The results show that emissions depend on loads and ethanol volume fraction. At low loads, ethanol has little effects on smoke. With the increase of ethanol, NOx decreases, but CO emissions increase. At high loads, smoke emissions reduce greatly with increasing ethanol, but NOx and total hydrocarbon (THC) emissions increase. With the increase of ethanol, ignition delays, combustion duration shortens. The maximum rates of heat release for the fuel containing 10 vol% ethanol (E10) and 30 vol% ethanol (E30) increase. Brake specific energy consumption (BSEC) of E10 and E30 is improved slightly only at full loads. Compared to smoke emissions obtained on the same engine using ethanol blended diesel fuels, the tendency of smoke reduction is similar to that of homogeneous charge combustion of ethanol at the same operating conditions. But the former is more effective in reducing smoke emissions. When the engine operated at high constant loads, smoke significantly decreases with increasing ethanol and smokeless combustion can be achieved at certain ethanol volume fraction dependent on loads. In the meantime, THC emissions gradually increase. Ignition retards, peak pressure increases first and then decreases again. Indicated mean effective pressures are close to those of diesel (E0), even higher than the latter at some cases. NOx emissions are dependent on loads and ethanol volume fraction, and even are less than those of E0 under certain conditions. The results obtained from an optical diesel engine show that ethanol also shortens combustion duration, decreases flame temperature and KL factor, which indicates that ethanol can suppress soot formation in combustion chamber.

Numerical Simulation of HCCI Engine With Multi-Stage Gasoline Direct Injection Using 3D-CFD With Detailed Chemistry

In this paper, the detailed chemical kinetics was implemented into the three-dimensional CFD code to study the combustion process in HCCI engines. An extended hydrocarbon oxidation reaction mechanism (89 species, 413 reactions) used for high octane fuel was constructed and then used to simulate the chemical process of the ignition, combustion and pollutant formation in HCCI conditions. The three-dimensional CFD / chemistry model (FIRE/CHEMKIN) was validated using the experimental data from a Rapid Compression Machine. The simulation results show good agreements with experiments. Finally, the improved multi-dimensional CFD code has been employed to simulate the intake, spray, combustion and pollution formation process of the gasoline direct injection HCCI engine with multi-stage injection strategy. The models account for intake flow structure, spray atomization, spray/wall interaction, droplet evaporation and gas phase chemistry in complex multi-dimensional geometries. The calculated results show the periphery of fuel-rich zone formed by the second injection ignited first, then the fuel-rich zone ignited and worked as an initiation to ignite the surrounding lean mixture zone formed by the first injection. The results provide a detailed insight into the processes governing combustion and pollutant formation in the HCCI engine.

Experimental study on filtration and continuous regeneration of a particulate filter system for heavy-duty diesel engines.

This study investigated the filtration and continuous regeneration of a particulate filter system on an engine test bench, consisting of a diesel oxidation catalyst (DOC) and a catalyzed diesel particulate filter (CDPF). Both the DOC and the CDPF led to a high conversion of NO to NO2 for continuous regeneration. The filtration efficiency on solid particle number (SPN) was close to 100%. The post-CDPF particles were mainly in accumulation mode. The downstream SPN was sensitively influenced by the variation of the soot loading. This phenomenon provides a method for determining the balance point temperature by measuring the trend of SPN concentration.

Control of a spark ignition homogeneous charge compression ignition mode transition on a gasoline direct injection engine

Abstract The hybrid combustion mode is an ideal operation strategy for a gasoline homogeneous charge compression ignition (HCCI) engine. A stable and smooth spark ignition (SI)/HCCI switch has been an issue in the research on multimode combustion. In this paper, the switch process has two key issues; the cam profile and throttle opening. With the developed two-stage cam system, the valve phase strategy can be switched within one engine cycle, from the normal cam profile for the SI mode to a negative valve overlap (NVO) profile for the HCCI mode, or vice versa. For a smoother and more stable switch, the throttle change was separated from the cam profile switch, which was called the stepped switch. The effect of throttle opening on HCCI combustion was studied, and the results showed that the concept of the stepped switch was reliable. With gasoline direct injection (GDI) the combustion mode switches from both SI and HCCI sides were smooth, rapid, and robust, without any abnormal combustion such as knocking and misfiring.

Experimental and computational studies on gasoline HCCI combustion control using injection strategies

Homogeneous Charge Compression Ignition (HCCI) has challenges of ignition control. In this paper, HCCI ignition timing and combustion rate were controlled by two-stage direct injection (TSDI) strategies on a four-stroke gasoline HCCI engine. TSDI strategy was proposed to solve the two major problems of HCCI application-ignition control and load extension. Both simulation and experiments were carried out on a gasoline HCCI engine with negative valve overlap (NVO). An engine model with detailed chemical kinetics was established to study the gas exchange process and the direct injection strategy in the gasoline HCCI engine with TSDI and NVO. Simulation results were compared with experiments and good agreement was achieved. The simulated and experimental results provided a detailed insight into the processes governing ignition in the HCCI engine. Using TSDI, the fuel concentration, temperature as well as chemical species can be controlled. The effects of different injection parameters, such as split injection ratio and start-of-injection (SOI) timing, were studied. The experimental results indicate that, two-stage direct injection is a practical technology to control the ignition timing and combustion rate effectively in four-stroke gasoline HCCI engines. Both the high load and low load limits of HCCI operation were extended.

Performance of straight-run naphtha single and two-stage combustion modes from low to high load

Double injection strategies with single-stage heat release and two-stage heat release process of straight-run naphtha were investigated on a single-cylinder diesel engine from low to high load. The two-stage combustion strategy is realized by split spray and combustion events around the compression top dead center with a dominant feature of “Combust After Injection End, Inject After Combustion End” to ensure the premixed compression ignition. The single-stage combustion is realized by the “spray–spray–combustion” process with the start of combustion separated from the end of injection. The straight-run naphtha has a research octane number of 58.8, and the compression ratio and displacement of the test engine are 16.7 and 0.5 L. Double injection strategy is used to generate the single- and two-stage combustion modes with different injection timing. NOx and total hydrocarbon emissions of the two-stage combustion mode are lower than that of single heat release mode in this study, and it is much easier to produce two-stage combustion mode at higher engine load. Diesel is also tested under double injection strategy just as the single heat release mode of straight-run naphtha, but the fuel efficiency and emission performance are worse than that of naphtha.

Optimization of gasoline hydrocarbon compositions for reducing exhaust emissions

Effects of hydrocarbon compositions on raw exhaust emissions and combustion processes were studied on an engine test bench. The optimization of gasoline hydrocarbon composition was discussed. As olefins content increased from 10.0% to 25.0% in volume, the combustion duration was shortened by about 2 degree crank angle (degrees CA), and the engine-out THC emission was reduced by about 15%. On the other hand, as aromatics content changed from 35.0% to 45.0%, the engine-out NOx emissions increased by 4%. An increment in olefins content resulted in a slight increase in engine-out CO emission, while the aromatics content had little effect on engine-out total hydrocarbon (THC) and CO emissions. Over the new European driving cycle (NEDC), the THC, NOx and CO emissions of fuel with 25.0% olefins and 35.0% aromatics were about 45%, 21% and 19% lower than those of fuel with 10.0% olefins and 40.0% aromatics, respectively. The optimized gasoline compositions for new engines and new vehicles have low aromatics and high olefins contents.