Mid-engine design

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

Benefits

The Lotus Europa S1 was based on a prototype built to compete for Henry Ford II's contract to build a Le Mans race car in the early 1960s.

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 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 makes 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. Mid-engine design is also 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 absorb more of the impact force to minimize penetration into the passenger compartment of the vehicle.

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.

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 the engine position is once again used to increase performance and the potentially smoother ride is usually more than offset by stiffer shock absorbers.

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

Underfloor mid-engine in a historical Hanomag truck chassis.

The largest drawback of mid-engine cars is restricted rear passenger space; consequently 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 buses. The mid-engine layout (with a horizontal engine) was common in single-decker buses in the 1950s and 1960s, e.g. the AEC Reliance. The Ferrari Mondial is to date the only successful example of a true mid-engined convertible with seating for 4 and sports car / supercar performance. A version of the Lotus Evora with a removable roof panel is anticipated but no definite date is known.

Like any layout where the engine is not front-mounted and facing the wind, the traditional "engine-behind-the-passengers" layout makes engine cooling more difficult. This has been a problem in some cars, but this issue 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.

Mid engined cars are more dangerous than front-engined cars if the driver loses control - although this may be initially harder to provoke due to the superior balance - and the car begins to spin. The moment of inertia about the center of gravity is low due to the concentration of mass between the axles (similar to standing in the middle of a playground roundabout, rather than at the edge) and the spin will occur suddenly, the car will rotate faster and it will be harder to recover from. Conversely, a front-engined car is more likely to break away in a progressive and controllable manner as the tires lose traction.

Variations

The term "mid-engine" has usually 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 vehicles' handling characteristics are more important than other features, such as capacity. Additionally the mechanical layout and packaging of an RMR car is substantially different from that of a front engine or rear engine car.

A subset of Front-Rear when the engine is in front of the driver, but fully behind the front axle line, the layout is sometimes called Front Mid engine Rear FMR layout instead of the less-specific term front-engine. In handling and vehicle layout FMR is substantially the same as FR. Some vehicles could be classified as FR or FMR depending on the factory installed engine (I4 vs I6). Historically most classical FR cars such as the Ford Models T and A would qualify as a FMR engine car. Additionally, the difference between FR and FMR may be as little as a few millimeters of engine protrusion in front of the front axle line as manufacturers mount engine as far back in the chassis as possible. Not all manufacturers use the Front-Mid designation.

Examples

FMR layout – Front Mid-engine / Rear-wheel drive

Front mid-engine position / Rear-wheel drive

These cars are RWD cars with engine placed between driver and front axle.

FM4 layout – Front Mid-engine / Four-wheel drive

This layout, similar to above FMR layout, with engine between driver and behind front axle, adds front wheel drive to become a four-wheel drive.

RMR layout – Rear Mid-engine / Rear-wheel drive

Rear mid-engine position / Rear-wheel drive

These cars use traditional engine layout between driver and rear drive axle. Typically, they're simply called MR layout cars.

M4 layout – Rear Mid-engine / Four-wheel drive

These cars uses mid-ship, four-wheel drive, with engine between axles.

MF layout – Mid-engine / Front-wheel drive

Front mid-engine position / Front-wheel drive

These cars are "mid-ship engined" vehicles, but they use front-wheel drive, with the engine in front of the driver. It is still treated as an FF layout, though, due to the engine's placement still being in the front of the car, contrary to the popular belief that the engine is placed in front of the rear axle with power transferred to the front wheels (an RMF layout). In most examples, the engine is longitudinally mounted rather than the transversely as is common with FF cars.

References

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