I was just thinking to myself today and realized a few of the car math equations that I and probably anyone else come across in our daily hobby involve knowing the volumetric efficiency of the motor in question.

So here's the questions

Has anyone ever calculated volumetric efficiency for the any/all b-series motors ?
How is volumetric efficiency effected when you change the bore of a motor, same question for the compression ratio...

I'm sure I could find all of this stuff on the net if I looked around but I figured it would be interesting to make a topic for everyone to read...Anyone who has equations/explanations please post.

 

Well (as you probably know, but for those who don't), volumetric efficiency is:

...the percent of the displacement of the cylinder that is filled with air. On a N/A Engine, there is always vacum because the piston pulles the air into the engine by way of the down stroke of the piston. So, a 1.8L engine that can be filled with 1.44L(@1atm) of air has a volumetric efficiency of 80%. The ENTIRE goal of building up the head, moding the exhaust and intake is to increase volumetric efficiency. The better the head, intake system, exhaust system flows, the more air they can move, the higher volumetric efficiency will rise. Because the more air you can get into the cylinder, the more gas you can get into the cylinder, the bigger the bang will be. That is exactly what Forced Induction does. They increase the volumetric efficiency of an engine... over 100%, up to and even over 200%. Boost is just a side effect. Shoving 2.7L(messured @ 1atm) into a 1.8L engine, will yeild 150% volumetric efficiency, and a boost reading of around 6-7psi.

Now, to answer you question:

I would imagine the volumetric efficiency would be difficult to calculate due to all the variables. You need to know the flow rate of the head, intake, exhaust systems. You can measure them individually on a flow bench, but when they are put together, the will minipulate each other. And various other factors will play into how well (efficiently) air will flow into and fill the cylinder (such as timing, displacement, cams). This is something that would be more easily messured than calculated. And unfortunetly... I do not know what the B-series motors are typically messured at. Anyone???????

As far as how displacement will effect the VF, I would think that it would actually go down. You will get more air into the cylinders because the large bore and large piston will crate a greater vaccum to pull the air into the cylinder. But your head and intake will still restrict the air flow.

For example:
Say you have seriously moded your intake, head, exhaust, and you are able to get 90% efficiency out of a 1.8L engine, your engine would suck in 1.62L (messured @ 1 atm) of air. Now if you decide your next mode is a nice re-sleeve, bore, and hone job... say to 2.0L. You will deffinitely see a great increase in power because your engine will be able to pull in more air, and use more gas. But because you are using the same head and intake, this will restrict the amount of air that will flow into the cylinder. So, your 2.0L engine may now be able to pull in 1.7L (@1atm) of air. And while you would have more air in the engine and create more power, your VF would end up being 85%. Which would be less that with your original 1.8L engine. Now, your goal is back to increasing the VF, and get the most of of your displacement.

Now as far as Compression Ratioes go, I would imagen they would have the opposite effect. Increasing the compression would decrease the displacement. And if the displacement is decreased and all other factors remain constant, I would imagine the VF would go up. But since the displacement lost due to increasing compression is usually relatively small, the VF increased would be negligable.

However, you must realize... that the VF is not intended to focused on (or a concern) when increasing displacement or compresssion. Infact, under these to sinarios, the concept of wanting to increase VF has the opposite effect on performance. The VF is a factor in the flow of the engine... ie. intake, head, exhaust.

I am not say to not to increase displacement. I think it is the best way to give your engine more potenti al (see my sig). But VF is a messure of getting the most out of your given displacement. You shouldn't be worried about VF when increasing displacement or Compression.

Well, that is my $.02 anyway.... ok, more like a $1.25 with all that typing. Fell free to elaborate or correcto me..... I'm hear to learn too!

PS Please do not rely on any of the figures I have given you. Or even assume they are educated guesses. I simply pulled them out of thin air to help explain the theory. I have no idea how much certain mods will increase your VF, or how much displament will increase your air intake or decease VF.

