Bob Sykes wrote:
> The recent threads on thrust washers, rocker oil feeds
> and death rattles have caused me to contemplate oil pressure.
> As I understand it, this "death rattle" is due to bearing play (wear).
> The noise some how goes away when oil pressure is built up.
> Here the part I'm having trouble with...
> Aren't the crank shaft and con rods in an area of the
> motor at ambient atmospheric pressure (blow-by excluded) ?
Yes.
> I always pictured the bottom end as being lubricated by oil
> slung around, draining from the top, on it's way to the sump.
No - well, maybe a bit. It was true of many early cars with low compression
ratios and low rpms. They didn't
have a sump but had a header tank of oil which was just dripped onto the moving
parts at a steady rate and then
out onto the road (great!) - known as a 'total loss lubrication system'. This
would be totally inadequate for more
modern high compression, high speed engines where the bearings need guaranteed
forced lubrication because
of the large forces imparted by the con rods every time the air/fuel mixture is
exploded. This, after all, is where all
the engine torque is generated by converting the linear movement of the piston
into the rotation of the crank.
Potential for rapid wear here is considerable and over the lifetime of an
engine (say 100k miles) the cranks wear
oval because the forces are uneven.
> How is oil under pressure forced into these bearings / bushings ?
I think I'd be correct in saying that the crankshaft is hollow with oil holes
in each of the 7 journals (on a 4 pot) or
10 on a 6 pot engine. The oil is forced into the crank at one of the main
bearings and is then fed to the remainder.
Of course, quite a lot of the oil is then lost back to the sump but what
survives comes out of another of the main
bearings at somewhat lower pressure to feed the rest of the engine.
> Where does the oil go next, after leaving the oil pump ? Where is the oil
>pressure sensor (schematically ) ?
The order is SUMP>PRESSURE SENSOR>FILTER>CRANKSHAFT>CAMSHAFT>ROCKER SHAFT (if
there's
any left!)>SUMP and round we go again. Somewhere along the line the other
ancillaries like the distributor,
timing chain, fuel pump etc. get the odd splash to keep them happy.
> In all my years of LBC wrenching I've never been into
> these parts of a motor.
Don't you mean 'retching'? Seriously, though, you don't want to unless it's
vital.
Phil
(Heralds and 2500S)
From: Phil Willson
Electronic Engineering
Queen Mary and Westfield College
Mile End Road
London E1 4NS
Tel +44 (0)171 975 5338
Fax +44 (0)181 981 0259
email p.j.willson@qmw.ac.uk
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