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Hybrid powertrain has been proven to be an effective fuel-saving technology in commercial vehicles, but many hybrid commercial vehicles still use conventional diesel engines, resulting in limited fuel savings. The main purpose of this study is to enhance the thermal efficiency of a dedicated hybrid diesel engine focusing on the characteristic operating conditions. Via fundamental thermodynamics process analysis of internal combustion engine, steel piston with high compression ratio, air system involving two-stage turbocharger(2TC) with an intercooler, and late intake valve closing(IVC) timing are proposed to improve the thermal efficiency of the engine. Experimental results show that high compression ratio and lower thermal conductivity of the combustion chamber surface lead to lower heat release rates, requiring optimization of piston profile to accelerate the mixing rate. Besides, high compression ratio also leads to higher mechanical losses.

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.

OH, soot and temperature distributions of wall-impinging diesel fuel spray were investigated in a high-temperature high-pressure constant volume combustion vessel. The ambient temperature (Ta) was set as 773 K, and the wall temperature (Tw) was set as 523 K, 673 K, 773 K, respectively. Three different injection pressures (Pi) of 60 MPa, 100 MPa, 160 MPa, and the ambient pressures (Pa) of 4 MPa were applied. The OH spatial distributions of wall-impinging spray were measured by the method of OH chemiluminescence imaging. Two-color pyrometry was applied to evaluate the spatial distributions of KL factor and flame temperature of wall-impinging spray. The results reveal that, OH chemiluminescence is observed in the region near the impingement point firstly. The regions of high OH chemiluminescence intensity and high KL factor appear in the location near the wall surface along the whole combustion process.

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.

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.

In order to achieve high-efficiency and clean combustion in HCCI engines, combustion must be controlled reasonably. A great variety of species with various reactivities can be produced through low temperature oxidation of fuels, which offers possible solutions to the problem of controlling in-cylinder mixture reactivity to accommodate changes in the operating conditions. In this work, in-cylinder combustion characteristics with low temperature reforming (LTR) were investigated in an optical engine fueled with low octane number fuel. LTR was achieved through low temperature oxidation of fuels in a reformer (flow reactor), and then LTR products (oxidation products) were fed into the engine to alter the charge reactivity. Primary Reference Fuels (blended fuel of n-heptane and iso-octane, PRFs) are often used to investigate the effects of octane number on combustion characteristics in engines.

Gasoline partially premixed combustion shows the potential to achieve clean and high-efficiency combustion. Injection strategies show great influence on in-cylinder air flow and in-cylinder fuel distribution before auto-ignition, which can significantly affect the combustion characteristics and emissions. This study explored the effects of various injection strategies, including port fuel injection (PFI), single direct injection (DIm), double direct injection (DIp+DIm) and port fuel injection coupled with a direct injection (PFI+DIm) on the combustion characteristics and emissions in a modified single cylinder heavy-duty diesel engine fueled with 92# gasoline at low load. The investigation consists of two parts. Firstly, the comparison among PFI, PFI+DIm, and DIp+DIm strategies was conducted at a fixed CA50 to explore the effects of PFI+DIm and DIp+DIm strategies on the thermal efficiency and combustion stability.

The flame structure and combustion characteristics of wall-impinging diesel fuel spray were investigated in a high-temperature high-pressure constant volume combustion vessel. The ambient temperature (Ta) was set to 773 K. The wall temperatures (Tw) were set to 523 K, 673 K and 773 K respectively. Three different injection pressures (Pi) of 600 bar, 1000bar and 1600bar, two ambient pressures (Pa) of 2 MPa and 4 MPa were applied. The flame development process of wall-impinging spray was measured by high-speed photography, which was utilized to quantify the flame luminosity intensity, ignition delay and flame geometrical parameters. The results reveal that, as the wall temperature increases, the flame luminosity intensity increases and the ignition delay decreases.

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.

On-board fuel reforming offers a prospective clean combustion mode for the engines. The flexible cylinder engine strategy (FCE) is a new kind of such mode. In this paper, the combustion of the primary reference fuel of PRF90 was theoretically investigated in a homogeneous charge compression ignition engine to validate the FCE mode, mainly focusing on the ignition delay time, the flame speed, and the emissions. The simulations were performed by using the CHEMKIN2.0 package to demonstrate the fuel reforming process in the flexible cylinder, the cooling effect on the reformed products, and the combustions of the mixture of the fresh fuel and the reformed products in the normal cylinders. It was found that the FCE mode decreased the ignition delay time of the fuel by about 35 crank angles at a typical engine condition.

