Wednesday, August 13, 2008

Thank Gears

TGIW - Thank Gears It’s Wednesday

Today we will learn about gears and how they play a vital role in improving the energy efficiency of vehicles. Not many of us can remember stick shifts that were on the steering column. Most vehicles in the 1940s had three forward gears and a reverse gear. The gear change lever was on the steering column. First gear is called the low gear because it has the lowest gearing ration between the engine and the drive wheel or wheels. The low gearing ratio is needed to commence the acceleration of the vehicle and allow the engine to enjoy the greatest “mechanical advantage”. Mechanical advantage is also used in pulleys and cranes to lift heavy items with the least amount of force. Likewise in a vehicle a low gearing ratio is used to allow the horsepower and torque of the engine to commence the acceleration of the vehicle. As the vehicle gains velocity the gearing ratio can be increased and the engine speed can be lowered to continue to propel the vehicle at a certain speed.

If you watch bicyclists who are climbing a hill, they pedal pretty fast and the bicycle is hardly moving. This is because they have engaged their lowest gear ratio between the sprocket on their pedals and the sprocket on the rear wheel. When the bike is on the flat or downhill the pedals do not have to be turned rapidly for the bicycle to move fast, this is because the chain now engages the smallest sprocket on the rear wheel and the largest sprocket on the pedals and allows one revolution of the pedals to drive multiple revolutions of the rear wheel. The cyclist would not be able to climb a hill while engaged in this high gearing ratio as the mechanical advantage has in fact become mechanical disadvantage and the cyclist is simply not powerful enough to exert sufficie nt force to climb a hill.

A human riding a bicycle has the power of about 50 watts or about one fifteenth of a horse. A Porche has over 300 horsepower so it needs many more forward gears than a 1940s vintage car. During the sixties it was common to have “four on the floor” meaning cars got four speed forward gearboxes and the gear shift lever was moved from the steering column to between the drivers seats. This is also because the front seats changed from being bench seats to two bucket seat. During the seventies and through the end of the last millennium five speed gearboxes were the most common for manual shift vehicles. Now six speed manual gearboxes are common on Porches and other high performance cars.

Automatic gearboxes were widely adopted in the 1960s. They were two speed gearing at first and then for many years were three speed. In the 1990s four and five speed automatic transmissions became available and in this decade more six and seven speed automatic transmissions have been engineered. This allows for a much higher final gearing ratio as well as a much smoother transition through the gears which in turn allows the vehicle to achieve much improved fuel efficiency. Nissan has developed a continuously or infinitely variable (CVT) transmission that does not have a discrete number of gearing ratios but rather has a system that allows the gearing ration to change in minute increments and hence is the ultimate in the smooth transition from low gear to high gear. We can thank gears for the smooth ride of modern vehicles and also for the improvement in fuel efficiency and more rapid acceleration. Nissan claims a 10% improvement in fuel efficiency for a vehicle equipped with CVT over a 4 speed transmission. Only a few of us will thankfully have the experience of “three on the column” where cars shuddered while accelerating and engines whined to gain moderate speed.

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