The engine is the vehicle’s main source of power. The engine uses fuel and burns it to produce mechanical power.

Chemical Energy converted into Mechanical Energy

The heat produced by the combustion is used to create pressure which is then used to drive a mechanical device.

Internal vs External

Prior to the 20th century, the burning or combustion of the fuel took place outside of the actual engine. The fuel, often coal, was burned to produce heat. This heat was then used to boil water to produce steam. The steam was held under pressure and then introduced into the engine where it forced the piston down in the cylinder. This is referred to as an External Combustion Engine or traditionally called a steam engine.

Today’s modern vehicles use an engine where the fuel is burned directly inside referred to as the Internal Combustion Engine. As the air/fuel mixture burns it expands rapidly causing the pressure inside the cylinder to increase. This increase in pressure forces the pistons down the cylinder thereby driving the connecting rod to turn crankshaft providing us with a continuous rotating motion with which to drive the vehicle and other components.

Reciprocating vs Rotary

Both the external and internal combustion engines use a piston housed in a cylinder which is attached to a connecting rod and then a crankshaft. The piston is forced down the cylinder which pushes on the connecting rod thereby turning the crankshaft. This type of engine is also referred to as a reciprocating engine because of the piston’s up and down movement.

In contrast to this engine is the rotary engine which uses a triangular-shaped rotor. The rotor is housed in an elliptical-shaped chamber and connected to a central main shaft (crankshaft). As the rotor moves around the chamber it draws in an air/fuel mixture, compresses it, burns, and then expels it. The movement of the rotor forces the main shaft to rotate.

4 Stroke vs 2 Stroke

The engine burns fuel to produce mechanical power. In order to accomplish this they must:

  • Draw in the necessary air/fuel mixture to be burned.
  • Compress it in order to increase its’ potential as well as allow for the positioning of the piston.
  • Ignite and burn it to release the energy.
  • Expel the burned/waste to allow for more air/fuel to enter.

These four (4) steps or cycles are more commonly referred to as:

  • Intake
  • Compression
  • Power
  • Exhaust

In a 4 stroke engine, each cycle is accomplished in a separate stroke of the piston as it moves up and down in the cylinder. However, in the 2 stroke engine, these 4 cycles are combined and sometimes overlapped in order to provide more power strokes in the same amount of time.

The 2 stroke engine uses the change in pressure below the piston to draw in the air/fuel mixture. It is then forced up a transfer port to the top of the piston where it is compressed and burned. As the piston moves down, the incoming air/fuel mixture forces out the burned exhaust gases. Because the engine draws the air/fuel mixture in through the lower half of the engine, the oil must be pre-mixed with the fuel to allow for proper lubrication.

Gasoline vs Diesel

Gasoline is by far the most popular fuel in use today. However, diesel fuel has been used in industrial vehicles and machinery for many years and is starting to increase in popularity in passenger cars. Diesel fuel contains more heat energy than gasoline making it far more economic but diesel fuel is thicker, heavier and does not vaporize as easily as gasoline, and must be used in high-pressure engines.

Because of this, the fuel must be sprayed directly into the cylinder. The fuel is introduced into the cylinder at the end of the compression stroke and ignites under the heat of compression eliminating the need for an ignition system. The exhaust produced is also very heavy and dirty like soot.

Engine Classification

The engine is usually classified in the three (3) main ways.

  • Displacement
  • Number of Cylinders
  • Cylinder Arrangement

The displacement refers to the volume of space that a piston moves through in a single stroke. It is calculated by multiplying the area of the piston by the length of its’ stroke. Stroke refers to the distance that the piston travels either upward or downward in the cylinder from the top (TDC) to the bottom (BDC). The cylinder arrangement of an engine falls into three (3) main formats.

In-line, V-type, or horizontally opposed. With the inline, all the cylinders are in a single file, one behind the other. V-type has half the cylinders off-center on one side (left bank) and the other half on the other side (right bank). The separation between the two (2) banks can be anywhere from >0 degrees to <180 degrees. When the separation is equal to 180 degrees the arrangement is referred to as horizontally opposed.

