Is it true that most of the engine wear during its life time occurs during a cold start up?

Question

I have heard this idea several times, but is it true?

Theories I read about why that might be true:

1. The engine is running rich at startup because it is running in open loop, which can cause damage to the piston rings as the excess fuel wipes oil from the walls

2. There is no oil at the upper parts of the engine (cylinder walls, cam shaft), so they are not lubricated enough, although in my 1993 miata the oil pressure gauge jumps up to 60 almost immediately after a cold start. Could that small gap of time cause significant wear?

3. Cold oil does not work as well in lubricating the engine, but shouldn't synthetic oils or even multi grade conventional oils solve this issue?

Answer 1

From advanced-auto-maintenance.com:

It’s widely acknowledged by automotive engineers that most engine wear occurs within this first ten minutes of driving before the engine has reached its highest, normal operating temperature.

From bloomberg.com:

When you first start the vehicle, do not race the engine. Accelerate gradually until the engine (and the rest of the drivetrain) has completely warmed up; it takes about 10-15 minutes, depending on the outside temperature. Most engine wear occurs at cold-start and during the first few minutes of operation. Revving a cold engine will greatly accelerate this wear and tear.

Basically, yes. It is true, and basically it's because the oil has settled and is no longer properly lubricating all the parts, creating a greater amount of friction and consequently, wear.

For a more official source, check out https://www.motorists.org/blog/things-we-do-to-cars/.

And Jim Kerr from the Automobile Journalists Association of Canada writes,

Most engine wear occurs in the first couple minutes after a cold start. Rich fuel mixtures wash lubrication from the cylinder walls.

Thick oil doesn’t spray onto moving parts as easily, so using a winter grade oil will help reduce engine wear.

When the engine is first started, the oil pump forces oil into the oil passages and through the oil filter.

The pleated filter element may restrict thick oil too much, so a bypass valve is designed into either the oil filter or the engine itself so the thick oil can bypass the filter.

Oil may bypass the filter for only a few seconds or for nearly a minute if temperatures are cold and the oil viscosity is high.

During this time, unfiltered oil flows to the engine, which is better than no oil, but it still allows dirt particles to flow to moving parts.

Changing to a winter grade oil with lower viscosity helps keep bypass times short and provides faster lubrication to moving parts.

And for a published resource, check out Vehicular Engine Design - Powertrain by Kevin L. Hoag, ISBN 1613-6349, p. 136.

Citations from the Society of Automobile Engineers:

http://papers.sae.org/850215/:

In this study, cold start wear tests were conducted in a cold room with temperature control ranging from +25°C to −40°C. Wear data of methanol engines, under starting conditions typical of the Canadian environment, are compared with data of a gasoline counterpart. The analysis of these data so obtained suggests that a temperature dependent theory is valid to explain the cold start wear results. Further, the cold start wear can be a significant portion of the total wear and is attributed to the direct attack of methanol on the cylinder walls in the first few seconds of engine operation.

and http://papers.sae.org/600190/:

Studies in laboratory engines equipped with radioactive piston rings show that wear is highest during a cold startup. Corrosion by condensed combustion products is responsible.Engine operating variables and additives in fuels and motor oils influence corrosion and, therefore, startup wear. Long shutdown periods, low engine temperature, and high intake-air humidity increase wear. In fuels, antirusts offer some control; for example, an amine dialkyl phosphate eliminates 40% of the wear.

NB: Special thanks to @DucatiKiller for his assistance

Answer 2

When your engine is cold, the gasoline is less likely to evaporate and create the correct ratio of air and vaporized fuel for combustion. Engines with electronic fuel injection have sensors that compensate for the cold by pumping more gasoline into the mixture. The engine continues to run rich in this way until it heats up to about 40 degrees Fahrenheit.

"That's a problem because you're actually putting extra fuel into the combustion chamber to make it burn and some of it can get onto the cylinder walls," Stephen Ciatti, a mechanical engineer who specializes in combustion engines at the Argonne National Laboratory, told Business Insider. "Gasoline is an outstanding solvent and it can actually wash oil off the walls if you run it in those cold idle conditions for an extended period of time."

Source

I agree with anonymous2's answer as well, but I am still searching for a credible source for that information.

Answer 3

The comments about fuel washing lubrication off cylinder walls are well documented. However, that isn't the only wear of substantial concern that occurs at startup. We should look at bearing wear which occurs in various places including the crankshaft, piston pins, cam bearings/followers and accessory drives. For large expensive (or both) engines, it is generally a little more costly to address bearing wear so you may want to take a look at the following piece. It shows something called the Streibeck Curve of efficiency as an engine transitions bearing lubrication from boundary lubrication (one oily surface sliding on another) to hydrodynamic lubrication where a rotating part rides on a pressure wave of its own making much like a waterskier. In steady rpm hydrodynamic regime, metal should theoretically not be touching metal. Ignoring fuel efficiency, you can intuitively see that in an engine where oil has drained out of the galleries and off the bearings, initial startup tends to see metal sliding on an incredibly thin film of oil which tends to cause more wear. Hence on startup we see more bearing wear than when in steady state hydrodynamic lubrication mode. Once the surfaces are roughened it becomes an increasingly problematic issue since the rougher surfaces beget even rougher ones. So, when an engine is cold, start very gently and don't rev it. Sure, new engines don't need to be warmed as did the old ones for fuel economy/smooth running sake, but for bearing wear purposes they do.
https://www.sciencedirect.com/science/article/pii/S0196890414001939/