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Natural gas has been used in spark-ignition (SI) engines of natural gas vehicles (NGVs) due to its resource availability and stable price compared to gasoline. It has the potential to reduce carbon monoxide emissions from the SI engines due to its high hydrogen-to-carbon ratio. However, short running distance is an issue of the NGVs. In this work, methodologies to improve the fuel economy of a heavy-duty commercial truck under the Japanese Heavy-Duty Driving Cycle (JE05) is proposed by numerical 1D-CFD modeling. The main objective is a comparative analysis to find an optimal fuel economy under three variable mechanisms, variable valve timing (VVT), variable valve actuation (VVA), and variable compression ratio (VCR). Experimental data are taken from a six-cylinder turbocharged SI engine fueled by city gas 13A. The 9.83 L production engine is a CR11 type with a multi-point injection system operated under a stoichiometric mixture.

We have proposed the engine featuring a new compressive combustion principle based on pulsed supermulti-jets colliding through a focusing process in which the jets are injected from the chamber walls to the chamber center. This principle has the potential for achieving relatively silent high compression around the chamber center because autoignition occurs far from the chamber walls and also for stabilizing ignition due to this plug-less approach without heat loss on mechanical plugs including compulsory plasma ignition systems. Then, burned high temperature gas is encased by nearly complete air insulation, because the compressive flow shrinking in focusing process gets over expansion flow generated by combustion.

A numerical study was carried out to investigate combustion characteristics of a dual-fuel gas diesel engine, using a multi-dimensional model combined with detailed chemical kinetics, including 43 chemical species and 173 elementary reactions. In calculations, the effects of initial temperature, EGR ratios on ignition, and combustion were examined. The results indicated EGR combined with intake preheating can favorably reduced NOx and THC emissions simultaneously. This can be explained by the fact that combustion mechanism is changed from flame propagation to HCCl like combustion.

A dual fuel natural gas diesel engine suffers from remarkably lower thermal efficiency and higher THC, CO emissions at lower load because of its lower burned mass fraction caused by the lean pre-mixture. To overcome this inevitable disadvantage at lower load, two methods of reducing the number of operating cylinders were examined. One method was to use the two cylinders operation while the second one was to use the quasi-two cylinders operation. As a result, it was found that the unburned hydrocarbons and CO emissions could be favorably reduced with the improvement of thermal efficiency by reducing the number of cylinders to half for a dual fuel natural gas diesel engine. Moreover, it was also found that the quasi-two cylinders operation could improve the torque fluctuation more compared to the two cylinders operation.

Natural gas pre-mixture is ignited by a small amount of pilot fuel in the dual fuel engine. In this paper, numerical studies were carried out to investigate the combustion and exhaust gas emissions formation process of this engine type by using a multi dimensional model combined with the detailed chemical kinetics including 57 chemical species and 290 elementary reactions. In calculation, the effect of the pre-mixture concentration on combustion was examined. The result indicated that the increased concentration of natural gas could improve the burning fraction and THC, CO emissions due to the increased pre-mixture consumption rate and the cylinders gas temperature.

It has been recognized that alternative fuels such as liquid petroleum gas (LPG) has less polluting combustion characteristics than diesel fuel. Direct-injection stratified-charge combustion LPG engines with spark-ignition can potentially replace conventional diesel engines by achieving a more efficient combustion with less pollution. However, there are many unknowns regarding LPG spray mixture formation and combustion in the engine cylinder thus making the development of high-efficiency LPG engines difficult. In this study, LPG was injected into a high pressure and temperature atmosphere inside a constant volume chamber to reproduce the stratification processes in the engine cylinder. The spray was made to hit an impingement wall with a similar profile as a piston bowl. Spray images were taken using the Schlieren and laser induced fluorescence (LIF) method to analyze spray penetration and evaporation characteristics.

For various densities of gas jets including very light hydrogen and relatively heavy ones, the penetration length and diffusion process of a single high-speed gas fuel jet injected into air are computed by performing a large eddy simulation (LES) with fewer arbitrary constants applied for the unsteady three-dimensional compressible Navier-Stokes equation. In contrast, traditional ensemble models such as the Reynolds-averaged Navier-Stokes (RANS) equation have several arbitrary constants for fitting purposes. The cubic-interpolated pseudo-particle (CIP) method is employed for discretizing the nonlinear terms. Computations of single-component nitrogen and hydrogen jets were done under initial conditions of a fuel tank pressure of gas fuel = 10 MPa and back pressure of air = 3.5 MPa, i.e., the pressure level inside the combustion chamber after piston compression in the engine.

