I see in my C180 owners manual that an "oil dilution" system was offered as an option for cold weather starts. From what I can tell, it looks like you could pump a couple of quarts of fuel in to the oil sump prior to starting the engine up. I imagine that the fuel boils off when the oil gets to temperature.

Anyone know anything about these systems? Is this method helpful for cold starts? For what reason(s) is this practice not implemented regularly in cold weather ops? Or is it? Seems easy enough to dump some fuel in to the oil after shut down if it allows the oil to flow when starting in cold temperatures.

This was a common practice up untill the early 1960's, I haven't heard of it being used recently. The procedure was to add gasoline to the oil sump and let the engine idle for a minute to mix it in before shutdown, and it would boil off as the engine warmed up durring the next flight. For a C172 they added 1 quart, a twin Beach added 1 gallon to each engine. Thanks to heated hangers, engine preheaters, and muti-weight oil it is history.

Dale Moul wrote:This was a common practice up untill the early 1960's, I haven't heard of it being used recently. The procedure was to add gasoline to the oil sump and let the engine idle for a minute to mix it in before shutdown, and it would boil off as the engine warmed up durring the next flight. For a C172 they added 1 quart, a twin Beach added 1 gallon to each engine. Thanks to heated hangers, engine preheaters, and muti-weight oil it is history.

Good Info Dale Moul. Do you know if there would be any any ill effects resulting from diluting oil in a pinch? Was it abandoned because of problems with the method, or advances in technology? Would a few diluted starts be worse than, say a few 20 degree F starts?

Scolopax wrote:Good Info Dale Moul. Do you know if there would be any any ill effects resulting from diluting oil in a pinch? Was it abandoned because of problems with the method, or advances in technology? Would a few diluted starts be worse than, say a few 20 degree F starts?

Multi-weight oil was a big part of it I'm sure, but I bet the other was an awareness that oil viscosity isn't the biggest part of the problem in cold starts. It's the differing contraction rates of the metals (steel v aluminium) so the colder it gets the tighter the gap between crankshaft and bearings gets.

The counter intuitive part of this is that it means an older engine actually suffers less damage during a cold start than a new one because the older one has looser tolerances than the freshly overhauled one.

this was also very common with the radial engines. they had a very small sump and an oil reserve tank so it took only a little gas to thin the oil.
I imagine there may have been a fire or two doing this too. Adding gas to a hot engine before shutdown might get sort of dicey.

Multi vis oils negate the need for it as well.

As mentioned the biggest problem is uneven expansion of metals, I think one of the best things a person can do with a cold start is to fire the engine, let it run for a few minutes and then shut it off. Let the engine sit for a little while until the heat from the cylinders soaks into the other components and brings everything up to a similar temperature then fire back up and take off. This will give the slower expanding parts a better chance to catch up to the fast ones.

shorton wrote:As mentioned the biggest problem is uneven expansion of metals, I think one of the best things a person can do with a cold start is to fire the engine, let it run for a few minutes and then shut it off. Let the engine sit for a little while until the heat from the cylinders soaks into the other components and brings everything up to a similar temperature then fire back up and take off. This will give the slower expanding parts a better chance to catch up to the fast ones.

I was taught that you wait at idle until the oil is 100F before you take off. That acts as a good proxy for heat being distributed throughout the engine.

Oil dilution was the best option available 50+ years ago, thanks to better refining and chemistry, (15W50 oil), it is now obsolete and there is no good reason to bring it back. For cold weather starts 15w50 will lubricate a lot better than single weight oil that hss been diluted to go thru the engine easier. If you still have the system in a plane remove it to save a few pounds, them mount the parts on a hanger wall and let your friends try to guess what it is.

I flew a Beaver for a number of years. Our maintenance folks insisted on running straight 50 weight oil. I didn't have electricity at the tidown, and didn't have electric heat anyway. I diluted on occasion, and hated doing it every time.

In this day and age of multi viscosity oils and great pre heat tools, why would ANYone use oil dilute? Bear in mind that oil dilution is something you have to plan in advance.....once that engine is cold soaked, you can't dilute. Why not do a little better job of planning, and remove that extra weight?

MTV