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In order to further study the effects of air and EGR dilution on the fuel economy improvement of natural gas engines under the premise of meeting EU6 legislation, a comparison between stoichiometric operation with EGR and lean burn operation with and without EGR has been conducted at 1600rpm 50% and 75% load. The conversion efficiencies of the catalysts for both NOx and CH4 emissions are assumed at 90% for lean burn operation. Experiment results indicate that under the condition of meeting both NOx and CH4 predetermined engine-out emissions limits for EU6 legislation, lean operation with a small fraction of EGR dilution enables more advanced combustion phasing compared to pure lean operation, which results in much better fuel economy, thus further improvement compared to stoichiometric operation is achieved.

A HCCI engine has been run at different operating boundaries conditions with fuels of different RON and MON and different chemistries. The fuels include gasoline, PRF and the mixture of PRF and ethanol. Six operating boundaries conditions are considered, including different intake temperature (Tin), intake pressure (Pin) and engine speed. The experimental results show that, fuel chemistries have different effect on the combustion process at different operating conditions. It is found that CA50 (crank angle at 50% completion of heat release) shows no correlation with either RON or MON at some operating boundaries conditions, but correlates well with the Octane Index (OI) at all conditions. The higher the OI, the more the resistance to auto-ignition and the later is the heat release in the HCCI engine. The operating range is also correlation with the OI. The higher the OI, the higher IMEP can reach.

This paper describes numerical simulations that compare the performance of two combustion CFD models against experimental data, and evaluates the effects of combustion and spray model constants on the predicted combustion and emissions under various operating conditions. The combustion models include a Characteristic Time Combustion (CTC) model and CHEMKIN with reduced chemistry models integrated in the KIVA-3Vr2 CFD code. The diesel spray process was modeled using an updated version of the KH-RT spray model that features a gas jet submodel to help reduce numerical grid dependencies, and the effects of both the spray and combustion model constants on combustion and emissions were evaluated. In addition, the performance of two soot models was compared, namely a two-step soot model, and a more detailed model that considers soot formation from PAH precursors.

The effects of cooled EGR on combustion and emission characteristics in HCCI operation region was investigated on a single-cylinder diesel engine, which is fitted with port injection of DME and methanol. The results indicate that EGR rate can enlarge controlled HCCI operating region, but it has little effect on the maximum load of HCCI engine fuelled with DME/methanol dual-fuels. With the increase of EGR rate, the main combustion ignition timing (MCIT) delays, the main combustion duration (MCD) prolongs, and the peak cylinder pressure and the peak rate of heat release decreases. Compared with EGR, DME percentage has an opposite effect on HCCI combustion characteristics. The increase of indicated thermal efficiency is a combined effect of EGR rate and DME percentage. Both HC and CO emissions ascend with EGR rate increasing, and decrease with DME percentage increasing. In normal combustion, NOX emissions are near zero.

Liquefied natural gas (LNG) engine could provide both reduced operating cost and reduction of greenhouse gas (GHG) emissions. Stoichiometric operation with EGR and the three-way catalyst has become a potential approach for commercial LNG engines to meet the Euro VI emissions legislation. In the current study, numerical investigations on the knocking tendency of several combustion chambers with different geometries and corresponding performances were conducted using CONVERGE CFD code with G-equation flame propagation model coupled with a reduced natural gas chemical kinetic mechanism. The results showed that the CFD modeling approach could predict the knock phenomenon in LNG engines reasonably well under different thermodynamic and flow field conditions.

Homogeneous charge compression ignition (HCCI) is considered as a high efficient and clean combustion technology for I.C. engines. Methanol is a potential fuel for HCCI combustion. In this research, a single cylinder diesel engine was applied to HCCI operation. Methanol and dimethyl ether (DME) were fueled to the engine by fuel injection system with an electric controlled port in dual fuel mode. The results show that the stable HCCI operation of DME/methanol can be obtained over a quite broad speed and load region. And compared with higher speeds, the load region is even wider at low engine speed. E.g., at the engine speed of 1000 r/min, the maximum indicated mean effective pressure(IMEP) can reach 0.77 MPa, while at 2000r/min it is 0.53 MPa. Both DME and methanol influence HCCI combustion strongly, and by regulating DME/methanol proportions the HCCI combustion process could be controlled effectively.

