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Fundamental engine research is primarily conducted under steady-state conditions, in order to better describe boundary conditions which influence the studied phenomena. However, light-duty automobiles are operated, and tested, under heavily transient conditions. This mismatch between studied conditions and in-use conditions is deemed acceptable due to the fundamental knowledge gained from steady-state experiments. Nonetheless, it is useful to characterize the conditions encountered during transient operation and determine if the governing phenomena are unduly influenced by the differences between steady-state and transient operation, and further, whether transient behavior can be reasonably extrapolated from steady-state behavior. The transient operation mode used in this study consists of 20 fired cycles followed by 80 motored cycles, operating on a continuous basis.

Ethanol and ethanol/gasoline blends are being widely considered as alternative fuels for light-duty automotive applications. At the same time, HCCI combustion has the potential to provide high efficiency and ultra-low exhaust emissions. However, the application of HCCI is typically limited to low and moderate loads because of unacceptably high heat-release rates (HRR) at higher fueling rates. This work investigates the potential of lowering the HCCI HRR at high loads by using partial fuel stratification to increase the in-cylinder thermal stratification. This strategy is based on ethanol's high heat of vaporization combined with its true single-stage ignition characteristics. Using partial fuel stratification, the strong fuel-vaporization cooling produces thermal stratification due to variations in the amount of fuel vaporization in different parts of the combustion chamber.

This work explores the high-load limits of HCCI for naturally aspirated operation. This is done for three fuels with various autoignition reactivity: iso-octane, PRF80, and PRF60. The experiments were conducted in a single-cylinder HCCI research engine (0.98 liter displacement), mostly with a CR = 14 piston installed, but with some tests at CR = 18. Five load-limiting factors were identified: 1) NOx-induced combustion-phasing run-away, 2) wall-heating-induced run-away, 3) EGR-induced oxygen deprivation, 4) wandering unsteady combustion, and 5) excessive exhaust NOx. These experiments at 1200 rpm show that the actual load-limiting factor is dependent on the autoignition reactivity of the fuel, the selected CA50, and in some cases, the tolerable level of NOx emissions. For iso-octane, which has the highest resistance to autoignition of the fuels tested, the NOx emissions become unacceptable at IMEPg = 473 kPa.

Multi-zone CFD simulations with detailed kinetics were used to model iso-octane HCCI experiments performed on a single-cylinder research engine. The modeling goals were to validate the method (multi-zone combustion modeling) and the reaction mechanism (LLNL 857 species iso-octane) by comparing model results to detailed exhaust speciation data, which was obtained with gas chromatography. The model is compared to experiments run at 1200 RPM and 1.35 bar boost pressure over an equivalence ratio range from 0.08 to 0.28. Fuel was introduced far upstream to ensure fuel and air homogeneity prior to entering the 13.8:1 compression ratio, shallow-bowl combustion chamber of this 4-stroke engine. The CFD grid incorporated a very detailed representation of the crevices, including the top-land ring crevice and head-gasket crevice. The ring crevice is resolved all the way into the ring pocket volume. The detailed grid was required to capture regions where emission species are formed and retained.

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.

Chemiluminescence imaging has been applied to investigate the naturally occurring charge stratification in an HCCI engine. This stratification slows the pressure-rise rate (PRR) during combustion, making it critical to the high-load operating limit of these engines. Experiments were conducted in a single-cylinder HCCI engine modified with windows in the combustion chamber for optical access. Using this engine, chemiluminescence images were obtained from three different view angles. These included both single-shot images with intensified CCD cameras and high-speed (20kHz) sequences with an intensified CMOS video camera. The engine was fueled with iso-octane, which has been shown to be a reasonable surrogate for gasoline and exhibits only single-stage ignition at these naturally aspirated conditions. The chemiluminescence images show that the HCCI combustion is not homogeneous but has a strong turbulent structure even when the fuel and air are fully premixed prior to intake.

We have conducted a detailed numerical analysis of HCCI engine operation at low loads to investigate the sources of HC and CO emissions and the associated combustion inefficiencies. Engine performance and emissions are evaluated as fueling is reduced from typical HCCI conditions, with an equivalence ratio ϕ = 0.26 to very low loads (ϕ = 0.04). Calculations are conducted using a segregated multi-zone methodology and a detailed chemical kinetic mechanism for iso-octane with 859 chemical species. The computational results agree very well with recent experimental results. Pressure traces, heat release rates, burn duration, combustion efficiency and emissions of hydrocarbon, oxygenated hydrocarbon, and carbon monoxide are generally well predicted for the whole range of equivalence ratios. The computational model also shows where the pollutants originate within the combustion chamber, thereby explaining the changes in the HC and CO emissions as a function of equivalence ratio.

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.

A diesel fuel injector has been modified to allow rotation around its axis, driven by an electric motor. This enables sweeping injections in a DI Diesel combustion system. It has earlier been shown that sweeping injection enhances the air entrainment into the spray. This is one reason for the reduced smoke level by counter-swirl rotation of the injector. The injected amount of fuel is small and this enables exploration of spray / wall interaction and the effects of reverse-squish. Flame visualization shows that normal, non-sweeping injection tends to build up fuel pockets where the sprays hit the piston bowl wall. This fuel burns quite slowly since it only to a limited extent benefits from the mixing effects of the reverse-squish flow. Increasing the air swirl ratio from 1.65 to 2.47 does not reduce the impact onto the piston bowl wall much. The decrease in smoke level with increasing swirl was attributed to enhanced mixing of the fuel that had accumulated under the piston bowl rim.

The injector in a DI diesel engine has been modified to allow rotation. The injector speed was varied within ± 4,000 rpm in the current study over 13 testpoints. Air swirl levels tested are 1.5 and 2.8. Rotating the injector adds a free parameter to the combustion system and enables lowest possible smoke emission from each given loadpoint. Smoke reduction up to 74% has been encountered. The mean reduction over all 10 testpoints with a swirl ratio of 1.5 is 55%. Increasing the air-swirl level decreases the smoke level with static injector. The further smoke reduction with counter-swirl rotation is significant albeit not as large as for the lower swirl case. A relationship between the injector speed and effective swirl ratio at TDC is strongly supported by the results and maximum spray stagnation onto the piston bowl wall explains the maximum smoke level when rotating co-swirl. Both counter-swirl rotation and increased air-swirl decreases the ignition delay.