Fuel Pressure Calculation: A Comprehensive Guide To Success

Calculating fuel pressure depends on a variety of factors, including the properties of the fuel, the flow rate, and the height between the fuel level in the tank and the inlet point to the fuel pump. Bernoulli's equation can be used to solve for fuel pressure, taking into account friction losses. However, it is important to note that fuel pressure is typically not calculated but rather measured directly, as it depends on various factors such as the state of the injector system and the pump, resistance of oil pipes, and viscosity of the oil.

Bernoulli's equation with friction losses

Bernoulli's equation is a useful tool for solving fluid flow problems, providing an easy way to relate the elevation head, velocity head, and pressure head of a fluid. It is based on the conservation of energy principle, which states that energy can be neither created nor destroyed. This is equivalent to the First Law of Thermodynamics, which states that the total head of the fluid is constant.

The simplified Bernoulli equation is derived from the general energy equation and the first law of thermodynamics. It is applied to a steady flow system in which no work is done on or by the fluid, no heat is transferred, and there is no change in the internal energy of the fluid. The simplified equation is as follows:

$$ {m g z_1 \over g_c} + {m v_1^2 \over 2 g_c} + P_1 V_1 = {m g z_2 \over g_c} + {m v_2^2 \over 2 g_c} + P_2 V_2 $$

Where:

• M = mass
• G = acceleration due to gravity
• Z = height above reference level
• V = average velocity of fluid
• P = pressure of fluid
• V = specific volume of fluid
• Gc = gravitational constant

Each term in the simplified Bernoulli equation represents a form of energy possessed by a moving fluid: potential energy, kinetic energy, and pressure-related energy. The sum of these energies is the total head of the fluid, and Bernoulli's equation states that this total head remains constant.

However, the simplified equation has some restrictions. Firstly, it does not account for fluid friction, limiting its practical application in real piping systems. Secondly, it does not allow for work to be done on or by the fluid, preventing its use in systems with pumps.

To address these limitations, the simplified Bernoulli equation can be modified to include gains and losses of head, resulting in the Extended Bernoulli equation:

$$ z_1 + {v_1^2 \over 2 g} + P_1 \nu_1 {g_c \over g} + H_p = z_2 + {v_2^2 \over 2 g} + P_2 \nu_2 {g_c \over g} + H_f $$

Where:

• Hp = head added by a pump
• Hf = head loss due to fluid friction

By including these additional terms, the Extended Bernoulli equation can be used to solve most fluid flow problems and is probably the most widely used fluid flow equation. It provides a way to calculate fuel pressure by considering the properties of the fuel, the flow rate, and the system geometry.

Fuel properties

Composition and Feedstocks

The chemical composition of fuel varies depending on its source and feedstocks. For instance, biodiesel is primarily composed of fatty acid methyl esters (FAME), derived from vegetable oils, animal fats, or waste cooking oil. On the other hand, gasoline is a blend of 90% gasoline and 10% ethanol, while diesel has a higher energy density. Additionally, the composition of biodiesel feedstocks can differ, with some sources having higher amounts of saturated fatty acids, such as coconut, palm, and tallow, while others are dominated by unsaturated fatty acids, like corn, rapeseed, and soy.

Energy Content

The energy content of fuel is typically measured in British thermal units (BTU) per gallon or its higher heating value (HHV). Biodiesel, for example, has a lower energy density compared to diesel, while propane has a lower energy density than gasoline. The energy content influences the overall efficiency and performance of the fuel in an engine.

Heating Value

The heating value of a fuel is a critical property, measured in BTU per gallon or kJ/g. It represents the amount of heat energy released during combustion. Vegetable oils, for instance, have higher heating values, which can be calculated using the saponification value (SV) and iodine value (IV). The equation for calculating HHV is: HHV = 49.43 − [0.041(SV) + 0.015(IV)].

Viscosity and Flow Properties

Viscosity is a key property that affects the flow characteristics of fuel. Biodiesel, for instance, has higher viscosity compared to petroleum fuels, which can impact engine performance. To address this, dilution with solvents or microemulsions can be used to lower the viscosity of vegetable oil fuels. Additionally, the transesterification process is commonly employed to reduce viscosity and improve fuel flow.

Specific Gravity

Specific gravity is the ratio of the density of a substance to the density of a reference material, typically water. It provides information about the weight and density of the fuel. This property is important for fuel calculation and can vary depending on the type of fuel.

Cetane Number and Iodine Value

The cetane number is a measure of the ignition quality of diesel fuel. It indicates how easily the fuel ignites under compression. A higher cetane number generally indicates better ignition quality. The iodine value, on the other hand, is a measure of the degree of unsaturation present in the fuel, particularly in biodiesel. It is used to determine the amount of iodine that will react with a given weight of the fuel.

