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<title>Engine efficiency</title>

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<span id="openzim-page-title" class="mw-page-title-main"><span class="mw-page-title-main">Engine efficiency</span></span>

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<p><b>Engine efficiency</b> of thermal <a href="Engine" title="Engine">engines</a> is the relationship between the total <a href="Energy" title="Energy">energy</a> contained in the <a href="Fuel" title="Fuel">fuel</a>, and the amount of energy used to perform useful work. There are two classifications of thermal engines-

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<ol><li><a href="Internal_combustion" class="mw-redirect" title="Internal combustion">Internal combustion</a> (<a href="Otto_cycle" title="Otto cycle">gasoline</a>, <a href="Diesel_cycle" title="Diesel cycle">diesel</a> and <a href="Gas_turbine" title="Gas turbine">gas turbine</a>-<a href="Brayton_cycle" title="Brayton cycle">Brayton cycle</a> engines) and</li>

<li><a href="External_combustion_engines" class="mw-redirect" title="External combustion engines">External combustion engines</a> (<a href="Steam_engine" title="Steam engine">steam piston</a>, <a href="Steam_turbine" title="Steam turbine">steam turbine</a>, and the <a href="Stirling_cycle" title="Stirling cycle">Stirling cycle</a> engine).</li></ol>

<p>Each of these engines has <a href="Thermal_efficiency" title="Thermal efficiency">thermal efficiency</a> characteristics that are unique to it.

</p><p>Engine efficiency, transmission design, and tire design all contribute to a vehicle's <a href="Fuel_efficiency" title="Fuel efficiency">fuel efficiency</a>.

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<div class="mw-heading mw-heading2"><h2 id="Mathematical_definition">Mathematical definition</h2></div>

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</style><div role="note" class="hatnote navigation-not-searchable">Main articles: <a href="Thermal_efficiency" title="Thermal efficiency">Thermal efficiency</a> and <a href="Carnot_cycle" title="Carnot cycle">Carnot cycle</a></div>

<p>The efficiency of an engine is defined as ratio of the useful <b><a href="Work_done" class="mw-redirect" title="Work done">work done</a></b> to the heat provided.

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<annotation encoding="application/x-tex">{\displaystyle \eta ={\frac {\mathrm {work\ done} }{\mathrm {heat\ absorbed} }}={\frac {Q_{1}-Q_{2}}{Q_{1}}}}</annotation>

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</math></span><img src="./59d2e9d036d6d07e513b6d4ae7b83d8b8ef9fe5e.svg" class="mwe-math-fallback-image-inline mw-invert skin-invert" aria-hidden="true" style="vertical-align: -2.338ex; width:31.7ex; height:5.843ex;" alt="{\displaystyle \eta ={\frac {\mathrm {work\ done} }{\mathrm {heat\ absorbed} }}={\frac {Q_{1}-Q_{2}}{Q_{1}}}}" loading="lazy"></span></dd></dl>

<p>where, <span class="mwe-math-element mwe-math-element-inline"><span class="mwe-math-mathml-inline mwe-math-mathml-a11y" style="display: none;"><math xmlns="http://www.w3.org/1998/Math/MathML" alttext="{\displaystyle Q_{1}}">

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</math></span><img src="./a8ea6463cb36d8278ff71214fb4d13127039ae53.svg" class="mwe-math-fallback-image-inline mw-invert skin-invert" aria-hidden="true" style="vertical-align: -0.671ex; width:2.893ex; height:2.509ex;" alt="{\displaystyle Q_{1}}" loading="lazy"></span> is the heat absorbed and <span class="mwe-math-element mwe-math-element-inline"><span class="mwe-math-mathml-inline mwe-math-mathml-a11y" style="display: none;"><math xmlns="http://www.w3.org/1998/Math/MathML" alttext="{\displaystyle Q_{1}-Q_{2}}">

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<annotation encoding="application/x-tex">{\displaystyle Q_{1}-Q_{2}}</annotation>

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</math></span><img src="./07f7b78269aae3fe5c60fc2eeebcdbe7fa852763.svg" class="mwe-math-fallback-image-inline mw-invert skin-invert" aria-hidden="true" style="vertical-align: -0.671ex; width:8.626ex; height:2.509ex;" alt="{\displaystyle Q_{1}-Q_{2}}" loading="lazy"></span> is the work done.

</p><p>Please note that the term <b>work done</b> relates to the power delivered <b>at the clutch</b> or <b>at the driveshaft</b>.

</p><p>This means the friction and other losses are subtracted from the work done by thermodynamic expansion. Thus an engine not delivering any work to the outside environment has zero efficiency.

