116 replies to this topic

[TrackWeapon]

yes, that is low and this is my point!! I'm thinking that the limitation of the RPM are more coming from the acceleration and not the piston speed

and what do you mean by corrected piston speed??

[VAR1016]

Originally posted by TrackWeapon
yes, that is low and this is my point!! I'm thinking that the limitation of the RPM are more coming from the acceleration and not the piston speed

and what do you mean by corrected piston speed??

Corrected Piston Speed: from Karl Ludwigsen's book "Classic Racing Engines":

"...Then in a method first set out by English engineer Frederick Lanchester, the mean piston speed is divided by the square root of the stroke/bore ratio. The resulting corrected piston speed gives a considerably more accurate representation of the actual stresses prevailing in the engine. The resulting values are higher than the uncorrected values where the engine is over-square... and lower where they are undersquare. The reflects the heavier [sic] piston mass of an oversquare engine for a given displacement."

A couple of examples from the tabulated data in the book:

V16 BRM: 49.5mm x 48.3mm Corrected piston speed: 3529 ft/min at 11000 rpm

Ferrari 312B 78.5 x 51.5mm corrected piston speed: 4840 ft/min at 11000 rpm

I note with interest that the corrected piston speed for the 1992 Honda V12 was 5964 ft/min at 14,000. Dimensions were 88 x 47.9mm


PdeRL

[TrackWeapon]

ok, but where do you put the limitation of the speed in those engine, where does it begin to be too much load on the parts?? does the rotating mass got a big importance?? or it's more the material strength?

another thing, where would be the weakest link, or which parts would need more attention??

btw you know that this V12 making that kind of rpm is playing with like 12,000 g of acceleration, that is with a 2.0 rod ratio

[VAR1016]

Originally posted by TrackWeapon
ok, but where do you put the limitation of the speed in those engine, where does it begin to be too much load on the parts?? does the rotating mass got a big importance?? or it's more the material strength?

another thing, where would be the weakest link, or which parts would need more attention??

btw you know that this V12 making that kind of rpm is playing with like 12,000 g of acceleration, that is with a 2.0 rod ratio

Well that's all I know; I hope that others here who are infinitely better qualified than I can answer your questions.

I imagine that for those last revs, that the test rig comes into operation!

I do know that engine research organisations like Ricardo have sufficiently powerful software that can make very accurate predictions about these things

PdeRL

[shaun979]

Originally posted by J. Edlund
[B]Usually it's hard to find engines that have a piston mean velocity much over 25 m/s (would give around 5000 ft/min or so), this independant on the piston acceleration.
Comparing different strokes a short stroke engine will have a higher piston acceleration for any given piston mean velocity, lets take three examples:

NASCAR engine, 83 mm stroke and 9000 rpm
24.9 m/s
4,000 g

F1 engine, 40 mm stroke and 19,000 rpm
25.3 m/s
9,800 g

3.5cc R/C racing engine, 16 mm stroke and 48,000 rpm (typical 30k-50k rpm dependning on application)
25.6 m/s
26,000 g

So it seems like, that for a racing engine the piston velocity is the limit, not the piston acceleration.

One can say that the friction loss is in relation to the mean piston velocity, so using high velocity will increase the friction loss. But I assume that there are also tribological reasons for this, which probably are the limiting ones when it comes to velocity. This is what for example limit the engine speed of a rotary engine and is also why Nikasil was developed which origianally was developed for rotarys to solve the sealing during high sliding speeds.

Might one cause of piston speed being a limit, be that for a given cylinder there is only so much intake valve area so can only flow so much air before it chokes? In the case of drag engines that exceed 5500fpm, perhaps it is because they sacrifice a lot on longevity as well on spread of power by making the intake port huge to the point that they aren't much good at anything lower than the narrow power band way up high?

[McGuire]

Originally posted by shaun979


Might one cause of piston speed being a limit, be that for a given cylinder there is only so much intake valve area so can only flow so much air before it chokes? In the case of drag engines that exceed 5500fpm, perhaps it is because they sacrifice a lot on longevity as well on spread of power by making the intake port huge to the point that they aren't much good at anything lower than the narrow power band way up high?

