Volumetric Efficiency or VE is a measure of the actual amount of air that is moved through an engine vs the engine’s cubic capacity.
You may have heard the saying that
“an engine is just an air pump”
, what that means is if we just forget for a moment about the process of combustion that happens when the valves are closed, for every engine cycle all we are really doing is taking air from the intake manifold - sucking it into the combustion chamber - and them pumping it out into the exhaust manifold.
Or simply, pumping air from intake to exhaust – thus the description of an engine as an air pump.
If we think of our engine as an air pump, we know that for every engine cycle our pump should suck in and pump out a fixed volume of air. The volume of air our engine pumps should be equal to the engine's cubic capacity, so a 350 cubic inch engine should move 350 cubic inches or 5.7 litres of air.
However, if we were to put a flow meter on the front of our air pump engine and actually measure the amount of air that was being pumped we would find it doesn’t always equal the engine's capacity.
It's normally a little bit lower – that’s because our engine is not 100% efficient at moving air from the intake to the exhaust. If it was, we would say the engine has 100% Volumetric Efficiency.
Much of the work in engine design and engine building is focused on improving this volumetric efficiency. Things like intake runners, exhaust headers, valves, and camshafts all play a big part in how efficient our engine air pump is at moving air from the intake to the exhaust.
So that’s what Volumetric Efficiency is: a measure of the actual amount of air that moves through an engine vs its cubic capacity.
Volumetric Efficiency tells us how much air is going into the engine and knowing how much air is going into the engine is essential for figuring out how much fuel to deliver to the engine.
The right mix of air and fuel (air to fuel ratio) is critically important to not just making good horsepower, but also in ensuring an engine runs reliably and predictably for many miles.
We have already established that engine design considerations like intake runners, exhaust headers, valves, and camshafts all affect VE, but once the engine is together, they are all relatively locked in and fixed and once the engine is up and running the biggest factors that affect how efficiently our engine can pump air are engine load and engine speed.
Knowing that we can now see that volumetric efficiency is important to an engine tuner because it tells us how much air is going into the engine and we can use that information to determine how much fuel to deliver.
Nothing you're about to read in the following paragraphs invalidates the previously stated definition of volumetric efficiency, but for the sake of better understanding, it's worth delving a bit deeper into it because, technically, it’s a really poor definition. Why? Because we don’t actually measure the
volume of air (or anything else) that goes through an engine.
Confused? Keep reading and it'll all become clear. Volume is a measure of 3-dimensional space, measured in cubic centimetres or cubic inches. Air is a gas (or rather a mixture of gasses) and you can’t measure gas as a volume.
It’s a bit like saying I am going to measure the weight of my car in dollars – it makes no sense, they are different things, one is a weight the other a currency.
However, if we said cars are worth $1 a pound and my car weighs 1000 pounds, I’ve got a $1000 dollar car. That equation works just fine.
That’s similar to what we actually do with volumetric efficiency – we have a known volume of the engine and we want to calculate the mass or weight of the air that passes through the engine in one engine cycle and compare that to the mass of air that would reside in the engine's total volume.
So perhaps a better description for Volumetric Efficiency wouble be: the actual mass of the air that passes through our engine in one engine cycle vs the mass of air that would be found in the cubic capacity of that engine.
But that’s all a bit wordy and possibly confusing and definitions should be short and to the point, so let's can stick with the earlier definition.
What we are most interested in is the mass of the air, and that's because mass is affected by temperature and pressure.
This isn’t really a problem, we can control this – we measure both air temperature with the intake air temperature sensor and air pressure with the MAP sensor. So running 15 or 30 or 60 psi of boost does not double, triple or quadruple our engine efficiency. How could it?
Our air pump didn’t get more efficient at pumping air from intake to exhaust just by adding air into the intake at a higher pressure. No, our air pump is equally efficient under boost, we simply make more power because of the higher
air mass
in the same
air volume
, but we are no more efficient at moving it from one point to another.
But wait, earlier on we stated that VE varies with RPM and Engine Load, and the fuel map axis are based on these two variables precisely because they change the VE. Isn't this directly contradicting it?
While on the surface it may sound contradictory it actually isn't. That's because boost alone does not change VE. To understand that let's take a look at where that boost comes from.
