Volumetric Efficiency

[Scott Clarke]

Anuone know what the VE is of 911 motors in various states of tune?
-Scott

[Wayne 962]

Here's something I found on the Internet:

In a four-stroke naturally aspirated engine, the theoretical maximum amount of air that each cylinder can ingest during the intake cycle is equal to the swept volume (displacement) of that cylinder (0.7854 x bore x bore x stroke). Since each cylinder has one intake stroke every two revolutions of the crankshaft, then the theoretical maximum volume of air it can ingest during each rotation of the crankshaft is equal to one-half its displacement. The actual amount of air the engine ingests compared to the theoretical maximum is called volumetric efficiency (VE).

There are many factors which determine the BMEP an engine can produce, but the fundamental determinant is the mass of air it can ingest into the cylinders, and there is a nearly-linear relationship between VE and BMEP. For contemporary naturally-aspirated, two-valve-per-cylinder, pushrod engine technology, a VE over 95% is excellent, and 100% is achievable, but quite difficult. Only the best of the best can exceed 110%, and that is by means of extremely specialized development of the complex system comprised of the intake passages, combustion chambers, exhaust passages and valve system components.

Generally, the RPM at peak VE coincides with the RPM at the torque peak. And generally, automotive engines rarely exceed 90% VE. There is a variety of good reasons for that performance, including the design requirements for automotive engines (good low-end torque, good throttle response, high mileage, low emissions, low noise, inexpensive production costs, restrictive form factors, etc.), as well as the allowable tolerances for components in high-volume production.

Knowing a claimed power and fuel-flow for a given engine, you can calculate an estimate of the VE required and use all these yardsticks for estimation of reasonableness.

Engine airflow at 100% VE, in sea-level-standard-day cubic feet per minute (scfm) is:

AIRFLOW (scfm) = DISPLACEMENT (ci) x RPM ÷ 3456

The air-to-fuel mass ratio at best power mixture is typically around 12.5 to 1. If you work through the arithmetic, you find that the airflow required at that mixture (12.5 to 1) is:

REQUIRED AIRFLOW (scfm) = FUEL FLOW (gph) x 2.72

So, if a 420 cubic inch engine produces 480 HP at 4400 RPM on a BSFC of .46, then the required airflow is 601 scfm and the VE is 107%. You already know that is a VERY big number.

Next Subject: Thermal Efficiency

I'm sure it's different for many different 911 engines.

-Wayne

[BK911]

I'm not sure what you mean by various states of tune. What are you trying to figure out?

Every engine will be a little different. Intake tract length, compression ratio, type of induction, air cleaner, cam timing, cam profile, exhaust, etc.. all have an impact. Surprisingly tract diameter doesn't greatly influence the peak VE, but changes when it occurs.

I can't remember the specifics, but I think a street car is about .8 and an F1 car is about 1.2. I would guess a 911 is closer to the .8 range. But I haven't measureed this (yet!) so hopefully somebody else has a better answer. I'm kind of curious too!

[snowman]

My all out 912 race engine simulation shows a volumetric efficentcy of about 110%. The engine actually makes almost 2 hp per cubic inch as measured on a dyno. Very difficult to do, requires 12:1 or higher compression or more and race gas, as well as a whole lot of bucks. Every thing is at the limit, ie piston almost hitting everything, ect.

In other words after 40 plus years of development by the whole world, this is the very best that can be had from a naturally aspirated 2 valve engine. Tuned everything.

The additional 10% that the F1 engines claim are due to mega mega bucks into one or two engines, not reproduceable at any other price.

[Scott Clarke]

See the link below for some interesting dyno experiments with mufflers and air filter assemblies. I'm trying to understand the behaviour of the stock airbox, as it appears to add volumetric efficiency to the motor over the middle of the RPM range. Specifically, I'm wondering if my airbox could or should be modified to suit the Short stroke 2.5 I'm building. Will the stock 2.2 airbox snorkle restrict flow for the new motor? There is a link in one of my posts in the thread referenced below to a PDF that describes a formula for determining the diameter of an inlet to a plenum. One of the variables is VE. The formula cited in Wayne's post seems to suggest that VE can be derived from HP. What is BFSC?

