Why do high revs kill engines ??

[Nick M]

 

Just pondering why higher engine revs tend to mean that engine longevity suffers ??

 

For example, if you have an engine that revs to 6,000rpm, what changes when you start running it at 8,000rpm ? Is it down to lubrication issues alone or a whole host of contributory factors such as bearing material and design, reciprocating mass, oil properties and behaviour, etc. ??

[tiddy1]

Nick in simple terms its all down to physics and Newton, ( he has a lot to answer for..)

 

Force = Mass x Acceleration,

 

As the revs go up the pistons have to slow down stop then accelerate in the oposite direction on each stroke much quicker than before, and as the mas of the pistons and rods has not changed the forces on the rods, pistons crank increase.

 

Hence fitting lighter pistons helps redress the equation and allows the engine to accelerate quicker.

 

If the force in the rods gets too high it will be able to overcome the hydrodynamic lubrication in the bearings (the layer of oil between the two rotating metal parts) and they will rub metal to metal and a nasty mess ensues

 

There are also increased stresses on all the valve gear etc

 

 

Simon

[Paul McKenzie]

Mainly due to increased forces exerted on the materials generated by greater velocity, acceleration and momentum of the components. Just imagine the difference in force exerted on the little end with a piston changing direction at TDC at 1 rpm or at 8000rpm and you get the message.

Paul

[westy]

Why do cream cakes make you fat?

 

 

 

 

It's just Gods way.

 

Always remember you're unique. Just like everyone else.

 

[Peter Carmichael]

Also to do with resonant frequencies. You get many more interesting modes of vibration as you add in a few revs. When your larger forces start being applied not quite in alignment you get big trouble.

 

It is easy to assume that big chunky objects like crankshafts are rigid, but they flex a lot with high revs. This can distort the bearing surfaces from being perfect cylinders, leading to the bearings picking up. (Just one example).

 

In a short stroke engine, the valvetrain suffers more than the bottom end. You can expect problems with trying to keep the followers touching the cam lobes, so you upgrade the springs. With high performance cams the lift can be higher too, so you need springs that make sure the full range of motion is possible before going coil bound. The material in these springs is highly stressed. Add in the resonance issues again and you can get problems with broken valve springs.

[Nick M]

Thx all - makes it much clearer now about why we do some of the things we do to engines to make them whizz round a bit quicker !

[Joe 90]

When you increase the RPM by 33% (e.g. from 6000 to 8000 RPM), the acceleration in creases with the square of the speed, i.e. by 78%. The crank flexes and the bearing loads shoot up as they struggle to keep the crank running true, putting the bearing wear up even faster. The moral is that a small increase in RPM gives a disproportionately large increase in wear rates.

 

Crash team to the Upgradeitis ward, the withdrawal symptons are getting worse ❗

[edmandsd]

Piston acceleration is probably a good measure of likely wear, being affected by the following factors in order of priority - rpm, stroke and to a lesser extent rod length/angularity. Obviously weight of reciprocating components is an important factor aswell.

 

Valve springs were mentioned quite rightly by Peter, having recently experienced a breakage myself and replacing the lot with virtually indestructible Schmithelm's as a consequence.

 

Crank rigidity can be improved by integrated sumps as per the Hart 420R engine or 4 bolt mains as per the BDT. Other techniques involve the use of a crank damper and balancing shafts. A properly counterbalanced crankshaft is obviously essential.

 

Home of BDR700

[YW Sin]

IMAGINE THIS.

 

TAKING A WALK FROM ARIZONA TO ALASKA vs RUNNING FLAT OUT FROM ARIZONA TO ALASKA.

 

YOU KNOW WHICH METHOD WILL GET YOU TO ALASKA.

 

Basically, the same principle applies to machines too. 😬

 

Edited by - YW SIN on 25 Jun 2003 13:58:02

[edmandsd]

It's amazing how reliable F1 engines are these days considering the distance they cover flat out in a race at getting on for 20,000 rpm. One of these in a club racer would actually probably last a long time between rebuilds. Therefore revs don't have to kill engines particularly considering some are designed better for revving than others (consider off the shelf bike engines for example). If there's a capacity limit that you're working within then rpm is one of the main factors in terms of increasing power.

 

Home of BDR700

[Bilbo]

There was an article years back in CCC say 1990

 

It talked about this very subject well reliability and extra power gained by more RPM. One thing that struck me was how much a piston weighing XX grams now = XX Tons at XXX miles per hour on the top of the stroke when it suddenly STOPS and reverses direction. I had never really thought about the concept that it STOPPED ❗ but when you think about it does.

 

It also explained how much the con rod stretched and the crank deformed. At 8000 rpm it does that 66.6 times a second ❗ On the down stroke it does the same thing 66.6 times a second and there is 3 more of them doing just the same thing. Then compared it with 6500 and 9500 rpm. Now at 20,000 rpm .

 

Chalenge for Peter Carmichael I am sure your up to filling in the XX bits. I bet you have nothing better to do 😬 😬 😬 subject for FAQ? Think I will pass on this one......I only make a mess of it 😳

 

It also gave a few of the forces on the bits and speeds of the valves made scary reading 😬 😬 😬

 

'Can you hear me running' ......... OH YES and its music to my ears 😬 😬 😬

1988 200 bhp, 146 ft lbs, 1700cc Cosworth BD? on Weber's with Brooklands and Clamshell wings, Freestyle Motorsport suspension.

[edmandsd]

Piston acceleration's the ultimate killer and my BD at 9,500 rpm with a 77.67 mm stroke has a maximum piston acceleration of 163,000 ft per sec sq. Same engine with say a 92 mm stroke would have a maximum piston acceleration of 201,000 ft per sec sq at the same rpm - such an engine would also probably require shorter rods making matters worse still. Therefore an 18% increase in stroke results in a disproportionate 23% increase in piston acceleration.

