Explain why prop oil B4 startup?

Re: Explain why prop oil B4 startup?

by MARK JACKSON » 14 years ago

MARK JACKSON

12 332 11

Hi Al

If the oil in the oil tank is above the level of the bottom of the cylinder, then any oil that syphons back into the engine while it is not running can fill the crankcase and then seep past the piston rings and settle in the cylinder between the piston and cylinder head.

If you try to start the engine with oil in this space you could cause a lot of damage as oil , unlike air, does not compress.

It is important that if you have a high mounted oil tank, you make sure the engine will turn over before trying to start it, and if there is a lot of oil in the crankcase, then this is pumped, by blow-by pressure, back to the oil tank.

On a completely separate issue, if you turn the prop backwards, there is a chance, because you are turning the oil pump backwards as well, that you could draw air from the crankcase into the oil system, and because air is compressible and oil isn't, the hydraulic tappets can collapse and not work properly.

I hope this helps explain it.

Cheers Mark

Re: Explain why prop oil B4 startup?

by bill martin » 14 years ago

bill martin

10 757 0

Al,

Mark is right. Next time you have a spark plug out, pull that piston to top dead center and look how little room you have in there. A few spoonfuls of oil would do it. If oil in there stops the piston from reaching TDC, all sorts of nasty and expensive things can happen like bent rods, bent cranks, etc.. Remember the WW2 documentaries where a gang of guys are pulling the props through on the bomber radial engines before starting? Thats to clear the bottom cylinders of oil before starting. Bigger problem with radials than with ours, but the same principle applies.

Bill.

Re: Explain why prop oil B4 startup?

by Al C » 14 years ago

Al C

63 138 106

Yup,
That helps explain it.
I understood oil doesn't compress.
I just didn't understand WHERE the "missing" oil might be pooling where it could cause mischief.
Now that I know that oil could be sitting in a cylinder all is enlightenment :) .

1200 cc's divided by 4 is 300 cc per cyclinder. At compression ratio of 1.5 to 1 at TDC there's only about 29 cc's space in that cylinder.

Thanks,
Al

Re: Explain why prop oil B4 startup?

by James Mason » 14 years ago

James Mason

11 14 0

Having just read this thread it would seem to me to suggest that a few turns of the prop would check that oil is not in any of the cylinders but a full burp would not be required.

Re: Explain why prop oil B4 startup?

by bill martin » 14 years ago

bill martin

10 757 0

James,

You should always pull the prop until it burps. It accomplishes two things. Hydraulic lock in a rotax is a very rare occurence but the results are catastrophic if the engine is started with oil in the cylinder. If your oil reservoir is located within the standard height limits rotax recommends, you will probably never have that problem. Burping is just one of those safety measures that can save you big bucks. Secondly, by burping, all of the oil is in the reservoir every time you check the oil level and you know exactly how much oil you have in the system.

Bill.