Fascinating point you raise. One good book that touches on piston velocity vs. mach index is Engines: An Introduction by Lumley. The title is misleading; it's fairly academic, while at the same time being essentially a free-ranging essay on aspects of engines the author happens to find interesting. Sort of casual and scholarly at the same time...sort of like LJK Setright but without the intellectual twaddle. I bet folks here would enjoy it.

Pro Stock drag racing engines are impressive freaks. From 500 CID with pushrod-operated 2V heads, they make over 1300 hp at close to 10,000 rpm, normally aspirated on gasoline with carburetors. Breathing through one enormous intake valve (58mm and up; the valve springs look they belong on the chassis of a small car) their intake ports displace over 340cc and can bench-flow something approaching 500 cfm each. Other curious features: Honda Civic rod bearings for minimum friction and recip mass and well over 20" hg of crankcase vacuum. Crazy stuff.

[Engineguy]

Yes, only one inlet valve... not only large diameter, but they open it very quick and they open it very, very far. 1300 HP sounds impressive, but it's only 158 HP/L... I was reading Duckworth today and he mentions the Cosworth SCA (single overhead cam, two valves per cylinder) that produced 143 HP/L... 40 years ago... and without the need to replace the valvetrain four times per mile.

Of course another way to look at it is that the SCA, at 997cc had four intake valves per liter, whereas the Pro Stock motor only has one intake valve per liter. Hmmm... I think I'll have to do a little Excel spreadsheet work to observe to what extent valve curtain area doesn't scale linerly with displacement, keeping valve dia/bore dia ratio and bore/stroke ratio constant.

[McGuire]

Well, there you go. An engine's potential is in large part defined by its basic topography -- the various ratios and relationships in area, volume, and geometry. It's just a pump, goes the old saw. Meanwhile very few racing engines actually spring from a clean sheet of paper, even to this day. Their destiny lies to a great degree in the the circumstances of their birth.

The Cosworth SCA (stands for "Single Cam, series A") was based on the 1496cc, 116E "intermediate" Kent engine from the Ford Cortina. It was then radically destroked, reducing its displacement by 50% to become a 997cc F2 engine, with the addition of Cosworth's single-stick cylinder head. Consequently the SCA had a very advantageous stroke/bore ratio..and naturally, valve area per unit of displacement as well.

On the other hand, the drag racing Pro Stock engine (like GM's DRCE engine, for example) is limited in stroke/bore ratio by the NHRA regulation which restricts bore center spacing to a maximum of 4.900." So the maximum bore is essentially limited to somewhere in the neighborhood of 4.500" (approx 115mm) with the stroke defined as the dimension required to achieve the maximum allowable displacement of 500 CID.

So while both engines use 2V heads, beyond that the comparison is decidedly "unfair." The Cosworth SCA displaces 61 cubic inches and has a total of 18.22 sq. in. of valve area. The DRCE displaces 500 cubic inches but despite its enormous intake valves, has a relatively paltry 58 sq. in. of intake valve area. Obviously, the SCA has far more intake valve area per unit of displacement. Again, it's all about the topography.

[Engineguy]

I didn't mean to dis the Pro Stock guys or imply it was a "fair" comparison... just happened to be simultaneously reading Chapter 4, "Duckworth's Design and Philosophy" (mostly in his own words) in The Story of the Ford Grand Prix Engine - Its Design and Development ©1971, the US version (which ironically I just bought on Ebay from someone in UK and had shipped to US) of the UK book Such Sweet Thunder. He was talking about HP/L of diverse engines (someone was even considering a 24 cylinder engine as the 3L F1 era began!), so I naturally was curious about the 1300HP from 500ci you mentioned... hence the idea to graph the nonlinear effect on valve area of scaling displacement.

[TrackWeapon]

well according to this article http://asia.vtec.net/article/Spoon1/ b16, with a 5 main block girdle and other mods, are able to run in the 12 000rpm, that's 31m/s (6100f/s), and I read on another forum eng-tips.com that in race some of them have been able to run as high as 32m/s (6300f/s), of course that would be a shift point.

another thing, how do you guys explain the missshift on certain stock engine, where they are seeing as high as 11K rpm, but only the valves are "crunched" because of the valve spring floating, or valve hitting the piston and nothing more, neither on the crankshaft nor the con rods??!!

