Volumetric Efficiency.

[Trevor]

Originally Posted by svxistentialist

Thanks Trevor

I know you are not angry with me, and I was actually putting across the point that anger feels good, but doing something about it feels better, if you catch my drift.

You are referring to the text filter, which is hindering your means of expression.

Etc ------

Speaking of which, where is Ron?

Joe

Joe,

Thanks Joe and I know exactly where you are coming from, but it would appear that the vocabulary of the filter, as set up, is a little narrow.

Harvey,

I am quite sure from the extensive evidence appearing here that all those posting exactly understand what VE means. i.e. Volumetric/efficiency, the RELATIVE efficiency of an internal combustion engine in respect of the available swept volume. Just why and for what agenda, are you suggesting confusion in this regard ?

Ron,

Where art thou ?

[SpoonChucker]

I have been reading this thread and the Stage III thread and seen repeated references to a magic 45 degree line.

“On a side note, that is why all of the dyno plots of engines with the whipple super charger are a 45 degree angle line.”

“Again this is why engines with positive displacement blowers have a power plot that is a 45 degree line which matches the blowers aircharge output.”

I think you are trying to say that the inlet and outlet volumetric flow rates are 1:1 for any rpm, but to say that the power curve with respect to rpm forms a 45 degree line is purely coincidental. First of all the units are not the same (hp and rpm) so it is impossible to claim that the power output is 1:1 with respect to engine speed. The fact the plots supplied by Whipple form a 45 degree line, is simply a product of the scaling. By changing the scale I can claim that the line is a 60 degree angle.

[longassname]

The reason I say it is a 45 degree angle line is because the output of air is 1 rise over one run which is a 45 degree angle line. If you turn the input shaft twice you get twice as much air, if you turn it three times you get 3 times as much air. Now if you wanted to play with the scales you could certainly make the line look like something other than 45 degrees. The HP plot with the positive displacement blower is the result of combusting just about the same amount of air with each revolution by whatever the rpms and thus results in the same upward sloping straight line. Stretch the scale to make it whatever angle you want it to be but it's still a straight upward sloping line not a curve like on the na engine.

Originally Posted by SpoonChucker

I have been reading this thread and the Stage III thread and seen repeated references to a magic 45 degree line.

“On a side note, that is why all of the dyno plots of engines with the whipple super charger are a 45 degree angle line.”

“Again this is why engines with positive displacement blowers have a power plot that is a 45 degree line which matches the blowers aircharge output.”

I think you are trying to say that the inlet and outlet volumetric flow rates are 1:1 for any rpm, but to say that the power curve with respect to rpm forms a 45 degree line is purely coincidental. First of all the units are not the same (hp and rpm) so it is impossible to claim that the power output is 1:1 with respect to engine speed. The fact the plots supplied by Whipple form a 45 degree line, is simply a product of the scaling. By changing the scale I can claim that the line is a 60 degree angle.

[It's Just Eric]

The projected baseline for sylables / confusion maxes out at 7 and 7, which is the 45 line they are talking about. The actuall baseline, though it varies from the projected baseline, is close. As you can plainly see, the higher ups are only maxing out at 2 confusion for 7 confusion. They dont even max out! This is the result of forcefully induced study. And just to finalize the point, I have incorperated the average for a naturally aspired to learn redneck. As you can see, theres not much power there.
Hopefully that clears up any confusion

[longassname]

This diagram does an excellent job of illustrating exactly why the ve of the engine with a positive displacement blower is determined by the blower and not the previous intake tuning of the na engine. I will comment throughout.

Originally Posted by oab_au

Well If we can get out of the cow paddock, and get the bulldung off our feet.

We seem to have a gap, that is not being crossed, so there is misunderstanding on both sides. I don't think people understand what VE is, we talk about it, but don't really appreciate its effects.

This is a scan of the diagram that is in the Workshop Manual that shows the engine as an air pump, measured as VE.



It looks a bit complicated, but the curve shows the amount of air that is trapped in the cylinder when the inlet valve closes. The curve with the round dots on it, is the amount of air trapped with out the IRIS valve opening, just the resonate system working. The curve with the triangle markers is the amount of air with the IRIS valve closed, just using the Inertia system.

