Manifold-pressure sensors measure pressure in the manifold. One common type has a small connecting hose, from the manifold to a sealed diaphragm within the sensor. Changing manifold pressure alters the position of the diaphragm, and the pressure it exerts on a sensing resistor. This provides an electrical signal, relative to the manifold pressure, which is sent to the control unit. Other devices can also be used to provide the air measurement, but all result in an electrical signal to the control unit.
Variations in air density according to temperature are measured by the air intake temperature sensor. It is installed in the main passageway, protruding into the air stream.
During cold starting, the engine needs extra fuel, and extra air also, to provide a suitable mixture, and increase idle speed. When the engine is cold, an auxiliary air valve lets air by-pass the throttle plate. As the engine warms up, the valve gradually closes. This keeps idle speed fairly close to the basic setting of the idle speed adjusting screw, which is on the throttle body.
Another method uses an idle speed control valve, with the bypass passageway controlled by the ECU. The passageway is varied for cold starting, and if the engine has extra loads such as headlights or air-conditioning. A pre-determined, programmed, idle speed can thus be maintained.
Variable inertia manifolds can vary the effective manifold length in 2 or 3 stages, to extend the torque curve over a wider RPM range. Generally, long intake pipes improve cylinder filling, and therefore engine torque, at lower engine RPM; shorter pipes improve cylinder filling at higher RPM. Cylinder filling can also be improved by using engine-driven supercharging, and by turbocharging.
In throttle-body systems, the air supply is led from the air filter into the central throttle body system. Here, the air mixes with the fuel from the injector, or injectors, and the mixture is carried through the manifold pipes into the cylinders.
Manifold design, however, is usually a compromise in order to keep the mixture vapourized. Heating the manifold by engine coolant is necessary, and the pipes tend to be smaller in cross-sectional area. This tends to limit the advantages of EFI compared to a carburetted system, but useful gains can still be achieved in power, and in reduced emissions.
As in a multi-point system, air entering the engine must be measured. This, together with the engine speed signal, provides the ECU with the basic information needed, to determine the pulse width of the injector.