Why It’s Almost Impossible to Rev to 21,000 RPM 

NONE of the thermal or stress issues are a deal breaker. Having worked in F1, analysing stresses in pistons and conrods and crankshafts and valves, I'm quite sure everything could easily run much faster - 30,000rpm would be no problem, particularly if exotic materials are allowed (like aluminium - beryllium). Turbochargers in humble road cars can hit 100k rpm while red hot... The killer factor is time. Higher RPM means less time per cycle, but the physics of fuel combustion are "fixed", in that flame fronts take finite time to propagate. The flame-shooting injection is a great idea - an advance on Alfa Romeo's "Twinspark" idea that used two spark plugs to start two flame fronts in the combustion chamber. The second problem is keeping everything synchronised. While the components can be designed to survive the loads and stresses, keeping the valve timing synchronised with the piston motion is harder than it might seem. (Quite apart from the problem of "when to start combustion" to get an optimal power output from the cylinder (fire too early, combustion works against you. Fire too late, most of the power goes out the exhaust. Ignition advance is a huge field all to itself. But I digress.) Because, nothing is rigid, and everything is flexible. And twistable. V12s and V16s have much longer crankshafts and camshafts, and will twist along their length, resulting in some very serious timing errors in the cylinders remote from where the shafts are linked together. Various efforts to deal with this include using link gears at both ends of the engine, or putting them in the middle. Which brings their own problems with "forced" positional control, that can induce their own breaking loads. This can all be engineered around, not least via novel valve technologies. But to even be aware that everything is flexible, and avoid the typical "rigid body" thinking that seemed to afflict everyone else working in the field, is a challenge in itself. For those that doubt if I know what I'm talking about, take a closer look at the bending crankshaft animation at 8:03. That is NOT showing "stress" (so, we don't even have to debate whether it is Maximum Principle Stress, or Von Mises stress, or some value of fatigue alternating stress - ALL of which MIGHT be the one dominating "failure".) It is showing a colour contour map of DISPLACEMENT. Because, the entire "red" part of the crank does NOT have similar stress - that would be concentrated at the fillet between crank web and big end... certainly not distributed equally over the whole section. Worse than that, the deformation is created by moving the centre main bearing while holding the mains at each end - a totally false load case that simply never happens in reality. At least, not to the extent it is shown here. Some movement is possible because these bearings are oil film (journal) bearings, and the oil film thickness allows a small amount of movement. But this animation is not intended to show that, since displacement at the centre WOULD cause movement in the end bearings too - the crankshaft stiffness forces that. One of the reasons I gave up doing what I did, was the disparaging remarks about being "the pretty pictures department" by folk that did not bother to look at my track record of "ZERO in-service failure" of any part I ever worked on. Sigh. But sadly, even 20 years later in 2024, it seems not only has nothing changed, but the "pretty picture" is not even a valid picture at all, but something created purely to look pretty. Sigh. TMI but for those still interested, "zero in-service failure" means inifinite life while in use in the engine. NOT "unbreakable parts" (you just have to subject them to loads they never see in service, to get them to break). This included redesigning finger followers that broke in 10 minutes, and conrods that snapped in an hour. All of these became "infinite life" parts that could last forever, just by fettling design details. And, making them lighter for better performance at the same time. For me, durability usually meant removing redundant material, and sometimes redistributing material, to better disperse loads (and hence stresses) throughout the structure. I never had to fix anything by adding any material to it. Which is contrary to intuition, and is the "go-to" response of about 99% of others doing this kind of work. "Beef up the weak bit" is NOT a good thing to hear. Just like the crankshaft that drew envy because it NEVER broke, not only lasting an entire race season, but also having the smallest main bearings of any crankshaft on the racetrack. (and smaller mains meant less parasitic torque was drained from the power output, giving more BHP at the flywheel...) But smaller bearings mean a huge increase in stress concentration, making it much more difficult to endure the loads any crankshaft has to survive. Anybody else that even tried to match our bearing size, never finished one race. When you get a handle on engineering, and materials, and stresses and strains, and loads, and resonance, and vibration, and heat transfer and temperatures, and... then designing engine parts for conditions thought impossible, becomes quite possible. There certainly are limits. Yield strengths of materials, for one. And when you factor in that the strength depends on temperature, too... But rather than get sidetracked further on "strain rate sensivity" and why F1 pistons survive stresses more than four times higher than their yield strength at operational temperature, just know that we are still quite far from reaching those boundaries with designs at 20k rpm. I mean, 99% of the piston is not even close to failure. It is the 1% detail problem where stresses concentrate that start the cracks that lead to failure. Same with that crankshaft. The challenge is fettling the details WHERE THEY MATTER. Sorry for the essay. Thanks for reading.

