crankshaft deflection

[malbeare]

I would like to get a handle on how much crankshaft rotational deflection occurs in engine operation .
I have heard that deflection could be in the order of 1 degree, say if number 1 cylinder has just fired and the resulting torque twists or winds up the crank to the flywheel end . I imagine there would be both positive and negative deflections.
Could anyone enlighten me to the actual measurments experienced in engines.
Thankyou

A tidy mind not intelligent as it ignors the random opportunities of total chaos. Thats my excuse anyway
Malbeare

 

[ivymike]

it varies significantly between engines. 1deg nose-to-tail is not unreasonable. Some large engines are designed to have less than 0.4 degree max deflection. google "torsional vibration" and you might find out more.

 

[GregLocock]

Yup, 1 degree pk to pk would be quite typical on a 4 litre I6. If you take the TV damper off you'll get a lot more.

Cheers

Greg Locock

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[gio1]

Shorter cranks are stiffer and tend to twist less.

For a I6 engine for cars I have experience of twist angles (end to end) of up to 1.5/2 deg, when rotating at high speed (7500/8000 rpm) with no engine load (driven by the dyno).
Twist is worse in this case because the 3rd order inertia torque (critical for a 6 cylinder) is not counteracted by firing torque.

 

[R1100S]

As with everything on this forum, it depends on the application. Top fuel engines see 15-20 degrees under power, enough that it has to be accounted for when you grind camshafts. 1 degree seems right for a long production crank.

 

[ivymike]

20 degrees of crank deflection? On a crank that stays in one piece?

 

[R1100S]

Yep, 20 degrees, keep in mind the engine is only at full power for a few seconds and rebuild every run, its lifespan is measured in hundreds of revolutions.

 

[GregLocock]

Isaac, we have halfshafts that do not fail if they are twisted up by 180 degrees! Now, admittedly they are made from nicer materal than your average crankshaft.

Cheers

Greg Locock

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[malbeare]

R1100S (Mechanical) 29 Aug 05 12:01
As with everything on this forum, it depends on the application. Top fuel engines see 15-20 degrees under power, enough that it has to be accounted for when you grind camshafts.

R1100S
Presumably the cam stays in time with the front cylinders as the drive for the cam is at the front of the engine .So do you have to retard the cam grind progressivly as you head towards the back cylinders?
Malbeare

A tidy mind not intelligent as it ignors the random opportunities of total chaos. Thats my excuse anyway
Malbeare

 

[ivymike]

and what happens at no- or part-load? or are the engines switched on and run immediately to full power?

 

[evelrod]

Heck, I can come to grips with a 20 degree twist quicker than I can visualize a 5000+ bhp at 10,000 rpm from an 8 litre V-8 engine pushing a car from 0 to 325+mph in 4+ seconds! If ya wanna see some BIG numbers, do the fuel flow calcs for ONE second.

Rod

 

[NormPeterson]

and what happens at no- or part-load?

I don't think its life cycle includes very many revolutions or much distance driven under those conditions either, so its "driveability" rating only needs to be a little better than "keeps running".

Norm

 

[ivymike]

yeah, but if the valve lift events are set for full load, and the deflections there are as large as mentioned, and valve-to-piston clearances are remotely close to "normal," then I can't see it making a single successful rev at idle.

 

[GregLocock]

Mal, I've seen timing problems introduced by camshaft twist (the grinder wasn't keying properly). You'd definitely have to do something funny if you were expecting 20 twist degrees on the crank.

Cheers

Greg Locock

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[patprimmer]

ivymike

Top Fuel dragsters run reasonably low static compression as they run high boost and high nitromethane levels.

This leaves plenty of room for a lot of piston to valve clearance.

Another concern is that at the valve spring pressures they run, the camshafts also "wind up" a bit, so I guess they would need to correct for both.

The crank would be torque and load dependant, while the cam will be rpm dependant, so the relationship will keep changing according to several independent factors.

Maybe someone will come up with individual cylinder crank position sensors to control spark and to control individual lobe "V tech". Just kidding, this is reasonably big budget, but is not F1 we are talking about.

Regards

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[kenre]

i read an article many years ago that the formula one engines had the camshaft ground with a 16 degree twist.. no wonder they idle rather strange...

Ken

 

[evelrod]

F1 engines really don't idle, unless you call 3 grand an idle.

Rod

 

[patprimmer]

Also, I would think that F1 had very tight piston to valve, being NA, high compression and big bore short stroke.

Maybe in the turbo era there was room to move the cam around.

Regards

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