Why is my bike using oil???

[embee]

Originally Posted by Nekkid

Crikey! Go Embee!!
Thanks!

it's what I do for a job, same as all you IT experts talk about your stuff.

Oil Consumption - Are bike engines designed in a similar way?
Stu

it's not that you really design an engine to use oil, and the theory that you allow more oil for bore lubrication is folklore.
It's simply that the higher the sliding speeds, rpm and therefore piston acceleration at reversal etc, the more difficult it gets to keep the ring pack under control.
You do design ring pack and piston grooves and lands (the bits between the ring grooves) to "manage" the gas leakage (blowby). You can use the gas pressures between and behind the rings to control the contact pressures on the bore and help to put off the point at which the rings lose contact with the bore.
Pistons tilt in the bore, typically reversing the tilt direction 7 times in a complete cycle. When this happens the rings slide in the grooves, and friction can make them stick momentarily. The tilt reversals are managed by offsetting the pin from the centre-line, typically a mm or so, usually towards the thrust side. This encourages the piston to tilt due to inertia forces at TDC and BDC, so you can get it sitting the right way before the firing pressure rises. This minimises the impact and thus noise.
You can make a ring pack work at very high speed but it necessarily ends up very light and slender, and durability becomes an issue.

A very significant factor in oil consumption is bore geometry. Bores are usually finish bored/honed (nowadays) with a dummy cylinder head bolted in place so that it's distorted just like when it's finally assembled. This means it's as round as you can get when the engine is cold, however they don't stay round nor straigght usually when it all heats up, so the rings have to accommodate a lobed tapered bore to some extent. This tends to give uneven oil film thicknesses round the bore surface and thus more oil consumption than you really need.

Emissions requirements tend to dominate engine development these days, and low oil consumption is crucial for long catalyst life.

my experience with oil consumption in my SV

if l check the sight glass religiously, and keep it topped off I go thru about 1quart-1½quarts between changes every 3500-4000 miles

however should I slip up and let it get below the sight glass, rate of oil consumption increases dramaticly. I have been almost 2 quarts low

heaviest rate of oil consumption is freeway steady high speed.

embee,

Question for you (slightly off topic sorry) would what you just explained be the reason MR2's suffer from servere piston slap until the engine has warmed up? Always remember them sounding like diesels for the first 10 mins or so...

Stu

My opinion is that most oil consumption is due to wear in the valve guides & seals

[embee]

Originally Posted by MavUK

embee,

Question for you (slightly off topic sorry) would what you just explained be the reason MR2's suffer from servere piston slap until the engine has warmed up? Always remember them sounding like diesels for the first 10 mins or so...

Stu

I spent a couple of years working on piston development in a family of car engines.
It's a complex subject, but yes, in basic terms the noise is from impacts of the piston on the bore at certain reversals. depending on which reversal is most significant you might do different things.

Pistons are very intricate objects, the skirts are not straight vertically, and they are not round, the ring lands are all different sizes and may be tapered and indeed need not be concentric with the skirt. Ring grooves have different back and side clearances. Oil drainage from the ring pack varies (internal/external). Pins are offset from the centreline (indeed pin bores aren't necessarily parallel, to allow for pin bending)

You try to get the profile so that it's suported by the skirt at all times, from cold starts at -30C up to full running temperature, allowing for the shape that the bore takes up too. However that can be difficult. The overriding requirement is avoiding scuff failures due to the skirt becoming too tight, and concentrating on optimising the contact area at normal working temperature. This may mean that cold start noise becomes an unfortunate compromise, though it's more and more unacceptable nowadays, and shouldn't be very noticeable in a well designed/developed engine.

The worst case is where the top land (crown) contacts the bore on the TDC firing reversal, that's the one that gives the diesel type rattle because the crown is very stiff and carries a significant amount of energy during the tilt due to the high rate of pressure rise.
If you can avoid the top land contact by extending the skirt support and altering the pin offset, you can turn a rattle into a "hollow growl", which is usually less objectionable. Most engines have some sort of piston growl from cold, in my experience. I've never known any mechanical damage from these sort of noise problems though, even in very noisy engines.
Emission requirements mean that top land clearance is kept to an absolute minimum (unburnt HC from "crevice volumes"), so it a close balancing act.

You can tweak cold start noise a bit by fiddling with ignition timing, but that's usually an emission/economy determined thing anyway.

Good block/bore structural design is crucial.

Modern valve stem seals are pretty good (actually metering devices, they are designed to control the oil flow not stop it completely), and unless they're damaged or completely knackered they'll contribute only a few percent to the overall consumption. Remember a piston is 10-15 times the diameter of a valve stem and slides nearly 10 times as far twice as often.

Breathers are usually no.2 on the list for oil consumption.