Mid-engine, four-wheel-drive layout

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Audi R8 V10 Plus (Type 4S), one example of an M4 vehicle 2018 Audi R8 Coupe V10 plus Front.jpg
Audi R8 V10 Plus (Type 4S), one example of an M4 vehicle

In automotive design, an M4, or Mid-engine, Four-wheel-drive layout places the internal combustion engine in the middle of the vehicle, between both axles and drives all four road wheels.

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It is a type of car powertrain layout. Although the term "mid-engine" can mean the engine is placed anywhere in the car such that the centre of gravity of the engine lies between the front and rear axles[ citation needed ], it is usually used for sports cars and racing cars where the engine is behind the passenger compartment. The motive output is then sent down a shaft to a differential in the centre of the car, which in the case of an M4 layout, distributes power to both front and rear axles.

The centre differential contained within many 4 wheel drive cars is similar to the conventional differential in a 2-wheel drive car. It allows torque to be distributed to both drive axles whilst allowing them to spin at different speeds, which vastly improves the cornering of a 4-wheel drive car on surfaces with high grip such as tarmac. However, unlike the differentials on the drive axles which are configured to provide torque equally to both wheels, the centre differential is usually set to have a bias to one set of drive wheels or the other, depending on application of the car.

Some 4 wheel drive cars use a centre viscous coupling unit that will provide most power to the rear wheels unless the amount of torque being supplied to the rear wheels is in excess of the traction limits of the rear tyres, such as in a Lamborghini Murciélago. [1] Others contain a computer that will decide how much power to distribute to any wheel at any time depending on the circumstances of each wheel. In general the M4 system is not widely used[ citation needed ] as it is suited toward sports cars and some off-road racing vehicles.

Benefits

The engine is usually where the weight of a car is most concentrated so placing it between the front and rear axles gives a car a much better handling balance. Assuming the engine is behind the passenger compartment, the engine will also be pushing down on the rear wheels. Because the weight of a car is shifted toward the rear under acceleration in all cars as a rule, this further improves the amount of grip on the rear wheels, increasing the amount of torque that can be supplied to the rear wheels before wheelspin occurs. [2] Because the engine is not in the front, the car can be designed with a minimum amount of frontal area perpendicular to the wind, greatly increasing aerodynamic efficiency.

A computer-controlled four-wheel-drive differential system allows a car to both accelerate and corner more quickly, since it can vary the amount of torque going to the front and rear wheels, and therefore vary how much the car behaves like a front- or rear-wheel-drive car. This means that through a fast corner the car is able to display more “neutral” handling - with less oversteer or understeer. This is a much more efficient means of turning and allows for faster cornering speeds as opposed to a two-wheel or conventional four-wheel-drive system. [3]

Drawbacks

Some cars are designed with a rear wheel drive powertrain where the power is sent to the rear wheels. This design is cheaper than the M4 design because of the limited parts required to make the system function. Manufacturers use a unique layout to balance the weight distributed between the front and rear axels. Having a front engine layout with a rear wheel drive powertrain almost evens out the weight distributed in order to optimize performance and handling.

Most mid-engine cars, because of the size and position of the engine and transmission, compromise heavily on both passenger and boot/trunk space.

Four-wheel-drive systems tend to be quite heavy and some of the engine's power can be lost through the various differentials in the car, in addition to the frictional losses of the powertrain.

The variable handling characteristics of a four-wheel-drive car mean that when travelling round a corner at high speeds the car may enter the corner and understeer and then half-way through the corner suddenly start to oversteer. [4]

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Axle Central shaft for a rotating wheel or gear

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A traction control system (TCS), also known as ASR, is typically a secondary function of the electronic stability control (ESC) on production motor vehicles, designed to prevent loss of traction of driven road wheels. TCS is activated when throttle input and engine torque are mismatched to road surface conditions.

Four-wheel drive type of drivetrain with four driven wheels

Four-wheel drive, also called 4x4 or 4WD, refers to a two-axled vehicle drivetrain capable of providing torque to all of its wheels simultaneously. It may be full-time or on-demand, and is typically linked via a transfer case providing an additional output drive shaft and, in many instances, additional gear ranges.

Understeer and oversteer vehicle dynamics terms

Understeer and oversteer are vehicle dynamics terms used to describe the sensitivity of a vehicle to steering. Oversteer is what occurs when a car turns (steers) by more than the amount commanded by the driver. Conversely, understeer is what occurs when a car steers less than the amount commanded by the driver.

Quattro (four-wheel-drive system) four-wheel drive system by Audi

quattro is the sub-brand used by the car brand Audi to indicate that all-wheel drive (AWD) technologies or systems are used on specific models of its Audi automobiles.

Automobile handling and vehicle handling are descriptions of the way a wheeled vehicle responds and reacts to the inputs of a driver, as well as how it moves along a track or road. It is commonly judged by how a vehicle performs particularly during cornering, acceleration, and braking as well as on the vehicle's directional stability when moving in steady state condition.

