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Natural gas/diesel dual fuel engines have potential for a high thermal efficiency and low NOx emissions. However, they have the disadvantages of high unburned species emissions and lower thermal efficiencies at low loads (at low equivalence ratio). A way to solve this problem is to properly distribute the pilot fuel vapor in a natural-gas premixture. The combustion chamber geometry affects the combustion process since it influences the distribution of the pilot fuel vapor. This study investigates the influence of injection conditions and the piston bowl geometry on the performance and emissions of a dual fuel engine. Experiments were carried out using two pistons with different bowl diameters, 52 mm and 58 mm, at single-and two-stage diesel-fuel injection. The results show that the larger bowl provides lower hydrocarbon emissions at a lower equivalence ratio in the case of single-stage injection.

This study tried to find a potential of dedicated EGR (d-EGR) system added to the four-cylinder spark ignition (SI) engine to decrease heat loss (Qheatloss) and improve thermal efficiency (ηth). Test fuels were chosen by methane and propane. PREMIX code in CHEMKIN-PRO was employed to calculate laminar burning velocity (SL) and flame temperature (Tf). Wiebe function and Wocshni's heat transfer coefficient were considered to calculate ηth. The results show that the d-EGR system increased ηth and it was higher than that of stoichiometric combustion of conventional SI engines due to the low Tf and fast SL.

The ignition characteristics of each component of natural gas and the chemical kinetic factors determining those characteristics were investigated using detailed chemical kinetic calculations. Ethane (C2H6) showed a relatively short ignition delay time with high initial temperature; the heat release profile was slow in the early stage of the ignition process and rapid during the late stage. Furthermore, the ignition delay time of C2H6 showed very low dependence on O2 concentration. In the ignition process of C2H6, HO2 is generated effectively by several reaction paths, and H2O2 is generated from HO2 and accumulated with a higher concentration, which promotes the OH formation rate of H2O2 (+ M) = OH + OH (+ M). The ignition characteristics for C2H6 can be explained by H2O2 decomposition governing OH formation at any initial temperature.

Engines using natural gas as their main fuel are attracting attention for their environmental protection and energy-saving potential. There is demand for improvement in the thermal efficiency of engines as an energy-saving measure, and research in this area is being actively pursued on spark ignition engines and HCCI engines. In spark ignition gas engines, improving combustion under lean condition and EGR (exhaust gas recirculation) condition is an issue, and many large gas engines use a pre-chamber. The use of the pre-chamber approach allows stable combustion of lean gas mixtures at high charging pressure, and the reduction of NOx emissions. In small gas engines, engine structure prevents the installation of pre-chambers with adequate volume, and it is therefore unlikely that the full benefits of the pre-chamber approach will be derived.

Recently, a potentiality of Dedicated EGR (D-EGR) concept SI engine has been studied. This concept engine had four cylinders and operated with exhaust gas supplied from the single cylinder to the intake manifold. Compared with conventional SI engines, it was able to increase thermal efficiency and decrease CO, HC, and NOx emission by the high D-EGR ratio 0.25. In this study, numerical analysis of a SI engine with D-EGR system with various D-EGR ratios was conducted for detailed understanding the potentiality of this concept in terms of thermal efficiency and NOx emission. #1 cylinder of assumed engine was used as D-EGR cylinder that equivalence ratio varied from 0.6 to 3.4. Entire exhaust gas from #1 cylinder was recirculated to the other cylinders. The other cylinders run with this exhaust gas and new premixed air and fuel with various equivalence ratios from 0.6-1.0.

A fuel design concept for an HCCI engine based on chemical kinetics to optimize the heat release profile and achieve robust ignition was proposed, and applied to the design of the optimal methane-based blend. Ignition process chemistry of each single-component of natural gas, methane, ethane, propane, n-butane and isobutane, was analyzed using detailed chemical kinetic computations. Ethane exhibits low ignitability, close to that of methane, when the initial temperature is below 800 K, but higher ignitability, close to those of propane, n-butane and isobutane, when the initial temperature is above 1100 K. Furthermore, ethane shows a higher heat release rate during the late stage of the ignition process. If the early stage of an ignition process takes place during the compression stroke, this kind of heat release profile is desirable in an HCCI engine to reduce cycle-to-cycle variation during the expansion stroke.

The ignition delay times and heat release profiles of CH4, C2H6, C3H8, i-C4H10, and n-C4H10 and dual-component CH4-based blends with these alkanes in air were determined using a detailed chemical kinetic model. The apparent activation energy of C2H6 in the relationship between initial temperature and ignition delay time is higher than those of the other alkanes because OH formation is dominated by H2O2(+M)=OH+OH(+M) from the beginning over a wide range of initial temperatures. The heat release rate of C2H6 is higher than those of the other alkanes in the late stage of ignition delay time because H2O2 is accumulated with a higher concentration and promotes the OH formation rate of H2O2(+M)=OH+OH(+M). These ignition characteristics are reflected in those of CH4/C2H6.

