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This article explains the basics of the inner workings of a naturally aspirated four stroke internal combustion engine and how it powers a car.  

The aim of a cars internal combustion engine is to create kinetic energy from the chemical potential energy stored in fuel. An internal combustion engine produces power by ‘combusting’ a fuel and air mixture. This combustion process burns and releases the chemical potential energy of the fuel in the cylinder at the end of the compression stroke, which causes the atmosphere inside the cylinder to expand.  This expansion (caused by the heat energy of the combustion process) pushes the piston back the other direction. In an average engine, around 33% of the heat / power is spent in pushing the piston.  Another 33% is lost out the exhaust (the exhaust gas, this is why turbos work!) and another 33% is lost through the radiator (heat transferred through the block and coolant passages). The rest of the energy is transferred to heat and sound via friction.

The 33% of the power that is spent pushing the piston in the other direction is transferred through a series of components, evenly making its way to the flywheel attached to the end of the crank shaft. The rotational kinetic energy possessed by the flywheel is then fed through the drive line where 15-17% for manual transmissions and 20-25% for automatic transmissions of energy is converted into heat and sound due to friction. The remaining energy is then harnessed and spent in the movement of the wheels and in turn the movement of the car.

A modern car engine is made up of numerous components, the main ones concerning the energy transformation are:

-       The Cylinder (Part of the block)

-       The Piston

-       The Connecting Rod

-       The Crankshaft

-       The Intake Valve (The Intake Valve opens at a precise time to allow the air/fuel mixture to enter the cylinder)

-       The Exhaust Valve (The Exhaust Valve opens at a precise time to allow the burned gases to leave the cylinder)

-       The Spark Plug (The Spark Plug ignites the air/fuel mixture in the cylinder)

-    The flywheel 

-       All affiliated nuts, bolts, washers and bearings

These are shown, assembled below in Figure 1.

Figure 1 – This image shows the assembled and exploded view of the piston assembly.

The force from the piston is then transferred to the Crankshaft through the Piston Rod (connecting rod). The piston rod converts the reciprocating motion of the piston, to the Rotating Motion of the crankshaft.

The four ‘strokes’ of these engines are as follows:

1. Intake: The intake valve opens allowing fresh oxygen rich air mixed with fuel to enter the cylinder. Ideally, an engine takes in Air (Oxygen and Nitrogen) and fuel (Hydrocarbons) and produces CO2, H2O, and the N2 just passes straight through. The chemical equation is as follows.

2 C8H18 (gas) + 25 O2 = 16 CO2 + 18H2O

This equation is representative of a stoichiometric air fuel ratio 14.7:1 (14.7 parts of oxygen for every 1 part of fuel), however under normal driving conditions an engine will encounter lean conditions (more air and less fuel) when cruising on the highway (better mileage) and rich conditions (more fuel and less air) when accelerating (better power). The lean condition results in oxide and harmful nitrogen compound production. Rich conditions result in carbon monoxide production.

For more infomation on fuels and the chemistry behind it, visit our Fuel Tech Page.

2. Compression: The piston is pushed upward by the flywheel's momentum compressing the air/fuel mix.

3. Combustion: Most superficial descriptions of automotive engines then say that the gas-air mixture is ignited at the exact moment of top dead centre (as seen in figure 2)  and that the even higher pressure of the exploding gas drives the piston down, turning the crankshaft. If you look at figure 2 for a while, you should be able to see that that is impossible! If the explosion (and all its effects) occurred exactly at the moment shown in figure 2, at TDC, the crankshaft would not

be given any rotation at all! Virtually the entire force of the explosion initially acts to try to drive the piston, connecting rod and crankshaft downward, out of the bottom of the engine, without giving it any rotation at all! (When this actually happens, VERY bad things tend to happen to the engine!)

Figure 2 - This image shows a pistion at top dead centre

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In reality the ignition spark occurs maybe 10° or 20° BEFORE top dead centre, so the explosion has a moment to build up its full power by the time it gets to top dead centre. Because of the sudden heat, the gases try to expand immediately, but they cannot, so the pressure in those hot gases greatly and rapidly increases. Due to the high compression of this mixture (typically around 190 PSI in a typical engine) it is very volatile. This pushes the piston downward, turning the crankshaft and the flywheel

4. Exhaust: After the Air/Fuel mix has been burnt the remaining chemicals in the cylinder (water and CO2 for the most part) must be removed so that fresh air can be brought in. As the piston goes back up after combustion the exhaust valve (right top of cylinder) opens allowing the exhaust gasses to be expelled.