59 Air flow measurement

An engine requires the correct air-fuel ratio to suit various conditions. With electronic fuel injection the ECM controls the air-fuel ratio and in order to do this it needs a constant flow of information about the amount of air flowing to the engine. With this information, and data stored in its memory, the ECM can then send out a signal to the injectors, so that they provide the correct amount of fuel. Air flow measurement is commonly performed by a 'flap'-type air flow sensor. The air flow sensor shown in Fig. 5.35 uses the principle of the potential divider (potentiometer).

Fig. 5.35 An air flow sensor

Figure 5.36 shows the theoretical form of a simple potential divider. A voltage, say 5 V, is applied across terminals A and B. C is a slider which is in contact with the resistor and a voltmeter is connected between A and C. The voltage VAC is related to the position of the slider C in the form VAC = VAB x x/l.

Potential divider

Potential divider

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