Read complete abstract The Need for D-Cycle Scooter Engines

Design... In designing the 50cc Scooter Engine, Yan Engines took into account the type of driving and load experienced by a typical scooter in populated areas of Asia. Much of the time, these scooters are engaged in start and stop city traffic and frequently, they can be seen carrying large loads (e.g. multiple people, bags of groceries, etc.). Understanding the type of demand this puts on low-end torque, we have designed our first scooter prototype in the following way…

In the conventional 4-cycle and 2-cycle, the engine size is product of bore and piston strokes and that is 2 times the crank arm. The torque is the product of cylinder force and crank arm. Therefore the engine torque has a direct relationship with the engine size. That is, the bigger the engine is, the higher the torque it produces. This is not true for the D-cycle, since its piston strokes are not tied to the crank arm. A small displacement D-cycle engine can produce high torque and save fuel. This is the design goal of the 50 cc D-cycle scooter.

The current popular 2-cycle 50 cc scooter is an economical and convenient transportation but produces tremendous amount of chemical and noise pollution in many crowded cities. The D-cycle 50 cc is to replace it providing the tens of millions of residents with economical, convenient, clean and quiet transportation.

50cc D-cycle piston-train scooter prototype

D-cycle allows optimal combinations of individual strokes and crank arm. We believe the engine size should be defined as the intake volume, because that determines the amount of fuel consumed. However, we limit the D-cycle’s maximum cylinder volume, which is the power stroke in this design, to 50 cc. The intake is about 30 cc from a baseline 80 cc scooter engine, as shown in the Figure. This design has the following advantages:

First, it has a torque arm equivalent to the baseline 80 cc engine. With a power stroke in every revolution, it can produce torque for driving equivalent to a 160 cc 4-cycle engine.

Second, it utilizes the upper 63% (50/80) of the cylinder for power stroke. The lower part of the cylinder contributes only about 14% of the torque capacity, everything else being equal. The D-cycle efficiency gain more than compensates for this loss. It can produce torque equivalent to its 80 cc baseline in every power stroke, with some charging from the air cooling system.

Third, it has only 30 cc intake stroke. A 2-cycle 50 cc has only about 30 cc intake volumes due to the ports. Since both D-cycle and 2-cycle produce a power stroke in every revolution, the D-cycle can potentially drive like a 2-cycle 50 cc even without charging. It is actually pusher - more torque.

Fourth, the exhaust and intake strokes take up ½ of the cycle time. This is to retain the original valve-train to save initial develop cost of a new valve cam. Investment in new valve cam with associated piston cam designs can further improve performance and efficiency.

Fifth, the exhaust stroke stops below the crank top dead center, retaining some burnt gas. The retained exhaust is used for emission control. This is like an EGR system in the cars, but for free. This makes the scooter inherently cleaner than its 4-cycle counter part.

Performance... The baseline 80 cc scooter peaks both torque and power at about 3200 rpm, with camshaft at 1600 rpm. The D-cycle keeps the same valve-train but with 1:1 gear ratio. The engine needs to operate at 1600 rpm to produce the same peak torque and power. The piston-train operates at about half speed of valve-train, or equal vent to 800 rpm for peak torque and power. Therefore, the 50 cc D-cycle will out-perform the obsolete 50 cc 2-cycle in torque and power, and much cleaner. See Performance Prediction for more information.

 

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