The Air Intake System
Last time we talked about how the fuel got to the cylinders. This time we'll talk about how the air the fuel needs to burn gets to the cylinders. Since all manufacturers do things in a different way, we'll look at these systems in general.
For an engine equipped with a carburetor, this is pretty straightforward. Air comes in the air filter housing, passes through the air filter, into the carburetor where the fuel is mixed with it. Then it passes through the intake manifold and is drawn into the cylinders. The most advanced part of the system was an Air Temperature Sensor in the air intake. It was used to measure the air temperature and, by opening and closing a flap, allow cool air in through the air horn or heated air piped in from around an exhaust manifold. This was to prevent carburetor icing that would cause the car to stall and die out. It also facilitated vaporization of the fuel into the air stream.
Very simple and straight forward. There was only one draw back; there was no way to accurately measure the incoming air for the proper fuel mixture. You could use an emission analyzer and adjust the fuel mixture, but that was only at idle and not under a load. At higher speeds the mixture would be incorrect. Some automobile manufacturers recommend adjusting the carburetor on a dynamometer with the engine under load.
In a fuel-injected car it's a whole different ball game. Air is drawn in through the air intake. This is usually a long plastic tube going into the air filter housing. The reason the intake tube is long is to get the air moving in a fairly steady, coherent stream. It then passes through the air filter and then through an Air Flow Meter.
There are two types of AFM's, a vane type and a hot wire type. The vane type has a flap that is pushed by the income air. The more air coming in, the more the flap is pushed backed. There is a potentiometer attached to the flap that sends a voltage signal to the Powertrain Control Module (PCM). There is also a second vane behind the main vane that fits into a closed camber that dampens the movement of the vane giving a more accurate measurement. There is also an Air Temperature Sensor built into the AFM and works exactly as it did in the carbureted engine.
The hot wire AFM uses a series of wires strung in the air stream. The resistance of the wire at normal operating temperature is a known constant and will allow a set voltage through it. As the air passes the hot wire, it cools. As it cools, the resistance of the wire decreases allowing more voltage to pass through it. This voltage signal then goes to the main EFI computer and allows the adjustment of the fuel mixture. From the AFM it goes to the throttle chamber.
The throttle chamber controls the airflow into the engine. When it's closed, the car idles. There is a small bypass chamber that allows a small amount of air to bypass the throttle plate and go into the engine. By adjusting the amount of air through the bypass, we can adjust the idle speed of the engine. There is also a Throttle Position Sensor (TPS) mounted on the throttle plate that sends a signal to the main EFI computer telling it weather it is at idle, midrange cruising or wide open throttle. From the throttle chamber it goes through the intake manifold and into the cylinders.
One last component is the Cold Start Valve. It is electrically heated and performs the same function as the choke on a carburetor. The Cold Start Valve is a valve with a bi-metallic element that expands as it's heated. When it's cold, it allows a large quantity of air to bypass the throttle plate thus causing an increase in idle speed. As the element heats up, it closes the bypass passage and lowers the idle speed as the engine obtains normal operating temperature.
Okay, so now you have an understanding of how the air gets to the engine. Next time we'll discuss how the PCM uses this information to adjust the fuel under different engine speeds and load. Until then, happy motoring!!!
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