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

 
  by Chao-Hsu Yao  

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Comparison of Transmission Systems

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The traditional automatic transmission system is much more complicated than the manual transmission system. It occupies more space, so normally a small car can easily be designed up to 5-speed manually, but only up to 3- or 4-speed automatically. The automatic transmission system is also heavier than the manual transmission system, which wastes more gasoline. The fluid torque converter also loses some power transferred from the engine due to indirect gear contact. The worst part of the automatic transmission is that it is easily broken due to its complicated design, and when there is a problem it is very costly to fix. However, an automatic transmission system is easy for a new driver to learn to use quickly, because engaging/disengaging the clutch by foot is not necessary, and the car will never stall during gear shifting. The CVT uses solid coupling instead of fluid coupling, which not only solves the power loss problem but also prevents the clutch from engaging/disengaging to perform the gear ratio change. However, the CVT is also heavy, and the strength of the driving belt is the critical issue which determines the CVT's performance. The infinite number of gear ratios makes gear change smooth, and the result is better fuel efficiency. However, some drivers who enjoy quick acceleration may not like the CVT's slow and smooth acceleration.

Two different semi-automatic transmission systems are the most advanced transmission systems used in race cars and luxury cars. They use computers to perform gear shift faster than conventional manual transmission, and also with better fuel economy. However, the direct shift transmission has too many machine parts, and also a complex clutch set, which makes the gear box heavy, and easily broken. The high repair cost is also an issue. The electrohydraulic manual transmission has a simpler gear structure with a rotating drum to perform gear ratio change. The drum can be rotated manually, or by computer control. This makes the gear shift extremely fast, and due to its simple structure, the gear box is not heavy, and only occupies a small space. That is why it's also used for manual transmission on motorcycles. However, if you need to make a sudden stop at high speed, it's impossible to shift from 6th gear directly to 1st gear. You will need to go through all gears from 5th to 2nd. Without a clutch design, the gear shift is not smooth. That's why it is only used for some high speed race cars like Toyota MR2 and Ferrari, which only consider speed but not comfort.

Go To The Future Development of Automotive Transmission Systems

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