Gas on Engine

Aug 09, 1996

RAY: Some years ago, I was working on something like a General Motors car, maybe like a Dodge Dart, and I was changing the customer's gas filter. Because our sign that said customers are not allowed to be in the area where we fix the cars was out being dry-cleaned, he was standing looking over my shoulder, making sure that I did the right thing. I was changing his gas filter, and as I did that gas spilled on the exhaust manifold.

TOM: And he leapt back in fear!

RAY: Actually what he did was run for the fire extinguisher.

TOM: And you said, "Cool out, man."

RAY: And he said, "You just spilled gasoline on the red-hot exhaust manifold. Aren't you afraid that it is going to catch fire?" I extinguished my cigarette and explained to him, "No, actually, I'm not afraid."

He said, "Well, I'll tell you an interesting story, young man." He said, "I spilled motor oil once on the manifold of this very car and it burst into flames."

I said, "That could possibly happen."

The question is, why is it that Ray could spill gasoline on the manifold without fear of a fire?


RAY: Let's assume that the manifold is the same temperature.

TOM: As what? The surface of the sun?

RAY: No. The motor oil burst into flames, yet I spilled gasoline, which everyone knows is quite flammable. Why is it that the gasoline does not catch fire, and the motor oil does?

TOM: Nuts!

RAY: Now we could have generalized this puzzler. Say you have a hot exhaust manifold, like 800 degrees, and you spill gasoline on one part of the manifold and you spill oil on another part of the manifold.

TOM: Right. And the oil bursts into flames and spreads across, and the gasoline catches fire and the whole car blows up.

RAY: But the reason the oil catches on fire and the gasoline doesn't is interesting. They both have what's called the same ignition point. That's the temperature at which they both will burst into flame; it's something like 752.73 degrees.

TOM: You looked it up?

RAY: Well, no, but it's something like that. They are both relatively close, which is surprising. The reason that the gasoline doesn't burst into flames is that it doesn't hang around long enough. The oil sits there and bubbles, getting hotter and hotter and hotter until it finally reaches what? The ignition point. And it bursts into flames. The gasoline, however, being so volatile, evaporates, so as it hits, the hot manifold it evaporates, because it has a very high flash point. So it's not there to burn.

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