The end result that operates for us, is seen by following the top of the combined curves. The IRIS valve closes at about 1500 rpm, to allow the Inertia system to build the VE, as you can see in the lower dia. on the left. This shows the air pressure in the collector/Plenum. You can see that the inlet valve opens to a positive pressure, the pressure dips as the piston accelerates down the cylinder, as the piston slows near BDC, the Inertia continues to force the air in to the cylinder, building a positive pressure, for the valve to close on, trapping this pressure in the cylinder. This action has raised the VE from 80% to 95%.

Here you explaining how this diagram illustrates how the iris system changes the ve by creating a small pressure at the intake valve. a half a psi. Forced induction, any kind of forced induction creates much more pressure and has a similar effect but on a much larger scale. Just as the iris system which has a small effect on the pressure at the intake valve noticeably effects ve, forced induction with it's large effect on the pressure at the intake valve is the dominant force in determining ve. Even with a turbo or centrifugal supercharger the ve and the power plot determined by it is dominated by the characteristics of the compressor.

Originally Posted by oab_au

As the speed increases the Inertia system is too slow, ( as can be seen in the lower middle dia, where the pressure at the opening and closing of the inlet valve is dropping to a zero value) and the VE dropping to 92%, so at 4200 it is turned off by opening the IRIS valve, to change the system to the Resonate system.

The resonate system is tuned to a set rpm, the dia shows it as 5200. (but it would seem to be lower in practice at 4800) This system uses sonic waves to change the pressure in the inlet system, as can be seen in the lower right hand dia. the inlet valve is now closing on a positive pressure of about 5 psi. This then raises the VE to 98%.

Just so nobody gets confused by the typo. It's point 5 psi not 5 psi.

Originally Posted by oab_au

So we have a VE curve that changes throughout the rev range of the engine. This is a function of the separate components that make up the inlet/exhaust system, It has a pressure the manifold inlet of 14.7 psi absolute. We can increase that pressure to 22 psi absolute and the system will still operate the same, the VE will still be lower at 2000 rpm, maximum at 5000 rpm, to drop lower at 6000 rpm. The applied pressure will not change this effect, it will just mirror the curve at a higher value. The applied pressure cannot change the functions of these components. They will vary slightly with the changes of the resonate frequency, due to the temp and pressure changes in the inlet air charge.

In the case of a positive displacement blower we have a fixed amount of air being forced into the cyllinders with each revolution. This defines the ve. When we put 1.5 times as much air in the cyllinders we have a ve of 150. When we put 1.77 times as much air in the cyllinders we have a ve of 177. This ratio of air is forced into the cyllinders by the positive displacement blower at any and all rpms. This gives you for all intents and purposes a constant ve and not a curve. The intake and exhaust characteristics have the effect of making it easier or harder for the blower to force this static amount of air into the cyllinders. This does not change ve. The volume of air being force into the cyllinders is not being changed--the force required to drive the blower and drive the air into the cyllinders is being changed. This is mechanical efficiency not vollumetric efficiency.

Originally Posted by oab_au

As the VE curve, produces the torque curve, the torque curve will still have a dip at 4000 rpm, as we saw in the graphs, when Michael did the Stage 2 Nitrous thread. Although the system had Nitrous pressure added. The torque curve still showed the dip that the VE graph shows. Though in the Stage 3 blower, it won't slow as the Inertia system has been removed.

Harvey.

As the static ve value created by the ratio at which we are driving the positive displacement blower produces the the torque plot it will no longer represent the curve of the na engine but the linear output of the positive displacement blower. Nitrous oxide does not work in this way. Nitrous oxide adds a basically stable rate of oxygen supply over time rather than rpm this of course creates a plot with the same basic shape of the engine into which it is being sprayed but raises the values.

[Trevor]

VE is simply a criterion enabling comparisons to be made on an equal basis.

A factor which appears not to have been discussed directly here, is that the degree of density (weight), of the air fuel mixture is the useful factor involved in any estimation of VE. It could be argued that the air fuel ratio etc, is also involved, but the word volume/volumetric, surely rules this out of an exact equation or assessment involving VE. This particular aspect can be confined to measurements and comparisons relating to power output.

The effective fuel density is directly dependent on pressure, first when the mixture is in transit, and finally and ultimately, at the last opportunity for cylinder filling. Just when this is, becomes an element of time. Compression ratio is not a factor of real consideration when assessing VE as such, even though it may have some side effects relative to cylinder filling.

