What determines MAX RPM

[RadioHowie]

Coming home from the office, I'm stuck in a massive highway-repair traffic jam. Cars are lined up literally for miles and my nearest "out" is about 3/4 of a mile away. My mind starts to wander on a variety of life/home/work related topics while counting how many minutes between fan-cycles, when a pirate pulls up next to me, rev-tuning the **** out of his Ukrainian Ditch Pump. It got me to thinking....after "what way can I murder this guy and not get caught" came this thought....."what determines a motor's MAX rpm?"

There are SO many things going on in an four-cycle motor -- induction, compression, ignition, exhaust, piston speed, crankshaft strength, valve-train weight, etc., etc., etc. -- that it made me wonder.

If Yamondasuzusaki was designing a motor for their brand-new "NutRoaster 6000", is the rev limit determined by the design of the motor?...the materials of the moving parts?...the amount of air/fuel that can be shoved in?...just what the hell is it? Is the horsepower goal designed in to a new motor, or does a company pick a displacement, divide by 4, build the thing then run it to destruction to determine the rev limit? Or is the rev limit built in to the design?

Why is the FJR rev-limited to 9k rpm while my ZRX is rev-limited to 11k? And some 6 cylinder Honda 125cc two-stroke from the 60s had something like a 25,000 rpm rev-limit. Why does the V-8 in a Ford Explorer have a rev limit of 4k while the same basic motor in a Mustang Cobra has an 8k rev limit?

I thought I'd ask in the appropriate forum, and some of you techie-types might answer with a reasonable amount of explanation.

 

[Bustanut joker]

I hear it has something to do with the type of oil used during the break in process.

 

[Rogue]

....."what determines a motor's MAX rpm?".......

Physics.

 

[beemerdons]

I hear it has something to do with the type of oil used during the break in process.

+1, Gunny; I am with Einstein here, it's the oil!

https://www.amazon.com/Top-Dead-Center-Cameron-Magazine/dp/0760327270 RadioHowie, if I thought you could read (other than ****o!) I would loan you my copy of "Top Dead Center - The Best of Kevin Cameron in Cycle World"!

 

[Geezer]

Many things:

The weight of the pistons and the strength of the connecting rods and crankshaft. Stronger parts are more expensive and must be made to closer tolerances. Pushing the limits causes reliability problems. Smaller engines with the same number of cylinders have smaller lighter pistons so they can spin faster.

The cam and valve springs are also factors. If the valve springs are strong enough to close the valves faster then they are causing more wear on the parts they act on. Again, pushing the limits causes reliability problems. This is where the F1 cars have an edge because they use actuators to open and close the valves so they don't have camshafts.

 

[TomInPA]

If you could just split lanes, you wouldn't need to worry about such stuff. Lane sharing is the get out of jail free card. :yahoo:

 

[TheZsdad]

E2 = (m c2)2 = (m0c2)2 + (pc)2

 

where pc = momentum in energy units (divide by c to get momentum) (note: the mass used in the momentum is the relativistic mass)

E = total energy

m = relativistic mass

m0 = rest mass

 

[beemerdons]

Many things:

The weight of the pistons and the strength of the connecting rods and crankshaft. Stronger parts are more expensive and must be made to closer tolerances. Pushing the limits causes reliability problems. Smaller engines with the same number of cylinders have smaller lighter pistons so they can spin faster.

The cam and valve springs are also factors. If the valve springs are strong enough to close the valves faster then they are causing more wear on the parts they act on. Again, pushing the limits causes reliability problems. This is where the F1 cars have an edge because they use actuators to open and close the valves so they don't have camshafts.

Very Nicely Done Greg, you have basically condensed into two paragraphs what Kevin Cameron used 321 pages in his book to explain!

 

[PhilJet09]

I don't know, but it reminds me of a video I saw of a motorcycle engine valve train at max rpm. Pretty amazing. I can't seem to find the video now though, dammit.

 

[Denver_FJR]

Many things:

The weight of the pistons and the strength of the connecting rods and crankshaft. Stronger parts are more expensive and must be made to closer tolerances. Pushing the limits causes reliability problems. Smaller engines with the same number of cylinders have smaller lighter pistons so they can spin faster.

The cam and valve springs are also factors. If the valve springs are strong enough to close the valves faster then they are causing more wear on the parts they act on. Again, pushing the limits causes reliability problems. This is where the F1 cars have an edge because they use actuators to open and close the valves so they don't have camshafts.

