AIR
How Does the Engine Know How Much Air It Is Breathing?
In Part 1, we looked at why modern diesel engines produce emissions and why manufacturers developed increasingly sophisticated after-treatment systems to deal with them.
Before we can understand how those systems work, we need to go back to the beginning and the earlier article: The basics – Part 2 – The internal combustion engine.
The engine itself
A diesel engine needs three basic things to produce power:
- Fuel
- Air
- Compression
We generally think about fuel first because that’s what we’re injecting into the engine. But, from an emissions point of view, the amount of air entering the engine is just as important.
The engine control system therefore needs to know how much air is actually entering the cylinders.
How much air is actually entering the cylinders?
There are several ways of finding out.
A diesel engine doesn’t normally control its power by restricting the amount of air entering the engine in the same way that a traditional petrol engine does. Instead, a diesel generally has plenty of air available and controls power primarily by changing the amount of fuel injected.
- More fuel = more power
- Less fuel = less power
Simple.
But there is a limit.
If we inject more fuel without providing enough air to burn it, combustion becomes incomplete and the result can be:
- Increased soot
- Black smoke
- Poor combustion
- Increased fuel consumption
- Increased emissions
So the engine control unit needs to know how much air is available before it decides how much fuel it can safely inject.
This is where our sensors come in.
The MAF Sensor
One way of measuring the air entering the engine is with a Mass Air Flow sensor, commonly called a MAF.
Its job is exactly what its name suggests, it measures the mass of air flowing into the engine.
The important word here is mass.
It doesn’t simply measure how fast the air is moving, it is trying to determine how many grams of air are passing through the intake system every second.
For example, the ECU might see:
20 g/s
Then under increased load:
80 g/s
And at high engine load:
200 g/s
The actual numbers will vary enormously depending on engine size and operating conditions, but the principle remains the same.
The ECU now has a measurement of the amount of air entering the engine.
Why Measure Mass Rather Than Volume?
To understand why the ECU is interested in mass of air, rather than simply the amount of air moving through the engine, we need to understand something very basic about air.
Air has weight.
You can’t see it, and you probably don’t think of it as having much weight, but every litre of air contains a certain amount of mass.
The amount of mass contained in that litre can change.
Let’s imagine two identical one-litre bottles.
We fill one bottle with air at sea level and we take the second bottle to the top of a very high mountain and fill it with air there.
Both bottles contain exactly one litre of air, but the air in the mountain bottle weighs less.
Why?
Because the atmosphere is thinner at high altitude.
The Atmosphere Is Pressing Down On Us
The Earth’s atmosphere is essentially a huge ocean of air. The air above us has weight, and that weight creates atmospheric pressure. At sea level there is a very large column of air above us, pressing down on everything below it.
As we climb higher, there is less air above us, and the weight of that column becomes smaller.
Therefore, atmospheric pressure decreases.
At sea level, atmospheric pressure is roughly 1013 mbar.
At a high altitude it might be considerably lower.
The important part for our engine is this:
“Lower pressure means fewer air molecules packed into the same space.”
So although both bottles contain one litre, the bottle at altitude contains fewer air molecules.
Fewer molecules means less mass.
Temperature Does the Same Sort of Thing
Temperature also affects air density.
Imagine a sealed room containing air.
Now heat the air.
The air molecules gain energy and move around more vigorously. If they are free to move, they spread further apart.
The result is less mass of air in a given volume.
Cool the air again and the molecules move less energetically and the air becomes denser.
This is why a litre of cold air contains more air mass than a litre of hot air, assuming the pressure conditions are comparable.
And this is particularly important to an engine.
Why Does the Engine Care?
Let’s imagine our engine is swallowing 500 litres of air every second. That sounds like a useful measurement, But it isn’t enough.
If that 500 litres consists of very dense, cold, high-pressure air, the engine is receiving a large mass of air.
If it consists of hot, low-pressure air, the engine is receiving a smaller mass of air.
- The volume is the same
- The mass is different
And combustion doesn’t care about litres.
The fuel molecules need oxygen molecules to burn.
So the ECU needs to know, directly or indirectly, how much actual air mass is available to mix with the fuel.
This is why a MAF sensor is so useful.
Rather than simply saying:
“A certain volume (litres) of air is flowing through the intake.”
it can tell the ECU:
“This much mass (grams) of air is entering the engine every second.”
The ECU now has information that it can actually use when deciding how much fuel can be injected.
But Not Every Engine Has a MAF
This is where things get particularly interesting.
If you’ve spent years working on heavy-duty diesel engines, you may have already noticed something.
Some engines simply don’t have one.
So how does the ECU know how much air is entering the engine?
Well, It calculates it.
And this brings us to another important sensor.
The MAP Sensor
MAP stands for Manifold Absolute Pressure. It measures the pressure inside the intake manifold. Unlike a MAF, it doesn’t measure airflow, It measures pressure.
This distinction is extremely important.
- A MAP sensor might report 1000 mbar absolute with the engine stopped
- The engine starts and the pressure changes
- The turbocharger then increases the pressure further as engine load increases
The ECU can therefore monitor the pressure available to the engine.
But pressure alone still doesn’t tell us exactly how much air is present.
We need one more piece of information.
Temperature Changes Everything
Remember our two one-litre containers? If one contains hot air and the other contains cold air, they don’t contain the same mass of air.
The same thing happens inside the engine.
This is why intake air temperature is important.
If the ECU knows:
- Pressure
- Temperature
- Engine speed
- Engine characteristics
it can calculate an estimate of the amount of air entering the engine.