We are left in the main with pressure above or below atmospheric, inlet tract resistance and time (RPM), in respect of the effective elements. The pressure of the charge in transit, is dependent on atmospheric or boost pressure against resistance, within and ending the inlet tract. The ultimate useful charging pressure and density, is directly dependent on this, together with the time available for cylinder filling to take place.

All aspects of engine design (including valve timing and valves), therefore have a part to play in establishing a degree of VE. However in the case of a supercharged engine, of whatever configuration, the available boost pressure, as considered against atmospheric pressure and within a specified time, is by far the most important factor.

This is all made clear as a basis of fact, when one considers the economics of choosing supercharging over other available alternative engine modifications. Joe, you and others are on the right track on this score, particularly in respect of a useful road car.

There appears to possibly be some confusion here, such that VE can be considered as a static element, intrinsic within a design. VE is an expression of a leval of operating efficiency and this must vary within the cycle of operation, time being an important element. When making any comparison, RPM must also be specified as part of the equation.

[shotgunslade]

In this discussion, we shouldn't forget that Volumetric Efficiency VE is merely a calculated parameter relating the mass of air in the cylinder at the start of the compression cycle when it is operating at a certain rpm to the mass of air that would fill the cylinder at atmospheric pressure if it were static in that position.

As rpm rises, and the velocity of air through the intake path increases, static and dynamic pressure losses will increase, generally reducing the density of the air in the intake manifold adjacent to the intake valve. However, the system is dynamic, in that the periodic opening intake valve causes a local pulsing negative pressure that propagates through the system. This localized negative pressure, if it is at the base of a tube will cause a slug of air to accelerate toward it. Getting that air moving and having it reach the intake valve takes a short time. If the intake valve has closed when the slug hits it, there is no advantage. If it has closed and reopened, the slug of air slides right through the valve into the cylinder. As a result, the density of the air in the cylinder is increased because more air crowds into the cylinder due to its pre-existing velocity. The rate at which this slug is developed and moves is a pretty constant function of the magnitude of the negative pressure and the diameter of the tube (f=ma), and the time it takes to get moving and hit the valve opening is therefore, additionally, a function of the length of the tube. That's why velocity stacks are customized to a specific speed range by their length. At a specific rpm, the air hits an open valve, at another rpm, with a different time period between valve openings, it hits a closed valve.

The IRIS system seems to be a primitive variable length intake system, that's why the VE as a function of rpm is different with the IRIS valve open and the valve closed. Having this variation enables creating a higher VE over a wider range of engine rotational speed.

So that is the basic idea, clearly it's a lot more complicated than that because a moving fluid is a wickely complex animal, turbulence and all sorts of other things. But the point is that this velocity stack phenomenon is pretty irrelevant in a postive displacement forced draft engine, because all that these dynamic effects do is marginally change the amount of power necessary to rotate the blower, they do not change the amount of air going into a cylinder. With a centrifugal blower, it's slightly different but not much, since volumetric flow through a centrifugal device is undefined, only mass flow is defined, and the mass flow is defined by a "fan curve," relating flow to rpm to the total pressure developed by the compressor. As a result dynamic effects that slightly change the pressure against which the compressor is working also change the mass flow through the compressor.

So that's my story and I'm sticking to it, but I forgot what we were arguing about.

[GreenMarine]

Originally Posted by It's Just Eric

The projected baseline for sylables / confusion maxes out at 7 and 7, which is the 45 line they are talking about. The actuall baseline, though it varies from the projected baseline, is close. As you can plainly see, the higher ups are only maxing out at 2 confusion for 7 confusion. They dont even max out! This is the result of forcefully induced study. And just to finalize the point, I have incorperated the average for a naturally aspired to learn redneck. As you can see, theres not much power there.
Hopefully that clears up any confusion

Hahahha That's creative

[SVXRide]

Originally Posted by shotgunslade

In this discussion, we shouldn't forget that Volumetric Efficiency VE is merely a calculated parameter relating the mass of air in the cylinder at the start of the compression cycle when it is operating at a certain rpm to the mass of air that would fill the cylinder at atmospheric pressure if it were static in that position.


The IRIS system seems to be a primitive variable length intake system, that's why the VE as a function of rpm is different with the IRIS valve open and the valve closed. Having this variation enables creating a higher VE over a wider range of engine rotational speed.

