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Making sense of Homogenous Charge Compression Ignition (HCCI)

Friday, July 27, 2018 - 07:00
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It’s hard to believe that the internal combustion engine has been around for more than 200 years. The design of such an engine is a combination of work from many different individuals, but basically we attribute the modern engine to Nikolas Otto. Nickolas was a German engineer who developed the compression charge internal combustion engine that ran on liquid petroleum gas. This basic design is what the modern engine is based from. Over the years, many individuals have put their twist on Nickolas’s engine design in order to enhance the reliability and performance. And it’s getting ready to be twisted again!

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(Photo courtesy of Mazda) At least one manufacturer, Mazda, is ready to put HCCI into production — the 2019 Mazda 3 with its Skyactiv-X engine will utilize spark plugs to ignite the air-fuel mixture at low revs, and piston compression at higher revs.

Introducing HCCI
As you already know, the modern internal combustion engine has seen a few twists. These twists are based on technological advancements in order to produce better performance and emission production. But perhaps you are not aware of one advancement the modern engine has seen — the Homogenous Charge Compression Ignition (HCCI) engine. Homogenous Charge (HC) refers to the charge state prior to ignition. A substance is homogeneous if its composition is identical wherever you sample it. This means that the charge mixture (fuel and air) has a uniform composition throughout the cylinder. Compression Ignition (CI) refers to the method that is used to drive the fuel past its autoignition point. When air is compressed rapidly, the molecules are accelerated off of the moving piston where they hit one another. The kinetic energy from the piston is turned into vibrational energy of the atoms causing a heating effect. This process is called Adiabatic Compression. The Adiabatic processes are characterized by zero heat transfer with the surroundings. In the case of rapid compression, the process occurs too fast for any heat transfer to occur. Heat transfer is a slow process. This rapid compression of the air creates a rapid heat increase that is used to drive the fuel well past its autoignition point. 

There are multiple ways used to combust fuel in the internal combustion engine. The fuel stock that is selected will set the method that will be used. In the automotive industry the fuels that we are most familiar with are gasoline and diesel. These fuel stocks have been around for many years and are used around the world. When using these fuel stocks, the ignition point is obtained with different methods. Gasoline will use the method of spark ignition, while diesel will use the method of compression ignition.

Spark ignition

In the spark ignition method, the charge prior to ignition is that of a homogenous charge. This means that the fuel/air charge is evenly mixed throughout the cylinder volume. In order to completely burn an evenly distributed mixture within the cylinder, the air/fuel ratio must be very close to that of stoichiometry. Stoichiometry refers to the weights of the chemicals that will react. In an internal combustion engine, the fuel is the reactant and the air is the oxidant. Air is comprised of 79 percent nitrogen, which is used as the working fluid, and 21 percent oxygen, which is used as the oxidant. The reaction will occur between the fuel, which is hydrocarbon based, and the oxygen. The stoichiometric ratio between the fuel and air is one where the hydrocarbons and oxygen are at a weight ratio that once they react with one another neither chemical will be present. This means that the hydrocarbons break apart becoming hydrogen and carbon. In the presence of oxygen, the hydrogen combines with the oxygen forming a new chemical; dihydrogen monoxide (H2O water). The carbon attaches to the oxygen forming a new chemical; carbon dioxide (CO2).

Figure 1

If the hydrocarbons and oxygen are at a stoichiometric ratio and react with one another then neither of these chemicals will remain present within the combustion gases (Figure 1). The chemical weight will be the same but the new chemicals formed during a complete reaction will be water and carbon dioxide. Although the mixture is at a stoichiometric ratio, in the real world there will not be a complete reaction between all of the chemicals so there will always be some hydrocarbons and oxygen left after the combustion process. This is due to the flame front being unable to get into the crevasses around the spark plug, valve pockets and piston rings.

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