I realize that we are not talking about gobs of power here. I'm mostly curious about the behaviour of the airbox as a Helmhotz resonator.
Thanks for all of your contributions,
-Scott

http://d240157.u39.zeonhost.com/forum/showthread.php?t=4752

[sammyg2]

the VE on my engine is around 140% from 3500 to around 4500 where it climbs a little to probably around 150%, then starts to drop down a little at 6000 rom, falls off to around 120% at 7000

[snowman]

Originally posted by sammyg2
the VE on my engine is around 140% from 3500 to around 4500 where it climbs a little to probably around 150%, then starts to drop down a little at 6000 rom, falls off to around 120% at 7000

Real engines don't need turbos or superchargers.

Anything over 100% VE is generated by some kind of "supercharging" the engine. Without an actual supercharger this is done by taking advantage of tuned ports and the fact that the engine is a pulsed system, ie the power is generated by relatively short bursts of power. If you look up the waveshape of a math function called sin x/x in the frequency (rpm) domain you will see a big hump in the center with symetrical decaying sine waves on each side. Every time a cylinder fires you get one of these things. If you carefully line up adjacent pulses you can use the mini pulses to add or subtract from the main pulse. This is what tuning and headers take advantage of. The mini pulses stuff more air in and help suck more exhaust out. but only over a narrow rpm range.
I beleive the maximum that can be done is about 120% and then only at ONE exact RPM. Thats my basic understanding of what goes on. An automotive engineer may be able to add to this and present a more precise or correct explanation.

By using extreemly sophisticated math one can broaden the usefull RPM range, reshape the pulse ect. But there is a very finite limit to what can be done without help from a turbo or external supercharger. The trick is also not to hurt performance, which is very very easy to do.
Jack

[BK911]

BSFC is brake specific fuel consumption. Basically, how much fuel an engine uses divided by how much power it produces.

The thread you referenced talks about a 3hp loss when removing the stock air cleaner assembly. That was on a ~200hp engine. I'm not sure how valid that test is. That's only like 1%. If you run the exact same engine twice in a row you'll get that much error between runs.

The whole principle behind helmholtz resonators is creating a pressure wave that reflects back toward the intake valve right before it closes. It is only good for a specific RPM.

[snowman]

Helmholtz resonators- Isn't it possible to design multiple resonators, lightly coupled in such a way to produce a broad band frequency response, much the same way filters are designed?

[sammyg2]

Snow, my engine is real.
Your posts are unreal.

[Scott Clarke]

BK911-
Thanks for the clarification. Please note that in the referenced thread a total of 6 dyno runs were executed. The results from each tested configuration seem logical and consistent. Thre presence of the stock airbox produced more power in the range of 3000 to 6000 RPM, and less above and below that range. Again, my interest is primarily to understand the priciples involved, but it does seem like the dyno results would be of no surprise to Porsche engineers. 3 HP is a nice gain when at the same time air is been filtered and induction noise is being reduced.

[snowman]

My posts represent the performance of anyone who has "drag raced" my 912 on the straight in numerous VARA and HSR west races. Even at 2250 lbs plus 250 lb driver, I have beat almost everyone. This leaves out the driver skill part, corners. There I have not beat almost everyone, but I still came in say 2nd in class at LA grand prix, over 60 cars in race. Not to mention dyno results the prove the point. My 912 engine is very real. And I admit I did expound a bit, the dyno 2000 program only came up with 109%, not quite 110% but close enough. All that of course is based on ACTUAL measured flow rates, compression, ect and the dyno results.

As to the speculation as to how the engine works its based on 30 plus years as an MSEE in microwave communications including pulsed systems, filtering, shaping and everything I have mentioned as to how an engine, a pulsed system, might work. I did do well enough in this field to retire at age 49 with over a 6 figure income, that was over 5 years ago. so what do you mean, exactly? Exactly what is unreal?

feeling slightly somewhat more than pissed with nimrods that act more like democrats, than someone who wants to discuss the facts.

Sorry, Wayne, but I just got P off.

As for democrats, so what, they lost.

[rswannabe]

As the purpatrator of the referenced dyno testing I thought I would chime in here.

Please note that in the referenced thread a total of 6 dyno runs were executed.

Actually Scott, there were 11 dyno runs altogether. I made multiple runs in each configuration (excepting the fully filterless configurations) and saw very consistent and repeatable results.

