engguy wrote:
sdennison
What you say makes some sense.
But the reason for turning the prop in the normal direction, is if there is a lock developing it can be felt. And then spark plugs removed to drain the oil. If turned backwards, things become a bit reversed. Meaning the exhaust then acts like the intake and vise versa. So if you turn it backwards what ever is in the cylinder at the time, will be forced into the intake and the same thing at the compression postition, if there is oil it would again lock since both valves are closed in that position. So nothing special is gained going backwards. Except for the fact that the intake tubes would get the oil forced into them. Then like the others say, if it is in the tubes, then as soon as it is spun the correct direction it sucks the oil back in, but this time there may be some inertia behind the turning effort, when the lock is encountered.
OK, so let me try this again. The four strokes are intake, compression, power, and exhaust. In the forward direction, the piston travels away from the head sucking in the intake charge. Next the compression stroke where the piston travels toward the head with both valves closed. Third is the power stroke with the piston traveling away from the head and finally, the exhaust stroke with the piston traveling toward the head forcing out the exhaust gases.
Now if you pull the engine through in either direction while in the compression stroke, you will have incompressable lock. This is when you stop and pull the plugs and drain the oil. However, with the exhaust and intake strokes being side by side and with oil possibly in the intake tubes, if you pull forward, the oil is sucked in and then you hit lock on the compression stroke. If you are pulling backward rotation, the piston is traveling away from the head sucking the oil in from the intake tubes with the next stroke now actually the exhaust, the piston travels toward the head and forces the oil into the exhaust manifold.
The next stroke is the power stroke, again a piston traveling away from the head (no lock issues) followed by again the compression stroke (now with a clear cylinder, no lock) followed by another intake stroke that could again draw any remaining oil into the cylinder from the intake tubes followed by the exhaust stroke forcing any additional oil into the exhaust manifold (no lock).
I'll grant you that if any cylinder is with oil on the compression stroke, you will have lock regardless of the direction you approach TDC, other than that, it would appear that reverse pull through is more advantageous than forward.
What am I missing here?