Most of the time, we are getting boost from a turbocharger, and what is a turbocharger if not just another air pump, an air pump with its own set of efficiencies on both the intake side and the exhaust side.
This turbo air pump directly affects the engine air pump efficiency because it's restricting both the intake and the exhaust side of the engine air pump.
It's worth noting here that this is not the case for superchargers. While they too are effectively air pumps with their own efficiency, they do not affect the engine air pump efficiency and therefore have no effects on engine VE.
Another big restriction, the one we intentionally put in place and whose sole purpose is to reduce engine efficiency, is the throttle.
Once the engine is bolted together VE will vary with RPM and engine load, but since the engine load is controlled by the throttle we could – and often do - use the Throttle Position as one of the axes in the VE table instead of the Manifold Pressure.
Most people will be more familiar with using Manifold Pressure as the load axis of the VE table which is fine because the only way to get into the vacuum areas of the VE map is to close the throttle and so manifold pressure correlates pretty consistently with throttle position in the vacuum areas.
On the positive pressure side of things, because a turbo acts as a restriction on both the intake and the exhaust side of our engine pump, and both the compressor and turbine wheels have their own separate efficiency curves so it's not uncommon to see changes in engine VE with a change in boost due to the turbocharger efficiency's effect on engine VE.
Therefore it's perfectly acceptable to be using either Manifold Pressure or Throttle Position as the load axis along with RPM when tuning the engine's VE map.
Then of course there are things like variable cam timing, the cooling effect of methanol on the incoming air charge, wastegate opening and exhaust backpressure and intake bleed valves and nitrous injections but we won't be covering these topics in this article. The underlying principles of tuning all VE-based systems are the same.
Now that the ECU is reading the actual intake air temperature from the air temp sensor and the air pressure from the manifold pressure sensor, it can calculate the mass of air that should be entering the engine based on the engine capacity.
The ECU then looks in the base fuel map or VE map to find out how efficiently the engine pumps air from intake to exhaust at any given RPM and load. With this information, an accurate calculation of the actual mass of air passing through the engine at any point in time can be made.
Of course, knowing the mass of air going through the engine isn’t the end goal here – delivering fuel to the engine is, so knowing the mass of air going through the engine the ECU looks over to the target air-fuel ratio map to determine the mass of fuel it needs to deliver.
NOTE: When we say we want an air to fuel ratio of say 14:1, we are actually talking about the relative mass of air going through the engine to the relative mass of fuel going through the engine. So 14:1 means for every 14 kilograms of air that passes through the air we need to deliver 1 kilogram of fuel.
Different fuels have different densities and stoichiometric air-fuel ratios and all VE-based systems will have a setting to let the ECU know the density of fuel you are using and somewhere to set or select the stoichiometric air-fuel ratio of your fuel.
Now the ECU has enough information to both determine the mass of air entering the engine and calculate the mass of fuel it needs to deliver out of the fuel injectors to give you that air-fuel ratio that you are targeting in the target air-fuel ratio map.
We can now measure to see if all that math worked. We do that with a Wideband O2 sensor in the exhaust. This sensor tells us what the actual mass of burned fuel is relative to the actual mass of air.
We take this reading of actual combusted air-fuel ratio and compare it to our target air-fuel ratio map and if the actual AFR does not match up to our target, one of our settings is wrong.
It could be any of the settings mentioned earlier; you have entered an incorrect flow rate for the injectors, you have told the ECU an incorrect engine capacity, your MAP sensor or air temp sensor are reading incorrectly. All these can be easily checked and fixed.
It's most likely that the VE number entered into the base fuel map is not actually the correct volumetric efficiency for your particular engine.
We are at a point now, where we move on to the actual process of tuning or calibrating the fuel map. Which, when you boil it all down, is just the process of adjusting each of the cells in the VE map until the actual air-fuel ratio that we read from the O2 sensor matches the target in the target air-fuel ratio map.
Traditionally we would do this on a dyno, but that's a subject for another article.
• Volumetric Efficiency or VE is a measure of the actual amount of air that is moved through an engine vs the engine’s cubic capacity.
• The engine's Volumetric Efficiency map is used in an engine control unit to calibrate how much fuel to deliver to the engine.
• It’s important to ensure that you enter the correct engine capacity, injector flow rate, and target air-fuel ratio if you want the system to respond how you would expect it to.