POWER FROM PETROL

Have you ever peered under the bonnet of your car into the mystifying jumble of metal that makes up the engine and wondered just what it is that gets those 'Goodyears' rolling? Well, the primary purpose of your car engine is to convert petrol into motion so that when your foot hits the gas you are catapulted from 0 to 100km/h in 4.5 seconds - at least for those of you who drive a Ferrari 360 Modena! This burst of energy is achieved through internal combustion. i.e: petrol burns inside the engine as opposed to old steam engines and steam boats which were driven forward when coal burnt outside the engine created steam, which in turn created motion inside the engine. Over time the efficiency and inexpensive nature of internal combustion engines has largely rendered their external combustion counterparts redundant. Today the advantages of an internal combustion engine beat any other existing technology for moving a car around.

So where does all that power come from?

Picture a movie where soldiers loaded cannons with gun powder and a cannon ball before lighting a fuse, sticking their fingers in their ears, BUFFFF!!!! and hundreds of their foe are tossed up into the air, somersaulting once or twice before falling lifeless to the ground. The firing cannon is the result of internal combustion. It is, however, hard to imagine that having anything to do with engines!

Now imagine what happens when a tiny amount of highenergy fuel like gasoline is placed in a small, enclosed space and ignited. Yes, you guessed it - an incredible amount of energy is released in the form of expanding gas. If you can create a cycle that allows you to set off explosions like this thousands of times per minute, and if you can harness that energy, you have the core of a car engine!

PETROL + AIR = EXHAUST GAS + WATER + ENERGY

Almost all car engines use a 4-Stroke cycle. Each movement of the piston up or down the cylinder is one stroke. Fuel and air are drawn into the combustion chamber (intake stroke) and compressed (compression stroke) before being ignited by a spark causing them to expand (power stroke) after which the burnt gases are expelled from the combustion chamber (exhaust stroke). It is the release of energy from the reaction of fuel and air under high temperature that causes the gaseous mixture to expand, pushing the piston down, creating the mechanical energy to drive your car.

More Power from your Engine

Optimum power out of your fuel/air mixture can be achieved by compressing it before it ignites (ie: during the compression stroke). This is because more mechanical energy is generated during the power stroke when the expansion of the combusted fuel takes place from a higher pressure. A compression ratio of 12:1 is 40% more efficient than a ratio of 6:1. Most modern petrol cars have a compression ratio between 8:1 & 12:1.

However, during compression the mixture can heat up to 600°C in a running engine. If you compress it too much it ignites itself. This is called "knocking" and can damage your engine.

The efficiency of your engine is therefore related to the compression ratio - how much you squish the fuel air mixture before igniting it. Ideally the compression ratio should be as high as possible without the fuel/air mixture igniting too early (before the spark plug has sparked) as this will damage your engine. Thus, a fuel that prevents that happening is required.