So that is the basic idea, clearly it's a lot more complicated than that because a moving fluid is a wickely complex animal, turbulence and all sorts of other things.

So that's my story and I'm sticking to it, but I forgot what we were arguing about.

Just to add some fuel to the fire I believe that the Porche engineers actually have a 3 stage variable length intake system (VarioRam) that they use on their 993 Carrerra

-Bill

[Trevor]

The man riding shotgun here, has his aim correct and has amplified my brief summing up, in a detailed and lucid manner which calls for compliment. The use of the term “slug of air” is excellent and befits a shotgun expert!

[oab_au]

Good to see that progress is being made. Eric has got it by the balls, as his clear and concise description of his graph, demonstrates. Interprets my jargon, into simplicity, that makes everything as clear as mud.

I'll just clear up the Typo, I did say about 5 psi.This is what I meant. I did not work this out, that is why I said aboutThis would be a bit on the Conservative side, at that engine speed. The following scan is from the book "The Scientific Design of Exhaust and Inlet Systems" by P Smith, and J Morrison. This is a research engine only running at 3000 rpm, but it attains a positive pressure at the end of the inlet stroke of about 5 or 6 psi. As our engine is operating at a much higher speed, I would expect the pressure at the end of the inlet stroke, to be at least 6 psi.





Shotgun post is very close to the mark, he has applied his fluid dynamics to the inlet action very well. This bit is not quite right.

Quote " The IRIS system seems to be a primitive variable length intake system, that's why the VE as a function of rpm is different with the IRIS valve open and the valve closed. Having this variation enables creating a higher VE over a wider range of engine rotational speed."

The system is actually two difference systems, the low speed system is an Inertia or ram charge system, using the inertia or mass flow in the inlet to ram the air into the cylinder. The high speed system is a tuned length system, using the pressure changes produced by the sonic waves.

The system that Porsche uses is a low speed Inertia, followed by a two stage resonate system. The Inertia effect that both engines use is only possible on a 3 cylinder bank of cylinders, due to the 240* inlet duration, 240 X 3 =720*.

Anyway back to the subject. It has been said that a blower will cancel out the effects of the mechanical attributes, that produce the VE curve that the dia shows. That the boost will produce a constant VE, because the pressure is constant and it will force its air output into the cylinder, regardless of the other actions.

If we look at the VE curve that I posted, we can see that although the air pressure acting on the inlet, is a constant 14.7 psi absolute, the VE curve is lower at each end than it is at the middle. We need to know why this is so, to be able to understand how the boost pressure will affect it. Looking at the low speed end first we see that at 1000 rpm, the VE is about 80%, this in real terms means that, instead of getting 553cc of air into the cylinder ( the swept volume of the cylinder), we are only trapping 442cc of air. So why? when the piston starts moving up on the compression stroke, from BDC, the inlet valve is still open, so the piston just pushes the air out of the cylinder through the open valve. As the inlet valve won't close till about 55* ABDC, we are loosing a lot of cylinder filling.

Ignoring the Inertia system for a minute looking at the normal inlet, as the blown version does not use it. By the time we have got to 3000rpm we are trapping 497cc, and at 5000 we trap 542cc. So why did this volume increase as the speed increased. As the speed increased the Inertia has increased also. so now. instead of the piston pushing the air out of the cylinder as it moves up the cylinder with the inlet valve open, there is a higher pressure in the inlet tract. !4.7 + the Inertia pressure. This combined pressure opposes the pumping of the piston to force the air out, so we trap more air in the cylinder.

Now if we increase the air pressure acting on the inlet, with a blower adding another 7 psi for a total of 22 psi absolute. At 1000 rpm we now get 663cc of air trapped, out of a possible 829cc, if all the air was trapped. 3000 rpm 705cc, 5000 rpm 813cc. We have certainly trapping more air, but it is still loosing a fair percentage at low speed due to the piston pushing it out the open inlet valve. The 50% increase in air pressure has not overcome this effect, its total pressure is still being reduced by the VE of the inlet system.

If we now look at the other end of the VE curve from 5000rpm on. It has been said that the blower will keep pushing air in to the cylinder regardless of what the mechanics of the engine do.

At 6000 rpm the VE has dropped to 95% and by 7000 rpm its back to 80%. this equates to trapping 525cc and 425cc respectively. The VE has been rising with engine speed, but now for some reason, it is falling why????

