• Variable valve actuation
• Variable exhaust gas percentage
• Variable fuel ignition quality
6.2 Variable Compression ratio
There are several methods for modulating both the geometric and effective compression ratio. The geometric compression ratio can be changed with a movable plunger at the top of the cylinder head. The effective compression ratio can be reduced from the geometric ratio by closing the intake valve either very late or very early with some form of variable valve actuation. Both of the approaches mentioned above require some amounts of energy to achieve fast responses. Additionally, implementation is expensive. Control of an HCCI engine using variable compression ratio strategies has been shown effective.
6.3 Variable…show more content… Ford motor company has an active research program in HCCI combustion. Researchers are using optical diagnostics in single-cylinder engines to explore viable HCCI operating regimes and to investigate methods of combustion control. In addition, chemical kinetic and cycle simulation models are being applied to better understand the fundamentals of the HCCI process and to explore methods of implementing HCCI technology.GM, at a research level, is evaluating the potential for incorporating HCCI combustion into engine systems. This work includes assessing the strengths and weaknesses of HCCI operation relative to other advanced concepts, assessing how best to integrate HCCI combustion into a viable power train, and the development of appropriate modeling tools. Work is focused on fuels, combustion control, combustion modeling, and mode transitioning between HCCI and traditional SI or CI combustion. GM is also supporting HCCI work at the university level. Cummins has been researching HCCI for almost 15 years. Industrial engines run in-house using HCCI combustion of natural gas has achieved remarkable emission and efficiency…show more content… Maintaining optimal ignition timing is more challenging for HCCI engines than for conventional engines because no positive mechanism, such as spark or fuel-injection, determines ignition timing.
In HCCI engines, ignition timing is determined by the chemical kinetic reaction rates of the mixture, which are controlled by time, temperature, and mixture composition. Of these parameters, ignition timing is most sensitive to temperature. As engine speed and load (time and mixture) are varied, the ignition timing will also vary, unless the charge temperature is adjusted to compensate. The amount of compensation required is a strong function of fuel type, with one-stage-ignition fuels (e.g. gasoline) requiring much less compensation for changes in speed and load than two-stage-ignition fuels (e.g. diesel).
Perhaps the most straightforward way to control charge temperature in an HCCI engine is to add a variable amount of hot EGR to the intake; however, the response is slow, and transients are not handled well. Alternatively, varying the temperature by mechanical variation of