Fuel Compensation

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Important Note

Do not use the fuel compensation to compensate for poor running resulting from other problems. Also note that changes made to the air and water temperature compensation tables may not take effect until the ignition is switched off, engine re-started and run for several minutes.

 

Air Temperature Compensation

The air temperature compensation tables determine how the ECU alters the amount of fuel delivered based on the intake air temperature.

fuel-iat-comp

There are two air intake temperature compensation tables; one for low load and idling conditions and another for medium to high load. As a guide the low load table is used below 3500 rpm at light manifold pressure, 2000 rpm at medium manifold pressure and 1000 rpm at high manifold pressure.

The compensation tables have two rows - temperature and correction factor. A higher correction factor will increase the amount of fuel delivered, a lower correction factor will decrease the amount of fuel delivered.

 

Theory of Air Temperature Compensation

 

Air density decreases with increasing temperature, so the corresponding amount of fuel needs to be less with hotter intake air temperatures. The calculation for the change in density (and theoretical change in fuel) is :

 

Density ratio = Temp 1

                     Temp 2

 

Both temperatures are measured in degrees Rankin (= degrees F + 459.67) or degrees Kelvin (= degrees C + 273)

 

e.g. if the air intake temperature changes from 80 degrees to 100 degrees, the air density change is 80 + 460 / 100 + 460 = 0.964 = -3.6 %

 

However in practice the fuel requirements of the engine are different from that calculated from the above air density calculation. In practice the decrease in engine fuel requirements is less than what the change in air density would suggest. What happens is that the speed of sound increases as the air temperature increases, and the engine will flow slightly better. Generally the air temperature compensation needs to be slightly less than half that calculated from the change in air density.

 

Tuning the Air Temperature Compensation Tables

 

The engine behaves differently at light load than high load, which is why there are two tables to tune the air temperature compensation. Generally at idle and light load the engine requires almost no reduction of fuel at higher air intake temperatures.

 

For the medium/high load table a rough guide of 1 % reduction in fuel for every 10 degrees F increase in air intake temperature. At high air intake temperatures it is usually best to not reduce the fuel as much, in order to cool the engine with a richer mixture.

 

Water Temperature Compensation

The water temperature compensation tables determine how the ECU alters the amount of fuel delivered based on the engine coolant temperature.

 

fuel-ect-comp

The engine requires much more fuel when the engine is cold, as some of the fuel will be lost as intake and cylinder wall deposits, and also in order to heat the catalytic converter up as rapidly as possible.

Note that the way the ECU buffers the water temperature in order to prevent fluctuating readings from air pockets etc. Consequently changes made to the water temperature compensation tables will not have an immediate effect. The ECU response time can be decreased by shorting the service connector.

 

Injector Phase

The injector phase (or injector timing) refers to the point at which the injector opens, and begins supplying fuel. Since the injected fuel takes some time to travel into the cylinder, typically the injector will open about 400 - 500 crankshaft degrees before TDC on the compression stoke of the engine. Intake / exhaust valve overlap is at 360 degrees before the power stroke, so this is 20-100 degrees before the intake valve opens.

 

Note that the injector phase values used in the K-Series ECU determine the end of the injector event.  ie when the injector closes.

 

fuel-phase

There are four injector phase values used: idle, cruise (low load), cruise (medium load) and WOT (high load).  The first three phase values are fixed (cannot be varied by rpm) and the full load values are indexed by rpm.

 

A table compensates the injector phase by the intake camshaft angle (VTC).  Typically as the intake camshaft is advanced the injector event is also advanced.