Gasoline direct injection is widely used for spark ignited automotive engines, how ever, small off-road spark ignited engines, which are used in the industrial, residential and transportation sectors for lawn and garden maintenance as well as for recreational purposes, still operate with carbureted or port fuel injection strategies. The overall objective of this investigation was to assess the potential benefits of gasoline direct injection combustion and compare its performance against port fuel injected over the 5-mode small off-road engine test protocol imposed by the Environmental Protection Agency (EPA) and the California Air Resources Board.
A Kohler water-cooled 4-stroke 747cc small-off road engine adapted for gasoline direct injection coupled to an emissions bench were used for this investigation. Comparison between PFI and GDI were made based on optima obtained using a design of experiments approach. Tests were performed at the stock (8.7) and, a higher compression ratio (10.2).
Comparisons were first made between optimal cases for each of the five different engine modes by constraining ISHC+ISNOx emissions to 8 [g/kW-hr] and achieving the lowest possible fuel consumption. Because emissions agencies apply a weighting factor criteria for each of the five operational modes, a global optimization was also conducted for each injection strategy. This global optimization was modeled in MATLAB using a minimization function. The PFI aggregated global optima with a 10.2 compression ratio showed a weighted total fuel mass flow of 0.334±0.002 [g/s] and an ISFC of 259.7 ± 3.1 [g/kW-hr]. This represented an improvement of 7.44% in ISFC when compared to PFI global optima with the stock compression ratio. For GDI, the aggregated global optima with a higher compression ratio yielded a fuel mass flow of 0.330±0.001 [g/s] and a ISFC of 258.7±2.7 [g/kW-hr]. This represented an improvement of 5.89% in ISFC when compared to GDI global optima with the stock piston. Overall, the GDI global optima with a higher compression ratio showed the lowest fuel mass flow (1.19% less than PFI with high compression ratio).