The fact that this video is still live after all the inaccuracies and wrong information speaks louder than the mistakes themselves.

The main reasons are actually the speed of the flamewall starts to be too slow, the combination of high compression and ultrashort stroke reaches mechanical limits, and the extremely narrow gap between head and piston at TDC prevents a clean burn. The mechanical load is a problem that is solvable.

Mass doesn't change with velocity, the piston doesn't magically become 2.5 tons. If you're talking engineering you absolutely have to be precise with your terms, man

The video never actually answered the question. It pointed out the difficulties of running at 20,000 rpm, but did not actually tell us why it could not go higher.

Short answer: mechanical stress

The only problem is that they DID 20K rpm... It CAN be done. Honda had a roadrace bike that would rev to 22K rom in the '60s.

An interesting side note: it's not uncommon for the massive diesel engines on cargo ships to be 2-cycle, but that's at least in part because they run slow enough to be able to effectively replace the exhaust with fresh air via blowers (which they have the room for) and that also allows them to not run oil mixed with the fuel (which might not be as big a deal given the fuel they run isn't that much different in weight than motor oil).

0:10 Mass of... Beg pardon? You need to look up the definition of mass. I'll give you a hint, acceleration doesn't change it.

Is this a challenge on how to get as many details wrong as possible in 13 minutes?

I remember being under the grandstand during USGP practice when Williams was very nearly 21,000 rpm. What a thrilling sound. Menacing, spine chilling, and the Mercedes McLaren engine still sounded more violent than the rest. Miss those V10s

To quote von Braun "I have become very careful about using the word 'Impossible'."

15 seconds in and you're killing me. Mass isn't changing, force is changing. There's no extra "stuff" just because it's accelerating so quickly.

You made it out to seem that there was a reason that specifically 20,000 rpm is the limit.

F1 engines were touching 22,000rpm in c.2006. Saw it on TV. The tach touched 22K from time to time,

No mention of the Honda air cooled, conventional valve springs, sixes of their racing bikes in the 60's, that routinely reved to 23k rpm? Relevant as they won at least 2 world championships and remained reliable. Also no mention of the, again Honda, CVICC system of the 70's? Strange because it is exactly the system you describe as new in 2015...

A couple of things that I noticed in the video. Engine friction wasn't mentioned but is a key reason for not increasing engine speed further. The NA F1 engines used port-fuel injection, direct injection came along in 2014. Current F1 engines do not use TJI as described in the video, they use a passive pre-chamber as only one injector per cylinder is allowed. The stoichiometric air-fuel ratio of gasoline isn't 14.7:1, it depends the formulation of the particular fuel.

My little Honda CBR250RR will rev to 21,000, it's stopped making power at 19,000 but I've seen it as high as 23,000 on the track. And that's 1990 technology and it's still going strong.

Renault and BMW both managed 21k in the V10 era of F1 before the FIA issued an order limiting engine RPMs to 19k.

When you explained the actions of the four stroke I was taken back many years, to when I started work at a Ford engine plant (I was 17. God that was so long ago!). My dad explained it using the "suck, squeeze, bang, blow" saying. It really helped me visualise it. And as you went from one part to the next, I was hearing those words in my mind just before you used the more technical ones 😂 Fascinating stuff!

