http://www.picoauto.com/tutorials/fuel-injection.html

Returnless Fuel Systems

Have been adopted by several motor manufacturers and differ from the conventional by having a delivery pipe only to the fuel rail with no return flow back to the tank.

The returnless systems, both the mechanical and the electronic versions, were necessitated by emissions laws. The absence of heated petrol returning to the fuel tank reduces the amount of evaporative emissions, while the fuel lines are kept short, thus reducing build costs.

Mechanical Returnless Fuel Systems

The ‘returnless’ system differs from the norm by having the pressure regulator inside the fuel tank. When the fuel pump is activated, fuel flows into the system until the required pressure is obtained; at this point ‘excess’ fuel is bled past the pressure regulator and back into the tank.

The ‘flow and return’ system has a vacuum supply to the pressure regulator: this enables the fuel pressure to be increased whenever the manifold vacuum drops, providing fuel enrichment under acceleration.

The ‘returnless’ system has no mechanical compensation affecting the fuel pressure, which remains at a higher than usual 44 to 50 psi. By increasing the delivery pressure, the ECM (Electronic Control Module) can alter the injection pulse width to give the precise delivery, regardless of the engine load and without fuel pressure compensation.

Electronic Returnless Fuel Systems

This version has all the required components fitted within the one unit of the submersible fuel pump. It contains a small particle filter (in addition to the strainer), pump, electronic pressure regulator, fuel level sensor and a sound isolation system. The electronic pressure regulator allows the pressure to be increased under acceleration conditions, and the pump’s output can be adjusted to suit the engine's fuel demand. This prolongs the pump’s life as it is no longer providing a larger than required output delivery.

The Electronic Control Module (ECM) supplies the required pressure information, while the fuel pump’s output signal is supplied in the form of a digital squarewave. Altering the squarewave’s duty cycle affects the pump’s delivery output.

To compensate for the changing viscosity of the fuel with changing fuel temperature, a fuel rail temperature sensor is installed. A pulsation damper may also be fitted ahead of or inside the fuel rail.

Here is one rendition of such a system in an auto application

Ed Anderson

Rv-6A N494BW Rotary Powered

Matthews, NC

eanderson@carolina.rr.com

http://www.andersonee.com

http://www.dmack.net/mazda/index.html

http://www.flyrotary.com/

http://members.cox.net/rogersda/rotary/configs.htm#N494BW

http://www.rotaryaviation.com/Rotorhead%20Truth.htm

From: Rotary motors in aircraft [mailto:flyrotary@lancaironline.net] On Behalf Of Al Wick
Sent: Monday, June 07, 2010 9:25 AM
To: Rotary motors in aircraft
Subject: [FlyRotary] Re: high/low pressure pumps question

I'm really concerned for your life risk. This is a very high risk fuel design. Puts your fuel pressure at pump inlet very very close to the vapor pressure of fuel. This suddenly makes the design sensitive to a whole bunch of environmental factors.

Ask this question: Millions of automobiles. When they designed automotive fuel systems, why did every single engineer return fuel to tank instead of pump inlet? So much more expensive to send to tank.

You can bench test your design and prove how close it is to failure. Just need to measure fuel pressure at pump inlet, measure pump temp. Use hair dryer to force pump to higher temp. Put all the numbers in spreadsheet, then calculate distance between vapor pressure and your readings. Adjust for worst case, which would be high altitude airport, hot day sitting on tarmac for 1 hour, heat soaked engine compartment, car fuel containing ethanol.

Marginal designs can fly for years without failure. This is the nature of failure. Use care, I'm concerned.

-al wick

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