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I am just quoting from memory of the days when Hailwood and Haslam were racing on the Isle of Man, Haslam on a short stroke four cylinder Honda and Hailwood on the Ducati long stroke but desmodromic V twin. The problems of the Ducati were all explained in terms of maximum not mean piston speed. In the end rider and rideability won.They key take-away is that crank speed and piston speed are related but NOT fixed across engines, because they are totally dependant on the stroke. Therefore a very high crankshaft speed can be achieved without a particuarly high mean piston speed by virtue of running a short stroke (this makes all sorts of other horrible things happen which are expensive to get right, because the valves then spend more time being close to banging into the piston crown....). Instantanious piston speed is basically the <Stroke/2 x Crank Speed> (its actually a horrible big formula but thats the basic working of it).
High mean piston speeds are not a very intelligent thing alone, what is intelligent is high crankshaft speeds with as short a stroke as possible to enable very high power outout with managable mechanical stresses. Mean piston speed is an irrelevance to power production, the power goes out through the crank, crank speed going up whilst maintaining torque is what good practise is, thats the maths ! Power (Watts) = Torque (N.m) x Crank Speed (6.28 x RPM/60)
I think I need to read that a few times. Great stuff."Internal stresses rarely limit racing designs"
- Thats so mindbendingly wrong its hard for me to actually answer, and it doesnt even make any sense with the rest of your own post either - but lets make some examples if you increase cyl pressure the rod will buckle if insufficienly strong, either as a beam if the critical length is too short in which case you use Eulers beam equation:
View attachment 524330
...thats called internal stresses limiting your racing engine.
Or even in actual compressive failure if the original rod was marginal enough, which you can get thorugh simple:
Stress = Force / area (area being the connecting rod cross sectional, and force being as per the graphs below)
...thats called internal stresses limiting your racing engine.
This gets very hard to fix because if you increase the section of the rod it normally bangs into the base of the liner. If you increase rpm the rod can stretch plastically, which occurs on the exhaust stroke as the gas pressure isnt resisting it, your crank and rod bearings will also start to go once the mean stress approaches 100Megapascals on the bearing shells, which is the fatigue limit, road cars would be 80Mpa, F1 its about 120Mpa. Thats basically nearly impossible to counter as you cant increase the bearing area easily so if it happens after you
have designed the engine - you`re stuffed.
....thats called internal stresses limiting your racing engine.
"As the stroke gets shorter and the rpms get bigger for the same piston speed, this breaks down because it is difficult to create valve gear that function at extreme high rpm"
- yes thats why nobody in their right mind since about 1930 has produced engines with pushrods. The japanese are perfectly happy at 15,000rpm with normal springs and
over head cams on finger followers, providing you know your camshaft mathematics and valvespring resonance theory - which they do.
"Higher crankshaft speeds do not produce more power as the stroke shortens.
- Yes they do because race engines use a larger bore and shorter stroke, so the capacity remains the same but crank speed rises. Increasing bore size is a much
better way of increasing capacity than stroke, because the capacity rises with the square of the bore radius, but the linear proportion of the stroke. In fact because
gas pressure is basically independant of speed, if we fix bore, fix boost and shorten the stroke and raise rpm, power does ALSO go up even with less
cylinder capacity, as I have calculated, graphed and proven below. This is because gas pressure is INDEPENDENT of crankspeed, so as speed goes up you make the torque
more quickly, even with less capacity you still win. Any slight loss in torque due to the shorter moment arm of the crank-throw is minimal compared to the gain from producing
more pops per second.
Note these graphs are from ONE cylinder of a DB601, so naturally x 12 for he engine total output, although the power shown is indicated power so of course you have to subtract
your mechanical losses to get brake output, call it -15%.
View attachment 524334
"The reason for a short stroke and a high crankshaft speed is weight."
-Nope, its to reduce the force in the rod from piston acceleration. Here are the main bearing forces for a DB601, I`ve made two versions here of the same
capacity, but with short stroke at high rpm, and long stroke a low rpm. Peak cylinder pressure is essentially independant of crankspeed so if you have a
larger bore and shorter stroke at high rpm you win because the engine makes a lot more power but the inertial forces didnt go up. Here is a
DB601 analysis which I`ve quickly done two versions of, same swept volume, 20% bigger bore and 30% smaller stroke. Makes the same power,
but I had to reduce boost pressure by 30% to equalise them on the short stroke/big bore high rpm! Notice that the combined gas and inertia forces have NOT changed, but we`re making
the same power with much less boost (because the revs have gone up with a larger bore).
View attachment 524321
View attachment 524328
....thats called internal stresses limiting your racing engine because the combined gas and interia force is basically what goes down the rod !
"The bottleneck for getting more oxygen into the cylinder is (all things being equal) the diameter of the cylinder"
- Well its actually the valve curtain area, which if you are building archaic 2valve engines is pitifully low, of course that only
applies with a naturally aspirated engine, as if you boost it charge density rises and therefore you can get as much charge into the
cylinder as you like through the same ports if you can keep the boost going up. Second last engine I was a designer on was a 1600cc V6 running
over 5Bar absolute manifold pressure; made quite a lot of power....and internal engine stresses had quite a lot of time spent on them in the
drawing office !
