Editing Two-stroke engine (section)

==Designs==


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.

===Inlet port variations===
====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 |title=Two-stroke Tuner's Handbook |chapter=Port timing |pages=75–90 |date=January 1973 |url=https://www.vintagesleds.com/library/manuals/misc/Two-stroke%20Tuner's%20Handbook.pdf |access-date=14 June 2024}}</ref>

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 (engine)|scavenging]].

====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).

Another form of rotary inlet valve used on two-stroke engines employs two cylindrical members with suitable cutouts arranged to rotate one within the other - the inlet pipe having passage to the crankcase only when the two cutouts coincide. The crankshaft itself may form one of the members, as in most glow-plug model engines. In another version, the crank disc is arranged to be a close-clearance fit in the crankcase, and is provided with a cutout that lines up with an inlet passage in the crankcase wall at the appropriate time, as in [[Vespa]] motor scooters.

The advantage of a [[rotary valve]] is that it enables the two-stroke engine's intake timing to be asymmetrical, which is not possible with piston-port type engines. The piston-port type engine's intake timing opens and closes before and after top dead center at the same crank angle, making it symmetrical, whereas the rotary valve allows the opening to begin and close earlier.

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.

===Scavenging variations===
====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" >
{{cite book
 |title=Two-Stroke Power Units
 |last=Irving |first=P.E.
 |author-link=Phil Irving
 |year=1967
 |publisher=Newnes
 |pages=13–15
 |ref={{harvid|Irving|Two stroke power units}}
}}</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
| Top dead center (TDC)
| Bottom dead center (BDC)
}}
{{legend|#10ff00|A: Intake/scavenging}}
{{legend|#639eff|B: Exhaust}}
{{legend|#ffae21|C: Compression}}
{{legend|#f00|D: Expansion (power)}}
]]
{{Main article|Schnuerle porting}}
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).

Often referred to as "Schnuerle" (or "Schnürle") loop scavenging after Adolf Schnürle, the German inventor of an early form in the mid-1920s, it became widely adopted in Germany during the 1930s and spread further afield after [[World War II]].

Loop scavenging is the most common type of fuel/air mixture transfer used on modern two-stroke engines. Suzuki was one of the first manufacturers outside of Europe to adopt loop-scavenged two-stroke engines. This operational feature was used in conjunction with the expansion chamber exhaust developed by German motorcycle manufacturer, MZ, and Walter Kaaden.

Loop scavenging, disc valves, and expansion chambers worked in a highly coordinated way to significantly increase the power output of two-stroke engines, particularly from the Japanese manufacturers Suzuki, Yamaha, and Kawasaki. Suzuki and Yamaha enjoyed success in Grand Prix motorcycle racing in the 1960s due in no small way to the increased power afforded by loop scavenging.

An additional benefit of loop scavenging was the piston could be made nearly flat or slightly domed, which allowed the piston to be appreciably lighter and stronger, and consequently to tolerate higher engine speeds. The "flat top" piston also has better thermal properties and is less prone to uneven heating, expansion, piston seizures, dimensional changes, and compression losses.

SAAB built 750- and 850-cc three-cylinder engines based on a DKW design that proved reasonably successful employing loop charging. The original SAAB 92 had a two-cylinder engine of comparatively low efficiency. At cruising speed, reflected-wave, exhaust-port blocking occurred at too low a frequency. Using the asymmetrical three-port exhaust manifold employed in the identical DKW engine improved fuel economy.

The 750-cc standard engine produced 36 to 42&nbsp;hp, depending on the model year. The Monte Carlo Rally variant, 750-cc (with a filled crankshaft for higher base compression), generated 65&nbsp;hp. An 850-cc version was available in the 1966 SAAB Sport (a standard trim model in comparison to the deluxe trim of the Monte Carlo).
Base compression comprises a portion of the overall compression ratio of a two-stroke engine.
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]] and [[Tuned exhaust]] 
-->
<gallery mode=packed heights=150px widths=200px>
File:Uniflow 2-stroke diesel animation.gif|Two-stroke diesel uniflow engine animation
File:Diesel engine uniflow.svg|Uniflow scavenging flow schematic
</gallery>

[[File:Ciclo del motore 2T unidirezionale.svg|thumb|right|The uniflow two-stroke cycle:
{{ordered list
| Top dead center (TDC)
| Bottom dead center (BDC)
}}
{{legend|#10ff00|A: Intake (effective scavenging, 135°–225°; necessarily symmetric about BDC; Diesel injection is usually initiated at 4° before TDC)}}
{{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.

The design using exhaust valve(s) is common in on-road, off-road, and stationary two-stroke engines ([[Detroit Diesel]]), certain small marine two-stroke engines ([[Gray Marine Motor Company]], which adapted the [[Detroit Diesel Series 71]] for [[Gray Marine 6-71 Diesel Engine|marine use]]), certain railroad two-stroke [[diesel locomotive]]s ([[Electro-Motive Diesel]]) and large marine two-stroke main propulsion engines ([[Wärtsilä]]). Ported types are represented by the [[opposed piston]] design in which two pistons are in each cylinder, working in opposite directions such as the [[Junkers Jumo 205]] and [[Napier Deltic]].<ref>{{cite web|url=http://www.iet.aau.dk/sec2/junkers.htm |title=junkers |publisher=Iet.aau.dk |access-date=2009-06-06 |url-status=dead |archive-url=https://web.archive.org/web/20080501215400/http://www.iet.aau.dk/sec2/junkers.htm |archive-date=May 1, 2008 }}</ref> The once-popular [[split-single]] design falls into this class, being effectively a folded uniflow. With advanced-angle exhaust timing, uniflow engines can be supercharged with a crankshaft-driven blower, either piston or Roots-type.<ref>{{cite web |url=https://www.enginehistory.org/Piston/Before1925/EarlyEngines/J/J.shtml |title=Selected Early Engines: Junkers |first=Kimble D. |last=McCutcheon |date=1 August 2022 |website=Engine History |access-date=14 June 2024 |quote=Junkers built experimental two-stroke opposed-piston diesel aircraft engines during WWI, which were derived from a stationary engine line. These featured two crankshafts at the cylinder ends connected by a gear train that also drove the propeller. Two pistons, working in opposite directions in each cylinder, uncovered inlet and exhaust ports near the ends of their strokes. The exhaust port was uncovered first, and when the inlet port was uncovered, a compressed air charge was forced through the cylinders, practically clearing all burnt gases.}}</ref><ref>{{cite web |url=https://airandspace.si.edu/collection-objects/junkers-jumo-207-d-v2-line-6-diesel-engine/nasm_A19660013000 |title=Junkers Jumo 207 D-V2 In-line 6 Diesel Engine |publisher=National Air and Space Museum |access-date=14 June 2024}}</ref>

===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.<ref>{{cite web | url=https://1library.net/article/stepped-piston-engines-basic-design-parameters-engine-geometry.qvvx8ngq | title=Stepped-Piston Engines - BASIC DESIGN PARAMETERS 3.1 Engine and Port Geometry }}</ref>