There are also two (2) ways of mounting an engine inside the vehicle. The conventional method is where the crankshaft and cylinders are inline with the vehicle from front to back. Transverse is where the engine has been turned sideways so the crankshaft and cylinders are inline from left to right.

The Lubrication System

The engine also incorporates the lubrication system and the cooling system. The lubrication system ensures that all the moving parts of the engine are kept well oiled in order to provide a long serviceable life. The lubrication system performs five important functions:

  • Lubricates – reduces friction between moving parts by providing a thin film of oil.
  • Cools – heat is transferred to the oil from the engine.
  • Cleans – as the oil washes over the inside of the engine it removes dirt and other particles.
  • Seals – fills in any small gaps inside the engine.
  • Absorbs Shock – it acts as a cushion between various parts inside the engine.

Smaller engines use a simplified system where the oil is thrown around the crankcase called a dipper and splash method. Larger, more powerful engines use a pressurized system which incorporates a pump, regulator and filter.

The Cooling System

The cooling system function is to maintain the engine at an ideal operating temperature. There are two methods of performing this function.

  • Air Cooled – fins are attached to the exterior of the engine thereby increasing the surface area where the heat is transferred to the surrounding air.
  • Liquid Cooled – the cylinders are surrounded by a chamber filled with a liquid called a water jacket. The heat is transferred to the liquid in the water jacket and then circulated to an external unit called a radiator. Like the air-cooled system, the radiator has fins that perform the same function.

Liquid cooling systems are far more efficient than air cooled systems but require many more parts and constant maintenance.

Key Terms and Definitions

  • Air/Fuel Mixture: The air/fuel ratio refers to the proportion of air and fuel present during combustion; approximately 14.7 to 1 by weight.
  • Compression Cycle: The movement of the piston from BDC to TDC, where the air/fuel mixture is compressed; follows the intake stroke.
  • Connecting Rod: A component used to attach the piston to the crankshaft.
  • Crankshaft: A component that changes the reciprocating motion of the pistons into rotating motion.
  • Displacement: The volume displaced by the pistons in moving from BDC to TDC.
  • Elliptical: Egg-shaped, oval or rounded like an egg.
  • Exhaust Cycle: The upward motion of the piston forcing burned gases out the open exhaust valve.
  • Expel: Force to leave or move out. Example exhaust gases
  • External Combustion Engine: An engine that burns the air-fuel mixture in a chamber outside the engine cylinder, such as a steam engine.
  • Horizontally Opposed: An engine that has two (2) banks of cylinders that are placed flat or 180 degrees apart.
  • Intake Cycle: A downward stroke of a piston that draws the air/fuel mixture into the cylinder.
  • Internal Combustion Engine: An engine that burns fuel within itself as a means of developing power.
  • Piston: An engine part that reciprocates in the cylinder and transfers the force of the expanding gases via the piston pin and connecting rod to the crankshaft.
  • Power Cycle: The piston stroke, with both valves closed, in which combustion takes place, forcing the piston from TDC to BDC.
  • Reciprocating: An up and down motion of the piston inside a cylinder.
  • Vaporize The process of turning a liquid, such as gasoline, into a vapor is often accomplished after the atomized fuel leaves the fuel injector.


When working on or around engine systems you must take the necessary precautions to ensure the safety of yourself and the people around you.

  • Do not wear loose clothing. These items may become entangled in pulleys or other moving parts causing serious injury.
  • Minimize distractions while working on your engine.
  • Never disconnect or unplug any electrical connector while the engine is running or the key is in the “on” position.
  • Wear safety glasses to prevent dirt and debris from falling into your eyes.
  • All engines and their parts have very sharp edges. To avoid possible, the injury does not grasp unfamiliar components too tightly.
author avatar
William Anderson (Schoolworkhelper Editorial Team)
William completed his Bachelor of Science and Master of Arts in 2013. He current serves as a lecturer, tutor and freelance writer. In his spare time, he enjoys reading, walking his dog and parasailing. Article last reviewed: 2022 | St. Rosemary Institution © 2010-2024 | Creative Commons 4.0


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