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The objective of this study is to improve the ignition quality of LPG (liquefied petroleum gas) in order to utilize LPG as a diesel fuel. First, the relationship between the cetane numbers and ignition delay periods of primary standard fuels (mixtures of n-cetane and heptamethylnonane) and diesel fuels were investigated by measuring the ignition delay periods using a constant volume combustion chamber. As a result, it was found that a good relationship between the cetane numbers and ignition delay periods could be obtained for a 550°C combustion chamber temperature and 4MPa pressure. Also, the cetane number estimation equation was established using the ignition delay data of n-paraffins. Next, the constant volume combustion chamber was modified to evaluate the ignition delay period of LPG with a cetane number improver, and these cetane numbers were then estimated.

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.