Lobo, J., Lee, J., Oswald, E., Lionetti, S. et al., "The Effect of Lean Operation, Ignition Advance, and Compression Ratio on the Performance and Emissions of a Propane Fueled Electronic Fuel Injected Engine," SAE Technical Paper 2016-32-0068, 2016, https://doi.org/10.4271/2016-32-0068.
The performance and exhaust emissions of a commercially available, propane fueled, air cooled engine with Electronic Fuel Injection (EFI) were investigated by varying relative Air to Fuel Ratio (λ), ignition timing, and Compression Ratio (CR). Varying λ and ignition timing was accomplished by modifying the EFI system using TechniCAL Industries’ engine development software. The CR was varied through using pistons with different bowl sizes. Strong relationships were recorded between λ and ignition timing and the resulting effect these parameters have on engine performance and emissions. Lean operation (λ > 1) has the potential to significantly reduce NOx production (110 g/kW-hr down to 5 g/kW-hr). Unfortunately, it also reduces engine torque by up to an order of magnitude (31 Nm down to 3 Nm). Moving ignition initiation to earlier in the compression stroke, 10o to 40o Before Top Dead Center (BTDC), improved engine performance considerably (25% improvement in brake torque) in the presence of excess air. Unfortunately, advancing the ignition also caused NOx production to increase. The effects these parameters have on engine performance are significant enough that the same engine can be used for vastly different applications with changes only to the control software. Compression ratio has a less significant effect on engine performance, but increasing CR does result in an increase engine torque. Increasing CR from 9.1:1 to 11:1 resulted in an increase in engine torque of approximately 10% for the operating parameters tested.