Also, I and I'm sure everyone else would appriciate it if anyone could supply any data, calculations, or figures for appoximating VF on engines and how different mods will affect it. Thanks!

[Modified by PSU-TEG, 1:42 PM 11/5/2002]


[Modified by PSU-TEG, 6:03 AM 11/6/2002]


[Modified by PSU-TEG, 9:41 AM 11/6/2002]

 

Volumetric efficiency is basically a measure of how well your car breathes. As the piston goes down it creates a vacum which draws air into the cylinder. In an ideal engine the cylinder would fill at ambient pressure, this would give a V.E. of 100%. But due to restrictions in the flow, (those pesky things like air filters and throtle bodies) the cylinder will never fill at atmospheric pressure. There is always a vacum in your intake. Except for the booosted folks who are running over 100% volumetric efficiency. There are many ways to calculate V.F. but the quickest and dirtiest is to attach a vacum gauge as close to the cylinder as possible, most probably the intake manifold. Record the amount of vacum and subtract it from the ambient pressure. Now divide this number by the atmospheric pressure where you live. This will give you a rough estimate of the V.F. of your engine.

 

I downloaded a VE calculator from http://www.turbofast.com. You need to know your HP rating and the RPM where it happens and it will spit out the VE for you. I used my dyno chart to figure the VE's of the motor at various RPM levels, the VE does change according to the RPM, but for other calculations people will use an average VE of the motor.

I also checked a few FI Hondas and they are way above 100% because the airflow is pressurized by the turbo.

 

Well put PSU-TEG sorry to post something similar to you I guess we replied at the same time. By the way, are you an Engineer as well. You write like one, no offence. I just had a couple more pennies to throw in. Engine mods work in one of two ways. Either it increases the efficiency with which the engine uses the fuel, i.e. increased compression. Or it increases the volumetric efficiency (head work, cams, intake etc.) There are more of these simply because they are much easier to do. The thing is that both of these factors contribute to engine power. They are dependant on each other. So if you do a combustion type mod and you don't get the power you think you should get, check how much airflow you are getting. And how you can improve it.

 

[QUOTE...the percent of the displacement of the cylinder that is filled with air. On a N/A Engine, there is always vacum because the piston pulles the air into the engine by way of the down stroke of the piston. So, a 1.8L engine that can be filled with 1.44L(@1atm) of air has a volumetric efficiency of 80%. The ENTIRE goal of building up the head, moding the exhaust and intake is to increase volumetric efficiency. The better the head, intake system, exhaust system flows, the more air they can move, the higher volumetric efficiency will rise. Because the more air you can get into the cylinder, the more gas you can get into the cylinder, the bigger the bang will be. That is exactly what Forced Induction does. They increase the volumetric efficiency of an engine... over 100%, up to and even over 200%. Boost is just a side effect. Shoving 2.7L(messured @ 1atm) into a 1.8L engine, will yeild 150% volumetric efficiency, and a boost reading of around 6-7psi.

[/QUOTE]

A good way to think about boost that I heard once is that any form of FI essentially artificially raises the ambient air pressure that the engine sees. It would be like running a NA car in Death Valley where the air pressure is slightly higher than at sea level. My point is, improving the flow capacity into and out of the engine always has benefits regardless of the setup.

 

Thanks, MrDyrten....

Yes, I am an Engineer.... well, an Engineering student (Senior).

I write like an engineer? Why, because my spelling, grammer, and typing suck?? hahaha.... No offence taken. I'm usually on here during work, so when I reply or make a post, I usually am trying to type it pretty fast, and I don't proff read it much at all. Oh well.... as I said, no offence taken....

 

Good posts...very informative, I do agree with you guys on how c/r or even bore/stroke changes wont really play a part in ve since ve is just a percentage (everybodys posts got me thinkin)

Looks like I'm sticking to using my 80-85% ve number for equations...