</p>

<div class="mw-heading mw-heading2"><h2 id="Compression_ratio">Compression ratio</h2></div>

<p>The efficiency of internal combustion engines depends on several factors, the most important of which is the expansion ratio. For any <a href="Heat_engine" title="Heat engine">heat engine</a> the work which can be extracted from it is proportional to the difference between the starting pressure and the ending pressure during the expansion phase. Hence, increasing the starting pressure is an effective way to increase the work extracted (decreasing the ending pressure, as is done with steam turbines by exhausting into a vacuum, is likewise effective).

</p><p>The compression ratio (calculated purely from the geometry of the mechanical parts) of a typical <a href="Petrol_engine" title="Petrol engine">gasoline (petrol)</a> is 10:1 (<a href="Octane_rating" title="Octane rating">premium fuel</a>) or 9:1 (regular fuel), with some engines reaching a ratio of 12:1 or more. The greater the expansion ratio, the more efficient the engine, in principle, and higher compression / expansion -ratio conventional engines in principle need gasoline with higher <a href="Octane_rating" title="Octane rating">octane</a> value, though this simplistic analysis is complicated by the difference between actual and geometric compression ratios. High octane value inhibits the fuel's tendency to burn nearly instantaneously (known as <a href="Engine_knocking" title="Engine knocking"><i>detonation</i> or <i>knock</i></a>) at high compression/high heat conditions. However, in engines that utilize compression rather than spark ignition, by means of very high compression ratios (14–25:1), such as the <a href="Diesel_engine" title="Diesel engine">diesel engine</a> or <a href="Bourke_engine" title="Bourke engine">Bourke engine</a>, high octane fuel is not necessary. In fact, lower-octane fuels, typically rated by <a href="Cetane_number" title="Cetane number">cetane number</a>, are preferable in these applications because they are more easily ignited under compression.

</p><p>Under part throttle conditions (i.e. when the throttle is less than fully open), the <i>effective</i> compression ratio is less than when the engine is operating at full throttle, due to the simple fact that the incoming fuel-air mixture is being restricted and cannot fill the chamber to full atmospheric pressure. The engine efficiency is less than when the engine is operating at full throttle. One solution to this issue is to shift the load in a multi-cylinder engine from some of the cylinders (by deactivating them) to the remaining cylinders so that they may operate under higher individual loads and with correspondingly higher effective compression ratios. This technique is known as <a href="Variable_displacement" title="Variable displacement">variable displacement</a>.

</p><p>Most petrol (gasoline, <a href="Otto_cycle" title="Otto cycle">Otto cycle</a>) and diesel (<a href="Diesel_cycle" title="Diesel cycle">Diesel cycle</a>) engines have an expansion ratio equal to the <a href="Compression_ratio" title="Compression ratio">compression ratio</a>. Some engines, which use the <a href="Atkinson_cycle" title="Atkinson cycle">Atkinson cycle</a> or the <a href="Miller_cycle" title="Miller cycle">Miller cycle</a> achieve increased efficiency by having an expansion ratio larger than the compression ratio.

</p><p>Diesel engines have a compression/expansion ratio between 14:1 and 25:1. In this case the general rule of higher efficiency from higher compression does not apply because diesels with compression ratios over 20:1 are <a href="Diesel_engine" title="Diesel engine">indirect injection diesels</a> (as opposed to direct injection). These use a prechamber to make possible the high RPM operation required in automobiles/cars and light trucks. The thermal and gas dynamic losses from the prechamber result in direct injection diesels (despite their lower compression / expansion ratio) being more efficient.

</p>

<div class="mw-heading mw-heading2"><h2 id="Friction">Friction</h2></div>

<p>An engine has many moving parts that produce <a href="Friction" title="Friction">friction</a>. Some of these friction forces remain constant (as long as the applied load is constant); some of these friction losses increase as engine speed increases, such as piston side forces and connecting bearing forces (due to increased inertia forces from the oscillating piston). A few friction forces decrease at higher speed, such as the friction force on the <a href="Camshaft" title="Camshaft">cam</a>'s lobes used to operate the <a href="Four-stroke_cycle_engine_valves" class="mw-redirect" title="Four-stroke cycle engine valves">inlet and outlet valves</a> (the valves' <a href="Inertia" title="Inertia">inertia</a> at high speed tends to pull the cam follower away from the cam lobe). Along with friction forces, an operating engine has <i>pumping losses</i>, which is the work required to move air into and out of the cylinders. This pumping loss is minimal at low speed, but increases approximately as the square of the speed, until at rated power an engine is using about 20% of total power production to overcome friction and pumping losses.

</p>

<div class="mw-heading mw-heading2"><h2 id="Oxygen">Oxygen</h2></div>

<p><a href="Earth's_atmosphere" class="mw-redirect" title="Earth's atmosphere">Air</a> is approximately 21% <a href="Oxygen" title="Oxygen">oxygen</a>. If there is not enough oxygen for proper combustion, the fuel will not burn completely and will produce less energy. An excessively rich fuel to air ratio will increase unburnt hydrocarbon pollutants from the engine. If all of the oxygen is consumed because there is too much fuel, the engine's power is reduced.

