What determines how high an engine can rev too...

In general, it's valve float and rod/piston/crank material strength.

Valves have to be opened to allow air in and exhaust out. Those valves are generally opened mechanically, and closed via springs. When the engine turns so fast that the springs cannot shut the valves before the next cycle, that's valve float. This results in decreased power, mixing of intake/exhaust gases, and in some cases contact between pistons and valves.

The strength of the material in the rods (and to a smaller extent, pistons and crank) determines how quickly they can spin before they distort, bend, or break. All of these are bad.

I am not an engineer, but I've broken cars


F1 engines these days have pneumatic valves which are quicker than mechanical valves, and insanely expensive and strong internal parts.

 

A good rule of thumb that has survived for decades is the piston speed per minute. The historical upper limit has been a "mile", or 5280 feet. Few engines will last past this more than an hour or two.

What this means is that if the stroke is, say 4 inches, one crank revolution is 8 inches. 5280 feet is 63360 inches. Divide 63360/8=7920 rpm. This holds pretty close for engines.

F1 cars rev to 18000 rpm or more due to their very short stroke. Compared to a regualr car engine, the F1 motor will have a large piston and a comparatively tiny stroke.

 

Valve springs...at least for F1 cars. As was mentioned above, F1 cars have a short stroke so they don't go as far "down" as a normal car. Rather than valve springs, they use compressed air. There was some trial and error done with magnetic valves, but I don't think they were ever used. They were looking to get way past 20k RPM with those.

 

In 1955 Mercedes Benz W196 driven by Stirling Moss used what is called a desmodronic valve system with a closing cam shaft lobe that pulled the valve closed rather than closing by spring pressure. In addition; they were rollerbearing cranks with solid rods (No caps) that were assembled onto the crankshaft by sections known as a Hirth style crank. 10K rpm was the norm. I have a photo of a crank from this car somewhere I will post it as soon as I find it.

 

The SLR motor was redlined at 7800 rpm. I believe the 2.5 L Grand Prix motor was as well. It was over revved once rather badly by one of the drivers in practice (somewhere in the 8500-8700 range) and the driver reported the incident straight away to Ulenhaut. It was not policy to over rev them. They had some RPM related problems with the valve adjusters that cost them several motors. Also multi piece cranks like the Hirth units are not strong enough to cope with the stresses of very high rpm and is the reason that technology has long been abondoned for plain bearing, one piece cranks. Rollers were only even used because oil and plain bearing technology of the time was not all that good.

 

I could never fathom how an engine could have a 20k+ RPM until I saw a picture of just how short the piston rods are.

Pic credit goes to www.m-power.eu

Here is a link to more pics.

https://www.team-bhp.com/forum/technical-stuff/38661-formula-1-engine-pics.html
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Once people wrap their head around the stroke and mean piston speed concept the next thing they think is "great, just make the bore huge and the stroke very small...rev the engine to the moon and make huge horsepower numbers..."

As with everything in engineering, there is no free lunch. You trade one thing for something else, and the "best" design is really the best compromise.

One way of comparing engines is the ratio of bore to stroke. F1 engines are the extreme example with ratios in the range of 2.5-3.0 or so. A Harley twin is on the other end of the spectrum with a bore a little less than stroke, or <1.0. Most sporty cars today are 1.0-1.3, while top line sportbikes are 1.5-1.7. For the real world, this is currently the upper limit of bore with respect to stroke. Therefore, for a given displacement the minimum stroke has a floor value based on the practical uper limit for bore/stroke based on what the engine is supposed to do.

I mentioned the compromise, and that is efficiency and driveability. An F1 car never sees the slow side of 10,000 rpm on the track, whereas a street car must idle and perform mundane tasks such as crawling in traffic. An F1 engine would be absolutely terrible on the street, with very poor volumetric efficiency and no power in lower revs, right where a street drive wants it. Sportbikes are somewhere in between an F1 and street engine, but are generally toys that are great on the open road and only tolerable in stop-and-go. 600 sportbikes that rev to 16k or more can barely get out of their own way below 8k.

 

^^Right tool for the right job...well said. I know a lot of people want those high performance light weight flywheels for their street cars. They forget the purpose of the heavier ones is to keep the momentum going while you're driving in traffic. It really only makes sense for the track as well.

