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25-643: In the 1930s DKW pioneered the Schnürle two-stroke loop scavenging process to dispense with the use of a deflector piston and improve efficiency of the combustion chamber. DKW also developed a highly efficient arrangement of transfer ports . These two features were included in the RT ;125 to great commercial advantage. Competitor companies such as Adler and TWN copied the adoption of flat-topped pistons and strove to develop equally efficient transfer port arrangements without infringing DKW's patent . Copies of

50-433: Is a design in which the fresh intake charge and exhaust gases flow in the same direction. This requires that the intake and exhaust ports be at opposite ends of the cylinder. As used by some two-stroke engines, the fresh charge enters through piston-controlled ports near the bottom of the cylinder and flows upward, pushing the exhaust gases out through poppet valves located in the cylinder head. Other uniflow engines - such as

75-422: Is a design used by most modern valveless two-stroke engines. The key difference compared to crossflow scavenging is that the transfer ports are located either side of the exhaust port and aimed at the opposite cylinder wall. As the fuel/air mixture enters the combustion chamber, it travels across the cylinder then up the cylinder wall opposite the exhaust port before looping over at the cylinder head and back down to

100-432: Is a misspelling. Adolf Schnürle was a prolific engineer and is named on many patent documents. 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 remaining exhaust gases can cause improper combustion for

125-427: Is a system to improve efficiency of a valveless two-stroke engine by giving better scavenging . The intake and exhaust ports cut in the cylinder wall are shaped to give a more efficient transfer of intake and exhaust gases. Gas flow within the two-stroke engine is even more critical than for a four-stroke engine , as the exhaust flow exits the chamber as the intake enters simultaneously. A well-defined flow pattern

150-484: Is required, avoiding any turbulent mixing . The efficiency of the two-stroke engine depends on effective scavenging , the more complete replacement of the old spent charge with a fresh charge. Apart from large diesels with separate superchargers , two-stroke engines are generally piston-ported and use their crankcase beneath the piston for compression. The cylinder has a transfer port (inlet from crankcase to cylinder) and an exhaust port cut into it. These are opened, as

175-751: The Second World War , DKW's factories in Zschopau were in the Soviet occupation zone . As such, they were under the control of the Soviet Union until they were handed over to the government of East Germany . The factory continued production of the RT 125 under the MZ ( Motorradwerk Zschopau ) brand into the 1950s. Meanwhile, DKW had reorganized itself in Ingolstadt , where it began production of

200-713: The Allies, the company reverse engineered it as the basis for their first motorcycle. Nippon Gakki formed the Yamaha Motor Company in 1955 to build their copy of the RT 125 as the Yamaha YA-1 , which was in production from 1955 to 1958. The YA-1 inherited design characteristics of RT 125 and, due to its thin body and chestnut brown tank, was affectionately nicknamed the Aka-tombo ( 赤トンボ , "Red Dragonfly") . Schnuerle porting Schnuerle porting

225-467: 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

250-716: The RT 125 (under SHL 125 and Sokół 125 brands), developed into 125/175 cc family motorcycles, produced until 1985. RT 125 plans were also taken to the United Kingdom where they became the basis of the BSA Bantam , and to the USA where they formed the basis of the Harley-Davidson " Hummer " (Hummer is really just a few specific years, but generally people call the Harley lightweights Hummers). After

275-581: The RT 125 were built by at least eight different entities in at least six countries. After World War II the Soviet Union took plans, tooling and even several dozen personnel as war reparations to MMZ in Moscow (later transferred to MMVZ and SMZ) and to a factory in Kovrov , and produced copies of the RT 125 as the M1A Moskva and K-125 respectively. WFM of Poland made a modified version of

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300-400: The RT 125W (for "West") in 1949. Variants of the RT 125W, usually with larger engines, were in production throughout the 1950s. In the 1950s, after reestablishing themselves as manufacturers of musical instruments, Nippon Gakki decided to use the manufacturing equipment left over from wartime production to make motorcycles. Since the copyright on the RT 125 had been voided by

