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Homogeneous Charge Compression Ignition HCCI


Published on Jan 10, 2016

Abstract

HCCI has characteristics 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.

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 spontaneously. Stratified charge compression ignition also relies on temperature increase and concentration resulting from compression, but combustion occurs at the boundary of fuel-air mixing, caused by an injection event, to initiate combustion.

The defining characteristic of HCCI is that the ignition occurs at several places at a time which makes the fuel/air mixture burn nearly simultaneously. There is no direct initiator of combustion. This makes the process inherently challenging to control. However, with advances in microprocessors and a physical understanding of the ignition process, HCCI can be controlled to achieve gasoline engine-like emissions along with diesel engine-like efficiency. In fact, HCCI engines have been shown to achieve extremely low levels of Nitrogen oxide emissions (NOx) without aftertreatment catalytic converter. The unburned hydrocarbon and carbon monoxide emissions are still high (due to lower peak temperatures), as in gasoline engines, and must still be treated to meet automotive emission regulations.

Introduction:

HCCI has characteristics 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. 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 temperature increase and concentration resulting from compression, but combustion occurs at the boundary of fuel-air mixing, caused by an injection event, to initiate combustion.

The defining characteristic of HCCI is that the ignition occurs at several places at a time which makes the fuel/air mixture burn nearly simultaneously. There is no direct initiator of combustion. This makes the process inherently challenging to control. However, with advances in microprocessors and a physical understanding of the ignition process, HCCI can be controlled to achieve gasoline engine-like emissions along with diesel engine-like efficiency. In fact, HCCI engines have been shown to achieve extremely low levels of Nitrogen oxide emissions (NOx) without after treatment catalytic converter.

The unburned hydrocarbon and carbon monoxide emissions are still high (due to lower peak temperatures), as in gasoline engines, and must still be treated to meet automotive emission regulations.The homogeneous charge compression ignition (HCCI) engine has caught the attention of automotive and diesel engine manufacturers worldwide because of its potential to rival the high efficiency of diesel engines while keeping NOx and particulateemissions extremely low. However, researchers must overcome several technical barriers, such as controlling ignition timing, reducing unburned hydrocarbon and carbon monoxide emissions, extending operation to higher loads, and maintaining combustion stability through rapid transients.

HCCI engines can operate using a variety of fuels. In the near term, the application of HCCI to automotive engines will likely involve mixed-mode combustion in which HCCI is used at low-to-moderate loads and standard spark-ignition (SI) combustion is used at higher loads. This type of operation using standard gasoline-type fuels requires a moderate compression ratio of 10:1 to 14:1 for SI operation and significant intake heating for HCCI operation.

Why HCCI?

The modern conventional SI engine fitted with a three-way catalyst can be seen as an very clean engine. But it suffer from poor partload efficiency. As mentioned earlier this is mainly due to the throttling. Engines in passenger cars operates most of the time at light- and partload conditions. For some shorter periods of time, at overtaking and acceleration, they run at high loads, but they seldom run at high loads for any longer periods. This means that the overall efficiency at normal driving conditions becomes very low.

The Diesel engine has a much higher part load efficiency than the SI engine. Instead the Diesel engine fights with great smoke and NOx problems. Soot is mainly formed in the fuel rich regions and NOx in the hot stoichiometric regions. Due to these mechanisms, it is difficult to reduce both smoke and NOx simultaneously through combustion improvement. Today, there is no well working exhaust after treatment that takes away both soot and NOx.

The HCCI engine has much higher part load efficiency than the SI engine and comparable to the Diesel engine, and has no problem with NOx and soot formation like the Diesel engine. In summary, the HCCI engine beats the SI engine regarding the efficiency and the Diesel engine regarding the emissions.

Advantages

• 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, NOx levels are almost negligible.

• Since HCCI runs throttleless, it eliminates throttling losses

Disadvantages

• High peak pressures

• High heat release rates

• Difficulty of control

• Limited power range

• High carbon monoxide and hydrocarbon pre-catalyst emissions

How to Accomplish the HCCI

Because of the high compression ratios in a diesel, the engine must be more robust to withstand the loads and the temperature of the combustion tends to be high enough to cause the nitrogen in the air to react with the oxygen resulting in NOx. As the name implies, homogeneous charge compression ignition (HCCI) relies on the high temperatures generated by compressing the intake stream to cause the fuel to auto ignite just like a diesel. The difference is that an HCCI engine runs on gasoline (or ethanol) instead of diesel fuel and has a significantly lower compression ratio.

HCCI

That lower compression ratio contributes to a lower combustion temperature and helps keep nitrogen oxide generation to a minimum. In order for this work, very precise metering of the fuel is required and that is now possible thanks to the latest direct injection technology. The fuel is injected directly into the cylinder and mixed with the air. Since gasolines vary in different regions and different times of the year, the timing.hcci operation and concentration has to be adjusted in real time. Having this capability built in also makes it easier to accommodate alternatefuel like ethanol.














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