Common Automotive and Definitions (Simplified)

23 May 2006

For the benefit of those not so well-versed in tech-speak, here are some simple explanations of the common terms used in the automotive industry.

Engine  This produces the power in the car. It uses fossil fuels like petrol, diesel or natural gas as its source of power. Some engines use hydrogen fuel cells, others use electricity. Hybrid vehicles use both electric and fossil fuel engines in a single car. Engines used in vehicles can be divided into three main categories:

  • 2-stroke: It takes two strokes (linear movements of the piston) to complete the combustion cycle. Mainly used in small engines found in lawn mowers and outboard motors.
  • 4-stroke: It takes four strokes to complete the combustion cycle. Most cars are using 4-stroke engines due to the fact that it is more fuel-efficient and has safer level of emissions compared to 2-stroke engines.
  • Rotary: In a rotary engine, a rotor is used instead of pistons for combustion. This design promises smooth high-rpm power from a compact, lightweight engine. However, these engines are criticized for poor fuel efficiency and poor exhaust emissions.

Engine Layout  This refers to how the engine cylinders are oriented. Most common layouts are the Inline or V.

  • Inline/Straight: The cylinders are arranged in a straight line, hence the name. E.g. Inline-4 in a Toyota Vios.
  •  V (Vee): Consists of two inline engine blocks (called banks) arranged at an angle to one another, and connected to a common crankshaft. Common angles include 60 and 90 degrees. E.g. V6 in a Nissan Cefiro.
  • Flat/Boxer - Simply put, this is a V engine with an angle of 180 degrees between banks. E.g. Flat-4 in a Subaru Impreza WRX.
  • W – This is two V engines connected to a common crankshaft. E.g. W16 in a Bugatti Veyron.

CC (Cubic Centimetres or cm3)  this is the total volume of the cylinders in an engine, also known as engine capacity.

Cylinders – This is where the combustion (burning of fuel) takes place. The piston is also found in the cylinder.

Bore & Stroke – Bore is the diameter of the cylinder and stroke is the length where the piston moves from the bottom to the top of its range. Bore and stroke are used to calculate volume of the cylinder.

Valves – These allow for openings in the cylinder to be sealed. They are two types: intake valves and exhaust valves. Intake valves allow fresh air and fuel to enter the cylinder, exhaust valves allow exhaust gases to exit the cylinder. Usually, the number of intake and exhaust valves per cylinder are equal (2 each, or 4 per cylinder). However, some engines have more intake than exhaust valves (3 intake, 2 exhaust).

Overhead Camshafts (OHC)  Camshafts control the valves directly, determining when they open and close. Overhead camshafts means the camshafts are located above the cylinders. Engine types vary according to the number of camshafts:

  • SOHC (Single Overhead Camshaft): This design has 1 camshaft actuating both intake and exhaust valves. 
  • DOHC (Double Overhead Camshafts): This design has 2 camshafts with one for intake and one for exhaust valves.
  • QOHC (Quad Overhead Camshafts): This does not denote four camshafts per cylinder. Rather, this term is used exclusively for V engines and just means two camshafts per cylinder. Because a V engine has two banks of cylinders, that makes a total of four camshafts. QOHC is a just a more sophisticated manner of saying "DOHC V engine".

Power – Measurement of energy that is produced by the engine. Unit of measurements are the horsepower (HP) and kilowatt (kW).

Torque – Measures how much turning force the engine produces. Torque translates directly to acceleration. Unit of measurement is usually the Newton metre (N m).

Drive Train Layout – This is the number of driven wheels, i.e. how many wheels are directly connected and powered by the engine:

  • Front Wheel Drive (FWD): engine is driving the front wheels, rear wheels are free-running.
  • Rear Wheel Drive (RWD): engine is driving the rear wheels, front wheels are free-running.
  • All Wheel Drive (AWD): engine is driving both the front and rear wheels at all times.
  • 4-Wheel Drive (4WD): choice of powering either front, rear or both front and rear wheels.

Transmission – This provides the means of transmitting the power from the engine to the wheels, hence the name. Namely divided into the following categories:

  • Manual: Manual transmissions require the user to manually select the gear. Also, a clutch is required to help change the gears. As such, all cars with a manual transmission have a clutch pedal.
  • Automatic: The engine computer selects the gear for you and a torque converter replaces the clutch. As such, the clutch pedal is missing in cars with an automatic transmission.
  • Semi-automatic: This is essentially an automatic gearbox with an option that allows the driver to choose which gear to put the car in. 

Turning Radius – This refers to the radius of the smallest circle that a car can perform while turning.

Suspension – The suspension supports the car chassis above the wheels. It also provides shock-absorption capability to ensure ride comfort. Suspension types vary accordingly:

  • Macpherson Strut: Usually used for the front wheel. It is a basic suspension setup where the shock absorber is connected to the wheel hub/tyre and one control arm.
  • Double wishbones: Can be used for both front or rear wheels, though the front is more common. In this setup, the shock absorber is connected to the hub via two control arms. This allows for greater steering geometry adjustments.
  • Multi-link: This suspension has multiple control arms linked to it, allowing for a greater degree of movement. The main advantage is that it can take varying degree of off-road conditions.
  • Torsion Beam: Also known as the torsion bar. One end is fixed to the chassis while the other end is connected to one of the control arms. The shear modulus and torsional rigidity of the bar provides the restoring force (in place of a spring).
  • Trailing Arm: This suspension uses one or more arms (or "links") which are connected between (and perpendicular to) the axle and the chasis. Commonly used for rear wheels as it takes up less space than other suspension setups, allowing for a flatter floor and more cargo room.

Compression Ratio – This is the ratio of the volume of the cylinder when the piston is at its lowest position to the volume of the cylinder when the pistion is at its highest position. Compression ratios tell you what type of aspiration and fuel types are used. Low compression ratios (<10) are used for vehicles with forced induction (FI), i.e. turbochargers or superchargers. High compression ratios are used for naturally-aspirated (NA) vehicles. These range from 10 to 15. Higher compression ratios of 20 to 30 are for diesel-powered engines.

Fuel Capacity – The fuel tank capacity, measured in litres or gallons.

Fuel Consumption – The rate at which the car consumes fuel. Unit is usually l/100 km or km/l. 

Car Type  The type of car, designed with specific purposes and characteristics in mind.

  • Sedan: 4-door car with a boot.
  • Hatchback: 5-door without a boot
  • Estate: 4-door sedan with additional roof top at the back boot.
  • Coupe: 2-door car with a boot. Hatchback Coupes have 3 doors and no boot.
  • Cabriolet: open-top car , also known as convertible.
  • MPV: Multi Purpose Vehicle 
  • SUV: Sports Utility Vehicle


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