Air Flow Meter/Sensor: Description and Operation

Fig. 1 CIS-Lambda Fuel Injection System:

The mixture control unit is located between the air filter and the throttle plate assembly and consists of the Air Flow Sensor and the Fuel Distributor.

The air flow sensor measures the amount of air entering the intake manifold and controls the amount of fuel injected by the fuel distributor. This quantity is modified, to adapt to various operating conditions, by the frequency valve, control pressure regulator, and the oxygen sensor, Fig. 1.

Fig. 3 Air sensor:

The slightest flow of air through the air cone lifts the sensor plate causing the plate lever to lift the control plunger inside the fuel distributor.
As the control plunger rises, it allows more fuel to be supplied to the injectors.
The movement of the plate, lever, and plunger is also controlled and dampened by a balance weight and the fuel "counter-pressure" on top of the control plunger.
The more air an engine uses, the more fuel it needs to run efficiently. Since the air sensor is constantly measuring air flow and controlling fuel quantity by raising and lowering the control plunger, the air/fuel mixture should always be correct.
The air sensor operation is based on the "floating body principle" which states that a floating body suspended in a cone moves in a line according to the rate of air flow through the cone.
The CIS "floating body" is the circular plate bolted to a lever which moves freely around a pivot point. The weight of the sensor plate and lever is countered by the balance weight, Fig. 3.
At "1", slight air flow at idle lifts the plate only enough to raise the control plunger slightly. The metering slit in the fuel distributor is opened to allow enough fuel for idle. As air flow increases, more of the metering slit is uncovered until at full throttle (3), fuel flow is at the mixture, Fig. 3.

Fig. 4 Theoretical air flow graph:

This graph, Fig. 4, shows an engine's basic air/fuel requirement. Notice that with CIS, the fuel flow is automatically correct for any volume of air flow from idle to full load.

Fig. 5 Actual air flow chart:

In actual use, the air-fuel requirements of an engine are not usually a straight line on a graph: this graph shows an engine's actual air/fuel requirement as a broken line. The basic requirement of fuel and air is still shown as a solid straight line, Fig. 5.
At idle the engine runs slightly rich.
At part load the engine can run leaner for best fuel economy.
At full load a richer mixture is needed for maximum power.
Correction stages are built into the air cone to tailor the air/fuel ratio for an engine's actual requirements.

Fig. 6 Air cone:

The corrected cone shown with broken lines, causes the sensor plate to float higher with the same air flow that it would in a simple cone at idle. This raises the control plunger slightly and enriches the air/fuel mixture slightly, Fig. 6.

At part load the sensor plate and control plunger float lower than in a simple cone so that the air/fuel mixture is leaner.

Fig. 7 Backfire core:

When the engine is not running, the sensor plate and lever rest on a spring-loaded stop. The control plunger completely closes the metering ports so that fuel cannot reach the injectors. If a backfire occurs in the intake manifold, the sensor plate is forced down against the spring-loaded stop - this creates a larger opening around the sensor plate to reduce pressure and prevent damage to the air sensor assembly, Fig. 7.

The metering slits can only control the amount of fuel to the injectors if the fuel pressure difference on either side of the metering slit is kept constant. A stainless steel diaphragm separates the two chambers of each regulating valve. About .1 bar (1-1/2 psi) of fuel pressure is "used up" to deflect the diaphragm and open the spring-loaded disk valve; for this reason, pressure on the injector side of the metering slit is always .1 bar lower than on the inlet side -- in this manner the control plunger can accurately control fuel volume without being affected by pressure changes.

Fig. 8 Pressure regulating valve:

System pressure in the lower half of the fuel distributor is controlled by a pressure relief valve in the body of the fuel distributor, Fig. 8.

Fig. 9 Pressure relief valve:

The delivery pump near the fuel tank supplies fuel to the fuel distributor. The relief piston controls system pressure by allowing excess fuel to flow back to the tank through the return line, Fig. 9.