This is often referred to as speed-density calculation.
The exact calculations are considerably more complicated than this, but the principle is quite simple.
Pressure + temperature + engine speed + engine characteristics = estimated air mass.
A Volvo Example
This is something you’ll see on many older Volvo diesel engines.
Take a Euro 3 Volvo engine. There is a sensor mounted on the intake pipe between the air filter and the turbocharger. Volvo refers to it as a vacuum sensor, and it measures air filter pressure and air filter temperature.
At first glance, you might think:
“Why does the engine need to know what’s happening before the turbo?”
There is actually some useful information there.
When the engine is stopped, the pressure at the sensor should be approximately the same as atmospheric pressure. Once the engine is running, the turbocharger is drawing air through the air filter. If the filter becomes restricted, the pressure on the turbocharger inlet side drops.
- A clean filter produces only a small pressure drop.
- A restricted filter produces a much larger pressure drop.
The ECU can therefore use the pressure signal to monitor the restriction of the air filter.
The sensor also measures the temperature of the air entering the turbocharger, giving the ECU another important piece of information about the air available to the engine.
Notice that this is not a MAF sensor. It isn’t measuring how much air is flowing into the engine. It is measuring the condition of the air entering the turbocharger , specifically its pressure and temperature.
What About the Sensor After the Turbo?
Now we move to the other side of the turbocharger.
The sensor commonly referred to as the boost pressure sensor or MAP sensor measures the pressure in the intake manifold.
This tells the ECU what pressure is actually available to the cylinders.
It can then compare that with what it expected.
For example:
Requested boost: 2.0 bar absolute
Actual boost: 1.4 bar absolute
Something isn’t right.
The ECU now has a reason to investigate.
It might be:
- A boost leak
- A turbocharger problem
- A VGT problem
- An air restriction
- An actuator problem
- A sensor problem
The important thing is that the ECU doesn’t necessarily know which component has failed.
It knows that the numbers don’t agree.
And that distinction is extremely important when diagnosing modern engines.
MAF and MAP Are Not the Same Thing
This is probably worth making absolutely clear.
A MAF tells the ECU:
“This is how much air is flowing into the engine.”
A MAP sensor tells the ECU:
“This is the pressure available in the intake manifold.”
They are measuring two completely different things.
You can have:
High airflow with low pressure
or
High pressure with relatively low airflow.
Pressure and flow are related, but they are not the same thing.
This is exactly the sort of distinction that becomes important when diagnosing a turbocharged engine.
So Why Have Both?
On an engine fitted with both sensors, the ECU has two independent pieces of information.
It can ask:
“How much air went in?”
and:
“What pressure did I create?”
If the answers don’t make sense together, we have a clue.
Imagine the MAF reports a large amount of air entering the engine, but the MAP sensor reports that the intake pressure is much lower than expected.
- Perhaps the turbo isn’t producing enough pressure.
- Perhaps there is a leak after the turbo.
- Perhaps the MAF is lying.
- Perhaps the MAP sensor is lying.
- Or perhaps another part of the system is affecting the result.
The ECU can’t simply assume that one sensor is correct.
It has to look at the whole picture.
This Is Where Diagnostics Begins
This is an important concept that will come up repeatedly throughout this series.
A sensor value by itself doesn’t necessarily tell you that something is wrong.
You need to ask:
“Does this value make sense when I compare it with everything else that is happening?”
For example, suppose a diagnostic tool tells you:
MAP = 1500 mbar
Is that good or bad?
There isn’t enough information to answer the question.
What is:
- Engine speed?
- Engine load?
- Atmospheric pressure?
- Requested boost?
- MAF?
- Turbo position?
- Fuel quantity?
- EGR position?
A pressure value without context can be almost meaningless.
The same applies to almost every sensor on a modern engine.
The ECU Is Building a Picture
Think of the engine control unit as trying to build a picture of what is happening inside an engine that it obviously cannot see.
It has no little man sitting inside the intake manifold with a clipboard.
Instead, it has sensors.
It knows:
- Engine speed – How fast is the engine turning?
- Air pressure – What pressure exists at different points in the intake system?
- Air temperature – How dense is that air likely to be?
- Air mass – How much air is actually entering, if a MAF is fitted?
- Fuel quantity – How much fuel is being injected?
- Turbo position – How much is the turbo being asked to work?
From all of this information, the ECU can calculate what it believes should be happening. It can then compare that with what its sensors are actually reporting.
And when the two don’t agree…
Diagnostics begins.
The Important Part
It is tempting to think of sensors as individual components that each have one simple job.
- The MAF measures air.
- The MAP measures pressure.
- The temperature sensor measures temperature.
But the ECU doesn’t really care about the sensors themselves. It cares about what those measurements tell it about the engine.
A MAF isn’t important because the MAF exists. It’s important because knowing the mass of air entering the engine allows the ECU to make better decisions about:
- Fuel quantity
- Smoke production
- EGR flow
- Turbocharger control
- Combustion
- Emissions
And that last point brings us neatly back to where we started.
Air management and emissions control are not separate systems.
They are deeply connected.
Coming Next
We’ve now established that the engine needs to know how much air is available. But getting the right amount of air into the cylinders is another matter entirely.
That’s where the turbocharger, variable geometry turbochargers, charge-air cooling and EGR come into the picture.
And this is where things start getting really interesting.
Because the turbocharger isn’t simply there to make boost. It is one of the tools the ECU uses to control airflow, combustion, emissions and even exhaust temperature.