Understanding Volumetric Efficiency and Beyond 100%

[5.0stang]

First off I want to say thanks to all of those who help begineers like me out on this board...but anyways...

In lamens terms, what is volumetric effeciency...how can one go above and beyond 100%.

I was asking, because I looked at a dyno graph of a higher revving 302 (8,000 rpm) engine and it was seeing numbers in the 108-109% range.

It just seems to me that 100% is as far as it would go?

Is it a good way of telling performance of an engine?

 

[Averagesupernova]

http://www.epi-eng.com/ET-VolEff.htm

To answer your question on how you can get above 100%:
-
Suppose you spin an engine up to 8000 or that neighborhood. Suppose the intake valve is timed so it closes well past bottom dead center. When the engine is spinning at this rate and moving a considerable amount of air, it is almost as if it becomes supercharged simply due to the momentum of the air. Depending on the size and length of the intake manifold passageways the engine may be tuned in such a way so that the momentum of the air causes more air to be pushed into the cylinder while the piston is starting to move upward after bottom dead center before the intake valve closes. This can lead to a net gain over closing the valve earlier on in the cycle. Obviously an engine built with this sort of performance at the top end will lack the performance lower in the RPM curve. Since the effective compression is lowered because of the late closing of the intake, you just can't get enough air in at the low end to make any power.

 

[brewnog]

Volumetric efficiency is:

V1/V2

Where V1 is the amount of air actually drawn into the cylinder, and V2 is the amount of air that the piston would draw into the cylinder if there were no restrictions in the inlet tract. It's essentially the ratio of how much mixture gets into the cylinders to how much would get in under static conditions.

In practical terms, volumetric efficiency measures the ability of the inlet and exhaust systems to get mixture into, and combustion products out of, the cylinders. This is exactly why gas-flowed heads, large valves, low-restriction air filters etc all work.

A turbocharged engine is a prime example of where you can have volumetric efficiencies much higher than 100%, because of the increased air pressure in the inlet manifold (you 'force' in more air than you'd get if you had a quasi-static system). However, in normally aspirated engines, the clever design of exhaust and inlet manifolds can use pressure pulse interations and resonances in the manifolds to achieve the same effect, along with optimised valve timing (overlap) in order to get the best possible scavenging of combustion products from the cylinder during and after the exhaust stroke.

Now, consider a normally aspirated engine. You may assume that volumetric efficiency is highest at low engine speeds. You'd partly be right, - you've got more time for the charge to enter the cylinders before being trapped by the valve closing. As you increase speed, you'd expect volumetric efficiency to drop as the increasing airflow struggles to match the demand caused by the piston intake stroke. However, the inlet valve will typically close after bottom dead centre, allowing the momentum of the charge to 'overfill' the cylinder slightly. The timing of 'inlet valve closing' can thus be used to alter the point in the engine's RPM range where volumetric efficiency is reached. Manufacturers took this factor, and developed variable valve timing (such as Honda's VTEC system) which can constantly alter the timing of this event depending on engine speed, getting a high volumetric efficiency throughout a wider range of speeds.

Clear as mud?!

edit: Curse you averagesupernova! At least the posts agree!

 

[Averagesupernova]

It's not the first time I've been cursed. I hope it's not the last. If it were I'd be worried about dying in the near future.

 

[Cougar5.0]

Funny, my 302 has 170% VE from 2000 to 6000 RPM

Amazing how the cam designers can design a cam to trap more air than the cylinder can hold under static conditions (in a narrow RPM range). I cheated & put a positive displacement supercharger on my 5L.

 

[brewnog]

Funny, my 302 has 170% VE from 2000 to 6000 RPM

Amazing how the cam designers can design a cam to trap more air than the cylinder can hold under static conditions (in a narrow RPM range). I cheated & put a positive displacement supercharger on my 5L.

I don't see how your volumetric efficiency stays at 170% over that wide a rev band. But yeah, you can use supercharging to create any volumetric efficiency you want. It doesn't mean the engine is going to work well.

 

[Danger]

It's not the first time I've been cursed. I hope it's not the last.

Then I hereby curse you as well, just to keep you happy.

 

[Cougar5.0]

I don't see how your volumetric efficiency stays at 170% over that wide a rev band. But yeah, you can use supercharging to create any volumetric efficiency you want. It doesn't mean the engine is going to work well.