</p><p>As combustion temperature tends to increase with leaner fuel air mixtures, unburnt hydrocarbon pollutants must be balanced against higher levels of <a href="Air_pollution" title="Air pollution">pollutants</a> such as nitrogen oxides (<a href="NOx" title="NOx">NOx</a>), which are created at higher combustion temperatures. This is sometimes mitigated by introducing fuel upstream of the combustion chamber to cool down the incoming air through evaporative cooling. This can increase the total charge entering the cylinder (as cooler air will be more dense), resulting in more power but also higher levels of hydrocarbon pollutants and lower levels of nitrogen oxide pollutants. With direct injection this effect is not as dramatic but it can cool down the combustion chamber enough to reduce certain pollutants such as nitrogen oxides (NOx), while raising others such as partially decomposed hydrocarbons.

</p><p>The air-fuel mix is drawn into an engine because the downward motion of the pistons induces a partial vacuum. A <a href="Gas_compressor" class="mw-redirect" title="Gas compressor">compressor</a> can additionally be used to force a larger charge (forced induction) into the cylinder to produce more power. The compressor is either mechanically driven <a href="Supercharging" class="mw-redirect" title="Supercharging">supercharging</a> or exhaust driven <a href="Turbocharging" class="mw-redirect" title="Turbocharging">turbocharging</a>. Either way, forced induction increases the air pressure exterior to the cylinder inlet port.

</p><p>There are other methods to increase the amount of oxygen available inside the engine; one of them, is to inject <a href="Nitrous_oxide" title="Nitrous oxide">nitrous oxide</a>, (N<sub>2</sub>O) to the mixture, and some engines use <a href="Nitromethane" title="Nitromethane">nitromethane</a>, a fuel that provides the oxygen itself it needs to burn. Because of that, the mixture could be 1 part of fuel and 3 parts of air; thus, it is possible to burn more fuel inside the engine, and get higher power outputs.

</p>

<div class="mw-heading mw-heading2"><h2 id="Internal_combustion_engines">Internal combustion engines</h2></div>

<div class="mw-heading mw-heading3"><h3 id="Reciprocating_engines">Reciprocating engines</h3></div>

<p>Reciprocating engines at idle have low thermal efficiency because the only usable work being drawn off the engine is from the generator.

</p><p>At low speeds, gasoline engines suffer efficiency losses at small throttle openings from the high turbulence and frictional (head) loss when the incoming air must fight its way around the nearly closed throttle (pump loss); diesel engines do not suffer this loss because the incoming air is not throttled, but suffer "compression loss" due to use of the whole charge to compress the air to small amount of power output.

</p><p>At high speeds, efficiency in both types of engine is reduced by pumping and mechanical frictional losses, and the shorter period within which combustion has to take place. High speeds also results in more drag.

</p>

<div class="mw-heading mw-heading4"><h4 id="Gasoline_(petrol)_engines">Gasoline (petrol) engines</h4></div>

<p>Modern <a href="Gasoline" title="Gasoline">gasoline</a> engines have a maximum thermal efficiency of more than 50%,<sup id="cite_ref-1" class="reference"><a href="#cite_note-1"><span class="cite-bracket">[</span>1<span class="cite-bracket">]</span></a></sup> but most road legal cars only achieve about 20% to 40% efficiency.<sup id="cite_ref-2" class="reference"><a href="#cite_note-2"><span class="cite-bracket">[</span>2<span class="cite-bracket">]</span></a></sup><sup id="cite_ref-3" class="reference"><a href="#cite_note-3"><span class="cite-bracket">[</span>3<span class="cite-bracket">]</span></a></sup><sup id="cite_ref-4" class="reference"><a href="#cite_note-4"><span class="cite-bracket">[</span>4<span class="cite-bracket">]</span></a></sup><sup id="cite_ref-5" class="reference"><a href="#cite_note-5"><span class="cite-bracket">[</span>5<span class="cite-bracket">]</span></a></sup> Many engines would be capable of running at higher thermal efficiency but at the cost of higher wear and emissions.<sup id="cite_ref-6" class="reference"><a href="#cite_note-6"><span class="cite-bracket">[</span>6<span class="cite-bracket">]</span></a></sup> In other words, even when the engine is operating at its point of maximum thermal efficiency, of the total heat energy released by the <a href="Gasoline" title="Gasoline">gasoline</a> consumed, about 60-80% of total power is emitted as heat without being turned into useful work, i.e. turning the crankshaft.<sup id="cite_ref-Baglione_2007_7-0" class="reference"><a href="#cite_note-Baglione_2007-7"><span class="cite-bracket">[</span>7<span class="cite-bracket">]</span></a></sup> Approximately half of this rejected heat is carried away by the exhaust gases, and half passes through the cylinder walls or <a href="Cylinder_head" title="Cylinder head">cylinder head</a> into the engine cooling system, and is passed to the atmosphere via the cooling system radiator.<sup id="cite_ref-8" class="reference"><a href="#cite_note-8"><span class="cite-bracket">[</span>8<span class="cite-bracket">]</span></a></sup> Some of the work generated is also lost as friction, noise, air turbulence, and work used to turn engine equipment and appliances such as <a href="Circulation_pump" class="mw-redirect" title="Circulation pump">water and oil pumps</a> and the electrical <a href="Alternator" title="Alternator">generator</a>, leaving only about 20-40% of the energy released by the fuel consumed available to move the vehicle.

