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"  Whether you drive a 500-hp sports car or a 96-hp economy hatchback, all that potency under your car&amp;#039;s or truck&amp;#039;s hood is useless if the engine&amp;#039;s torque doesn&amp;#039;t get to the drivewheels through a complex maze of gears. In fact, the drivetrain may be the least understood part of a vehicle. New innovations in four-wheel and all-wheel drive have only made that confusion worse for many drivers. Here&amp;#039;s a primer to help explain that mystery under the floorboards: what really happens when you press down on the accelerator. FRONT-WHEEL DRIVE  Though front drive can be found in such classics as the 1929 Cord, the overwhelmingly popular modern configuration is based on the 1959 Mini. Its creator, Sir Alec Issigonis, put the small engine transversely--sideways--under the hood, mounted the transmission and differential in one unit called a transaxle, and installed that underneath and to the rear of the engine. While some front-drivers have a longitudinally--front-to-rear--mounted drivetrain, all the components are still up front. Because the front wheels must steer as well as propel, they are connected to the axle halfshafts via complex universal joints, called constant velocity joints, which can transmit power smoothly while severely articulated. PRO \u2022 More room for people and cargo. \u2022 Better fuel economy due to reduced weight. \u2022 Improved wet-weather traction thanks to the weight over the drivewheels. CON \u2022 Increased wear on front tires and suspension. \u2022 Cramped engine compartment makes service difficult. \u2022 Limits to amount of power the front wheels can handle without making steering unpredictable. \u2022 Reduced wet-weather traction on upgrades. Transaxle  The operation of any transaxle is exactly the same as that of any transmission. The difference is this: Instead of being connected via a long driveshaft to the rear axle, the transmission&amp;#039;s output shaft drives a large gear that meshes directly with the differential&amp;#039;s ring gear. And the differential itself (which would be mounted on the rear axle in a rwd car) is located in the transaxle housing, mounted parallel to the transmission. As power is applied, the differential distributes it to the two front wheels via halfshafts. Continuously Variable Transmission (CVT)  CVTs are gaining popularity and are used in several new Fords, Saturns and Audis. Instead of gears, the CVT uses a belt between two pulleys. One is driven by a shaft from the engine, the other drives a shaft to the differential unit and drive axles. Both pulleys are split so that their halves can slide closer together and farther apart. As the belt rides higher and lower in the pulleys, the effective gear ratios between the driving and driven shafts change. REAR-WHEEL DRIVE  Still the classic, rear drive was basically the only drivetrain system for many years. A longitudinally mounted engine, with the transmission bolted directly to it, sends power via a driveshaft to a differential unit at the rear axle. The differential turns the power 90\u00B0 and sends it to the rear wheels. (Some sports cars such as Corvettes, Ferraris and Porsches place a combined transmission and differential--or transaxle--in the rear.) The driveshaft connects via yoke-type universal joints and a splined expansion joint to allow for vertical and longitudinal suspension movement. PRO \u2022 Better front/rear weight distribution results in nimbler handling. \u2022 Ease of service thanks to spread-out components. \u2022 Less wear and tear because the front tires don&amp;#039;t have to both steer and pull the car. CON \u2022 Poor wet-road traction and stability without sophisticated electronic controls. \u2022 Reduced passenger and cargo room. Manual Transmission  The gearbox is mated to the engine via a spring-loaded clutch plate faced on both sides with friction material. The clutch must be disengaged to shift gears, and the transmission must be in Neutral, or the clutch disengaged, for the vehicle to be stopped while the engine is running. The transmission consists of an input shaft from the engine and an output shaft to the drivewheels. The input gears can slide back and forth to mesh with their output mates. Synchronizer cones between the sliding gears and shaft allow smooth shifting. Reverse gear is on its own shaft. Automatic Transmission  An oil-filled torque converter that multiplies engine torque inside the transmission bellhousing allows some slippage so the vehicle can be stopped while the engine runs. A friction clutch built into the center of the converter locks its input and output shafts to the same speed for highway cruising. Computer-controlled hydraulic pressure selects which combination of gears within several planetary sets can rotate, changing the ratios between the