Fuel pump/filter heat shielding

[stringfellow]

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.

[airguy]

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

[DanH]

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.

[rcpaisley]

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

[rv6ejguy]

Originally Posted by rcpaisley

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

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.

[upperdeck]

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

[airguy]

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?

[DanH]

Originally Posted by rcpaisley

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.

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.

[rv6ejguy]

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.

[DanH]

Originally Posted by rv6ejguy

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.

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.