EFI : BUSY (THROTTLE) BODIES

By Dr. Horizon Gitano CBT

Back in the day when cars had carburetors, we expected to see a bunch of complex mechanical parts around the throttle. There were venturies and fuel jets spraying fuel into the air flow, choke valves, vacuum pressure lines, fuel lines, electrical fuel cutoff valves, vacuum controlled throttle props, and etc. But once we switched over to electronic fuel injection (EFI) the carburetor was replaced by a relatively simple throttle body (and of course the fuel injection system down stream of the throttle body). Basically this is a very basic device; it is just a “butterfly valve” that lets you control the amount of air entering the engine, and in some simple EFI systems that’s it. Well, almost.

The EFI system needs a way to estimate the air flow going into the engine. That’s were the first “addition” to the humble throttle body comes in:  the Throttle Position Sensor (TPS) measures the position of the throttle so the EFI controller can tell how hard your pushing the peddle. In order to supply the correct amount of fuel, the EFI controller needs to know how much air is going into the engine. In some simple systems the air flow is estimated by the throttle position and the engine speed. This isn’t the most accurate way of measuring the actual air flow, for a number of reasons, not the least of which is the non-linear air flow of a butterfly valve. From idle to wide open the throttle is rotated a little less than 90 degrees. The airflow changes a lot in the first 10 degrees of rotation (ie. from idle to 10 deg. open), and almost none in the last 10 degrees (from 80 deg. to Wide Open). Air is compressible, and tends to “squeeze” around the throttle, so that by the time it is rotated half way (45 deg.) the airflow is almost the same as wide open throttle

The actual amount of air drawn into the cylinders depends on the air density in the intake manifold. The air density is a function of air temperature and pressure. Most modern fuel injection systems measure the pressure in the manifold (MAP or Manifold Air Pressure) as well as the manifold air temperature (MAT). Some times the MAP sensor is connected to the throttle body or manifold by a small hose, otherwise it may be located directly on the throttle body or intake manifold. The MAT sensor has to be located in the air flow, so it is usually located directly on the throttle body or manifold. With these additional sensors the EFI controller can more accurately estimate of the actual amount of air entering the combustion chamber.

While the speed and throttle position are sufficient to estimate the airflow into the engine, using a “speed-density” technique (which requires the MAT and MAP sensors) generally gives better results. The TPS sensor is still used, however. It serves to indicate when the throttle is at idle, and is also is used to detect rapid changes in the throttle position, like during rapid throttle opening or closing. Modern EFI controllers have gotten pretty sophisticated and usually they can detect if one of the sensors is faulty or disconnected. If, for example, the MAP sensor goes bad, the controller can always fall back on the TPS sensor and estimate the airflow into the engine, allowing you to drive it back to a mechanic with an OBD (On Board Diagnostics) reader who can directly read the “faulty MAP sensor” code, and fix it. Well, at least that is how it is supposed to go anyway!

Not satisfied with the air density measurements some EFI systems have gone on to include air flow sensors, or Air Mass Flow Sensors. These sensors are generally of 2 types: 1) a “hot wire anemometer” or 2) an air flow deflection plate. The hot wire anemometer is a thin heater wire in the air flow with a temperature sensor. As air flows past it, it cools the temperature sensor and wire. A control system is measuring the temperature of the sensor, and controlling the current to the wire heater to maintain a constant temperature. So as air flow increases the current required to maintain the temperature of the sensor increases, thus the current is a fairly direct measurement of air flow. The deflection type air flow sensor is simply a “flap” or vane in the air flow. As more air flows past the flap, it is rotated out of the way. The position of the flap can be measured by a device similar to the throttle position sensor. Either of these systems can give a good measurement of air flow into the engine and are used as the fundamental air flow measurement in many EFI systems. 

You know how when you click on the air con the engine either surges, or slows down? Well in the old days there was usually a solenoid actuated vacuum line connected to the throttle of the carburetor. Switching on the AC also turned on the solenoid valve, propping open the throttle slightly. If the adjustment at the carburetor end was done right the engine would speed up slightly, and if it was wrong the engine would usually slow noticeably. Modern EFI engines have different ways of doing the same thing. Usually there is an idle air flow screw on the throttle body, or more rarely an idle throttle stop adjustment, but you’ll often notice an electronic Idle Speed Control (ISC) or Air Bypass Valve (ABV) device allowing air to bypass the throttle and enter the intake manifold. These can be simple solenoid valve devices, or motor driven valves. They are used to maintain a constant engine idle speed, or even run at a higher idle speed when cold or just after starting the engine. Of course they can also be used to compensate for the air conditioning load on the engine as well.

These idle speed controls are often used in a “closed loop” mode, that is the EFI controller is measuring the actual idle speed, and comparing it to the ideal speed for the given operating condition. From the TPS sensor the controller knows that the throttle is closed, so the target idle speed is selected based on the temperature of the engine, and whether or not the air conditioning is on. If the engine is running faster than the desired idle speed the idle control valve is closed slightly (and slowly). If the engine is too slow, the idle control valve is opened slightly. A common mistake made by “old school” mechanics is trying to adjust the idle speed of a modern engine using the idle air bypass screw: usually the EFI controller will simply adjust the air flowing through its electronically controlled idle speed valve to counteract whatever the mechanic is doing.  It is a “closed loop” control because the idle speed is actually measured, and the valve is then adjusted to bring the idle speed back to the desired valve. The old AC controlled solenoid valve throttle bias was a case of an “open loop” control system: it could affect the output (idle speed) but wasn’t directly controlled by the idle speed.

Some systems have a motor connected to the throttle to adjust the throttle position at idle for the same purpose. “Drive By Wire” systems take it a step further: the throttle valve is directly driven by a motor with no mechanical connection to the accelerator peddle at all. In these systems when you “stomp on it” all you are doing is pushing a sensor that requests torque from the engine. The controller actually opens the throttle via the motor connected to it. While this may sound like an unnecessary extravagance, it actually makes sense: this way the controller can open the throttle at a known rate (rather than simply reacting to how the user opens it) helping to maintain better control over the engine’s emissions. Also now the functions of the idle speed control can be taken over by the throttle motor, somewhat reducing the complexity of the throttle body.

While throttle bodies in modern EFI engine are fairly busy, they are actually a lot less complicated than the vacuum hose, solenoid valve adorned carbureted monstrosities of the 1980’s. Also the EFI systems last longer, and require almost no adjusting ever and provide much better control over fueling.