Front-engine, front-wheel-drive layout Term used in automotive technology

In automotive design, an FF, or front-engine, front-wheel-drive (FWD) layout places both the internal combustion engine and driven roadwheels at the front of the vehicle.

Rear-engine, rear-wheel-drive layout term used in automotive technology

In automotive design, an RR, or rear-engine, rear-wheel-drive layout places both the engine and drive wheels at the rear of the vehicle. In contrast to the RMR layout, the center of mass of the engine is between the rear axle and the rear bumper. Although very common in transit buses and coaches due to the elimination of the drive shaft with low-floor bus, this layout has become increasingly rare in passenger cars.

Rear mid-engine, rear-wheel-drive layout type of drivetrain layout for automobiles

In automotive design, a RMR or Rear Mid-engine, Rear-wheel-drive layout is one in which the rear wheels are driven by an engine placed just in front of them, behind the passenger compartment. In contrast to the rear-engined RR layout, the center of mass of the engine is in front of the rear axle. This layout is typically chosen for its low moment of inertia and relatively favorable weight distribution. The layout has a tendency toward being heavier in the rear than the front, which allows for best balance to be achieved under braking. However, since there is little weight over the front wheels, under acceleration, the front of the car is prone to lift and cause understeer. Most rear-engine layouts have historically been used in smaller vehicles, because the weight of the engine at the rear has an adverse effect on a larger car's handling, making it 'tail-heavy'. It is felt that the low polar inertia is crucial in selection of this layout. The mid-engined layout also uses up central space, making it impractical for any but two-seater sports cars. However, some microvans use this layout, with a small, low engine beneath the loading area. This makes it possible to move the driver right to the front of the vehicle, thus increasing the loading area at the expense of slightly reduced load depth.

Locking differential

A locking differential is designed to overcome the chief limitation of a standard open differential by essentially "locking" both wheels on an axle together as if on a common shaft. This forces both wheels to turn in unison, regardless of the traction available to either wheel individually.

Transaxle

A transaxle is a single mechanical device which combines the functions of an automobile's transmission, axle, and differential into one integrated assembly. It can be produced in both manual and automatic versions.

A mid-engine layout describes the placement of an automobile engine between the rear and front axles and generally behind the passenger compartment.

A swing axle is a simple type of independent suspension designed and patented by Edmund Rumpler in 1903. This was a revolutionary invention in the automotive industry, allowing wheels to react to irregularities of road surfaces independently, and enable the vehicle to maintain a strong road holding. The first automotive application was the Rumpler Tropfenwagen, later followed by the Mercedes 130H/150H/170H, the Standard Superior, the Volkswagen Beetle and its derivatives, and the Chevrolet Corvair, amongst others.

BMW xDrive four-wheel drive system developed by BMW

BMW xDrive is the marketing name for the all-wheel drive system found on various BMW models since 2003. The system uses an electronically-actuated clutch-pack differential to vary the torque between the front and rear axles. Models with the DPC torque vectoring system also have a planetary gearset to overdrive an axle or rear wheel as required.

Torque steer is the unintended influence of engine torque on the steering, especially in front-wheel-drive vehicles. For example, during heavy acceleration, the steering may pull to one side, which may be disturbing to the driver. The effect is manifested either as a tugging sensation in the steering wheel, or a veering of the vehicle from the intended path. Torque steer is directly related to differences in the forces in the contact patches of the left and right drive wheels. The effect becomes more evident when high torques are applied to the drive wheels either because of a high overall reduction ratio between the engine and wheels, high engine torque, or some combination of the two. Torque steer is distinct from steering kickback.

ATTESA is a four-wheel drive system used in some automobiles produced by the Japanese automaker Nissan, including some models under its luxury marque Infiniti.

Super Handling-All Wheel Drive or SH-AWD is a full-time, fully automatic all-wheel drive traction and handling system combines front-rear torque distribution control with independently regulated torque distribution to the left and right rear wheels to freely distribute the optimum amount of torque to all four wheels in accordance with driving conditions." The system was announced in April 2004, and first introduced in the North American market in the second generation 2005 model year Acura RL, and in Japan as the fourth generation Honda Legend.

The layout of a motorised vehicle such as a car is often defined by the location of the engine and drive wheels.

The following outline is provided as an overview of and topical guide to automobiles:

Torque vectoring is a technology employed in automobile differentials. A differential transfers engine torque to the wheels. Torque vectoring technology provides the differential with the ability to vary the torque to each wheel. This method of power transfer has recently become popular in all-wheel drive vehicles. Some newer front-wheel drive vehicles also have a basic torque vectoring differential. As technology in the automotive industry improves, more vehicles are equipped with torque vectoring differentials. This allows for the wheels to grip the road for better launch and handling.

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