The more power strokes you can squeeze into a minute,( rpm ) the more power you make -But - the volumetric efficiency gets worse as you try to speed things up ( that whole getting the air and fuel into the cylinder and mixed , thing ) . There’s a point at which at which the gains by higher rpm get overtaken by the loss in volumetric efficiency . Had a tutor who said “ at 20,000 rpm pistons don’t go up and down - they just vibrate “ 😂

6:25 Massive blunder here, con-rod length does not influence stroke and therefore the bore/stroke ratio and engine „squareness“, only the crank can influence stroke. Rod length to stroke ratio is a whole other matter.

Internal combustion piston engines are just insane, I mean 20,000 rpm means each piston is changing direction 666.6 times *EVERY SECOND* (at BDC and TDC, i.e. twice per rotation). Pretty mind-boggling.

How about a episode on F-1 pre-chamber ignition? Possibly, Micah McMahan as a resource?

3:55 ah yes, rice.

My first bike was a 1987 Yamaha FZR250R. It had a 19k recline and while not particularly fast, it was absolutely glorious. F1 sounds at reasonable speeds. I miss the simplicity of youth.

I find it kind of funny that they just created prechamber ignition for F1 even though this is ancient diesel technology where early Cat diesel engines used precups or Pre-combustion chambers that would start the burn before entering the cylinder .

I did some ball park math years ago. A high reving engine, the piston can travel up to 80 MPH. Fairly impressive, but it changes direction (up-down) about 500 times PER SECOND.

Because they don't want or need to. Everything about F1 is rules and limitations, if they had to rev to 21000, they'd figure it out. ( and we'd probably see some new rotary designs because pistons are horrifically inefficient when scaled with speed, because the reciprocating load scales with revs )

I rarely post comments like this but... At 8:18 - Aluminum has a better strength to weight ratio than steel, but is more vulnerable to fatigue, not less. Strength and weight are completely unrelated to fatigue in general. You actually picked the two most extreme examples to be wrong about - in that some steels will literally never fail due to fatigue as long as they aren't overloaded, but with aluminum any repeated load will eventually cause a fatigue failure no matter how small. Some ferrous titanium alloys can behave more like steel - but it's because they're ferrous, not because they're titanium.

Frame front velocity of the fuel is what determines rpm limit. Different fuels have different flame front velocities, and thus different rpm limits for the same engine.

My remote control car with a nitro engine does 37,000 rpm. When I run it with the exhaust pipe off its like 10 times louder than a full sized car.

Time for f1 to move to Rotary engines.

Our small 3.5cc model competition engines are 2 stroke and are able to run 48.000 rpm on the track and w/o load they can shoot over 50.000 rpm.

Ive never learnt so much about engines before. Your Videos are awesome.

I remember back when I was kart racing. I started off with 100cc karts but I never raced them. While racing with TKM and Rotax engines, I always had older 100cc engines laying about. I remember my Vortex VR / CW rotary valved engine. That engine would SCREAM. Highest I ever had it was 23,000rpm. Although I never raced it. Occasionally I'd stick on a larger rear sprocket on the colder dry days and let it sing. You really seen the temp go up. If I managed 5 laps in a row achieving 22k rpm, water temp would easily go over 70c and you'd struggle getting 20k. It was all about managing the carb and temp. Leaner engine, more power, more temp. On the really hot days I kept it rich but it was still achieving 18k to 20k. Really loved those days... I miss the smell and noise.

Surprised no one has tried an engine with Oval Pistons like Honda did with the NR500/750 8v per cylinder and two conrods per piston?

20k is amazing, heck 9000 peak on an average car or truck defies belief. Into the realm of 10, 12, 15, and 18,000 RPM's is astonishing.

