Fuel pump/filter heat shielding

I'm posting in the fuel injection systems forum because I figure most people here will be familiar with the FlyEFII dual fuel pump unit. I've decided to mount mine on the engine side of the firewall (4-place Bearhawk) both for convenience reasons (hard to fit under floor) and to keep the high-pressure portion of the fuel system apart from the cockpit.

The unit came with a simple aluminum plate as a heat shield, though it leaves the sides of the pumps exposed. While I will try to keep everything mounted as high as possible on the firewall to get distance from the exhaust, will this simple heat shield be adequate? Should I fabricate a new one with "wings" to wrap around the sides? Am I dealing with direct radiation heating from the exhaust? Or the overall equilibrium of air temp inside the cowling?

One inherent advantage to a fuel rail system is that you are moving a lot of fuel all the time, transporting a lot of heat away, but I'm sure preventing much heat absorption is beneficial. What have others' experiences been with shielding their electric pumps and post-filter on the firewall been?

Thanks.

Hot starts are going to be a very ugly situation for you with the pumps forward of the firewall.

The heat risk is found at the line feeding the pump module, as it runs at below ambient pressure. After the pumps the line pressure is roughly 40 psi.

The EFII-brand hose loop past all the exhaust pipes is a big fuel heater, but it seems to work if the tank return is set up properly. SDS does it conventionally.

fuel pumps

The fuel pumps are self cooled by the 35gph of cold fuel running through the system. There is no such thing as hot starts with our system. There is always cold fuel at the injectors. The heat shield is to block the radiant heat from the exhaust. This is all that is needed.

The SDS injector mount is where hot start issues occur since the injectors sit above the engine within the baffle area. Their injectors also sit at the end of one way distribution lines, another hot start contributor. After engine shut down, all fuel components sitting above then engine experience heat soak and fuel vaporization. As Dan says, their setup is conventional - including the hot starts.

Robert

rcpaisley said:

The fuel pumps are self cooled by the 35gph of cold fuel running through the system. There is no such thing as hot starts with our system. There is always cold fuel at the injectors. The heat shield is to block the radiant heat from the exhaust. This is all that is needed.

The SDS injector mount is where hot start issues occur since the injectors sit above the engine within the baffle area. Their injectors also sit at the end of one way distribution lines, another hot start contributor. After engine shut down, all fuel components sitting above then engine experience heat soak and fuel vaporization. As Dan says, their setup is conventional - including the hot starts.

Robert

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Since the fuel pressure is over 40 psi, the boiling point of the fuel is raised substantially. The top location does not have the proximity of the exhaust stacks to contend with.

In hot weather testing by Dave Anders at 108F ambient, the engine was purposely left for between 5 and 15 minutes repeatedly. Hot starts were instant in every case with both bottom and top mount injector placements.

We published Dave's injector temperature measurements also last year on VAF.

I disagree about the hot starts. This should be one of the areas that the EFII system excels.

I'm thinking about the pumps pulling fuel forward once they are heat-soaked. That has been tested and found to be a non-issue?

rcpaisley said:

The SDS injector mount is where hot start issues occur since the injectors sit above the engine within the baffle area. Their injectors also sit at the end of one way distribution lines, another hot start contributor. After engine shut down, all fuel components sitting above then engine experience heat soak and fuel vaporization. As Dan says, their setup is conventional - including the hot starts.

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Hmmm.

Dan also says that hot starts are nonexistent in current automobiles, unless the system has a leak to bleed off rail pressure. And that's with mogas, which has a much higher vapor pressure than avgas.

At 185F, a realistic upper deck temperature, 100LL's true vapor pressure is only 29 psia, waaaay less than 40 psig fuel rail pressure under all conditions (roughly 55 psia at sea level, WOT, and maybe 45 psia cranking at a high altitude airport, throttle at idle). If vapor pressure is less than rail pressure, it can't boil. I doubt either rail system (EFII brand or SDS) can boil.

As Greg and I both note, if there is a problem with the OP's firewall forward pump install, it will be at the fuel pump inlet. Temperature there must be held to less than roughly 140F, assuming a 1 psi drop.

