Fuel Science: The Stoichiomteric Ratio

The Stoichiomteric Ratio

‘Stoichiometry’ is a strange word but its relevance to your car, in particular to its fuel system, will soon become clear. The word comes from the Greek stoicheion meaning element or principle, and metron, meaning measure.

Now we know what stoichiomtry means, what’s this stoichiometric ratio about? In the simplest of terms, it describes the chemically correct ratio of air to fuel necessary to achieve complete combustion of the fuel.

In an engine, the fuel is a mixture of hydrocarbons and for petrol, the ratio of air mass to hydrocarbon mass, the AFR, is between 14.6 and 14.7 to 1. This value is the stoichiomteric ratio

In practice the difference between the engine’s actual “air fuel ratio”(AFR) to the “stoichiometric air fuel ratio” (SAFR) is known (and is represented by the Greek letter), lambda.

This probably rings a bell – we’ve all heard of a lambda sensor. In relation to one of these devices, lambda is normally given a value of “1” and the stoichiometric air fuel ratio is sometimes refered to as “lambda” for simplicity where in reality lambda is the ratio between AFR and SAFR.

For maximum efficiency, it’s necessary to maintain the stoichiomteric ratio as accurately as possible. In a modern engine, two devices are used, the abovementioned lambda sensor and a catalytic converter.

If the actual mixture fed to the engine contains more air (a leaner ratio), then some oxygen is left in the exhaust. On the other hand, if the fuel is in excess, (a rich mixture ratio), then complete combustion achieved. In reality this produces a mixture of CO and CO2 in the exhaust.

The catalytic converter in the exhaust removes this CO along with any hydrocarbons left over due to poor combustion. It also removes any other pollutants such as nitrogen oxides (NOx). A catalytic converter, however, can only work correctly when the mixture is very close to the stoichiomteric ratio.

For practical purposes, the lambda figure is calculated this way:
For a petrol engine, the optimal operational value of the fuel to air ratio is 14.7 parts of air, to 1 part of fuel. By mass, that’s 14.7 kilograms of air to each kilogram of fuel. If we give lambda a specific value when the fuel and air are at the ideal ratio, then we can crunch the numbers accordingly. So, if lambda equals 1 when the fuel mixture is correct, then ‘lambda = 1’ means the same as ‘the air to fuel ratio = 14.7 to 1’. This figure’s being the same as the stoichiomteric ratio is no accident!
Now for the lambda sensor, also known as the exhaust gas oxygen sensor. This lives in or near the exhaust manifold, where it can measure the percentage of oxygen in the exhaust gas the engine produces. Should too much oxygen be present, this means that there is too little fuel in the engine. Equally, too little oxygen in the exhaust gas suggest too much fuel is entering the engine.

In either event, the lambda sensor is looking for a lambda figure as near to 1 as possible. If the figure is significantly awry, the lambda sensor will communicate the fact to the fuel system’s ECU, which will put matters right.

In practice the system is regulated so the average lambda lies between about 0.996 (richer mixture) and 1.003 (leaner, or weaker mixture) at all times. In the real world, engines are often run slightly on one side or the other of this perfect mix for a variety of reasons.

This is a highly simplified explanation of how fuel is admitted to an engine. In reality, there are many other factors at work and the levels of speed and accuracy involved in maintaining the stoichiomteric ratio are, quite simply, remarkable. But the fact remains: the control in a modern engine works with the manufacturing techniques employed to make it as efficient as possible.


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