[Engineguy]

Originally posted by TrackWeapon
well according to this article http://asia.vtec.net/article/Spoon1/ b16, with a 5 main block girdle and other mods, are able to run in the 12 000rpm, that's 31m/s (6100f/s), and I read on another forum eng-tips.com that in race some of them have been able to run as high as 32m/s (6300f/s), of course that would be a shift point.

another thing, how do you guys explain the missshift on certain stock engine, where they are seeing as high as 11K rpm, but only the valves are "crunched" because of the valve spring floating, or valve hitting the piston and nothing more, neither on the crankshaft nor the con rods??!!

I know there are some guys in NASCAR that will be glad to hear that all they need is a Magic Block Brace, and they too can run 12,000 RPM. Seriously, even the overly-enthusiastic author says he's never seen anybody else attain 10,000 RPM with the same type engne. Tell me me the engine's stroke and we can compare its piston acceleration at 12,000 to that of an F1 engine at 19,000. If it's even close Takuma Sato might be interested in their rod and piston source.

Regarding over revved engines... most failures are fatigue life determined... a rod might survive a given stress 200 times (i.e 1 second at 11,000 RPM), maybe 400 times, but the 575th time it sees that level of stress it go boom.

A given rod, with a given stroke may tolerate 575 cycles at 11,000 RPM, a million cycles at 10,500 RPM, 30 million cycles at 10,000, and virtually infinite cycles at 9,500 RPM. And 200 cycles at 11,000 RPM may weaken it so that it will only last another million cycles at 9,500 RPM.

[TrackWeapon]

that would be 3.047" stroke (B16), making it around 8000 g of acceleration at that speed, so it's not that close to those in F1 which are 10 000g

so does your answer is indicating that it would be the acceleration causing the damaged?

[Greg Locock]

As a rule of thumb, fatigue life varies inversely proportional to stress to the power of 5 or 6 (and that sort of uncertainity indicates how flaky the data is).

Your accelerations, and so your peak force, vary with rpm^2

so, 1 minute at 11000 rpm is doing 1.1 times as many cycles at 1.21 times the loading which will cause 2.6 times the damage per cycle compared with 1 minute at 10000 rpm, ie you will have nearly 3 times as much fatigue damage.

Well OK, that doesn't sound like a bad tradeoff, rebuilding three times a season instead of once, say, for a notional 10% improvement in power.

However, as you approach low cycle fatigue failures these neat relationships fall apart completely.

[ZoRG]

Engineguy I know a few B16 guys that go to 10,000rpm and beyond. No problems so far ...

[McGuire]

Originally posted by Greg Locock
However, as you approach low cycle fatigue failures these neat relationships fall apart completely.

Indeed. :

[McGuire]

Originally posted by Engineguy
I didn't mean to dis the Pro Stock guys or imply it was a "fair" comparison... just happened to be simultaneously reading Chapter 4, "Duckworth's Design and Philosophy" (mostly in his own words) in The Story of the Ford Grand Prix Engine - Its Design and Development ©1971, the US version (which ironically I just bought on Ebay from someone in UK and had shipped to US) of the UK book Such Sweet Thunder. He was talking about HP/L of diverse engines (someone was even considering a 24 cylinder engine as the 3L F1 era began!), so I naturally was curious about the 1300HP from 500ci you mentioned... hence the idea to graph the nonlinear effect on valve area of scaling displacement.

No worries, not taken that way at all so I put "unfair" in quotes. I'm not representing for any particular engine or builder.

I lost my copy of Such Sweet Thunder (author Blundsen?) years ago. My job takes me to several dozen races per year and one of my hobbies is to search the used book stores in each area, looking mostly for automotive titles. At Wine Country Motorsports (which is right inside the paddock at Sears Point) last month I found another copy but the price was totally outrageous, $100 or something. "Nice musuem you got here," I told the guy. It's a necessary book and I might have paid it, but you never know...I could find another copy in some downtown used book store somewhere for $10 the very next week.

[McGuire]

I see that earlier I referred to the Cosworth 997cc SCA as an "F2 engine." My bad; it was really built for F3. Sorry about that. There was a long-stroke SCC version produced later, with few of the attributes of the original of course.