The dual-fuel Reactivity Controlled Compression Ignition (RCCI) combustion could achieve high efficiency and low emissions over a wide range of operating conditions. However, further high load extension is limited by the excessive pressure rise rate and soot emission. Polyoxymethylene dimethyl ethers (PODE), a novel diesel alternative fuel, has the capability to achieve stoichiometric smoke-free RCCI combustion due to its high oxygen content and unique molecule structure. In this study, experimental investigations on high load extension of gasoline/PODE RCCI operation were conducted using late intake valve closing (LIVC) strategy and intake boosting in a single-cylinder, heavy-duty diesel engine. The experimental results show that the upper load can be effectively extended through boosting and LIVC with gasoline/PODE stoichiometric operation.

Studies on combustion process of Dimethyl Ether (DME) were carried out on a constant volume combustion bomb (CVCB) and a visualization engine, and the photograph of combustion of DME was taken by high speed digital CCD. The results show that the ignition delay of DME is shorter than that of diesel fuel. When the fuel delivery amounts of DME and diesel in volume are the same, the combustion duration of DME is shorter than that of diesel fuel, and the flame temperature of DME is lower than that of diesel. At the end of combustion, the second injection occurred. The results of high-speed photograph in visualization engine show that, as soon as DME leaves the nozzle, it evaporates rapidly, and under the effect of air swirl, the spray“core” is blown off. Compared to diesel, the penetration of DME is shorter, and the wall combustion of DME is apparently smaller.

This work concerns the oxidation of biodiesel surrogates in a CI engine. An experimental study has been carried out in a single-cylinder common-rail CI engine with soybean biodiesel and two biodiesel surrogates containing neat methyl decanoate and methyl decanoate/n-heptane blends. Tests have been conducted with various intake oxygen concentrations ranging from 21% to approximately 9% at intake temperatures of 25°C and 50°C. The results showed that the ignition delay and smoke emissions of neat methyl decanoate were closer to that of soybean biodiesel as compared with methyl decanoate/n-heptane blends. A reduced chemical kinetic mechanism for the oxidation of methyl decanoate has been developed and applied to model internal combustion engines. A KIVA code, coupled with the Chemkin chemistry solver, was used as the computational platforms. The effects of various intake oxygen concentrations on the in-cylinder emissions of OH and soot were discussed.

The effects of various injection timing of methanol on thermo-atmosphere combustion of methanol by port injection of dimethyl ether (DME) and direct injection of methanol were experimentally investigated. The experiment results show that, as injection timing is at 6 degree before TDC, the combustion process comprises three stages: low temperature heat release of DME, high temperature heat release of DME and diffusion combustion of methanol. As injection timing increases, premixed combustion proportion of methanol is increased and diffusion combustion proportion is decreased. As injection timing increases to 126 degree before TDC, diffusion combustion of methanol disappears. At this time, the combustion process shows typical two stages heat release of HCCI combustion. As injection timing increases, required DME rate is increased, combustion efficiency and indicated thermal efficiency all first increase and then decrease.

The application of oxygen-enriched or oxy-fuel combustion coupled with carbon capture and storage technology has zero carbon dioxide emission potential in the boiler and gas turbine of the power plant. However, the oxygen-enriched combustion with high oxygen level has few studies in internal combustion engines. The fundamental issues and challenges of high oxygen level are the great differences in the physical properties and chemical effects compared with the combustion in air condition. As a consequence, the diesel spray combustion characteristics at high oxygen level were investigated in an optical constant volume vessel. The oxygen volume fraction of tested gas was from 21% to 70%, buffered with argon. The high-speed color camera was used to record the natural flame luminosity.

This work numerically investigates the detailed combustion kinetics of partially premixed combustion (PPC) in a diesel engine under three different premixed ratio fuel conditions. A reduced Primary Reference Fuel (PRF) chemical kinetics mechanism was coupled with CONVERGE-SAGE CFD model to predict PPC combustion under various operating conditions. The experimental results showed that the increase of premixed ratio (PR) fuel resulted in advanced combustion phasing. To provide insight into the effects of PR on ignition delay time and key reaction pathways, a post-process tool was used. The ignition delay time is related to the formation of hydroxyl (OH). Thus, the validated Converge CFD code with the PRF chemistry and the post-process tool was applied to investigate how PR change the formation of OH during the low-to high-temperature reaction transition. The reaction pathway analyses of the formations of OH before ignition time were investigated.

