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Inautomotive engineering, amid-engine layout describes the placement of anautomobileengine in front of the rear-wheelaxles, but behind the front axle.
The mid-engine, rear-wheel-drive format can be considered the original layout of automobiles.[citation needed] A 1901Autocar was the first gasoline-powered automobile to use adrive shaft and placed the engine under the seat. This pioneering vehicle is now in the collection of theSmithsonian Institution.[1]
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 orspin 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 ofall-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 designed to absorb more of the impact force in a frontal collision in order to minimize penetration into the passenger compartment of the vehicle.
In most automobiles, and in sports cars especially, idealcar 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 gearbox 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 stiffershock absorbers.
This layout also allows the motor, gearbox, and differential to be bolted together as a single unit. Together with independent suspension on the driven wheels, this removes the need for the chassis to transfer engine torque reaction.
The largest drawback of mid-engine cars is restricted rear or front (in the case of front-mid layouts) passenger space; consequently, most mid-engine vehicles are two-seat vehicles. The engine in effect pushes the rear passenger seats forward towards the frontaxle (if the engine is behind the 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 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. theAEC Reliance. TheFerrari 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 theLotus 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,[citation needed] but this issue seems to have been largely solved in newer designs. For example, theSaleen 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.
Super, sport, and race cars frequently have a mid-engined layout, as these vehicles'handling characteristics are more important than other requirements, such as usable space. In dedicated sports cars, aweight distribution of about 50% front and rear is frequently pursued, to optimise the vehicle's driving dynamics – a target that is typically only achievable by placing the engine somewhere between the front and rear axles. Usually, the term "mid-engine" has been primarily applied to cars having the engine located between the driver and therear drive axles. This layout is referred to asrear mid-engine, rear-wheel drive, (or RMR) layout. The mechanical layout and packaging of an RMR car are substantially different from that of a front-engine or rear-engine car.
When the engine is in front of the driver, but fullybehind the front axle line, the layout is sometimes called a front mid-engine, rear-wheel-drive, orFMR layout instead of the less-specific term front-engine; and can be considered a subset of the latter. In-vehicle layout, FMR is substantially the same as FR, but handling differs as a result of the difference in weight distribution.
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 an FMR engine car. Additionally, the distinction between FR and FMR is a fluid one, depending on the degree of engine protrusion in front of the front axle line, as manufacturers mount engines as far back in the chassis as possible. Not all manufacturers use the Front-Mid designation.
These cars are RWD cars with the engine placed between the driver and the front axle.
This layout, similar to the above FMR layout, with the engine between driver and the front axle, adds front-wheel drive to become a four-wheel drive. An engineering challenge with this layout is getting the power to the front wheels past the engine - this would normally involve raising the engine to allow apropshaft to pass under the engine, or in the case of theFerrari FF taking power from both ends of thecrankshaft with two separate gearboxes.
These cars use a traditional engine layout between driver and rear drive axle. Typically, they're simply called MR; formid-rear (engined), ormid-engine, rear-wheel-drive layout cars.
These cars use mid-ship,four-wheel-drive, with an engine between the axles.
These cars are "mid-ship engined" vehicles, but they usefront-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 transversely as is common with FF cars.
