Content deleted Content added
→Lubrication: Qualify the main part of this section. (It comes directly after the diesel section, and is followed by another mention of sump lubrication.) |
→Direct injection: Corrected punctuation Tags: Mobile edit Mobile web edit |
||
| (28 intermediate revisions by 26 users not shown) | |||
Line 1:
{{Short description|Internal combustion engine type}}
{{redirect|2-cycle|the mathematical concept|Cyclic permutation}}
[[File:Two-Stroke Engine.gif|thumb|Animation of a two-stroke engine]]
A '''two-stroke''' (or '''two-stroke cycle''') '''engine''' is a type of [[internal combustion engine]] that completes a [[Thermodynamic power cycle|power cycle]] with two strokes
Two-stroke engines often have a high [[power-to-weight ratio]] due to having power strokes twice as often as a four-stroke engine. Two-stroke engines can also have fewer [[moving parts]] than four-stroke engines, and thus be cheaper to manufacture and weigh less. In countries and regions with stringent emissions regulation, two-stroke engines have been phased out in automotive and motorcycle uses. In regions where regulations are less stringent, small displacement two-stroke engines remain popular in [[moped]]s and motorcycles.<ref>{{Cite web |title=Docker Maroc |url=https://docker.ma/ |access-date=2023-04-12 |language=fr-FR}}</ref> They are also used in power tools such as [[Chainsaw|chainsaws]] and [[Leaf blower|leaf blowers]].
== History ==
The first commercial two-stroke engine involving cylinder compression is attributed to [[Scotland|Scottish]] engineer [[Dugald Clerk]], who patented his design in 1881.<ref>See:
* Clerk, Dugald
* Clerk, Dugald [https://pdfpiw.uspto.gov/.piw?Docid=00249307 "Motor worked by combustible gas or vapor,"] U.S. patent no. 249,307 (filed: September 2, 1881
* Day, Joseph
* Day, Joseph
* Day, Joseph [https://pdfpiw.uspto.gov/.piw?docid=00543614 "Gas-engine"] US patent no. 543,614 (filed: May 21, 1892
* {{cite journal |last1=Torrens |first1=Hugh S. |title=A study of 'failure' with a 'successful innovation': Joseph Day and the two-stroke internal combustion engine |journal=Social Studies of Science |date=May 1992 |volume=22 |issue=2 |pages=245–262|doi=10.1177/030631292022002004 |s2cid=110285769 }}</ref><ref>Joseph Day's engine used a reed valve. One of Day's employees, Frederic Cock (1863–1944), found a way to render the engine completely valve-less. See:
* Cock, Frederic William Caswell
* Cock, Frederic William Caswell [https://pdfpiw.uspto.gov/.piw?docid=00544210 "Gas-engine"] US patent no. 544,210 (filed: March 10, 1894
* The Day-Cock engine is illustrated in: {{cite journal |last1=Dowson |first1=Joseph Emerson |title=Gas-power for electric lighting: Discussion |journal=Minutes of Proceedings of the Institution of Civil Engineers |date=1893 |volume=112 |pages=2–110 |doi=10.1680/imotp.1893.20024 |url=https://babel.hathitrust.org/cgi/pt?id=hvd.hxgrpu&view=1up&seq=60}}
Two-stroke [[gasoline]] engines with electrical [[spark ignition]] are particularly useful in lightweight or portable applications such as [[chainsaws]] and motorcycles. However, when weight and size are not an issue, the cycle's potential for high [[thermodynamic efficiency]] makes it ideal for [[diesel fuel|diesel]] [[compression ignition]] engines operating in large, weight-insensitive applications, such as [[marine propulsion]], [[locomotive|railway locomotives]], and [[Diesel generator|electricity generation]]. In a two-stroke engine, the exhaust gases transfer less heat to the cooling system than a four-stroke, which means more energy to drive the piston, and if present, a turbocharger.
Line 22 ⟶ 23:
== Emissions ==
Crankcase-compression two-stroke engines, such as common small gasoline-powered engines, are lubricated by a [[petroil]] mixture in a [[total-loss oiling system|total-loss system]]. Oil is mixed in with their petrol fuel beforehand, in a fuel-to-oil ratio of around 32:1. This oil then forms emissions, either by being burned in the engine or as droplets in the exhaust, historically resulting in more exhaust emissions, particularly hydrocarbons, than four-stroke engines of comparable power output. The combined opening time of the intake and exhaust ports in some two-stroke designs can also allow some amount of unburned fuel vapors to exit in the exhaust stream. The high combustion temperatures of small, air-cooled engines may also produce [[NOx|NO<sub>x</sub>]] emissions.
