Vapor lock

Vapor lock (also known as vapour lock) is a problem that mostly affects gasoline-fueled internal combustion engines. It occurs when the liquid fuel changes state from liquid to gas while still in the fuel delivery system. This disrupts the operation of the fuel pump, causing loss of feed pressure to the carburetor or fuel injection system, resulting in transient loss of power or complete stalling. Restarting the engine from this state may be difficult. The fuel can vaporise due to being heated by the engine, by the local climate or due to a lower boiling point at high altitude. In regions where higher volatility fuels are used during the winter to improve the starting of the engine, the use of "winter" fuels during the summer can cause vapor lock to occur more readily.

Causes and incidence

Vapor lock was far more common in older gasoline fuel systems incorporating a low-pressure mechanical fuel pump driven by the engine, located in the engine compartment and feeding a carburetor. Such pumps were typically located higher than the fuel tank, were directly heated by the engine and fed fuel directly to the float bowl inside the carburetor. Fuel was drawn under negative pressure from the feed line, increasing the risk of a vapor lock developing between the tank and pump. A vapor lock being drawn into the fuel pump could disrupt the fuel pressure long enough for the float chamber in the carburetor to partially or completely drain, causing fuel starvation in the engine. Even temporary disruption of fuel supply into the float chamber is not ideal; most carburetors are designed to run at a fixed level of fuel in the float bowl and reducing the level will reduce the air:fuel mixture delivered.

Carburetor units may not effectively deal with fuel vapor being delivered to the float chamber. Most designs incorporate a pressure balance duct linking the top of the float bowl with either the intake to the carburetor or the outside air. Even if the pump can handle vapor locks effectively, fuel vapor entering the float bowl has to be vented. If this is done via the intake system, the mixture is, in-effect, enriched, creating a mixture control and pollution issue. If it is done by venting to the outside, the result is direct hydrocarbon pollution and an effective loss of fuel efficiency and possibly a fuel odor problem. For this reason, some fuel delivery systems allow fuel vapor to be returned to the fuel tank to be condensed back to the liquid phase. This is usually implemented by removing fuel vapor from the fuel line near the engine rather than from the float bowl. Such a system may also divert excess fuel pressure from the pump back to the tank.

Most modern engines are equipped with fuel injection, and have a high pressure electric fuel pump in the fuel tank. Moving the fuel pump to the interior of the tank helps prevent vapor lock, since the entire fuel delivery system is under high pressure and the fuel pump runs cooler than if it is located in the engine compartment. This is the primary reason that vapor lock is rare in modern fuel systems. For the same reason, some carbureted engines are retrofitted with an electric fuel pump near the fuel tank.

Other solutions to vapor lock are rerouting of the fuel lines away from heat generating components, installation of a fuel cooler or cool can, shielding of heat generating components near fuel lines, and insulation of fuel lines.

A vapor lock is more likely to develop when the vehicle is in traffic because the under-hood temperature tends to rise. A vapor lock can also develop when the engine is stopped while hot and the vehicle is parked for a short period. The fuel in the line near the engine does not move and can thus heats up sufficiently to form a vapor lock. The problem is more likely in hot weather or high altitude in either case.

Do not assume that a gravity feed fuel system is immune to vapor lock simply because there is no fuel pump to "upset.” Much of the foregoing applies equally to a gravity feed system; if vapor forms in the fuel line, its lower density reduces the pressure developed by the weight of the fuel. This pressure is what normally moves fuel from the tank to the carburetor, so fuel supply will be disrupted until the vapor is removed, either by the remaining fuel pressure forcing it into the float bowl and out the vent or by allowing the vapor to cool and recondense.

Vapor lock has been the cause of many a forced landing in aircraft. That is why aviation fuel (AVGAS) is manufactured to far lower vapor pressure than automotive gasoline (petrol). In addition aircraft are far more susceptible because of their ability to change altitude rapidly. Liquids boil at lower temperatures when in lower pressure environments.

Incidence with other fuels

The higher the volatility of the fuel, the more likely it is that vapor lock will occur. Historically, gasoline was a more volatile distillate than it is now and was more prone to vapor lock. Conversely, diesel fuel is far less volatile than gasoline and thus these engines hardly ever suffer from vapor lock. However, diesel engine fuel systems are far more susceptible to air locks in their fuel lines as standard diesel fuel injection pumps rely on the fuel being non-compressible. Air locks are caused by air leaking into the fuel delivery line or entering from the tank rather than the fuel evaporating in the system. Eliminating such air locks requires an extended period of turning over the engine using the starter motor or manually bleeding the system. One of the avenues that is available to diesels is a fuel system that filters the fuel from air/vapor before reaching the engine. These systems have been highly effective giving way to increased horsepower, fuel economy, torque and lowers the emissions output.

Cultural References

*In The Simpsons episode "Bart Star" while Bart is upset about his poor quarterback skills, Joe Namath's car breaks down nearby; Namath's wife reveals that the vehicle was merely immobilized due to vapor lock and he leaves without helping Bart.
*In the Bewitched episode "Okay, Who’s the Wise Witch?", Samantha and Darrin get sealed inside their house because of a vapor lock created by the lack of magic being done in the house, and cannot leave until a spell has been performed from outside the house to release the lock.