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).

Shifting technique notwithstanding (I'm surprised you assume all cars will behave the same way yours will, Frank), it could be a case of the alternator working harder on start-up due to 12v drain during the coasting portion. If the belt's old or a little loose, it may squeal under those conditions but not in "normal" use.

Regardless, not being sure whether its tire squeal or belt squeal would have me wondering whether this technique may be outside your comfort zone.

Golf has totally different clutch grab properties launching vs starting. I have no Idea why.

Launching there is a WIDE range of Slip, Starting (engine) there is about 1/4" of slip, and if you try and release the clutch linearly without slowing at that point you will chirp the tires. lol


Check your belt. if you want to be totally ghetto you can put some valve grinding compound on it, but that will wear it (and your pulley) out quickly. <- Used car salesmen trick.

if your are not comfortable with this, put it on the shelf until you feel that you are ready to try again.

Really? When I shift, I noticed I deliberately slow down when I get to the clutches engagement point, IME the clutch engaging over 500ms seems correct, at least for bump starts. 100-200ms would be popping it for me, at least in terms of releasing it. That being said, one thing that irks me is the difference in engaging a hydraulic versus a cable actuated clutch, and I could see a hydraulic clutch cutting down on the engagement time since IME it does.

I recall driving a mid-90's Jetta (fairly new). I was driving a '96 Civic 5-speed at the time with a distinct engagement point. The advice of the VW owner was to almost dump the clutch to eliminate slip. It was odd, but it worked.

Same with a new '03 Golf TDi I test drove. Fast clutch pedal travel for seamless engagement.

Is yours the same, comparatively?

RH77

I still find it's smoother to clutch start in a higher gear and then shift down to the "driving" gear. Obviously we have different interpretations of "smooth".

Exactly how do you double clutch a stopped engine? The only way to RPM match to zero is to hold the clutch in for a few seconds before shifting and let it spin down.

Anyway, here's what I do to bump start. If I'm going 20mph or above, I put it in 5th, set my throttle to where I think RPMs should be in 5th and let out the clutch quickly but smoothly. I never pop it, it's hard on pretty much everything under the hood.

Do you guys ever experiance booster runout? With the engine off and there possible being heavy traffic, aren't you afraid you'll run outta vaccum on your booster? This the whole reason hybrids need a special braking system, so that you have boosted pressure if you need it.

Maybe it is common practice not to cut the engine in heavy traffic?

The 'double clutching' here was not in reference to shifting gears. It was referring to bump starting by letting out the clutch gently just till the engine turns over and starts and then pushing the clutch back in quickly so as not to force the motor to spin up to a high RPM in a few miliseconds.

In many vehicles, simply letting the clutch out all the way to start the motor, all in one move, will cause a jerk and rebound situation that I find most unsettling.

Hi AnDoireman.
I'm a fitter and whenever i hear a belt squealing, it's probably loose. If its almost worn out and its 03:00 i just horse some belt spray in there. Works a treat!
Otherwise, like what was said above, dont dump the clutch and bump start in 4th or 5th.
By the way, welcome! I'm from Slane, Co.Meath.

ollie

Hello vtec-e. I haven't been on the site for a while, and coming back yesterday or so, noticed yourself actually, and was reading about your solar panel. Quite interesting, but was a bit dissapointed to see some comments that it might not be that helpful. The belt issue - I had of course assumed it was a belt squealing. I know the difference in sound, but the reason I had mentioned the tyres was because of the harsh momentum loss when almost dropping the clutch, and because I had recently checked my belts, and found them to be on tight. I also couldn't see why that would make the belts squeal. Do you know of any Irish or European sites similar to this one? No harm to this site, as there are loads of intelligent people who know their stuff on here from the looks of things, but of course, a good proportion of the information is US-Centric, and it would be nice to be able to talk to more local people about fuel use, where our roads, cars and all that are different.

My vacuum booster is good for two full stops before the pedal gets hard - really hard. Of course, by the time I've come to a complete stop, there's a better-than-even chance I'm going to need the engine to get up to speed again, so there's my booster restocked with, um, nothingness. :D

Lots of light taps on the brakes will run the booster down, too, but if I find I'm having to tap the brakes while coasting, I'll engage the tranny to spin the engine in high gear, which provides some drag, recharges the booster, and give me plenty of spin to refire the engine if needed.

As to the original thread: It sounds like one of your engine accessories doesn't want to spin up as fast as you're trying to make it spin. Either you're dumping the clutch for engine restarts, which could be alleviated with a gentler reengagement, or one or more accessories is experiencing a lot of drag. Also consider checking the belt tensions.

His last post makes me think he's dumping the clutch a little too hard. To the op: you might want to try letting the clutch out very slowly until you figure out where EXACTLY you have to release it to, to get the engine spinning again. Make sure, also, that when the engine spins up you are dis-engaging the clutch almost immediately to take any excess strain off the engine and allow you time to RPM match and put it in the proper gear for your speed.

Yeah, this was quite a while ago though. It's no longer an issue, when I ease the clutch out. It doesn't feel very good for the car to do it though, so I haven't really been doing it much lately.

This is the closest i could find:

http://www.askaboutmoney.com/showthread.php?p=630785

It's not a hypermiling forum, just a thread in the askaboutmoney froum. Interesting to read what the general public think about "certain" techniques!
Like coasting. Thats where the real gains are to be made!!
Anyway, i don't think the Irish are quite ready for all this carry on just yet....which is crazy given the cost of fuel. Heads in the sand i reckon.

ollie

Hey, stick around! 1/4 of my ancestors came to Canada from Ireland. :P

Of course i will! I learn far more here at ecomodder than i could anywhere else!

ollie

Honestly, the U.S. and Canada seems to generate the same kind of response. On the flip side, there is probably a a larger number who view the information and begin to consider it.

It's like the comment card at a restaurant -- most of the feedback is generally from complaints. People don't bother to fill it out if their in the middle or the experience was good. If it was good to great, people will tell their friends, etc.

Brake Booster:
I often run a brake booster down to nothing. An example: I became stuck in a highway traffic jam on a long downhill last week (it took 30 minutes to go 1/4 mile). The car will still stop, but it takes extra effort (both feet to push). It's like the days of "manual brakes" -- you just have to replace vacuum with your own energy.

It takes quite a bit of attention. Once the third car ahead starts moving, it's time to coast. Leaving enough space, it's possible to either use the foot pedal or emergency brake. In this case, the e-brake was the ratcheting foot style, which is a pain.

Granted, top speed was 4 mph in this case -- so higher speeds typically require that vacuum for panic stops and steers.

Good to see that you're staying! :thumbup:

RH77