Originally Posted By: edhackett
Quote:
That should really read: 90% of engine wear occurs during the first 20 minutes after a cold start.
Oil flow is a minor contributor to start-up wear.
Ed
So if it's not that oil flow is less right after start up, then what causes wear?
The largest contributor to wear when cold is the fact that the operating clearances are not to specification. Different metals expand at different rates. Until the engine is fully heat stabilized the clearances are off. The most striking example is pistons being oval. They are machined out of round so that as the various thicknesses of metal heat up the piston achieves a round shape. A cold piston can have clearances that are four to five times normal operating clearance on one axis.
Some anti-wear additives are not effective until the oil warms up.
Rich mixtures wash oil from cylinder walls. This is less of an issue with fuel injection, but is returning as a real problem with direct injected engines.
Ed
What Ed wrote is an accurate descripion of what causes start-up wear. In addtion, the top ring (compression) is the most prone to wear. Generally, the ring is coated but studies have determined that the coating is worn off by 60k miles. Hot restart does not have the same high wear rate as cold start. Ideal oil temperature range for least wear is 80C to 100C. Engine design plays a role in how much wear occurs during cold start (ie, low tension rings, thermal expansion rates, piston to cylinder bore clearances, etc). Inorganic friction modifiers are not active till around 80C. Organic FMs are not as effective at the boundary condition. Generally, starts at 60F ambient and above do not contribute significant increases in wear rate. Below 60F, wear rates increase dramatically.
Quote:
That should really read: 90% of engine wear occurs during the first 20 minutes after a cold start.
Oil flow is a minor contributor to start-up wear.
Ed
So if it's not that oil flow is less right after start up, then what causes wear?
The largest contributor to wear when cold is the fact that the operating clearances are not to specification. Different metals expand at different rates. Until the engine is fully heat stabilized the clearances are off. The most striking example is pistons being oval. They are machined out of round so that as the various thicknesses of metal heat up the piston achieves a round shape. A cold piston can have clearances that are four to five times normal operating clearance on one axis.
Some anti-wear additives are not effective until the oil warms up.
Rich mixtures wash oil from cylinder walls. This is less of an issue with fuel injection, but is returning as a real problem with direct injected engines.
Ed
What Ed wrote is an accurate descripion of what causes start-up wear. In addtion, the top ring (compression) is the most prone to wear. Generally, the ring is coated but studies have determined that the coating is worn off by 60k miles. Hot restart does not have the same high wear rate as cold start. Ideal oil temperature range for least wear is 80C to 100C. Engine design plays a role in how much wear occurs during cold start (ie, low tension rings, thermal expansion rates, piston to cylinder bore clearances, etc). Inorganic friction modifiers are not active till around 80C. Organic FMs are not as effective at the boundary condition. Generally, starts at 60F ambient and above do not contribute significant increases in wear rate. Below 60F, wear rates increase dramatically.
