To control a modern diesel engine you need to control:

Fuel injected quantity

Fuel injection timing

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.

Mass of Air Flowing (MAF)

Fuel Injected quantity

Fuel injection timing

Fuel injection duration

Engine speed (rpm)

These factors are controlled by the accelerator pedal via the engine ECU.

Electronic accelerator pedals send 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 (wot).

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. 800 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.

Understanding the Facts and figures. (based on 1.9 tdi pd VAG engine.)


There is no real air control !!!

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)

As all four cylinders should be identical 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.

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 475 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 1milligram/cm3 and 1.2 mg/cm3.

It is easier to think of it as 1.0 milligram/cm3.

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

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

The engine doesn’t need to measure this figure because this is what it will get without trying.

So why does the engine have a Mass Air Flow meter when it is getting 474 mg/stroke Mass Air Flow?

The MAF or (Air Mass Meter. AMM) is a meter to measure Exhaust Gas Recirculation (EGR).

The easiest way to measure EGR flow is to measure MAF.

The ECU knows the engine is getting 474 mg/stroke as long as it turns over and the valves open.

So the MAF measurement must be 474 mg/stroke. If the MAF measurement falls to 274 mg/stroke the cylinder must be still getting 474 mg/stroke in order to run. So where is the other 200 mg/stroke coming from?

The other 200 mg/stroke is coming from the EGR valve as exhaust gas recirculation.

So the engine gets 200 mg/stroke EGR + 274 mg/stroke MAF which is a normal total of 474 mg/stroke.

So the ECU “knows”

The MAF figure should be 474 mg/stroke with NO exhaust gas recirculation. (EGR valve shut) and LESS than 474 mg/stroke with the EGR valve open.

So if the MAF figure stays constantly high (near 474 mg/stroke) the EGR valve is stuck shut.

If the MAF figure stays constantly low (near 274 mg/stroke) the EGR valve is stuck open.

So why does the engine have this EGR valve.

EGR helps cut down on nitrogen oxides (NOx) which pollute the atmosphere.

The 474 mg/stroke of air contain more OXYGEN than the FUEL needs to burn.

When the fuel burn is finished some of the WASTE gas produced reacts with the spare OXYGEN to make NITROGEN OXIDES.

This problem can be reduced by reducing the amount of oxygen in the 474 mg/stroke of AIR.

The easiest way to do this is to have less air.

The easiest way to have less air is to add Exhaust Gas instead. So that’s why you have an EGR valve and a MAF sensor.


Air pressure and temperature varies depending on where you live in the world, weather etc.

The following assumes an air temperature around 20 °C and an air pressure of 1000 millibar (mbar).

Let’s assume our engine cylinder is receiving air at 1000 mbar pressure and 20 °C temperature at a rate of 474 mg/stroke. (To keep things simple, let’s assume the EGR is not involved)

A typical turbocharger boost value is to add an extra 1000 - 1500 mbar of air pressure. So a typical turbo boost pressure graph against rpm will run from 1000 mbar (no boost) up to 2500 mbar max boost. (That’s an extra 1500 mbar boost)

The extra air pressure means extra air so if we have 474 mg of air at 1000 mbar we can have 948 mg of air at 2000 mbar. So with twice as much air we start with more pressure in the cylinder and can burn twice as much fuel at the same efficiency as before.

This results in the engine developing more power.

The turbo needs to be controlled because the engine design and fuelling MAP’s assume a certain BOOST level.

The engine therefore has a boost pressure sensor (manifold absolute pressure (MAP)sensor.) and Intake Air Temperature (IAT) sensor.

These sensors allow the ecu to compare current boost pressure with boost pressure MAP’s stored in the ecu.

The ecu also has a Single Value Boost Limiter (SVBL) which acts like an emergency cut off for boost.

A turbo boost MAP will be used to increase injection quantity in line with the increased boost.

Another boost MAP will compare actual boost increase (from map sensor) with the required boost increase on the built in boost MAP. (BOOST LIMITER)

The actual boost MUST roughly follow the shape of the required boost MAP. If the actual boost consistently stays too high or too low, the ecu will switch off the turbo. (Limp mode)

The turbo will also be switched off if the actual boost goes above the Single Value Boost Limiter. (SVBL).


As diesel engines don’t control their air intake, the engine needs to be controlled by the FUEL.

You can’t suck fuel into a diesel because it needs to be inserted when the cylinder has squashed the air. So the diesel fuel is injected into the cylinder. The air pressure in the cylinder is very high (squashed air) so the fuel must be injected at very high pressure.


The ECU knows how much fuel to inject because it knows how much air is in the cylinder.

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

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. The DRIVERS WISH for more engine speed is controlled by the accelerator pedal.

At idle the accelerator pedal will be set at 0 %, so no EXTRA injection will occur.

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.


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.


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

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

The ideal point to inject the fuel is generally taken as Top Dead Center. 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 will be 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 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.

So the Start of Injection (SOI) has to be ADVANCED 8 degrees of crankshaft rotation.

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 as calculated from the injection quantity.

Assuming that the engine has NO fuel pump, fuel pressure, fuel injector faults, 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 (°CR), starting exactly on time.

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).

If only life really was that simple.

If the engine only ever ran at one precise temperature and air pressure, things would be this simple.


You can’t ignore the laws of physics. Gases and liquids behave differently at different temperatures and Pressures.

So the ecu MAPs must contain correction MAPs for changes in temperature and pressure.

Pressure changes don’t concern most people living near sea level. Temperature changes concern everyone because all engines start cold and get hot.

A cold engine will inject more fuel which it should.

A hot engine which thinks it is cold will inject more fuel. This may show up as increased fuel consumption and more smoke from the exhaust.