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VE vs engine volume

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I've recently watched the video by Haltec where Scot says that a 5L engine with 100 percent efficiency consumes 5L of air every cycle. Therefore, following that logic a 5L engine with 50 percent efficiency (VE 50) would be consuming only 2.5L of air each cycle. But that does not make any sense to me as we have fixed engine displacement of 5L and regardless of how much air we put in that engine the volume of that air is always going to be 5L but with different density. That is why we have different vacuum reading on the gauge at different throttle position.

The only way we could think of engine volume to be changing is when we recalculate engine air density to standard conditions and use it as the point of reference to obey ideal gas low but still the physical engine volume would stay the same in reality.

Is there anything I'm missing here?

yes, the VE is relative to the ambient air, so only 2.5L of that air (at the ambient density) is consumed.

That is theoretically, whilst in reality those theoretical 2.5L of air would still be distributed within 5L of engine displacement but with lower density - is that correct?

One cycle meaning one otto cycle or one revolution? From context, I would assume the former?

Your reference to the vacuum gauge is exactly why the 'tuner' use of the term is WRONG! They are using a established term incorrectly and introducing exactly this confusion! For the same manifold pressure, one engine with 50% VE will only allow 50% of the potential air charge into the cylinder, whereas one with 100% VE will fully fill the cylinder. What the 'tuner'actually means is the increase in air mass compared to the original figure - it should be Mass Efficiency, if anything!

In other words, the true measure of volumetric efficiency is the actual cylinder filling vs the potential. For instance, if the engine had 85% VE at x rpm, then the cylinder would have 85% of the air mass it would have if it was fully filled. In short, it is how efficiently the cylinder is filled. It is NOT the same thing as the "VE" that some use for mapping, which is a BS use of the term - for example, if the engine's true VE is improved and it is restricted to the same mass of air entering it, the manifold pressure will drop and torque will normally increase. Using the BS "VE" there is no change, using the true VE, the engine breathing has improved.

It may help to think of it as lower density IN THE CYLINDER but it is a dynamic process, and density will vary on the piston position which is also compressing the air.

Gord, Otto cycle of course.

What is making me confused is the statement that depending on engine efficiency the volume of air getting inside of the engine varies. To me the volume doesn't change at all as it is fixed by engine displacement. What changes is air density within that displacement thus mass of air. But when we're talking air mass we should call it engine load, not VE.

Density change relatively to piston movement should not be taking to consideration when we are discussing cylinder volume since we are only interested in BDC position which represents full volume available.

The SWEPT volume stays the same. The ACTUAL volume of air drawn into the cylinder will vary - you no doubt are very familiar with the changes in torque and power that a camshaft change will make - the swept volume stayed the same, but because the changes made it easier for the air to enter the cylinder, it is going to be able hold more air, the volumetric efficiency has improved. Conversely, that same set of camshafts may also allow more of the air in the cylinder to be pushed back out ot low rpm, so the cylinder doesn't hold as much air and the volumetric efficiency will be less.

I assume you are familiar with the term 'dynamic compression'? If you think of that as representing how efficiently the cylinder is filled - the greater the filling, the efficiency, the greater the dynamic compression.

Again, this is why it should NEVER have been used for the fuelling, or mass air flow, of forced induction engines. It's even worse than the people who use ft.lbs instead of lbs.ft for torque - the first is work done, NOT torque - they are two different things!

Ok, let's talk specific here. We have 5 Litres engine which at 100 percent efficiency consumes 5 Litres of air at 20 degrees C and 1 Bar absolute pressure in intake manifold (NA engine) per two crank revolutions. That would fill all cylinders with air of 20 degrees C and 1 Bar of absolute pressure if we slowly rotate crankshaft until the position is at BDC.

Now, we restrict the air in intake manifold to 0.5 Bar but same 20 degrees C and slowly rotate crankshaft 2 times. The air entering engine will have 2 times less air molecules since the pressure is 2 times less. What is going to happen with the density of that air once it gets inside those 5 liters comparing to the first example with 1 bar of absolute pressure in intake manifold? Lets not talk VE but discuss air status.

It doesn't quite work like that, as it's with a running engine, but if I follow you correctly...

The engine will still be operating with 100% VE, because it is filled with 100% of the 0.5 bar air from the manifold through the ports and valves. If the manifold had 2 bar pressure in it, the cylinder would be filled with 100% of 2 bar air. However, it is the running filling that is what actually counts.

You cannot really separate the air mass/density and VE, because the former depends largely on the latter and the manifold to cylinder pressure ratio.

Try thinking of it this way - you do a compression test and get, say, 10 bar on the gauge, you adjust the camshaft timing a little and do another compression test and get 9 bar, then you adjust the camshaft timing the other way and get 11 bar. In all three cases the swept volume of the engine is the same, the manifold pressure is the same as you have the throttle wide open, yet the engine has drawn in different amounts of air into the cylinder to be compressed. The volumetric efficiency, the actual volume (mass) of air in the cylinder compared to the cylinder swept volume (or equivalent mass), is going to be greatest with the highest compression and the lowest with the lowest.

You did no answer the question)) forget about VE and everything else. What is going to happen with the air entering 5 l engine at 0.5 bar absolute pressure and 20 degrees C? How much volume is it going to occupy and at what density comparing to 1 bar in intake manifold?

Read what I said... then think about it.

Your questions, as phrased, cannot be answered in the way you seem to want. Whatever the manifold pressure is, the swept cylinder volume is a constant. The former will have 50% of the air density of the latter, and at the same point of crank rotation therefor have half the density in the cylinder, all else being equal.

Thank you very much. I read everything you are posting.

Now back to the subject. I'm glad we finally came to the point when density of air in the cylinder can be called twice less than the air density at different conditions. But as you absolutely correctly said that difference in air density was not caused by engine efficiency but something else (restriction of air pressure in intake manifold).

Now let's introduce engine efficiency to that story. Let's say the same 5l engine is only 75 percent efficient at 5000 RPM because of very tight camshafts. It means that the time the intake valve stays open is good enough to fill up the cylinder only 75 percent of its maximum available volume ( lets put aside other factors that influence efficiency for now). It means that for some time the intake valve stays closed whilst the position is still going down towards BDC still trying to suck air in but no more air is available since intake valve is already shut in. That causes the restriction in intake system effecting the air density just like in my previous example. The only difference between those two is that in first example we restricted air pressure and in this case we introduced mechanical restriction such as intake valve. But the results is the same - less air molecules have to spread out in the same 5l engine volume resulting in less air density when the piston is at BDC.

To understand that we can demonstrate a little example with plastic syringe -picture attached. It can represent cylinder volume, piston and intake valve. Try to move the inner piston quickly from top to bottom side and then the piston is half way through seal the intake plastic port with your finger. You will feel some vacuum inside the syringe as the finger skin will be sucked in as well as the piston will tend to move back to the top side. That is the same principle of intake valve closing out too early causing the less air occupy the same 5 l volume and as the result making the air density less. That is all inline with Ideal gas low that tells us if the volume is fixed then temperature and pressure will be changing. When pressure and temperature change so does density.

That is my vision of what is happening...

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Sorry about taking so long to get back to you, a few other things on.

I feel we're talking about the same thing, but from different perspectives - if I follow you, yes, that's exactly what is happening. You get more/less air mass from the manifold into the cylinder to burn the fuel and make torque/power.