Board logo

Fuel Injection return pipe
chris68 - 1/2/15 at 09:57 AM

Hi all,

This is just a question out of interest. I assume all fuel injection systems have a return pipe to the tank - thinking mainly of petrol engines - but was wondering if anyone knows how much fuel is actually returned. I guess it will depend how open the throttle is?

Anyone know?

rusty nuts - 1/2/15 at 10:17 AM

A very rough guide would be taking the delivery rate of the pump and deducting the amount of fuel actually used?

r1_pete - 1/2/15 at 10:26 AM

I tried to calculate this, and gave up, satisfying myself with the answer - most of it!

In my Jag Im fitting a 200lph pump, which could theoretically empty the 50l fuel tank in 15 minutes.

If I drive at 70 mph for 15 minutes I'd travel 17.5 miles, and to make things simple probably achieve 17.5 mpg, maybe a little more.

So Id really use 4.5 litres of fuel, and return 45.5 litres to the tank.

I know there are more factors in play, but I was happy that the return would keep my in tank swirl pot filled up.

[Edited on 1/2/15 by r1_pete]

Oddified - 1/2/15 at 10:57 AM

As above, most of it get returned unless your on full throttle and/or the pump flow/rate is on the limit.

I did some testing a couple of weeks ago as i was making a new tank for my own car with a built in an internal swirl pot. I have a Bosch 044 pump on the car and with a 5mm jet in the return line into the swirl pot venturi was enough to fill the internal swirl pot (which is 3L) in seconds when the venturi intake is only just covered with fuel. That requires a reasonable jet of fuel to achieve that.

mark chandler - 1/2/15 at 12:05 PM

All of it!

Set your engine to zero fuel on overrun, when just idling 99.9%

The maths are just about getting enough at WOT, you do not fuel backing up in the return as it will affect the FPR.

Oddified - 1/2/15 at 01:25 PM

quote:
Originally posted by mark chandler
The maths are just about getting enough at WOT, you do not fuel backing up in the return as it will affect the FPR.


As long as the pressure in the return line is significantly lower than the fpr pressure it makes no difference and that's how many oem fuel systems operate.

Ian

Ugg10 - 1/2/15 at 02:35 PM

Slight ot but iirc the jag v6 (3.0l duratech based) have a none return fuel injection rail. There is an ecu controled valve/regulator on the inlet and I think there is a pressure sensor at the outlet end of the rail and using some closed loop trickery keeps the fuel rail pressured to the right level without a return.

But again, typically the normal return system return most ofnthe fuel back to the tanK.

big_wasa - 1/2/15 at 03:20 PM

Lots of modern cars have a return less system for emissions purposes.

If you don't have some restriction in the return on a kit set up with a swirl pot the lift pump gets hot.

Dooey99 - 1/2/15 at 03:40 PM

not all fuel injection cars have a return either.
the majority do and the majority of the fuel gets returned back to the tank.
just fit a friggin huge pump like a 200-255lph then your safe.

pewe - 2/2/15 at 09:08 AM

Safest way surely is to size the return pipe the same as the delivery pipe.
That way you keep it simple.
Not being a student of fluid dynamics I'd assume the fuel will take the least line of resistance, flowing to the fuel rail when there's demand but returning when there isn't - correct me if I'm wrong.
Cheers, Pewe10

Ugg10 - 2/2/15 at 09:24 AM

quote:
Originally posted by pewe
Safest way surely is to size the return pipe the same as the delivery pipe.
That way you keep it simple.
Not being a student of fluid dynamics I'd assume the fuel will take the least line of resistance, flowing to the fuel rail when there's demand but returning when there isn't - correct me if I'm wrong.
Cheers, Pewe10


Not far off but -

The fuel pump provides fuel at a much greater pressure and flow rate than is required to run the engine even at full chat.

At the end of the fuel injector rail is a pressure regulator that maintains the fuel pressure at the injector (the key here is that for the injectors to reliably deliver the amount of fuel required, determined by the ecu, the pressure must remain constant, the amount of fuel is govened by the amount of time the injector is open) therefore for any given demand there is always an excess of fuel in the fuel pump side of the pipe which has to "escape" to maintain the pressure, this is returned to the tank.

The return pipe is effectively at the same pressure as the fuel rail at one end and atmospheric pressure at the other so although the diametre of the return pipe will help, beacause of the pressure difference it is not a vital factor so long as you are sensible. That said, most fuel rails have the same size connector at both ends so this points towards your logical conclusion of using the same size pipe for both sdes.

Hoep this helps.

chris68 - 2/2/15 at 06:29 PM

Thats interesting thanks - all makes much more sense.

Whilst on the subject maybe you have an opinion. The SR2 i'm building uses the complete MX-5 fuel pump and sender unit with just a tweek to the float level arm where it sits in the tank. The return pipe which is solid runs down just behind the pump and exits just above the base of the pump, now this return pipe fouls the tank but not by much. My question is this does the return pipe have to exit right down at the base? If I cut the solid return pipe above where it fouls the tank will that be a problem? I would then end up with the return exiting about half way up the tank on a ledge so the fuel returning would just need to drain down to pump take off and as the MX-5 has an in tank pump and the whole fuel line is high pressure from pump to return there will be plenty of pressure to get the fuel back asap.

Would this be ok?

Dooey99 - 2/2/15 at 06:48 PM

The return can go where ever you want. On a trials 4x4 I built a while ago had the return half way up the filler neck and that worked okay was abit of a pikey installation but worked a treat as long as the fuel can get back in the tank, the closer to the pump pick up the better as when the fuel level gets low it will reduce fuel surge and starvation

SPYDER - 2/2/15 at 08:01 PM

quote:
Originally posted by Ugg10


...At the end of the fuel injector rail is a pressure regulator that maintains the fuel pressure at the injector ... the pressure must remain constant...


Not strictly true in many instances. Many installations feature a vacuum tube from the pressure regulator to the inlet manifold. The pressure in the fuel rail varies at the same rate as that in the manifold.
Some aftermarket systems do run fixed pressure but will be mapped to take it into account. One reason for having variable pressure (apparently) is that the lower pressure at idle can make mixture adjustment easier, particularly with large injectors.
I do remember, however, turning up at John Eales with our race TR7V8 engine for a dyno session and the first thing he did was to remove said vacuum pipe. "Waste of time", he said.

I recently fitted GSXR throttle bodies to my Toyota 3SGE engined car and decided to leave the vacuum pipe in place. Excess fuel from the rail goes to the front mounted swirl pot via a cooling coil. The return from pot to fuel tank is 6mm but has a restrictor fitted. Without this restrictor the low pressure pump feeding the swirl pot runs unloaded and sounds stressed. I have a Malpassi Filter King on the LP side of the swirl pot maintaining about 2psi in the pot.
Sounds complicated but the system works fine.