For this you use Eulers beam equation:
View attachment 524330
...thats called internal stresses limiting your racing engine.
Or the rod can also have compressive failure (which is just about cross sectional area, not the stiffness of its section) if the original rod design was marginal enough, which you can get through simple:
Stress = Force / area ("area" being the connecting rod cross sectional area, and "force" being as per the graphs below, the force down the rod axis)
...thats called internal stresses limiting your racing engine.
This gets very hard to fix because if you increase the cross-section of the rod it normally bangs into the base of the liner at about 90degrees crank angle. If you increase rpm the rod can stretch plastically, which occurs on the exhaust stroke as the gas pressure isnt resisting it, your crank and rod bearings will also start to go once the mean stress approaches 100Megapascals on the bearing shells, which is the typical fatigue limit on modern shell bearings, road cars would be 80Mpa, F1 its about 120Mpa+. Thats basically nearly impossible to counter as you cant increase the bearing area easily so if it happens after youhave designed the engine - you`re stuffed.
....thats called internal stresses limiting your racing engine.
"As the stroke gets shorter and the rpms get bigger for the same piston speed, this breaks down because it is difficult to create valve gear that function at extreme high rpm"
- yes thats why nobody in their right mind since about 1930 has produced engines with pushrods because the rods bend and whip everwhere and the valve motion bears no resemblance at all to the actual camprofile anymore. The japanese motorbike engines are perfectly happy at 15,000rpm with perfectly normal steel valve-springs and
over head cams on finger followers, providing you know your camshaft mathematics and valvespring resonance theory - which they do.
"Higher crankshaft speeds do not produce more power as the stroke shortens."
- Yes...they DO for two reasons. Race engines use a larger bore and shorter stroke, so the capacity can remain the same but crank speed rises. Increasing bore size is a much
better way of increasing capacity than stroke, because the capacity rises with the square of the bore radius (pi x r^2), but only the linear proportion of the stroke. In fact because
gas pressure is basically independant of crank-speed, if we fix bore, fix boost and shorten the stroke and raise rpm, power does ALSO go up even with less
cylinder capacity, as I have calculated, graphed and proven below.
Note these graphs are from ONE cylinder of a DB601E, so naturally x 12 for he engine total output, although the power shown is indicated power so of course you have to subtract
your mechanical losses to get brake output, call it -15%. (so 1350ish bhp)
""The reason for a short stroke and a high crankshaft speed is weight."
-Nope, its to reduce the force in the rod from piston acceleration. Here are the main bearing forces for a DB601 (two graphs down), I`ve made two versions here of the same
capacity, but with short stroke at high rpm, and long stroke a low rpm.
"....thats called internal stresses limiting your racing engine because the combined gas and interia force is basically what goes down the rod !
"The bottleneck for getting more oxygen into the cylinder is (all things being equal) the diameter of the cylinder"
- Well its actually the valve curtain area, which if you are building archaic 2valve engines is pitifully low, of course that only
applies with a naturally aspirated engine, as if you boost it charge density rises and therefore you can get as much charge into the
cylinder as you like through the same ports, because flow is velocity limited not density limited.
Stress was definitely a limiting factor in many WW II aircraft engines. Since every designer was striving for the best power to weight ratio engines were built as lightly as they could be.
Even back in the 1930s race car engines often had different limits depending on use.
Just a basic question Snowy. Do you take compressive and tensile yield strength to be the same value, did they have equipment for accurately measuring both in the 1930s?I ask you a very serious question, without the answer to which I am not going to contribute to this thread further.
Please tell us how many engines you have designed. I`ll patiently await your list.
Please don't make your contributions dependent on someone else's posts. I for one greatly welcome your posts, and read them with great interest. What Biff is to the air combat threads, you are to the engine design threads.I ask you a very serious question, without the answer to which I am not going to contribute to this thread further.
Please tell us how many engines you have designed. I`ll patiently await your list.
I spent my life in mechanical testing. In pipeline engineering the material is tested in a tensile test although most loads are actually compressive. On one project this became a major issue and so there were tests of both compressive and tensile loads using test samples and full sized pipes, while similar they were not the same.Entirely material dependant, for example cast-iron is specifically VERY different in tension and comression as when its formed its full of micro-cracks, which is one reason its very good as a bore material because the cracks retain tiny oil particles. Under compression these cracks are simply forced shut, and so basically dont matter but under tension mean the value with be VERY different to that under compression.
Wrougt materials will typically be similar in tension and compression.
