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Mid-engine design

Mid-engine in a Hanomag-Fahrgestell.


A mid-engine layout describes the placement of an automobile engine between the rear and front axles. Another term for this is mid-ship, though that term is used mostly by British and Japanese manufacturers.

Benefits

The mid-engine layout is typically chosen for its relatively favorable weight distribution. The heaviest component is nearer to the center of the vehicle, reducing the vehicle's moment of inertia and making it easier and faster to turn the vehicle to a new direction. Also the engine weight is more evenly carried by all the wheels with this layout. As a result, vehicle stability, traction, and ride quality are naturally improved when turning, braking, and accelerating.

Mounting the engine in the middle instead of the front of the vehicle puts more weight over the rear tires so they have more traction and provide more assistance to the front tires in braking the vehicle, with less chance of rear wheel lockup and less chance of a skid or spin out. If the mid-engine vehicle is also rear-drive (as almost all of them are) the added weight on the rear tires can also improve acceleration on slippery surfaces, providing much of the benefit of all wheel drive without the added weight and expense of all wheel drive components. The mid-engine layout make ABS brakes and traction control systems work better, by providing them more traction to control. The mid-engine layout may make a vehicle safer, since an accident can occur if a vehicle cannot stay in its own lane around a curve or is unable to stop quickly enough. And additionally, mid-engine design is a way to provide additional empty crush space in the front of the automobile between the bumper and the windshield, which can then be used in a frontal collision to spread out the impact force over time.

In most automobiles, and in sports cars especially, ideal car handling requires balanced traction between the front and rear wheels when cornering in order to maximize the possible speed around curves without sliding out. This balance is harder to achieve when the heavy weight of the engine is located far to the front or far to the rear of the vehicle. Some automobile designs strive to balance the fore and aft weight distribution by other means such as putting the engine in the front and the transmission and battery in the rear of the vehicle. Some of the same benefits are gained, but at the cost of greater moment of inertia compared to the mid-engine layout, making it harder and less responsive to turn the vehicle to a new direction.

Another benefit comes when the heavy mass of the engine is located close to the back of the seats. It makes it easier for the suspension to absorb the force of bumps so the riders feel a smoother ride. But in sports cars this benefit is once again utilized to increase performance and is usually more than offset by stiffer shocks.

This layout also allows the transmission and motor to be directly bolted to each other - with independent suspension on the driven wheels this removes the need for the chassis to transfer engine torque reaction.

Drawbacks

The largest drawback of mid-engine cars is packaging; most mid-engine vehicles are two-seat vehicles. The engine in effect pushes the passenger compartment forward towards the front axle (if engine is behind driver). Exceptions typically involve larger vehicles of unusual length or height in which the passengers can share space between the axles with the engine, which can be between them or below them, as in some Toyota vans, large trucks and busses.

Like any layout where the engine is not in the front of the car facing the wind, the traditional "engine-behind-the-passengers" layout makes engine cooling more difficult, and this has been a problem in some cars such as the Porsche 914, which is air cooled rather than having a front mounted coolant system. But this problem seems to have been largely solved in newer designs. For example, the Saleen S7 employs large engine-compartment vents on the sides and rear of the bodywork to help dissipate heat from its very high-output engine.

Variations

Traditionally, the term mid-engine has been applied to cars having the engine located between the driver and the rear drive axles. This layout is referred to here as RMR layout. Sports and racing cars typically have this mid-engine layout, as these vehicle's handling characteristics are more important than other features, such as practicality.

When the engine is in front of the driver, but fully behind the front axle line, the layout is sometimes called front-mid-engine instead of the less-specific term front-engine. That layout is referred to here as FMR layout and MF layout.

Examples

RMR layout, Mid-Engine

Traditional engine between driver and rear drive axle

FMR layout, Front-Mid-Engine

RWD with engine between driver and front axle

MF layout

Midship Front-wheel drive, with engine infront/parallel to driver

FMA layout

Midship, four-wheel drive, with engine parallel to driver


Automobile layouts
Layouts:
FF | FMR | FR | MF | RMR | RR | F4
Engine positioning:
Front-engine | Mid-engine | Rear-engine
Drive:
Front-wheel drive | Rear-wheel drive | Four-wheel drive | Six-wheel drive