An automobile, autocar, motor car or car is a wheeledmotor vehicle used for transporting passengers, which also carries its own engine or motor. Most definitions of the term specify that automobiles are designed to run primarily on roads, to have seating for one to eight people, to typically have four wheels, and to be constructed principally for the transport of people rather than goods.[3] Propulsion is a means of creating force leading to movement. Look up propulsion in Wiktionary, the free dictionary. A propulsion system has a source of mechanical power (some type of engine or motor, muscles), and some means of using this power to generate force, such as wheel and axles, propellers, apropulsive nozzle, wings, fins or legs. Other components such as clutches,gearboxes and so forth may be needed to connect the power source to the force generating component. Ground propulsion is any mechanism for propellingsolid bodies along the ground, usually for the purposes oftransportation. The propulsion system often consists of a combination of an engine or motor, a gearbox Automotive engineering is an applied science that includes elements of Mechanical engineering, Electrical engineering, Electronic Engineering, Software Engineeringand Safety engineering as applied to the design, manufacture and operation of automobiles, buses and trucks and their respective engineering subsystems.and wheel and axles in standard applications. n automobiles, the wheelbase is the horizontal distance between the center of the front wheel and the center of the rear wheel. At equilibrium, the total torque of the forces acting on the car is zero, and thus the wheelbase is related to the force on each pair of tires by the following formula: where is the force on the front tires, is the force on the rear tires, is the wheelbase, is the distance from the center of gravity (CG) to the rear wheels, is the distance from the center of gravity to the front wheels ( + = ), is the mass of the car, and is the gravity constant. So, for example, when one loads the truck with heavy goods, the center of gravity shifts rearward and the force on the rear tire increases causing it to sink to the extent that depends on the stiffness of thesuspension. If the automobile is accelerating or decelerating, extra torque is placed on the rear or fronttire respectively, and the equation relating the wheelbase, height above the ground of the CG, and the force on each pair of tires becomes: where is the force on the front tires, is the force on the rear tires, is the distance from the CG to the rear wheels, is the distance from the CG to the front wheels, is the wheelbase, is the mass of the car, is the acceleration of gravity (approx. 9.8 m/s2), is the height of the CG above the ground, is the acceleration (or deceleration if the value is negative). So, as is common experience, when the automobile accelerates, the rear usually sinks and the front rises depending on the suspension. Likewise, when braking the front noses down and the rear rises.:[1] Because of the effect the wheelbase has on the weight distribution of the vehicle, wheelbase dimensions are crucial to the balance and steering of the automobile. For example, a car with a much greater weight load on the rear tends to understeer due to the lack of the load (force) on the front tires and therefore the grip (friction) from them. This is why it is crucial, when towing a single-axle caravan, to distribute the caravan's weight so that down-thrust on the tow-hook is about 100 pounds force (400 N). Likewise, a car may oversteer or even "spin out" if there is too much force on the front tires and not enough on the rear tires. Also, when turning there is lateral torque placed upon the tires which imparts a turning force that depends upon the length of the tire distances from the CG. Thus, in a car with a short wheelbase, the short lever arm from the CG to the rear wheel will result in a greater lateral forceon the rear tire which means greater acceleration and less time for the driver to adjust and prevent a spin out or worse. Wheelbases provide the basis for one of the most common vehicle size class systems. The axle track in automobiles and otherwheeled vehicles which have two or more wheels on an axle, is the distance between the centreline of two roadwheels on the same axle, each on the other side of the vehicle. In a case of the axle with dual wheels, the centerline in the middle of the dual wheel is used for the axle track specification. In a car, or any vehicle, with two axles, this will be expressed as "front track" and "rear track". However the front wheels and/or rear wheels on either side of a vehicle do not necessarely have to be mounted on the same axle for the distance that they are apart to be called the Front wheel drive Front-wheel-drive layouts are those in which the front wheels of the vehicle are driven. The most popular layout used in cars today is the front-engine, front-wheel drive, with the engine in front of the front axle, driving the front wheels. This layout is typically chosen for its compact packaging; since the engine and driven wheels are on the same side of the vehicle, there is no need for a central tunnel through the passenger compartment to accommodate a prop-shaftbetween the engine and the driven wheels. As the steered wheels are also the driven wheels, FF (front-engine, front-wheel-drive layout) cars are generally considered superior to FR (front-engine, rear-wheel-drive layout) cars in conditions such as snow, mud or wet tarmac. The weight of the engine over the driven wheels also improves grip in such conditions. However, powerful cars rarely use the FF layout because weight transference under acceleration reduces the weight on the front wheels and reduces their traction, putting a limit on the amount of torque which can be utilized. Electronic traction control can avoid wheelspin but largely negates the benefit of extra torque/power. A transverse engine (also known as "east-west") is commonly used in FF designs, in contrast to FR which uses a longitudinal engine. The FF layout also restricts the size of the engine that can be placed in modern engine compartments, as FF configurations usually have Inline- 4 and V6 engines, while longer engines such as Inline-6 and 90° V8 will rarely fit. This is another reason luxury/sports cars almost never use the FF layout. Exceptions do exist, such as the Volvo S80 (FWD/4WD) which uses transversely mounted inline 6 and V8 engines, and the Ford Taurus, available with a 60° V8 and all-wheel drive. Most Audis are FF layout cars, but with longitudinal engines, such as the Audi A4 and Audi A6, however "FrontTrak" front-wheel-drive models are only entry-level trims in the United States and Canada; most Audis usually come with "quattro" all-wheel drive. The Audi A3 is a FF layout car with a traverse engine mounting, as it does not share a platform with more expensive offerings in the marque. [edit]Characteristics Front-wheel drive gives more interior space since the powertrain is a single unit contained in the engine compartment of the vehicle and there is no need to devote interior space for a driveshaft tunnel or reardifferential, increasing the volume available for passengers and cargo. [1] There are some exceptions to this as rear engine designs do not take away interior space (see Porsche 911, and Volkswagen Beetle). It also has fewer components overall and thus lower weight.[1] The direct connection between engine and transaxle reduces the mass and mechanical inertia of the drivetrain compared to a rear-wheel-drive vehicle with a similar engine and transmission, allowing greater fuel e conomy.[1] In front-wheel- drive cars the mass of the drivetrain is placed over the driven wheels and thus moves the center of gravity farther forward than a comparable rear-wheel-drive layout, improving traction and directional stability on wet, snowy, or icy surfaces.[1][2][3] Front-wheel-drive cars, with a front weight bias, tend toundersteer at the limit, which according to, for instance, Saab engineer Gunnar Larsson, is easier since it makes instinct correct in avoiding terminal oversteer, and less prone to result in fishtailing or a spin.[3][4] According to a sales brochure for the 1989 Lotus Elan, the ride and handling engineers at Lotus found that "for a given vehicle weight, power and tire size, a front-wheel-drive car was always faster over a given section of road."[5] However, this may only apply for cars with moderate power-to-weight ratio.[2][6][7] According to road test with two Dodge Daytonas, one FWD and one RWD, the road layout is also important for what configuration is the fastest.[3] Weight shifting limits the acceleration of a front-wheel-drive vehicle. During heavy acceleration, weight is shifted to the back, improving traction at the rear wheels at the expense of the front driving wheels; consequently, most racing cars are rear-wheel drive for acceleration. However, since front-wheel-drive cars have the weight of the engine over the driving wheels, the problem only applies in extreme conditions in which case the car understeers. On snow, ice, and sand, rear-wheel drive loses its traction advantage to front or all-wheel-drive vehicles which have greater weight over the driven wheels.