HCCI
Homogeneous Charge Compression Ignition, or 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 chemical energy into a sensible form that can be translated by an engine into work and heat.
HCCI has characteristics from each of the two most popular forms of combustion used in IC engines: homogeneous charge spark ignition (gasoline engines) and stratified charge compression ignition (diesel engines). As in homogeneous charge spark ignition the fuel and oxidizer are mixed together and sufficient heat is added to initiate combustion. However, rather than using an electric discharge to ignite a portion of the mixture, the concentration and temperature of the mixture are raised by compression until the entire mixture reacts simultaneously. Stratified charge compression ignition also relies on the heat and concentration resulting from compression, but combustion occurs at the boundary of unmixed fuel which is injected to initiate combustion.
The defining characteristic of HCCI is that there is no direct initiator of combustion. This makes the process inherently challenging to control. However, with advances in microprocessors and a physical understaning of the ignition process, HCCI can achieve gasoline engine like emissions with a diesel engine like efficiency.
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History
HCCI engines have a long history, even though HCCI has not been as widely implemented as spark ignition or diesel injection. It is essentially an Otto combustion cycle. In fact, HCCI was popular before electronic spark ignition was used. One example is the hot-bulb engine which used a torch-heated head to add heat to the inducted gases. The extra heat combined with compression induced the conditions for combustion to occur.
Operation
Methods
A mixture of fuel and air will ignite when the concentration and temperature of reactants is sufficiently high. The concentration and/or temperature can be increased several different ways:
- High compression ratio
- Pre-heat induction gases
- Forced induction
- Retain or reinduct exhaust
Once ignited, combustion occurs very quickly. In fact, when auto-ignition occurs too early or with too much chemical energy combustion is too fast. In such cases, high in-cylinder pressures can destroy an engine. For this reason, HCCI is typically operated at lean overall fuel mixtures.
Benefits
- HCCI is closer to the ideal Otto cycle than spark ignited combustion.
- Lean operation leads to higher efficiency than in spark ignited gasoline engines
- Homogeneous mixing of fuel and air leads to cleaner combustion and lower emissions. In fact, due to the fact that peak temperatures are significantly lower than in typical spark ignited engines, levels of NOx emissions are almost negligible.
Control
HCCI is more difficult to control than other popular modern combustion methods.
In a typical gasoline engine, a spark is used to ignite the pre-mixed fuel and air. In diesel engines, combustion begins when the fuel is injected into compressed air. In both cases, the timing of combustion is explicitly controlled. In an HCCI engine, however, the homogeneous mixture of fuel and air is compressed, and combustion begins whenever the appropriate conditions are reached. This means that there is no well-defined combustion initiator that can be directly controlled. An engine can be designed so that the ignition conditions occur at a desirable timing. However, this would only happen at one operating point. The engine could not change the amount of work it produces. This could work in a hybrid vehicle, but most engines change their energy production to meet user demand.
To achieve dynamic operation in an HCCI engine, the control system must change the conditions that induce combustion. Thus, the engine must control either the compression ratio, inducted gas temperature, inducted gas pressure, or quantity of retained or reinducted exhaust.
Difference from Knock
Engine knock or pinging occurs when some of the unburnt gases ahead of the flame in a spark ignited engine detonate. The detonation is caused by a shock wave passing at the speed of sound through the compressed gases. In HCCI, the chemical reaction occurs everywhere simultaneously - not induced by either a deflagration or detonation wave.
See Also
External links
- Research at Chalmers University of Technology
- Research at Stanford University
- Research at University of California, Berkeley