Editing Diesel engine (section)

== Classification ==
There are several different ways of categorising diesel engines, as outlined in the following sections.

=== RPM operating range ===
Günter Mau categorises diesel engines by their rotational speeds into three groups:<ref name="Mau_1984_15" />
* High-speed engines (> 1,000 rpm),
* Medium-speed engines (300–1,000 rpm), and
* Slow-speed engines (< 300 rpm).

; High-speed diesel engines
High-speed engines are used to power [[truck]]s (lorries), [[bus]]es, [[tractor]]s, [[automobile|cars]], [[yacht]]s, [[Gas compressor|compressors]], [[pump]]s and small [[electrical generator]]s.<ref name="Reif_2014_11" /> As of 2018, most high-speed engines have [[Direct fuel injection|direct injection]]. Many modern engines, particularly in on-highway applications, have [[common rail]] [[Direct fuel injection|direct injection]].<ref name="Tschöke_2018_295" /> On bigger ships, high-speed diesel engines are often used for powering electric generators.<ref name="Mau_1984_42" /> The highest power output of high-speed diesel engines is approximately 5&nbsp;MW.<ref name="Mau_1984_43" />

; Medium-speed diesel engines
[[File:12 Cylinder Diesel Engine.jpg|thumb|right|Stationary 12 cylinder turbo-diesel engine coupled to a generator set for auxiliary power]]
Medium-speed engines are used in large electrical generators, railway [[diesel locomotive]]s, ship propulsion and mechanical drive applications such as large compressors or pumps. Medium speed diesel engines operate on either diesel fuel or heavy fuel oil by direct injection in the same manner as low-speed engines. Usually, they are four-stroke engines with trunk pistons;<ref name="Mau_1984_33" /> a notable exception being the [[EMD 567]], [[EMD 645|645]], and [[EMD 710|710]] engines, which are all two-stroke.<ref>{{Cite conference |last=Kettering, E.W. |date=29 November 1951 |title=History and Development of the 567 Series General Motors Locomotive Engine |url=https://books.google.com/books?id=QuUiAQAAMAAJ |conference=ASME 1951 Annual Meeting |location=Atlantic City, New Jersey |publisher=Electro-Motive Division, General Motors Corporation}}</ref>

The power output of medium-speed diesel engines can be as high as 21,870&nbsp;kW,<ref name="Mau_1984_136" /> with the effective efficiency being around 47-48% (1982).<ref name="Mau_1984_121" /> Most larger medium-speed engines are started with compressed air direct on pistons, using an air distributor, as opposed to a pneumatic starting motor acting on the flywheel, which tends to be used for smaller engines.<ref name="Merker_2014_280" />

Medium-speed engines intended for marine applications are usually used to power ([[Roll-on/roll-off|ro-ro]]) ferries, passenger ships or small freight ships. Using medium-speed engines reduces the cost of smaller ships and increases their transport capacity. In addition to that, a single ship can use two smaller engines instead of one big engine, which increases the ship's safety.<ref name="Mau_1984_33" />

; Low-speed diesel engines
[[File:5S50MC.jpg|thumb|right|The MAN B&W 5S50MC, a two-stroke, low-speed, [[straight-five engine|inline five-cylinder]] marine diesel engine on board a 29,000 tonne chemical carrier]]

Low-speed diesel engines are usually very large in size and mostly used to power [[ship]]s. There are two different types of low-speed engines that are commonly used: Two-stroke engines with a crosshead, and four-stroke engines with a regular trunk-piston. Two-stroke engines have a limited rotational frequency and their charge exchange is more difficult, which means that they are usually bigger than four-stroke engines and used to directly power a ship's propeller.

