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Fuel mixture ratio

Figure 3- The Perfect or 'Stoichiometric' Ratio

For perfect, efficient combustion to occur, the ratio of air to fuel should be 14.7 to 1. That is 14.7 parts air to one part fuel. The lambda sensor is fundamentally important in allowing the ECU to maintain this ratio, as the sensor is designed to detect this crossover point from below 14.7:1 - ie. too much fuel = mixture too rich, to above 14.7:1 - ie. too much air = mixture too weak. We denote this with the Greek letter "L" or "lambda". The letter "lambda" represents a quantity called the 'excess air factor'. The perfect ratio is Lambda =1.0. The scientific name for this perfect ratio is the 'Stoichiometric Ratio' . It means there is neither too little, nor too much air. The value 14.7 is calculated from the chemical composition of the fuel, and varies slightly amongst fuel types, for example LPG and Diesel may have differing values, but the principle is the same.

If there is too little air, the mixture is too rich, and there will be fuel left over after combustion. This pollutes the environment. If there is an excess of air the mixture is too lean, and more NOx pollutants are produced. We will see more about the main pollutant groups later. However, the actual amount of air that the engine can suck in at any time depends upon many things, for example the altitude, the air temperature, the engine temperature, the barometric (atmospheric) pressure, the engine load, and so on.

Figure 4- A Catalytic Converter

In theory it is possible for the ECU to 'guess' how well the combustion is occurring. Other sensors in the engine show how much air is being sucked in, and the ECU should admit the correct amount of fuel via the fuel injectors. Old cars ran perfectly well without the need for a lambda sensor. But this was before the introduction of catalytic converters. They require precise mixture control to ensure best performance and minimum emissions and to maximise catalyst life. Catalytic converters have been introduced to combat air pollution from vehicles.

Modern ECU's use the lambda sensor to provide best performance over a range of driving conditions, and also to protect the cat, which is an expensive and delicate item.

When the lambda sensor fails, the ECU can no longer sense the air/fuel ratio, so it ends up guessing. It may accompany this with a light on the dashboard to let you know something is amiss, or because of the gradual deterioration of the sensor it may not notice there is a problem. Your car then performs poorly and uses more fuel than it needs to.
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