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1974

Spark ignited internal combustion engines

Ofodike A. Ezekoye

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


The spark ignited internal combustion engine (SI-ICE) has been another transformational technology. SI-ICE are used in a variety of power delivery systems ranging in size from leaf blowers and edge trimmers to passenger land vehicles, propeller-driven airplanes, and small boats. The vast majority of passenger vehicles are powered by this technology. The most basic model for the SI-ICE is the air-standard Otto cycle. The SI-ICE can be operated in a variety of modes (e.g., 2-stroke or 4-stroke modes). The basic operational features are that an air flow and a fuel stream are ingested into a piston cylinder system that is configured with inlet and exhaust valves. The air and fuel can be mixed prior to entry into the cylinder through a fuel-injection process in the inlet port regions or can be mixed in-cylinder through a direct fuel injection process. In both cases, it is desirable to have molecularly mixed fuel and air prior to ignition of the flammable mixture. The flammable mixture is ignited by a spark-plug, which informs the SI portion of the SI-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 ai-standard reversible Otto cycle thermodynamic model is shown below:


The elements of the Otto cycle are a fast combustion process that is modeled as being a constant volume heat addition process in which the cylinder pressure rapidly increases from the fully compressed value to a much larger value that has been 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 pressure processes.

Important research and development topics for the SI-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 SI-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. More detailed modeling of the SI-ICE

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