 

On a side note, it IS possible to get over 100% VE in an NA motor (at, or above, sea level)

 

How is that, martini??

Please, do explain!!!!!!


[Modified by PSU-TEG, 9:42 AM 11/6/2002]

 

is kinda hard to calculate, but i worked with a Subaru 2.5 RS and it had a TECII computer, i learned a lil out of this, and believe me it wasn't nice at all. We needed to come up with the volumetric efficiency at some given RPM, so when i first read your question i thought it was imposible due to the fact that VE would be different because it varies do to RPM (as well as air temp, atmospheric presure, blah blah blah, THIS IS IN A NA ENGINE, REMEMBER THAT!), NA engines are not as effcient at high RPM because they don't have time to breath that much (this is where porting and polishing IM, TB and heads kicks in), but the engine speeds kinda compensate for this. I don't really know what my point is, but i sure hope it helps developing this thread.

I guess what i'm saying is that when you calculate VE on a engine you are just doing an Average of what that engine does, and it could be low comparing of what that same engines does at low RPMs.


[Modified by GZERO, 12:57 PM 11/6/2002]

 

Yeah, VE is extremely dependent on RPM (unless you happen to have infinitely variable cam timing, lift, intake runners, and intake/exhaust diameter). For example, the VE of a B16 at 500 rpms is going to be pretty awful because the cams are designed to open the valves in a manner that befits a much higher rpm. An engine's torque curve exactly follows an engine's volumetric efficiency.

I believe the 85% VE figure that's thrown around for modern engines is typical of volumetric efficiency at their torque peak. But I could be mistaken - it might be for like a 4,000 range around the torque peak.

And compression ratio does not change displacement at all. Like MrDyrten said, compression ratio affects thermal efficiency, not volumetric. Air intake temperature is another factor in an engine's thermal efficiency - the higher the temperature, the less oxygen is in it per capita, which means you can't use as much fuel. So whoever said that forced induction increases air temperature to get more power was very, very, very wrong. Increased IAT is a negative side-effect of pressurizing air, one which people try to alleviate by intercooling.

 

VERY TRUE, that's why FI people have intercoolers which makes the car (usually) lose one PSI of boost or around, because they make air colder and colder air takes less space than hot air.

 

No it defys the laws of physics and thermodynamics

Gawd I'm a nerd.


[Modified by MrDyrten, 12:27 PM 11/6/2002]

 

That's what I say.

But I'm opened minded. I'd like to hear your case, martini.

Maybe he is just of base with his idea VE. And if he is not, I'd really like to hear how!!

martini???????

 

[QUOTE]I believe the 85% VE figure that's thrown around for modern engines is typical of volumetric efficiency at their torque peak. But I could be mistaken - it might be for like a 4,000 range around the torque peak.
[QUOTE]

Most of the equations I had in mind when I made this post were either using redline RPM ve or some % of redline RPM...90% of the time to calculate CFM

And whoever said hot air is good needs to put down the crack pipe

 

Hot air is good coming out of the crack pipe.

 

Actually, yes it does.

How do you think piston compressions change? By increasing or deceasing the dome size. you increase the dome size to get a higher compression. and by doing that the displacement is also decreased (although it is a very small decrease.. and as I said, it is neglagable!).

Compression Ration = CR= Volume BDC / Volume at TDC

(don't rely on these numbers, again I just used them to explain the theory)

CR = 10.0 sq. in. / 1.0 sq. in. = 10:1 CR Right??

Well, lets decrease the volume of each just a little due to a larger dome. Both would decrease the same amount. Well say the new pison domn has a larger area of just 0.1 sq. in.

CR = 9.9 sq. in. / .9 sq. in. = 11:1 CR

This is how you raise you compression by replacing your pistons. And there fore, you loose displacement. (but like I siad, it only decreased by 1%... somewhat negligable)

And the same goes for when you use thinner head gaskets to incease compression. you are taking away a small part of the legth of that cylinder. And therefore the displacement is smaller (all be it, it is a neglagable lost).

