Gasoline engines combine homogeneous charge (HC) with spark ignition (SI), abbreviated as HCSI.
Modern direct injection diesel engines combine stratified charge (SC) with compression ignition (CI), abbreviated as SCCI.
HCCI engines achieve extremely low levels of oxides of nitrogen emissions (NOx) without a catalytic converter.
Recent research has shown that the hybrid fuels combining different reactivities (such as gasoline and diesel) can help in controlling HCCI ignition and burn rates.
RCCI, or reactivity controlled compression ignition, has been demonstrated to provide highly efficient, low emissions operation over wide load and speed ranges.
One example is the hot-bulb engine which used a hot vaporization chamber to help mix fuel with air.
When auto-ignition occurs too early or with too much chemical energy, combustion is too fast and high in-cylinder pressures can destroy an engine.
In an HCCI engine, however, the homogeneous mixture of fuel and air is compressed and combustion begins whenever sufficient pressure and temperature are reached.
Exhaust gas is very hot if retained or re-inducted from the previous combustion cycle or cool if recirculated through the intake as in conventional EGR systems.
Hot combustion products conversely increase gas temperature in the cylinder and advance ignition.
[13] Variable valve actuation (VVA) extends the HCCI operating region by giving finer control over the temperature-pressure-time envelope within the combustion chamber.
Another means to extend the operating range is to control the onset of ignition and the heat release rate[14][15] by manipulating the fuel itself.
[16] Examples include blending of commercial gasoline and diesel fuels,[17] adopting natural gas [18] or ethanol.
Partially Pre-mixed Charge Compression Ignition (PPCI) also known as Premixed Charge Compression Ignition (PCCI) is a compromise offering the control of CIDI combustion with the reduced exhaust gas emissions of HCCI, specifically lower soot.
This is done by timing the injection event such that a range of air/fuel ratios spread across the combustion cylinder when ignition begins.
[21] The adoption of high EGR and diesel fuels with a greater resistance to ignition (more "gasoline like") enable longer mixing times before ignition and thus fewer rich pockets that produce soot and NOx[20][21] In a typical ICE, combustion occurs via a flame.
The high compression ratio in the auxiliary combustion chamber causes the auto-ignition of the homogeneous lean air-fuel mixture therein (no spark plug required); the burnt gas bursts - through some "transfer ports", just before the TDC - into the main combustion chamber triggering its auto-ignition.
However, in HCCI engines increasing the fuel/air ratio results in higher peak pressures and heat release rates.
[25] Because HCCI operates on lean mixtures, the peak temperature is much lower than that encountered in SI and diesel engines.
This low peak temperature reduces the formation of NOx, but it also leads to incomplete burning of fuel, especially near combustion chamber walls.
[17][26][27] This is largely because ignition is more sensitive to chemical kinetics than to turbulence/spray or spark processes as are typical in SI and diesel engines.
Computational models have demonstrated the importance of accounting for the fact that the in-cylinder mixture is actually in-homogeneous, particularly in terms of temperature.