Vapor Lock: Carburetor vs Fuel Injection

[121Dispatch]

I'm having a bit of a tough time coming up with a clear explanation of this, which tells me I don't fully understand it myself.

On the surface, it seems vapor lock would be less of a problem with a fuel injected engine since the fuel is pumped under pressure into the cylinders. However, I keep reading about vapor lock being MORE of an issue in fuel injected aircraft (having only flown old-school carbureted engines I have no first-hand experience).

Is the vapor lock caused because the engine gets so hot the fuel in the fuel lines near the nozzles vaporizes when it is shut down, and then only pumps fuel vapor into the cylinder, making a difficult hot-start? Is this a problem with an aux fuel pump as well, or only an aircraft with an engine driven pump (shouldn't the plan have both pumps though?)

In contrast, the carbureted engine sends fuel from a "pool" through a discharge nozzle as a result of low pressure above.

In a nutshell, does this answer the question of why a fuel injected engine is more likely to encounter vapor lock than a carbureted engine? I must say, I've had my share of hard times hot-starting a carbureted 172!

 

[Skip Miller]

Vapor Lock most frequently occurs when the engine is shut down. The problems we encounter is when we try to restart the hot engine.

Heat rises. Most carburetors are on the bottom of the engine = cooler location while on the ground. In addition, the float bowl contains enough fuel that it is not likely that all of the float bowl will be vaporized and escape, so there is enough fuel available for the engine to be started, where the combination of the electric pump (if installed) and the mechanical pump can draw cool fuel from the tank and "flush out" any vapor. Having the carb in warm air in flight helps prevent carb ice.

In most injected engines, the fuel system is on top of the engine. This is better while flying, as the cool air passes from the top of the engine down through the cylinder fins (heating it up). So in flight, the air on top of the engine is cooler than that below. However, having the injection plumbing on top of the engine is worse when you shut down and heat rises. The problem is vapor in the injection pump and fuel distribution spider. The pump can't pump vapor = no start.

As you probably know, one solution to this is to intentionally flood the engine with the primer pump, and then use a flooded start procedure. Flooding the engine both gets fuel where it needs to be, and cools the injector pump and spider.

-Skip

 

[121Dispatch]

Thanks Skip. Excellent explanation!

 

[CJones]

As you probably know, one solution to this is to intentionally flood the engine with the primer pump, and then use a flooded start procedure. Flooding the engine both gets fuel where it needs to be, and cools the injector pump and spider.

-Skip

Common practice of cropdusters flying those big IO-720's on hot summer days. It's easier to just go ahead and flood it and start from a 'known situation' rather than *hope* to get enough fuel in it, then crank crank crank, and THEN flood it while cranking.

 

[2256Q]

Vapor Lock most frequently occurs when the engine is shut down. The problems we encounter is when we try to restart the hot engine.

Heat rises. Most carburetors are on the bottom of the engine = cooler location while on the ground. In addition, the float bowl contains enough fuel that it is not likely that all of the float bowl will be vaporized and escape, so there is enough fuel available for the engine to be started, where the combination of the electric pump (if installed) and the mechanical pump can draw cool fuel from the tank and "flush out" any vapor. Having the carb in warm air in flight helps prevent carb ice.

In most injected engines, the fuel system is on top of the engine. This is better while flying, as the cool air passes from the top of the engine down through the cylinder fins (heating it up). So in flight, the air on top of the engine is cooler than that below. However, having the injection plumbing on top of the engine is worse when you shut down and heat rises. The problem is vapor in the injection pump and fuel distribution spider. The pump can't pump vapor = no start.

As you probably know, one solution to this is to intentionally flood the engine with the primer pump, and then use a flooded start procedure. Flooding the engine both gets fuel where it needs to be, and cools the injector pump and spider.

-Skip

Wonder if it couldn't have a lot to do with the engine design, as well. I've got several thousand hours flying the GTSIO-520s in 400 series Cessnas and never had a vapor lock worth mentioning. I just use the normal hot-start procedure and if the engine hasn't caught within 4 or 5 blades I bump the prime switch for half a second and we're off.

 

[Dan Thomas]

I'm having a bit of a tough time coming up with a clear explanation of this, which tells me I don't fully understand it myself.

On the surface, it seems vapor lock would be less of a problem with a fuel injected engine since the fuel is pumped under pressure into the cylinders. However, I keep reading about vapor lock being MORE of an issue in fuel injected aircraft (having only flown old-school carbureted engines I have no first-hand experience).

Is the vapor lock caused because the engine gets so hot the fuel in the fuel lines near the nozzles vaporizes when it is shut down, and then only pumps fuel vapor into the cylinder, making a difficult hot-start? Is this a problem with an aux fuel pump as well, or only an aircraft with an engine driven pump (shouldn't the plan have both pumps though?)

In contrast, the carbureted engine sends fuel from a "pool" through a discharge nozzle as a result of low pressure above.

In a nutshell, does this answer the question of why a fuel injected engine is more likely to encounter vapor lock than a carbureted engine? I must say, I've had my share of hard times hot-starting a carbureted 172!

True vapor lock is a result of warm fuel under low pressure. A low-wing airplane is most likely to suffer vapor lock with the boost pump turned off so that the engine-driven pump pulls the fuel up from the tanks, lowering the pressure on the fuel in the line and thereby lowering its boiling point. The fuel, if warm enough, will boil in the lines and nothing but vapor reaches the engine. It won't matter whether the engine is carbureted or injected; neither will run on vapors. A boost pump pushes the fuel up from the tanks, so there's more pressure on it and it doesn't boil so easily. High-wing airplanes, with gravity flow at work, don't get vapor lock easily either.

Dan
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R1100S