How Does Petrol Make An Engine Work?

We all take for granted that we put petrol in and that the car will function. We take a look at what actually happens under the bonnet.
Sherryn de Vos
2017-03-16
Your car is not only a machine that helps you get from A to B. This is a machine that uses fuel, like petrol or diesel, and turns it into mechanical energy. If you think about it carefully, it is actually an intricate contraption that converts mechanical energy into kinetic energy. How does it do this though? Believe it or not, it does it by creating tiny explosions in the engine. Although this does sound dangerous, it is very much controlled and is, in fact, the reason that your car gets propelled forward. We take a look at, specifically, how a petrol engine works.

How Does Fuel Work?

Let’s start with the simple principle. When you sit around a fire and become warm, you are actually absorbing the energy that the fire is giving off. Tiny molecules of energy are given off by the burning fire, which are transferred to you. Wood and paper and coal contain hydrocarbons. Inside these molecules are atoms of oxygen, carbon, and hydrogen. These hydrocarbons, when burnt, split apart, and the energy released from this creates heat. The principle is the same with fuels. These too contain hydrocarbons. When these are burnt, they split and release heat energy. When the fuel is burnt in the engine, it is contained in small compartments, meaning the energy cannot go anywhere. All of this heat and energy is, therefore, captured in the engine and converted into mechanical energy.
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What Makes Up An Engine?

The engine has a set of cylinders. You can get anything from two to 12 cylinders. Think of cylinders as cooking pots. The fuel burns inside these, as they are made of string metal and are sealed shut. On one end, they have openings that open and close like bicycle pumps. Pistons that work as plungers slide up and down inside them. At the top of each cylinder are two valves; an inlet and outlet. These open and close really quickly, and allow air and water into the inlet, and release exhaust gases out of the outlet. The fuel comes in the cylinder through a fuel injector or carburetor. At the top of a cylinder is a spark plug, which is a controlled device that sets fire to the fuel. At the bottom is a piston, which is attached to a constantly-turning axle, called the crankshaft. This powers the gearbox and makes the wheels turn.
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What Makes The Fuel Drive The Engine?

There are four stages to convert the fuel into energy.

1. Intake

The piston is pulled down inside the cylinder by the momentum of the crankshaft. The inlet valve opens and a mixture of fuel and air is let in.

2. Compression

The inlet valve closes. The piston moves back up the cylinder and squeezes the air. The pressure of this compression makes it more flammable. As the piston reaches the top, the spark plug fires.

3. Power

The spark plug ignites the fuel-air mixture, causing the mini-explosion we spoke about earlier. The explosion gives off a hot gas that pushes the piston back down. This then powers and turns the crankshaft.

4. Exhaust

The outlet valve opens. As the crankshaft turns, the piston is forced back into the cylinder and forces the gases out of the outlet. The gases are created from the burning of the fuel. At one point, the cylinders will be firing at different times. This keeps the crankshaft permanently moving. The more cylinders, the more powerful the engine. This is why a 12 cylinder engine will usually be found in a supercar.