Schnuerle porting is a system to improve efficiency of a valveless

two-stroke engine A two-stroke (or two-stroke cycle) engine is a type of internal combustion engine that completes a power cycle with two strokes (up and down movements) of the piston during one power cycle, this power cycle being completed in one revolution of t ...

by giving better

scavenging Scavengers are animals that consume dead organisms that have died from causes other than predation or have been killed by other predators. While scavenging generally refers to carnivores feeding on carrion, it is also a herbivorous feeding be ...

. The intake and exhaust ports cut in the

cylinder A cylinder (from ) has traditionally been a three-dimensional solid, one of the most basic of curvilinear geometric shapes. In elementary geometry, it is considered a prism with a circle as its base. A cylinder may also be defined as an infin ...

wall are shaped to give a more efficient transfer of intake and exhaust gases.

Description

Gas flow within the two-stroke engine is even more critical than for a

four-stroke engine A four-stroke (also four-cycle) engine is an internal combustion (IC) engine in which the piston completes four separate strokes while turning the crankshaft. A stroke refers to the full travel of the piston along the cylinder, in either directio ...

, as the two flows are both entering and leaving the combustion chamber simultaneously. A well-defined flow pattern is required, avoiding any

turbulent mixing In fluid dynamics, turbulence or turbulent flow is fluid motion characterized by chaotic changes in pressure and flow velocity. It is in contrast to a laminar flow, which occurs when a fluid flows in parallel layers, with no disruption between ...

. The efficiency of the two-stroke engine depends on effective

, the more complete replacement of the old spent charge with a fresh charge. Apart from large

diesel Diesel may refer to: * Diesel engine, an internal combustion engine where ignition is caused by compression * Diesel fuel, a liquid fuel used in diesel engines * Diesel locomotive, a railway locomotive in which the prime mover is a diesel engin ...

s with separate

supercharger In an internal combustion engine, a supercharger compresses the intake gas, forcing more air into the engine in order to produce more power for a given displacement. The current categorisation is that a supercharger is a form of forced induct ...

s, two-stroke engines are generally piston-ported and use their crankcase beneath the piston for the compression needed to fill the cylinder with fuel/air mixture. The cylinder has a transfer port (inlet from crankcase to cylinder) and an exhaust port cut into it. These are opened, as the piston moves downwards past them; with the higher exhaust port opening earlier as the piston descends; and closing later as the piston rises. The simplest arrangement is a single transfer and single exhaust port, opposite each other. This "

cross scavenging Scavenging is the process of replacing the exhaust gas in a cylinder of an internal combustion engine with the fresh air/fuel mixture (or fresh air, in the case of direct-injection engines) for the next cycle. If scavenging is incomplete, the ...

" performs poorly, as there is tendency for the flow to pass from the inlet directly to the exhaust, wasting some of the fuel mixture and also poorly scavenging the upper part of the chamber. Before Schnuerle porting, a deflector on top of the piston was used to direct the gas flow from the transfer port upwards, in a U-shaped loop around the combustion chamber roof and then down and out through the exhaust port. Apart from the gas flow never quite following this ideal path and tending to mix instead, this also gave a poorly shaped combustion chamber with long, thin flame paths. In 1926, the German engineer Adolf Schnürle developed the system of ports that bears his name. The ports were relocated to both be on the same side of the cylinder, with the transfer port being split into two angled ports, one on either side of the exhaust port. A deflector piston was no longer required. The gas flow was now a circular loop, flowing in and across the piston crown from the transfer ports, up and around the combustion chamber and then out through the exhaust port. With Schnuerle porting, the piston crown may be of any shape, even bowl shaped. This permits a far better combustion chamber shape and flame path, giving better combustion, particularly at high speeds.

Loop scavenging

As Schnuerle porting encourages flow in a loop, it is termed " loop scavenging". Historically, the deflector piston form of cross scavenging was termed "loop scavenging", after the supposed shape of the flow. Schnuerle flow was termed "reverse loop scavenging". As the first of these was realised to be inaccurate, the later form adopted the simpler name. These original terms are now obsolete and no longer used.

Adolf Schnürle

The system is named after its inventor, Adolf Schnürle. Either "Schnürle" or the more common

Anglicisation Anglicisation is the process by which a place or person becomes influenced by English culture or British culture, or a process of cultural and/or linguistic change in which something non-English becomes English. It can also refer to the influen ...

as "Schnuerle" are generally acceptable. It also appears as "Schnürrle", but "Schneurle" is a misspelling. Adolf Schnürle was a prolific engineer and is named on many patent documents.

References

{{Reflist Two-stroke engine technology