The Influence of PRF and Commercial Fuels with High Octane Number on the Auto-ignition Timing of an Engine Operated in HCCI Combustion Mode with Negative Valve Overlap

Despite the fact that HCCI engines are distinguished by mixture homogeneity, some degree of stratification always appears inside a combustion chamber. It is especially applied to residual effect engines utilizing negative valve overlap. Mixture stratification is a result of the imperfect mixing of fresh air with trapped residuals. Direct fuel injection introduces stratification as well, due to fuel vaporization. As a consequence, the temperature within the combustion chamber is uneven. Thermal stratification affects auto-ignition timing and combustion evolution in a high extent. The purpose of this study was to evaluate a degree of thermal stratification in HCCI engine utilizing negative valve overlap. Investigations were performed using three-dimensional CFD model of the combustion system, made by using AVL FIRE software. Simulations were realized for various timings of fuel injection into the cylinder. It was found that fuel injection timing had a significant effect on the thermal stratification and resulting auto-ignition timing.

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Effects of spark assist on HCCI combustion

Combustion Engines ◽

10.19206/ce-116894 ◽

2015 ◽

Vol 161 (2) ◽

pp. 73-77

Author(s):

Jacek HUNICZ ◽

Michał GĘCA ◽

Paweł KORDOS ◽

Alejandro MEDINA

Keyword(s):

Flame Propagation ◽

Pressure Rise ◽

Spark Discharge ◽

Combustion Stability ◽

Experimental Heat ◽

Hcci Combustion ◽

Auto Ignition ◽

Negative Valve Overlap ◽

Compression Ignition Combustion ◽

Valve Overlap

HCCI (homogeneous charge compression ignition) combustion is initiated by compression temperature and is independent of spark discharge. However, spark discharge can be applied under certain conditions to achieve hybrid combustion, where combustion by flame propagation is followed by auto-ignition of the unburned mixture. Spark assist can be applied to improve combustion stability at low loads or to reduce pressure rise rates under high load regime. In the current study variable spark ignition timing was applied for stoichiometric HCCI combustion, achieved using negative valve overlap technique. Under investigated conditions increase of nitrogen oxides emissions, due to flame propagation, was not observed. To provide more insight into combustion evolution, double Wiebe function was fitted to experimental heat release rates. It was found that only less than 10% of mixture was burned by flame propagation, even for very advanced spark discharge.

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Numerical study on combustion characteristics of six-stroke-cycle gasoline compression ignition engine with continuously variable valve duration valve technology

International Journal of Engine Research ◽

10.1177/1468087419838390 ◽

2019 ◽

Vol 22 (1) ◽

pp. 165-183 ◽

Cited By ~ 1

Author(s):

Oudumbar Rajput ◽

Youngchul Ra ◽

Kyoung-Pyo Ha ◽

You-Sang Son

Keyword(s):

Numerical Study ◽

Power Stroke ◽

Compression Ignition ◽

Ignition Timing ◽

Compression Ignition Engine ◽

Engine Operation ◽

Wide Range ◽

Negative Valve Overlap ◽

Variable Valve ◽

Valve Overlap

Engine performance and emissions of a six-stroke gasoline compression ignition engine with a wide range of continuously variable valve duration control were numerically investigated at low engine load conditions. For the simulations, an in-house three-dimensional computational fluid dynamics code with high-fidelity physical sub-models was used, and the combustion and emission kinetics were computed using a reduced kinetics mechanism for a 14-component gasoline surrogate fuel. Variation of valve timing and duration was considered under both positive valve overlap and negative valve overlap including the rebreathing of intake valves via continuously variable valve duration control. Close attention was paid to understand the effects of two additional strokes of the engine cycle on the thermal and chemical conditions of charge mixtures that alter ignition, combustion and energy recovery processes. Double injections were found to be necessary to effectively utilize the additional two strokes for the combustion of overly mixed lean charge mixtures during the second power stroke. It was found that combustion phasing in both power strokes is effectively controlled by the intake valve closure timing. Engine operation under negative valve overlap condition tends to advance the ignition timing of the first power stroke but has minimal effect on the ignition timing of second power stroke. Re-breathing was found to be an effective way to control the ignition timing in second power stroke at a slight expense of the combustion efficiency. The operation of a six-stroke gasoline compression ignition engine could be successfully simulated. In addition, the operability range of the six-stroke gasoline compression ignition engine could be substantially extended by employing the continuously variable valve duration technique.

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Effect of Fuel Injection Timing During Negative Valve Overlap Period on a GDI-HCCI Engine

Volume 1: Large Bore Engines; Fuels; Advanced Combustion ◽

10.1115/icef2018-9653 ◽

2018 ◽

Author(s):

Sok Ratnak ◽

Jin Kusaka ◽

Yasuhiro Daisho ◽

Kei Yoshimura ◽

Kenjiro Nakama

Keyword(s):

Fuel Injection ◽

Direct Injection ◽

Single Pulse ◽

Injection Timing ◽

Pulse Injection ◽

Hcci Combustion ◽

Fuel Injection Timing ◽

Intake Stroke ◽

Negative Valve Overlap ◽

Valve Overlap

Gasoline Direct Injection Homogeneous Charge Compression (GDI-HCCI) combustion is achieved by closing early the exhaust valves for trapping hot residual gases combined with direct fuel injection. The combustion is chemically controlled by multi-point auto-ignition which its main combustion phase can be controlled by direct injection timing of fuel. This work investigates the effect of single pulse injection timing on a supercharged GDI-HCCI combustion engine by using a four-stroke single cylinder engine with a side-mounted direct fuel injector. Injection of primary reference fuel PRF90 under the near-stoichiometric-boosted condition is studied. The fuel is injected during negative valve overlap (NVO) or recompression period for fuel reformation under low oxygen concentration and the injection is retarded to intake stroke for the homogeneous mixture. It is found that the early fuel injection in NVO period advances the combustion phasing compared with the retarded injection in the intake stroke. Noticeable slower combustion rate from intake stroke fuel injection is obtained compared with the NVO injection due to charge cooling effect. Zero-dimensional combustion simulations with multiple chemical reaction mechanisms are simulated to provide chemical understanding from the effect of fuel injection timing on intermediate species generations. The species such as C2H4, C3H6, CH4, and H2 are found to be formed during the NVO injection period from the calculations. The effects of single pulse injection timings on combustion characteristics such pressure rise rate, combustion stability, and emissions are also discussed in this study.

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