Editing Two-stroke engine (section)

===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piston cooling and achieving an effective combustion chamber shape more difficult is why this design has been largely superseded by uniflow scavenging after the 1960s, especially for motorcycles, but for smaller or slower engines using direct injection, the deflector piston can still be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>