Zunqing Zheng

EXPERIMENTAL STUDY ON HOMOGENEOUS CHARGE COMPRESSION IGNITION COMBUSTION WITH PRIMARY REFERENCE FUEL

Abstract By mixing iso-octane with octane number 100 and normal heptane with octane number 0, it was possible to obtain a primary reference fuel (PRF) with octane rating between 0 and 100. The influences of PRF fuels octane number on the combustion characteristics, operation range, performance, and emissions characteristics of homogeneous charge compression ignition (HCCI) engine were investigated. The experiments were carried out on a modified single cylinder direct injection diesel engine. The test results show that, with the increase of the octane number, the ignition timing delays, the combustion rate decreases, the combustion duration prolongs, and the cylinder pressure decreases. The HCCI combustion can be controlled, then the HCCI operating range can be extended by burning different octane number fuel at different engine modes, in which engine burns low octane number fuel at small load mode and large octane number fuel at high load mode. There exists an optimum octane number that achieves the highest indicated thermal efficiency at different engine load. With the increase of the PRF fuel octane number, NOx, HC and CO emissions increase, especially for HC emissions.

An Experimental and Numerical Study on the Effects of Fuel Properties on the Combustion and Emissions of Low-Temperature Combustion Diesel Engines

Experimental and numerical investigations on the effects of fuel properties on combustion and soot emissions under both conventional and premixed low-temperature combustion (LTC) conditions have been conducted. Three different fuels, diesel, gasoline, and n-butanol, were used to formulate five fuels with different fuel properties. Computational fluid dynamic (CFD) simulations were conducted to predict the combustion processes and soot emissions of the various fuels. The results show that under both conventional and premixed combustion conditions, the cetane number (CN) has the dominant effect on the ignition delay; the volatility, aromatic, and oxygen contents only have a minor influence on ignition delay. High CN fuels need much higher exhaust gas recirculation (EGR) to provide a sufficiently long ignition delay compared to the fuels with lower CN. As a result, the carbon monoxide (CO) and unburned hydrocarbon (UHC) emissions of the high CN fuels are higher than the low CN fuels due to the lower intake oxygen concentrations. The volatility can be important under high, mixing-controlled, conventional combustion conditions, and the high volatility and oxygen content are also beneficial for CO and UHC reduction under high EGR premixed LTC conditions. The CN plays a dominant role in soot emissions, followed by the oxygen content and the volatility under low oxygen concentration conditions. The experiments show that the trade-off between NOx and soot can be totally eliminated by optimizing the diesel/gasoline/butanol blended fuel. A reduced primary reference fuel (PRF)-n-butanol-polycyclic aromatic hydrocarbon (PAH) mechanism was formulated to predict the combustion and soot emissions of the tested fuels, and the effects of fuel chemistry properties on the combustion processes and soot emissions were well predicted. The simulation results show that the mixing process can be greatly improved by adjusting the fuel chemistry properties, which leads to improved combustion and low soot emissions.

Reaction Mechanisms and HCCI Combustion Processes of Mixtures of n-Heptane and the Butanols

A reduced primary reference fuel (PRF)-Alcohol-Di-tert-butyl Peroxide (DTBP) mechanism with 108 species and 435 reactions, including sub-mechanisms of PRF, methanol, ethanol, DTBP and the four butanol isomers, is proposed for homogeneous charge compression ignition (HCCI) engine combustion simulations of butanol isomers/n-heptane mixtures. HCCI experiments fuelled with butanol isomer/n-heptane mixtures on two different engines are conducted for the validation of proposed mechanism. The mechanism has been validated against shock tube ignition delays, laminar flame speeds, species profiles in premixed flames and engine HCCI combustion data, and good agreements with experimental results are demonstrated under various validation conditions. It is found that although the reactivity of neat tert-butanol is the lowest, mixtures of tert-butanol/n-heptane exhibit the highest reactivity among the butanol isomer/n-heptane mixtures if the n-heptane blending ratio exceeds 20% (mole). Kinetic analysis shows that the highest C-H bond energy in the tert-butanol molecule is partially responsible for this phenomenon. It is also found that the reaction tC4H9OH+CH3O2 =tC4H9O+CH3O2H plays important role and eventually produces the OH radical to promote the ignition and combustion. The proposed mechanism is able to capture HCCI combustion processes of the butanol/n-heptane mixtures under different operating conditions. In addition, the trend that tert-butanol /n-heptane has the highest reactivity is also captured in HCCI combustion simulations. The results indicate that the current mechanism can be used for HCCI engine predictions of PRF and alcohol fuels.

