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Everything You Should Know About Homogeneous Charge Engines

written by: Tarun Goel • edited by: Lamar Stonecypher • updated: 9/30/2011

A homogeneous charge engine, in which fuel is mixed with air and is compressed to be ignited, possesses characteristics of both diesel and gasoline engines. The popularity of these engines may increase because they are capable of reducing emissions.

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    This type of internal combustion engine works on the principle of homogeneous charge compound ignition (HCCI). It compresses the well-mixed fuel and air just to the point of auto-ignition and as in other forms of combustion, this exothermic reaction releases chemical energy that can be used to perform mechanical actions by the engine. Like gasoline engines, HCCI engines mix the air and fuel together, but like diesel engines, they use no spark plug to ignite the mixture. Instead, the temperature raised by the compression is used instead. During the HCCI process, ignition occurs at several places, resulting in the burning of mixture of fuel and air nearly simultaneously. Advances in microprocessors and the physical understanding of the ignition process means HCCI can be controlled to achieve a gasoline-like emission with diesel engine-like efficiency.

    Working of the Engine

    The HCCI is essentially an Otto combustion cycle.1 The air and fuel mixture will ignite when the concentration and temperature of the reactant is high and will increase if there is a high compression ratio, pre-heating induction of gases, forced induction and retained or re inducted exhaust gases. Combustion occurs quickly when the auto-ignition occurs too early or with too much chemical energy. With the high in-cylinder pressure it produces, the engine could be destroyed; hence HCCI is operated at lean overall fuel mixtures.

    Because of the simultaneous burning of air and fuel, increasing the power of an HCCI engines is a challenge. This is because increasing the fuel and air ratio, as was used in other engines could result to higher peak pressures and heat release rates. One way of increasing the power is to use a fuel with different auto-ignition properties that will lower the heat release rate and peak pressures and will make it possible to increase the equivalence ratio. Thermally stratifying the charge is also applicable for different points in the compressed charge will have different temperatures and will burn at times lowering the heat release rate. Lastly is to run the engine at an HCCI mode only at part load conditions and run on Diesel and spark ignition engines at full or near full load conditions.

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    Advantages and Disadvantages of HCCI

    Advantages of HCCI engines over conventional engines are listed2.

    1. As compared to conventional gasoline engines, HCCI engines are far more efficient because they have least throttling losses, shorter combustion duration, and high compression ratios that make engine work faster and better.
    2. The obvious advantage of fuel flexibility, which means diesel and gasoline, can be used equally. Less dependency on a single fuel.
    3. HCCI provides up to 15 % fuel savings, which means less burden on your pocket.
    4. It is compatible with ethanol fuel as well, can be used with the E85 gasoline too.
    5. Clean burning and reduced emissions are its major advantages. As compared to conventional engines, the HCCI engines do not affect the environment greatly. Less quantity of soot is released by HCCI engines.
    6. HCCI engines cost less since they use lower-pressure-fuel-injection equipment.

    Like every other technology, HCCI also has certain drawbacks and limitations. Major disadvantages of using a HCCI engine are mentioned.

    1. Controlling of ignition timing has always remained a major concern. For different ranges of speed and load, the engine might behave poorly.
    2. Hydrocarbon and monoxide emissions are more in HCCI engines as compared to conventional spark plug engines. However using oxidation catalysts can solve this problem and reduce emissions.
    3. Engine wear and tear could happen because of high pressure and heat release.

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    Recent developments in the HCCI technology have given very positive results to overcome the limitations of this technology. The technology has huge scope of use and it is used in a wide range of industries, which makes it a promising technology for the coming generations. Automobile giants like GM, Ford and Cummins have been exploring the possibilities in the HCCI technology for more than 15 years. General Motors has started educational programs in various Universities to promote the research work in this technology3. HCCI has also enabled engineers to experiment with different blends of fuel mixture so that performance and efficiency of HCCI engines can be tested with different combinations of non conventional fuels. As the demand of conventional fuels is increasing, scope of research and experimentation in HCCI technology will increase only with time.

    Of late, many companies have launched HCCI based automobiles in the market among which Mercedes F700, Volkswagen Touran, Opel Vectra, and Saturn Aura the prominent names.

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