Internal combustion engine
An internal combustion engine (ICE) is a type of heat engine that generates power through the combustion of a fuel and an oxidizer, producing expanding gases that provide kinetic energy. As of the early 2020s, ICEs powered approximately 90% of automobiles and trucks, although their prevalence is declining due to the rise of electric vehicles (EVs) and hybrid technologies. There is a growing belief that advancements in battery technology, particularly lithium-ion batteries, will lead to a significant shift toward electric propulsion in the automotive industry to address climate change. Internal combustion engines can be categorized into various types, including reciprocating engines, which can be further divided into continuous (like jet engines) and intermittent combustion engines (such as gasoline and diesel engines).
The ICEs operate through different mechanisms, such as spark ignition and compression ignition, with variations like two-stroke and four-stroke cycles. Another variant, the rotary engine, exemplified by the Wankel engine, utilizes continuous rotary motion to enhance power efficiency but has faced challenges related to emissions and fuel economy. Gas turbine engines, while offering advantages like high efficiency and reduced emissions, also present drawbacks such as high noise levels and material costs. Overall, the internal combustion engine has played a crucial role in transportation, but evolving technologies and environmental concerns are pushing the industry towards greener alternatives.
Internal combustion engine
Summary: The technology of the internal combustion engine applies the reaction of an oxidizer and a fuel in a combustion that produces the working fluids (in the form of expanding gases) to supply kinetic energy.
In the early years of the 2020s, the internal combustion engine was used in about 90 percent of all automobiles and trucks, although that percentage was declining as electric vehicles began to take away a growing share of the market. There was a rising opinion that the incorporation of electric motors would supersede the use of internal combustion engines, at least in automobiles, in the not-so-distant future. Advancements in battery technology, specifically regarding efficient lithium-ion batteries, combined with global efforts to fight climate change led to some arguments that it would not be long before carmakers would be shifting more to electric technology—or at least more of a hybrid between the two. As electric cars are simpler to make and maintain and drastically reduce carbon emissions as opposed to cars that use internal combustion engines, the call for electric cars may further increase. Yet others claim that with some companies working on methods for making the internal combustion engine more efficient, it will continue to remain in use for some time.
An internal combustion engine is one in which the working fluids are comprised of reactants of combustion—oxidizer and fuel—along with the products of combustion. The heat released by combustion of the oxidizer-fuel mixture propels the gaseous products of combustion against the moving surfaces of the engine, whether a piston, turbine blade, or other device. The chief types of internal combustion engines are reciprocating and rotary (Wankel). Gas turbine types are also in use.
Reciprocating Engines
Reciprocating engines come in three general categories:
• Continuous- and intermittent-combustion engines: A steady flow of fuel and oxidizer into the engine is the signature of the continuous-combustion engine. A stable flame is maintained; jet engines are typical. The intermittent-combustion engine applies periodic ignition of air and fuel; this is most often called a reciprocating engine; gasoline and diesel piston engines are the most common examples.
• Spark ignition/compression ignition engines: Fuel and air are mixed prior to the intake stroke, or just after inlet valve closure, in the electric spark ignition, or Otto, engines. In compression ignition versions, the fuel—diesel—is injected after the compression process; rather than a spark, the fuel is ignited by the high temperature of the compressed gas.
• Four-stroke (4s) and two-stroke (2s) engines: The work cycle in a four-stroke engine takes two crankshaft revolutions, divided into intake, compression, expansion, and exhaust stages. This is sometimes called the Otto cycle, in honor of inventor Nikolaus Otto. One crankshaft revolution is sufficient for a complete two-stroke cycle, which has no intake or exhaust strokes. Gas exchange in these engines occurs when the piston is near the bottom center position, between the expansion and compression strokes.
Rotary Engines
The rotary-piston engine, invented by Felix Wankel, generates power in the familiar four-stroke cycle of compression, ignition, and expansion of a gasoline-air mixture. However, the moving parts work in a continuous rotary motion, instead of a reciprocating movement.
The Wankel rotary engine delivers one power stroke for each full crankshaft rotation. Thus its displacement volume is used twice as often as in four-stroke engines. This allows the average Wankel to be engineered to roughly half the size and weight of a conventional engine. There are also far fewer components, usually about 40 percent of the number of moving parts a V-8 engine would have. There is a similar kind of advantage, although not as dramatic, over a four-cylinder engine. By reducing the number of components and the complexity of their interactions, engine manufacture costs are also reduced.
Ironically, the Wankel engine also embodies some manufacturing drawbacks, mainly the need for expensive materials and the requirement for higher-precision manufacturing techniques. For all that, the Wankel has tended toward low fuel economy and high emissions of incompletely combusted hydrocarbons. The high hydrocarbon emissions result from poor sealing between the rotor and housing. However, design and engineering improvements since 2003 have brought about production models of the Wankel type that meet contemporary fuel economy and emissions standards.
Gas Turbine Engines
The gas turbine, or combustion turbine, engine is a rotary, continuous internal combustion engine wherein the fuel flows to a burner supplied with an excess of compressed air. The expanding combustion gases apply pressure to turbine blades; the turbine power is transferred via gearing to an output shaft.
The gas turbine engine embodies four main operations, as described in Mehrdad Ehsani et al:
• Compression: Air enters the gas turbine and is compressed.
• Heat exchange: Heat is drawn from the exhaust gases and communicated to the compressed air.
• Combustion: Fuel is mixed with hot air and ignited. The pressure increases.
• Expansion: The hot exhaust gases drive the turbine, thus releasing their energy. The turbine turns the compressor and the output shaft.
The advantages of gas turbines include high rotational speed, yielding a very compact engine; rotating movement yields vibration-free operation; ability to operate on a wide variety of fuels; and continuous combustion yields reduced hydrocarbon and carbon monoxide emissions compared to internal combustion engines.
However, these advantages must be balanced against the following drawbacks when gas turbines are considered for automotive applications: quenching of the gases by the turbine and compressor yields high noise levels; low efficiency of the dynamic compressor and turbine at smaller scales yields relatively higher fuel consumption; to obtain the high rotating speeds needed to maximize efficiency, sophisticated and costly materials must be incorporated; and high cost materials must also be used to tolerate the higher temperature levels compared to other engine types.
Comparisons With External Combustion Engines
An internal combustion engine is a heat engine; its thermal energy is derived by a chemical reaction within the working fluid. The working fluid itself is then exhausted to the environment, aiding excess heat rejection. In external combustion engines, heat is transferred to the working fluid through a solid wall and also expelled to the environment via another solid wall. Steam engines are in this class.
Internal combustion engines have two intrinsic advantages: Except for auxiliary cooling, they require no heat exchangers, thereby reducing weight, volume, cost, and complexity. With no requirement for high-temperature heat transfer through walls, the design of internal combustion engines permits the maximum temperature of the working fluid to exceed maximum allowable wall material temperature.
Intrinsic disadvantages include the fact that for all practical purposes the working fluids are limited to air and products of combustion. Nonfuel heat sources, such as waste heat, cannot be used to generate motive energy. Additionally, there is little flexibility in combustion conditions—they are largely set by engine requirements. This factor makes achieving low-emissions combustion more difficult.
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