A combined experimental and modeling study has been conducted to investigate the sources of CO and HC emissions (and the associated combustion inefficiencies) at low-loads. Engine performance and emissions were evaluated as fueling was reduced from knocking conditions to very low loads (ϕ = 0.28 - 0.04) for a variety of operating conditions, including: various intake temperatures, engine speeds, compression ratios, and a comparison of fully premixed and GDI (gasoline-type direct injection) fueling. The experiments were conducted in a single-cylinder engine (0.98 liters) using iso-octane as the fuel. Comparative computations were made using a single-zone model with the full chemistry mechanisms for iso-octane, to determine the expected behavior of the bulk-gases for the limiting case of no heat transfer, crevices, or charge inhomogeneities.
Experimental results show that as fueling is reduced to equivalence ratios (ϕ) below 0.20, CO emissions begin to increase substantially, reaching levels corresponding to more than 60% of all fuel carbon at idle loads (ϕ = 0.1 - 0.12). As this occurs, combustion efficiency falls from 94% to less than 55%. These high CO levels are in very good agreement with those predicted by the model, indicating that the high CO emissions and the associated combustion inefficiencies are due to incomplete bulk-gas reactions. HC emissions also rise, but the increase does not become pronounced until ϕ < 0.14. In addition, the model indicates that significant emissions of oxygenated hydrocarbons (e.g., formaldehyde) should occur as bulk-gas reactions become less complete. This prediction is supported by the experimental exhaust carbon balance. Intake temperature significantly affects the onset of incomplete bulk-gas combustion; however, engine speed and compression ratio have only small effects for the fuel studied here. Fuel stratification by late GDI injection was investigated and found to have good potential for improving combustion efficiency at low loads.