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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 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.

In this work the gasoline compression ignition (GCI) combustion characterized by both premixed gasoline port injection and gasoline direct injection in a single-cylinder diesel engine was investigated experimentally and computationally. In the experiment, the premixed ratio (PR), injection timing and exhaust gas recirculation (EGR) rate were varied with the pressure rise rate below 10 bar/crank angle. The experimental results showed that higher PR and earlier injection timing resulted in advanced combustion phasing and improved thermal efficiency, while the pressure rise rates and NOx emissions increased. Besides, a lowest ISFC of 176 g/kWh (corresponding to IMEP =7.24 bar) was obtained, and the soot emissions could be controlled below 0.6 FSN. Despite that NOx emission was effectively reduced with the increase of EGR, HC and CO emissions were high. However, it showed that GCI combustion of this work was sensitive to EGR, which may restrict its future practical application.

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.

An experimental study is carried out to investigate the coupling between dual loop EGR (DL-EGR) and variable geometry turbocharger (VGT) on a heavy-duty commercial diesel engine under different operating conditions and inlet conditions. The effects of VGT rack position and high-pressure (HP) proportion in DL-EGR on engine performance and emissions are studied. The boosting system is a series 2-stage turbocharger with a VGT as the HP-stage. The HP-Proportion in DL-EGR is swept from 0% to 100% while several intake pressure values and EGR rates are fixed by adjusting the VGT position. Results demonstrate that the VGT and HP EGR both have great influence on the exhaust enthalpy and turbocharger efficiency. The exhaust enthalpy and the intake demand have great influence on the DL-EGR split strategy.

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.

An experimental study is carried out to compare the effects of high-pressure-loop, low-pressure-loop and dual-loop exhaust gas recirculation systems (HPL-EGR, LPL-EGR and DL-EGR) on the combustion characteristics, thermal efficiency and emissions of a diesel engine. The tests are conducted on a six-cylinder turbocharged heavy-duty diesel engine under various operating conditions. The low-pressure-loop portion (LPL-Portion) of DL-EGR is swept from 0% to 100% at several constant EGR rates, and the DL-EGR is optimized based on fuel efficiency. The results show that the LPL-EGR can attain the highest gross indicated thermal efficiency (ITEg) in the three EGR systems under all the tested conditions. At a middle load of 0.95 BMEP, 1660 r/min, the pumping losses of LPL-EGR lead to the lowest BTE among the EGR systems. The HPL-EGR can achieve the best brake thermal efficiency (BTE) and emissions within the EGR rate of 22.5% mainly due to the reduced pumping losses.

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.

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 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.

The effects of Exhaust Gas Recirculation (EGR) and octane number of PRF fuel on combustion and emission characteristics in HCCI operation were investigated. The results show that EGR could delay the ignition timing, slow down the combustion reaction rate, reduce the pressure and average temperature in cylinder and extend the operation region into large load mode. With the increase of the fuel/air equivalence ratio or the fuel octane number (ON), the effect of EGR on combustion efficiency improves. With the increase of EGR rate, the combustion efficiency decreases. The optimum indicated thermal efficiency of different octane number fuels appears in the region of high EGR rate and large fuel/air equivalence ratio, which is next to the boundary of knocking. In the region of high EGR rate, HC emissions rise up sharply as the EGR rate increases. With the increase of octane number, this tendency becomes more obvious.

By mixing iso-octane with octane number 100 and normal heptane with octane number 0, it was possible to obtain a PRF fuel with octane rating between 0 and 100. The influence of PRF fuel’s octane number on the combustion characteristics, performance and emissions character of homogeneous charge compression ignition (HCCI) engine was investigated. The experiments were carried out in a single cylinder direct injection diesel engine. The test results show that, with the increase of the octane number, the ignition timing delayed, the combustion rate decreased, and the cylinder pressure decreased. The HCCI combustion can be controlled and then extending the HCCI operating range by burning different octane number fuel at different engine mode, which engine burns low octane number fuel at low load mode and large octane number fuel at large load mode. There exists an optimum octane number that achieves the highest indicated thermal efficiency at different engine load.

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.