Why are MFI injector fuel lines equal length?

[BURN-BROS]

Originally Posted by groundeffect

The lines are only full of fuel during an injection event. The fuel is metered to each cylinder by the injection pump, not the injectors as in EFI. Read Dempsey's book.

Wrong. Fuel is in the line at a minimum pressure of approx 24 psi.

[rootmatt]

Has anyone considered the inertia that a column of fuel presents. During an injection event, the fuel must be accelerated from rest then brought back to a complete stop in as little as 1min/4000 or .015 seconds. Inertia could have an effect on this process. (Think of "water hammer") If the lengths are all equal, it would eliminate a possible source of mixture variation as the mass of each column would be equal.

Matt

[Chuck Moreland]

Originally Posted by rootmatt

Has anyone considered the inertia that a column of fuel presents. During an injection event, the fuel must be accelerated from rest then brought back to a complete stop in as little as 1min/4000 or .015 seconds. Inertia could have an effect on this process. (Think of "water hammer") If the lengths are all equal, it would eliminate a possible source of mixture variation as the mass of each column would be equal.

Matt

The volume of fuel that moves with each event is very small (anyone have a range?). I would therefor expect inertial and head impact to be insignificant.

I'm with Ingo on the manufacturing economies.

[groundeffect]

Originally Posted by BURN-BROS

Wrong. Fuel is in the line at a minimum pressure of approx 24 psi.

If so, this "residual pressure" is 10% of the injection pressure...but you miss the point. If a 220-240 psi fuel impulse is released from the pump, there will be a propagation delay (travel time)through the injection hose as the fuel impulse travels to the cylinder. If all the hoses are different lengths, the arrival times of the fuel impulses will be different. It may not matter much in the real world, but I would make my injector lines the same length.

[Eagledriver]

Your "impulse" (if there was one) would travel at the speed of sound in fuel (I'm guessing 1800 FPM). Not much time required to go through a 2 foot line. The fact is there is no impulse. The injection pump uses a cam to push a piston that squirts the fuel. The cam accelerates the piston at a reasonable rate just like the cams in your valve train. You wouldn't call a valve opening an impulse event.

The injectors are just spring loaded valves, they are not controlled in anyway. As the pressure in the line rises above 200 PSI or so the spring compresses allowing the fuel to flow until the pressure drops back below 200 PSI. The pressure is supplied by the piston moving in a cylinder full of fuel inside the injector pump. You could think of these springs in the injectors as the equivalent of the valve springs in your cylinder heads.

-Andy

[CruiseControl]

The valve opening (inlet/exhaust) is not a good analogy. Tuned induction and exhaust systems are bases on using the "impulse(s)".....

Lets expand on the effects of different length fuel lines. If you have two lenghts of hose, one is 1 foot long and the other is 1 mile long. With an identical supply source (pressure and pulse) the resulting timing and flow at 1 mile is different to 1 foot. This example is a major exaggeration, to make the point that there is a difference. Now back to the MFI, what diifference does it make if there are unequal lenght fuel lines? not much, but there is a difference. On diesel engines that use a common mechanical pump with individual fuel lines to each injector this difference is more important as it also effects the firing time for cylinders, hence the fuel lines are equal length as in the below photo.



If all converted to EFI then this thread would fade away

Paul

[Wayne 962]

Originally Posted by groundeffect

The lines are only full of fuel during an injection event. The fuel is metered to each cylinder by the injection pump, not the injectors as in EFI. Read Dempsey's book.

Sorry, that's not true - the lines are full of fuel at all times when the engine is running. If they weren't full of fuel, then you'd have a compressible situation (with air in the lines), and you wouldn't get the pressures that you needed out of the lines. It works like a hydraulic line at that point. You can have fuel in the lines but not under pressure at a certain moment.

-Wayne

[pjh69911]

Mercedes used unequal length fuel lines on their MFI cars. Fuel is in the lines at all times, and relatively incompressible. The MFI pump is a positive displacement pump. A tuned intake relies on a compressible fluid.
I vote that it was for production efficiency...

[Bobboloo]

Think of it as a pipe with a row of ball bearings inside. When you push a ball bearing in one end of the pipe a ball bearing will fall out the other end of the pipe at the same exact time of the one being pushed in.

