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Vapor lock occurs in piston engines especially during summers.. before starting engine we prime it and the engine starts but as soon as we reduce it to near idle or just above idle, engine tends to shut down. to prevent engine from shutting down, we quickly turn on the electric fuel pump and after turning on fuel pump and increasing throttle a bit, engine doesn't shut. curious to know what's happening behind it ?

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  • $\begingroup$ What have you found in your research? $\endgroup$
    – Jim
    Oct 22 '21 at 18:18
  • $\begingroup$ You don't start with the boost pump on already? Normally you have the pump running and use the mixture to prime, then idle cutoff while cranking, boost pump still on, and go to rich as soon as it catches. $\endgroup$
    – John K
    Oct 22 '21 at 23:37
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  1. what causes vapor lock?

Fuel pumps found on GA planes are designed to pump liquid fuel. They won’t pump fuel vapor. Vapor lock occurs when fuel in the supply plumbing gets heated above the fuel’s boiling point, turning it into vapor. No bueno.

2)Turning on the electric pump may help in one or both ways:

A) increasing fuel pressure in the vapor locked area, thus condensing the vapor back into liquid, and/or

B) pumping liquid fuel around the vapor bubble, letting the engine start and flush the vapor bubble into the float chamber or the intake manifold. This assumes the fuel plumbing permits this.

-Skip

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    $\begingroup$ Important component of the vapor lock is the suction. The fuel pipe does not get heated above boiling point of the fuel at normal pressure, but the suction reduces pressure, and with it reduces the boiling point. Of course it is more likely to happen if it is warm, but if the pipe is partly clogged, it will cause vapor lock at lower temperature. $\endgroup$
    – Jan Hudec
    Oct 22 '21 at 21:23
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As the temperature of gasoline increases, so does its vapor pressure (or vapour pressure, if you prefer). If the vapor pressure is higher than the local absolute pressure in the fuel, then bubbles will start to form, like bubbles on the side of a glass of champagne. Vapor lock is what happens when enough bubbles of gasoline vapor collect in a particular location to prevent a pump that is designed to pump liquid fuel, not vapor, from delivering fuel to an engine.

The problem is exacerbated by suction pumps, which reduce the absolute pressure of the fuel. (Flow restrictions would also tend to lower the absolute pressure of the fuel locally, as @JanHudec points out.) Vapor lock problems had been common in automobiles in hot weather, until engineers switched from suction fuel pumps to compression fuel pumps, which increase the pressure of the fuel. Compression fuel pumps must be guaranteed a flow of fuel, which is why they are usually located inside the fuel tank in cars. A pump inside a fuel tank is nearly universally powered by an electric motor, because a mechanical connection would be more difficult to seal against fuel leaks.

I don't know much about general aviation engines, but it sounds as though the main pump is a suction pump, which introduces the possibility of vapor lock, and I'll guess that it's mechanically-driven by the engine for reliability reasons. I'll also guess that the electric fuel pump is a compression pump; if so, running it would increase the absolute pressure of the fuel, helping to prevent vapor lock.

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    $\begingroup$ Vapour outside the US lol. Also, Z is "zed" not "zee". Anyway, boost pumps are usually suction pumps installed upstream of the EDP, typically on the firewall. On GA airplanes that's all you really need and it avoids multiple pumps installed in fuel tanks as is done on transport airplanes. The EDP on the case gets really hot from the case itself. My plane (a homebuilt) runs on Mogas and I have had vapour lock problems in very hot weather (mostly pressure fluctuations). I installed a cooling shroud on the EDP fed with cold air from the cowl inlet and my VL problems have largely gone away. $\endgroup$
    – John K
    Oct 23 '21 at 4:42

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