Editing Diesel engine (section)

==Operating principle==

=== Overview ===
The characteristics of a diesel engine are<ref name="Pischinger_2016_348" />
* Use of [[compression ignition]], instead of an ignition apparatus such as a [[Spark-ignition engine|spark plug]].
* Internal mixture formation. In diesel engines, the mixture of air and fuel is only formed inside the combustion chamber.
* Quality torque control. The amount of torque a diesel engine produces is not controlled by throttling the intake air (unlike a traditional spark-ignition petrol engine, where the airflow is reduced in order to regulate the torque output), instead, the volume of air entering the engine is maximised at all times, and the torque output is regulated solely by controlling the amount of injected fuel.
* High [[Air–fuel ratio#Air–fuel equivalence ratio (λ)|air-fuel ratio]]. Diesel engines run at global air-fuel ratios significantly leaner than the [[Stoichiometry#Stoichiometric ratio|stoichiometric ratio]].
* [[Diffusion flame]]: At combustion, oxygen first has to diffuse into the flame, rather than having oxygen and fuel already mixed before combustion, which would result in a [[premixed flame]].
* [[Heterogeneous]] air-fuel mixture: In diesel engines, there is no even dispersion of fuel and air inside the cylinder. That is because the combustion process begins at the end of the injection phase, before a homogeneous mixture of air and fuel can be formed.
* Preference for the fuel to have a high ignition performance ([[Cetane number]]), rather than a high knocking resistance ([[octane rating]]) that is preferred for petrol engines.

=== Thermodynamic cycle ===
{{technical|section|date=July 2022}}
[[File:Model Engine Luc Viatour.jpg|thumb|Diesel engine model, left side]]
[[File:Model Engine B Luc Viatour.jpg|thumb|Diesel engine model, right side]]
{{See also|Diesel cycle|Reciprocating internal combustion engine}}

The diesel internal combustion engine differs from the gasoline powered [[Otto cycle]] by using highly compressed hot air to ignite the fuel rather than using a spark plug (''compression ignition'' rather than ''spark ignition'').

In the diesel engine, only air is initially introduced into the combustion chamber. The air is then compressed with a compression ratio typically between 15:1 and 23:1. This high compression causes the temperature of the air to rise. At about the top of the compression stroke, fuel is injected directly into the compressed air in the combustion chamber. This may be into a (typically [[toroid]]al) void in the top of the piston or a ''pre-chamber'' depending upon the design of the engine. The fuel injector ensures that the fuel is broken down into small droplets, and that the fuel is distributed evenly. The heat of the compressed air vaporises fuel from the surface of the droplets. The vapour is then ignited by the heat from the compressed air in the combustion chamber, the droplets continue to vaporise from their surfaces and burn, getting smaller, until all the fuel in the droplets has been burnt. Combustion occurs at a substantially constant pressure during the initial part of the power stroke. The start of vaporisation causes a delay before ignition and the characteristic diesel knocking sound as the vapour reaches ignition temperature and causes an abrupt increase in pressure above the piston (not shown on the P-V indicator diagram). When combustion is complete the combustion gases expand as the piston descends further; the high pressure in the cylinder drives the piston downward, supplying power to the [[crankshaft]].

As well as the high level of compression allowing combustion to take place without a separate ignition system, a high [[compression ratio]] greatly increases the engine's efficiency. Increasing the compression ratio in a spark-ignition engine where fuel and air are mixed before entry to the cylinder is limited by the need to prevent [[pre-ignition#Pre-ignition|pre-ignition]]<!-- detonation from occurring after ignition and is more a fuel quality or combustion chamber design issue -->, which would cause engine damage. Since only air is compressed in a diesel engine, and fuel is not introduced into the cylinder until shortly before top dead centre ([[Top Dead Center|TDC]]), premature detonation is not a problem and compression ratios are much higher.

