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This study compares the role of the in-cylinder flow field for spray-guided stratified-charge combustion and for traditional well-mixed stoichiometric operation, both using E70 fuel. The in-cylinder flow field is altered by changing the engine speed between 1000 and 2000 rpm. The stratified operation with the ethanol blend enabled “head ignition” of the fuel sprays, thus minimizing the available fuel/air-mixing time prior to combustion, creating a highly stratified combustion event. For well-mixed stoichiometric operation, the heat-release rate (HRR) scales proportionally with engine speed due to increased in-cylinder turbulence, as is well-known from literature. In contrast, increasing the engine speed influences the stratified combustion process very differently. Ensemble-averaged over 500 cycles, the time-based HRR in kW remains comparatively unchanged as the engine speed increases. However, cyclic variability of the stratified combustion increases substantially with engine speed.

The lean-burn stratified-charge DISI engine has a strong potential for increased thermal efficiency compared to the traditional throttled SI engine. This experimental study of a spray-guided stratified-charge combustion system compares the engine response to injection-timing retard for gasoline and E85. Focus is on engine-out NO and soot, and combustion stability. The results show that for either fuel, injection-timing retard lowers the engine-out NO emissions. This is partly attributed to a combination of lower peak-combustion temperatures and shorter residence time at high temperatures, largely caused by a more retarded combustion phasing. However, for the current conditions using a single-injection strategy, the potential of NO reduction with gasoline is limited by both elevated soot emissions and the occurrence of misfire cycles. In strong contrast, when E85 fuel is used, the combustion system responds very well to injection-timing retard.

Fuel stratification has been investigated as a means of improving the low-load combustion efficiency in an HCCI engine. Several stratification techniques were examined: different GDI injectors, increased swirl, and changes in injection pressure, to determine which parameters are effective for improving the combustion efficiency while maintaining NOx emissions below U.S. 2010 limits. Performance and emission measurements were obtained in an all-metal engine. Corresponding fuel distribution measurements were made with fuel PLIF imaging in a matching optically accessible engine. The fuel used was iso-octane, which is a good surrogate for gasoline. For an idle fueling rate (ϕ = 0.12), combustion efficiency was improved substantially, from 64% to 89% at the NOx limit, using delayed fuel injection with a hollow-cone injector at an injection pressure of 120 bar.

HCCI-combustion with direct injection of gasoline using a standard GDI-injector is investigated in this work. The test engine is a 6-cylinder heavy-duty diesel engine with one cylinder operating in HCCI-mode. Exhaust gases from one of the diesel cylinders serve as simulated EGR. Electric heaters are used to raise the inlet temperature when no EGR is applied. The piston bowl is modified to match the hollow-cone spray better than the original re-entrant piston. Spray imaging outside the engine shows the characteristics of the fuel spray. Injection timing sweeps show that a homogeneous charge is created when the injection is performed in the middle of the intake stroke for a moderate fuel/air-equivalence ratio of 0.29. This leads to low emissions of NOx and Smoke. Using a homogeneous mixture when the fuel/air-equivalence ratio is reduced to 0.20 leads to low combustion efficiency with associated high levels of CO and HC emissions.

Large-eddy Simulations (LES) have been carried out to investigate spray variability and its effect on cycle-to-cycle flow variability in a direct-injection spark-ignition (DISI) engine under non-reacting conditions. Initial simulations were performed of an injector in a constant volume spray chamber to validate the simulation spray set-up. Comparisons showed good agreement in global spray measures such as the penetration. Local mixing data and shot-to-shot variability were also compared using Rayleigh-scattering images and probability contours. The simulations were found to reasonably match the local mixing data and shot-to-shot variability using a random-seed perturbation methodology. After validation, the same spray set-up with only minor changes was used to simulate the same injector in an optically accessible DISI engine. Particle Image Velocimetry (PIV) measurements were used to quantify the flow velocity in a horizontal plane intersecting the spark plug gap.

Nitric Oxide (NO) can significantly influence the autoignition reactivity and this can affect knock limits in conventional stoichiometric SI engines. Previous studies also revealed that the role of NO changes with fuel type. Fuels with high RON (Research Octane Number) and high Octane Sensitivity (S = RON - MON (Motor Octane Number)) exhibited monotonically retarding knock-limited combustion phasing (KL-CA50) with increasing NO. In contrast, for a high-RON, low-S fuel, the addition of NO initially resulted in a strongly retarded KL-CA50 but beyond the certain amount of NO, KL-CA50 advanced again. The current study focuses on same high-RON, low-S Alkylate fuel to better understand the mechanisms responsible for the reversal in the effect of NO on KL-CA50 beyond a certain amount of NO.