Vapour lock
Encyclopedia
Vapor lock is a problem that mostly affects gasoline-fueled internal combustion engine
s.
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 vaporize 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.
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 fuel to air mixture delivered to the engine.
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, or using an active carbon filled canister where fuel vapor is absorbed. 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 an electric submersible
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 positive 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.
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 heat up sufficiently to form a vapor lock. The problem is more likely in hot weather or high altitude in either case.
Gravity feed fuel systems are not immune to vapor lock. 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 re-condense.
Vapor lock has been the cause of forced landings in aircraft. That is why aviation fuel
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 and associated ambient pressure rapidly. Liquids boil at lower temperatures when in lower pressure environments.
, since the machines still utilize gasoline and carburetors. A car which runs out of fuel on the track often experiences difficulty restarting once it has been refueled in the pits
. Crew members usually attempt to restart the car with the aid of ether
and push starting
. If a driver is aware he is running out of fuel on the track, to help avoid vapor lock, he may turn off the engine (before it stalls) and coast to the pit area for refueling. This allows for a small amount of fuel to remain in the tank and fuel lines to make refiring easier.
Vapor lock is less common in other forms of motorsports such as Formula One
and IndyCar
, due to the use of fuel injection and alcohol fuels (ethanol
or methanol
), which have a lower vapor pressure (less evaporation at a given temperature). See Reid Vapor Pressure
.
Modern diesel injection systems that feature electric FI pumps are able to self-bleed, thus eliminating the "air lock" issue.
Internal combustion engine
The internal combustion engine is an engine in which the combustion of a fuel occurs with an oxidizer in a combustion chamber. In an internal combustion engine, the expansion of the high-temperature and high -pressure gases produced by combustion apply direct force to some component of the engine...
s.
It occurs when the liquid
Liquid
Liquid is one of the three classical states of matter . Like a gas, a liquid is able to flow and take the shape of a container. Some liquids resist compression, while others can be compressed. Unlike a gas, a liquid does not disperse to fill every space of a container, and maintains a fairly...
fuel
Fuel
Fuel is any material that stores energy that can later be extracted to perform mechanical work in a controlled manner. Most fuels used by humans undergo combustion, a redox reaction in which a combustible substance releases energy after it ignites and reacts with the oxygen in the air...
changes state from liquid to gas
Gas
Gas is one of the three classical states of matter . Near absolute zero, a substance exists as a solid. As heat is added to this substance it melts into a liquid at its melting point , boils into a gas at its boiling point, and if heated high enough would enter a plasma state in which the electrons...
while still in the fuel delivery system. This disrupts the operation of the fuel pump
Fuel pump
A fuel pump is a frequently essential component on a car or other internal combustion engined device. Many engines do not require any fuel pump at all, requiring only gravity to feed fuel from the fuel tank through a line or hose to the engine...
, causing loss of feed pressure to the carburetor
Carburetor
A carburetor , carburettor, or carburetter is a device that blends air and fuel for an internal combustion engine. It is sometimes shortened to carb in North America and the United Kingdom....
or fuel injection
Fuel injection
Fuel injection is a system for admitting fuel into an internal combustion engine. It has become the primary fuel delivery system used in automotive petrol engines, having almost completely replaced carburetors in the late 1980s....
system, resulting in transient loss of power or complete stalling
Stall (engine)
A stall is the slowing or stopping of a process, and in the case of an engine, refers to a sudden stopping of the engine turning, usually brought about accidentally....
. Restarting the engine from this state may be difficult.
The fuel can vaporize due to being heated by the engine, by the local climate or due to a lower boiling point
Boiling point
The boiling point of an element or a substance is the temperature at which the vapor pressure of the liquid equals the environmental pressure surrounding the liquid....
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 pumpFuel pump
A fuel pump is a frequently essential component on a car or other internal combustion engined device. Many engines do not require any fuel pump at all, requiring only gravity to feed fuel from the fuel tank through a line or hose to the engine...
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 fuel to air mixture delivered to the engine.
