Would like to understand logic behind hot start suggestions

[YankeeClipper]

I am not in need of suggestions for hot starting procedures--there are many posted indeed. But I want to understand those procedures and in turn better my understanding of the fuel injection system. My plane is an early model, and while the engines hadn't changed materially in this regard (to my knowledge) I know that some of the fuel plumbing may have. So if there are any substantive differences in the later models my diagram and logic here may not apply.

Looking at the attached, it looks like the throttle ("power") control is downstream of the mixture's on the fuel's journey to fulfillment. If that is so, why the suggestion (from some) to close the mixture fully, but open the throttle, when "priming" with the electric pump for a few seconds (the 15 or so often suggested)? Any setting after the mixture seems a moot point if you've closed the mixture.

[Learjetter]

Pretty good article on the system. There are others and YouTube videos on the topic as well.

https://www.avweb.com/ownership/cont...uel-injection/

Throttle = air
Mixture = fuel

You’re correct: priming with the electric pump does not result in fuel pressure or flow downstream of the throttle body if the mixture lever is in the cutoff position. It does ensure slightly less warm fuel is pressurizing the mechanical pump intake.

I’ll take a swing at it to start the discussion. I’m mostly a jet guy, but here’s my unlearned idea.

In many hot start situations, the problem (as I understand it, and I may be wrong!) is the liquid fuel in the divider and the nozzle lines becomes vapor-ed by ambient engine heat and isn’t effectively atomized under pressure at the nozzles…leading to an unburnable concoction in the intake. Shutting mixture off for a few spins of the prop while allowing full air into the cylinders “clears out” the cylinder so when fresh, less warm, liquid fuel is pushed under pressure to the divider when you push the mixture lever up, the result is a burnable concoction.

Essentially, by closing the mixture you’re ensuring no more warm/hot liquid fuel gets to the throttle body or the divider. The electric pump pressurizes the lines to the mechanical pump with cooler liquid fuel. And with the throttle full open you’re ensuring the max airflow into the cylinders to clear them out of whatever unburnable fog is in there. Then, while motor spinning, you open mixture and apply the choke by reducing throttle…and “bob’s your uncle” — she starts.

Usually. 60% of the time, it works every time.

Happy to be educated by folks that know more!
-LJ

[n86121]

The basic requirements are obviously the right ratio of fuel and air for whatever conditions need to be such that combustion will occur.

An old grizzled mechanic, (Danny Fragass, Clinto Aero MD) taught me what he uses on every aircraft that always works, hot cold or in between.

Steps and my thoughts on why it works.

1. Mixture and props forward.
The prop, obviously to keep starter load min, and it should be left that way
Mixture open so you can pressurize fuel system all the way to nozzles

2. Throttle closed
Starting point, ...read on

3. Elec boost pump on
Pressurize fuel system with cold fuel all the way to injectors.

4. Move throttle from closed to open slowly, then back to closed. Maybe 5 secs
Fills fuel system with slug of cool pressurized fuel
My shadin goes up to 17 gph or so at peak before going back to closed,

5. Elec pump off
Cool pressurized fuel is now in the entire fuel system ready for start

6. While hitting starter move throttle forward slowly.
Fires every time.

I BELIEVE
The early steps put a slug of cool pressurized fuel in the entire system, from tank to nozzles.
As the throttle is then slowly re-opened it always hits the right mixture to fire.

And fire she always does!

[YankeeClipper]

Thanks Lear-jetter.

1) Your Vapor-lock explanation checks out with any understanding I've had on the topic

2) Because of the return lines, I also get that at least upstream a bit you'll get cooler fuel primed and ready to feed further downstream once the mixture is opened again--at least if we can assume that tanks are cooler than hot engines. Hopefully we can.

3) Open throttle: this is where I don't understand the procedures. They do not say to rotate the engine in the closed throttle position (not the ones I've read here and elsewhere). They say to close the throttles to cracked before rotating the engine.

4) the relatively gradual opening of the mixture also makes sense, ensuring that you don't get a flooding condition.

[YankeeClipper]

"Move throttle from closed to open slowly, then back to closed. Maybe 5 secs"

So this procedure (different from the closed throttle ones I am confused about) starts to make more sense to me. Opening the throttle purges a sacrificial "slug" of hot and vaporized fuel from the lines, into the chamber, and pinches the line back off again just upstream of the divider.

However, given the potential of a somewhat flooded chamber (great or small, depending on circumstances), I would think it makes more sense to stay "cracked" as they always say, but pull the mixture back instead. Then start cranking, and advance the mixture to ensure a healthy firing when the needed mixture is arrived at. Using the throttle to do this would advance both fuel and air, and--I would think--not have the benefit of a lean beginning that transitions to a correctly enriched mix, facilitating a healthy fire up. In other words, as you start your crank with throttle cracked and mixture back, there would be a brief time to un-flood the chamber (assuming there is a flood, but no harm in a momentary lean condition in any case), and then start enriching with mixture advancement.