Small Off-road Engines represent a large population of engines that produce a significant amount of the nation’s gross pollutant emissions. Because of their low-cost, technology advancements for these engines tends to be limited. However, EPA emissions regulations have become more strict forcing manufacturers to improve their engines. Gasoline Direct Injection (GDI) provides a potential solution to improve fuel economy while meeting emissions requirements.
To assess the potential for GDI in small engines, calibrations of GDI and Port-Fuel Injection (PFI) were compared at three speed/load operating conditions in a four-stroke utility engine. Spark timing and equivalence ratio were individually optimized for each fueling system. To identify the lowest possible cost, the low injection pressure limit was determined at each operating condition. At low and part loads, a split-injection strategy was investigated.
For low and part loads, the lowest acceptable injection pressure for GDI was 10 bar. Comparing optimized results showed no fuel consumption advantage for GDI at these conditions. Split-injection testing showed no improvements over single-injection GDI at the low injection pressure.
At high load, knock-limited spark timings were identified for PFI and GDI to provide a boundary for the calibrations. Because of the charge-cooling effect, GDI showed more advanced knock-limited spark timings. The lower injection pressure limit was 15 bar, which came from a limitation in the functionality in the engine controls software. At an injection pressure of 15 bar, the calibrations showed no performance advantage for GDI. At an injection pressure of 60 bar, there was a 4% reduction in fuel consumption and a 4% increase in IMEP for GDI relative to PFI.
A high compression ratio piston, which increased the compression ratio from 8.7 to 10.2, was also tested. The low injection pressure limit, which was found to be 15 bar ifor high load. At low load, GDI gave no fuel consumption advantage. For high load at 15 bar, there was no performance advantage for GDI, but at 60 bar, there was a 4% reduction in fuel consumption and a 5% increase in IMEP for GDI relative to PFI.