325-637: The Ricardo Dolphin marine engine - use a downward flow direction, with the fresh air/fuel mixture entering at the top of the cylinder and the exhaust gases exiting at towards the bottom of the cylinder. Yet another design uses piston-controlled ports at both ends of the cylinder and two opposed pistons in each cylinder moving in opposite directions to compress the charge between them. The uniflow method of scavenging has been often used for two-stroke diesel engines in motor vehicles, marine vessels, railway locomotives and as stationary engines. Its drawback

350-413: 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. As Schnuerle porting encourages flow in a loop, it is termed " loop scavenging ". Historically, the deflector piston form of cross scavenging

375-416: The combustion chamber. This arrangement had the advantage of simplicity, but it also directed the incoming charge directly towards the exhaust port. To improve the emptying of the cylinder of exhaust gases and retain more of the incoming charge in the cylinder, a deflector piston was often used. This piston shape directed the intake gases towards the top of the cylinder to push the exhaust fumes down and out

400-433: The cylinder, the exhaust port through which the combustible mix leaves. The first engines deliberately designed to encourage scavenging were gas engines built by Crossley Brothers Ltd in the United Kingdom in the early 1890s. These Crossley Otto Scavenging Engines were made possible by the recent change from slide valves to poppet valves , which allowed more flexible control over valve timing events. The closing of

425-466: The exhaust pipe is long enough to contain the gas slug for the entire duration of the stroke. As the Crossley engine was so slow-revving, this resulted in an exhaust pipe with a length of 65 feet (20 m) between the engine and its cast-iron 'pot' silencer. Crossflow cylinder heads are used by most modern 4-stroke engines, whereby the intake ports are located on one side of the combustion chamber and

450-431: The exhaust port. This long flow path and opposite directions of intake and exhaust flows minimizes the mixing of the fresh and spent gases and limits the amount of fresh charge which escapes the cylinder prior to the ports closing. This scavenging method does require a greater understanding of the 3-dimensional gas flow in the cylinder and more care in the placement, size, and angle of the various ports. Uniflow scavenging

475-475: The exhaust port. However, the deflector piston was not very effective in practice - much of the gas flow took a shortcut path and still failed to reach the top of the cylinder - and the shape of the piston compromised the shape of the combustion chamber by causing long flame paths and excessive surface area. Therefore, vertical loop scavenging is rarely used in modern two-stroke engines. Schnuerle scavenging (sometimes called "loop scavenging" or "reverse scavenging")

500-444: The exhaust ports are on the other side. The momentum of the gases assists in scavenging during the 'overlap' phase (when the intake and exhaust valves are simultaneously open). For two-stroke engines, crossflow scavenging was used in early crankcase-compression engines, such as used by small motorcycles. The transfer port (where the fuel/air mixture enters the combustion chamber) and the exhaust port were located on opposite sides of

525-448: The exhaust valve occurred more than 30 degrees later than on earlier engines, giving a long 'overlap' period (when both the intake and exhaust valves are open). As these were gas engines they did not require a long period of valve closure during the compression stroke. The exhaust gases were drawn from the engine by a partial vacuum following in the wake of a 'slug' of exhaust gas from the previous combustion cycle. This method requires that

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550-446: The next cycle, leading to reduced power output. Scavenging is equally important for both two-stroke and four-stroke engines . Most modern four-stroke engines use crossflow cylinder heads and valve timing overlap to scavenge the cylinders. Modern two-stroke engines use either Schnuerle scavenging (also known as "loop scavenging") or uniflow scavenging. The scavenge or scavenging port refers to that port through which clean air enters

575-424: 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 " 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

600-437: 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,

625-500: 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. The system is named after its inventor, Adolf Schnürle  [ de ] . Either "Schnürle" or the more common Anglicisation as "Schnuerle" are generally acceptable. It also appears as "Schnürrle", but "Schneurle"

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