Need help with a fluid system debate -Bernoulli

In summary: This is an undesirable effect and is why the last injector is typically disabled.""In summary, this proposed solution won't work because the pressure in the fuel rail will rise and the engine will run lean."
  • #1
Zooomer
2
0
I am debating with a group in a message forum about fueling an engine.

Currently there is a pump running to a filter. The filter has two outlets, one returns fuel through a 60psi regulator then to the tank. The other outlet runs to the front of a car. All lines are 5/16". The fuel rail is 9/16" in diameter and 14" long and there are 4 injectors.

The debate is that they are claiming that the injector on the end of the fuel rail will put out the least fuel and starve that cylinder of gas. The solution proposed is to not return the fuel from the filter but to run all the way to the rail, then out the other side of the rail and back to the regulator before dumping in the tank.

This didn't make any sense to me as I proposed the fuel pressure would remain the same at all spots in the rail.

I figure it falls under Pascal's law or Bernoulli's principal
 
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  • #2
The relative flow rates through the four injectors will depend on the exact geometry of the rail and also on the overall flow rate. Also, whether the injection is constant or intermittent (timed). But I think you are probably more correct than the others. They are likely relying on their 'intuition' which tells them the flow 'will follow the path of least resistance.' This intuition is misleading and really not correct.

Look for 'manifolds' in fluid mechanics textbooks to see how these are treated mathematically. One thing to realize is that the pressure in the manifold (rail) downstream of a given branch may increase due to the deceleration (since some fluid exits through the branch, the velocity in the (constant area) rail drops, and this raises pressure).

Also, friction losses in your 14 inch long, 9/16 diameter rail are going to be negligible relative to the losses in the long smaller diameter line from the filter to the rail.
 
  • #3
This is what they are saying is happening and I told them it's not possible:

http://www.stevetek.com/Images/Figures/stockFuelSystem.gif

The explantion
"Notice in figure 1 that in this case the fuel pressure sender is in the small diameter restrictive section of the delivery system. Now the diameter of the fuel rail is much larger than the fuel line leading up to it. So when the injectors are at a high enough duty cycle as to approach the maximum capacity of the preceding delivery system, the pressure in the fuel rail will see a drop off at each successive injector as indicated by the wavy blue lines. There will be enough pressure to deliver sufficient fuel for the first two cylinders and probably the third. But the last cylinder (#1) will be starved and thus risks a lean condition. "
 
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  • #4
The pressure in a pipe only drops when there is flow, due to friction between the walls of the pipe and the fluid, and due to viscosity within the fluid. This pressure drop is small since the flow through that last fuel injector is momentary and also small compared to the related flow in the relatively large diameter fuel rail. Due to harmonics and momentum of the fluid, it's possible that at certain frequencies of injector operation, the pressure is actually higher at that last injector.
 

FAQ: Need help with a fluid system debate -Bernoulli

What is Bernoulli's principle and how does it relate to fluid systems?

Bernoulli's principle is a fundamental concept in fluid dynamics that states that as the speed of a fluid increases, its pressure decreases. This principle is based on the conservation of energy and can be used to explain the behavior of fluids in various systems, such as pipes, pumps, and airplanes.

How does Bernoulli's principle apply to real-life scenarios?

Bernoulli's principle can be observed in many everyday situations, such as when blowing air over a piece of paper to make it rise or when a shower curtain is pulled towards a person standing in a shower. It is also essential in the design of airplane wings, as the shape of the wing creates a difference in air pressure and allows the plane to generate lift.

What are some common misconceptions about Bernoulli's principle?

One common misconception is that Bernoulli's principle only applies to liquids. In reality, it is a fundamental principle that applies to both liquids and gases. Another misconception is that Bernoulli's principle only applies to objects moving in a straight line. However, it can also be applied to objects moving in a curved path.

How is Bernoulli's principle related to the Venturi effect?

The Venturi effect is a phenomenon that occurs when a fluid flows through a narrow section of a pipe, causing its speed to increase and its pressure to decrease. This effect is based on Bernoulli's principle, as the faster-moving fluid has lower pressure according to the principle.

What are some practical applications of Bernoulli's principle?

Bernoulli's principle has many practical applications, such as in the design of carburetors for cars, where it is used to mix air and fuel for combustion. It is also crucial in the design of pressure gauges and flow meters. In addition, it is used in the field of aerodynamics for designing efficient aircraft and in the medical field for understanding blood flow through blood vessels.

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