The transmission system of the vehicle is an elaborate assembly of gears, shafts, and other parts that transfers selected amounts of engine to the vehicles wheels. The transmission enables the vehicle to accelerate forward or backward or to maintain high cruising speeds-all while the engine operates at efficient speeds and within safety limits. In the development of cars, transmission evolution veered the vehicle towards comfort and convenience.
MANUAL TRANSMISSIONIn 1927, Warner Gear introduced a four-speed manual gearbox. The extra gear made high-speed cruising more comfortable due to less noise and vibration, increased fuel economy, and reduced engine wear. Oddly, the H-shaped gear pattern was opposite to today's pattern.
The manual transmission was, by the 1930s, a fairly mature and reliable technology. But the opportunity to improve on its operation took automakers into different directions. An unusual clutch spring was used in the Buick Series 40 for 1939. The cone-shaped spring was claimed to exert a high-pressure on the clutch plates to prevent slipping, while at the same time give a light clutch pedal feel for extended stop-and-go driving. It was an effective, but short-lived solution, doomed by durability problems.
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Chrysler took an even more radical approach, with the adoption of its "fluid drive." Although the vehicle had a clutch pedal, it operated two vaned plates that transmitted power through shearing action in low-viscosity mineral oil. The design allowed drivers to stop without using the clutch, even leaving the gear selector in high. Providing quick acceleration was also not needed. Chrysler touted the design as being easier to drive, quieter, smoother, less prone to stalling and safer. Yet fluid drive flopped.
Traditionally, the transmission was placed directly behind the engine. It delivered the engine power to a drive shaft that powers the rear axle, a metal shaft to which one or more wheels were attached.
The simplest manual transmission, used in some cars, especially the smaller ones, is the sliding-spur gear type with three or more forward speeds and reverse. The desired gear ratio is selected by manipulating a shift lever that slides a spur gear into the proper position to engage the various gears. Early devices of this type required considerable skill on the part of the operator to shift the gears smoothly and without clashing the teeth.
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Ease of shifting was improved by the use of synchronizing clutches that caused the two portions of a positive clutch to turn in unison before the driving and driven gear teeth touched each other during engagement. The only difficulty remaining in the operation of the sliding-gear transmission was the need for simultaneously operating the accelerator pedal, the clutch pedal, and the gearshift lever. The automatic transmission was developed to eliminate this manipulation.
AUTOMATIC TRANSMISSIONAutomatic transmission systems consist of an arrangement of gears, brakes, clutches, a fluid drive, and governing devices that automatically changes the speed ratio between the engine and the wheels of an automobile. Gears are shifted in an automatic transmission by changes in hydraulic pressure, in some case, the pressure of transmission oil created by the impeller. A governor valve routes the pressurized oil to shift valves. The shift valve, in turn, controls the clutches and bands that shift and hold gears in place. Shift valves also contain pistons that move in and out, depending on the amount of hydraulic pressure. As the vehicle's speed changes, the governor valve changes the hydraulic pressure at the appropriate shift valves to cause a gearshift.
Since its introduction in 1939, the fully automatic transmission has become optional or standard equipment on most passenger cars. When the transmission is in the drive position, the driver has only to depress the accelerator pedal, and as the car gathers speed the transmission will shift automatically through its entire forward range of gears from low to high, (i.e., ratios of the speeds of drive shaft and engine shaft) until the two shafts are directly connected through the oil in the fluid drive, which may be either a two-element fluid coupling or a three-element torque converter. When the car loses speed, the transmission automatically shifts back from high to low gear.
It was in 1940 that Oldsmobile first introduced the fully automatic transmission to the car-buying public. That Hydra-matic transmission was manufactured by what was to become the Hydra-matic Division of General Motors. That first automatic brought new levels of comfort and convenience to driving by eliminating the clutch pedal and the need to shift gears.
In 1948, Buick offered the first automatic transmission with a torque converter. The torque converter added a reaction element (or stator) within the fluid coupling, which multiplied engine torque at low engine rpm. This gave the automatic much more take-off power. It also allowed the engine to idle without the need to disengage the transmission.
In 1965, the Hydra-matic Division offered the world's first front-wheel drive transaxle (automatic transmission) in the Oldsmobile Toronado. Front-wheel drive brought many benefits, including better traction, no hump in the passenger compartment due to the drive train, and more vehicle styling options. The transmission was located on the axle between the rear wheels, providing better weight distribution. In conventional transmissions and transaxles, the gears and other parts that transfer engine power to the wheels function in the same manner.
In 1995, GM Powertrain revealed its newest innovation in the Dexron line of transmission fluid. DEXRON III is a transmission fluid that is so durable that it never needs replacing under normal driving conditions. It is touted to be the long-life automatic transmission fluid.
MANUAL VERSUS AUTOMATIC SUSPENSIONManual transmission systems continue to be installed in 50% of the cars produced worldwide, especially the economy models. Apart from the price factor, a number of drivers prefer the manual transmission to automatic for greater control over shifting and a more physical sense of operating the vehicle. Other drivers prefer automatics for the convenience of not having to think about shifting forward gears.
Most drivers consider manual systems to be more reliable than the complex automatics. Manuals are also considered to be more fuel-efficient. However, some car enthusiasts believe that the era of the manual transmitters may soon be over with modern automatic transmissions that are computer-controlled and capable of thinking for themselves, thereby making gearboxes redundant.
Increasingly congested streets and highways that make shifting a nuisance for commuters are partially responsible for the wide-ranging popularity of automatics in the West. Prices have come down as well; to the point that few buyers opt for a manual simply to reduce the cost of the car. Modern technology is also playing a role in making automatics popular. As a result of technological improvements, automatic transmission is no longer a power-wasting, gas-sucking slush box, sneered at by those of us who honed our skills on clutch and gearbox. The three-speed automatic of yesteryears has given way to four-speeds, with even five-speeds coming on stream in expensive European imports. (As with a manual, more gears mean better performance and fuel economy.) Computers, linked to the car's electronic system, cause the transmission to adapt to driving situations like hills, corners, power-off coasting.
Even though some diehard drivers continue to sneer at the automatic systems as the mark of a novice, there is little doubt that the automatic may soon become the norm, rather than the exception.
