Belt squeal, engine on and off (Pulse & Glide)
 

I was trying some P&G last night, out some back roads where I'd travel up to around 50, switch off my engine, keep the ignition on, then let go of the clutch once I got to a certain speed to bump start it again. However, whilst it started, there was a fair bit of squealing. Not sure if it was a belt or my tyres (would doubt that), but it was certainly concerning. Is this something common? Would many of you use the starter to get the car going again when gliding? The idea of using the starter to me seemed a little silly, putting increased wear on the car - especially if you were running without the engine all the time.

I always bump start without problems. I only use the starter if I coast to a stop. I would check your belts. I do find it odd though. Usually belts squeel when they're cold, not once they are warmed up.

There aren't an awful lots of Belts on my car... what would cause one to squeal when it was being bump started anyway?

Your starter is only designed to turn over your engine at about 500rpm. When you bumpstart it, you are instantly turning at 1500-2000 rpm. Do your belts squeal if you perform a quick throttle rap up?

I don't know what that is, sorry.

Do your belts squeal any other time? The shadow had an old stretched alternator belt that was almost nothing at first and eventually worked its way up to a constant SHREEEEEEEK.

I meant does it squeal if you rev up the engine really fast.

Hold on a second....

1. What speed is "a certain speed"
2. What gear are you bump starting?
3. How are you bump starting (are you just letting the clutch engage?)


Bump start in the highest gear possible, and don't let the clutch completely engage. You want to let it bite just enough to spin up the motor and then push in on the clutch pedal as soon as that happens. Then, tap your throttle (to rev match) and put yourself back in the gear suitable for the speed you're traveling.

DO NOT just let the clutch pedal go and call it a day... That's a LOT of strain on the clutch assembly (ever seen a clutch blow out? it's pretty cool looking o.0). It also puts a lot of strain on the rest of the rotating components that weren't designed for a quick change in torque (such as belts/pulleys).

It is an excessive torque load ;) 0 to 1xxx+ in a few milliseconds puts a huge amount of stress there.... When you shut off your engine, does the engine stop spinning immediatly? No, there's a lot of rotating mass that keeps it going - dumping the clutch magnifies that load.


In any case, here's a more complete thread on bump starting...
http://ecomodder.com/forum/showthread.php?t=1141

So you got me thinking.... just how much torque is there? So some engineering maths.....


Lets assume we only have to spin up a flywheel... that is, no piston friction, accessory load, etc. etc. Just spinning up a ~20lb mass in a 12" disk. I'm going to do this in SI and convert to ft-lb because, lets face it, Slugs are a retarded.

mass = 9kg (slightly less than 20kg)
radius = .3048m (12 inches)
assume uniform weight distribution (this is not true, there's typically more weight near the out edge)

So Angular momentum (L):
L=I*(omega)
where
I=Moment of Inertia (again, assume uniform distribution - also assume a 0 height disk)
(omega) = angular velocity

angular velocity = 136.78 rad/sec - about 1500rpm

So,
L= 57.18 (kg*rad/s*m^2)

Torque is the differential of angular momentum with respect to time (dL/dt)
Time from 0 to 1500 rpm....

Staring with 50ms and converting to ft-lbs
843.52746 ft-lb

421.76373 foot pounds for 100ms
210.881865 foot pounds for 200ms

And 200ms is starting to get slow for something like this.... Add on top of this that, in reality, these numbers are higher when friction, and accessory loads, reciprocating and other rotating mass are considered.... I'm fairly confident that the load will be more like a gradient - the closer you are to 0rpm, the higher the torque and the closer you are to your final rpm, the lower the torque (approaching 0) with the value above being the average over the entire range.

Now, my car makes a max of 122ft-lb at 2600rpm.....

Not to gloat... but the last owner of my car (and only other owner) babied the transmission... I too am nice with my transmission.... I've got 115K miles on the factory clutch :D I'm fairly confident that's not from dumping the clutch :p

Feel free to check the maths - remember, it's a simplified calculation (something I could do using notepad and the calc that comes with windows :p).