Our concerns are rarely on the pressure side of the pumps regarding vapor lock issues but rather on the suction side. Problems are potentially more likely on low wing designs using mogas which has a much lower boiling point.

We recommend mounting the pumps in a cool location, as low as possible, with the pumps horizontal or with the pressure side higher than the suction side (never the other way around), with no restrictive fittings on the inlet and returning fuel to the tanks for maximum heat sink.

Regarding injector temperatures. With the top mounted type, instrumentation shows they run 11-25F over ambient in flight, compared to about 160F over ambient with the induction tube mounting since they're bathed in hot air from the heads and in close proximity to the exhaust stacks. In one test, Dave saw 274F.

Maximum measured injector temp after a 10 minute hot shutdown was 174F with the top mounted type. Bottom mount type were seeing over 250F in most cases after 5-15 minutes of heat soak.

We've mounted them in both places without issues when operating on 100LL. Some Jabirus in Australia were running them low mounted and reported no issues at 111F ambient temps, running and hot starting.

We are concerned with injector temps exceeding around 260F for extended periods though.

rv6ejguy said:

Regarding injector temperatures. With the top mounted type, instrumentation shows they run 11-25F over ambient in flight, compared to about 160F over ambient with the induction tube mounting since they're bathed in hot air from the heads and in close proximity to the exhaust stacks. In one test, Dave saw 274F.

Maximum measured injector temp after a 10 minute hot shutdown was 174F with the top mounted type. Bottom mount type were seeing over 250F in most cases after 5-15 minutes of heat soak.

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I can't verify actual injector temperatures, but Dave's observations do correspond well with my own in-cowl air temperature measurements. Anything in the lower cowl space runs hotter, longer, as compared to components in the upper cowl space.

Eggenfellner had some issues with EFI pumps mounted on the engine side of the firewall in the early days. Not sure what they had for cooling them if anything. There was at least one forced landing as a result and a change to the design with a bypass orifice.

One of our vendors had pumps firewall mounted with improper orientation (pressure side down). The pumps were continuously cavitating at the inlets when hot and the pumps suffered very short lives.

You want the fuel as cool as possible flooding the pump inlets (preferably not having to be pulled in by the pump) and you want the pump bodies as cool as possible as well. Anything which warms the fuel at the pump inlet reduces your margin from vapor lock issues.

Just a few days ago I was looking at a semi-famous RV which developed a new fuel vapor problem when engine compartment temperatures were raised to a level still less than usual for a typical RV. It's a low pressure system (not electronic fuel injection) with fuel lines running all over the place. They pick up heat...more line feet, more heat.

The moral is keep low pressure fuel lines as short as possible, with as little heat exposure as possible. Will the OP have a problem? Depends on the details.

Here is my data, full Dual EFII system, engine side mounted pumps.......no hot restart issues....at all. like Robert noted, there is a lot of cold gas circulating the instant that fuel pump switch comes on. you can observe this with the live fuel pressure readout, instantly 35 psi, before you crank, if there ever was a vapor lock issue you would see little to no fuel pressure before you start. If that ever happened I would imagine the system would purge that fuel-vapor instantly.

DanH said:

The heat risk is found at the line feeding the pump module, as it runs at below ambient pressure. After the pumps the line pressure is roughly 40 psi.

The EFII-brand hose loop past all the exhaust pipes is a big fuel heater, but it seems to work if the tank return is set up properly. SDS does it conventionally.

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You are correct on this issue Dan, just for S and G I fabricated small heat shields at these locations, I didn't like the idea of 1400 deg pipe so close to the fuel lines, the fuel lines are all heat shielded and there are many running just fine like that, but I added heat shields for extra insurance.

As for hot starts, I've never had a problem at all. Even with high ambient temps its' always started in no more than 2 blades. I did tie the plane down in the summer and run static until I saw the highest CHTs I ever seen in my plane 370 degrees and an oil temp of 230, shut it down waited for 5 minutes and it fired before the second blade and idled like it normal. i tried to make it have a problem and it didn't.

Just a thought,

DanH said:

<snip>Will the OP have a problem? Depends on the details.

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It is a Bearhawk - high wing with good static pressure head on the pump. So, with recirculation back to tank, the op should not have any issue if radiation shielding is used. IMO.