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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.

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.

Combustion phasing is one important issue that must be addressed for HCCI operation. The intake temperature can be adjusted to achieve ignition at the desired crank angle. However, heat-transfer during induction will make the effective intake temperature different from the temperature measured in the runner. Also, depending on the engine speed and port configuration, dynamic flow effects cause various degrees of charge heating. Additionally, residuals from the previous cycle can have significant influence on the charge temperature at the beginning of the compression stroke. Finally, direct injection of fuel will influence the charge temperature since heat is needed for vaporization. This study investigates these effects in a systematic manner with a combination of experiment and cycle simulation using WAVE from Ricardo.