Because the screw type blower I am using will fill the cylinders with ~1.8 times atmospheric pressure starting at about 2000 RPM and doesn't stop until ~ 6000 RPM (my redline) due to the high-flow heads and slightly hotter cam I am using. Based on some data logs I have, I can only verify 2500 - 5800 RPM @ about 165% VE. I may have exaggerated slightly at the extremes

 

[polar]

It’s possible to obtain greater than 100% volumetric efficiency in a naturally aspirated (non-supercharged) engine by using tuned intake/exhaust systems. How all this works isn’t all that complicated, but it takes a while to explain it properly. I think the first use of tuned intake runners were the vertical velocity stacks. They were generally of a length that put the open bell mouth somewhere around 15 to 18 inches above the back side of the intake valve. Although both the length and diameter of the runner are important, the length is what determines the specific rpm at which the runner is tuned to provide peak efficiency.

The velocity stack utilized the fact that air is a compressible fluid to produce its boost. At wide open throttle, with the engine turning at high rpm (the rpm that the intake is tuned for), a column of air is moving at a high rate of speed down the intake pipe while the intake valve is open. The column of air moves toward the cylinder in response to differential pressure; the pressure in the cylinder is lower than the pressure at the open end of the velocity stack. When the intake valve closes, the inertia of the column of air causes it to continue moving down the intake tube, stacking up against the back of the closed intake valve, causing the intake air to compress, and creating a higher pressure right up against the valve.

Now if the high pressure air sitting at the intake valve would just stay there until the valve opens again, life would be very simple. Only it can’t, because the pressure is now lower at the inlet to the runner. So the high pressure air bounces off the closed valve and tries to move backwards toward the inlet. Since the valve is closed, the entire column of doesn’t really start flowing backwards; instead it is more like a high pressure wave propagating back toward the inlet. This high pressure wave (or pulse) leaves a low pressure behind it, and when it finally reaches the inlet, the pressure at the inlet is now greater than the pressure in the intake tube. As a result of low pressure in the tube, air starts moving back into the tube, its inertia causing it to stack up against the intake valve again, which is still closed. If the engine is turning the proper rpm (whatever the intake tube is tuned for), the intake valve opens when this higher than ambient pressure is present at the valve.

Some of the design considerations are pretty obvious. If the diameter of the tube is too large, the velocity of the column of air will be too slow to create a good inertial pulse, or reflected wave. If the tube is too narrow it will restrict the airflow and cause a performance decrease. The length of the runner determines the rpm where any boost effect will occur. Earlier it was noted that the typical velocity stacks on old race cars were around a foot long. Since I think the reflected waves that set up inside the runner propagate at the speed of sound, I think the length is such that it is three times as long as the calculations would indicate it should be. These older types used a third order harmonic, or in other words, several of these waves would be bouncing back and forth inside there at any given time. Maybe some math people here can sort out whether this is part is correct or not. I think they used to use a foot (or foot and a half) long for an rpm around 5 or 6k. It seems like there used to be a formula that was used to make some sort of preliminary “length to rpm” calculation. Testing would still be needed to fine-tune a particular setup.

The way it was explained to me, using a length tuned to the third-order harmonic gives a very deep peak when you hit the resonate rpm, in other words a big kick. The problem with this is that it is not effective when you get off of (above or below) that rpm for which it is tuned. The much shorter stacks that are common today, and the tuned induction systems seen on a lot of cars must be using first- or second-order harmonics, I’m not sure, but that would have the purpose of making the thing effective over a significantly wider rpm band, but at the cost of not producing quite as high of a peak boost.

Exhaust systems are similar, but I think they are not quite as twitchy to get right as the intake runners are. The exhaust pulses have a significantly higher pressure differential, or power pulse, that you are dealing with, to begin with. But the process is similar, except the thing is tuned to have the exhaust valve open when the low pressure pulse is present at the back of the valve.

 

[brewnog]

Maybe some math people here can sort out whether this is part is correct or not. I think they used to use a foot (or foot and a half) long for an rpm around 5 or 6k. It seems like there used to be a formula that was used to make some sort of preliminary “length to rpm” calculation. Testing would still be needed to fine-tune a particular setup.

Indeed, the maths is simple resonance theory, considering one end to be closed. Match the natural frequency of the air column to the firing frequency of one single cylinder (or an order thereof) and you won't be too far away.

Anyone who's interested can look up "Helmholtz resonators".

Incidentally, many car manufacturers now use variable length inlet runners to achieve a wider rev band over which this effect can occur. Audi's V8 engines, Alfa's 2 litre petrol engines, the 2.0l Renault Clio, and most modern Volvos (as well as some others) use some kind of arrangement to do this, although the benefits from swirl control gained from using variable inlet geometry are greater than the benefits from employing pressure-pulse interaction.

 

[polar]

Anyone who's interested can look up "Helmholtz resonators".

Aha, the resonance is the same as blowing across the mouth of an empty beer (or soda) bottle, and can be calculated the same way. Thanks brewnog!