The thread you referenced talks about a 3hp loss when removing the stock air cleaner assembly. That was on a ~200hp engine. I'm not sure how valid that test is. That's only like 1%. If you run the exact same engine twice in a row you'll get that much error between runs.

BK911, the difference in performance between the stock airbox and the K&N filters were consistent over all runs made. The stock airbox made more power up to 6,600 rpm, the greatest difference being 7 hp at 6,200 rpm. This was not merely an instance of run to run variation.

Brooke

[beepbeep]

Originally posted by snowman
Helmholtz resonators- Isn't it possible to design multiple resonators, lightly coupled in such a way to produce a broad band frequency response, much the same way filters are designed?

Yupp.

Mazda had telescoping intake ducts, Porsche had Varioram...all this is just another way of adjusting resonance frequency and ramming little more air down the cylinder.

Actually, it's not just "resonance" that is raising VE, otherwise you would see sharp jumps in torque only at certain RPM's. Ait has certain inertia so it keeps on moving trough intake ducts even as valve closes. Careful shaping of intake ducts will broaden this effects to other RPM's.

Snowman:
So no "real" engines are turbocharged? Your "race" 912 engine is great? Pistons almost hitting things? You dynoed it to 2hp per cubic inch? Ohh wait...it's simulated!

You're pulling so many things out of your a$$ and doing it in such arrogant way that it's often very tiring to read your posts. Fortunately, there is a ignore-option.

[BK911]

BK911, the difference in performance between the stock airbox and the K&N filters were consistent over all runs made. The stock airbox made more power up to 6,600 rpm, the greatest difference being 7 hp at 6,200 rpm. This was not merely an instance of run to run variation.

Oops! I just went back to reread the other post. I didn't realize there was more than one page. My bad. Maybe next time I'll know all the facts before I open my mouth. Nah, why start now!

Beautiful car.

[sammyg2]

Yep, one more for the "ignore post" club.
Bye Bye snowman, btw your resume means nothing to me, hopefully it means something to you. it appears you are trying to compensate.

[snowman]

Originally posted by beepbeep
Yupp. ...

Snowman:
So no "real" engines are turbocharged? Your "race" 912 engine is great? Pistons almost hitting things? You dynoed it to 2hp per cubic inch? Ohh wait...it's simulated!
...
.

The dyno was real, not simulated. Simulation was done as well, agrees with actual performance.

Isn't a turbo or supercharger something like viagra? Can't quite get the job done without some help.

sg
I seem to irritate some people. Good.

"telescoping intake ducts, Porsche had Varioram" these are crude, very very crude attempts to broaden the band or resonance. Sophisticated ways involve multiple resonant cavities that can remain fixed and therefore free of mechanical difficulties involved with variable anything. As to sophistication I am also talking a couple of decades of development beyond the mere application of additional resonators, to really sophisticated broad banding techniques used today. If you understand what I just said then we might be able to talk sensibly.

I am not trying to "talk down" to anyone, but outlandish criticism of something that one dosen't understand is also unwarrented. There is problably someone who DOES unserstand what I have stated, and I would LOVE to discuss it with them. I have made an honest effort to present my theory in a way that almost anyone who tries to think about engine theory can try to participate in the discussion without advanced anything.

And even an absolute moron could figure out that I really have nothing against supercharging or turbocharging. If my statements that only "real" engines do not have super chargers or turbochargers somehow caught in your graw, you may neee some psycharitric help to overcome your t urbo super charged mentality. The original topic was naturally aspirated engines. For someone to try and interject a supercharged engine into the discussion without mention of the effect on efficiency is just plain ---irrelavent, ie dumb.=

[1980 911 sc]

wow a supercharged pissing contest!!

[snowman]

NAW! Naturally aspirated pissing contest.

And just in case the super charged ignore artists didn't notice I didn't claim to have invented anything, I just did the same thing that umteem years of dedicated Porsche finatics did and copied them as well as I could. Thats all that I am claiming, what they have already proven. I didn't do any better, but I did do as good as they did, and they did very well indeed.

[RoninLB]

Originally posted by snowman

NAW! Naturally aspirated pissing contest.

my pissing contest is how can I further atomize accelerator fuel in carbs.. huh?