Environmental Impact

The environmental impact of different fuels is an important consideration. Biodiesel, for example, is considered renewable, non-toxic, and eco-friendly alternative to traditional diesel. It reduces petroleum use throughout its lifecycle and has a positive impact on the carbon-neutral economy. Additionally, feedstocks like algae have less polluting nature due to their high energy content and low levels of polyunsaturated fatty acids.

Flow rate

To calculate the flow rate, you need to consider the power output of the engine, the type of fuel used, and the specific fuel consumption. The Brake-Specific Fuel Consumption (BSFC) is a measure of the fuel needed per horsepower per hour, and it varies depending on the load and RPM. By knowing the BSFC for your fuel type, you can calculate the fuel requirements for different horsepower levels.

For example, let's consider a 400 hp gasoline engine with a BSFC of 0.50 lbs/hp/hr. We can calculate the fuel consumption as follows:

400 hp x 0.50 = 200 lbs/hr

To convert this value to gallons per hour, we divide by the conversion rate from pounds to gallons, which is 6.25 lbs to 1 gallon of gasoline:

200 lbs/hr / 6.25 lbs/gal = 32 gph

So, for this gasoline engine, the approximate peak fuel flow rate is 32 gallons per hour.

It's important to note that fuel flow rates can be influenced by various factors, including throttle position, RPM, and aspiration type. Additionally, the thermodynamics of the burner in the case of aircraft engines also play a significant role in determining the fuel flow rate.

Height between fuel level and pump

The height between the fuel level in the tank and the inlet point to your fuel pump is a crucial factor in calculating fuel pressure. This height needs to be corrected for the specific gravity (SG) of the fuel. The pressure at the pump is directly influenced by the height of the fuel column above the pump inlet.

To calculate the height of the fuel column, you can use the formula:

> L = P / (ρ0 · SG · g)

Here, L represents the height of the liquid column, P is the pressure reading at the bottom of the column, ρ0 is the density of fresh water (1000 kgm-3 at 4°C), SG is the specific gravity of the fuel, and g is the acceleration due to gravity (standard gravity is 9.80665 ms-2).

It's important to note that the pressure value used in the formula is the difference in pressure between the measurement point and the top of the fuel column. In most cases, the top of the fuel column is vented to the atmosphere, so the measured pressure is typically relative to atmospheric pressure. However, if the top of the fuel column is sealed, you will need to determine the pressure difference to correct for any positive or negative pressure acting on the fuel surface.

The specific gravity of the fuel is a crucial factor in these calculations. It represents the ratio of the density of the fuel to the density of fresh water. For example, fresh water has an SG of 1, while some oils have an SG of around 0.83, indicating that they are less dense than water.

By inputting the relevant values into the formula, you can calculate the height of the fuel column, which is essential for determining the fuel pressure at the pump inlet.

Additionally, the height between the fuel level and the pump can be influenced by factors such as the pipe resistance in the suction pipe, the number of bends in the pipe, and the internal suction losses of the pump. These factors can impact the overall fuel pressure and should be considered when designing a fuel system or troubleshooting fuel pressure issues.

Fuel pump inlet height

The fuel pump inlet height is a critical factor in calculating fuel pressure. The pressure at the pump is determined by the height between the fuel level in the tank and the inlet point of the fuel pump. This height must be corrected for the specific gravity of the fuel.

To calculate the pressure at the pump, you need to know the height of the fuel pump inlet in relation to the tank. This is a basic principle of fluid dynamics, where the pressure of a fluid is directly related to the height at which it is positioned. In the context of a fuel tank and pump system, the pressure at the pump is influenced by the height of the fuel column above the inlet.

For example, let's consider a scenario described in an online engineering forum. A user seeks assistance in calculating the fuel pressure for a diesel generator fuel pump. The tank dimensions are provided as 2450 mm wide x 1520 mm long x 1520 mm high, and it sits on a 600 mm stand. The user wants to determine the pressure at the pump, which is located approximately 400 mm above the ground level.

Artisi, one of the forum members, provides a response. They explain that the pressure at the pump is determined by the height between the fuel level in the tank and the inlet point of the fuel pump. This height needs to be corrected for the specific gravity of the fuel. In this case, the user mentions that the fuel is standard gas oil, with a specific gravity of around 0.83 depending on the temperature.

By taking into account the height of the fuel pump inlet in relation to the tank and making the necessary corrections for the specific gravity of the fuel, one can calculate the pressure at the pump. This calculation is crucial for ensuring proper fuel delivery and engine performance.

Frequently asked questions