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introduction | Two-Stroke Motorcycle Engines:
In a Two-Stroke engine the piston goes down, compressing the fuel mixture under the piston and blowing it into the cylinder. As this mixture blows in it also blows the burnt exaust gases out. The fuel mixture is blown into the cylinder through passages ( Ports ) in the cylinder walls. The piston comes up, covering the ports in the cylinder walls and compressing or squeezing the mixture. This also creates a vacuum in the crankcase under the piston, sucking the fuel mixture into the crankcase. The spark then ignites the mixture and the burning gases push the piston down, starting everything again.
This is all done in two strokes of the piston.
Piston down... Piston up.
Two-Strokes.
Image Courtesy of Wikipedia |
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characteristics | Two-Stroke Single Cylinder Engines
Description: Powerful with lots of torque, inexpensive to manufacturer, high RPM's and light in weight for its little size they were popular in sport bikes and dirt bikes till 1985. Their one major disadvantage, pollution, killed them over the four-strokes since both oil and gas are burned at the same time to make the motorcycle engine run.
Two-Stroke Parallel Twin
Description: More power, more torque, more speed, more RPM range… unfortunately as with the Two-Stroke Single, the manufacturers could not get over its pollution, they did give it a great effort but lost to very stringent government regulations. Produced up to 500cc sizes, they were probably the best bike engines designed for their weight, efficiency and small size. |
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history | Walter Kaaden, the chief engineer for MZ motorcycles in the 1950s, is generally considered the father of the modern two stroke expansion chamber. He reportedly said "You'll know when you have the design right, because the chamber will then be impossible to fit on the motorcycle without having it drag the ground, burn the rider's leg, or force the relocation of one or more major components."
Ernst Degner was a famous Eastern-German motorcycle Grand Prix racer who rode for the East German manufacturer MZ/DKW from 1956-61, and also a trained engineer who had played an important development role for the make. He had a real chance of winning the 125 cm3 World Championship for MZ in 1961, but instead he committed Landesflucht - he moved to West-Germany, it is believed he escaped from the East Germany in the boot of a car - just before the last race of the season, the Argentine GP and so the title went to Australian Tom Phillis. He was signed by Suzuki who probably took with him the intimate knowledge and designs of the competitive bikes developed by MZ/DKW, and went on in his Grand Prix career, racing for the Japanese team in the World Motorcycle Championship until 1966. Degner won the 50 cm3 World Motorcycle Championship in 1962, and in the same year he was the winner of the 50 cm3 race at the Isle of Man Tourist Trophy, at his debut in the Mountain circuit. During all his career, Degner achieved 59 starts with 7 wins in 50 cm3 class, and 8 wins in 125 cm3 class. Degner’s post-racing career was held in West Germany, where he provided in service technical training to Suzuki dealership staff. He died on 10 September 1983 at the age of 51.
The fast right bend before the underpass at Suzuka circuit was named Degner Curve after him: it was the spot at which Ernst Degner crashed his Suzuki while leading the 250 cm3 in a All-Japan event in 1966, which effectively ended his career. Many historical sources give the reason for the official designation of the Suzuka's Degner Curve as "rider Degner was killed at Suzuka". Degner actually survived his fiery crash, naming the corner after him was really just a matter of Japanese graciousness. |
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tuning | Two strokes are all about the exhaust. When combustion occurs in a two stroke, the piston is pushed down by the expanding gases uncovering the exhaust port. The exhaust gas rushes out into the header pipe and a pressure wave is created. This wave can be either positive or negative in pressure. If it is a positive wave and hits a change in diameter in the tubing of the exhaust pipe it changes to a negative wave. Hit another change in diameter and the wave changes back to positive. This wave pressure can be as high as seven pounds per square inch (Both positive and negative waves) and travels at 1600 to 1700 feet per second.
If the pipe is made just right, a wave with negative pressure reaches the exhaust port just as it is fully open. This sucks the exhaust AND part of the new intake charge into the exhaust pipe. Then, just before the piston closes the exhaust port, this negative wave changes to a positive wave and pushes the fresh intake fuel charge BACK into the cylinder. All this happens in three to four thousands of a second. This sort of supercharges the engine, giving the high power out put of a two stroke engine. Without that properly designed exhaust pipe called an "Expansion Chamber", the two stroke engine will have no real power!
There is only one draw back to all this and that is that this supercharging only happens at the specific RPM that the pipe was designed for. Running the engine at any RPM above or below the designed RPM and the supercharging effect will be much less effective. That is why two strokes have such a narrow power band. |
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