NA Engines: Factors that increase torque

[Zmechanic]

oooOOOO! I love this thread already!

Can you define "intertial ramming" for me? I'm surprised Wave Tuning effects are listed as so low...


The the stuff about Piston dome "Creepage" it sounds like what you've said implies that a flat top piston will produce more torque than a domed piston because the flame doesn't travel as far- is this part of what you're saying?


Adam

Inertial ramming as it's meant there is the recovery of the energy in the velocity of the flow as pressure. Energy is stored in the mass of the air and fuel charge, you can conserve enthalpy and convert this to pressure. This is why you'll hear pressure recovery discussed. A proper chamber design is essential to maximizing this effect.

The dome vs flat top thing is a tradeoff. Can you get an adequate C/R with a flat top? Maybe, maybe not. If you can't, then a dome will probably still win. But if you can get the adequate C/R with a flat top, it has a good chance of outperforming the dome. Imagine a flat desert, now imagine a mountain. Which is a larger distance to cross? I say creepage because that's exactly what the term is used for in electronics. The literal linear distance electricity must travel along the surface between two points. The flame front propagates through deflagration, which takes time. If you can get the mixture burning/burnt at the end of the chamber before the boundary layer has disappeared and it's heated up excessively, you win. Having the plug wayyyyy off to one side won't make that happen.

[Brian P]

Can you define "intertial ramming" for me? I'm surprised Wave Tuning effects are listed as so low...

Piston motion sets up a column of air moving in the intake runner, then towards the end of the intake stroke when the piston stops moving down that column of air is still moving into the cylinder due to its inertia, which stuffs a little more in. Shutting the intake valve at the right time afterward traps the overpressure inside the cylinder. That's inertia ramming.

Inertia ramming and wave tuning are somewhat related, because if you get the wave tuning right (and in the correct RPM range), it ensures that the charge is moving as fast as possible into the cylinder towards the end of the intake stroke and that maximizes the inertia ramming effect.

The intake runners need to be pretty long for this to happen in a useful RPM range. On the engine that I've been playing around with (single cylinder motorcycle engine), it seems to want the total intake runner length to be around 280mm long to get a useful torque bump in the 8000 - 9000 rpm range. At 280mm runner length and 340 m/s speed of sound, it takes 1.6 milliseconds for a pressure wave to go from one end of the intake runner to the other (which is inside the air filter housing) and be reflected back again (in opposite phase). At 8000 rpm that's 79 degrees of crank rotation. At 9000 rpm it's 89 degrees - call it 90 degrees. Now think about some repercussions of that. Near the end of the exhaust stroke the inertia of the outgoing column of gases in the exhaust pipe has brought the pressure in the cylinder well below atmospheric, so crossing TDC during valve overlap there's a strong negative pressure that's sent outward in the intake runner through the opening intake valve. That gets reflected back as a positive pressure which arrives somewhere near 90 degrees after TDC - halfway down the intake stroke when the valve is opened close to maximum - speeding up the flow and helping to fill the cylinder. And the other thing is that the pull-down of cylinder pressure from the piston accelerating down at its peak, let's say from about 30 degrees ATDC to about 90 degrees ATDC, gets reflected back as a positive pressure throughout the end of the intake stroke - 120 to 180 degrees ATDC - so now you have fast motion into the cylinder in the intake port, a cylinder that has already been filled, and an intake valve that is still open for a wee bit longer.

At 7500 rpm and below, there doesn't seem to be much effect, but this engine lives mostly above that anyhow. It seems to run pretty good up to redline (11,000). If it signs off above that, it's not a problem.

If you scale this back to normal car-engine rev range, double the lengths to get half the revs. Lots of newer fuel-injected engines have manifolds with intake runners folded back on themselves to get them long enough so that this effect happens in a useful rev range.

[panic]

I hate to be the wet blanket (but it's the role I was meant to play, don't you agree?) but that Faberge egg of a piston dome shown above is properly called "not ready for prime time".
All those jewel-like precise "as CNC machined" sharp edges are trip wires that cause flow to fall on its face, move away, etc. and follow some path other than the one you intended.
If you're really lucky, that mistake will reverse another mistake (too much overlap), but it's more likely that it will produce the other effect: overlap flow is both delayed and reduced, and the engine will produce more peak power with a tighter LSA (which would be detrimental with a less intrusive piston dome, or reduced static CR).
No professional engine builder should use pistons in that state, except as window dressing for the unwary buyer.

