Fuel pumps

[Discorde]

Quick tech question: what are the relative merits of gear-type fuel pumps over diaphragm pumps? Do most small aero-engines use the latter and, if so, why?

[Uplinker]

Not an aircraft engineer or designer, but gear pumps give a smooth constant delivery. Diaphragm pumps give a pulsed delivery, and rely on a rubber/nitrile diaphragm, which can perish or develop holes and leaks. A gear type can give a much larger volume flow and higher delivery pressure.

Depending on the fuel system and engine demand; each type has its uses. So a small carburettor engine might be fine with a diaphragm pump, whereas a large and/or fuel injected engine would probably use a gear pump, and large turbine engines certainly do.

[EXDAC]

Originally Posted by Discorde

Quick tech question: what are the relative merits of gear-type fuel pumps over diaphragm pumps? Do most small aero-engines use the latter and, if so, why?

Whether a fuel pump is needed depends on the engine type and the position of the fuel tank(s) relative to the engine.

High wing aiplanes with carburetted engines typically have no fuel pump as the location of the tanks provides adequate pressure at the carb. E.g. Cessna 182.

Low wing aiplanes with carburetted engines typically require a fuel pump and, if one is required, a second is usually provided as a backup. E.g. PA-28-180 which has an engine driven diaphragm pump and and an electric reciprocating pump.

Fuel injected engines typically require a fuel pump regardless of fuel tank position as the required pressure at the injection servo is much higher than can be provided by gravity feed. E.g. FX-3 Carbon Cub which has an engine driven diaphragm pump and an electric gerotor pump. In this aircraft any pressure pulsing with the engine driven diaphragm pump is a non issue.

[Jan Olieslagers]

Rotax 912 (and probably some of its derivatives) uses an automotive diaphragm pump, from a small Opel or VW or such. The advantage of the diaphragm system is that it can stand it when no fuel flow is possible, perhaps due to the float in the carburettor closing off the entrance.
I suppose gear pumps need to have some kind of bypass to handle this situation adequately.

My 912-powered Apollo Fox was one of the few planes at the field not having an extra electrical fuel pump, even most high-wingers with a Rotax had one. Totally needless, the fuel pump on the engine - which is there anyway - is more than sufficient. Perhaps even in a low- or midwing plane, though I'd not trust myself to speculation in that corner.

PS not to be forgotten: the fuel pump is not merely for pumping fuel from the tank(s) to the engine, it has an important function of keeping sufficient pressure in the fuel tubes so as to avoid vapour-locking.

[Pilot DAR]

If the engine requires a fuel pump for operation (always the case for fuel injected engines, and generally the case for low wing airplanes), a second fuel pump, powered by a different power source, is required for certified airplanes. Thus, engine driven pumps (obviously powered mechanically from the engine) are supplemented by (usually) an electric pump. I have observed that commonly, the type of pump will be different for each.

For my experience, the engine driven diaphragm pumps on Lycoming engines are remarkably reliable. Similarly, the electric pulse pumps (Facet brand) are really good. The electric motor fuel pumps on many of the larger Cessna singles are less than ideally reliable, and very expensive to rebuild.

Going back 75 years in design, I brought back an original DHC-2 Beaver a few weeks ago - the second fuel pump is a hand operated (wobble) pump, which you use to bring up the fuel pressure for the start, and then help along as the engine driven pump picks up the pressure. If the engine driven pump won't, you're hand pumping the rest of the flight, and I've been told that's been done a few times!

[Jhieminga]

Originally Posted by Pilot DAR

If the engine driven pump won't, you're hand pumping the rest of the flight, and I've been told that's been done a few times!

At least that keeps the crew awake.... 😉

Most of the pros and cons have been listed already. I can only add that I find the continuous whine of a gear type pump a more reassuring sound than that from a diaphragm pump.

[EXDAC]

Just in case anyone was wondering - Gerotor, as used to describe an electric pump in my earlier post, was not a typo. Gerotor pumps are not the same as gear pumps. They do have their own distinctive normal sounds and, as I found out, a quite different sound when they are failing.

https://en.wikipedia.org/wiki/Gerotor
https://en.wikipedia.org/wiki/Gear_pump

[wrench1]

Originally Posted by Discorde

Quick tech question: what are the relative merits of gear-type fuel pumps over diaphragm pumps?

In most cases none. In general, the pump requirements are determined at the design stage and selected accordingly.

Do most small aero-engines use the latter and, if so, why?

My guess, the simplicity, durability, and cost effectiveness of diaphragm pumps is the reason they are more popular on small aircraft engines.