So what's holding Ducati and their desmodoohicky motors from wrapping up to 45k RPM?

 

[RaYzerman19]

Here's the video.......

 

[wfooshee]

This is where the F1 cars have an edge because they use actuators to open and close the valves so they don't have camshafts.

Not quite. They have camshafts, but use pneumatics to close the valves rather than metal springs.

I've seen that Renault has experimented with electro-hydraulic valves without cams, but that's not been used in the real world.

So what's holding Ducati and their desmodoohicky motors from wrapping up to 45k RPM?

Desmodronic systems are extremely sensitive to proper adjustment, and if you spin them too fast, the parts start to flex, which is, as they say, not good.

To the original question, my understanding is that it's primarily valve gear. If you can't close the valve fast enough to keep the piston from ramming it, then bad things happen that R. Howie is all too familiar with.

Once you handle the valvetrain, then conrods come into play. That big flat thing at the end that goes up and down has mass, and the conrod has to make that mass stop and go the other way over and over and over. That's easier if you have a short stroke, so a short stroke can give you a free-revving engine, i.e. Formula One engines at 18,000 RPM at 2.4 liters.

His question about how much air can be shoved into it plays in, too. When GM was running the Corvette in the LeMans GT1 class, they actually reduced the engine size to fit the rules better. The larger engine would have required supersonic airflow through the intake restrictor.

A 2-stroke limit should have an obvious answer: no valvetrain.

As for the Explorer vs. Mustang, the Explorer motor would probably turn the revs if it wasn't built to power a truck. Its ECU, accessories, etc. would be designed for lower RPM, partly for economy, partly for NVH (noice, vibration, and harshness) and partly because its requirement would emphasize torque over horsepower. Remember that revs equal horsepower, and the Mustang driver is looking for power, not trailer-towing torque.

 

[RadioHowie]

Okay, SOME good answers, but the ones that aren't..... :finger: :****: :finger: ......you know who you are.

But some of my original question remains unanswered...and maybe can't be. Does a manufacturer design its engine for a specific Max RPM, or do they build what they want, run it on a dyno till it 'splodes, then put the redline 1k under the 'splode point?

And ARE motors built to rev specifications, or is it a matter of "let's slap the most expensive parts we can afford together and see how far it goes?"

 

[worney]

Good timing and a bad memory on my part...

Sometime in the past 4/5 months in of the m/c mags I have bought talked about this very question in their repair section q and a. They stated that it was piston speed, which relates to the force exerted on all of the engine internals. I'll try to find it. Of course the mag writer may have been speaking out of his butt.

-worney

 

[MCRIDER007]

I don't think anyone has mentioned piston speed, which for a long time was generally limted to about 4,000 ft per minute...but I think the limit now is about 4,400 feet per minute. If you want higher rpms you have to shorten the stroke and then find a valve train that can keep up with rpms.

 

[ionbeam]

This is an overview of a design process. In spite of the length of this, it is just an overview leaving out many things.

Someone in marketing says to the Big Boss, "We have an opportunity to sell..." A good marketing guy will say that they have survey results which tells them they can sell 50,000 units a year if they build this exact combination of performance, appearance and perhaps a bit of comfort. The Big Boss will tell the engineers to make it so. Now the engineers have a design target which is to build a 200 lb motorcycle that makes 200 hp, will out perform a track bike, ride like a Gold Wing, cost $2k to build and sell for $20k. After a lot of bitter fighting between management and those incompetent engineers they compromise on a 500 lb motorcycle that makes 190 hp, will work on the street, is rideable for at least 100 miles at a sitting, will last for 100k miles, cost $7k to build and will sell for $14k.

Now to figure out the power curve of the motorcycle and match it to the style and intent of the bike. For this pure sport bike it is decided that the best way to meet the power curve, with a sporting feel above all else, the engine will need to displace 1,000 cc and be a short stroke design to meet the fast revving specification. To rev quickly the reciprocating mass needs to be low with low flywheel weight. After a lot of math, using the displacement as the only known, it is determined that the engine will have to have this bore and stroke combination with an eye on piston speed. This combo now determines what the revs will have to be to meet the HP goal. Also, at this time the engineers discover that to meet the high HP sport feel and HP goals that the engine will need to have a volumetric efficiency of 1.25:1 so some special intake and exhaust design will be needed.

