The Mystery Behind Pressurized Fuel Tanks In Hybrids

Hybrid vehicles have pressurized fuel tanks to prevent fuel vapours from escaping into the atmosphere. This is achieved by temporarily storing excess vapour in a charcoal canister, which is periodically purged into the engine to be burned. This process is known as corking and was mandated by the EPA in 1971 to reduce hydrocarbon emissions from vehicles. While modern fuel tanks are sealed, a tiny amount of fuel can still escape, and this has led to the development of pressurized tanks that can tolerate internal pressure. Pressurized tanks also help to preserve the gasoline, as the pressure prevents oxygen and condensation from entering the tank, thus stabilizing the fuel.

Pressurized fuel tanks prevent vapours from escaping into the atmosphere

Pressurized fuel tanks are an essential component of hybrid vehicles, and they play a crucial role in preventing vapours from escaping into the atmosphere. This system is designed to significantly reduce emissions and improve overall fuel efficiency.

The pressure inside the tank prevents fuel vapours from escaping, ensuring that they remain within the closed system. This is particularly important for hybrids, as they can operate in pure electric mode, and without a method to purge and burn accumulated fuel vapours, these vapours could escape into the atmosphere. By keeping the vapours contained, pressurized fuel tanks help hybrids maintain compliance with emissions requirements.

The pressure inside the fuel tank is carefully regulated. When the engine is running, the Fuel Tank Isolation Valve (FTIV) and purge valves work in tandem to manage the vapour. During this process, the FTIV opens to allow pressurized fuel vapours to pass through the charcoal canister, where they are filtered before being vented to the atmosphere. This ensures that only a minimal amount of fuel vapour escapes, reducing the vehicle's overall emissions.

Additionally, the pressure in the tank helps to stabilize the fuel. By preventing oxygen and condensation from entering, the pressurized system extends the fuel's longevity, reducing the need for frequent refuelling. This is especially beneficial for hybrid vehicles, as they may go through longer periods without using fuel.

While the primary purpose of pressurized fuel tanks is to prevent vapours from escaping, it's worth noting that this technology also offers weight savings. Plastic tanks, for example, can be up to 20% lighter than their steel counterparts, contributing to improved fuel economy.

This is achieved by temporarily storing excess vapour in a charcoal canister

Hybrid vehicles have pressurised fuel tanks to prevent vapour from escaping and causing excessive emissions. This is achieved by temporarily storing excess vapour in a charcoal canister.

The charcoal canister is a critical component of a vehicle's evaporative emissions system, which includes hoses, valves, and the canister itself. Mounted in the engine compartment, the charcoal canister contains activated carbon, a substance with a highly porous surface area that can effectively absorb gases and toxins. The canister is connected to the fuel system and fuel tank through a supply line and has an output line that leads back to the engine.

The activated carbon in the charcoal canister acts like a sponge, soaking up hydrocarbons in the vapours. The engine control computer then periodically purges the canister of fumes by releasing them back into the engine through a purge valve to be burned. This process not only reduces hydrocarbon emissions but also prevents unburned fuel vapours from escaping into the atmosphere, contributing to air pollution.

The charcoal canister plays a crucial role in maintaining the proper functioning of the engine. When the purge valve is stuck open, it introduces too much fresh air into the engine, altering the fuel-air ratio and causing a rough idle and difficulty starting. On the other hand, when the purge valve is stuck closed, it leads to a rich-running condition, along with similar issues such as difficulty in starting and a rough idle.

It is important to note that certain practices, such as topping off the fuel tank, can damage the charcoal canister by saturating it with fumes. This saturation reduces the canister's ability to absorb and purge vapours effectively, potentially leading to problems during refueling and causing the "check engine" light to illuminate. Therefore, it is essential to follow recommended fuelling practices and maintain the charcoal canister to ensure optimal vehicle performance and compliance with emissions standards.