Reactivity controlled compression ignition (RCCI) is a potential combustion strategy to achieve high engine efficiency with ultra-low NOx and soot emissions. Fuel stratification can be used to control the heat release rate of RCCI combustion. But the in-cylinder combustion process of the RCCI under different fuel stratification degrees has not been well understood, especially at a higher engine load. In this paper, simultaneous measurement of natural flame luminosity and emission spectra was carried out on a light-duty optical RCCI engine under different fuel stratification degrees. The engine was run at 1200 revolutions per minute under a load about 7 bar indicated mean effective pressure (IMEP). In order to form fuel stratification degrees from low to high, the common-rail injection timing of n-heptane was changed from -180° CA after top dead center (ATDC) to -10° CA ATDC, while the iso-octane delivered in the intake stroke was fixed.

The combustion in low-speed two-stroke marine diesel engines can be characterized as large spatial and temporal scales combustion. One of the most effective measures to reduce NOx emissions is to reduce the local maximum combustion temperature. In the current study, multi-dimensional numerical simulations have been conducted to explore the potential of Miller cycle, high compression ratio coupled with EGR (Exhaust Gas Recirculation) and WEF (water emulsified fuel) to improve the trade-off relationship of NOx-ISFC (indicated specific fuel consumption) in a low-speed two-stroke marine engine. The results show that the EGR ratio could be reduced combined with WEF to meet the Tier III emission regulation. The penalty on fuel consumption with EGR and WEF could be offset by Miller cycle and high geometric compression ratio.

In this work, both the ‘SCR-only’ and ‘EGR+SCR’ technical routes are compared and evaluated after the optimizations of both injection strategy and turbocharging system over the World Harmonized Stationary Cycle (WHSC) in a heavy duty diesel engine. The exhaust emissions and fuel economy performance of different turbocharging systems, including wastegate turbocharger (WGT), variable geometry turbocharger (VGT), two-stage fixed geometry turbocharger (WGT+FGT) and two-stage variable geometry turbocharger (VGT+FGT), are investigated over a wide EGR range. The NOx reduction methods and EGR introduction strategies for different turbocharger systems are proposed to improve the fuel economy. The requirement on turbocharging system and their potential to meet future stringent NOx and soot emission regulations are also discussed in this paper.

Gasoline partially premixed combustion (PPC) is a potential combustion concept to achieve high engine efficiency as well as low NOx and soot emissions. But the in-cylinder process of PPC is not well understood. In the present study, the double injection strategy of PPC was investigated on a light-duty optical engine. The fuel/air mixing and combustion process of PPC was evaluated by fuel-tracer planar laser-induced fluorescence (PLIF) and high-speed natural luminosity imaging technique, respectively. Combustion emission spectra of typical double injection case were analyzed. The primary reference fuel, PRF70 (70% iso-octane and 30% n-heptane by volume) was chosen as the lower reactivity fuel like gasoline. Double injection strategies of different first fuel injection timing and mass ratio of the two fuel injections were comparatively studied.

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.

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.

Effects of fuel physical and chemical properties on combustion and emissions were investigated on both metal and optical diesel engines. The new generation oxygenated biofuels, n-butanol and DMF (2,5-dimethylfuran) were blended into diesel fuel with 20% volume fraction and termed as Butanol20 and DMF20 respectively. The exhaust gas recirculation (EGR) rates were varied from zero to ∼60% covering both conventional and low temperature combustion. Meanwhile, the reference fuels such as n-heptane, cetane, and iso-cetane were also used to isolate the effects of different fuel properties on combustion and emissions. In addition, to clarify the effects of oxygenated structures on combustion and emissions, a fundamental partially premixed burner was also used. Results based on metal and optical diesel engines show that fuel cetane number is the dominated factor to affect the auto-ignition timing and subsequent combustion process.

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.

To investigate the effects of fuel volatility on combustion and emissions in a diesel engine, a high-volatility fuel of n-heptane was blended into diesel fuel with different volumetric fractions (0%, 40%, 70%, 100%). A wide range of EGR rates from 0% to 65% were investigated, which covered both the conventional diesel combustion and low temperature combustion. Experiments under two engine load conditions, ∼5.2 bar and ∼10.5 bar gross IMEP were performed at 1500 rpm. The injection timing was fixed at 8°CA BTDC for all test cases. Results show that even if the ignition delay and combustion duration are nearly the same for all tested fuels, the premixed combustion fractions are increased for higher volatility fuels due to the improvement on mixing process during the ignition delay period. The indicated specific fuel consumption is decreased as using high-volatility fuels. The effect of fuel volatility on soot emissions depends on engine loads.

In order to achieve high efficiency and clean combustion indiesel engines, many advanced combustion concepts have been developed to simultaneously reduce NOx and soot emissions with high efficiency. However, the benefits of these combustion modes are limited to low loads because the energy release ratesaretoo fast at high loads. Recently, Dual-fuel highly premixed charge combustion (HPCC) strategies with the port injection of gasoline and direct injection of diesel have demonstrated advantages in terms of extending the operating range by the flexible control of fuel chemical reactivity and charge stratification. However, the extension to high-load in a turbocharged multi-cylinder diesel engine with the HPCC is a critical challenge due to excessive pressure rise rates. Mean while it suffers from the excessive of CO/HC emissions at low loads.