[[File:DieselCycle PV.svg|thumb|right|pV diagram for the ideal diesel cycle (which follows the numbers 1–4 in clockwise direction). The horizontal axis is the cylinder volume. In the diesel cycle the combustion occurs at almost constant pressure. On this diagram the work that is generated for each cycle corresponds to the area within the loop.]]
The [[pressure–volume diagram]] (pV) diagram is a simplified and idealised representation of the events involved in a diesel engine cycle, arranged to illustrate the similarity with a [[Carnot cycle]]. Starting at 1, the piston is at bottom dead centre and both valves are closed at the start of the compression stroke; the cylinder contains air at atmospheric pressure. Between 1 and 2 the air is compressed adiabatically&nbsp;– that is without heat transfer to or from the environment&nbsp;– by the rising piston. (This is only approximately true since there will be some heat exchange with the [[Cylinder (engine)|cylinder walls]].) During this compression, the volume is reduced, the pressure and temperature both rise. At or slightly before 2 (TDC) fuel is injected and burns in the compressed hot air. Chemical energy is released and this constitutes an injection of thermal energy (heat) into the compressed gas. Combustion and heating occur between 2 and 3. In this interval the pressure remains constant since the piston descends, and the volume increases; the temperature rises as a consequence of the energy of combustion. At 3 fuel injection and combustion are complete, and the cylinder contains gas at a higher temperature than at 2. Between 3 and 4 this hot gas expands, again approximately adiabatically. Work is done on the system to which the engine is connected. During this expansion phase the volume of the gas rises, and its temperature and pressure both fall. At 4 the exhaust valve opens, and the pressure falls abruptly to atmospheric (approximately). This is unresisted expansion and no useful work is done by it. Ideally the adiabatic expansion should continue, extending the line 3–4 to the right until the pressure falls to that of the surrounding air, but the loss of efficiency caused by this unresisted expansion is justified by the practical difficulties involved in recovering it (the engine would have to be much larger). After the opening of the exhaust valve, the exhaust stroke follows, but this (and the following induction stroke) are not shown on the diagram. If shown, they would be represented by a low-pressure loop at the bottom of the diagram. At 1 it is assumed that the exhaust and induction strokes have been completed, and the cylinder is again filled with air. The piston-cylinder system absorbs energy between 1 and 2&nbsp;– this is the work needed to compress the air in the cylinder, and is provided by mechanical kinetic energy stored in the flywheel of the engine. Work output is done by the piston-cylinder combination between 2 and 4. The difference between these two increments of work is the indicated work output per cycle, and is represented by the area enclosed by the pV loop. The adiabatic expansion is in a higher pressure range than that of the compression because the gas in the cylinder is hotter during expansion than during compression. It is for this reason that the loop has a finite area, and the net output of work during a cycle is positive.<ref name="Reif_2014_18" />

=== Efficiency ===

The [[fuel efficiency]] of diesel engines is better than most other types of combustion engines,<ref name="Dubbel_1981_712" /><ref name="Reif_2014_10" /> due to their high compression ratio, high [[air-fuel ratio#Air–fuel equivalence ratio (λ)|air–fuel equivalence ratio (λ)]],<ref name="Pischinger Kell Sams p. 137–138">{{Cite book |last1=Pischinger |first1=Rudolf |title=Thermodynamik der Verbrennungskraftmaschine |last2=Kell |first2=Manfred |last3=Sams |first3=Theodor |date=2009 |publisher=Springer-Verlag |isbn=978-3-211-99277-7 |publication-place=Wien |pages=137–138 |language=de |oclc=694772436}}</ref> and the lack of intake air restrictions (i.e. throttle valves). Theoretically, the highest possible efficiency for a diesel engine is 75%.<ref name="Hemmerlein_1991" /> However, in practice the efficiency is much lower, with efficiencies of up to 43% for passenger car engines,<ref name="vB_2017_755" /> up to 45% for large truck and bus engines, and up to 55% for large two-stroke marine engines.<ref name="Reif_2014_13" /><ref name="EPA_2004" /> The average efficiency over a motor vehicle driving cycle is lower than the diesel engine's peak efficiency (for example, a 37% average efficiency for an engine with a peak efficiency of 44%).<ref name="Soimar_2000" /> That is because the fuel efficiency of a diesel engine drops at lower loads, however, it does not drop quite as fast as the Otto (spark ignition) engine's.<ref name="Karle p. 53">{{Cite book |last=Karle |first=Anton |title=Elektromobilität Grundlagen und Praxis; mit 21 Tabellen |date=2015 |isbn=978-3-446-44339-6 |publication-place=München |page=53 |language=de |oclc=898294813}}</ref>