Reactivity Controlled Compression Ignition (RCCI) has been shown to be an attractive concept to achieve clean and high efficiency combustion. RCCI can be realized by applying two fuels with different reactivities, e.g., diesel and gasoline. This motivates the idea of using a single low reactivity fuel and direct injection (DI) of the same fuel blended with a small amount of cetane improver to achieve RCCI combustion. In the current study, numerical investigation was conducted to simulate RCCI and HCCI combustion and emissions with various fuels, including gasoline/diesel, iso-butanol/diesel and iso-butanol/iso-butanol+di-tert-butyl peroxide (DTBP) cetane improver. A reduced Primary Reference Fuel (PRF)-iso-butanol-DTBP mechanism was formulated and coupled with the KIVA computational fluid dynamic (CFD) code to predict the combustion and emissions of these fuels under different operating conditions in a heavy duty diesel engine.

In the current, numerical study RCCI combustion and emission characteristics using various fuel strategies are investigated, including methanol, ethanol, n-butanol and gasoline as the low reactivity fuel, and diesel fuel as the high reactivity fuel. A reduced Primary Reference Fuel (PRF)-alcohol chemical kinetic mechanism was coupled with a computational fluid dynamic (CFD) code to predict RCCI combustion under various operating conditions. The results show that a higher quantity of diesel was required to maintain the same combustion phasing with alcohol-diesel fuel blends, and the combustion durations and pressure rise rates of methanol-diesel (MD) and ethanol-diesel (ED) cases were much shorter and higher than those of gasoline-diesel (GD) and n-butanol-diesel (nBD) cases. The simulations also investigated the sensitivities of the direct injection strategies, intake temperature and premixed fuel ratio on RCCI combustion phasing control.

With a zero-dimensional detailed chemical kinetic model, a numerical study was carried out to investigate the chemical reaction phenomena encountered in the homogenous charge compression ignition process of dimethyl ether (DME) and methane dual fuel. The results show that the DME/methane dual fuel elementary reactions affect each other. The low temperature reaction (LTR) of DME is inhibited, the second molecular oxygen addition of DME is restrained, and β -scission plays a dominant role in DME oxidation. Hydrogen peroxide (H2O2) is controlled by DME oxidation and almost has no correlation with methane oxidation. The rich H2O2 concentration makes methane oxidation occurs at low initial temperature. Most of the formaldehyde (CH2O) is produced from H-abstraction of methoxy (CH3O) rather than from LTR of the DME. However, the heat release of methane oxidation promotes the hot flame reactions of DME which make the reactions with high activation energy occur.

A very competitive alcohol for use in diesel engines is butanol. Butanol is of particular interest as a renewable bio-fuel, as it is less hydrophilic and it possesses higher heating value, higher cetane number, lower vapor pressure, and higher miscibility than ethanol or methanol. These properties make butanol preferable to ethanol or methanol for blending with conventional diesel or gasoline fuel. In this paper, the spray and combustion characteristics of pure n-butanol fuel was experimentally investigated in a constant volume combustion chamber. The ambient temperatures were set to 1000 K, and three different oxygen concentrations were set to 21%, 16%, and 10.5%. The results indicate that the penetration length reduces with the increase of ambient oxygen concentration. The combustion pressure and heat release rate demonstrate the auto-ignition delay becomes longer with decreasing of oxygen concentrations.

Biodiesel is a widely used biofuel in diesel engines, which is of particular interest as a renewable fuel because it possesses the similar properties as the diesel fuel. The pure soybean biodiesel was tested in an optical constant volume combustion chamber using natural flame luminosity and forward illumination light extinction (FILE) methods to explore the combustion process and soot distribution at various ambient temperatures (800 K and 1000 K) and oxygen concentrations (21%, 16%, 10.5%). Results indicated that, with a lower ambient temperature, the autoignition delay became longer for all three oxygen concentrations and more ambient air was entrained by spray jet and more fuel was burnt by premixed combustion. With less ambient oxygen concentration, the heat release rate showed not only a longer ignition delay but also longer combustion duration.

A yc6112ZLQ turbocharged 6 cylinder engine with intercooler was converted to operate in dual fuel mode with compressed natural gas (CNG) and pilot diesel. The influence of the CNG ratio, pilot diesel injection advance (ADC) and intake temperature after intercooler on the combustion process, emissions and engine performance was investigated. The results show that the combustion process of dual-fuel engines is faster than diesel engine. Both the ignition timing of the pilot fuel and the excess air ratio of total fuel λ dominate the combustion characteristics of duel-fuel engines. With the increase of CNG ratio, the pressure and temperature in cylinder decrease at rated mode, but increase at torque and low speed modes. With advanced the pilot injection timing or increased the intake temperature, the cylinder pressure and temperature increase.