==Applications==
Line 30 ⟶ 29:
[[File:BritishSeagull2.JPG|thumb|right|upright|Lateral view of a two-stroke Forty series [[British Seagull]] [[outboard engine]], the serial number dates it to 1954/1955]]
Two-stroke gasoline engines are preferred when mechanical simplicity, light weight, and high [[power-to-weight ratio]] are design priorities.
A number of mainstream automobile manufacturers have used two-stroke engines in the past, including the Swedish [[Saab Automobile|Saab]]
Two-stroke engines are still found in a variety of small propulsion applications, such as [[outboard motor]]s, small on- and [[dirt bike|off-road]] [[motorcycles]], [[moped]]s, [[scooter (motorcycle)|scooter]]s, [[motorized bicycle]]s, [[tuk-tuk]]s, [[snowmobile]]s, [[
Due to their high power-to-weight ratio and ability to be used in any orientation, two-stroke engines are common in handheld outdoor power tools including [[
Two-stroke [[diesel engine]]s are found mostly in large industrial and marine applications, as well as some trucks and heavy machinery.
==Designs==
[[File:Kunmadaras Motorsport 2021. szeptember 19. JM (69).jpg|thumb|Two-stroke motorbike with an expansion chamber exhaust system that increases the cylinder charge]]
Line 46 ⟶ 45:
Although the principles remain the same, the mechanical details of various two-stroke engines differ depending on the type. The design types vary according to the method of introducing the charge to the cylinder, the method of scavenging the [[cylinder (engine)|cylinder]] (exchanging burnt exhaust for fresh mixture) and the method of exhausting the cylinder.
===
====Piston-controlled inlet port====
[[Piston]] port is the simplest of the designs and the most common in small two-stroke engines. All functions are controlled solely by the piston covering and uncovering the ports as it moves up and down in the cylinder. In the 1970s, [[Yamaha Motor Company|Yamaha]] worked out some basic principles for this system. They found that, in general, widening an exhaust port increases the power by the same amount as raising the port, but the power band does not narrow as it does when the port is raised. However, a mechanical limit exists to the width of a single exhaust port, at about 62% of the bore diameter for reasonable piston ring life. Beyond this, the piston rings bulge into the exhaust port and wear quickly. A maximum 70% of bore width is possible in racing engines, where rings are changed every few races. Intake duration is between 120 and 160°. Transfer port time is set at a minimum of 26°. The strong, low-pressure pulse of a racing two-stroke expansion chamber can drop the pressure to -7 psi when the piston is at bottom dead center, and the transfer ports nearly wide open. One of the reasons for high fuel consumption in two-strokes is that some of the incoming pressurized fuel-air mixture is forced across the top of the piston, where it has a cooling action, and straight out the exhaust pipe. An expansion chamber with a strong reverse pulse stops this outgoing flow.<ref>{{cite book |first=Gordon |last=Jennings
A fundamental difference from typical four-stroke engines is that the two-stroke's [[crankcase]] is sealed and forms part of the induction process in gasoline and [[hot bulb engine]]s. Diesel two-strokes often add a [[Roots blower]] or piston pump for scavenging.▼
▲A fundamental difference from typical four-stroke engines is that the two-stroke's [[crankcase]] is sealed and forms part of the induction process in gasoline and [[hot
===Reed inlet valve===▼
▲====Reed inlet valve====
{{Main article|Reed valve}}
[[File:Old Cox Babe Bee engine dissasembled.JPG|thumb|upright=1.0|A [[Cox Models|Cox]] Babe Bee {{convert|0.049|in3|cm3|abbr=on}} reed valve engine, disassembled, uses glow-plug ignition. Its mass is 64 g.]]
The reed valve is a simple but highly effective form of [[check valve]] commonly fitted in the intake tract of the piston-controlled port. It allows asymmetric intake of the fuel charge, improving power and economy, while widening the power band. Such valves are widely used in motorcycle, ATV, and marine outboard engines.
====Rotary inlet valve====
The intake pathway is opened and closed by a rotating member. A familiar type sometimes seen on small motorcycles is a slotted disk attached to the [[crankshaft]], which covers and uncovers an opening in the end of the crankcase, allowing charge to enter during one portion of the cycle (called a disc valve).