</p><p>A gasoline engine burns a mix of gasoline and air, consisting of a range of about twelve to eighteen parts (by weight) of air to one part of fuel (by weight). A mixture with a 14.7:1 air/fuel ratio is <a href="Stoichiometric" class="mw-redirect" title="Stoichiometric">stoichiometric</a>, that is when burned, 100% of the <a href="Fuel" title="Fuel">fuel</a> and the <a href="Oxygen" title="Oxygen">oxygen</a> are consumed. Mixtures with slightly less fuel, called <a href="Lean_burn" class="mw-redirect" title="Lean burn">lean burn</a> are more efficient. The <a href="Combustion" title="Combustion">combustion</a> is a reaction which uses the <a href="Oxygen" title="Oxygen">oxygen</a> content of the air to combine with the fuel, which is a mixture of several <a href="Hydrocarbon" title="Hydrocarbon">hydrocarbons</a>, resulting in <a href="Water_vapor" title="Water vapor">water vapor</a>, <a href="Carbon_dioxide" title="Carbon dioxide">carbon dioxide</a>, and sometimes <a href="Carbon_monoxide" title="Carbon monoxide">carbon monoxide</a> and partially burned hydrocarbons. In addition, at high temperatures the oxygen tends to combine with <a href="Nitrogen" title="Nitrogen">nitrogen</a>, forming <a href="Nitrogen_oxide" title="Nitrogen oxide">oxides of nitrogen</a> (usually referred to as <i>NOx</i>, since the number of oxygen atoms in the compound can vary, thus the "X" subscript). This mixture, along with the unused nitrogen and <a href="Atmospheric_chemistry" title="Atmospheric chemistry">other trace atmospheric elements</a>, is what is found in the <a href="Exhaust_system" title="Exhaust system">exhaust</a>.

</p><p>The most efficient cycle is the Atkinson Cycle, but most gasoline engine makers use the Otto Cycle for higher power and torque. Some engine design, such as Mazda's <a href="Skyactiv" title="Skyactiv">Skyactiv-G</a> and some hybrid engines designed by Toyota utilize the Atkinson and Otto cycles together with an electric motor/generator and a traction storage battery. The hybrid drivetrain can achieve effective efficiencies of close to 40%.

</p>

<div class="mw-heading mw-heading4"><h4 id="Diesel_engines">Diesel engines</h4></div>

<p>Engines using the Diesel cycle are usually more efficient, although the Diesel cycle itself is less efficient at equal compression ratios. Since diesel engines use much higher compression ratios (the heat of compression is used to ignite the slow-burning <a href="Diesel_fuel" title="Diesel fuel">diesel fuel</a>), that higher ratio more than compensates for air pumping losses within the engine.

</p><p>Modern turbo-diesel engines use electronically controlled common-rail <a href="Fuel_injection" title="Fuel injection">fuel injection</a> to increase efficiency. With the help of geometrically variable turbo-charging system (albeit more maintenance) this also increases the engines' torque at low engine speeds (1,200–1,800 rpm). Low speed diesel engines like the <a href="MAN_B%26W_Diesel" class="mw-redirect" title="MAN B&W Diesel">MAN</a> S80ME-C7 have achieved an overall <a href="Energy_conversion_efficiency" title="Energy conversion efficiency">energy conversion efficiency</a> of 54.4%, which is the highest conversion of fuel into power by any single-cycle <a href="Internal_combustion" class="mw-redirect" title="Internal combustion">internal</a> or <a href="External_combustion" class="mw-redirect" title="External combustion">external combustion</a> engine.<sup id="cite_ref-bmmWW_9-0" class="reference"><a href="#cite_note-bmmWW-9"><span class="cite-bracket">[</span>9<span class="cite-bracket">]</span></a></sup><sup id="cite_ref-mhi_10-0" class="reference"><a href="#cite_note-mhi-10"><span class="cite-bracket">[</span>10<span class="cite-bracket">]</span></a></sup><sup id="cite_ref-11" class="reference"><a href="#cite_note-11"><span class="cite-bracket">[</span>11<span class="cite-bracket">]</span></a></sup> Engines in large diesel trucks, buses, and newer diesel cars can achieve peak efficiencies around 45%.<sup id="cite_ref-12" class="reference"><a href="#cite_note-12"><span class="cite-bracket">[</span>12<span class="cite-bracket">]</span></a></sup>

