Fuel Pressure Sensitivity To Temperature: What's The Deal?

Fuel pressure and temperature are closely related. When the temperature of a container increases, the kinetic energy of the gas molecules inside it also increases, leading to more frequent collisions with one another and the container walls. This results in an increase in pressure. Similarly, in a car engine, the temperature affects the fuel pressure. On cold days, the engine control unit (ECU) sends more fuel, leading to higher pressure. Conversely, on warm days, the pressure tends to be lower. These changes in fuel pressure with temperature can be observed in vehicles like the Subaru Impreza.

Fuel pressure and temperature compensation

Fuel pressure is affected by temperature. As the temperature of a container increases, the kinetic energy of the gas molecules inside it also increases, leading to an increase in the collision rate of the particles. This, in turn, increases the pressure across the container wall.

In the context of fuel pressure and temperature, compensation refers to the process of correcting or adjusting for these changes in pressure and temperature to ensure accurate measurements and optimal performance. This is particularly important in flow meters used to measure the flow rate of gases, liquids, and steam.

Principles of Temperature and Pressure Compensation

The principle behind temperature and pressure compensation is based on understanding how physical parameters such as density and viscosity of the fluid change with variations in temperature and pressure. These changes can impact the accuracy of measurements.

#### Temperature Compensation

Temperature compensation focuses on adjusting the reference temperature of the free end of a sensor to make it more suitable for the application. Sensors typically have temperature coefficients, which means their parameters will change with fluctuations in ambient temperature. Temperature compensation techniques aim to prevent the sensor's indicators and performance from being affected by these temperature variations.

#### Pressure Compensation

Pressure compensation addresses the changes in the quality of the medium under different pressures when detecting flow. While density remains relatively constant under temperature changes, even slight changes in pressure can cause significant alterations in density. Therefore, pressure detection is necessary to calculate the medium's density accurately.

Implementation of Temperature and Pressure Compensation

Temperature and pressure compensation can be implemented in various systems, including flow totalizers, PLC control systems, and DCS (Distributed Control Systems).

#### Flow Totalizers

When configuring a flow totalizer, attention should be given to selecting the appropriate density compensation module and ensuring channel correctness and unit consistency. If the site is a pulse instrument, unit consistency is also crucial when setting the instrument coefficient.

#### PLC Control Systems

Implementing temperature and pressure compensation in PLC control systems is more complex. Custom function blocks must be created according to the respective compensation formulas, and application programs must be written using a textual high-level programming language.

#### DCS

Mainstream DCS systems, such as Emerson DeltaV, Yokogawa CS3000, and Hollysys MACCS, have built-in functional blocks for mathematical functions and logical operations, making program implementation more straightforward.

Examples of Temperature and Pressure Compensation

Let's consider an example of H2 gas flow measurement using an orifice calibrated for a specific design pressure and temperature. If the input pressure and temperature deviate from the design conditions, pressure and temperature compensation calculations can be applied to derive the compensated flow. These calculations use equations based on Euler's equation of continuity and Bernoulli's principle.

For instance, if the design pressure is 35 Bara and the actual pressure is 30 Bara, the compensated flow due to the change in pressure can be calculated using the following equation:

> Flow Compensated = Original Flow * SQRT(Actual Pressure / Design Pressure)

Similarly, if the design temperature is 300 K and the actual temperature is 325 K, the compensated flow due to the change in temperature can be calculated as:

> Flow Compensated = Original Flow * SQRT(Design Temperature / Actual Temperature)

By applying these compensation calculations, we can account for deviations from the design conditions and obtain more accurate flow measurements.

Fuel pump and temperature

Fuel pumps are affected by temperature, both in terms of their functionality and the accuracy of the fuel volume they dispense.

Fuel Pump Functionality

Fuel pumps are susceptible to problems caused by high temperatures. This is known as "vapour lock", where the fuel in the pump vaporises and won't be pumped to the engine. This can cause the car to lose power and even stop running. Vapour lock is more likely to occur when a car is driving at low speeds, as slower movement means the fuel sits in the pump for longer and has more time to heat up.

To prevent vapour lock, some drivers install an electric fuel pump near the fuel tank to keep the fuel cool. Cooling the fuel pump with ice or water can also help.

Fuel Volume Accuracy

The volume of fuel dispensed by pumps is affected by temperature. Fuel expands as it gets warmer, so a litre of cold fuel will weigh more than a litre of warm fuel. However, fuel pumps measure volume, not weight, so the amount of fuel dispensed is the same regardless of temperature.

Fuel is usually stored in underground tanks at filling stations, which helps to maintain a fairly constant temperature.

Fuel filter and temperature

Fuel filters are an important component of modern engines, ensuring the fuel entering the engine is free from contaminants. The fuel we put into our vehicles can contain impurities such as small metal pieces, rust, sand, organic deposits, and other harmful particles. These impurities can cause significant damage to the engine and its components, including the fuel pump and injectors, if not properly filtered.

