Volumetric Efficiency

[Mr Beau]

Well Snowman (if that is your real name...), you've said some assine things and have pulled me into this ridiculous thread...

Originally posted by snowman

"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.

OK Snowy, take your Sin X/X calculations (in the frequency domain) you like to mention, and do some Helmholtz calculations. After doing these obviously simple calculations on the back of any used stamps you have lying around, you'll see that there are finite resonate points for a set of given dimensions. If you do a good job of packing everything under the hood, you'll find that in the engine speeds you'll be using, have one strong resonance and probably a second smaller. That's it. If you want more than two of these peaks, you need to change intake dimensions. This includes lengths and volumes. Three guesses how to do this, but the first two don't count, and you're not allowed to reread first sentence I quoted above...

Do you know what a Helmhotz resonator is to begin with??

I am not trying to "talk down" to anyone, but outlandish criticism of something that one dosen't understand is also unwarrented.

So why do it???

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.

An honest effort includes a certain level of humility when one does not fully understand a topic.

[snowman]

Well a Helmhotz resonator is nothing more than a simple jug with a small necked opening on it. But if you link several, say 3 such resonators togather with a set COUPLING factor, say a specific size and length of tubing, you can create a much broader band resonance effect, and you can do so without giving up much (unfortunately you must give up some but not a whole lot) of the peak. By broader band I mean RPM range in the case of engines.

An typical engine is a PULSED system, that is it is not continuous power, but short bursts of power produced each time a cylinder fires. At any given time, a very short time, the engine is operating at a fixed RPM. Say over a one half second period. If you look at the performance of the engine in the " frequency domain" ie RPM not time domain it can be analyzed with fourier analysis of the pulses produced by the cylinders firing at constant RPM. The shape of the pulse produced by the cylinder burning its charge, the length of the burn, and the frequency that the cycle is repeated can be presented graphically using something that will look like this sine x/x thing I have mentioned. You just have to look up the math function sine X/X to see what one looks like.

OK this is a lot of hokey pokey but why do it? Well it lets us look at how the previous and next cylinder firing will affect the engine. If you try and discribe whats happening by a sequence of events its totally confusing, but by showing it in this way you can just place each graph next to each other and see how every little ripple from the previous and even next cylinder firing affects each other. And even better you can use very high powered math to taylor the exact shape of these interactions and force them to match a predetermined shape, in the case of an engine the broadest higest power band possible.

The names for these responses are Chebeychev, maximally flat, gaussian, as well as dozens of others, each with their own special chacteristics. The math is well established, the use of the math for making filters, the mating of several resonators using some kind of coupling makes a filter, is also very well established. The use of this type of math and theory for engines is not known to me as I am not an automotive engineer, but it must include at least some of this technology at this time. Very advanced use of this technology for filters I am familiar with, and I so no reason that it dosen't apply to engines. The very advanced part simply allows a broader band, ie broader range of RPM to be used at maximum performance.

I want to try and keep discussions on line to visual conceptual discussions, off line I would consider mathematical discussions.

Everyone knows tuned headers make more power, how? well they line up the adjacent pulses in this sine x/x pattern produced by the engine so they add up. Stepped headers make power over a wider RPM range, how, they take into account of a range of pulses and make it so they can also add up in phase to make more power. How do we know how long to make stepped headers, how many steps, ... the math of the pulsed systems I am talking about tells us, the graphs and discription in the frequency domain (rpm range) helps us see how.

[snowman]

Here is one of the sinx/x things I have been talking about.

The most interesting thing about this is NEGATIVE time is shown. The cylinder fired at the center, the effects both BEFORE as well as after are shown.

So if you place the next cylinder to fire where it belongs next to this one,you can see the effect of the cylinder that fired before the next one on the next one as well as the ones that follow. Try to do that with some time sequence of events and keep track of EVERYTHING.

It really does make sense. Say the intake valve closed and generated a back wave, the next intake pulse will run into the previous intake pulse won't it. Not really negative time, but a way to show the impact on a current event by past events.

[dwightp]

Here we go again...

Head flow question

There is a reason I dropped that thread.

I probably should have exercised restraint here. Naaaaaaah.

