Fuel Tank Pressure: Why It's Necessary And How It Works

Fuel tanks are pressurised to maintain a constant flow of fuel to the engine, ensuring smooth and efficient engine performance. This pressure prevents the fuel from vaporising, which can cause engine malfunctions, and is necessary for high-altitude flights to compensate for decreased atmospheric pressure. Pressurisation is achieved through pumps, valves and regulators, and the pressure required depends on the type of fuel and engine. For example, carbureted engines need around 4-7 psi, while modern fuel-injected engines may require 60 psi or more. However, pressurisation is not always necessary, and some vehicles, like Formula 1 cars, do not use it.

Pressure helps control emissions by preventing vapours from being released

Pressure is an important factor in controlling emissions from fuel tanks. By maintaining pressure inside the tank, fuel vapours can be prevented from being released into the atmosphere, thereby reducing emissions. This is particularly relevant for storage tanks containing crude oil and gas condensate, which typically operate at or near atmospheric pressure.

When introduced into a storage tank, crude oil and condensate experience a pressure drop, causing gases dissolved in the liquid to vaporize and collect in the vapor space between the liquid and the roof of the tank. These vapours, known as "flashing losses", account for most emissions from storage tanks. To prevent this, pressurized storage tanks can be used, which significantly reduce emissions of methane and other volatile organic compounds (VOCs).

In the context of automotive fuel tanks, pressure plays a similar role in controlling emissions. Gasoline is composed of various hydrocarbons, which vaporize when running, creating pressure in the tank. This pressure can be managed through the use of an Evaporative Emission Control (EVAP) system, which traps fuel vapours in a vapor canister, preventing their release into the atmosphere. This system is designed to prevent fuel vapours from escaping and contributing to air pollution.

Additionally, the pressure within a fuel tank can be regulated through the use of a Fuel Tank Pressure (FTP) sensor. This sensor measures the air pressure in the fuel tank, allowing the Engine Control Module or Powertrain Control Module (ECM/PCM) to test for leaks in the EVAP system. By sealing the fuel tank and EVAP system, the ECM/PCM can detect changes in pressure over time, indicating the presence of an air leak. This technology helps ensure that fuel vapours remain contained within the system, preventing their release into the atmosphere.

Overall, pressure plays a crucial role in controlling emissions from fuel tanks by preventing the release of fuel vapours. This is achieved through the use of pressurized storage tanks, EVAP systems, and FTP sensors, which work together to reduce emissions and mitigate their environmental impact.

It is normal to have pressure on the gas tank

In a typical fuel system, the fuel tank is sealed to contain the fuel vapours and prevent their release into the atmosphere. This sealed system is crucial for both safety and environmental reasons, as it prevents the escape of volatile organic compounds that contribute to air pollution and potential health hazards. By containing the vapours, the system also mitigates the risk of explosions or fires caused by the ignition of vapours outside the tank.

The pressure in the gas tank is carefully regulated to maintain a balance between the vapour pressure and the atmospheric pressure. This regulation is achieved through the use of a vapour recovery system, which includes components such as carbon canisters and vapour recovery valves. These components work together to adsorb and store the fuel vapours, ensuring that the pressure in the tank remains within safe limits.

Additionally, the fuel pump plays a crucial role in managing the pressure within the gas tank. The pump is designed to generate sufficient pressure to overcome the vapour pressure and ensure the smooth flow of fuel from the tank to the engine. This prevents issues such as fuel starvation, which can occur when the engine does not receive enough fuel to sustain combustion, resulting in engine stalling or poor performance.

Overall, the presence of pressure in a gas tank is not only normal but also essential for the proper functioning of a vehicle's fuel system. This pressure is carefully managed through various components and systems to ensure the safe and efficient delivery of fuel while also addressing environmental and safety concerns associated with fuel vapours. Understanding the importance of pressure in the gas tank can help vehicle owners and technicians effectively maintain the fuel system, contributing to the overall reliability and performance of the vehicle.

High pressure can lead to dangerous conditions

High pressure in a fuel tank can lead to dangerous conditions. Here are some of the potential issues:

Firstly, high pressure can cause the fuel tank to bulge, which can lead to fuel line leaks and fuel pump problems. This can result in a lack of fuel being delivered to the engine for combustion, leading to engine performance issues such as rough idling or even stalling.