Now if you have two pipes of unequal length and you push a ball bearing in both at the exact same time a ball bearing will fall out the other end of each at the exact same time. What is different between them, however, is the amount of pressure required to push the ball bearing out the other end. The longer pipe requires more pressure.

Timing is not an issue in different lengths pressure is the issue.

The weight difference of the fuel between different length lines in an MFI setup is of no consequence at a 220 psi pressure. There is more than enough pressure to push both easily.

[groundeffect]

Originally Posted by Wayne at Pelican Parts

Sorry, that's not true - the lines are full of fuel at all times when the engine is running. If they weren't full of fuel, then you'd have a compressible situation (with air in the lines), and you wouldn't get the pressures that you needed out of the lines. It works like a hydraulic line at that point. You can have fuel in the lines but not under pressure at a certain moment.

-Wayne

Thanks Wayne, you are of course correct. What I was thinking, and should of said, is the lines are not pressurized all the time like EFI.

[RWebb]

This has been a fascinating discussion. My personal opinion is that the lines are equal length because they are subject to the conservation of mass law of physics.

B-b-loo's ball bearing analogy is a good one...

[groundeffect]

Originally Posted by Eagledriver

Your "impulse" (if there was one) would travel at the speed of sound in fuel (I'm guessing 1800 FPM). Not much time required to go through a 2 foot line. The fact is there is no impulse. The injection pump uses a cam to push a piston that squirts the fuel. The cam accelerates the piston at a reasonable rate just like the cams in your valve train. You wouldn't call a valve opening an impulse event.

The injectors are just spring loaded valves, they are not controlled in anyway. As the pressure in the line rises above 200 PSI or so the spring compresses allowing the fuel to flow until the pressure drops back below 200 PSI. The pressure is supplied by the piston moving in a cylinder full of fuel inside the injector pump. You could think of these springs in the injectors as the equivalent of the valve springs in your cylinder heads.

-Andy

Fuel impulse=Pressure impulse
Absolutely I would call a valve opening an impluse event. In fact, the ideal cam "profile" would be a rectangle...fast open to max height, dwell, then fast close. Similar to the improvment attained by roller cams over flat tappets.

[audi2.7t]

Originally Posted by Eagledriver

Your "impulse" (if there was one) would travel at the speed of sound in fuel (I'm guessing 1800 FPM).

-Andy

Actually more like 1200 MPS...(meters per second)..the denser the fluid, the faster the speed. I think that @ 220psi the expansion and contraction of the steel line is more of a factor than compression (yes, it does compress) or weight of the fuel...equal length lines will give you the same rate of fuel with linear movement and pressure drop...all of which, combined, will affect timing to one extent or another...why have any variation when you can just make them all the same and be done with it. Theoretically, spark plug wires should be of equal length also .

[Eagledriver]

I agree that a rectangular cam profile would result in an impulse event

-Andy

[silverc4s]

I am gonna bump this in hopes that some more of our experts will join the discussion. As far as I can interpret, we still don't know what the real reasons are here.

[jcge]

I suspect the answer is the expansion of the LINE during the injection stroke. With an increase in line pressure, the line itself will expand in diameter, and therefore the mass of the fuel in the line will increase. Albeit by a small amount in steel lines...but more so in the flexible RSR/Racing lines.

I'm sure somebody here has the inclination to do the calcs for steel vs nylon lines at the working pressure...

I believe the Bosch test procedure calls for equal length flexible test lines 6x2x600....test fluid at 45°C (can anybody confirm this ?? and or post a copy of the procedure??)

The change in the mass of fuel in the line with each injection stroke will be in direct proportion to the length of the line.

With respect to presure drop due to flow rate (and line internal roughness) - the flow rate is too low to introduce an appreciable pressure differential

In any event, equal length lines with minimise variances due to all effects - (in)compressibility of fuel, line expansion both diameteral & linear, and flow rate pressure drops)

Now if you could only guarantee that all of the line were at the same temperature........

Anyway, thats my 2 cents worth...

[911pcars]

I think we're overanalyzing this knowing that other manufacturers used different lengths.

As Bobby and Ischmitz suggest at the beginning, it's just a production step. I tend to agree. PAG can order x thousand straight lines from their vendor, then specify 6 bend patterns for each. It's easier on inventory. Instead of ordering 6 different part numbers and keeping track of how each line gets bent, all lines are bent 6 different ways.

Sherwood