Line 64 ⟶ 65:
Rotary valve engines can be tailored to deliver power over a wider speed range or higher power over a narrower speed range than either a piston-port or reed-valve engine. Where a portion of the rotary valve is a portion of the crankcase itself, of particular importance, no wear should be allowed to take place.
===
====Cross-flow scavenging====
[[File:Two-stroke deflector piston (Autocar Handbook, 13th ed, 1935).jpg|thumb|upright|[[Deflector piston]] with cross-flow scavenging]]
In a cross-flow engine, the transfer and exhaust ports are on opposite sides of the cylinder, and a [[deflector piston|deflector]] on the top of the piston directs the fresh intake charge into the upper part of the cylinder, pushing the residual [[exhaust gas]] down the other side of the deflector and out the exhaust port.<ref name="Irving, 13" >
Line 77 ⟶ 79:
}}</ref> The deflector increases the piston's weight and exposed surface area, and the fact that it makes piston cooling and achieving an effective combustion chamber shape more difficult is why this design has been largely superseded by uniflow scavenging after the 1960s, especially for motorcycles, but for smaller or slower engines using direct injection, the deflector piston can still be an acceptable approach.
====Loop scavenging====<!-- This section is linked from [[Exhaust pulse pressure charging]] -->
[[File:Ciclo del motore 2T.svg|upright=1.3|thumb|The two-stroke cycle
{{ordered list
Line 91 ⟶ 93:
This method of scavenging uses carefully shaped and positioned transfer ports to direct the flow of fresh mixture toward the combustion chamber as it enters the cylinder. The fuel/air mixture strikes the [[cylinder head]], then follows the curvature of the combustion chamber, and then is deflected downward.
This not only prevents the fuel/air mixture from traveling directly out the exhaust port, but also creates a swirling turbulence which improves [[combustion efficiency]], power, and economy.
Usually, a piston deflector is not required, so this approach has a distinct advantage over the cross-flow scheme (above).
Line 108 ⟶ 110:
Work published at SAE in 2012 points that loop scavenging is under every circumstance more efficient than cross-flow scavenging.
====Uniflow scavenging====
<!-- This section is linked from [[Exhaust pulse pressure charging]]
-->
[[File:Diesel engine uniflow.svg|thumb|upright=0.5|Uniflow scavenging]]▼
<gallery mode=packed heights=150px widths=200px>
[[File:Ciclo del motore 2T unidirezionale.svg|right|thumb|The uniflow two-stroke cycle▼
File:Uniflow 2-stroke diesel animation.gif|Two-stroke diesel uniflow engine animation
</gallery>
{{ordered list
| Top dead center (TDC)
Line 119 ⟶ 126:
{{legend|#639eff|B: Exhaust}}
{{legend|#ffae21|C: Compression}}
{{legend|#f00|D: Expansion (power)}}]]
In a uniflow engine, the mixture, or "charge air" in the case of a diesel, enters at one end of the cylinder controlled by the piston and the exhaust exits at the other end controlled by an exhaust valve or piston. The scavenging gas-flow is, therefore, in one direction only, hence the name uniflow.
===Stepped piston engine===
The piston of this engine is "top-hat"-shaped; the upper section forms the regular cylinder, and the lower section performs a scavenging function. The units run in pairs, with the lower half of one piston charging an adjacent combustion chamber.
The upper section of the piston still relies on total-loss lubrication, but the other engine parts are sump lubricated with cleanliness and reliability benefits. The mass of the piston is only about 20% more than a loop-scavenged engine's piston because skirt thicknesses can be less.
==Power-valve systems==
Line 135 ⟶ 143:
==Direct injection==
{{Main article|Gasoline direct injection#In two-stroke engines}}
Direct injection has considerable advantages in two-stroke engines. In carburetted two-strokes, a major problem is a portion of the fuel/air mixture going directly out, unburned, through the exhaust port, and direct injection effectively eliminates this problem. Two systems are in use
Since the fuel does not pass through the crankcase, a separate source of lubrication is needed.
{{See also|API-TC}}
Line 205 ⟶ 191:
==External links==
* {{
* [http://science.howstuffworks.com/two-stroke.htm Two-Stroke Engine] at [[How Stuff Works]]
* {{Citation |last=Sherman |first=Don |date=December 17, 2009 |title=A Two-Stroke Revival, Without the Blue Haze |url=https://www.nytimes.com/2009/12/20/automobiles/20STROKE.html |work=[[The New York Times]] |ref=none}}.
| |||