Secondly, the increased pressure can create hazardous situations when opening the fuel tank filler cap, as the built-up pressure may be released suddenly. This can also lead to fire hazards, as the vapors escaping from the tank may ignite.

Additionally, high pressure in the fuel tank can be caused by a failure in the evaporative emission control (EVAP) system, which is responsible for stabilizing the pressure. If the EVAP system becomes clogged, restricted, or fails altogether, it can lead to a dangerous buildup of pressure in the tank.

Furthermore, high pressure can affect the fuel injectors, especially if there is debris in the fuel. The high pressure can push the debris through the injectors, causing damage and potentially leading to fuel leaks.

It is important to note that while some pressure is normal in a fuel tank due to the vaporization of gasoline during vehicle movement, excessive pressure can lead to these dangerous conditions. Therefore, it is crucial to maintain the fuel tank and EVAP system properly to prevent such issues.

A pressure test can be performed on a fuel tank

Step 1: Gather the Required Tools and Materials

• Hose pinch pliers
• An inflatable pipe plug
• A low-pressure air pressure gauge that can be tee-ed into a 3/8" fuel hose with a Schrader valve
• An air pump or a tank of compressed air

Step 2: Gain Access to the Fuel Tank

Locate the fuel tank and its components, including the fuel fill hose, vent hose access ports, and fuel pick-up access. Most boats have access ports that allow access to these sections. However, for older boats, you may need to remove certain parts, such as a leaning post with a livewell, to reach the access port.

Step 3: Install the Inflatable Pipe Plug

Insert the inflatable pipe plug into the fuel fill port and pump it up to shut off the fuel fill hose. Ensure that the plug is seated properly and there are no spots where air can leak out of the hose.

Step 4: Pinch Off the Fuel Tank Vent Hose

Use the hose pinch pliers to pinch off the vent hose, making sure to squeeze tightly so that no air can escape. Be careful not to damage the hose, especially if it is old and beginning to dry rot.

Step 5: Remove the Fuel Hose from the Pickup Tube

Slice a small section of the fuel hose connected to the pickup tube to easily remove it. This step is important as, over time, the fuel hose can shrink and stick to the hose barb, making it difficult to remove.

Step 6: Install the Low-Pressure Air Pressure Gauge

Attach the low-pressure air pressure gauge and hose to the pickup tube. Ensure that it is securely connected to the hose barb.

Step 7: Pump Air into the Fuel Tank

Start pumping air into the tank but do not exceed 3 lbs of pressure. Depending on the amount of fuel in the tank, you may need to apply more or less air to pressurize it. Work your way up to 2 psi and hold it there to check for any pressure loss.

Step 8: Locate Any Leaks

If the pressure drops, locate the leak by spraying soapy water on the sender, fittings, and ports on the tank. Look for air bubbles, which indicate a leak. If no leaks are found on the top of the tank, check the tools and hoses for any potential leaks.

Step 9: Interpret the Results

If the pressure in the tank remains stable within the specified range and no leaks are detected, the test is successful. However, if the pressure drops or leaks are found, further inspection and repairs may be necessary.

A pressurised fuel tank is necessary for high-altitude flights

In most light aircraft, the fuel tanks are located inside the wings. A filler cap on top of the wing allows them to be filled; drains in the bottom allow fuel samples to be drawn for inspection and any moisture to be removed. Vents allow internal pressures to equalise. In high-wing aircraft, gravity propels the fuel from the tank to the carburettor, and fuel pumps might not be necessary. However, a low-wing aeroplane—and high-wing craft with fuel-injected engines—will need an engine-driven pump to move the fuel. There should also be an electrically powered pump for engine start and use as a backup.

Aircraft fuel systems are either gravity-fed or employ fuel pumps. Gravity-fed systems are primarily used in high-wing aircraft. Fuel pumps are required by low-wing aircraft and those with fuel-injected engines.

The fuel tank also consists of suction valves that allow fuel to be drawn by the engines in the event of tank pump failure. This requires the pilots to descend to a lower altitude, which prevents low-pressure fuel boiling.

In aircraft capable of flying at high altitudes, the tank pumps are a necessity because the reduced pressure at altitudes can cause fuel to boil, causing vapor locks that can prevent fuel from entering the engine-driven pump.

Frequently asked questions