A swirl-chamber diesel engine has an indirect injection system in which fuel is injected into a pre-chamber called the swirl-chamber that is separated from the main chamber. Indirect fuel injection systems can be directly mechanically controlled by the camshaft, which is cheaper than electronic control. For these reasons, they are used in diverse industrial applications and automobiles. However, optimization of the swirl-chamber shape and performance tests have been mainly experimental, and there has been insufficient verification of the accuracy of simulations. Thus, we have attempted to verify simulations using a rapid compression and expansion machine that can reproduce the combustion in one engine cycle, with a chamber like a swirl chamber in the cylinder head to visualize the behavior of evaporative sprays and the combustion process. In this study, the authors focused on the wall impingement of the fuel spray and took photos of its liquid phase and ignition.

A variable valve timing (VVT) mechanism was applied to achieve premixed diesel combustion at higher load for low emissions and high thermal efficiency in a light duty diesel engine. By means of late intake valve closing (LIVC), compressed gas temperatures near the top dead center are lowered, thereby preventing too early ignition and increasing ignition delay to enhance fuel-air mixing. The variability of effective compression ratio has significant potential for ignition timing control of conventional diesel fuel mixtures. At the same time, the expansion ratio is kept constant to ensure thermal efficiency. Combining the control of LIVC, EGR, supercharging systems and high-pressure fuel injection equipment can simultaneously reduce NOx and smoke. The NOx and smoke suppression mechanism in the premixed diesel combustion was analyzed using the 3D-CFD code combined with detailed chemistry.

Pre-chamber spark ignition technology can stabilize combustion and improve thermal efficiency of lean burn natural gas engines. During compression stroke, a homogeneous lean mixture is introduced into pre-chamber, which separates spark plug electrodes from turbulent flow field. After the pre-chamber mixture is ignited, the burnt jet gas is discharged through multi-hole nozzles which promotes combustion of the lean mixture in the main chamber due to turbulence caused by high speed jet and multi-points ignition. However, details mechanism in the process has not been elucidated. To design the pre-chamber geometry and to achieve stable combustion under the lean condition for such engines, it is important to understand the fundamental aspects of the combustion process. In this study, a high-speed video camera with a 306 nm band-pass filer and an image intensifier is used to visualize OH* self-luminosity in rapid compression-expansion machine experiment.

Natural gas is a promising alternative fuel for internal combustion engines because of its clean combustion characteristics and abundant reserves. However, it has several disadvantages due to its low energy density and low thermal efficiency at low loads. Thus, to assist efforts to improve the thermal efficiency of spark-ignited (SI) engines operating on natural gas and to minimize test procedures, a numerical simulation model was developed to predict and optimize the performance of a turbocharged test engine, considering flame propagation, occurrence of knock and ignition timing. The numerical results correlate well with empirical data, and show that increasing compression ratios and retarding the intake valve closing (IVC) timing relative to selected baseline conditions could effectively improve thermal efficiency. In addition, employing moderate EGR ratios is also effective for avoiding knock.

Pre-chamber (PC) natural gas and hydrogen (CH4-H2) combustion can improve thermal efficiency and greenhouse gas emissions from decarbonized stationary engines. However, the engine efficiency is worsened by prolonged combustion duration due to PC jet velocity extinction. This work investigates the impact of cylindrical PC internal shapes to increase its jet velocity and shorten combustion duration. A rapid compression and expansion machine (RCEM) is used to investigate the combustion characteristics of premixed CH4 gas. The combustion images are recorded using a high-speed camera of 10,000 fps. The experiments are conducted using two types of long PC shapes with diameters φ=4 mm (hereafter, longφ4) and 5 mm (hereafter, long φ5), and their combustions are compared against a short PC shape (φ=12 mm). For all designs of the PC shapes, the PC holes are 6 with 2 mm in diameter.

To identify ways of achieving good mixture formation and heat release in diesel spray combustion, we have performed Large Eddy Simulation (LES) using a detailed chemical reaction mechanism to study the temporal and spatial distribution of the local equivalence ratios and heat release rate. Here we characterize the effect of the fuel injection rate profile on these processes in the combustion chamber of a diesel engine. Two injection rate profiles are considered: a standard (STD) profile, which is a typical modern common rail injection profile, and the inverse delta (IVD) profile, which has the potential to suppress rich mixture formation in the spray tip region. Experimental data indicate that the formation of such mixtures may extend the duration of the late combustion period and thus reduce thermal efficiency.

We have proposed a new compressive combustion principle leading to the auto-ignition of fuel by focusing compression due to the collision of the pulsed supermulti-jets. This principle has the potential of nearly-complete air insulation due to encasing burned gas around the center of the combustion chamber and a high compression ratio around the chamber center while suppressing vibration and noise levels. We have developed the first prototype engine having a very small combustion chamber of a diameter of 18 mm and also 14 side passages for the supermulti-jets colliding at the chamber center. Combustion experimental results indicating air insulation effect and high thrust over 100 N were obtained as basic data for various types of applications, including automobiles and aerospace usage such as for rockets. However, it was found that higher compression due to more jets is necessary to get stabler combustion.