A specific real-world example: Larry Widmer's work on the Honda B16 4-valve DOHC has shown (at least to his satisfaction, and he's smarter than I am) that a single slot connecting the 2 intake valve reliefs improves power despite the chamber volume increase and resultant loss of static CR - the flow leaving the valves improves by getting dome material far away from them whenever possible.

</rant>

[MadBill]

if an engine creates more power at a given rpm it will also create more tq at that rpm and vice versa. as far as airspeed, your target airspeed will come from your displacement and port cross section, and those partially determine where that will happen along with the camshaft. as a general rule, at an engines peak tq you are no where near your target peak airspeed. cylinder fill creates power. whether you want to measure as tq or hp, that doesn't really matter.

This. The listed factors and others affect both power and torque at any given RPM, not one or the other. The focus is usually to maximize them over a specified RPM range to meet the designer's goals. If the intent was truly just to adjust various factors to achieve the highest peak torque number, you would want to tweak all of them to maximize some mid-RPM value that best trades off wave and inertial effects against friction and pumping losses, both of which increase more than linearly with RPM. (Good chart here: http://web.mit.edu/2.61/www/Lecture%20n ... bology.pdf )

That said, there is far more scope for increasing power than torque in a given engine. A good street 350" SBC might make 425 HP @ 6,000 RPM and 425 lb-ft. at 4500. A built-to-the-hilt 350" could be 1,000 HP @ >10,000 RPM, but peak torque might go up 'only' to 560 lb-ft. @ perhaps 8500 RPM, thus a power increase of 135% but a torque gain of only ~30%.

[Biteme]

1: airspeed
2: correct cam timing
3: compression ratio as high as octane will allow
4: exhaust

[Newold1]

NewbVetteGuy

I think you left two of the most important factors off your large list:

DISPLACEMENT

BORE AND STROKE

I think you were meaning bore and stroke when you listed cylinder length and width?

I think its important for you to set the range of torque and horsepower you hope to achieve with your particular engine plans and then study, learn and apply the best overall package of displacement, bore and stroke, rod length, compression ratios, camshaft designs, cylinder head performance, intake system performance and exhaust system performance for your particular use and requirements. Understanding and applying all the science involved in studying and applying all the items in your list and offered by others or making this a life long serious hobby will take some serious engineering, mathematics and physics education and time. If you are leaning towards this type of formal education and career then by all means get that education. However if you are just trying to understand the best torque and power production for your particular application then I would suggest you not getting your head fogged and spinning from trying to apply all the technology and science on this subject, keep it a little simpler and just keep asking questions and learning what others have achieved with your desired engine package and work toward creating that result! Try not to over think your needs, you'll most likely confuse and possibly frustrate your efforts. JMO As an older experienced one I have seen and worked with many younger enthusiasts who get caught up in trying to over question and engineer their needs.

[Rizzle]

NewbVetteGuy

I think you left two of the most important factors off your large list:

DISPLACEMENT

BORE AND STROKE

I think you were meaning bore and stroke when you listed cylinder length and width?

I think its important for you to set the range of torque and horsepower you hope to achieve with your particular engine plans and then study, learn and apply the best overall package of displacement, bore and stroke, rod length, compression ratios, camshaft designs, cylinder head performance, intake system performance and exhaust system performance for your particular use and requirements. Understanding and applying all the science involved in studying and applying all the items in your list and offered by others or making this a life long serious hobby will take some serious engineering, mathematics and physics education and time. If you are leaning towards this type of formal education and career then by all means get that education. However if you are just trying to understand the best torque and power production for your particular application then I would suggest you not getting your head fogged and spinning from trying to apply all the technology and science on this subject, keep it a little simpler and just keep asking questions and learning what others have achieved with your desired engine package and work toward creating that result! Try not to over think your needs, you'll most likely confuse and possibly frustrate your efforts. JMO As an older experienced one I have seen and worked with many younger enthusiasts who get caught up in trying to over question and engineer their needs.

He said displacement was obvious at the top of the first post.
Bore and stroke are just ways to increase displacement so that mostly falls under the same category.

Maybe he's just asking a general question, and a good one. I don't understand why you said most of the rest of your post?

