can you inform us about what is of the valves and camshafts, or cylinders air filling, or crankshaft , or else, most limiting revs, in what order and proportion?
from what i have read it is mostly due to the strength and material mass of the reciprocating components in the engine. Mostly the pistons and con rods. the higher the rpm the higher the g force and the greater the strength needed which means more material which equates to more reciprocating mass and greater forces and so the nasty cycle goes. So its a balance between needed material mass and forces from rpm. Valve springs used to be the problem but they got around that by moving to a pneumatic system. Wish i had some first hand knowledge. It must be quite the challenge to develop these motors.
Right now it appears the valves are the limiting factor in maximum RPM. With the introduction of pneumatic valve closing systems, the metal wound spring seemd to have reached a plateau, and the pneumatic was a wonderful innovation that got around a lot of inertia issues.
But the new V-8's seem to have terrible vibration issues, it gets worse as RPM climbs.
The problem is, everything is affected by other demands. You could build a crankshaft that survives 30,000 RPM,, but it may be weigh just too much,and have so much inertia that it's unworkable. And each part if designed and constructed for it's projected environment. A piston designed to survive just two races is quite different than one designed for a qualifying engine, where it's life was measured in minutes.
And each time the RPM climbs, just about everything has to be completely redesigned, and problems may arise. A connecting rod that was good at 15,000 RPM would have lots of trouble at 19,000. It would then probably have to be much stronger, and with more mass. And then maybe the crankshaft would have to be stouter, and that would affect the block, and so on and so on. If you open and close the valves quicker, then where the camshafts contact the valve buckets would have to be redesigned. And maybe a less aggressive lift profile might have to be adopted. It's a very complex intereaction of relationships between parts.
For induction and air flow, the new restricted V-10's definitely have an upper limit. As the air velocity increases through the restrictor, it eventually reaches the transonic region, and shock waves appear. That pretty well stops any further progress in increasing air flow. In an unrestricted engine, supersonic flow and shock waves just aren't there, the velocities aren't as high. But the inertia of the air has to be dealt with, and the cam profile, intake trumpet lengths, and interior of the intake itself have a major influence on how much air you can get into the combustion chamber each stroke.
Fuel burn time is relevant. On the Audi LeMans diesel it's practical limit is 5,000 RPM because the diesel fuel just doesn't burn fast enough to go much faster in RPM's. I personally believe that this factor is near the limit on gasoline. Not limiting yet, but we may see it become a limiting factor if RPM's continue to climb. And ignition systems may be coming to a plateau too. The ignition source is limited by the rules, and as RPM's climb, you want a good hot spark each time.
Right now, the valves are the limit. The valvetrain itself is probably a close second, with piston ring seal an issue that's in jeapordy. For the engineers, all they can do is work on the weakest link, and keep building in more RPM, until another part issue rears it's ugly head.
I guess that the major fact that limits the developement of more RPM is the rules. If only we had the chance of using more cylinders instead of 8 or 10 and if we had the chance of running valve arrangements as used by honda in the 70's in their motorcycle engine program, which had 8 valves and an eliptical cylinder shape i guess that we were in the range of 25,000 rpm or something
I'd point my finger towards plasma or laser ignition and direct fuel injection(especially direct mixture injection). More advanced ignition combined with all 4 valves being outlet (direct mixture injection) would certianly raise rpm.
Wow
This looks like a great topic to follow.
My two cents worth is really I don't know.
My guess would have to be vibrations. Vibrations will beat things to death.
Dave points how you need less mass to diminish inertia and, at the same time, more mass to increase resistance. I've read pistons rods have to take 9.000 G's (can it be true?)...
Even if you can control vibrations and stress somehow, I am intrigued by this (Dave also talks about "inertia of the air"): speed of sound is around 340 m/s. In 1/20.000th of a second, a sound wave moves only a couple of centimeters. Of course, you can add 0.6 m/s per each degree centigrade you heat the air.
But this could mean that, when you close the exhaust valve, the increase in pressure has no time to reach the intake valve before the exhaust opens again.
Aren't we hitting here some kind of basic limitation? I mean, can you close and open the valve so fast that the rest of the air column "does not feel it"?
Are pressure waves in the column of air faster than sound? I have some experience on the difference between explosions and detonations, so I can imagine the mechanism (BTW, I really love dynamite, it is one of the fun things about road building!), but I do not see how can you apply it here.
But, seriously, is there a "sound barrier" piston engines cannot overcome?
by my calcs its somewhere around 157,000 revs/min with a crank radius of 25cm and a air temp of 150°C, but i didnt take into acount pressure changes in the airbox and just took it to be atmospheric.
thats for the inlet stroke, i'm not gonna get anywhere near the calcs for the combustion stroke. But it should be still valid.
Last edited by Crabbia on 15 Jun 2006, 01:56, edited 1 time in total.
And i'm gonna go with valves on this one. I've heard that the mid twenties could be possible with magnetically operated valve systems and the limiting factor being the respose times of the pnuematic systems currently used.
magnetically opperated valves could in theory work directly of the control unit outputs, one needing only to amplify the signal to actuate the valves. problem with this system is that the force due to these solenoids increase exponentially with respect to thier position to the poles of the solenoid. simply meaning that at the open and closed position the force is largest. This is a problem becuase it will break the valve right off the stem if you can't get it exactly right.
ofcourse this systems is extremely untessted and the few times i've heard of it working it didnt work for long
and the way things are going with two race engines and the points system set up as it is, it pays to have reliablility over HP.
Isn't the maximum rpm possible gonna depend directly, at least by the regulations, on the cylinder spacing?!
If the distance between the centre of the bores is fixed or limited, that will limit the maximum bore size limiting the stroke (to maintain the displacement) and therefore the piston speeds and inertia forces on the rods, and also the size of the valves compromising the airflow to the cylinder!
What do you think?!
Ciro Pabón wrote:
But, seriously, is there a "sound barrier" piston engines cannot overcome?
Yup, the current technology is only able to handle air moving below the speed of sound. That's the kicker in restricted engines, because at the restrictor, when the engine is asking for more air, it's when the velocity of the intake reaches the speed of sound, then pressure waves at the restrictor basically choke it off. But once past that point, the volume then expands, velocity decreases inside the plenum and intake ports.
A lot has been mentioned about pressure waves, and such, but air also has mass, and inertia. It's not much, but it takes a little bit to get it moving, and of course, it tends to keeep moving.
Ciro, you are right about the speed of sound being a basic limitation. I remember reading a very indepth article about air restrictors and their aero dynamics. Basically everything fell apart as the air speed through the restrictor aproached the speed of sound.
With regard to the momentum of the air (and exhaust gases) it is still very significant as the volume of air being drawn through the engine and its speed is very high.
What limited rpm 10 and 20 years ago seamed insurmountable then, much as things may appear today, but thats the beauty of engineering and material science, they evolve contineously