I don't think F1 cars idle below 9k, which is pretty ridiculous since most cars redline a couple thousand before that. I'm a low-end torque fan myself.

 

I love this post, especially the short 4th paragraph.

One night years ago I came up with the idea for a rotary valve system that eliminated float. The sky would be the limit on rpm. I sketched, drew, plotted, dreamed of combustion chamber shapes and the huge fortune I was going to make.

Then Google shattered my dreams in 5 seconds by showing me that smowmobiles have been doing that for decades. Ouch.

 

Stress in rotating machinery is function of the square of the speed the mass and the radius of the mass (stroke). If you double the speed, the stress is four times that of where you started if the geometry is the same. As you can see, small changes is speed result in big changes in stress. Another way of saying it is a 20% increase in speed increases the stress in the parts more than 40%... Since the stress is a linear function of stroke, you have to cut the stroke a lot to make up for the speed squared effect. That's why you see super short stroke engines in F1.

While putting in pneumatic valve springs raised the allowable speed of the valve train, the next thing that happened is that the connecting rods became the weak link. Titanium rods are used because they weigh less and have a higher strength to weight ratio than steel, but right now, they are still the weak link in the system. The designers try to remove as much weight in the piston because that is what is pulling on the rods. End result is you get something like what is shown in the picture above. It's just the engineers working on the levers that they have to optomize the system.

 

Might just make sense to modify a jet engine at that point...they can rev to infinity it seems like compared to the good ole' piston and rod design. :/ Sucks about the lag, though. Maybe use electric motors for low speed and the jet for high?

 

I stand corrected... I misread the article... Michael Riedner's book lists 13000 rpm as a theoretical RPM... Good call.

 

I'm most impressed by the piston design.
The skirts, to keep the piston straight in the bore, are almost non-existent.
The material area below the bottom ring on the sides looks too thin to support a ring at 18000 rpm.

 

Turbines have the same issue, the rotational speed is higher, but you still run into limits as to how fast you can spin it. If you spin it too fast it slings it's guts out.

All internal combustion engines are basically air pumps. Turbomachinery is a much better way to pump air than a reciprocating engine, but at the power levels that you need for a car most of the time a turbine is running at a very low percentage of max power and it isn't efficient there.

A hybrid that uses a small gas turbine for steady state power and an electric motor to drive the wheels is probably a much lighter hybrid than a reciprocating engine system, but it won't be cheap.

 

Isn't titanium a bit flexible vs steel? Yes stronger & lighter, but I have seen titanium bolts that are dificult to get fully tightened and stay that way.

 

Titanium isn't as stiff as steel, it has an elastic modulus of about 17 million psi as compared to steel that has a modulus of about 30 million. The stiffness to weight ratio of titanium is essentially the same as steel. Steel has a density of about .3 pounds per cubic inch and titanium is about .17 pounds per cubic inch, so the ratio of modulus to weight ratio is the same. All body centered cubic materials (aluminum too) have the same stiffness to weight ratio. aluminum weighs about .1 pound per cubic inch and has a modulus of 10 million. Stiffness isn't the issue with these parts, what is important is the strength to weight ratio.

What makes titanium the good stuff for rotating parts is the strength to weight ratio. An alloy like 6-4 titanium has an ultimate strength of about 170,000 pounds per square inch. For steel to have the same strength to weight ratio, it would have to be capable of 300,000 psi, so titanium wins hands down in that regard.

Titanium bolts need to be turned further than a similar steel bolt because the material is about half as stiff, so to put the same stretch and force in a titanium bolt it takes a almost twice the angle of turn to put the same amount of force in the bolted joint.

The way it works is that each part of the rotating assembly is pulling on the chain so to speak. For that reason, the piston is made from the lightest material and doesn't need to be as strong as the rod. The rod needs to be stronger than the piston, but weight is still important, so that's made of titanium. The crank is made of a higher strength material than titanium, (high strength steel forging that has a strength of over 200,000 psi) so that it can hold on to the stress caused by the piston and rod. So long as the rod doesn't buckle, the stiffness of the rod isn't an issue. The shorter the rod the less it will tend to buckle (like by a cube if I remember correctly) so as long as the rod is short titanium is stiff enough.

 

Thanks!

 

Your wish is Jaguar's command!
The Gas powered micro turbine electric engine. http://www.youtube.com/watch?v=V6GK3qdqv20