Leaning and Fuel Injection

[uwochris]

Hey guys,

Got some Qs regarding mixture control and fuel injection. These come after reading the "mixture magic" article on avweb (pelican's perch).

1. Let's say I'm flying at 9,000 ft and I've leaned to peak EGT. If I change my power, will I have to relean it or enrichen it again? I always thought it would be necessary to enrichen your mixture with power increases (more power= more air= more fuel needed). Or, if its a fuel injection engine, will the fuel air control unit take care of this? Does it differ for carb engines?

2. Why is it that fuel injection engines pose more problems with hot starts? What makes them more prone to fuel vaporization? ie. why is it that it is recommended to wait 45 minutes to restart a fuel injected engine after shutdown?

3. The author of the article suggested that if you have problems trying to start a hot fuel injected engine, you should turn the fuel pump on and bring the mixture to ICO for 60-90 seconds- how does this do anything? I understand that if the mixture was rich you would certainly flood the engine, but if the mixture is at ICO, can you still get fresh cool fuel flowing thru the lines (i.e if the mixture control is at ICO, I thought all fuel flow is stopped?) I've never heard of this technique before and I found it odd.

4. Why is it that when starting the plane, if its a cold start, the mixture control is full rich, and if it's a hot start, the control is at ICO? There's probably a simple answer to this one which I am completely overlooking.

Also, if anyone read the article, what do you think of it? Do you agree with his recomendations and opionion? I really liked his article on Manifold Pressure and Props, but this one was a little tougher to follow.

Thanks in advance.

 

[rfeathe1]

If you are leaned to peak at any altitude you have to re-lean when you change the power setting. Hot starts on FI engines can be a problem because the " spider" ( the little gizmo on top of the engine with the FI lines running out of it) is subject to lots of residual heat after the engine is shut down. This has a tendency to boil the fuel in the spider & lines & cause them to vapor lock. Running the boost pump with the mixture in ICO will circulate cold fuel through the spider & back out through the return line which cools most of the fuel system. On a cold start a rich mixture is desireable because a cold engine needs more fuel to get started.

 

[uwochris]

Thanks for the response.

It just seemed weird for me... I know that fuel injected engines have a "fuel control unit" whose job is to adjust the fuel/air ratio with power changes- I thought that this device would imply that there would be no need to relean or enrichen with power changes... maybe I am misinterpreting the FCU and wha it does???