Engine types and classifications

There are many ways to classify an engine, even though the fundamental parts (block, pistons, crankshaft, camshaft are basically the same. These design differences, however, can greatly affect how the engine performs and how it is served.

Modern automotive engines are normally classified by:

1. Number of cylinders and engine swept volume (4,6,8,12,3,5,2), (cc, L)

2. Arrangement of cylinders (In-line, V-type, Slant, Horizontally).

3. Type of fuel burned (Gasoline, Diesel, LPG,…).

4. Ignition classifications (Spark, Compression ignition).

5. Method of fuel entry into engine(Carburetor, Injection).

6. Method of air-fuel mixture entry into engine (Normal aspiration , Turbo and Supercharging).

7. Cooling system type (Liquid, Air cooling systems).

8. Number of valves per cylinder. (2, 4-Valve combustion chamber).

9. Cam shaft location (OHC, DOHC).

10. Number of stroke per cycle. (2, 4-storke engine).

Engine position

Front Engine:

Apart from tradition there are a number of reasons for sitting the engine at the front of a car as shown in the figure.

o The large mass of an engine at the front of the car gives the driver protection in the event of a head-on collision,

o Engine cooling is simpler to arrange,

o The cornering ability of the vehicle is normally better if the weight is concentrated at the front.

Rear Engine:

By placing the engine at the rear of the vehicle it can be made as a unit that incorporates the clutch, gearbox and final drive assembly. With such an arrangement it is necessary to use some form of independent rear suspension. Most rear-engine layouts have been confined to comparatively small cars, because the heavy engine at the rear have adverse effect on the “handling” of the car by making it ‘tail-heavy’. Also it takes up a good deal of space that would be used on a front-engined car for carrying luggage. Most of space vacated by the engine at the front end can be used for luggage, but this space is usually less than that available at the rear.

Central and mid-engine:

These engine situations generally apply to sports cars because the engine sitting gives a load distribution that achieves both good handling and maximum traction from the driving wheels. These advantages, whilst of great importance for special cars, are outweighed in the case of everyday cars by the fact that the engine takes up space that would normally be occupied by passengers.

General layout of cars

Many different layouts are used with each arrangement offering specific advantages. Variation occurs in the location of the engine and the driving arrangement, i.e. the number and position of the driving wheels.

Front engine and Rear-wheel drive (RWD):

The traditional layout shown in the figure has the engine situated with its output shaft set longitudinally. In this arrangement the rear wheels act as the driving wheels and the front wheels swivel to allow the vehicle to be steered. In the past rear wheel drive was a natural choice because of the difficulty of transmitting a drive to a wheel that had to swivel for steering purposes. Spacing out the main components in this layout makes each unit accessible but a drawback is the intrusion of the transmission components into the passenger compartment. Using the rear wheels to propel the car utilizes the load transfer that takes place from the front to rear of vehicle when the car is climbing a hill or accelerating. Good traction is obtained but when the wheels lose adhesion, the driving wheels move the rear of the car sideways.

Front engine and Front-wheel drive (FWD):

The compactness of the layout shown in the figure has made it very popular for use on cars. The advantage associated with the engine being placed across the vehicle, i.e. mounted transversely, and has spread the use of front-wheel drive to many cars. Accommodating all the main components under the bonnet (hood) in one compartment give maximum space within the car for the occupants; also absence of floor bulges and tunnel provides more room for the rear passengers. Transverse mounting of the engine simplifies the transmission, because the output shafts from the engine and gearbox move in a similar direction to the wheels. This avoids the need for a bevel-type final drive; instead a simple reduction gear, incorporating a differential, transits the power by short drive shafts to the road wheels. One criticism of front-wheel drive is that the driving wheels have less grip on the road when the vehicle is accelerating and hill-climbing. Although this characteristic can be partly corrected by placing the engine well forward to increase the load on the driving wheels, the car is then liable to become ‘nose-heavy’. The effect of this is to make the steering of the car more arduous. In cases where the driver’s steering effort is considered excessive, the car is often fitted with power-assisted steering.

 Using the front wheels for steering allows the driving force to act in the same direction as wheel is pointing. This feature, together with the fact that the vehicle is being ‘dragged’ behind the front driving wheels, improves vehicle handling especially in slippery conditions.

Rear Engine and rear wheel drive (RWD):

In the now less popular rear engined car, the engine is mounted either fore-and aft behind t transaxle or transversely behind the gearbox and final drive unit. One advantage attributed to a rear-engined layout is that it increases the load on the rear driving wheels, giving them better grip of the road.

Four-wheel drive (4x4) and (AWD):

This arrangement, shown in the figure, is safer because it distributes the drive to all four wheels. The sharing of the load between the four wheels during acceleration reduces the risks of wheel spin. Also the positive drive to each wheel during braking minimizes the possibility of wheel lock-up.

A further advantage of this layout is shown when the vehicle is driven on slippery surfaces such as snow and mud.

The 4x4 (4WD) gives the driver the choice of operating in either 2WD or 4WD through the use of a shift lever or shift button. The AWD system operates in continuous 4WD.