Abstract
Keywords - DI-Diesel Engine, New Combustion Concept, GTL fuel, SULEV, Low-temperature condition
Abstract - Experimental investigations were previously conducted with a direct-injection diesel engine with the aim of reducing exhaust emissions, especially nitrogen oxides (NOx) and particulate matter (PM). As a result of that work, a new combustion concept, called Modulated Kinetics (MK) combustion, was developed that reduces NOx and smoke simultaneously through low-temperature combustion and premixed combustion to achieve a cleaner diesel engine. The MK concept was then combined with an exhaust after- treatment system and applied to a test vehicle. The results indicated the attainment of ULEV emission levels, albeit in laboratory evaluations.
Subsequent in-vehicle evaluations have made it clear that reducing cold-start emissions is an issue that must be addressed to attain a further improvement in the emission performance of a diesel engine that combines the MK combustion concept with an after-treatment system. MK combustion with a low compression ratio tends to deteriorate under a cold-start condition on account of the increased ignition delay due to the cool combustion chamber and cylinder walls. Moreover, since the catalytic converter does not exhibit sufficient conversion performance until it lights off, engine-out exhaust gas is discharged into the atmosphere without reacting with the catalyst. Therefore, an attempt was made to apply lambda control for hastening catalytic converter light-off by raising the exhaust gas temperature with a lower lambda under a cold-start condition. However, there is a trade-off between a higher exhaust gas temperature and a noticeable increase in the hydrocarbon (HC) level.
In the present work, an investigation was made of the use of a high cetane number gas-to-liquid
(GTL) fuel combined with a lower compression ratio, which can provide the optimum ignition delay as the MK combustion. The effect of this combination on reducing cold-start exhaust emissions was confirmed. The results indicated that the use of GTL fuel can reduce the HC level substantially under a low-temperature condition because of its good ignitability due to high levels of paraffin compounds and a high cetane number. In addition, the applicable range of lambda control for quickening catalytic converter light-off can be expanded because the trade-off between exhaust emissions and the exhaust gas temperature is improved. As a result, the combination of MK combustion and GTL fuel has the possibility of attaining SULEV emission levels.