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The effects of fuel injection timing on fuel-air mixture formation, combustion and emissions for a PREmixed lean DIesel Combustion (PREDIC) engine has been studied numerically by the KIVA II modeling package. The software was modified with an improved autoignition and combustion submodel, which describes the formation of combustible or ignitable fuel-air mixtures by turbulent mixing, and describes four chemical reactions, including low-temperature oxidation. The results indicate that the present computational model reproduces major features of two-stage autoignition and experimentally observed trends in NO and unburned fuel emissions. The relationships among in-cylinder distributions of fuel sprays, fuel-air equivalence ratio, temperature and mass fractions of NO and unburned fuel were demonstrated by graphically imaged results. A method of fuel-air mixture characterization has been introduced and used to analyze the numerical results.

The authors designed and produced a new dual fuel injector that allows two different kinds of fuel to be injected. This injector contains both a throttle type nozzle and a hole type which are located coaxially. The injection timing as well as the fuel quantity can be controlled individually. The running test using two lines of gas oil brought a good reduction of NOx and exhaust smoke. The experiment using gas oil and alcohol also brought a satisfactory reduction of exhaust emission.

It has been clarified that diesel fuel properties have a great effect on the exhaust emissions and fuel consumption of a conventional diesel combustion regime. And as other diesel combustion regimes are applied in order to improve exhaust emissions and fuel consumption, it can be supposed that the fuel properties also have significant effects. The purpose of this study is to propose the optimum diesel fuel properties for a premixed compression ignition (PCI) combustion regime. In this paper, the effect of the auto-ignitability of diesel fuels on exhaust emissions and fuel consumption was evaluated using a heavy-duty single-cylinder test engine. In all experiments, fuels were injected using an electronically controlled, common-rail diesel fuel injector, and most experiments were conducted under high EGR conditions in order to reduce NOx emissions.

In this study, diesel exhaust emission characteristics were investigated as GTL (Gas To Liquid) fuel was applied to a heavy-duty diesel truck which had been developed to match a Japanese new long-term exhaust emission regulation (NOx < 2.0 g/kWh, PM < 0.027 g/kWh). The results in this study show that although the test vehicle has advanced technologies (e.g. high pressure fuel injection, oxidation catalyst, and urea-SCR aftertreatment system, etc.) which are applied to reduce diesel emissions, the neat GTL fuel has a great advantage to reduce particulate matter emissions and poly aromatic hydrocarbons. And regarding nano-size PM emissions, nuclei mode particles emitted during idling are significantly decreased by using the GTL fuel.

A study of the mixing mechanism of fuel with surrounding air is necessary in order to clarify the combustion process. In this study, the flow field near non-evaporating diesel spray as well as spray surface were observed and analyzed using a Nd-YAG laser light sheet. A single shot fuel spray was injected into a high pressure vessel and photographed under double-pulse laser illumination. The images of dispersed particles in the vessel were processed and velocity vectors were obtained by the auto-correlation method. Measured results showed temporal variation in the air movement around the spray. Just after the start of injection, air near the nozzle was pushed outward by the spray tip, after which the flow direction reversed. The air velocity ahead of spray tip was very low compared to fuel spray tip velocity. At a stable injection condition, air near the nozzle tip was pulled by the spray movement and flowed uniformly, and the spray-air boundary was smooth.

Low NOx combustion is possible by PREDIC (PREmixed lean DIesel Combustion) in which fuel is injected at a very early stage of the compression stroke and the combustion starts at near the top dead center by self-ignition. To simplify the phenomenon of the PREDIC process, the test engine was operated with gaseous fuels added to intake air to realize combustion of a perfectly homogeneous mixture. The rich limit was observed around λ=2.0∼2.4. This limit was determined by considering the increase in NOx, and the steep pressure rise. During high load operations is not only the ignition timing but also the combustion rate should be controlled. By comparing the homogeneous charge and direct injection case, the mixture heterogeneity could be found to have an influence on the ignition timing and combustion rate, the engine speed and injection timing also had an influenced on these.

Previous research in our laboratory has shown that NOx emissions can be sharply reduced by PREDIC (PRE-mixed lean DIesel Combustion), in which fuel is injected very early in the compression process. However some points of concern remained unsolved, such as a large increase in THC and CO, higher fuel consumption, and an operating region narrowly limited to partial loads, compared to conventional diesel operation. In this paper, the causes of PREDIC's problem areas were analyzed through engine performance tests and combustion observation with a single cylinder engine, through fuel spray observation with a high-pressure vessel, and through numerical modeling. Subsequently, measurable improvements were achieved on the basis of these analyses. As a result, the ignition and combustion processes were clarified in terms of PREDIC fuel-air mixture formation. Thus, THC and CO emissions could be decreased by adopting a pintle type injection nozzle, or a reduced top-land-crevice piston.

