REFLECTOR:IO360 to TSIO520

[reflector@tvbf.org]

> Does the return line get air in it? Or do you mean that it 
> help to keep air from getting in to the fuel pickup line?
> 

I am not sure about if this applies to automotive systems or not but ...

On the TCM's as the fuel passes through the engine compartment it is under pressure and it picks up a lot of heat as it passes through the engine driven fuel pump and the fuel controller which are both hot from being connected to or close by the hot engine.  

Then the return line empties into the sump or the tank and the pressure comes off all at once. The rapid drop in pressure of the *heated* fuel can cause vapor bubbles to form. In extreme cases vapor lock occurs! Total engine stoppage. This problem is aggravated by 100 degree temps on the ground in the summer time. 

This is true on all TCM fuel injected engine but not Lycoming. TCM and Lycoming used different FI systems. In fact, Lycomings do not even have a fuel/vapor return.

The older 210's have the sumps under the floor so this is an old problem for them. Older Bonanza's have the vapor return line going into the left tank so they rarely (if ever) have any engine problems because of bubbles. (it does makes fuel management tricky for drivers of the older Bo's though because while you are feeding from the right tank the left tank gets fuller! :) Also if you have a fuel totalizer in an old Bo you may have only burned 30 gallons of 40 available in the right tank but it's empty because the other 10 gallons got 'returned' to the other tank.) 

> I am installing fuel injected LT1 V8 that require a return 
> line. I put the return down at the bottom of the sump. 
>
> Should this be changed ?

Well, I don't *know* for certain on your system how much heat the fuel will pick up going through the engine compartment. 

However, if it was my plane I would change it! As a true-blue  experimenter and being the first person to use a particular engine setup in that airframe you are going to discover unique new problems in the course of shaking out that installation. No need for you to experience common old problems too! :)

When I was still planning to use the Mazda engine in my plane I plumbed return lines to each strake tank. Now I did that so I could use them for a aux fuel system AND vapor return- to get a twofer- but had I not put them directly into the strakes I for sure would have gone into the TOP of the sump tank. 

YMMV though! 

Rob

PS.  The same heat pick that causes vapor in flight is also why TCM engines can be such an unholy bear to hot start! 
After shut down the engine driven pump heat soaks and gets much hotter than it ever does with the engine running and air moving around it and a steady flow of incoming cool fuel. 

When it heat soaks and the steady flow of "fresh" cool fuel stops all the fuel boils out of the pump. 

Then when you try to start up the fuel flashes into vapor on contact with the hot fuel pump. You can't get it pressurized enough to prevent it from vaporizing. 

The solution? If you know you are facing a hot start on a TCM then as soon as you get into the plane set the throttle wide open and the mixture to IDLE cut off and kick on the boost pump (high setting if you have it).  Then go about getting belted in, getting the headsets on etc., etc. The normal prestart up stuff. 

After at least 60 seconds of letting the boost pump run do a normal start. 

Why does that work? With the throttle open and mixture closed the fuel controller sends ALL the incoming fuel back down the return line. Circulate 20 GPH of cool fuel through the engine driven pump and fuel controller for 60 seconds or more and you can cool them down enough that fuel does not flash into vapor on contact!