The light-medium load operating range (4-7 bar net IMEP)
presents many challenges for advanced low temperature combustion
strategies utilizing low cetane fuels (specifically, 87-octane
gasoline) in light-duty, high-speed engines. The overly lean
overall air-fuel ratio (Φ≺0.4) sometimes requires unrealistically
high inlet temperatures and/or high inlet boost conditions to
initiate autoignition at engine speeds in excess of 1500 RPM. The
objective of this work is to identify and quantify the effects of
variation in input parameters on overall engine operation. Input
parameters including inlet temperature, inlet pressure, injection
timing/duration, injection pressure, and engine speed were varied
in a ~0.5L single-cylinder engine based on a production General
Motors 1.9L 4-cylinder high-speed diesel engine.
With constraints of combustion efficiency, noise level (pressure
rise rate) and emissions, engine operation sensitivity due to
changes in inlet temperature between 50-90C was first examined for
fixed fueling rates. This experiment was then repeated at different
inlet pressures and engine speeds. Finally, constant load
experiments were performed in which perturbations in injection
strategies (timing, duration, and pressure) were executed to assess
overall system sensitivity. These experiments revealed primary and
secondary effects with respect to changes in engine operation. In
addition, an assessment of combustion robustness was made as
well.
Based on the results, we conclude that input parameters can be
effectively manipulated to maintain low NOx emissions ≺0.6
g/kg-fuel with good combustion stability (COV of IMEP ≺3%) over a
wide inlet temperature range. Further optimization (with respect to
combustion efficiency and CO/UHC emissions) was realized with
additional adjustment of these input parameters. Interestingly,
gross ISFC remained relatively unaffected by changes in input
parameters (185-190 g/kWh). This last observation leads to the
assessment that GCI combustion can provide robust,
high-fuel-efficiency, low-emissions light-medium load operation in
a light-duty engine application.