The only way you can increase compression well as increase displacement is with a larger stroke, and that is a whole other ball of wax.

[Modified by PSU-TEG, 12:46 PM 11/6/2002]


[Modified by PSU-TEG, 12:48 PM 11/6/2002]

 

Ditto



[Modified by PSU-TEG, 1:09 PM 11/6/2002]

 

Static compression ratio is changed when you adjust dome size, yes. But this doesn't affect displacement. Displacement does not include the size of the combustion chamber.

[(3.1416/4) x bore^2 x stroke] x (# of cylinders) = Displacement

You'll notice how neither compression ratio nor combustion chamber size is included in that equation.

Likewise, changing the stroke shouldn't affect compression ratio, unless you use the wrong size connecting rod or wrist pin location.

 

PMI, but I don't believe that forced induction necessarily increases your VE to 100%+. If you have a stock GS-R motor intake manifold is filled with 1.8L of |25psi| air, you won't necessarily ingest 1.8L of it in one complete intake stroke. I know it's getting nitpicky, but atmospheric air is already pressurized. By pressurizing it a little more, it doesn't automatically mean that it'll flow into the intake ports, past the intake valves, and into the cylinder any faster. At least that's what Corky Bell says. Iono.

 

It does make sense that you could achieve a higher than 100% VE at a certain point (or points, using variable valve technology) in the RPM band.

Air/fuel is being pulled in by the piston. So if you leave the intake valve open a tiny bit longer when the piston reaches BDC, intake velocity and resonance tuning can allow air/fuel to continue to pile into the cylinder while the piston dwells at the bottom. Since the a/f is a compressible substance, with an air intake velocity high enough, more a/f can be brought into the cylinder than it would normally hold at atmospheric pressure. It then becomes trapped when the intake valve snaps shut, before the piston begins to push it back out the top. This is the goal of the VTC feature in iVTEC - it continually adjusts the intake cam's timing to maximize this effect, giving the engine a wider torque curve - an improved VE across the powerband.

Valve overlap can also help this phenomenon - or exhaust scavenging. By leaving the exhaust valve open when the intake valve opens, the vacuum created by the exhaust leaving the cylinder can help to accelerate the process of a/f entering the cylinder. This reduces pumping losses & helps push VE up another couple percentage points.

 

Ok, true... the rated displacement of the block is not effected.

When I am refering to displacement... I was talking about the total displacement. I guess I should refer to it as the total volume of the a cylinder. I'm talking about the reality of what displacement/volume our engine actually sees Not a rated number.

And the stroke DOES effect the compression ratio. Compression Ratio is the amount that the air (volume of air) is compressed. And by increasing the stroke. you are increasing the volume at BDC, and decreasing the volume at TDC. And that increases the CR. It has nothing to do with the connecting rod or wrist pin location. Those are adjustments that can be made to compinsate for limited deck height and to optimize RS Ratios.

 

It's not that the F.I. makes the air flow any faster into the cylinder. It's at a higher pressure. So for a gas in a given volume, more pressure means litterally more gas. More molecules in the cylinder than at atmospheric pressure. This is how F.I. makes power, more air (on a mass basis) allows for more fuel to be burned in each stroke as well as adding energy to the air fuel mixture. Faster airflow is good, dont get me wrong, it will help the V.F. and fuel air mixture by bringing the cylinder filling pressure closer to local atmospheric pressure. But that local pressure is the best that you can manage naturally aspirated.

 

Yes, many newer NA motors have greater than 100% VE at specific rpms. It is not physicly and thermodynamicly impossible. Desktop Dynos has one of the better explanations of finite amplitude wave theory and how it applies to intake manifolds and ram tuning.

Daemione explanation is essentially right, but it has less to do with variable valve timing as just overlap and air volocity in the intake. Variable valve timing just allows you to flatten out the VE curve.