Search

A variable valve timing mechanism has been applied in a high-speed direct injection diesel engine. The effective compression ratio ( εeff) is lowered by means of late intake valve closing, while keeping the expansion ratio constant. Premixed charge compression ignition (PCCI) combustion, adopting the Miller cycle, was experimentally realized and numerically analysed. Significant improvements in NO x (nitrogen oxides) and soot emissions were achieved for a wide range of engine speeds and loads frequently used in a transient mode test. The operating range of the Miller-PCCI combustion has been expanded up to an indicated mean effective pressure of 1.3 MPa.

A variable valve timing (VVT) mechanism is 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 centre are lowered, thereby preventing too early ignition and increasing ignition delay to enhance fuel-air mixing. The variability of an 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, exhaust gas recirculation (EGR), supercharging systems, and high-pressure fuel injection equipment can simultaneously reduce NO x and smoke. The NO x and smoke suppression mechanism in the premixed diesel combustion is analysed using a three-dimensional computational fluid dynamics (3D-CFD) code combined with detailed chemistry. LIVC can achieve a significant NO x and smoke reduction due to lowering combustion temperatures and avoiding local overrich regions in the mixtures respectively.

NOx conversion performance of a urea-selective catalytic reduction (SCR) system comprising V2O5/TiO2 catalyst under steady state operating conditions of an 8-litre, common-rail turbo direct injection (TDI) diesel engine was investigated. It was shown that the urea-SCR system achieves 70–90 per cent NOx conversion under medium and high load conditions at 1440 r/min and that NOx conversion is low under low load conditions because of the low catalyst temperatures and the NO/NO2 ratio being higher than unity. It was also shown that NOx conversion exceeds 90 per cent when the catalyst temperature is higher than 530 K. To investigate the details of the chemistry and thermofluid dynamics within the urea-SCR system, a computational fluid dynamics (CFD) code that incorporates detailed surface chemistry was developed based on the modified subroutines of CHEMKIN-II. The spatial variations of chemical species including NO and NH3 in a thin catalyst channel was calculated using the model. The calculated result of NO conversion showed relatively good agreement with experimental results.

Dimethyl ether (DME) has been recognized as an alternative fuel for better combustion and less pollutant formation than diesel, even though they have a cetane number. In this study, characteristics of DME spray were observed experimentally under various ambient conditions in a constant-volume chamber with a common-rail injection system. To understand the combustion characteristics of DME, radicals from the flame and exhaust gases were also investigated. Radicals from DME combustion were measured by a high-speed camera with stereoscope. Flame spectra were taken near 310 and 400-530 nm emitting from OH and CH or C2 radicals respectively. Flames of diesel were found to be much stronger than those of DME because of the smoke emitted from CH and C22 radicals. Moreover, NOx emission is substantially decreased without increasing in the smoke emission with a high exhaust gas recirculation (EGR) rate in DME combustion. It was confirmed from these results that DME has the potential for a low emission fuel alternative to diesel.

This paper investigates the effects of combustion system and fuel properties on spray, mixture formation, combustion process, and emissions, using a constant volume chamber and single- cylinder engine. In a combustion system, high-pressure injection, exhaust gas recirculation (EGR), and impingement spray are applied while visualizing and analysing exhaust gas. Also, light diesel fuel, which has approximately the same cetane value as the diesel but has low values of PAHs, boiling point, density, viscosity, and surface tension, were used. Spray and combustion images were taken using a high-speed video camera and analysed by their penetration and evaporation characteristics in comparison with current diesel fuel. The results showed that the mixture formation of the high- pressure injection, impingement spray, and light diesel fuel is faster than that of diesel fuel, depending on physical properties. In the engine test, a considerable reduction of smoke emission was observed using the high-pressure injection with supercharging, due to the enhanced fuel atomization and air entrainment. In addition, a high reduction of NOx emission can be achieved with a high EGR rate, and with supercharging can achieve a reduction of NOx emission without deteriorating BSFC.

It has been recognized that alternative fuels, such as liquid petroleum gas (LPG), have less polluting combustion characteristics than diesel fuel. Furthermore, when LPG direct-injection, stratified-charge combustion can be adopted in the spark-ignition engine in place of the conventional premixed spark-ignition combustion, it will be possible to achieve less polluting and more efficient combustion characteristics. However, the physical properties of LPG, such as boiling point (—40-0°C) and viscosity, differ largely in comparison with current fuels. Therefore, there are many unknown phenomena in the behaviour of spray, mixture formation, and combustion of LPG in the cylinder. These issues make the development of a high-efficiency LPG engine difficult. Thus, in this study, a numerical method has been adjusted to predict the mixture formation and combustion of LPG. KIVA-3 code was used for this purpose, and tuning factors were adjusted by comparing them with the measurements. In the combustion model, the eddy-dissipation combustion model proposed by Magnussen and Hjertager [1] is adopted. The predicted results show favourable agreement with the measurements and the possibility of useful methods to improve engine performance.

The homogeneous charge compression ignition (HCCI) combustion has been attracting growing attention in recent years due to its potential for simultaneous improvement of exhaust gas emissions and fuel consumption in diesel engines. For practical application of HCCI to internal combustion (IC) engines, precise control of auto-ignition of pre-mixtures during the compression stroke is inevitable. This paper discusses the auto-ignition processes in an HCCI engine operated with n-heptane/air mixtures using a zero-dimensional combustion model including a detailed kinetics. The model proposed is validated first by a comparison between calculated and experimental pressure diagrams, and then the effects of initial charge conditions, compression ratio and excess air ratio on ignition and combustion are investigated. It was found from the parametric study that HCCI combustion of n-heptane/air mixtures is classified into three types of combustion: complete combustion, only low-temperature reaction and misfire, depending on the compression ratio and excess air ratio at which the engine is operated. Finally, the major paths of the HCCI reaction occurring in the engine cylinder were clarified by a sensitivity analysis of chemical reactions involved in the HCCI reaction scheme.