The dislikes are from the pistons harmed in the making of this video

At 06:29 The length of the conrod has nothing to do with the stroke. The stroke is determined by the offset of the taps on the crankshaft only.

Meanwhile engineers: hold my beer I bet they could do impossible, if they had a chance

........except Ferrari had F1 engines running at 24.000 rpm ....and could have raced them. Paolo Martinelli was in charge at that time from 2002 - 2012 and was only limited by supply of pistons. Mahle delivered the best material....but quantities were limited. So at races with high full throttle mapping the full power was executed. At the rest of the races standard engines up to 21.000 revs were employed. The glorious years of "rev festivals" are over for the moment. But are coming back as " electric turbines" , brushless inrunner with easily 30.000 revs maybe soon...but of course mainly silent as efficiency is king!

Nope. BMW once did 21,005RPM.

Standard Kawasaki ZXR250, 19,000rpm redline, but happy to rev higher. The speed of flame front is so slow an engine would not work above a reasonably low rpm. However, there's something called "squish velocity". In performance 2-strokes it's critical to get it just right. I suspect it's the same in 4-strokes. The velocity of the gas in the chamber is what reduces the burn time to allow high rpm and efficiency. It makes enough difference that ignition timing can be backed off at least 5°. I'd imagine modern materials and manufacturing techniques would allow for higher rpm if allowed.

Quit common on nitromethane piston engines for model cars I have one that revs to 33,000 RPM

Please man, review your scripts better

peak cylinder pressure in a race engine 1500psi. probably even more win a race engine, so with 80mm bore that's over 15000kgf pushing on the piston

RC model engine regularly run around 30.000 rpm, and some even higher, i've seen up to 45k being suggested. Granted they run on nitromethane, which burns fast, but the mechanical aspect is still here. Just a little info to temper the "absolut limit" idea that seems to be played here

Just commenting to support the channel and video. Keep up the great work Scott. I really hope you keep getting good viewer numbers. I’m concerned you took a major hit from the algorithm after the overdrive debacle.

ICE are fundamentally air pumps that you squirt fuel into. For a given displacement higher rpm means more air flow and more capacity to burn fuel. They are limited by mechanical loads and flame speed. At very high rpm both of those factors come into play even for very oversquare engines. Two strokes move something approaching twice as much “effective air” for a given rpm. Turbo and superchargers pressurize the input air and effectively increase displacement for a given rpm.

A great vid! And it's not only about the final answer, but about all we learn in between.

Raikkonen in 2006 quali in Imola reach 22k ...there's an onboard video

Very effective explanation about engines in general especially 2 vs 4 stroke

Honda 50 cc twin cilnder racing engine and the 125cc 5 cilinder were able to run at 35000 rpm in tests in races at around 30.000 max this in 1967

At 10:13 A complete 4-stroke cycle takes 2*60/20000 = 0.006 s You forgot that a full 4-stroke cycle takes 2 revolutions of the crankshaft. Still a very short time...

We want 20,000 RPM engines back in F1. They sound powerful and made for some great racing. Who cares if the hybrid cars are slightly quicker, it's a show and we want the music to go with it. These modern cars sound like shit.

I have a theory: If you use a powerful enough electric fuel pump, you can move the pistons just by squirting fuel really hard at them. 🙂

Nice video. The problem that normally isn't one. The crystal clear comparison of translation and rotation would answer the question very quickly. Certain hybrid engines and their modifications would be very serious competitors, just like back then. You know what I mean and the history of the topic in racing with combustion engines. Peter

Thanks for making this video. Many times people blaming MGU for less screaming on current PU, whereas it's actually it's rev/minutes, as current PU rarely reach 13000 rpm.