The engineers now start designing the intake, spark control, fuel and exhaust system. The aim is to have one narrow peak intake/exhaust resonance occurring between 11,000 and 12,500 rpm where peak HP will be generated and allowed to continue with 'empty revs' up to 14,000 rpm where torque is falling so quickly there will be no more acceleration. Resonance is where the intake pressure waves and the exhaust pressure waves in conjunction with the mechanical valve and piston motion reenforce the viscous mass of gasses being flowed. The inertia of the intake gasses will actually let it continue to flow after the piston reaches bottom dead center allowing the cylinder to over fill, achieving the 1.25:1 volumetric efficiency at peak resonance.

To retain acceptable low rpm power the engineers discover they will have to have some way to control the size of the intake opening and exhaust opening. They calculate that the intake will need to be narrowed and the exhaust narrowed by This Amount from 1,000 rpm to 6,000 rpm then over the next 1,500 rpm the intake and exhaust needs to be fully opened. Now the intake tract and exhaust pipes have flow valves designed in.

The critical cylinder head design is drawn up to meet the cam, valve, spark, compression and flame front requirements. The valve train is calculated so that the engine will have acceptable low rpm driveability yet meet both the peak power and volumetric efficiency. Valve lift, duration and rate are calculated. The engineers discover that the valves will need to have a three angle grind on the valve edge to create better initial un-seat flow without having to stress valve springs and have a radical cam profile.

With all the engine functions determined it is time to start working on the materials that will meet these criteria. The operating range of the engine will be from an ambient of 0° - 130°F. The engine will be designed to meet 100k miles of durability. It must be rideable from a cold start with all engine components cold and contracted up to full operating temperature with all engine components hot and expanded. From the engine specifications the cylinder block and cylinder head temperature can be calculated and coolant passages and coolant flow determined which will be incorporated in the castings. Knowing piston mass/speed, crank mass/speed, valve mass/speed and maximum engine part's acceleration rate, materials can be selected. The engine block and cases can be designed with materials meeting the stiffness and strength requirements. The engine will be designed run at full specifications for 100,000 miles. To meet this the materials are selected to have sufficient strength to not wear out, break, bend or explode as long as the engine is operated within specification. Since the engine will almost never actually see this kind of operation it is technically 'over designed'.

The engineers tell The Boss they can meet the engine specifications quite easily using titanium and magnesium for many of the parts but the cost will go up by $30,000. The Boss regains his ability to speak and the blood vessels on his forehead stop pulsing. They discuss using typical metal alloys and compromise by allowing the engine to be both slightly larger in external size and slightly over the target design weight.

Lots of computer time is used. Metal chips fly, molten metal flows. The transmission guys show up with a box of gears and some clutch plates. Assembly takes place -- all parts fit as intended. Off to the test cell where they will see if the engine meets design specifications. Now that the engine has met performance specifications it is time to start reliability and durability requirements. To achieve these test goals in as short a period of time as possible they will run some engines at a normal operational profile across all expected conditions. Other engines will be run to destruction for failure analysis. In 6 months of testing and a number of design and material changes the engine is certified.

 

[Bustanut joker]

Lots of computer time is used. Metal chips fly, molten metal flows. The transmission guys show up with a box of gears and some clutch plates. Assembly takes place -- all parts fit as intended. Off to the test cell where they will see if the engine meets design specifications. Now that the engine has met performance specifications it is time to start reliability and durability requirements. To achieve these test goals in as short a period of time as possible they will run some engines at a normal operational profile across all expected conditions. Other engines will be run to destruction for failure analysis. In 6 months of testing and a number of design and material changes the engine is certified.

 

[beemerdons]

Lots of computer time is used. Metal chips fly, molten metal flows. The transmission guys show up with a box of gears and some clutch plates. Assembly takes place -- all parts fit as intended. Off to the test cell where they will see if the engine meets design specifications. Now that the engine has met performance specifications it is time to start reliability and durability requirements. To achieve these test goals in as short a period of time as possible they will run some engines at a normal operational profile across all expected conditions. Other engines will be run to destruction for failure analysis. In 6 months of testing and a number of design and material changes the engine is certified.

Thank Goodness "The Smart Guy" Alan Ionbeam finally showed up!

I could see this RH thread careening into Manatee mating rituals!

 

[Donal]

The engine will be designed run at full specifications for 100,000 miles.

Is that 100,000 miles between oil changes or.......... ?

Don

The engine will be designed to run at full specifications for 100,000 miles.

Is that 100,000 miles between oil changes or.......... ?

Don