Recently, dimethyl ether (DME) has been attracting much attention as a clean alternative fuel, since the thermal efficiency of DME powered diesel engine is comparable to diesel fuel operation and soot free combustion can be achieved. In this experiment, the effect of ambient pressure on DME spray was investigated with observation of droplet size such as Sauter mean diameter (SMD) by the shadowgraph and image processing method. The higher ambient pressure obstructs the growth of DME spray, therefore faster breakup was occurred, and liquid column was thicker with increasing the ambient pressure. Then engine performances and exhaust emissions characteristics of DME diesel engine were investigated with various compression ratios. The minimum compression ratio for the easy start and stable operation was obtained at compression ratio of about 12.

This work aimed to develop an LPG fueled direct injection SI engine, especially in order to improve the exhaust emission quality while maintaining high thermal efficiency comparable to a conventional engine. In-cylinder direct injection engines developed recently worldwide utilizes the stratified charge formation technique at low load, whereas at high load, a close-to-homogeneous charge is formed. Thus, compared to a conventional port injection engine, a significant improvement of fuel consumption and power can be achieved. To implement such a combustion strategy, the stratification of mixture charge is very important, and an understanding of its combustion process is also inevitably necessary. In this work, a numerical simulation was performed using a CFD code (KIVA-3), where the shape of a combustion chamber, swirl intensity, injection timing and duration, etc. were varied and their effects on the mixture formation and combustion process were investigated.

The oxidation mechanism of DTBP (Di-tertiary-butyl peroxide) and its role in butane oxidation have been investigated, as it pertains to the development of an LPG DI diesel engine. Ignition delay contours were analyzed to investigate the role of DTBP (ϕ≈0.2 to the total oxygen) in butane oxidation. At higher pressure and lower temperature regions, it was apparent that the addition of DTBP significantly enhances the ignition delay of butane, whereas at lower pressures and higher temperatures, this effect diminishes. Results of this study showed that the role of DTBP to enhance the ignition delay of the base fuel is through rapid heat release, rather than by radicals produced by decomposition during the base fuel ignition delay. Formaldehyde is a principal species involved in reactions for heat release in the higher pressure lower temperature region, comparable to diesel engine operating conditions.

To date, the DME combustion mechanism has been investigated by in-cylinder gas sampling, numerical calculations and observation of combustion radicals. It has been possible to quantify the emission intensities of in-cylinder combustion using a monochromator, and to observe the emitting species as images by using band-pass filters. However, the complete band images were not observed since the broadband (thermal) intensity may be stronger than band spectra intensities. Emission intensities of DME combustion radicals from a pre-mixed burner flame have been measured using a spectroscope and photomultiplier. Results were compared to other fuels, such as n-butane and methane, then, in this study, to better understand the combustion characteristics of DME, emission intensities near CH bands of an actual DI diesel engine fueled with DME were measured, and band spectra emitted from the engine were defined. Near TDC, emission intensities did not vary with wavelength.

Flame propagation characteristics, in a heavy-duty type LPG lean burn SI engine, were investigated by simulation methodology, using the global one step and the ten step chemical kinetic reaction mechanisms, respectively. The swirl ratio and equivalence ratio were varied to investigate their effects on flame front speed. The effect of increased swirl intensity on flame speed was very minor at ranges of equivalence ratio of this study. Flame front shape, however, was affected by swirl intensity. Circular flame front formed for a higher swirl ratio, which is in a qualitative accordance with that of measurements. Comparison between calculation and measurements of flame propagation characteristics shows a good agreement for both the global one step and the ten step chemical kinetic model. This work concludes that the reduced chemical kinetic reactions, consisting of ten steps, is useful for flame propagation study in an LPG SI engine.

For better understanding of the in-cylinder combustion characteristics of DME, combustion radicals of a direct injection DME-Fueled compression ignition engine were observed using a spectroscopic method. In this initial report, the emission intensity of OH, CH, CHO, C2 and NO radicals was measured using a photomultiplier. These radicals could be measured with wavelength resolution (half-width) of about 3.3 nm. OH and CHO radicals appeared first, and then CH radical emission was detected. After that, the combustion radicals were observed using a high-speed image intensified video camera. C2 and CH radicals were able to observe roughly as images. However, the emission intensity of DME combustion was not strong enough to take OH, CHO and NO radical images. CH radical combustion occurred near the chamber wall and burned like a ring, as combustion progress, indicating active heat release occurred near the chamber wall.

The spray characteristics of n-butane were analyzed numerically using KIVA-3V code and compared with those of diesel under the same boundary conditions. The transient behavior of hollow cone spray was calculated not only in a constant volume chamber under various ambient conditions, but also in a premixed compression ignition engine. The spray characteristics were evaluated in terms of spray tip penetration and droplet size distribution. Various atomization sub-models such as TAB, Wave breakup and Wave-KH (Kelvin-Helmholtz) model were implemented in the code and validated by comparison with experimental data. The results show that mixture formation for butane proceeds faster than diesel fuel primarily due to a higher evaporation rate caused by butane's higher diffusivity in air. Furthermore, in a premixed compression ignition engine, the mixture of butane becomes more homogeneous than diesel by the end of compression stroke.

