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There are three basic types of engines found in use on motorcycles. They are the two stroke, four stroke, and to a lesser degree, the wankel rotary engine. In todays world, most motorcycles are four stroke and make use of cutting edge technology to produce very high rates of horsepower. If you strip off the computer's, sensors, fuel injectors, and liquid cooling, delete a few valves from each combustion chamber, and add some metal to the crank and pistons, you end up with the type of four stroke engine found in classic motorcycles.
When the Japanese began taking over the motorcycle market, circa 1960, all but Honda made exclusive use of the two stroke across their respective product lines. Keep in mind that most all of these bikes were 250cc and under. Honda, on the other hand, vowed not to build two strokes or "stink wheels", and offered a full line with four stroke engines.
Most of the two strokes were of the rotary disk valve type, where the carburetor was mounted on the side of the engine, and as a rotating disk opens, the fuel mixture enters the crank case, and from there, gets processed like any other two stroke. This style of two stroke was dropped by Yamaha and Suzuki in favour of a newer design, known as piston port. Kawasaki, on the other hand, continued to use, and refine the rotary disk engine, and it was very sucessful for them. At one point, just about every motorcycle in the Kawasaki line up was a rotary disk type, and the final model produced with the disk was the KE-100, which was phased out in 2001. With other models from Kawasaki, the switch was made first to piston port, then to reed valve in the early to mid 1970's. The advantage of the disk was that the fuel was delivered directly to the crank case, rather than having to travel through ports to get to the crank case, and in theory, made the process faster allowing the engine to produce horsepower faster. From personal experience, they were right, but I also found that the engine would peak early, allowing the piston port equipped engine to pass it on the top end of the power curve. In the world of street bikes circa mid to late 1960's, Kawasaki and Bridgestone motorcycles used a multi cylinder, multi disk engine set up that allowed for some very fast, very powerful motorcycles. It was also very widely used in the road racing curcuit into the 1970's.
Yamaha and Suzuki moved onto piston port two stroke engines. In this configuration, the carburetor was mounted on the back of the cylinder. The fuel charge would get sucked into the intake port by vacuum created in the crank case, by the up stroke of the piston. Once the fuel charge gets to the crank case, it gets processed like any other two stroke. The two stroke engine is illustrated below. Click on the illustration for animation.
 The next big step was the introduction of the reed valve by Yamaha in 1974, followed by Suzuki in 1977. Yamaha went the piston port, reed valve route, branded as "tourque induction" , where a reed valve was mounted between the carburetor and the intake port. It was a vast improvement over the piston port design allowing the engine to produce more bottom end tourque, and also keeping the fuel charge from back firing out the carburetor, sometimes igniting the air filter. In time this system would be copied industry wide. Suzuki introduced its reed valve system in 1977. Known as "case reed induction" , their reed was mounted on the bottom of the cylinder at the end of the intake port, and worked equally as well as the piston port system. the only drawback, was that to get access to the valve, the cylinder had to be removed from the engine block, which caused issues with the racing bikes. If a reed petal needed replacing mid race, your race was over. if you were riding a Yamaha, it was a five minute fix, simply because of location and accessibility.
With four stroke engines , there are many variants. The earlier ones made use of a two valve cylinder head, with a single cam shaft either operating rocker arms or push rods to open the valves. The rocker arm system usually meant that the camshaft was positioned above the cylinder head, and was known as " overhead cam, or ohc". As time went on, some models had two cams mounted over head, and were known as " double overhead cam or dohc". In the case of the push rod system, the cam shaft was located in the crank case, near the crank shaft. As the cam rotates its lobes push up on rods, which in turn operate a series of rocker arms, which open the valves at the correct time. The cam shaft is usually driven by the crank shaft by chain, and is timed to it to allow the valves to open at the correct time. The advent of the DOHC engine did away with rocker arms as the lobes directly pushed open the valves from above. This system made use of an inverted bucket over each valve stem, and a series of shims, which were used to set the clearance. While it was warmly embraced by the super bike crowd, and did away with several moving parts, it also made setting valve clearances much more difficult and time consuming. The four stroke engine is illustrated below. Click the illustration for animation.
 In the 1950's, Dr Felix Wankel designed a totally new type of combustion engine, and infact, was the only new type put into production in the 20th century. It was developed at N.S.U. in Germany and was used in automobiles, motorcycles, small equipment, and in aircraft. Basically you have a three sided disk that rotates eccentrically on a shaft, inside a specially shaped combustion chamber. Apex seals are used to seal the three voids created by the disk. Without them, the result would be blow by, and the engine would run poorly if at all. What made this engine so revolutionary is the fact that it produced smooth power from idle to a what seemed like an unlimited top end. Unlike a piston engine which loose momentum when the piston reverses direction, the rotary just continues to spin. Some drawbacks to it is the fact that it generated tremendous heat when in operation due to the fact that it doesn't take in a large volume of cool air every cycle like a piston engine. Another issue were the apex seal wearing out, as well as the chamber loosing its shape due to wear. Much of this was the result of expecting too much of the materials available at the time, which have improved tenfold since the 1950's.
In terms of it's use in motorcycles, Van Veen, Hercules, Suzuki, Norton and Yamaha have produced or built prototypes. Suzuki produced the RE-5 , beginning in 1975, and ending production in 1976, which nearly bankrupt the firm. The bikes very heavy, and ugly, and didn't catch on with prospected buyers. Yamaha built a prototype, but it never went into production. Mazda continued to use and improve the rotary engine in their RX series of automobiles, and with years of research and developement, and modern materials, the rotary engine is reliable. Please click the drawing below for animation.
 NOTE: The illustrations on this page were found at wikipedia . I give full credit for them to their respective creators and use them here to help people better understand the contents of this page. If you click on these illustrations they are animated and give you a realtime look at what goes on in a running engine. ' |
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