PRIMARY COMBUSTION INTERMEDIATES IN LOW-PRESSURE PREMIXED LAMINAR 2,5-DIMETHYLFURAN/OXYGEN/ARGON FLAMES

Primary combustion intermediates in low-pressure premixed laminar 2,5-dimethylfuran (DMF)/oxygen (O2)/argon (Ar) flames with equivalence ratios of 0.8 and 1.5 were investigated by using tunable synchrotron vacuum ultraviolet photoionization and molecular-beam mass spectrometry. DMF is a promising biofuel, with properties similar to those of gasoline. However, the combustion chemistry of DMF is not well-studied. Possible reaction pathways of DMF and its primary combustion derivatives were proposed based on the combustion intermediates identified in this work. Photoionization efficiency curves (PIEs) of the combustion intermediates in the DMF/O2/Ar flames were recorded. Ionization energies (IEs) were measured from the PIEs. The combustion intermediates were identified by the agreement of the measured IEs with those reported in the literatures or calculated at G3B3 level. H abstraction and the consecutive reaction products were identified, including 5-methylfurfural, (Z)-1-oxo-1,3,4-pentatriene, and 2-ethyl-5-methylfuran, etc. Furan was not observed in the low-pressure DMF flames. H and OH addition products were also identified, including 2-methylfuran, (2Z,3E)-1-oxo-1,3-pentadiene, and 2-oxo-2,3-dihydro-5-methylfuran. The extra methyl side chain may explain the lower laminar burning velocity of DMF relative to that of 2-methylfuran.

Experimental Study on Homogeneous Charge Compression Ignition Operation by Burning Dimethyl Ether and Methanol

In this paper, a new approach to burning methanol in engine is proposed, in which the engine burns dimethyl ether (DME) and methanol dual fuel in homogeneous charge compression ignition (HCCI) mode, and DME is converted from methanol. Combustion, engine performance, and pollutant emissions of the new HCCI combustion system were investigated. The results show that the stable HCCI operation of DME/methanol can be obtained over a quite broad speed and load region. Both DME and methanol affect HCCI combustion strongly, and by regulating DME/methanol proportions the HCCI combustion process could be controlled effectively. NOx emissions are, very low overall, while HC and CO emissions are much higher than that in conventional compression ignition engines. An appropriate optimal HCCI operation can be obtained by controlling the DME and methanol supply according to operating conditions.

Effect of Fuels with Different Distillation Temperatures on Performance and Emissions of a Diesel Engine Run at Various Injection Pressures and Timings

AbstractIn the current study, a commercial diesel fuel was redistilled and four fuels with different distillation temperatures were obtained. The effects of fuels with different distillation temper...

Experimental Investigation of Injection Strategies on Low Temperature Combustion Fuelled with Gasoline in a Compression Ignition Engine

The present study focuses on the experimental investigation on the effect of fuel injection strategies on LTC with gasoline on a single-cylinder CI engine. Firstly, the engine performance and emissions have been explored by sweeping SOI1 and split percentage for the load of 0.9 MPa IMEP at an engine speed of 1500 rpm. Then, the double-injection strategy has been tested for load expansion compared with single-injection. The results indicate that, with the fixed CA50, the peak HRR is reduced by advancing SOI1 and increasing split percentage gradually. Higher indicated thermal efficiency, as well as lower MPRR and COV, can be achieved simultaneously with later SOI1 and higher split percentage. As split percentage increases, emission decreases but soot emission increases. CO and THC emissions are increased by earlier SOI1, resulting in a slight decrease in combustion efficiency. Compared with single-injection, the double-injection strategy enables successful expansion of high-efficiency and clean combustion region, with increasing soot, CO, and THC emissions at high loads and slightly declining combustion efficiency and indicated thermal efficiency, however. MPRR and soot emission are considered to be the predominant constraints to the load expansion of gasoline LTC, and they are related to their trade-off relationship.

Combustion Mode Design with High Efficiency and Low Emissions Controlled by Mixtures Stratification and Fuel Reactivity

This paper presents a review on the combustion mode design with high efficiency and low emissions controlled by fuel reactivity and mixture stratification that have been conducted in the authors’ group, including the charge reactivity controlled homogeneous charge compression ignition (HCCI) combustion, stratification controlled premixed charge compression ignition (PCCI) combustion, and dual-fuel combustion concepts controlled by both fuel reactivity and mixture stratification. The review starts with the charge reactivity controlled HCCI combustion, and the works on HCCI fuelled with both high cetane number fuels, such as DME and n-heptane, and high octane number fuels, such as methanol, natural gas, gasoline and mixtures of gasoline/alcohols, are reviewed and discussed. Since single fuel cannot meet the reactivity requirements under different loads to control the combustion process, the studies related to concentration stratification and dual-fuel charge reactivity controlled HCCI combustion are then presented, which have been shown to have the potential to achieve effective combustion control. The efforts of using both mixture and thermal stratifications to achieve the auto-ignition and combustion control are also discussed. Thereafter, both charge reactivity and mixture stratification are then applied to control the combustion process. The potential and capability of thermal-atmosphere controlled compound combustion mode and dual-fuel reactivity controlled compression ignition (RCCI)/highly premixed charge combustion (HPCC) mode to achieve clean and high efficiency combustion are then presented and discussed. Based on these results and discussions, combustion mode design with high efficiency and low emissions controlled by fuel reactivity and mixtures stratification in the whole operating range is proposed.