input and output shafts. Open Differential  While cornering, the outside wheels cut a wider arc than the inside. The differential needs to ensure that the outside and inside wheels are allowed to turn at different speeds\u2014hence the name\u2014while still supplying power to both wheels. The basic differential housing contains a large ring gear that meshes with a small pinion gear driven by the driveshaft. The ration between the ring and pinion gear is known as the final-drive ratio or rear-axle ratio. The ring gear also spins a carrier containing perpendicularly meshing spider gears that allow the left and right axle shafts to spin independently. Downside: The wheel with the least traction limits power applied to the road. Limited-Slip Differential  The concept of providing traction to the nonslipping drivewheel with a limited-slip differential dates back at least to the late 1950s. Though there are now many wrinkles to the old theme, the essentials are the same. The spider gears are mechanically linked to share torque regardless of conditions. This can be done simply by adding spring-loaded clutch packs that keep the spider gears from spinning. Power then flows to both wheels to the limit of the clutch packs&amp;#039; capacity. The spiders can also be pneumatically or electrically locked together\u2014but this defeats the differential function. FOUR- &amp;amp;amp; ALL-WHEEL DRIVE  From a traction point of view, the best of all worlds is when both front and rear wheels are propelling the vehicle. However, the front and rear axles rotate at different speeds except when driving in a perfectly straight line. So the only way both can power the vehicle on dry-road turns is if there&amp;#039;s a differential between them. (On slippery surfaces the tire slippage compensates for the wheel-speed differences.) Many awd vehicles share much of their drivetrain with similar front-drive models but add a compact center differential, driveshaft and rear differential. Four-wheel-drive vehicles use a transfer case placed after the transmission that directs power to both the front and rear axles when needed. When engaged, the transfer case drives two separate driveshafts that operate individual differentials. No center differential is used on true 4wd vehicles going off-road in 4wd mode. PRO \u2022 Maximum traction on a variety of surfaces. CON \u2022 Added weight, which reduces fuel economy. \u2022 More things to break. \u2022 Higher cost. FUTURE DRIVE Two decades from now, you can expect that the power to move your vehicle will be electric. It will likely have a relatively small electric motor for each wheel, and the concepts of front-, rear- and all-wheel drive will become obsolete. The electronics will be able to direct power to any one wheel, to all wheels at the same time, or to any combination. Either a hydrogen fuel cell or an internal-combustion engine running on hydrogen turning a generator will supply the electricity. As fuel cell development costs are still staggering, a more economic alternative might be to replace gas stations with hydrogen filling stations. Viscous Coupling  This is a differential with no gears. The input shaft from the transaxle (at the front wheels) and the output shaft to the rear wheels each carry a series of plates that are alternately intertwined and closely spaced. all the plates swim in a special fluid that transfers power from the input to the output plates when needed. If the front-drive wheels begin to slip, their shaft and plates spin more quickly than the others. This speed differential within the housing churns and heats the fluid, which thickens it and more tightly bonds the alternating plates. Some torque is now sent to the grippier wheels until spinning ones regain traction. Torsen Differential  The Torsen dates back to 1983. Since then it has been used by various carmakers, including Audi and Hummer. The Torsen multiplies what torque is available from the axle that is starting to spin or lose traction, and sends it to the slower-turning axle with better traction. The gears allow a torque-bias ratio of 4:1, which means they can deliver four times as much power to the nonslipping axle than can be supported by the slipping axle. One big advantage of Torsen systems is that because they are purely mechanical, they react very quickly to slip. Transfer Case  This is a separate gearbox mounted behind the transmission. Power goes to the transfer case to be directed to the rear wheels only or to both front and rear. A separate driveshaft connects the transfer case to a differential in the front axle. Most transfer cases also offer two gear ratios, for a High and Low range. While many vehicles still have a manually engaged transfer case, several now offer electrically activated engagement. "^^<http://www.w3.org/2001/XMLSchema#string>
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