We have a constant pressure of 14.7psi acting on the inlet, it has not reduced, so the reason has to be the time available, to get the air into the cylinder. The Piston is now moving very fast, so fast in fact that the air pressure acting on the inlet cannot get the air moving fast enough, to fill the cylinder before the valve is closing. So we are loosing out at the high speed end of the cycle also. ( we have addressed this problem with the 8mm lift cam, that opens the valve wider allowing more air to enter in the time available).

When we add another 7psi to the atmospheric 14.7, we can squeeze more air in, so that at 6000 rpm we trap 788cc, and at 7000 rpm its 663cc. Now it has been held that with a blower the pressure in the manifold will rise to overcome the time problem. This is so, but along with the rise in pressure there is also a rise in temperature that reduces the mass flow, and can bring on detonation. So the VE will still reduce at the high speed end of the rev range.

Harvey.

[longassname]

According to the factory service manual the iris system produces a point 5 psi momentary pressure at the inlet valve at it's highest. That is just a momentary pressure at the inlet valve, I believe the manifold remains under vacuum.

It's not that the pressure is kept constant, a constant amount of air is forced into the manifold and has no where to go but into the cyllinders. As a result pressure in the manifold and the cyllinders is increased. If there is more resistance the pressure is higher and it takes more power to drive the blower but the same ratio of air is being force into cyllinders otherwise where does it go? If it didn't go into the cyllinders manifold pressure would continue to escalate.

[Jade Dragon]

i agree with the green marine. Very large and overly confusing words. lol. and as shotgun said. fluid dymanics ia a b!tch of a subject. that is higher level engineering/physics class. OW my brain hurts reading all this stuff...

[shotgunslade]

OK. One more final thought. We're really beating this dead horse to hamburger, aren't we. I think I saw this thread lying in the middle of Route 1; looked like someone dropped 25 lb of raw meat on the road.

Anway, I think Harvey has a good point about the duration of the intake valve opening. At high RPM's that duration is very short, and a lot of air has to be accelerated very quickly into the cyclinder. Assuming the positive displacement blower continues to operate at approximately the same multiple of volumetric flow rate as the engine, and that the intake restrictions don't significantly reduce the density of the intake charge on the blower, then the blower is going to have to develop enough pressure to accelerate this air into the engine cylinder. As rpm's increase, and intake valve opening duration drops, the required acceleration of the air charge and the static pressure necessary to force this acceleration are going to go through the roof, causing the parasitic losses of the blower to go way up. It seems to me that a little tuning of the intake manifold could result in a local ram air effect at high rpm's, to reduce the amount of manifold pressure required to get the air into the cylinder, thus reducing the power draw of the blower. This modification is intended to be a benefit only at high rpm's when the manifold pressure might tend to get very high. Remember, there's no boost control on this blower, and that the manifold pressure is a function of the volumetric flow rate of the blower, the volumetric flow rate of the engine, and the flow restriction into the engine. Flow restriction on the engine at high rpms could increase faster that the blower because of the periodicity of the intake valve opening. I think a ram tube tuned at 6000 to 7000 rpm to deliver the air slug to the intake valve when it is open, rather than closed, would decrease the static pressure in the manifold. Just esoterica for those of us building a top fuel dragster.

[oab_au]

Sorry for the "Very large and overly confusing words". I did try to keep it as easy as I could.

Shotgun, a ram system can't work fast enough at the engine speed that this time problem exists. After all it is the mass of the air that is slowing the filling, in the first place.

I don't think that this engine will hit this problem, at the rev range that it is going to operate at. I reckoned that it will comfortably run power to 7500 at least. The torque will still peak at the same rpm, + or - a bit, but it will not drop off as quickly as the standard curve. So the HP will still continue to climb. If anything, the exhaust would be the first to restrict it.

The engine has very good air flow as it is. If at higher boost, or a higher power at the same boost is wanted, the 240* 8mm lift inlet cam will allow the VE to curve off a lot slower at the higher rpms. Supporting the torque to higher rpms.

Personally speaking, when you see how this Stage 3 set up runs, you would need to be brave to want more. It has the potential to take more boost, either with a lower comp ratio, or with boost controller, reducing the boost at lower rpms, and water controlling detonation at higher rpms.

A supercharged engine makes very usable power, right through the rev range, a joy to drive.

Harvey.