Homogeneous charge compression ignition (HCCI) is a form of internal combustion in which well-mixed fuel and oxidizer (typically air) are compressed to the point of auto-ignition. As in other forms of combustion, this exothermic reaction releases energy that can be transformed in an engine into work and heat.
As in HCSI, HCCI injects fuel during the intake stroke. However, rather than using an electric discharge (spark) to ignite a portion of the mixture, HCCI raises density and temperature by compression until the entire mixture reacts spontaneously.
Stratified charge compression ignition also relies on temperature and density increase resulting from compression. However, it injects fuel later, during the compression stroke. Combustion occurs at the boundary of the fuel and air, producing higher emissions, but allowing a leaner and higher compression burn, producing greater efficiency.
Controlling HCCI requires microprocessor control and physical understanding of the ignition process. HCCI designs achieve gasoline engine-like emissions with diesel engine-like efficiency.
HCCI engines achieve extremely low levels of Nitrogen oxide emissions (NO
x) without a catalytic converter. Unburned hydrocarbon and carbon monoxide emissions still require treatment to meet automotive emission regulations.
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.
A mixture of fuel and air ignites when the concentration and temperature of reactants is sufficiently high. The concentration and/or temperature can be increased in several different ways:
- Increasing compression ratio
- Pre-heating of induction gases
- Forced induction
- Retained or re-inducted exhaust gases
Once ignited, combustion occurs very quickly. 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. For this reason, HCCI is typically operated at lean overall fuel mixtures.
- Since HCCI engines are fuel-lean, they can operate at diesel-like compression ratios (>15), thus achieving 30% higher efficiencies than conventional SI gasoline engines.
- Homogeneous mixing of fuel and air leads to cleaner combustion and lower emissions. Because peak temperatures are significantly lower than in typical SI engines, NOx levels are almost negligible. Additionally, the technique does not produce soot.
- HCCI engines can operate on gasoline, diesel fuel, and most alternative fuels.
- HCCI avoids throttle losses, which further improves efficiency.
- Achieving cold start capability.
- High heat release and pressure rise rates contribute to engine wear.
- Autoignition is difficult to control, unlike the ignition event in SI and diesel engines, which are controlled by spark plugs and in-cylinder fuel injectors, respectively.
- HCCI engines have a small power range, constrained at low loads by lean flammability limits and high loads by in-cylinder pressure restrictions.
- Carbon monoxide (CO) and hydrocarbon (HC) pre-catalyst emissions are higher than a typical spark ignition engine, caused by incomplete oxidation (due to the rapid combustion event and low in-cylinder temperatures) and trapped crevice gases, respectively.