The Hot Manifold

Oct 30, 2021

It's time for the new puzzler. This is a puzzle of old.

I remember, some years ago, I was working on something like a General Motors car--maybe like a Dodge--and I was changing the customer's gas filter. And because our sign was out being dry cleaned where it says customers are not allowed in the area where we fix the cars, he was standing looking over my shoulder making sure that I did the right thing. So I was changing his gas filter and as I did that gas spilled onto the exhaust manifold.

And he ran for the fire extinguisher, came back, and pointed it right at me. And he said, "You just spilled gasoline on the red hot exhaust manifold! Aren't you afraid it's going to catch fire?" And I explained, "No, I'm actually not afraid of that!"

And he said, "Well, I'll tell you an interesting story, young man! I spilled motor oil once on the manifold of this very car. And it burst into flame." And I said, "Yeah, that's true that could possibly happen."

And the question is, why is it that he could spill motor oil on the manifold? Let's assume the manifold is the same temperature as it was the time I spilled the gas onto it. And when he spilled motor oil on it, it burst into flames. Yet when I spilled gasoline, which everyone knows is quite flammable,  the gasoline did not catch fire?



The reason that oil catches fire and the gasoline doesn't is interesting. They both have what's called an ignition point (that is the the temperature at which they both will burst into flame) and it's like 752.73 degrees. I looked it up! And while I don't remember the exact temperature, the ignition point for both is relatively close.

The reason the gasoline doesn't burst in the flame is that it doesn't hang around long enough. The oil sits there and bubbles and gets hotter and hotter and hotter until it finally reaches its ignition point and bursts into flame. Gasoline, however, being so volatile, evaporates. So it hits, it evaporates and disappears because it has a very high flash point. So it's not there to burn.


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