 

A safe max is usually considered at around the 150,000 ft per sec sq mark.

 

An F1 car with 43mm stroke and the same rod length as above, at 18,500 rpm would result in a maximum piston acceleration of 308,000 ft per sec sq. Interestingly a piston in say a vx with double the stroke but half this rpm (9,250) would travel the same distance as the F1 piston over any set period of time (and hence have the same mean piston speed) but with a maximum piston acceleration of only 175,000 ft per sec sq.

 

Home of BDR700

 

Edited by - edmandsd on 25 Jun 2003 20:57:47

[Peter Carmichael]

If you ever get me in the same room as my laptop, I have an accurately modelled engineering animation of bottom end forces.

 

Dave Edmands has given an example that allows me to emphasise what I mentioned earlier. With a short stroke F1 engine, the crank is still a familiar crank and the rods are still familiar rods. The valves however are pneumatic sprung because you cannot build coil spirngs to do the job. Short stroke engines of any given capacity have bigger bores. The bigger bores allow bigger ports and bigger valves and hence bigger power potential. In order to reach the flow potential the engine needs to turn bigger revs. According to some calculations I did, the bottom end accelerations need not be that much worse, but the valve accelerations are horrendous.

[Bilbo]

Hi Peter

 

That's a cop out so err .............. NO I am not going to do it 😬 😬 😬

 

I am off to eat diner in a minute, not sure what you mean by familiar take it that's its a program that a 'modelling' related to our types of engines 🤔

 

However, I am not disagreeing with you and edmandsd. I was just trying to get over the magnitude of speed, stress and related forces. Loads in relation of the output from the engine.

 

'Can you hear me running' ......... OH YES and its music to my ears 😬 😬 😬

1988 200 bhp, 146 ft lbs, 1700cc Cosworth BD? on Weber's with Brooklands and Clamshell wings, Freestyle Motorsport suspension.

 

 

Edited by - bilbo on 25 Jun 2003 22:39:09

[Peter Carmichael]

The fault in dave E's analysis is that the engine with the, say, 92mm stroke would require fewer strokes per second (rpm) to pump the same volume into the cylinders... therefore it achieves the same power potential at lower revs and the net effect is no different for piston acceleration. You get into considering "dwell" given by particular rod lengths/angles etc. determining combustion efficiency.

 

Piston mass of 439 grammes. 9200 rpm in an 1798cc K-series (89.33mm stroke, 133.1mm conrod)...

... small end load of... 2.4 tonnes at top dead centre. 1.2 tonnes at BDC

[edmandsd]

'According to some calculations I did, the bottom end accelerations need not be that much worse, but the valve accelerations are horrendous.'

 

An interesting point Peter - you'll appreciate from my calculations that my BD can spin roughly 500rpm higher than a standard stroke vx, but have no greater piston acceleration. As a consequence I run relatively mild valve acceleration cam profiles albeit with 'big' springs. The good vx's appear to run lower peak rpm but with much wilder valve acceleration cam profiles. I also run only four big injectors whereas the top vx's run 8 smaller injectors. I suppose it's all about making the most out of what you're presented with.

 

As an aside the relatively big bore BD's make more use of high octane racing gasoline than an equivalent smaller bore vx, with my engine now giving peak power some 500rpm higher than it used to simply as a result of increasing the ignition timing.

 

Home of BDR700

[edmandsd]

'The fault in dave E's analysis is that the engine with the, say, 92mm stroke would require fewer strokes per second (rpm) to pump the same volume into the cylinders... therefore it achieves the same power potential at lower revs and the net effect is no different for piston acceleration.'

 

If the engines have 4 cylinders and the same capacity then the capacity of each cylinder will be the same 🤔......in any event I didn't mention power, only piston acceleration.

 

If you're referring to my earlier post I did actually say 'If there's a capacity limit that you're working within then rpm is ONE of the main factors in terms of increasing power the other' obviously being volumetric efficiency.

 

 

Home of BDR700

[Peter Carmichael]

Sorry. Use of the word "fault" was a bit forthright. I was aiming to illuminate an aspect of the problems associated with revs that was not being emphasised.

[edmandsd]

Peter - If you're still interested in one of the Elite boxes drop me a line as I might have something that interests you - david@edmands.fsbusiness.co.uk

 

I can assure you this ISN'T a sales pitch but the box is awesome (you'll be aware that I swapped over from the prototype box recently) and with the powershift the advantages over a conventional H pattern box are significant. If you're only a circa a second under your time with the less powerful engine at Curborough i'd bet that you could beat the big engine's time with one of these boxes installed.

 

Home of BDR700

[caterhamnut]

Bilbo - somewhere I have all these facts and figures in an article for an F1 engine - it does make interesting reading. The g-load on the components etc!

 

F1 is getting over the problem of valve movement by developing electo-magnetic valves ( I think that is the right name) rather then mechanical or pneumatic. Being developed by Renault ISTR. Does this mean - in very simplified terms - using relays as valves?

 

New site! mycaterham.com

here

42,000 in 23 months!

 

Edited by - angus&tessa on 26 Jun 2003 00:47:08

[edmandsd]

...so a crank and rod engine will rev even higher than currently as the restriction remains to be the valve train, eventhough its currently pneumatic...simply astounding.

 

Home of BDR700

[Graham Perry]

With electric valves the need for a camshaft will disapear which will mean fewer mech losses. Just imagine you will be able to 'map' your valve timing/opening/overlap/emmissions/power/torque, all at different revs. Indeed you could then challenge the need for traditional shaped valves,and some sort of rotary port may be possible instead

 

Edited by - Graham Perry on 26 Jun 2003 08:26:38