</p>

<div class="mw-heading mw-heading3"><h3 id="Gas_turbine">Gas turbine</h3></div>

<p>The <a href="Gas_turbine" title="Gas turbine">gas turbine</a> is most efficient at maximum power output in the same way reciprocating engines are most efficient at maximum load. The difference is that at lower rotational speed the pressure of the compressed air drops and thus thermal and <a href="Fuel_efficiency" title="Fuel efficiency">fuel efficiency</a> drop dramatically. Efficiency declines steadily with reduced power output and is very poor in the low power range.

</p><p><a href="General_Motors" title="General Motors">General Motors</a> at one time manufactured a bus powered by a gas turbine, but due to rise of crude oil prices in the 1970s this concept was abandoned. <a href="Rover_(marque)" title="Rover (marque)">Rover</a>, <a href="Chrysler" title="Chrysler">Chrysler</a>, and <a href="Toyota" title="Toyota">Toyota</a> also built prototypes of turbine-powered cars. Chrysler built a short prototype series of them for real-world evaluation. Driving comfort was good, but overall economy lacked due to reasons mentioned above. This is also why gas turbines can be used for permanent and peak power electric plants. In this application they are only run at or close to full power, where they are efficient, or shut down when not needed.

</p><p>Gas turbines do have an advantage in <a href="Power_density" title="Power density">power density</a>—gas turbines are used as the engines in heavy armored vehicles and armored tanks and in power generators in jet fighters.

</p><p>One other factor negatively affecting the gas turbine efficiency is the ambient air temperature. With increasing temperature, intake air becomes less dense and therefore the gas turbine experiences power loss proportional to the increase in ambient air temperature.<sup id="cite_ref-13" class="reference"><a href="#cite_note-13"><span class="cite-bracket">[</span>13<span class="cite-bracket">]</span></a></sup>

</p><p>Latest generation gas turbine engines have achieved an efficiency of 46% in <a href="Simple_cycle_combustion_turbine" title="Simple cycle combustion turbine">simple cycle</a> and 61% when used in <a href="Combined_cycle" class="mw-redirect" title="Combined cycle">combined cycle</a>.<sup id="cite_ref-gtEfficiency_14-0" class="reference"><a href="#cite_note-gtEfficiency-14"><span class="cite-bracket">[</span>14<span class="cite-bracket">]</span></a></sup>

</p>

<div class="mw-heading mw-heading2"><h2 id="External_combustion_engines">External combustion engines</h2></div>

<div class="mw-heading mw-heading3"><h3 id="Steam_engine">Steam engine</h3></div>

<div role="note" class="hatnote navigation-not-searchable">Main article: <a href="Steam_engine" title="Steam engine">Steam engine</a></div>

<dl><dd><dl><dd>See also: <a href="Steam_engine#Efficiency" title="Steam engine">Steam engine#Efficiency</a></dd>

<dd>See also: <a href="Timeline_of_steam_power" title="Timeline of steam power">Timeline of steam power</a></dd></dl></dd></dl>

<div class="mw-heading mw-heading4"><h4 id="Piston_engine">Piston engine</h4></div>

<p>Steam engines and turbines operate on the <a href="Rankine_cycle" title="Rankine cycle">Rankine cycle</a> which has a maximum <a href="Carnot_efficiency" class="mw-redirect" title="Carnot efficiency">Carnot efficiency</a> of 63% for practical engines, with steam turbine power plants able to achieve efficiency in the mid 40% range.

</p><p>The efficiency of steam engines is primarily related to the steam temperature and pressure and the number of stages or <i>expansions</i>.<sup id="cite_ref-Thurston1875_15-0" class="reference"><a href="#cite_note-Thurston1875-15"><span class="cite-bracket">[</span>15<span class="cite-bracket">]</span></a></sup> Steam engine efficiency improved as the operating principles were discovered, which led to the development of the science of <a href="Thermodynamics" title="Thermodynamics">thermodynamics</a>. See graph:<a rel="nofollow" class="external text" href="http://www.cuug.ab.ca/branderr/eeepc/017_coal.html">Steam Engine Efficiency</a>

</p><p>In earliest steam engines the boiler was considered part of the engine. Today they are considered separate, so it is necessary to know whether stated efficiency is overall, which includes the boiler, or just of the engine.