In the 30`s they had reasonably advanced test machinery even for fatigue and torsional testing and suchlike. So yes I think 100% all good engine designers from the late 20`s onwards would have certainly had a very good grasp of all that stuff. I think the chief disadvantage then was that materials standarisation almost didnt exist in the very early days so you`d
have to do tests on the exact stuff you got from your chosen mill, as it was probably specific to them and not something you could order from other mills. This changed slowly between WW1 and WW2. Much of the work of the Royal Aircraft Factory in WW1 was on materials standarisation and testing, so the designers would be able to make better parts.
I spent my life in mechanical testing. In pipeline engineering the material is tested in a tensile test although most loads are actually compressive. On one project this became a major issue and so there were tests of both compressive and tensile loads using test samples and full sized pipes, while similar they were not the same.
I used to have to visit the Salzgitter Mannesmann Vallourec "Vorschung" in Duisburg, it is like a place owned by a Bond villain, hundreds of machines nodding and whirring constantly, but hardly a human to be seen lol. They managed to prepare the test pieces to the wrong standard, so no one is perfect lol.A selection of torsional fatigue testers is visible in this aviation industry torsional testing laboratory in Germany, 1941 at the Schenck company. Caption on the wall reads: "We are inescapably bound to the great eternal laws of nature and must obey them, whether we like it or not"
I`ll say this is still JUST on-topic as it maybe illustrates that the means to develop higher speed engines was certainly not limited by materials testing know-how.
View attachment 525335
And you use Euler's beam equation to design a stouter conrod. Problem solved. Internal stress does not limit the engine.
.
"The bottleneck for getting more oxygen into the cylinder is (all things being equal) the diameter of the cylinder"
.
(Con-rod length 259mm in both cases, cylinder pressure kept same)
Standard DB601E, 2700rpm, mean piston speed = 14.4m/s
:Bore = 150mm - Stroke = 160mm - Cylinder Swept Volume = 2.827 Litres (33.9L total)
Power (assuming 85% mechanical efficiency) = 1358bhp
De-Stroked DB601, higher rpm, 4200rpm, mean piston speed = 15.4m/s (only 7% higher because the stroke has gone down).
:Bore 150mm - Stroke = 110mm - Cylinder Swept Volume =1.944 Litres (23.32L total, 68% of the size of the standard engine)
Power (assuming 85% mechanical efficiency) = 1400bhp
(The crossover point is about 4000rpm, when the power is identical even with 10 litres swept volume missing.)
If we take it a bit further and go towards (but NOT at) the piston speeds Jumo were looking at in 1944 with the 213J.
De-Stroked DB601, very high rpm, 4500rpm, mean piston speed = 16.5m/s
:Bore 150mm - Stroke = 110mm - Cylinder Swept Volume =1.944 Litres (23.32L total, 68% of the size of the standard engine)
Power (assuming 85% mechanical efficiency) = 1490bhp
If your destroke engine ran at the same piston speed it would have the same power? ie at 3,927rpm?
Using the same rod length would, I assume, lead to a small improvement in efficiency because it would have less extreme angles in the short stroke version.
What you are essentially trying to say is something like "with modern materials the limits experienced by old designers dont exist and modern designers can basically do what they want without creative boundaries imposed from stress issues, if they have enough money to use the very most advanced materials"
This is about half right, the limits for what can be done have of course moved very significantly since 1945 in reciprocating engines, but - they have NOT become unrestrained by them on any level, in any component of an engine, right up to F1 or NASCAR (incidentally nether of which actually make very good studies for the state of the art from a materials OR engine layout perspective as both racing series are extremely heavily restricted legally by the regulations imposed by the race regulators).
So its very dangerous to say things like "yea but in NASCAR..."
I did a few different models, the graphs are showing if you keep capacity constant but increase RPM, this showed that the reduction in stroke (and the associated reduction in piston acelerations) actually made it possible to keep stresses in-check despite the higher speeds. In order to keep the power "matched" between both engines I had to actually reduce the boost on the high rpm, big-bore short-stroke engine.
Mean piston speed is an irrelevance to power production, the power goes out through the crank, crank speed going up whilst maintaining torque is what good practise is, thats the maths ! Power (Watts) = Torque (N.m) x Crank Speed (6.28 x RPM/60)
I was (along with the point about curtain area), demonstrating that this is pretty meaningless because since flow through the valve IS velocity limited, NOT
density limited, you can shove almost any amount of air you want through any set of valves by increasing boost, as its just higher density. So more
energy is contained within the same volume when it passes through the valve. Hence bore diameter really doesnt come into it. I think you just
didnt read that properly as you seem to be throwing the exact point I made back at me.
Your point covering that base by stating "all things being equal" is sadly not very useful to make as its never possible to do, for example if you change the bore size the only way to fit bigger valves without altering all sorts of other things is to go to a shallower included valve angle, and so on etc etc. Sometimes its a useful phrase, but not when discussing bore sizes, too many variables which just make it meaningless, as there are just too many knock-on factors.
If you can figure out a way to increase either the BMEP or the piston speed, you can make more power for the same displacement (something known to top racing teams for decades).
We can then proceed to a discussion of how long a fighter engine needs to last between overhauls when it is subject to be shot down every sortie.