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Automotive basics

The need for a transmission in an automobile is a consequence of the characteristics of the internal combustion engine. Engines typically operate over a range of 600 to about 6000 revolutions per minute (though this varies from design to design and is typically less for diesel engines), while the car's wheels rotate between 0 rpm and around 2500 rpm.

Furthermore, the engine provides its highest torque outputs approximately in the middle of its range, while often the greatest torque is required when the vehicle is moving from rest or travelling slowly. Therefore, a system that transforms the engine's output so that it can supply high torque at low speeds, but also operate at highway speeds with the motor still operating within its limits, is required. Transmissions perform this transformation.

Most transmissions and gears used in automotive and truck applications are contained in a cast iron case, though sometimes aluminum is used for lower weight. There are three shafts: a mainshaft, a countershaft, and an idler shaft.

The mainshaft extends outside the case in both directions: the input shaft towards the engine, and the output shaft towards the rear axle (on rear wheel drive cars). The shaft is suspended by the main bearings, and is split towards the input end. At the point of the split, a pilot bearing holds the shafts together. The gears and clutches ride on the mainshaft, the gears being free to turn relative to the mainshaft except when engaged by the clutches.

Manual transmission

Manual transmissions come in two basic types: a simple unsynchronized system where gears are spinning freely and must be synchronized by the operator to avoid noisy and damaging "gear clash", and synchronized systems that will automatically "mesh" while changing gears. The former type is only used on some rally cars nowadays.

Manual transmissions dominate the car market outside of North America. They are cheaper, lighter, usually give better performance and fuel efficiency (although the latest sophisticated automatic transmissions may yeld results slightly closer to the ones yelded by Manual transmissions) and it is customary for new drivers to learn, and be tested, on a car with a manual gearchange. In Germany, the UK, and France at least, a test pass using an automatic car does not entitle the driver to use a manual car on the public road unless a second manual test is taken. In most of the other European nations like Italy and The Netherlands, obtaining a drivers licence is only possible by passing a drivers test driving a car with manual transmission.

Automatic transmission

Most modern North American cars have an automatic transmission that will select an appropriate gear ratio without any operator intervention. They are primarily using hydraulics to select gears, depending on pressure exerted by fluid within the transmission assembly. Rather than using a clutch to engage the transmission, a torque converter is put in between the engine and transmission. It is possible for the driver to control the number of gears in use or select reverse, though precise control of which gear is in use is usually not possible.

Automatic transmissions are easy to use. In the past, automatic transmissions of this type have had a number of problems, they were complex and expensive, and sometimes had reliability problems (which sometimes caused more expense in repair), and often have been less fuel-efficient than their manual counterparts. With the advancement of modern automatic transmissions this has changed. With computer technology, considerable effort has been put into designing gearboxes based on the simpler manual systems that use electronically-controlled actuators to shift gears and manipulate the clutch, resolving many of the drawbacks of a hydraulic automatic transmission.

Automatic transmissions have always been extremely popular in the United States, where perhaps 19 of 20 new cars are sold with them (many vehicles are not available with manual gearboxes anymore). In Europe automatic transmissions are gaining popularity as well.

Attempts to improve the fuel efficiency of automatic transmissions include the use of torque converters which lock-up beyond a certain speed eliminating power loss, and overdrive gears which automatically actuate above certain speeds; in older transmissions both technologies could sometimes become intrusive, when conditions are such that they constantly cut in and out as speed and such load factors as grade or wind vary slightly. Current computerized transmissions possess very complex programming to both maximize fuel efficiency and eliminate any intrusiveness.

For certain applications, the slippage inherent in automatic transmissions can be advantageous; for instance, in drag racing, the automatic transmission allows the car to be stopped with the engine at a high rpm (the "stall speed") to allow for a very quick launch when the brakes are released; in fact, a common modification is to increase the stall speed of the transmission. This is even more advantageous for turbocharged engines, where the turbocharger needs to be kept spinning at high rpm by a large flow of exhaust in order to keep the boost pressure up and eliminate the turbo lag that occurs when the engine is idling and the throttle is suddenly opened.

Semi-automatic transmission

The creation of computer control also allowed for a sort of half-breed transmission where the car handles manipulation of the clutch automatically, but the driver can still select the gear manually if desired. This is sometimes called "clutchless manual". Many of these transmissions allow the driver to give full control to the computer.

There are some specific types of this transmission, including Tiptronic, Geartronic and Direct Shift Gearbox.

There are also sequential transmission which use the rotation of a drum to switch gears. A great example of this is the 7-speed sequential transmission on the Bugatti Veyron, a supercar that puts out 1,001 horsepower (746 kW) and goes 254 miles per hour (409 km/h).

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