</p><p>Comparisons of efficiency and power of the early steam engines is difficult for several reasons: 1) there was no standard weight for a bushel of coal, which could be anywhere from 82 to 96 pounds (37 to 44 kg). 2) There was no standard heating value for coal, and probably no way to measure heating value. The coals had much higher heating value than today's steam coals, with 13,500 BTU/pound (31 megajoules/kg) sometimes mentioned. 3) Efficiency was reported as "duty", meaning how many foot pounds (or newton-metres) of work lifting water were produced, but the mechanical pumping efficiency is not known.<sup id="cite_ref-Thurston1875_15-1" class="reference"><a href="#cite_note-Thurston1875-15"><span class="cite-bracket">[</span>15<span class="cite-bracket">]</span></a></sup>

</p><p>The first piston steam engine, developed by <a href="Thomas_Newcomen" title="Thomas Newcomen">Thomas Newcomen</a> around 1710, was slightly over one half percent (0.5%) efficient. It operated with steam at near atmospheric pressure drawn into the cylinder by the load, then condensed by a spray of cold water into the steam filled cylinder, causing a partial vacuum in the cylinder and the pressure of the atmosphere to drive the piston down. Using the cylinder as the vessel in which to condense the steam also cooled the cylinder, so that some of the heat in the incoming steam on the next cycle was lost in warming the cylinder, reducing the thermal efficiency. Improvements made by <a href="John_Smeaton" title="John Smeaton">John Smeaton</a> to the Newcomen engine increased the efficiency to over 1%.

</p><p><a href="James_Watt" title="James Watt">James Watt</a> made several improvements to the <a href="Newcomen_atmospheric_engine" title="Newcomen atmospheric engine">Newcomen engine</a>, the most significant of which was the external condenser, which prevented the cooling water from cooling the cylinder. Watt's engine operated with steam at slightly above atmospheric pressure. Watt's improvements increased efficiency by a factor of over 2.5.<sup id="cite_ref-16" class="reference"><a href="#cite_note-16"><span class="cite-bracket">[</span>16<span class="cite-bracket">]</span></a></sup>

The lack of general mechanical ability, including skilled mechanics, <a href="Machine_tool" title="Machine tool">machine tools</a>, and manufacturing methods, limited the efficiency of actual engines and their design until about 1840.<sup id="cite_ref-17" class="reference"><a href="#cite_note-17"><span class="cite-bracket">[</span>17<span class="cite-bracket">]</span></a></sup>

</p><p>Higher-pressured engines were developed by <a href="Oliver_Evans" title="Oliver Evans">Oliver Evans</a> and <a href="Richard_Trevithick" title="Richard Trevithick">Richard Trevithick</a>, working independently. These engines were not very efficient but had high <a href="Power-to-weight_ratio" title="Power-to-weight ratio">power-to-weight ratio</a>, allowing them to be used for powering locomotives and boats.

</p><p>The <a href="Centrifugal_governor" title="Centrifugal governor">centrifugal governor</a>, which had first been used by Watt to maintain a constant speed, worked by throttling the inlet steam, which lowered the pressure, resulting in a loss of efficiency on the high (above atmospheric) pressure engines.<sup id="cite_ref-18" class="reference"><a href="#cite_note-18"><span class="cite-bracket">[</span>18<span class="cite-bracket">]</span></a></sup> Later control methods reduced or eliminated this pressure loss.

</p><p>The improved valving mechanism of the <a href="Corliss_steam_engine" title="Corliss steam engine">Corliss steam engine</a> (Patented. 1849) was better able to adjust speed with varying load and increased efficiency by about 30%. The Corliss engine had separate valves and headers for the inlet and exhaust steam so the hot feed steam never contacted the cooler exhaust ports and valving. The valves were quick acting, which reduced the amount of throttling of the steam and resulted in faster response. Instead of operating a throttling valve, the governor was used to adjust the valve timing to give a variable steam cut-off. The variable cut-off was responsible for a major portion of the efficiency increase of the Corliss engine.<sup id="cite_ref-Hunter_1985_19-0" class="reference"><a href="#cite_note-Hunter_1985-19"><span class="cite-bracket">[</span>19<span class="cite-bracket">]</span></a></sup>

</p><p>Others before Corliss had at least part of this idea, including <a href="Zachariah_Allen" title="Zachariah Allen">Zachariah Allen</a>, who patented variable cut-off, but lack of demand, increased cost and complexity and poorly developed machining technology delayed introduction until Corliss.<sup id="cite_ref-Hunter_1985_19-1" class="reference"><a href="#cite_note-Hunter_1985-19"><span class="cite-bracket">[</span>19<span class="cite-bracket">]</span></a></sup>

</p><p>The Porter-Allen high-speed engine (ca. 1862) operated at from three to five times the speed of other similar-sized engines. The higher speed minimized the amount of condensation in the cylinder, resulting in increased efficiency.<sup id="cite_ref-Hunter_1985_19-2" class="reference"><a href="#cite_note-Hunter_1985-19"><span class="cite-bracket">[</span>19<span class="cite-bracket">]</span></a></sup>

</p><p><a href="Compound_engine" title="Compound engine">Compound engines</a> gave further improvements in efficiency.<sup id="cite_ref-Hunter_1985_19-3" class="reference"><a href="#cite_note-Hunter_1985-19"><span class="cite-bracket">[</span>19<span class="cite-bracket">]</span></a></sup> By the 1870s triple-expansion engines were being used on ships. Compound engines allowed ships to carry less coal than freight.<sup id="cite_ref-20" class="reference"><a href="#cite_note-20"><span class="cite-bracket">[</span>20<span class="cite-bracket">]</span></a></sup> Compound engines were used on some locomotives but were not widely adopted because of their mechanical complexity.