Don't forget the Honda CBR 250rr MC22 from the 90s which could also rev up to 20000 rpm while it was street legal too. Other japanese bikes from that era were close to that too

"With an engine revving at 20,000 RPM this piston has a mass of 2.5 tonnes." No it does not. It's mass is it's mass. It does not change. Under acceleration, it's apparent weight will change, not it's mass. It's mass is constant, not taking into account any negligible amount it may shed due to normal wear within normal operating parameters. For a channel that overtly portrays itself as digging into the technical, it sure seems odd that so many basic technical details are so often just overtly wrong. And in such instances, technicalities are not quibbles, they are significant issues that are germane to a true understanding of the topic, with significant ramifications, in terms of derived conclusions, if misunderstood.

Scobby rev'd his dad's Astra Belmont above that one night, when dad took it to church on Sunday the engine fell out.

You stated with 2 strokers combine intake and exhaust in a single stroke and compression and power in a single stroke. It’s actually exhaust/compression then power/intake.

It's mind boggling to me that they can even work at such high revs. Engineers who design and make these are insanely talented.

(Thumbnail-pic showing the 2 pistons, one long, one short) "As the RPM increases, the piston gets closer to the speed of light and therefore becomes shorter" :P

Wrong. My 1992 Kawasaki ZXR250 revved to 21K. Admittedly the redline was 19k, but that was just a bit of ink on a dial face. The engine went to 21K. On a road vehicle. That survived at least 20 years - I sold it early 2012 with no known issues. 4 stroke. 4 cyl. 16 valves. 4 carbs.

Well, mechanical constraints do not limit you to 20k RPM, if going above that RPM is your only goal. What limits your RPM is time. It takes time to exchange gasses, build a flammable mixture, ignite the mixture, and then you want time for the mixture to push the cylinder down. A longer power stroke will extract more energy inside the engine and send less energy down the exhaust. So it's a design balance between energy sent to the turbo/MGU-H and energy kept inside the engine. Air exchange can happen close to the speed of sound, which is close to 350m/s at ambient and close to 700m/s at exhaust temperature. Reaction time is a view µsec. The speed of the flame of 0.5-3.5 m/sec is the limiting factor. So you don't want to rev higher because it would reduce the amount of energy you get at your crankshaft at a certain point, and you want to rev even lower to avoid heat and save on mass you would otherwise need for cooling. PreChamberIgnition (PCI) was also designed to save a lot of weight. With it, you only need a flammable mixture inside the prechamber. That flame can travel through mixtures that would be too rich or too lean to ignite. In an engine, the mixture is way too lean. So you get much more air to a slightly lower temperature and pressure. An engine that runs rich to stay cool at high rpm would burn 2-4x as much fuel. Which does not work with the current fuel limit and would still be irrelevant if you could use as much fuel as you would like. It would only be interesting if the cars would be significantly (200kg

Now this makes me think, if we used a rotating valve instead of regular ones. Some people have tried it, with varying success, but it has never been done by big companies that have the funds and equipment to make something ridiculous.

There is an upper limit, but I think it's controlled by gas dynamics, not by kinetics of the metal rotating parts. I'm pretty sure that they could make a machine that will rev faster than pressure disturbances can travel through a gas medium.

A lot of talk in the comments about "mass" vs "weight" but not a lot about how that "0.003s" figure in the opening line of the video is for a full rotation of the crank and not the acceleration of the piston. That's a shame because the actual 0-60 time of the piston is even more impressive than 0.003s!

So basically, in 2006 they were pushing the physical limits of natural combustion engines in terms of material strength and the combustion itself.

Wait, did I miss it? Why do modern F1 engines only rev to 13k if they're now unrestricted?

My stock 1999 Honda CBR250RR with 55,000km on the odometer routinely hits 21,000rpm and they are known to hit 23,000rpm without issue. Redline starts at 19,000rpm

First of all, the RPMs in any car or bike racing should be ABSOLUTELY LIMITED, it is a STUPID thing to go over 6000 on the streets, 10000 racing; NO POINT on earth to go over 10000, that is just stupid, pointless and temporary,with NO practical application; Secondly, another fantastic video, thanks, and ignore "those" comments lol

Are most F1 pistons 2 ring or 3 ring, or both types?

