Combustion diagnostics of a dual fuel CI engine : an experimental and theoretical study
The term 'dual fuel' refers to a compression ignition engine where a small quantity of diesel fuel called the pilot is used to ignite a second gaseous fuel which is the primary energy source. The motivation to use dual fuel has traditionally been economic, as the primary fuel is often less expensive than the distillate fuel it replaces. However, some benefits in terms of the reduced emissions of smoke and oxides of nitrogen (NOx) can also be achieved. In this research, a small, direct injection diesel engine was converted to dual fuel operation. This engine is typical of those used in stationary power generation applications. A review of literature revealed that whilst performance and emissions trends were well established for indirect injection engines, little research had been conducted on a direct injection engine. In particular, this class of small, high speed industrial engine had been somewhat neglected, partly because they have been subject to less stringent emissions legislation than their automotive counterparts. By performing a detailed investigation ' into the errors and assumptions that have a bearing on the three zone technique, it was possible to challenge some previous assumptions regarding the dual fuel combustion process. Namely, the theory that the pilot bums in two separate initial stages was found to be a deficiency of previous analysis techniques and therefore incorrect. It was found that as the proportion of the gaseous fuel was increased, the combustion process retained similar characteristics and magnitudes of mass burned to diesel until all but the highest equivalence ratios. At this point, the premixed and diffusion burning periods merged, but continued to show a fundamental dependence on the pilot ignition and the combustion processes were never independent of the pilot. The range of equivalence ratios over which the transition between the two patterns occurs is firstly a function of the primary fuel, and secondly a function of the operating conditions (such as in cylinder temperature). It is proposed that the dual fuel combustion process is better described as a diesel combustion process with a modified diffusion burning period that results from the gaseous fuel concentration and type. By using this explanation, it was identified that the emissions characteristics of the engine could be modified through the use of a second fuel. The primary fuel can reduce the initial mass burning rates (to reduce NOx) and simultaneously elevated the diffusion burning rates (to reduce smoke emissions). This provides an alternative, beneficial means by which the classic diesel NOx-Particulate trade-off can be manipulated. Butane was found to be unsuitable for this type of engine, and propane consistently yielded the best performance and emissions trends. Additionally, it was found that the addition of small quantities of methane or propane can result in disproportionately large reductions in smoke and NOx without the penalty of increased carbon monoxide and unburned hydrocarbons.