If You Understand Volumetric Efficiency You Understand Engines

The volumetric efficiency table is perhaps the most important table inside any ECU. Our vertical axis is engine load which in this case is MAP or manifold absolute pressure, in other words this is simply the air pressure inside your intake manifold. Our horizontal axis is engine rpm or rotations per minute. This data can come from a crankshaft position sensor which counts the number of engine revolutions or it can come from something like an ignition coil, the ECU counts the number of ignition coil firings per minute and knows the rpm based on this.

At idle and other low load scenarios inside the intake manifold we will find vacuum, or air pressure below that of the atmospheric air pressure outside the engine. This occurs because the throttle plate is closed and prevents entry of large amounts of outside air into the intake manifold while at the same time the engine is running and the downward motion of the pistons is rapidly creating a void or absence of air above it. The same air is then mixed with fuel, compressed and burned. In other words it’s consumed. So the engine is consuming more air than the throttle plate is allowing inside the manifold, meaning that we actually have less air per unit of volume inside the manifold than in the ambient atmosphere outside the engine. In other words a cubic inch or cubic centimeter or cubic whatever of air inside the intake manifold at idle actually contains less molecules of air than that same cubic inch or cubic centimeter of ambient atmosphere air outside the engine. Because we have less air we have less air pressure, or in other words, a vacuum inside the intake manifold.

But as the throttle plate opens more and more outside air is allowed into the engine and pressure gradually increases, it transitions from vacuum to atmospheric pressure. The engine of course can’t consume the entire atmosphere and therefore the pressure inside the intake manifold becomes atmospheric when the throttle is fully opened. But notice that on our map atmospheric pressure is displayed as zero. Below zero is vacuum. Above zero is boost. A naturally aspirated engine will never go significantly beyond this point whereas a turbocharged or supercharged engine will venture into this area and how high it will go depends on how much boost is generated.

Speaking of what the engine is capable of doing we must ask what do the numbers in the table itself actually mean? 87 what? Well, this is volumetric efficiency of the engine at that particular intersection of manifold pressure or engine load and engine rpm which means that this is not 87 of some unit, it’s 87 percent. 87 percent of what? 87% of the volume of the engine, or it’s displacement. In other words 87 means that 87 of the engine’s displacement has been filled with air. 100% would mean that all of the engine’s displacement has been filled with air. As you know the displacement of an engine is actually the volume of the cylinder and the combustion chamber above the piston. 100% volumetric efficiency means that the engine has managed to ingest enough air to completely fill this space with air. What does a volumetric efficiency of 110 mean? As you can see this occurs at boost, in other words a forced induction device, aka turbo or supercharger is forcing more air into the engine than the engine would be capable of aspirating naturally. Due to the action of the forced induction device the volume of air inside the cylinder is greater than the volume of the cylinder and so the pressure of air inside the cylinder and consequently the intake manifold increases.

What’s interesting to observe is that volumetric efficiency actually starts dropping off as RPM increases. Shouldn’t the engine be ingesting more air the faster it spins? Well yes, but up to a point and this table very accurately reflects the anatomy of the engine. At low rpm we have low piston speed and because the pistons are moving slowly the velocity of the air entering the engine is also reduced so we’re unable to fill the cylinder. As piston speeds increase air velocity increases and we reach a point where we achieve maximum volumetric efficiency. But as piston speeds increase even further the intake and exhaust valves are open for ever shorter amounts of time because the opening and closing of the valves is synced to the speed of the piston via a cam chain or cam belt. At some point the valves are no longer open long enough for enough air to enter the engine and volumetric efficiency starts falling off. The engine simply can’t breathe fast enough to match the rpm.

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