An engine can be located at the
- front
- middle, or
- rear of a vehicle
An engine located at the front can be mounted longitudinally, and can drive either the front, or the rear wheels. In 4-wheel-drive applications, it can drive both the front and rear wheels.
Alternatively, the engine can be transverse, and drive either the front wheels only, or, in 4-wheel-drive applications, the front and rear wheels.
Mid-engined vehicles have the engine in front of the rear wheels. The engine can be transverse or longitudinal, and usually drives the rear wheels only.
Rear-engined vehicles have the engine mounted behind the rear wheels. The engine can be transverse or longitudinal, and usually drives the rear wheels only.
The drive train transfers turning effort from the engine to the driving wheels.
A drive train can include a clutch for manual transmission, or a torque converter for automatic transmission; a transmission, a drive shaft, final drive and differential gears, and driving axles.
Alternatively, a transaxle may be used. A transaxle is a self-contained unit, with the transmission, final drive gears and differential located in one casing.
It is usually used on front-engined, front-wheel-drive vehicles, or, rear-engined, rear-wheel-drive vehicles.
It can also be used on front-engine, rear-wheel-drive vehicles, connected to the engine by a propeller shaft.
Vehicles can be described by the number of axles. Most light vehicles only have 2 axles. Most vehicles have 4 wheels but only 2 driving wheels.
Some have 4 driving wheels, and these are commonly called 4-wheel-drive vehicles.
On commercial vehicles, the load carried on a single axle is limited by law, so vehicles with extra axles are common. This layout has 6 wheels to support the vehicle but only 2 drive it. The extra axle at the rear is only used to support the weight of the vehicle. This extra axle is sometimes called a lazy axle. This is called a 6-by-2 vehicle.
If the lazy axle is changed to a driving axle, this becomes a 6-by-4 vehicle.
Some heavy goods vehicles have an extra steering axle, which allows more weight to be carried.
The location of the driving axle determines whether the vehicle is classified as
- rear-wheel drive
- front-wheel-drive
- 4-wheel drive
- all-wheel-drive
Rear-wheel-drive vehicles can use a conventional layout with the engine at the front of the vehicle. The torque from the engine is transmitted to the rear-mounted driving axle by propeller, or drive shaft. This spreads the weight of components throughout the vehicle.
Some rear-wheel-drive vehicles have the engine at the rear, driving the wheels through a combination transmission and rear axle called a transaxle. The transaxle is lighter than a separate transmission and rear axle.
Moving the engine to the rear allows a lower bonnet line, which improves aerodynamics. The increase in weight over the rear wheels can improve their traction.
A vehicle which has the engine located behind the operator’s cabin, but forward of the rear driving axle is called a mid-engine, rear-wheel-drive vehicle.
A mid-engine design locates the mass of the engine behind the driver but forward of the rear axle. This allows for a low bonnet profile, and good handling.
Most goods-carrying, rigid commercial vehicles locate the engine near the front, and drive the rear axle by a 2-piece propeller shaft. The rear axle supports most of the goods, or payload.
Mini-buses commonly locate the engine at the front of the vehicle, beneath the operator’s cabin, and drive the rear axle by a propeller-shaft.
Larger buses and coaches locate the engine at the rear. This allows the vehicle to have a low floor, and removes much of the noise and vibration from the passenger compartment.
Front-wheel-drive vehicles use the front wheels to pull the vehicle along. In light passenger vehicles it gives lighter body weight, and increased interior room.
The engine and transaxle are at the front, and can be mounted laterally, that is, the engine is parallel to the front axle, or longitudinally where the engine is inline with the centre line of the vehicle.
Transmissions & final drives
A vehicle with a manual transmission uses a clutch to engage and disengage the engine from the drive train.
Engine torque is transmitted through the clutch to the transmission or transaxle. The transmission contains sets of gears that increase or decrease the torque, before it is transmitted to the rest of the drive train. The lower the gear ratio selected, the higher the torque transmitted.
A vehicle starting from rest needs a lot of torque, but once it is moving, it can maintain speed with only a relatively small amount of torque. A higher gear ratio can then be selected, and engine speed reduced.
A conventional vehicle with the engine at the front and driving wheels at the rear, uses a drive shaft, called a propeller shaft, to transmit torque from the transmission to the final drive.
The final drive provides a final gear reduction, to multiply the torque before applying it to the driving axles.
On front-engined, rear-wheel-drive vehicles, the final drive changes the direction of the drive by 90 degrees.
Inside the final drive, a differential gear set divides the torque to the axles, and allows for the difference in speed of each wheel when cornering.
Commercial vehicles may use a 2-speed final drive. The low range provides more torque for lower speeds, and heavy loads.
High range is used for higher speeds, and light loads. Axle shafts transmit the torque to the driving wheels.
In a rear-wheel-drive vehicle, the axles can be solid, or contain joints to allow for movement of the suspension.
For a front-wheel-drive vehicle, the drive shaft has universal joints to allow for suspension and steering movement.
An automatic transmission or transaxle performs similar functions to a manual transmission or transaxle, except that gear selection is controlled either hydraulically, or electronically.
The automatic transmission uses a torque converter, which acts as a hydraulic coupling to transfer the drive.
4 wheel drive transmission
A 4-wheel-drive vehicle has a propeller shaft, a final drive and differential gears, and axles for both the front and rear axle assemblies. A transfer case is attached to the transmission.
Part-time 4-wheel-drive means the vehicle is usually in 2-wheel-drive, and switched to full-time when needed by engaging the transfer case. It locks the propeller shafts together, and directs torque through them to both axles. When disengaged, the vehicle transfer case is coupled to 1 propeller shaft only.
When 4-wheel-drive is disengaged, most part-time, 4-wheel-drive vehicles drive the rear wheels.
Constant-4-wheel-drive uses a third differential in the transfer case. It allows for the difference in speed between the front and rear wheels during cornering.
The driver can still lock the front and rear axles together by moving a separate lever as in a conventional 4-wheel-drive or by moving the main gear selector. This is called a differential lock.
Some full-time, 4-wheel-drive sedans use a front engine and transaxle, with a propeller shaft connected to drive the rear wheels.
These cars are lighter and less rugged than conventional off-road types, and usually operate at higher speeds. The drive to all wheels provides better balanced handling, and traction for cornering in slippery conditions.
Chassis
A
chassis (plural: "chassis") consists of a framework which supports an inanimate object, analogous to an animal's skeleton; for example in the construction of an automobile or of a firearm.
Examples of use
- In the case of an automobile, the term chassis can refer to the frame and to the "running gear" like engine, transmission, driveshaft, differential, and suspension.
- In an airplane the chassis consists only of the landing gear or undercarriage, not the airframe itself.
- A tank's chassis (hull) comprises the bottom part of the tank, which includes the tracks, power plant, driver's seat and crew compartment. In most general usages when one says "tank chassis" one means the tank without the turret. Tank chassis often serve as basis for tank-converted platforms such as armored personnel carriers, combat engineering vehicles, etc.
- A chassis in a television, radio, or other electronic device consists of the metal frame on which the circuit boards and other electronic components are mounted. In the absence of a metal frame the chassis refers to the circuit boards and components themselves, not the physical structure.
- In computers, the chassis refers to the rigid framework onto which the assembly-worker mounts the motherboard, memory, disk drives and other equipment. It also supports the shell-like case: the housing that protects all of the vital internal equipment from dust, moisture, and tampering. The term "case modding" refers to the artistic styling of otherwise rather functional and plain computer encasings
Transmission components
When changing gears in a manual transmission, a stick shift and selectors are utilized.
The selectors are mounted in the transmission, and allow the various gear ratios to be selected.
In this example, the selector levers are connected to the stick shift inside the vehicle by selector rods.
Inside the vehicle of a manual transmission, a stick shift is provided to allow the various gear positions to be selected.
The stick shift is located on the floor beside the driver.
On a Rear Wheel Drive vehicle with the stick shift located on the floor, it is normally mounted directly into the transmission itself.
In other vehicles, the stick shift can be attached to the steering column, they can be connected to the transmission by means of selector rods or cables.
Inside the vehicle equipped with automatic transmission, a gear selector is provided to allow the various gear positions to be selected. The gear selector can be located beside the driver on the floor, often referred to as a ‘T Bar’ automatic. In other vehicles the gear selector is located on the steering column. In either situation, the selector is connected to the transmission by selector rods or, in some applications, by cables.
Chassis configurations
Vehicles can be described by the number of axles. Most light vehicles only have 2 axles.
This vehicle has 4 wheels but only 2 driving wheels.
This one has 4 driving wheels, and is commonly called a 4-wheel drive vehicle.
The location of the driving axle determines whether the vehicle is classified as: rear-wheel drive, front-wheel drive, 4-wheel drive, or all-wheel drive.
Rear-wheel drive vehicles can use a conventional layout with the engine at the front of the vehicle. The torque from the engine is transmitted to the rear-mounted driving axle by propeller, or drive shaft. This spreads the weight of components throughout the vehicle.
This rear-wheel drive vehicle has the engine at the rear, driving the wheels through a combination transmission and rear axle called a transaxle. The transaxle is lighter than a separate transmission and rear axle.
Moving the engine to the rear allows a lower hood profile, which improves aerodynamics. The increase in weight over the rear wheels can improve their traction.
This vehicle has the engine located behind the operator's cabin, but forward of the rear driving axle. This is called a mid-engine, rear-wheel drive vehicle. A mid-engine design locates the engine behind the driver but forward of the rear axle. This allows for a low hood profile, and good handling.
Front-wheel drive vehicles use the front wheels to pull the vehicle along. In light passenger vehicles it gives lighter body weight, and increased interior room. The engine and transaxle are at the front, and can be mounted laterally, that is, the engine is parallel to the front axle, or longitudinally where the engine is inline with the center line of the vehicle.
