Compression ignition internal combustion engines

Ofodike A. Ezekoye

J.T. MacGuire Professor
Dept. of Mechanical Engineering, The University of Texas at Austin Austin, TX 78712

Combustion ignition internal combustion engines (CI-ICE) are considered to be durable and reliable power generation engines. CI-ICE power electric generators, passenger vehicles, trucks, busses, and ships, and submarines. The most basic model for the CI-ICE is the air-standard Diesel cycle. This is an idealization of the actual combustion cycle that replaces a combustion process with a heat addition process. The basic operational features are that an air flow is ingested into a piston cylinder system that is configured with inlet and exhaust valves. The piston movement from the largest cylinder volume to the smallest cylinder volume compresses the air, causing it to heat up. When the piston has compressed the air to the smallest volume, an atomized fuel is sprayed into the cylinder. The fuel is burned in the hot air environment. The flammable mixture is ignited by the hot compressed air, which informs the CI portion of the CI-ICE's name. Typically, the ignition process occurs slightly before the piston has compressed the reactant mixture to it's minimum volume (maximum pressure). This is termed top-dead-center. Once ignited, the flame expands, first as a so-called flame kernel and then eventually as a turbulent combusting front. The expansion process generates burned gas products which are at higher temperature and lower density than the unburned material. The expanding flame drives the piston and does work. Once the piston has swept though the available cylinder volume, the exhaust valve(s) open and further cylinder motion upward drives the burned products through the exhaust valves and into the exhaust port. Once these gases have been exhausted the piston motion cycles back from top-dead-center towards bottom-dead-center while ingesting a fresh reactant mixture and repeating the cycle. The air-standard reversible Diesel cycle thermodynamic model is shown below:

The elements of the Diesel cycle are a slow combustion process that is modeled as being a constant pressure heat addition process in which the cylinder volume increases from the fully compressed value to a much larger value that is generated by the combustion process. After the flame propagation process ends, there is an isentropic expansion process. At the end of the expansion process, the exhaust process occurs and is modeled as being a constant volume process. As previously noted, in reality, the exhaust process occurs as the piston sweeps from bottom-dead-center to top-dead-center and this is followed by the intake (reactant ingestion) process. These two processes do not meaning fully affect the amount of work produced by the engine and are typically shown on thermodynamic indicator diagrams as parallel constant atmospheric pressure processes.

Important research and development topics for the CI-ICE include the need to improve the thermodynamic efficiency of the cycle while also reducing the harmful products of combustion generated while it is in operation. The CI-ICE has a thermal efficiency of approximately 30%. In an ideal combustion scenario, the only products of combustion are carbon-dioxide and water vapor. Recognition of the role that CO2 plays in global warming and climate change are leading to questions and opportunities to improve the engine's thermal efficiency and reduce the amount of CO2 produced. Because the combustion process is a CI-ICE is poorly mixed, the combustion process results in incomplete products of combustion which include carbonaceous solid particles (soot), polycyclic aromatic hydrocarbons (PAHs), carbon monoxide, and other pollutants. Research and development activities seek to reduce the incomplete products of combustion by improving the in-cylinder fluid mechanics and mixing and developing technologies for oxidizing the exhaust products.

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