The pressure inside the tank increases when the engine is not running

Hybrid vehicles have pressurised fuel tanks to prevent fuel vapours from escaping and to reduce emissions. When the engine is not running, vapours are stored in a charcoal canister. However, the canister can only hold a limited amount of vapour. If the canister becomes saturated and the fuel system is open to the atmosphere, vapours will escape, causing the vehicle to fail emissions requirements.

To prevent this, the fuel system is sealed, and the pressure inside the tank is increased when the engine is not running. This prevents vapours from filling the canister and ensures that they are fed into the engine to be burned when the vehicle is next operated. This technology is not yet in production, but working prototypes exist.

The pressure inside the tank increases as the temperature of the fuel rises. This can create a risk of injury and excessive emissions when opening the filler cap during refuelling. To address this, some hybrid vehicles have a Fuel Lid Opener Switch that opens a Fuel Tank Isolation Valve (FTIV) to allow pressurised fuel vapours to pass through the charcoal canister and vent to the atmosphere. Once the pressure has normalised, the fuel filler door can be opened safely.

During refuelling, the FTIV remains open to filter refueling vapours through the canister. When refuelling is complete, the FTIV closes again, sealing the fuel tank. At this point, the pressure in the tank is the same as the atmospheric pressure. The pressure in the tank will only increase if the fuel temperature rises.

This technology helps to reduce emissions and fuel evaporation

Pressurized fuel tanks in hybrid vehicles are designed to reduce emissions and fuel evaporation. This technology is particularly relevant for hybrid vehicles as they can operate in pure electric mode, meaning there is no method of purging and burning accumulated fuel vapors. In the past, vapors emanating from the tank made up a significant portion of a car's overall hydrocarbon emissions.

In a non-pressurized fuel tank, fuel vapors would escape and cause the vehicle to fail emissions requirements. By pressurizing the fuel tank, fuel vapors are prevented from escaping and are instead fed into the engine and burned. This technology also helps to preserve the gasoline, as the pressure prevents the evaporation of lighter hydrocarbons.

Additionally, the relocation of fuel-delivery components from the exterior to the interior of the tank also contributes to reduced emissions. This design feature addresses the issue of leaks, which can still occur even in closed-off systems.

Overall, the use of pressurized fuel tanks in hybrids is an important step towards reducing emissions and fuel evaporation, ensuring that hybrid vehicles are more environmentally friendly and efficient.

Pressurized tanks can also save weight compared to steel tanks

Pressurized fuel tanks in hybrid vehicles are not a new concept. They have been around for a while and are used to prevent fuel vapors from escaping into the atmosphere, which could lead to excessive emissions. This is achieved by venting the vapors through a charcoal canister, which filters and stores them before they are purged into the engine to be burned. This system is known as the EVAP system.

In addition to reducing emissions, pressurized fuel tanks can also offer weight savings compared to steel tanks. For example, TI Automotive, a supplier of fluid systems and delivery hardware to automakers, developed a plastic fuel tank that is about 20% lighter, or five pounds lighter, than an equivalent steel tank. This weight saving is significant, especially in the face of rising fuel economy standards where every ounce of weight savings counts.

The use of plastic tanks is becoming more prevalent, with roughly 98% of European vehicles and 75% of US vehicles currently utilizing them. This trend is likely to continue as the benefits of reduced weight and emissions become more important.

Furthermore, pressurized tanks can also help to preserve the gasoline in hybrid vehicles. For example, the BMW X5 45e has a pressurization system that stabilizes the fuel, allowing it to last longer than in conventional fuel tanks. This is particularly useful for hybrids that can operate for long periods solely on electric power, as it prevents the fuel from degrading due to condensation and evaporation.

Overall, the use of pressurized fuel tanks in hybrids offers several advantages, including weight savings compared to steel tanks, reduced emissions, and improved fuel stability. These benefits contribute to the overall efficiency and performance of hybrid vehicles, making them a popular choice for automakers and consumers alike.

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