1. Diesel engine control

DIESEL ENGINE CONTROL

If you are new to the idea of playing with ECU maps. Read this guide before you start.

Do not play with ECU maps until you understand what they do. Mistakes can be expensive!

A BRIEF GUIDE TO ECU CONTROL OF A DIESEL ENGINE.

To control a modern diesel engine you need to control:

Fuel injected quantity (IQ)

Fuel injection timing (SOI)

Fuel injection duration

In order to control fuel injection you must know how much air is flowing into the engine and you must know the engine speed.

So you have 5 factors very closely linked.

1. Mass of Air Flowing (MAF)

2. Fuel Injected quantity

3. Fuel injection timing

4. Fuel injection duration

5. Engine speed (rpm)

The following guide based on a 1.9 tdi pd VAG engine.

This engine has a capacity of 1.9 litres or 1900 cubic centimetres (cm³).

The exact figure is actually 1897 cm³.

The engine has FOUR cylinders so each cylinder is 474 cm³. (1897/ 4)

All four cylinders are identical so we only need to deal with one.

If one cylinder has a volume of 474 cm³, the maximum amount of air & fuel it can hold is 474 cm³.

Fuel request (Drivers wish).

The electronic accelerator pedal sends a signal to the ECU showing how much the driver is pushing the pedal down.

The measurement is usually a percentage.

0 % is no push so the engine idles. 100 % is full or Wide Open Throttle.

So at 0 % throttle the injectors must give a fixed QUANTITY of fuel, for a fixed DURATION starting at a fixed TIME. This results in a pre-set idle speed. e.g. 900 rpm.

So the idle speed has been Mapped to a specific value for MAF, QUANTITY, DURATION and TIMING.

At 100 % throttle the injectors must give a fixed quantity of fuel, for a fixed duration starting at a fixed time. So 100 % throttle has been Mapped to a specific value for MAF, QUANTITY, DURATION and TIMING.

This means that every other percentage from 1 % to 99 % will also need to be Mapped to a specific value for MAF, QUANTITY, DURATION and TIMING.

How much fuel should be Injected?

The ECU knows how much fuel to inject because it knows how much air is in the cylinder. So we need to know about AIR before we decide about FUEL.

If our cylinder has a volume of 474 cm³, the maximum amount of air & fuel it can hold is 474 cm³.

If we ignore fuel for a moment, it means the maximum amount of air that the cylinder can hold is 474 cm³

If air was a liquid, life would be easy. The amount of a liquid that fits into 474 cm³ is 474 cm³.

Air is a gas and so you can fit different amounts of air into the same space.

So how much air fits into 475 cm³ ?

This is determined by the density of the air and the density of the air depends on the surrounding temperature and pressure.

The density of air at sea level and on a warm day is between 1mg/cm³ and 1.2 mg/cm³.

Let’s assumes air density is 1.0 mg/cm³.

So our 474 cm³ cylinder will hold 474 x 1.0 mg of air, which is 474 mg of air.

So every stroke of one piston will suck in 474 mg of air. This is referred to as 474 mg/stroke.

So now we know how much air is in our cylinder (474 mg. We can inject some fuel.

Injecting fuel (IQ)

Our cylinder holds 474 mg of air.

Diesel burns at maximum efficiency at roughly 14.6 mg of air to 1 mg of fuel. So 474 mg of air can efficiently burn 32.5 mg of diesel fuel. (474 / 14.6)

So we inject 32.5 mg of fuel and off we go. Not really.

This doesn’t mean the injectors inject 32.5 mg of fuel per stroke (mg/stroke).

32.5 mg/stroke is the ideal maximum, assuming a normal air supply (EGR shut)

If the injectors inject more than 32.5 mg/stroke, some of the fuel won’t burn properly and will come out of the engine as black smoke. (This is often described as the smoke limit).

The injectors can inject any amount of fuel less than 32.5 mg/stroke and that’s what they do.

At idle the injectors may be injecting as little as 6.0 mg/stroke.

To make the engine speed rise the INJECTION QUANTITY is increased

The injection quantity is controlled by a map in the ECU often called Drivers Wish.

At idle the accelerator pedal will be set at 0 %, so no EXTRA injection will occur because idle speed is controlled by an idle speed map, not the Drivers Wish map.

When fully pressed down (wide open throttle. WOT) the accelerator pedal will be 100 %.

So the ecu receives a signal varying between 0 % and 100%.

If you apply 30 % accelerator pedal the ecu consults the built in DRIVERS WISH MAP checks the required INJECTION QUANTITY and injects that amount.

SIMPLE.

Unfortunately this is not simple because diesel engines don’t really measure how much fuel they inject.

Fuel injection is very complicated these days, so this is a very simple explanation.

Imagine a fuel injector is like a doctor’s syringe loaded with 100 mg of fuel.

The driver presses the accelerator pedal and WISHES for 30 %. The ecu consults the DRIVERS WISH MAP and decides to inject 16 mg of fuel. SIMPLE.

BUT

When do you inject the fuel and how long will the injection take?

Engine designers measure time in degrees of rotation of the CRANKSHAFT. That is why you hear people referring to engine timing.

The ideal point to inject the fuel is generally taken as Top Dead Center.(TDC). This is the point when both valves are usually shut and the air has been squashed to its maximum.

TDC is often referred to as Degrees Before Top Dead Center (BTDC) or Degrees After Top Dead Center. They are both the same thing, just opposites of each other.

So 4°BTDC is the same as -4° ATDC. (Only BTDC is used here)

Injecting 16 mg of fuel will take time (DURATION) and because the piston is going up and down, you need a START OF INJECTION point.

Lets assume 2 mg of fuel takes 1 degree of crankshaft rotation (°CR) to inject.

Assuming that the injection best time is 0°BTDC and 16mg will take 8°CR to inject. (DURATION)

Injection will need to start at 8°BTDC instead of at 0°BTDC.

Start of Injection (SOI) has to be 8 degrees BTDC so that all the fuel has been injected by 0°BTDC.

So the ecu needs maps to decide on;

INJECTION QUANTITY as requested by the accelerator position.

INJECTION DURATION as calculated from the injection quantity

INJECTION START (SOI) as calculated from the injection quantity.

Assuming that the engine is in perfect condition, the maps for Injection Quantity, Injection Duration and Start of Injection will be accurate.

So a precise amount of fuel will be injected for the correct amount of time (DURATION), starting exactly on time.(Start Of Injection)

The ecu can be sure of this because the crankshaft and camshaft sensors give precise details of the piston positions. These measurements end up on the dashboard as engine speed measured in Revolutions Per Minute (rpm).

Below is a graph showing the amount of fuel (IQ) being injected into a cylinder to cause the engine rpm to rise.

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