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 centimeters 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 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 decscription 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 affects VE?
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.
Fuel Map Axes and VE
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 back pressure 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.
VE and your ECU
The ECU needs to know a few basic parameters to do its job: