Editing Two-stroke engine (section)

===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.