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, or using an active carbon filled canister where fuel vapor is absorbed. 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
Fuel injection
Fuel injection is a system for admitting fuel into an internal combustion engine. It has become the primary fuel delivery system used in automotive petrol engines, having almost completely replaced carburetors in the late 1980s....
, and have an electric submersible
Submersible pump
A submersible pump is a device which has a hermetically sealed motor close-coupled to the pump body. The whole assembly is submerged in the fluid to be pumped. The main advantage of this type of pump is that it prevents pump cavitation, a problem associated with a high elevation difference between...
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 positive 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.
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 heat up sufficiently to form a vapor lock. The problem is more likely in hot weather or high altitude in either case.
Gravity feed fuel systems are not immune to vapor lock. 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 re-condense.
Vapor lock has been the cause of forced landings in aircraft. That is why aviation fuel
Aviation fuel
Aviation fuel is a specialized type of petroleum-based fuel used to power aircraft. It is generally of a higher quality than fuels used in less critical applications, such as heating or road transport, and often contains additives to reduce the risk of icing or explosion due to high temperatures,...
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 and associated ambient pressure rapidly. Liquids boil at lower temperatures when in lower pressure environments.
Motorsports
Vapor lock is common in NASCARNASCAR
The National Association for Stock Car Auto Racing is a family-owned and -operated business venture that sanctions and governs multiple auto racing sports events. It was founded by Bill France Sr. in 1947–48. As of 2009, the CEO for the company is Brian France, grandson of the late Bill France Sr...
, since the machines still utilize gasoline and carburetors. A car which runs out of fuel on the track often experiences difficulty restarting once it has been refueled in the pits
Pit stop
In motorsports, a pit stop is where a racing vehicle stops in the pits during a race for refuelling, new tires, repairs, mechanical adjustments, a driver change, or any combination of the above...
. Crew members usually attempt to restart the car with the aid of ether
Ether
Ethers are a class of organic compounds that contain an ether group — an oxygen atom connected to two alkyl or aryl groups — of general formula R–O–R'. A typical example is the solvent and anesthetic diethyl ether, commonly referred to simply as "ether"...
and push starting
Push start
Push starting, also known as bump starting, pop starting or crash starting is a method of starting a motor vehicle with an internal combustion engine by engaging the manual transmission through the motion of the vehicle...
. If a driver is aware he is running out of fuel on the track, to help avoid vapor lock, he may turn off the engine (before it stalls) and coast to the pit area for refueling. This allows for a small amount of fuel to remain in the tank and fuel lines to make refiring easier.
Vapor lock is less common in other forms of motorsports such as Formula One
Formula One
Formula One, also known as Formula 1 or F1 and referred to officially as the FIA Formula One World Championship, is the highest class of single seater auto racing sanctioned by the Fédération Internationale de l'Automobile . The "formula" designation in the name refers to a set of rules with which...
and IndyCar
IndyCar Series
The IZOD IndyCar Series is the premier level of American open wheel racing. The current championship, founded by Indianapolis Motor Speedway owner Tony George, began in 1996 as a competitor to CART known as the Indy Racing League . Citing CART's increasing reliance on expensive machinery and...
, due to the use of fuel injection and alcohol fuels (ethanol
Ethanol
Ethanol, also called ethyl alcohol, pure alcohol, grain alcohol, or drinking alcohol, is a volatile, flammable, colorless liquid. It is a psychoactive drug and one of the oldest recreational drugs. Best known as the type of alcohol found in alcoholic beverages, it is also used in thermometers, as a...
or methanol
Methanol
Methanol, also known as methyl alcohol, wood alcohol, wood naphtha or wood spirits, is a chemical with the formula CH3OH . It is the simplest alcohol, and is a light, volatile, colorless, flammable liquid with a distinctive odor very similar to, but slightly sweeter than, ethanol...
), which have a lower vapor pressure (less evaporation at a given temperature). See Reid Vapor Pressure
Reid Vapor Pressure
Reid vapor pressure is a common measure of the volatility of gasoline. It is defined as theabsolute vapor pressure exerted by a liquid at 100 °F as determined by the test method ASTM-D-323...
.
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.Modern diesel injection systems that feature electric FI pumps are able to self-bleed, thus eliminating the "air lock" issue.