**Overcoming Engine Speed Limits**

In the pursuit of power and speed, engineers face several challenges when designing high-performance engines. One major limitation is the ability to rev high without compromising engine integrity. To understand why, let's examine the factors that contribute to this issue.

Firstly, valves play a crucial role in determining an engine's rev limit. The traditional overhead valve design or overhead cam designs are common in most vehicles. However, as engines speed up, the valves must snap shut faster to avoid flow. To address this problem, engineers use stiffer springs, but this can lead to increased wear and tear on the engine.

An alternative solution is to switch to a pneumatic spring, which uses nitrogen-filled pistons to retract the valve more quickly and prevent oscillation. This system is more complex and expensive than traditional designs. Another innovative approach is electronic valves, as seen in Koenigsegg's free valvet technology. While this offers improved performance, it comes at a significant cost.

**The Role of Combustion**

Combustion itself takes time, and the speed at which it occurs is known as flame propagation. The moving line of combustion is the flame front, and its speed is determined by the type of fuel used. Every type of fuel has an intrinsic burning velocity, which can vary depending on factors such as temperature, pressure, turbulence, and oxygen distribution.

To illustrate this point, let's compare the flame speeds of two fuels: standard ethanol-based fuel and 91% isopropyl alcohol. The results show that isopropyl alcohol burns faster than ethanol under ideal conditions. However, actual flame speed can be affected by a range of factors, including temperature, pressure, turbulence, and fuel distribution.

**The Impact on Rev Limits**

When an engine reaches high RPMs, the piston speeds can reach 40 or 50 meters per second, which is significantly faster than the flame propagation. This means that under certain conditions, the combustion process cannot keep up with the piston speed, effectively limiting the engine's rev capacity.

F1 engines, for example, use incredibly short stroke lengths to achieve high RPMs. This design choice reduces piston speeds and prevents outrunning the flame front, allowing the engine to reach remarkable rev limits of 20,500 RPM. However, this comes at a cost: low power output at lower RPMs.

**The Trade-Offs**

When designing engines for high performance, engineers must weigh the pros and cons of various design choices. Damage to the bottom end from rapid acceleration and deceleration can be mitigated through careful design, but top-end damage caused by valves that don't retract quickly enough is another challenge. Additionally, pistons may outrun combustion, limiting the engine's rev capacity.

While it's difficult to achieve 20,000 RPM in road cars, understanding the underlying factors that limit engine speed can help engineers develop innovative solutions for high-performance vehicles. By carefully managing valve actuation, combustion processes, and piston speeds, engineers can push the boundaries of what's possible in engine design.