Recently, producing energy with reduced CO₂ emissions has been one of the most important topics of global discussion. Combined heat and power (CHP) systems, which use natural gas to produce thermal energy and electric energy simultaneously, are considered to contribute significantly to the efficient use of energy and environmental conservation. The prevalence of CHP systems is expected. From an energy security point of view, promoting the introduction of distributed energy systems, which use natural gas, is highly recommended. In a CHP natural gas engine, enhancing engine efficiency is required to save energy. The challenge with spark ignition (SI) engines is how to improve destabilizing firing in the situation of a mixed lean burn and exhaust gas recirculation (EGR) condition. This is especially important in large size of lean combustion natural gas engines, which use a pre-chamber to maintain spark ignition performance, while also avoiding knocking.

The purpose of this study is to determine a pilot injection control strategy for the improvement of dual-fuel combustion with a lean natural gas/air mixture. Experiments were performed using a single cylinder test engine equipped with a common-rail injection system. The injection pressure, timing and quantity were varied at a fixed overall equivalence ratio of 0.5. The results of single-stage-injection experiments show that middle injection timings (−20 to −10 degATDC) produce low emissions of unburned species, because the pilot-fuel vapor spreads into the natural-gas lean mixture and raises the effective equivalence ratio, which leads to fast flame propagation. Early injection (−35degATDC) is advantageous for low NOx emission; however, increased emissions of unburned species are barriers.

The possibility of turbocharging into a natural gas homogeneous charge compression ignition (HCCI) engine is investigated experimentally and by simulation. Experiments are performed using a four-cylinder naturally aspirated engine fitted with an external supercharger and a butterfly valve for back pressure control to simulate a turbocharger with efficiency of 0.64. Based on the test results, the performance and emission characteristics are studied in detail through numerical one-dimensional cycle simulations. The results indicate that the thermal efficiency can be improved by raising the engine compression ratio and lowering the turbocharging pressure. At an engine compression ratio of 21 and turbocharging pressure of 1.9 bar, the brake thermal efficiency reaches 0.43, with NOx emissions of only 10 ppm or less.

Since there is a trade-off relationship between NOx and particulates in exhaust gas emitted from a diesel engine, simultaneous reduction of the amounts of NOx and particulates in a combustion chamber is difficult. However, the amount of particulates produced in the combustion process could be reduced in a state of almost complete combustion, and the amount of NOx produced during the combustion process could be reduced by the use of a catalyst and reducing agent in the exhaust process. It has been demonstrated that the use of ethanol as a reducing agent on a silver-base catalyst in the presence of oxygen is an effective means for reducing NOx, although the mechanism of the reduction has not been elucidated. Therefore, in the present study, an NOx-reduction apparatus was conducted, and model experiments on NOx reduction were carried out in an atmosphere simulating exhaust gas emitted from a diesel engine and at the same catalyst temperature as that in a combustion chamber.

Small two stroke SI engines supplied with natural gas in the intake port are advantageous for low maintenance and low cost when used in co-generation systems for residential use. However in the engines with port injection systems, the unburned HC emissions are higher and thermal efficiency is lower than with 4 stroke engines. To overcome these disadvantages, an in-cylinder injection with a special low pressure injection nozzle system was attempted. The results showed that improvements in unburned HC emissions and thermal efficiency are possible due to the remarkable reduction in scavenging loss and the lean combustion.

Lean-burn engines now being developed employ in-cylinder injection which requires high pressures and so necessitates expensive injection equipment. The injection system proposed here is an air assisted in-cylinder injection system which is injecting a mixture of air and fuel in the cylinder during the intake stroke and allowing atomization at lower injection pressures than those necessary in compressing fuel with a usual solid injection. This time, the experiments used a testing engine of a 4 stroke gasoline OHV type replacing the Side Valve type. Performance with a small depression in the main combustion chamber was investigated with a spark plug and reed valve installed in the depression. The engine was operated then following the same method as last year (SAE 982698). As a result, the lean burn method employed here was possible over a wide range of engine speeds and loads. Moreover, it was also shown that this operation was possible with a fully opened throttle valve.