Most internal combustion engines use a fuel filter to protect the fuel system. The filter is designed to catch and remove foreign particles and liquids from the fuel before it enters the engine. The filters are typically made of synthetic or cellulose fabric in the form of a corrugated cartridge. They are also designed with a specific direction of flow for the fuel, depending on whether it is a petrol or diesel engine.

The presence of water in the fuel can be a major issue, especially in diesel engines. Water can damage injector components and reduce the lubricity of the fuel, leading to harm to components that work with close tolerance, such as fuel pump assemblies. Diesel engines often use a bowl-like design at the bottom of the filter to collect water, which can then be drained off. This is because diesel floats on top of water, so the water can be easily separated and removed.

In cold weather, diesel fuel is particularly susceptible to the formation of wax crystals, which can plug the fuel filter. This occurs when the temperature drops below the fuel's cloud point, causing wax precipitation. To prevent this, fuel suppliers blend diesel fuel based on anticipated cold weather conditions, and fuel conditioners can also be used to shield the fuel from wax crystal formation.

It is important to maintain and replace fuel filters at regular intervals to ensure the smooth and efficient operation of the engine. A clogged or dirty fuel filter can lead to reduced engine performance, impaired reaction to pressing the accelerator pedal, and even engine stuttering or turning off at idle. Therefore, it is recommended to pay attention to the manufacturer's guidelines for fuel filter replacement intervals and to use high-quality fuel filters to ensure the longevity and optimal performance of the engine.

Fuel regulation and temperature

Temperature regulation is crucial in fuel systems, especially in aircraft and vehicles, to ensure optimal performance and safety. Fuel temperature affects fuel density, volume, and weight calculations, which are vital for efficient aircraft performance. Deviations from the standard temperature can cause discrepancies in fuel quantity readings, leading to inaccurate fuel consumption calculations.

In aircraft, as the plane climbs to higher altitudes, the outside temperature typically drops, causing significant temperature changes in the fuel. Cold fuel can increase viscosity, making it harder to pump, and potentially causing engine operation issues. Low temperatures can also cause waxing and crystallization, clogging fuel filters and lines. Conversely, warm fuel can result in vapor lock, disrupting fuel flow and engine function. Therefore, pilots and ground crews must closely monitor fuel temperatures and take appropriate actions, such as using fuel heaters or adjusting flight parameters, to maintain the ideal fuel temperature.

In vehicles, hydrogen fuel cells' temperature plays a vital role in their overall efficiency and durability. Extreme temperatures can lead to reduced performance and even irreversible damage to components. Effective temperature management is necessary to ensure the fuel cell operates within the optimal temperature range. This involves dissipating heat during operation to avoid overheating and maintaining a minimum temperature in colder environments to prevent freezing. Various systems and designs, such as cooling systems, insulation materials, and hydrogen storage temperature control mechanisms, are employed to regulate fuel temperature in vehicles effectively.

Additionally, the temperature can impact the octane sensitivity of fuels used in spark ignition engines. A study found that at higher loads, knock resistance became proportional to the octane sensitivity of the fuel, with the higher sensitivity fuel exhibiting better resistance. This highlights the importance of understanding the temperature-related behaviour of different fuels to optimize engine performance and avoid issues like knocking.

Fuel ignition and temperature

The ignition temperature of a substance is the minimum temperature required to start or cause combustion, regardless of an igniting element. It is an important property of any fuel because the combustion reaction of the fuel becomes self-sustaining only above this temperature. Exothermic chemical reactions can take place at room temperature, but the reaction rate is very slow at low temperatures. As the temperature of a substance increases, so does the kinetic energy of its molecules, leading to an increased collision rate. This results in a rise in pressure, which, in the case of fuel, can lead to ignition if the temperature is high enough.

The ignition temperature is generally lower for fuels with higher volatile matter content. For example, biomass particles have a higher volatile matter content than coal, giving them a significantly lower ignition temperature. Additionally, the ignition temperature depends on several factors, including oxygen partial pressure, particle size, rate of heating, and the thermal surroundings of a particle.

The auto-ignition point of regular gasoline can range from about 257°C to as low as 221°C. Ethanol has an auto-ignition point of 329°C, and petrol (gasoline) has a fairly low auto-ignition temperature of 280°C. Diesel fuel has an auto-ignition temperature of 210°C, while kerosene falls between petrol and diesel at 220°C.

It is important to understand the difference between the flash point and auto-ignition temperature of a fuel. The flash point is the temperature at which a fuel gives off enough flammable vapors to ignite when a source of ignition, such as an open flame, is introduced. The fire point, which is often confused with the flash point, is the temperature at which the vapors of the fuel will continue to burn for a minimum of five seconds after the source of ignition is removed. The auto-ignition temperature, on the other hand, is the temperature at which the fuel will spontaneously ignite without the need for a spark, flame, or any other external ignition source.

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