In automotive design

Automotive design

Automotive design is the profession involved in the development of the appearance, and to some extent the ergonomics, of motor vehicles or more specifically road vehicles. This most commonly refers to automobiles but also refers to motorcycles, trucks, buses, coaches, and vans...

, an M4, or Mid-engine

Mid-engine design

A mid-engine layout describes the placement of an automobile engine between the rear and front axles. Another term for this is mid-ship.-Benefits:The mid-engine layout is typically chosen for its relatively favorable weight distribution...

, Four-wheel drive

Four-wheel drive

Four-wheel drive, 4WD, or 4×4 is a four-wheeled vehicle with a drivetrain that allows all four wheels to receive torque from the engine simultaneously...

 layout places the internal combustion engine

Internal combustion engine

The internal combustion engine is an engine in which the combustion of a fuel occurs with an oxidizer in a combustion chamber. In an internal combustion engine, the expansion of the high-temperature and high -pressure gases produced by combustion apply direct force to some component of the engine...

 in the middle of the vehicle, between both axles and drives all four roadwheels.

It is a type of car powertrain

Powertrain

In a motor vehicle, the term powertrain or powerplant refers to the group of components that generate power and deliver it to the road surface, water, or air. This includes the engine, transmission, drive shafts, differentials, and the final drive...

 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, it is usually used for sports cars and racing cars

Racing Cars

Racing Cars are a Welsh pop band, formed in the Rhondda Valley, Wales in 1973.-Career:They were signed to one of the biggest British record labels of the time, Chrysalis Records. Racing Cars's debut album yielded their only hit single with "They Shoot Horses, Don't They?"...

 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 axle

Axle

An axle is a central shaft for a rotating wheel or gear. On wheeled vehicles, the axle may be fixed to the wheels, rotating with them, or fixed to its surroundings, with the wheels rotating around the axle. In the former case, bearings or bushings are provided at the mounting points where the axle...

s.

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, dependng on application of the car.

Some 4 wheel drive cars use a centre viscous coupling unit

Viscous coupling unit

A viscous coupling is a mechanical device which transfers torque and rotation by the medium of a viscous fluid. It is made of a number of circular plates with tabs or perforations, fitted very close to each other in a sealed drum. Alternate plates are connected to a driving shaft at one end of the...

 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

Lamborghini Murciélago

The Lamborghini Murciélago is a two-door, two-seat sports car produced by Italian automaker Lamborghini between 2001 and 2010. Successor to the Diablo and flagship of the automaker's lineup, the Murciélago was introduced as a coupé in 2001. The automaker's first new design in eleven years, the car...

. 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 as it is suited toward sports cars and some off-road racing vehicles

Off-road vehicle

An off-road vehicle is considered to be any type of vehicle which is capable of driving on and off paved or gravel surface. It is generally characterized by having large tires with deep, open treads, a flexible suspension, or even caterpillar tracks...

.

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

Car handling

Automobile handling and vehicle handling are descriptions of the way wheeled vehicles perform transverse to their direction of motion, particularly during cornering and swerving. It also includes their stability when moving at rest. Handling and braking are the major components of a vehicle's...

 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.
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

Aerodynamics

Aerodynamics is a branch of dynamics concerned with studying the motion of air, particularly when it interacts with a moving object. Aerodynamics is a subfield of fluid dynamics and gas dynamics, with much theory shared between them. Aerodynamics is often used synonymously with gas dynamics, with...

 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-

Front-wheel drive

Front-wheel drive is a form of engine/transmission layout used in motor vehicles, where the engine drives the front wheels only. Most modern front-wheel drive vehicles feature a transverse engine, rather than the conventional longitudinal engine arrangement generally found in rear-wheel drive and...

 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

Understeer

Understeer and oversteer are vehicle dynamics terms used to describe the sensitivity of a vehicle to steering. Simply put, oversteer is what occurs when a car turns by more than the amount commanded by the driver...

. 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.

Drawbacks

Such a system is very difficult and expensive to design and engineer, which is why it is only usually found on race cars and very expensive sports cars.

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

Trunk (automobile)

The trunk or boot of an automobile or car is the vehicle's main storage, luggage, or cargo compartment. Trunk is used in North American English and Jamaican English; boot is used elsewhere in the English speaking world. Trunk is also primarily used in many non-English speaking regions, such as...

 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

Powertrain

In a motor vehicle, the term powertrain or powerplant refers to the group of components that generate power and deliver it to the road surface, water, or air. This includes the engine, transmission, drive shafts, differentials, and the final drive...

.

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.