</p><p>A very well-designed and built steam locomotive used to get around 7–8% efficiency in its heyday.<sup id="cite_ref-seEfficiency_21-0" class="reference"><a href="#cite_note-seEfficiency-21"><span class="cite-bracket">[</span>21<span class="cite-bracket">]</span></a></sup> The most efficient reciprocating steam engine design (per stage) was the <a href="Uniflow_steam_engine" title="Uniflow steam engine">uniflow engine</a>, but by the time it appeared steam was being displaced by diesel engines, which were even more efficient and had the advantages of requiring less labor (for coal handling and oiling), being a more dense fuel, and displaced less cargo.

</p>

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</style><blockquote class="templatequote"><p>Using statistics collected during the early 1940s, the <a href="Atchison%2C_Topeka_and_Santa_Fe_Railway" title="Atchison, Topeka and Santa Fe Railway">Santa Fe Railroad</a> measured the efficiency of their fleet of steam locomotives in comparison with the FT units that they were just putting into service in significant numbers. They determined that the cost of a ton of oil fuel used in steam engines was $5.04 and yielded 20.37 train miles system wide on average. Diesel fuel cost $11.61 but produced 133.13 train miles per ton. In effect, diesels ran six times as far as steamers utilizing fuel that cost only twice as much. This was due to the much better thermal efficiency of diesel engines compared to steam. Presumably the trains used as a milage standard were 4,000 ton freight consists which was the normal [tonnage] at that time.</p></blockquote><div class="templatequotecite"><p style="display: inline; padding-left: 2.3em;">— Jim Valle, "How efficient is a steam engine?"<sup id="cite_ref-seEfficiency_21-1" class="reference"><a href="#cite_note-seEfficiency-21"><span class="cite-bracket">[</span>21<span class="cite-bracket">]</span></a></sup></p></div>

<div class="mw-heading mw-heading4"><h4 id="Steam_turbine">Steam turbine</h4></div>

<p>The <a href="Steam_turbine" title="Steam turbine">steam turbine</a> is the most efficient steam engine and for this reason is universally used for electrical generation. Steam expansion in a turbine is nearly continuous, which makes a turbine comparable to a very large number of expansion stages. Steam <a href="Fossil_fuel_power_station" title="Fossil fuel power station">power stations</a> operating at the <a href="Critical_point_(thermodynamics)" title="Critical point (thermodynamics)">critical point</a> have efficiencies in the low 40% range. Turbines produce direct rotary motion and are far more compact and weigh far less than reciprocating engines and can be controlled to within a very constant speed. As is the case with the gas turbine, the steam turbine works most efficiently at full power, and poorly at slower speeds. For this reason, despite their high power to weight ratio, steam turbines have been primarily used in applications where they can be run at a constant speed. In AC electrical generation maintaining an extremely constant turbine speed is necessary to maintain the correct frequency.

</p>

<div class="mw-heading mw-heading3"><h3 id="Stirling_engines">Stirling engines</h3></div>

<p>The <a href="Stirling_engine" title="Stirling engine">Stirling engine</a> has the highest theoretical efficiency of any thermal engine but it has a low output power to weight ratio, therefore Stirling engines of practical output tend to be large. The size effect of the Stirling engine is due to its reliance on the expansion of a gas with an increase in temperature and practical limits on the working temperature of engine components. For an ideal gas, increasing its absolute temperature for a given volume, only increases its pressure proportionally, therefore, where the low pressure of the Stirling engine is atmospheric, its practical pressure difference is constrained by temperature limits and is typically not more than a couple of atmospheres, making the piston pressures of the Stirling engine very low, hence relatively large piston areas are required to obtain useful output power.

