ABSTRACT
Mainly for economic and environmental reasons, the size of the diesel fuel pilot needs to be minimized relative to that of the cheaper fuel gas. This is to be done while maintaining optimum combustion and engine performance characteristics. When the regular diesel engine injection system is retained and employed for pilot delivery in dual-fuel operation, the desirable size of the pilot may be too small and outside the normal operating ranges of the injector and fuel pump systems. The pilot quantity often may be much below the amount of fuel needed to keep the diesel engine idling at the same speed, which leads to off-design operation and poor injection system performance. The employment of such small quantities can contribute to increased cyclic variations accentuated by variations in the mass of fuel injected per cycle, with the need to improve heat transfer, avoid overheating of the injector tip, and guard against occasional backfire. Reducing the relative size of the pilot will also be associated with conditions where the injection of the fuel may be completed well within the ignition delay period. With further increases in the pilot fuel size, longer injection times will be involved, which then tend to produce only secondary performance effects. With the employment of relatively large pilots (e.g., greater than 30% of normal diesel operation), as shown, for example, in Figure 11.1 for a dual-fueled engine operating on propane, engine performance with respect to power and specific energy consumption can become superior even to those corresponding values of normal high load diesel performance.