Is it possible to watch an entire F1 race without falling asleep?

I think you got the temperature of an F1 engine wrong. You said it can get up to 2600C 8:34 Titanium melts at 1668C, and aluminum, iron and steel all melts at lower temperatures than titanium. I might be wrong, but imo 2600 is impossible.

If you get rid of valve springs,RPM is much easier. My ''street'' Ducati Panigale revs to 16500 RPM daily.)))

The fact we can even make a 20k RPM engine is insane to me.

I've seen this video on every car and engineering channel on RU-vid already but welcome to the party.

Thank you for fixing the voiceover sound. Both room treatment and the microphone

There is no magical RPM ceiling. It's always a compromise between maximum RPM/HP, reliability, fuel and oil chemistry, cost, necessary fuel mileage and materials technology. All of these factors have to be weighed against each other in light of whatever technical rules the engine is subject to.

Great video, but what happened to the Cosworth piston design video?

Would it be it be possible to premix and inject the complete quantity of fuel and air instead of having an injector and an air inlet valve? This would ensure perfect mixing of air/fuel when it’s injected, and reduce the need for one valve?

advice from me: dont watch the "what if f1 had no rules" video. At the end he just says: "meh i dont want to say a horsepower figure bcs it would be too much to handle for the driver". Brotha i dont care just give me a rough estimate, i watched a 15 min video for this.

fantastic video thank you for making it🎉

Impossible? Nearly impossible? All of the RX8 guys are laughing right now.

It would be interesting if that TGI ignition could be applied to old cars. What effect it would has on performance and efficiency.

The highest rpm ever measured in a V8 was achieved by Mercedes in the V8 at 20.232 rpm. However, this was only achieved on the test bench. The engine was never used in a race.

The strict theoretical limit is friction. At some RPM frictional losses in the moving assembly, *but more fundamentally in intake and exhaust gasses* become the limit aka 'pumping losses' and pumping losess increase faster than power gain as RPM increases. Friction *cannot be zero* for gases as gasses can't be Bose condensates. So theoretical RPM limit is when Power = Friction Power. Flame front v, material strength etc.. are all practical engineering limits, but only engineering limits...

its the mass and strength of the reciprocating parts that define the acceleration and deceleration of the piston and connecting rod that limits the max rpm, just like a helicopter rotor and blade cannot exceed a certain speed because the rotary wing aircraft is already moving in air at a certain speed while the machine is also adding to that speed while the other hemisphere of the rotor is doing just the opposite so there is a point of imbalance so that a helicopter cannot go over 2-3 hundred knots. However, if the reciprocating mass is smaller, rpms can exceed 20k, it just depends on the balancing of the crankshaft and pistons so there are no harmonic imbalances.

You forget the one thing that actually IS the limiting factor. Flame front speed!

what if you ceramic coat the pistons and block and use rotary valves in the head this is more air to

0:12 Pretty sure the mass doesn't change because the piston is experiencing G forces. Yes, I know what he meant, but it was just wrong and this is seemingly meant to be an engineering video. Edit: I see in the other comments I wasn't the only one disappointed.

"With an engine revving at 20,000 RPM this piston has a mass of 2.5 tonnes." thats....not how that works.

I miss the 20,000RPM days

I used to have a nitro RC car with a tiny aluminum single cylinder engine that did 29,000 RPM. That thing was awesome!!! HPI has a 3.0cc 2 stroke engine that will max out at 32,000 RPM!!!

You talked about increasing RPM and therefore increasing power leading to faster laps. So it's logical to conlude that power makes the car faster... I believe it's something that needs to be explained in videos more often to people.

Ironic how the pre-chamber that was the bain of the diesel engine has now found its way into the spark-ignition engine...

At 20,000 rpm the crank shaft is spinning at 333 rps (revolutions per second) that means the piston is going up and down 666 times in one second ... you need a revolution computer to even control that.