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Automotive Transmissions: Efficiently Transferring Power from Engine to Wheels
(Released January 2008)

 
  by Chao-Hsu Yao  

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Before the steam engine was invented, all of the physically demanding jobs like construction, agriculture, shipping, and even traveling, were done by strong animals or human beings themselves. The invention of the steam engine prompted the Industrial Revolution, at which time human beings started using automated machines to reduce human work load and increase job efficiency. In 1705 Thomas Newcomen invented the first version of the steam engine, which is also called atmospheric engine [2]. Figure 1 shows the animation of how the Newcomen steam engine works. From this design, water (blue) is boiled and vaporized into steam (pink), which pushes the closed right valve (red) open (green). The steam pushes the piston to move up, which causes the pressure inside the cylinder to decrease. Gravity will push the water from the upper tank to open the left valve, and splash the water into the cylinder to cool steam. The steam inside the cylinder therefore is condensed, which turns the cylinder vacuum and sucks back the piston. The descending piston shuts two valves and finishes one cycle.

steam engine diagram
Figure 1: Newcomen Steam Engine.
The Newcomen Steam Engine was only used to pump water out of mines at that time. In 1769, James Watt improved the function of the steam engine and made it practical in the real world [1], which is why most people still think Watt invented the steam engine.

James Watt's steam engine is designed so that water goes into a high temperature boiler, is boiled and vaporized, and turns into high pressure steam. This steam pushes the piston, generating a forward and backward motion (see http://static.howstuffworks.
com/flash/steam-engine.swf
for an animated picture) [3]. Because the combustion room is located outside the engine, the steam engine is also called the external combustion engine.

According to the physics rule of motion, when an object is in static status it needs a larger force to overcome friction. When the object starts moving, the needed driving force becomes smaller and smaller, and the speed becomes faster and faster. Therefore, to move the piston in a steam engine from static position, very high pressure must be generated to push the piston. When the piston starts moving, the pressure decreases, because it is released from the exhaust by the movement of the piston, before it can be compressed into high pressure air. At low speed, the engine creates high pressure steam to push the piston, while at high speed, the steam pressure becomes low. That's why the old steam powered locomotives start very slowly, but still can reach a very high speed.

The steam engine is very efficient at generating power based on the physics rule of motion; however, it takes awhile before the machine can reach its highest efficiency. Another drawback is that the steam engine occupies too much space. Therefore, scientists tried to develop an engine with smaller size, but that can instantly generate the power needed. The internal combustion engine, which has been used for most machinery including vehicles, was invented. Several kinds of internal combustion engines have been widely used for vehicles, for example, in the two-stroke combustion cycle, four-stroke combustion cycle, and rotary engines. The first engine to use a four-stroke combustion cycle successfully was built in 1867 by N. A. Otto [9]. The design of the internal combustion engine is much more complicated than the steam engine, however. All internal combustion engines need to go through the following procedures to finish the combustion cycle: intake, compression, combustion, and exhaust. The shock wave file http://static.howstuffworks.com/flash/engine.swf [11] shows how the four-stroke internal combustion engine works. First, gasoline comes from 'C' and moves the piston downward. Second, the piston moves upward and compresses the air. Third, the compressed air is fired and moves the piston downward again. Finally, the fired air is exhausted through 'L' and moves the piston upward again. While fired once every two cycles for a four-stroke cycle internal combustion engine, a two-stroke combustion cycle internal engine is fired once per cycle, which can be seen on the shock wave file http://static.howstuffworks.com/flash/two-stroke.swf [10]. The internal combustion design can instantly convert the power generated by the explosion of burning fuel into high pressure air to push the piston. Unlike the steam engine, for an internal combustion engine to move the piston faster and faster, more and more fuel is needed to generate higher pressure. In other words, for an internal combustion engine, high pressure is needed to keep the piston running at a high speed, while at low speed, only low pressure is necessary. This is just opposite to the function of the steam engine.

Even though it solves the dimension and slow start issues of the steam engine, the internal combustion engine generates another serious problem. When the piston is running at high speed, the pressure needed is also high, which violates the physics rule of motion. Running an engine at high speed with high pressure is not efficient, and also decreases the engine life. To solve this problem, the transmission system was invented.

To transfer engine power efficiently, the gear ratio between the engine and wheels plays a very important role. When we use a screwdriver, the portion we hold has a larger diameter, while the portion contacting with the screw has smaller diameter. This design makes users use less force to unscrew a screw while applying force on a larger diameter portion of the screw driver. Therefore, attaching a smaller gear to the engine side and connecting it to a larger gear to deliver power to wheels helps overcome friction when moving a static vehicle. Figure 2 shows that the large gear of the wheels needs less force to drive it. However, it also shows that when the engine gear turns one circle, the wheel gear only turns about one half. The car won't run as fast as possible.

gears from engine to wheels
Figure 2

Consider the following situation from Figure 3: the wheel gear has a smaller size, which needs more force to move it while the car is static. It won't even be possible to move the car if the engine power is not large enough. However, when the engine gear turns 1 cycle, the wheel gear may turn 2, which makes the car run faster.

gears from engine to wheels
Figure 3

Based on the physics rule of motion, after the object starts moving, the driving force needed becomes smaller. Therefore, if the car can run on the large gear condition (Figure 2) when starting, but change to a small gear (Figure 3) when moving, that is, applying a large force when starting, but a small force when moving, this will makes the power transmission much more efficient.

Go To Different Kinds of Transmission Systems Used for the Automobile

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