ABSTRACT
Care is needed when evaluating the suitability of different fuels for dual-fuel engine applications. Different conclusions may be arrived at depending on the basis on which the evaluation is being made. For example, a comparison of fuels based on their heating values on a mass basis may produce a different ranking from one made similarly, but on a volume basis or on the basis of the energy release per unit of volume of fuel-air mixture. Similar yet different rankings may be obtained if the comparison were to be extended to include considering the energy expenditure in the fuel production and its dispensing. Also, in any fuel application, the use of the available energy should be optimized whether with respect to the direct chemical energy of the fuel or that arising also from its state, such as the compression work of compressed natural gas (CNG) or the cold in liquefied natural gas (LNG) applications. Moreover, in transport applications, the energy expenditure due to the increased mass of the vehicle arising from the additional weight of the fuel tanks needs to be considered. Similarly, the introduction into the engine of gaseous fuels such as methane or hydrogen displaces a significant portion of the intake air. Also, the enhancement of the volumetric efficiency in liquid fuel applications due to the evaporative cooling of the fuel will be missing whenever gaseous fuels are employed. These would lead to a reduction in the total energy that can be released in comparison to liquid fuels unless special additional remedial measures are taken to compensate for this reduction. In any case, the conversion of engines to operate on alternative fuels often tended to lack the degree of refinement and optimization of conventional diesel engines, which have benefited greatly from the continued support of much research and development over the years. Additionally, government policies of support and taxation of the use of the different fuels, together with some remaining technical challenges, have controlled and greatly distorted on occasion the merits of a wider employment of an alternative fuel. On this basis, the evaluation of the relative benefits of the conversion of a diesel engine to operate on an alternative fuel can be made largely on a tentative basis. The benefits and associated costs will vary widely with the types of fuel and engine, field of application, fuel availability, and infrastructure required, and whether significant design changes to the engine are needed or not.