Lean burn engines now being developed employ in-cylinder injection which requires high pressures and so necessitates expensive injection equipment. The experiments reported here used air assisted in-cylinder injection, and injected a mixture of air and fuel during the intake stroke, so allowing atomization at lower injection pressures than those necessary in compressing fuel with solid injection. The experiments confirmed that operation in this manner resulted in similar output and fuel consumption as with a carburetor. Next, a divided combustion chamber was installed and connected to the main combustion chamber and air assisted in-cylinder injection from a reed type injection nozzle was attempted. With this arrangement, stable idling operation was possible to air-fuel ratios (A/F) of 70. Lean burn at A/F = 22 to 35 was also achieved at maximum rated outputs (3.7 kW at 4200 min-l) of 6 - 18 %.

A critical factor in improving performance of crankcase-scavenged two-stroke gasoline engines is to reduce the short-circuiting of the fresh charge to the exhaust in the scavenging process. To achieve this, the authors developed a reciprocating exhaust control valve mechanism and direct air-fuel injection system. This paper investigates the effects of exhaust control valve and direct air-fuel injection in the all aspect of engine performance and exhaust emissions over a wide range of loads and engine speeds. The experimental results indicate that the exhaust control valve and direct air-fuel injection system can improve specific fuel consumption, and that HC emissions can be significantly reduced by the reduction in fresh charge losses. The pressure variation also decreased by the improved combustion process. CRANKCASE SCAVENGED two-stroke gasoline engines suffer from fresh charge losses leading to poor fuel economy and it is a reason for large increases of HC in the exhaust.

Extensive experiments were conducted on a low emission DI diesel engine by using Dimethyl Carbonate (DMC) as an oxygenate fuel additive. The results indicated that smoke reduced almost linearly with fuel oxygen content. Accompanying noticeable reductions of HC and CO were attained, while a small increase in NOx was encountered. The effective reduction in smoke with DMC was maintained with intake charge CO2, which led to low NOx and smoke emissions by the combined use of oxygenated fuel and exhaust gas recirculation (EGR). Further experiments were conducted on an optically accessible combustion bomb and a thermal cracking set-up to study the mechanisms of DMC addition on smoke reduction.

This paper uses NO Reaction Kinetic to determine NO formation characteristics in diesel engines. The NO formation was calculated by Extended Zel'dovich Reaction Kinetics in a diffusion process. The results show that the NO formation rate is independent of the mixing of the combustion gas, and that internal EGR (combustion gas mixing in a cylinder) has no effect on NO reduction. The paper also shows the potential of two stage combustion, and its effect strongly depends on the time-scale of mixing. Additionally the paper investigates the mechanism of increased NOx emissions in high pressure fuel injection.

This paper presents a theory and its experimental validation for air entrainment changes into fuel sprays in DI diesel engines. The theory predicts air entrainment changes for a variety of swirl speeds, number of nozzle holes, nozzle diameters, engine speeds, injection speeds and fuel densities. The formulae of the theory are simple non-dimensional equations, which apply for different sized engines. Experiments were performed to compare theoretical predictions and experimental results in six different engines varying from 85 to 800mm bore. All results showed good agreement with the theoretical predictions for shallow-dish piston engines. However the agreement became poor in the case of deep cavity piston engines. With the theory, it is possible to interpret a variety of combustion phenomena in diesel engines, providing additional understanding of diesel combustion processes.

Dimethyl ether (DME) has very good compression ignition characteristics, and can be converted from methanol using a γ - alumina catalyst. A previous report investigated a compression ignition methanol engine with DME as an ignition improver. The results showed that the engine operation was sufficiently smooth without either spark or glow plugs. Two methods were studied, one was an aspiration method, and the other was a torch ignition chamber method (TIC method). The aspiration method allows a simple engine structure, but suffers from poor engine emissions and requires large amounts of DME. With the TIC method where the DME was introduced into a torch ignition chamber (TIC) during the intake stroke, the diffusion of the DME into the main combustion chamber was limited, and significant reductions in both the necessary quantity of DME and emissions were obtained [1][2].

Exhaust Particulate emitted from diesel engines is a serious problem form the point of view of the environment and energy saving. Exhaust particulate is consist of dry soot and SOF (soluble organic fraction). To clarify the formation process of SOF in the combustion chamber of diesel engines, first lower temperature column condensed method was investigated. The gas from combustion chamber was collected to the sampling column using this method, and the cracked as well as the condensation polymerized components were analyzed with gas chromatography. The sampling condition of the low temperature column condensation method are length of condensation column 600mm, cooling temperature 198K, and dilution ratio 5. The diesel fuel injected into the combustion chamber, first cracks into lower boiling point hydrocarbons, this is followed by dehydrogenation and formation of benzene ring compounds through condensation polymerization. This is followed by the formation of PAH.