[camgrinder]

Something I have wondered about is the formulas people use to determine Volumetric effciency. Seems like most of them ask for the engine size based on swept volume. The problem I see is the cylinder is about 10% larger (depending on compression ratio) with the piston at the bottom than the swept volume indicates. The incoming charge is going to fill the swept volume area and the combustion chamber area. An engine with low compression and a larger combustion chamber volume might be 12% larger, and an engine with high compression and smaller combustion chamber volume might only be 8% larger than the stated engine size.
Just wondering if this is taken into account when calculating the VE.

[RoninLB]

not for nothing, but sometimes I'd float a simple few sentences that may or may not spur interest. If someone shows interest I'd answer or ask my question in more detail. I'm not the only one around here doing this. That said, we are all lucky to have helpful brains available answering from different directions imo.

[beepbeep]

Originally posted by snowman
An typical engine is a PULSED system, that is it is not continuous power, but short bursts of power produced each time a cylinder fires. At any given time, a very short time, the engine is operating at a fixed RPM. Say over a one half second period. If you look at the performance of the engine in the " frequency domain" ie RPM not time domain it can be analyzed with fourier analysis of the pulses produced by the cylinders firing at constant RPM. The shape of the pulse produced by the cylinder burning its charge, the length of the burn, and the frequency that the cycle is repeated can be presented graphically using something that will look like this sine x/x thing I have mentioned. You just have to look up the math function sine X/X to see what one looks like.

Nice. But what has "cylinder burning its charge, the length of the burn" has to do with intake tuning?? We were talking about VE which has lot's to do with intake tuning. Intake doesn't "feel" the burning phase at all as valves are closed when expansion circle occurs. Tuning the intake is mostly about careful shaping of plenum chamber and cam timing so it maximizes "filling" of cylinders. Engine is a big airpump, and "cold" intake-side is "inhaling" air in gulps. VE will be therefor affected mostly by revs, volume of air "inhaled" (and thus throtlle opening) and cam timing. it isnt's affected by burn/expansion phase artifacts, at least not as long as cams aren't so wild that you get blowby into intake.

And what's a telescoping intake duct if not a Helmhotz resonator with variable length?

My understanding of your problem with you, snowman, is that you know couple of things and have a need to discuss them. Unfortunately, you happend to do it in veryarrogant in "You guys don't know nothing, look at me" -way, and even happend to be wrong couple of times. Now if you were a John Walker, I would believer you...problem is that there are people here who are newcomers and they might assume that all posts "made by that chestthumping gentleman" are correct...which leads to problems.


Worst of all, you don't seem to have any sort of self-criticism. Even if proven wrong you try to swing discussion into area where your knowledge is better instead of meeting the criticism and accepting that you are wrong some times.

I've seen this kind missfortunate type of behaviour before and it's very hard on all involved.

For some reason, It often affects people who's main job was of individual technical nature (engineering, research etc.) and who got sacked or retired.

So my guess is that you are a retired person with technical background, fiddling with motors on a free time desperatly trying to convince world & dog that you are still a hot shot...despite the facts that motors weren't quite your proffession.

Some humility and respect for others opinions would do you good.

[RoninLB]

fwiw.. ignition spark duration and flame speed is another subject for another time.

[snowman]

Everything that happens affects the input tuning of an engine. Some things much more than others. The time that it takes to charge the cylinder is affected by how long the valves are open, when they are closed, and since any performance engine has significant intake/exhaust overlap by the exhaust valve timing as well as the exhaust tuning. I would suggest reading D. Elgins paper
http://www.elgincams.com/ Camshafts, a paper by Dema Elgin for more details. This is the long, difficult to understand (all the interactions) way of seeing how an engine works, in my opinion. The magic of the frequency domain is it gives another view that uncomplicates many interactions in that I can draw a picture of what I want an engine to look like (performance wise) and then apply all the fancy math available to help get the desired shape and then deconstruct ie go back to the time domain and see if I can acutally make it do what I need it to do. The down side of looking at an engine in the frequency domain is you loose site of what the parts, valves, pistons... are really doing.

Bottom line a very powerful, usefull tool is added by using the frequency domain and add to that many more tools can be added when one treats the engine as a pulsed system, which it really is.

With information like this a telescoping intake might be replaced by a couple of small chambers (helmholtz resonators) with a specific size and shape pipe connecting them togather, yielding the same or better performance. This better performance might be due to higher Q of a fixed design over a variable one. Or is it 3 small resonators that work better? ANd then how will we ever know?