Fuel and Power

We all know that engines produce power through burning fuel inside the combustion chamber. This process requires the fuel to be mixed with a certain amount of air, which is done by the carburettor or the fuel-injection system. According to the amount of air that enters the induction system, a certain amount of fuel is atomised into the airstream to form a combustible mixture.


If we break the burning process down into a chemical reaction, we will see that we need 14.7 parts of air to completely burn up one part of fuel. This is called the "stoichiometric" ratio or chemically correct mixture. A mixture that contains more fuel (or less air) than this, is referred to as being a rich mixture, while a mixture that contains less fuel is referred to as a lean mixture.



Due to the nature of the internal combustion engine, it produces maximum power on a slightly richer mixture than this - around 13 parts of air to one part of fuel will satisfy most engines. Maximum economy occurs on a mixture that is slightly leaner than the chemically correct proportion - around 16:1. Under normal operating conditions, the fuel system has to regulate the mixture around these two points, depending on whether we demand power from the engine, or are just cruising at a steady speed on the highway.

From the graph it will be clear that mixtures richer than 13:1 will only succeed in wasting fuel : one part of fuel will burn up 14.7 parts of air. If we add two parts of fuel to 14.7 parts of air, only one part of fuel will burn up and do work, because there is not enough air for the second part of fuel to burn. A very important point to note, is that the second part of fuel, which is not going to do any work, takes up space in the combustion chamber - valuable space which could have been filled by a combustible mixture. So, where we could have had a combustion chamber that was filled 100% with a combustible mixture, we now only have say 80% of that space available. The other 20% is taken up by excess fuel which is not going to burn. That leaves us with an engine that will only deliver 80% of the power it is capable of, while using twice the amount of fuel it should. I hope that this explanation finally dispels the "more petrol, more power" - myth. If we want more power, we need to fill the combustion chamber with more mixture.


From the above, it should be clear that the only practical, reliable way to ensure that an engine is getting the right amount of fuel, is to use a loading type dynamometer and an accurate exhaust gas analyser. The engine can be run under load, and the exhaust gasses can be analysed to see if the air/fuel ratio is within acceptable limits (which it is mostly not). A quick look at our dyna tuning statistics will reveal that more than 80% of the vehicles that was dyna tuned by us started out with mixtures that was so rich that it was robbing the engine of power, and costing the owners dearly in wasted fuel. The dynamometer is not only good for getting the full-power mixtures right. It is also just as good a tool to ensure that the fuel system leans out the mixture under steady-speed cruising conditions, when economy is important. This shows that the dynamometer is not only for the benefit of the racers. Ordinary motorists have much to gain, especially in the fuel-economy department.