Effects of pre-spark heat release on engine knock limit

This paper presents an experimental study on controlled ASSCI (Assisted Spark Stratified Compression Ignition) for engine knock suppression in a GDI engine with high compression ratio. The direct injection is used for forming desired stoichiometric stratified mixture at WOT condition without turbo-charging. The engine is filled with 20% cooled external EGR and the ignition timing is maintained at MBT point. The combustion characteristics of the desired stoichiometric stratified mixture show two-stage heat release, where the first stage is caused by spark ignition and the second stage is due to moderate auto-ignition. Compared with engine knock, the second stage heat release of controlled ASSCI shows smooth pressure curve without pressure oscillation. This is due to the low energy density mixture around the cylinder wall caused by cooled external EGR. The stratified mixture could suppress knock. Fuel economy and combustion characteristics of the baseline and the controlled ASSCI combustion were compared. The baseline GDI engine reaches a maximum of 8.9 bar BMEP with BSFC of 291 g/(kWh), the controlled ASSCI combustion achieves a maximum of 8.3 bar BMEP with BSFC of 256 g/(kWh), improving the fuel economy over 12% while maintaining approximately the same power. CA50 (the crank angle of 50% heat release) of the controlled ASSCI is detected at 8.4° CA ATDC, which is 17.4° CA advanced than that of the baseline while the combustion duration of the controlled ASSCI is 52.84dG CA, 16.6° CA longer than that of the baseline caused by diluted mixture and two-stage heat release. The COV of the controlled ASSCI is 1.4%, 2.1% lower than that of the baseline. The peak pressure (Pmax) and the maximum pressure rise rate (PRRmax) of the controlled ASSCI are 59.7 bar and 2.2 bar/° CA, 22.9 bar and 1.5 bar/° CA higher than that of the baseline respectively. The crank angle of Pmax and PRRmax of the controlled ASSCI are 11° CA ATDC and −1° CA ATDC, 15.4° CA and 17.2° CA earlier than that of the baseline. The results show that controlled ASSCI with two-stage heat releases is a potential combustion strategy to suppress engine knock while achieving high efficiency of the high compression ratio gasoline engine.

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Suppressing Knock of the Dual-Fuel Engine by Injection Timing under Pressure Boundary Conditions

Applied Mechanics and Materials ◽

10.4028/www.scientific.net/amm.291-294.1648 ◽

2013 ◽

Vol 291-294 ◽

pp. 1648-1652

Author(s):

Cheng Wei Zhang ◽

Bing Xiao

Keyword(s):

Boundary Conditions ◽

Heat Release ◽

Heat Release Rate ◽

Release Rate ◽

Dual Fuel ◽

Injection Timing ◽

Engine Knock ◽

Intake Pressure ◽

Pressure Boundary Conditions ◽

Engine Reliability

The engine knock has direct relation with the energy release rate. The faster combustion speed is, the higher heat release rate is. If heat release rate is too high, it will deteriorate reliability of the engine. A dual-fuel engine combustion mechanism model is established and intake pressure boundary conditions influence on the dual-fuel engine reliability is studied. Injection timing can supress engine knock and inprove engine reliability. Studies have shown that the greater the intake pressure, the smaller injection timing should be selected and the smaller the intake pressure, the larger injection timing should be selected. Appropriate injection timing can ensure reliability and power of the engine.

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Controlled SSCI With Moderate End-Gas Auto-Ignition for Fuel Economy Improvement and Knock Suppression

Journal of Engineering for Gas Turbines and Power ◽

10.1115/1.4030101 ◽

2015 ◽

Vol 137 (10) ◽

Cited By ~ 9

Author(s):

Hui Liu ◽

Zhi Wang ◽

Jianxin Wang ◽

Mengke Wang ◽

Wanli Yang

Keyword(s):

Heat Release ◽

Compression Ratio ◽

Fuel Economy ◽

Direct Injection ◽

High Compression Ratio ◽

Two Stage ◽

Second Stage ◽

Auto Ignition ◽

Engine Knock ◽

Gdi Engine

Hybrid combustion mode including flame propagation induced by spark ignition (SI) and auto-ignition could be an effective method to improve fuel economy and suppress engine knock simultaneously. An experimental research on controlled spark-assisted stratified compression ignition (SSCI) for this purpose was conducted in a gasoline direct injection (GDI) engine with high compression ratio. At wide open throttle (WOT) and minimum spark advance for best torque (MBT) condition without turbocharging, direct injection was used to form desired stoichiometric stratified mixture while 20% cooled external exhaust gas recirculation (e-EGR) was sucked into the cylinder. The combustion characteristics of controlled SSCI show two-stage heat release, where the first stage is caused by SI and the second stage is due to moderate auto-ignition. Compared with engine knock, the second stage heat release of controlled SSCI shows smooth pressure curve without pressure oscillation. This is due to the low energy density mixture around the cylinder wall caused by cooled e-EGR. The stratified mixture could suppress knock. Fuel economy and combustion characteristics of the baseline and the controlled SSCI combustion were compared. The baseline GDI engine reaches a maximum of 8.9 bar brake mean effective pressure (BMEP) with brake specific fuel consumption (BSFC) of 291 g/(kWh), and the controlled SSCI combustion achieves a maximum of 8.3 bar BMEP with BSFC of 256 g/(kWh), improving the fuel economy over 12% while maintaining approximately the same power. The results show that controlled SSCI with two-stage heat releases is a potential combustion strategy to suppress engine knock while achieving high efficiency of the high compression ratio gasoline engine.

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