Four-stroke engines on ships are usually used to power an electric generator. An electric motor powers the propeller.<ref name="Mau_1984_15" /> Both types are usually very [[Stroke ratio#Undersquare or long-stroke engine|undersquare]], meaning the bore is smaller than the stroke.<ref name="Mau_1984_129" /> Low-speed diesel engines (as used in ships and other applications where overall engine weight is relatively unimportant) often have an effective efficiency of up to 55%.<ref name="Reif_2014_13" /> Like medium-speed engines, low-speed engines are started with compressed air, and they use heavy oil as their primary fuel.<ref name="Merker_2014_280" />

=== Combustion cycle ===
[[File:Uniflow 2-stroke diesel static.svg|thumb|right|Schematic of a two-stroke diesel engine with a roots blower]]
[[File:Detroit Diesel timing.jpg|thumb|Detroit Diesel timing]]

[[Four-stroke engine]]s use the combustion cycle described earlier. Most smaller diesels, for vehicular use, for instance, typically use the four-stroke cycle. This is due to several factors, such as the two-stroke design's narrow powerband which is not particularly suitable for automotive use and the necessity for complicated and expensive built-in lubrication systems and scavenging measures.<ref name=enginebuilder>{{cite web | url = https://www.enginebuildermag.com/2015/01/cars-get-two-stroke-diesels/ | archive-url = https://web.archive.org/web/20221209084324/https://www.enginebuildermag.com/2015/01/cars-get-two-stroke-diesels/ | archive-date = 2022-12-09 | date = 2015-01-11 | title = Could Our Cars Get Two Stroke Diesels? | first = Sterling | last = Shriber | work = Engine Builder | publisher = Babcox Media Inc. }}</ref> The cost effectiveness (and proportion of added weight) of these technologies has less of an impact on larger, more expensive engines, while engines intended for shipping or stationary use can be run at a single speed for long periods.<ref name=enginebuilder/>

[[Two-stroke diesel engine|Two-stroke engines]] use a combustion cycle which is completed in two strokes instead of four strokes. Filling the cylinder with air and compressing it takes place in one stroke, and the power and exhaust strokes are combined. The compression in a two-stroke diesel engine is similar to the compression that takes place in a four-stroke diesel engine: As the piston passes through bottom centre and starts upward, compression commences, culminating in fuel injection and ignition. Instead of a full set of valves, two-stroke diesel engines have simple intake ports, and exhaust ports (or exhaust valves). When the piston approaches bottom dead centre, both the intake and the exhaust ports are "open", which means that there is atmospheric pressure inside the cylinder. Therefore, some sort of pump is required to blow the air into the cylinder and the combustion gasses into the exhaust. This process is called ''scavenging''. The pressure required is approximately 10-30&nbsp;kPa.<ref name="Mau_1984_50" />

Due to the lack of discrete exhaust and intake strokes, all two-stroke diesel engines use a [[scavenge blower]] or some form of compressor to charge the cylinders with air and assist in scavenging.<ref name="Mau_1984_50" /> Roots-type superchargers were used for ship engines until the mid-1950s, however since 1955 they have been widely replaced by turbochargers.<ref name="Mau_1984_23" /> Usually, a two-stroke ship diesel engine has a single-stage turbocharger with a turbine that has an axial inflow and a radial outflow.<ref name="Mau_1984_pp53" />

==== Scavenging in two-stroke engines====
In general, there are three types of scavenging possible:
* Uniflow scavenging
* Crossflow scavenging
* [[Schnuerle porting|Reverse flow scavenging]]

Crossflow scavenging is incomplete and limits the stroke, yet some manufacturers used it.<ref name="Mau_1984_148" /> Reverse flow scavenging is a very simple way of scavenging, and it was popular amongst manufacturers until the early 1980s. Uniflow scavenging is more complicated to make but allows the highest fuel efficiency; since the early 1980s, manufacturers such as MAN and Sulzer have switched to this system.<ref name="Mau_1984_16" /> It is standard for modern marine two-stroke diesel engines.<ref name="Grote_2018_P93" />

=== Fuel used ===
{{Main|Bi-fuel vehicle#Diesel conversions}}
So-called dual-fuel diesel engines or gas diesel engines burn two different types of fuel ''simultaneously'', for instance, a gaseous fuel and diesel engine fuel. The diesel engine fuel auto-ignites due to compression ignition, and then ignites the gaseous fuel. Such engines do not require any type of spark ignition and operate similar to regular diesel engines.<ref name="Karim_2015_2" /><ref>{{cite web|url=http://www.dualfuel.org/wp-content/uploads/2022/03/DFPS-Brochure.pdf|title=DFPS Brochure|website=dualfuel.org}}</ref>