Exploring the Potential of Dual Fuel Diesel-CNG Combustion for Passenger Car Engine

摘要

The concept of dual fuel Diesel-CNG is well known in heavy duty applications as the high octane number of methane allows converting easily existing CI engines without varying compression ratio, thus reducing adaptation costs. However, with this approach, Diesel fuel substitution ratio is quite limited and the benefits on CO_2 savings and on thermal efficiency are not exploited. The objective of this paper is to highlight experimentally the potential of a different approach for dual fuel Diesel/CNG combustion applied to smaller passenger car engines. The objective of this concept is to maximize CO_2 savings compared to traditional Diesel fuel operation by using the optimal amount of Diesel fuel and optimizing engine efficiency. The study was based on experiments carried out on a CI single cylinder engine modified for allowing dual fuel operation with methane port fuel injection. A first part of the study investigates the behaviour of dual fuel combustion process at different equivalence ratios. Then a second part described how the combustion mode needed to be adapted to the engine load: for example, lean dual fuel with EGR mode and stoichiometric dual fuel mode were addressed depending on the engine operating point. Special attention was given at the results at stoichiometric full load. Indeed, in these conditions, the dual fuel combustion is optimal compared to conventional Diesel operation: less noise, no smoke, faster end of combustion. This study demonstrated that optimizing an engine for dual fuel Diesel-CNG combustion is a challenging task. The control of Diesel fuel autoignition delay is crucial to enhance thermal engine efficiency at low load. THC emissions need to be drastically reduced to comply with stringent emissions standards. Further studies need to be carried out to analyze in more details the combustion process in dual fuel mode.