WEBVTTKind: captionsLanguage: en- A few months ago, we didan episode on why it's almostimpossible for car to goa thousand miles per hour.Now I'm not gonna spoil it for you,it's a good episode, butit turns out that top speedof a car isn't limited by physics,but rather by mother nature.So I started to think about thelimitations of engine speed.And as it turns outthe fastest revving motorpeaks at about 20,000 RPM.(car engine revving)So why is that?Well today, we're going to figureout what limits engine speeds,how engines can be built to spin faster,and why it's so hard to hitthat 20,000 RPM ceiling.And if you don't careabout any of those answers, don't worry.I'm going to also light some fuel on fire.Let's go.(upbeat music)- A big thanks to Keeps forsponsoring today's video.You know, I wanted to make2021 about bettering myself,so I started working out,even joined the dating app.My nephew Nolan thinksI'm having some sortof midlife crisis.So I decided to go totherapy and to be honestI really 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horsepower.Horsepower is torque timesrevolutions per minutewhich means that while anengine is producing torque,the faster it spins, themore horsepower it makes.But all motors have somelimit to how fast theycan spin typically becauseof friction heat and wear.These problems affect everythingfrom jet turbines to electric motors.But some of those canexceed 1 million RPM.So why do piston enginesstruggle to reach 20,000 RPM?Well, there are three main problems.Uno, dos, tres.Now whether it's a Camryor a Mercedes W10 F1 car,all four stroke engines turn fuel intohorsepower using the same four steps.Suck, squeeze, bang, blow.Each step begins with the pistonat a dead stop at top deadcenter or at bottom dead center.The piston accelerates fromzero to its top speed as muchas 95 miles per hour or42.8 meters per second.And then it decelerates back to zeroso it can change directionin the next step.This produces the first limiting factorfor how fast an engine can spin.It's reciprocating masseswe need to overcome.The moving object has an effective weightequal to its mass timesany G force exerted on it.but a piston only weighswhat, maybe 500 grams.How bad could that be?Well, take the Cosworth V8Formula One engine for example,that's one of the few enginesto ever hit 20,000 RPM.At that speed, the piston is accelerating,decelerating and reversingover 600 times a secondand producing 9,500 G's.That gives a 500 gram pistonan effective weight of a thousand pounds.But it's not just the piston that has towithstand those G-forces,oh no, it's the risk pins,it's connecting rods, thecrankshaft, the bearings,they all got to keepthat rapid accelerationand deceleration of the piston in check.So one reason why reciprocatingengines have a limitto how fast they can spin is becausebeyond a certain speed,the effective weightof a piston will breakordinary bottom end parts.But we can solve this problemof reciprocating mass andthat's by beefing things up.You can strengthen the bottom end to dealwith the additional weight that comeswith higher engine speeds.Using Ford's parts insteadof cast for example,but stronger often means heavierand bottom end parts are alsoreciprocating or rotating.So you generally have to dealwith the trade-off betweenstrength and lightweight.You can switch to materialsthat are stronger and lighterbut that's going to be more expensive.(upbeat music)The limits of bottom end partsaren't the only reasonsengines have rev limits.The top end is vulnerable too,and the second problem thathigh revs create is valve flow.An engine valve is pushedopen when the camshaftor rockers press down on the valve shaft,pushing it into the cylinder.Once the cam lobe or rocker isno longer applying pressure,the valve is closed by a spring.So valve is also a reciprocating mass.It travels in one directionstops then reverses,which takes time.In an overhead cam motorlike the S2000, that's it.But in the case of a push rod motorlike a Chevy small block, thetapits, push rods, and rockersthey are also reciprocating.The valve shaft is supposed to remainin contact with the cam lobeor lifter even when closed,but springs, they take time to expand.If the camshaft is moving too quicklybecause the engine is spinning too fastit remains open for too longprotruding into the cylinderwhen it shouldn't be or rapidly openingand closing as the undammedvalve spring oscillates.Just sits there, just in a constant statelooking compressed.The same thing happens in a push rod motorexcept now all the other partsbetween the valve and thecan, can end up floating too.Now when a valve floatsthe best case scenario isthe valve remains open a bittoo long, reducing efficiencybecause the cylinder doesn't seal properlyfor the compression stroke,but many engines have aninterference design wherethe valves and the pistonsoccupy the same spacejust at different times.You can, you can probablysee where the problem is.If an interference engineis spinning too fastand a valve stays open for too longthe piston can hit that valve,called fricking crap day.It's your crappy day, okay.It crushes the valve and you gota ton of repair work to do so.How do you make valves that can copewith increased enginespeeds and avoid flow?Well, you can use stiffer springsso if the valve snap shut faster,of course that meansmore force is required toopen those valves asthey open more slowly.The solution in someracing engines is to switchto a pneumatic spring,a nitrogen filled pistonretracts the valve more quicklyand doesn't oscillatelike a mechanical spring.The downside, that's a much more complexand expensive system than the traditionaloverhead valve designor overhead cam designs.There's also some electronic valveswhich is pretty interesting or Koenigsegg,they have their free valvetechnology again, very cool,but very expensive.So you might have noticed a trend so far,the factors that limit engine speedall can relate to carefullytimed movements of its parts.But it's not just the movementof engine parts that haveto be carefully timed.Third problem for revving highis that combustion itself takes time.(upbeat music)Combustion starts at an ignition point.The spark plug in anengine and moves outwardinto a process called flame propagation.The moving line ofcombustion is the flame frontand how fast it moves is the flame speed.You can see the flame propagate downwardfrom the ignition pointat the top of this bottle,that's like the flame front insidean engine cylinders movingdown from the spark plug.Every type of fuel has anintrinsic burning velocity.It's flame speed under aset of ideal conditions.So we have two fuels that we tested.We used a standard ethanol based fuel.Super cas.(fuel exploding)And we also use isopropyl,alcohol, 91% isopropyl alcohol.(fuel exploding)Pretty potent stuff.You're not going to find thishigh quality isopropyl alcoholat your local CVS, actually yeah we did,we bought it at CVS.If we compare the speedof the flame propagationbetween these two, we can seethat this fuel burnsfaster than this fuel.Different fuels, different burn rates.But burning velocityis just the beginning.Actual flame speed isaffected by temperature,pressure, turbulence in the fueland the distribution of oxygen.When I used compressed airI was creating turbulence,which slows flame speedbut I was also vaporizing the fuel,kind of like a fuel injector.So what does all this mean for rev limits?At 1500 RPM and at a 12to one compression ratiothe flame speed in agasoline engine is around 16and a half meters per second.But with changes in temperature,turbulence, and fuel air mixturesflame speeds can vary between10 and 80 meters per second.Piston speeds can reach40 or 50 meters per secondmeaning that under the rightconditions at higher revsthe piston will outrunthe flame propagation.If that happens, the combustionisn't exerting any forceon the piston and the enginesimply can't spin any faster.It's sort of a built-in rev limiterthat particularly affectsengines that have a long strokebecause of their higher piston speeds.In fact, one of the primary reasonswhy F1 engines can rev so highis because they haveincredibly short stroke,just a bit over 53millimeters, that's two inches.A short stroke keeps pistonspeeds low, prevents outrunningthe flame front, and keepseffective masses manageablewithout heavy bottom end partsas you approach the 20,000 RPM ceiling.It also increases the space for valvesmeaning you can feed moreair into the cylinders.Large boards, small stroke is partof how Bruno's 2006 F1 enginescapable of revving to a record 20,500 RPM.But there's always a couple of downsidesand one of them is with a short strokevery little power is made at lower RPM.That's why an F1 engine has to idleat 5,000 RPM just to stay running.And one reason why we don'thave 20,000 RPM enginesin our road cars.So now we know that if we want to rev highwe've got three problems to overcome.Damage to the bottom endfrom rapid acceleration anddeceleration of the piston.We got damage to the top endfrom valves that aren'tretracting quickly enough.And we got pistons that outrun combustion.The reason why every car doesn't red lineat 20,000 RPM is because the solutionsto these problems allcome with trade-offs.You're a trade-off, okay.Your parents traded offhaving a fun life for,thank you guys so much forwatching this episode of B2B.We're getting up towards5 million subscribers.Thank you guys so muchfor being part of the Donut family.We love you.Until next week, bye for now.