I'm surprised variable cam timing hasn't been mentioned. Can be used to improve low end power, and as the engine speed increases, shift to make more power up top. This is for single phase vvt. There are some interesting uses at part throttle as well.
Once you get control of the intake and exhaust separately, (mostly dohc, but the viper v10 had cam in cam vvt) the ability to improve the average power is great.
I believe Mast had a comparison article for an LS3 type application showing cam advanced, straight, and retarded vs the same cam using variable cam timing, with graph overlays. I'd find it if I wasn't on my phone.

[Rizzle]

In the same vein: If faster burns increase torque does that mean that higher octane fuel, which burns slower, decreases torque?


Adam

Higher octane fuel does not necessarily burn slower. Careful with some of the common conceptions, there are many that are incorrect, or have drawn the wrong conclusion from the change.

[Newold1]

Rizzle, you got me again! I should use you as my editor before I make poor reads and posting faux pas! He did mention displacement and such in his opening statement. My Bad!

However, what I was trying to suggest in the rest of my post was not to be meant as a criticism but as a piece of experienced advice. It seems from NewbVetteGuy's previous posts and contributions here on Speedtalk is that he is a younger or newer participant in engine performance discussion and I just wanted to inject the idea that sometimes when newer enthusiasts get started in the world of performance discussion and inquiry they sometimes get there head spinning around with all the science, math and physics of the technical aspects of understanding and applying torque in engines in general. I've seen this sometimes end up with a person bouncing around looking like a billiard ball on a pool table trying to formulate an actual build for their needs. If he loves and thrives on these subjects and has a passion for learning a lot of this somewhat complicated knowledge, then for all reasons he should carry on and DO IT!

[Brian P]

The original poster would also do well to look up and understand the concept of "brake mean effective pressure" - and understand why a well-designed naturally aspirated 4-stroke gasoline engine will have a peak BMEP in a similar range, even for engines of widely differing sizes, operating speeds, and design philosophies.

[statsystems]

oooOOOO! I love this thread already!

Can you define "intertial ramming" for me? I'm surprised Wave Tuning effects are listed as so low...


The the stuff about Piston dome "Creepage" it sounds like what you've said implies that a flat top piston will produce more torque than a domed piston because the flame doesn't travel as far- is this part of what you're saying?


Adam

Inertial ramming as it's meant there is the recovery of the energy in the velocity of the flow as pressure. Energy is stored in the mass of the air and fuel charge, you can conserve enthalpy and convert this to pressure. This is why you'll hear pressure recovery discussed. A proper chamber design is essential to maximizing this effect.

The dome vs flat top thing is a tradeoff. Can you get an adequate C/R with a flat top? Maybe, maybe not. If you can't, then a dome will probably still win. But if you can get the adequate C/R with a flat top, it has a good chance of outperforming the dome. Imagine a flat desert, now imagine a mountain. Which is a larger distance to cross? I say creepage because that's exactly what the term is used for in electronics. The literal linear distance electricity must travel along the surface between two points. The flame front propagates through deflagration, which takes time. If you can get the mixture burning/burnt at the end of the chamber before the boundary layer has disappeared and it's heated up excessively, you win. Having the plug wayyyyy off to one side won't make that happen.

Zmechanic...how does spark plug location affect creepage?

[77cruiser]

Then there's always NITRO.

[MadBill]

..Zmechanic...how does spark plug location affect creepage?

If combustion begins at one side of the chamber, the flame front has to travel the full bore diameter to consume all the mixture. If it starts at the center it just travels the radius in all directions.

Also, the term 'creepage' may have its place in electronics but in ICE's it's just a confusing misnomer for how compact and fast burning a chamber might be. Later I'll post a pic of just how bad it can be...

[statsystems]

..Zmechanic...how does spark plug location affect creepage?

If combustion begins at one side of the chamber, the flame front has to travel the full bore diameter to consume all the mixture. If it starts at the center it just travels the radius in all directions.

Also, the term 'creepage' may have its place in electronics but in ICE's it's just a confusing misnomer for how compact and fast burning a chamber might be. Later I'll post a pic of just how bad it can be...

Thanks. I'd love to see the pics.

If you look at 2 obvious spark plug locations, say he I verses 23* sbc, the hemi doesn't really care about a dome, where th SBC would. Am I correct?

[Zmechanic]

Also, the term 'creepage' may have its place in electronics but in ICE's it's just a confusing misnomer for how compact and fast burning a chamber might be. Later I'll post a pic of just how bad it can be...

Yeah, I'll concede it wasn't a good choice of term.. My mind just didn't seem up to the task of finding the right one.