=== Emissions ===
{{see also|Diesel exhaust}}
Diesel engines are combustion engines and, therefore, emit combustion products in their [[exhaust gas]]. Due to incomplete combustion,<ref name="List_1939_1" /> diesel engine exhaust gases include [[carbon monoxide]], [[hydrocarbons]], [[Particulates|particulate matter]], and [[nitrogen oxides]] pollutants. About 90 per cent of the pollutants can be removed from the exhaust gas using exhaust gas treatment technology.<ref name="Dubbel_2018_1191" /><ref name="Reif p. 329">{{Cite book |last=Reif |first=Konrad |title=Diesel engine management : systems and components |date=2014 |publisher=Springer-Verlag |isbn=978-3-658-03981-3 |publication-place=Wiesbaden |page=329 |oclc=884504346}}</ref> Road vehicle diesel engines have no [[sulfur dioxide]] emissions, because motor vehicle diesel fuel has been sulfur-free since 2003.<ref name="Reif p. 331">{{Cite book |last=Reif |first=Konrad |title=Diesel engine management : systems and components |date=2014 |publisher=Springer-Verlag |isbn=978-3-658-03981-3 |publication-place=Wiesbaden |page=331 |oclc=884504346}}</ref> Helmut Tschöke argues that particulate matter emitted from motor vehicles has negative impacts on human health.<ref name="Tschöke Mollenhauer Maier p. 813">{{Cite book |last1=Tschöke |first1=Helmut |title=Handbuch Dieselmotoren |last2=Mollenhauer |first2=Klaus |last3=Maier |first3=Rudolf |date=2018 |publisher=Springer Vieweg |isbn=978-3-658-07697-9 |publication-place=Wiesbaden |page=813 |language=de |oclc=1011252252}}</ref>

The particulate matter in diesel exhaust emissions is sometimes classified as a [[carcinogen]] or "probable carcinogen" and is known to increase the risk of heart and respiratory diseases.<ref>{{Cite web |title=What Are Diesel Emissions? Diesel Engine Exhaust Emissions |url=https://www.nettinc.com/information/emissions-faq/what-are-diesel-emissions |access-date=9 July 2022 |website=www.NettTechnologies.com}}</ref>

=== Electrical system ===
In principle, a diesel engine does not require any sort of electrical system. However, most modern diesel engines are equipped with an electrical fuel pump, and an electronic engine control unit.

However, there is no high-voltage electrical ignition system present in a diesel engine. This eliminates a source of [[Electromagnetic interference|radio frequency emissions]] (which can interfere with navigation and communication equipment), which is why only diesel-powered vehicles are allowed in some parts of the American [[National Radio Quiet Zone]].<ref name="NRAO" />

===Torque control===
To control the torque output at any given time (i.e. when the driver of a car adjusts the [[accelerator pedal]]), a [[governor (device)|governor]] adjusts the amount of fuel injected into the engine. Mechanical governors have been used in the past, however electronic governors are more common on modern engines. Mechanical governors are usually driven by the engine's [[Serpentine belt|accessory belt]] or a gear-drive system<ref name="buckman" /><ref name="Rochester" /> and use a combination of springs and weights to control fuel delivery relative to both load and speed.<ref name=buckman/> Electronically governed engines use an [[electronic control unit]] (ECU) or electronic control module (ECM) to control the fuel delivery. The ECM/ECU uses various sensors (such as engine speed signal, intake manifold pressure and fuel temperature) to determine the amount of fuel injected into the engine.

Due to the amount of air being constant (for a given RPM) while the amount of fuel varies, very high ("lean") air-fuel ratios are used in situations where minimal torque output is required. This differs from a petrol engine, where a [[throttle]] is used to also reduce the amount of intake air as part of regulating the engine's torque output. Controlling the timing of the start of injection of fuel into the cylinder is similar to controlling the ignition timing in a petrol engine. It is therefore a key factor in controlling the power output, fuel consumption and exhaust emissions.