Development of LPG SI and CI engines for heavy duty vehicles has been carried out. In order to measure the performance and emissions of an LPG lean burn SI engine, the piston cavity, swirl ratio, and propane-butane fuel ratio were varied and tested. Compared to the bathtub and dog dish cavities, the nebula type cavity showed the best performance in terms of cyclic variation and combustion duration. High swirl improved combustion by achieving a high thermal efficiency and low NOx emissions. A feasibility study of an LPG DI diesel engine also has been carried out to study the effectiveness of the selected cetane enhancing additives:Di-tertiary-butyl peroxide (DTBP). When more than 5 wt% DTBP was added to the base fuel, stable engine operation over a wide range of engine loads was possible. The thermal efficiency of LPG fueled operation was found to be comparable to diesel fuel operation at DTBP levels over 5 wt%.

In order to reduce environmental disruption due to exhaust PM and NOx emissions from diesel engines of dimethyl ether (DME) has been proposed the use for the next generation vehicles, because the discharge of the atmospheric pollutants is less. In this study, DME is used to fuel a retrofit type diesel engine, and operational tests were carried out using a rotary distributor fuel injection pump. In this experiment, comparison and examination of the effects of fuel injection pressure, nozzle hole diameter, and injection timing. When using DME as an alternative fuel, the fuel temperature affects engine operation. And diameter of the injector nozzle hole and larger injection quantity is regarded as factors affecting the improvement in engine performance. In addition, for understanding the DME spray in the cylinder, DME was sprayed in a constant volume chamber where atmospheric temperature and pressure increased simultaneously, and the result is compared and examined with diesel fuel.

In Japan there is currently a strong social demand for exhaust emissions reduction from heavy-duty diesel engines. Therefore, new Long-Term Regulation will come into effect in October 2005, setting the NOx standard at 2.0 g/kWh and the PM standard at 0.027 g/kWh. At the same time, customers always demand exceptional fuel economy from heavy-duty commercial vehicles. A urea-based Selective Catalytic Reduction (SCR) system was developed to satisfy both these demands, and will be introduced in the fall of 2004. The operating conditions of vehicles in Japan are different from those in the US and Europe. Basically, average vehicle speeds are significantly lower. To improve the low temperature SCR performance, an oxidation catalyst was located upstream of the SCR, and an additional oxidation catalyst was located downstream of the SCR for emergency NH3 slip. The muffler size with all three catalysts was similar to a conventional muffler.

A feasibility study of an LPG DI diesel engine has been carried out to study the effectiveness of two selected cetane enhancing additives: Di-tertiary-butyl peroxide (DTBP) and 2-Ethylhexyl nitrate (EHN). When more than either 5 wt% DTBP or 3.5 wt% 2EHN was added to the base fuel (100 % butane), stable engine operation over a wide range of engine loads was possible (BMEPs of 0.03 to 0.60 MPa). The thermal efficiency of LPG fueled operation was found to be comparable to diesel fuel operation at DTBP levels over 5 wt%. Exhaust emissions measurements showed that NOx and smoke levels can be significantly reduced using the LPG+DTBP fuel blend compared to a light diesel fuel at the same experimental conditions. Correlations were derived for the measured ignition delay, BMEP, and either DTBP concentration or cetane number. When propane was added to a butane base fuel, the ignition delay became longer.

The authors tried to use LP gas, mainly butane, as the main fuel of diesel engines to reduce soot and to maintain high thermal efficiency. LP gas was injected in the direction of the intake valve directly as a spray to prevent knocking and to preserve high charging efficiency. The newly developed electronic fuel injection provided accurate fuel control and injection timing. As a result, the dual-fuel operation produced high thermal efficiency almost identical to that of diesel engines. Soot in engine exhaust was almost negligible. Three quarters of maximum output was obtained with butane, and only small amount of gas oil for idling, in spite of an high compression ratio of 17 for gas engines. Increasing the proportion of gas oil resulted in maximum output from a diesel engine and almost no soot output.

Since dimethyl ether (DME) is a synthetic fuel, it is possible to make it from natural gas, coal and biomass. It is a low-emission, oxygenated fuel, which does not generate soot in the exhaust. Therefore, it has recently been identified as a possible replacement for diesel fuel. In Japan, the new short-term emissions regulations will be enforced beginning in 2003, and the long-term emissions regulations are scheduled to be enforced in 2005. In order to meet these more stringent emissions regulations, existing diesel engines would not be as widely used in the near future as they currently are. This will thus bring about a more widespread use of DME engines due to their low emissions potential. Moreover, when the modification of existing diesel engines into DME engines is available at a moderate cost, the wider use of DME engines can be expected. This study targeted development and application of DME engine technology for diesel engine retrofit, in a used diesel vehicle.