What is HCCI Engine | Working , Advantages and Disadvantages
What Is HCCI?
- HCCI is a combustion process. HCCI is not an engine concept. HCCI must be incorporated in an engine concept.
- HCCI is a low temperature chemically controlled (flameless) combustion process.
- HCCI can be considered as a hybrid form between the diesel and Otto combustion process.
- However combustion process is different. So there is neither Diffusion flame (as in a diesel engine) nor a flame front traveling through a premixed charge ( as in SI engine).
Why HCCI Engine ?
- High efficiency and ultra low emission with respect to conventional Diesel engine.
- To achieve near zero NOx and soot emission
- To achieve latest Euro Norms ( E5)
- To reduce fuel consumption, greenhouse gas emission.
Law of Diesel HCCI –
- Every one percent increase of diesel HCCI car saves 90 million liters of fuel per year.
- This corresponds to emission saving of some 210000 metric tones of CO2
WORKING PRINCIPLE OF HCCI
Homogenous charge is drawn in to the cylinder during suction and compress to high enough temperature to achieve spontaneous ignition of the charge. Combustion starts in almost whole volume of C.C Two degree before TDC After Combustion initiation the temperature rapidly increases and whole fuel burn simultaneously As whole mixture burns simultaneously and no flame propagation , combustion temperature can be controlled less than 700 deg Centigrade and thus NOx formation is avoided.
- Higher load HCCI operation is difficult.
- To achieve Transient operation and cold starting are issues.
- Start of combustion (SOC) is difficult to control because there is no direct ignition source such as a spark plug or fuel injection timing.
- Instead SOC is controlled by mixture properties at intake valve closing.
- Controlling how rapidly the energy is released is also a problem. For this reason significant EGR dilution is needed.
- CO and UHC emissions are also high at light loads
- Ultra-low NOx emissions because there is no propagating flame front and the burning is both locally and globally lean.
- Extremely low PM emissions because the mixture is homogeneous and there are no rich pockets
- Significant part load efficiency gains over SI because the engine is operated unthrottled and load is controlled by fuelling and varying the fuel to air ratio.
- Homogeneous Charge Compression Ignition (HCCI) engines have the potential of producing a high thermal efficiency with extremely low PM and NOx emissions.
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