</p>

<ul><li><a href="Chrysler_Turbine_Car" title="Chrysler Turbine Car">Chrysler Turbine Car</a> (1963)</li>

<li><a href="Fuel_efficiency" title="Fuel efficiency">Fuel efficiency</a></li>

<li><a href="Specific_fuel_consumption_(shaft_engine)" class="mw-redirect" title="Specific fuel consumption (shaft engine)">Specific fuel consumption (shaft engine)</a></li>

<li><a href="Specific_impulse" title="Specific impulse">Specific impulse</a></li></ul>

<div class="mw-heading mw-heading2"><h2 id="References">References</h2></div>

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<li id="cite_note-17"><span class="mw-cite-backlink"><b><a href="#cite_ref-17">^</a></b></span> <span class="reference-text"><cite id="CITEREFRoe1916" class="citation book cs1">Roe, Joseph Wickham (1916). <a rel="nofollow" class="external text" href="https://archive.org/details/englishandameri01roegoog"><i>English and American Tool Builders</i></a>. New Haven, Connecticut: Yale University Press. <a href="ISBN_(identifier)" class="mw-redirect" title="ISBN (identifier)">ISBN</a> <bdi>978-0-917914-73-7</bdi>. <a href="LCCN_(identifier)" class="mw-redirect" title="LCCN (identifier)">LCCN</a> <a rel="nofollow" class="external text" href="https://lccn.loc.gov/16011753">16011753</a>.</cite> <span class="cs1-hidden-error citation-comment"><code class="cs1-code">{{cite book}}</code>: </span><span class="cs1-hidden-error citation-comment">ISBN / Date incompatibility (help)</span>. Reprinted by McGraw-Hill, New York and London, 1926 (<a href="LCCN_(identifier)" class="mw-redirect" title="LCCN (identifier)">LCCN</a> <a rel="nofollow" class="external text" href="https://www.loc.gov/item/27024075">27-24075</a>); and by Lindsay Publications, Inc., Bradley, Illinois, (<a href="ISBN_(identifier)" class="mw-redirect" title="ISBN (identifier)">ISBN</a> <bdi>978-0-917914-73-7</bdi>).</span>

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<li id="cite_note-18"><span class="mw-cite-backlink"><b><a href="#cite_ref-18">^</a></b></span> <span class="reference-text"><cite id="CITEREFBenett1986" class="citation book cs1">Benett, Stuart (1986). <i>A History of Control Engineering 1800-1930</i>. Institution of Engineering and Technology. <a href="ISBN_(identifier)" class="mw-redirect" title="ISBN (identifier)">ISBN</a> <bdi>978-0-86341-047-5</bdi>.</cite></span>

</li>

<li id="cite_note-Hunter_1985-19"><span class="mw-cite-backlink">^ <a href="#cite_ref-Hunter_1985_19-0"><sup><i><b>a</b></i></sup></a> <a href="#cite_ref-Hunter_1985_19-1"><sup><i><b>b</b></i></sup></a> <a href="#cite_ref-Hunter_1985_19-2"><sup><i><b>c</b></i></sup></a> <a href="#cite_ref-Hunter_1985_19-3"><sup><i><b>d</b></i></sup></a></span> <span class="reference-text"><cite id="CITEREFHunter1985" class="citation book cs1">Hunter, Louis C. (1985). <i>A History of Industrial Power in the United States, 1730-1930, Vol. 2: Steam Power</i>. Charlottesville: University Press of Virginia.</cite></span>

</li>

<li id="cite_note-20"><span class="mw-cite-backlink"><b><a href="#cite_ref-20">^</a></b></span> <span class="reference-text"><cite id="CITEREFWells1891" class="citation book cs1">Wells, David A. (1891). <a rel="nofollow" class="external text" href="https://archive.org/details/recenteconomicc01wellgoog"><i>Recent Economic Changes and Their Effect on Production and Distribution of Wealth and Well-Being of Society</i></a>. New York: D. Appleton and Co. <a href="ISBN_(identifier)" class="mw-redirect" title="ISBN (identifier)">ISBN</a> <bdi>0-543-72474-3</bdi>.</cite> <span class="cs1-hidden-error citation-comment"><code class="cs1-code">{{cite book}}</code>: </span><span class="cs1-hidden-error citation-comment">ISBN / Date incompatibility (help)</span></span>

</li>

<li id="cite_note-seEfficiency-21"><span class="mw-cite-backlink">^ <a href="#cite_ref-seEfficiency_21-0"><sup><i><b>a</b></i></sup></a> <a href="#cite_ref-seEfficiency_21-1"><sup><i><b>b</b></i></sup></a></span> <span class="reference-text"><cite id="CITEREFJ._Fallon2011" class="citation web cs1">J. Fallon (10 January 2011). <a rel="nofollow" class="external text" href="http://cs.trains.com/trn/f/740/t/185359.aspx">"How efficient is a steam engine?"</a>. Trains.com<span class="reference-accessdate">. Retrieved <span class="nowrap">25 April</span> 2017</span>.</cite></span>

</li>

</ol></div></div>

<div class="mw-heading mw-heading2"><h2 id="External_links">External links</h2></div>

<ul><li><a rel="nofollow" class="external text" href="http://www.viragotech.com/fixit/FuelEconomyEngineEfficiencyPower.html">Fuel Economy, Engine Efficiency & Power</a></li></ul></div><div><div class="zim-footer">

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