How long for a hot start

[M20F]

One has never lived till they have flown with pressure carbs….

[Vance Harral]

3 hours ago, EricJ said:

Don Maxwell made a vid

The first engine start in that video that takes place within the first minute is interesting - the one he calls "kind of a dirty start".  Note that after that first start he "runs it up" and "clears it out", as he does in every subsequent run.

I don't think all the other starts in that video are interesting.  The amount of time the engine is stopped in these subsequent demos is about 20 seconds, 30 seconds, 60 seconds, 5 seconds, and 15 seconds.  He talks about fuel "percolating" in the flow divider during those shutdowns, but he's not not actually waiting long enough for that to happen.  The fresh fuel flowing through the system when he's running the engine and "clearing it out" cools the pump, flow divider, lines, etc.  Once the engine is stopped, it takes several minutes for the hot cylinders to convectively reheat the cooled components and the fuel inside them, and cause the problem we're trying to solve.

In summary: good description of the problem, good discussion of theory, but this circus trick of restarting the "hot" engine just a few seconds after shutting it down isn't very relevant to the hot start problem - it's just indirectly telling you not to let the problem occur in the first place.

 

[Bobaran]

Early on with my 67 Mooney M20F and its IO360 engine, I was told by the previous owner to hot start in this manner:  Throttle forward all the way (open), and with mixture fully back (closed) crank it until it catches and push mixture in pulling throttle back at the same time.  While this might work at times, it doesn't seem to be good for the engine. This causes an unnecessary engine surge to very high RPMs . 

After watching the Maxwell video mentioned in this post on page 4, that is what I follow:

1. Throttle at idle position. 

2. Mixture (red knob) all the way back (closed).

3. Turn the starter key. (duh)

4. After a turn or two of the prop, Crack (open)  the Mixture slightly (push in 1/8 inch or slightly more).

5. When the engine starts, slowly push (open) the Mixture (red knob) toward the engine (about an inch or a bit more) to a smooth idle.  If you push the mixture in too far, the engine might stop (I'm not sure why, but maybe from getting too much fuel?),  If the engine does stop, repeat the above.  I personally keep my mixture very lean during idle and during taxi per the A&P videos I've watched from Savvy Aviation.

Battery Note:  You need a good battery to hot start your IO360.  I ditched my wet cell lead battery for the earthX battery ETX900-TSO-35-M20 STC.   This battery gives me all the high amperage juice I need to get a great prop spin.  If I don't get it started on the first try, I know I will have the same amperage on the next try.  Also the battery provides a long supply of  electricity for instruments if the engine alternator ever decides to quit during flight.  While the initial cost of the earthX TSO package is around $900, I have saved so much time, money, and aggravation from old lead acid battery dying and needing recharging (I am not an employee or earthX, nor do I sell or distribute earthX batteries).   

 

[skykrawler]

20 minutes ago, Bobaran said:

Early on with my 67 Mooney M20F and its IO360 engine, I was told by the previous owner to hot start in this manner:  Throttle forward all the way (open), and with mixture fully back (closed) crank it until it catches and push mixture in pulling throttle back at the same time.  While this might work at times, it doesn't seem to be good for the engine. This causes an unnecessary engine surge to very high RPMs . 

 

The wide open throttle/mixture off is really a flooded start.   Which is what happens during a shutdown that is long enough to percolate the fuel out of the divider and lines into the intakes.   It doesn't take much practice to poise your hand on the mixture and shove it in when the engine fires and move left and immediately pull out the throttle.  I quit trying to start a hot IO-360 any other way because it's that reliable.

[Bobaran]

2 hours ago, skykrawler said:

The wide open throttle/mixture off is really a flooded start.   Which is what happens during a shutdown that is long enough to percolate the fuel out of the divider and lines into the intakes.   It doesn't take much practice to poise your hand on the mixture and shove it in when the engine fires and move left and immediately pull out the throttle.  I quit trying to start a hot IO-360 any other way because it's that reliable.

Thanks, for the information.   I agree that it is pretty easy to pull back the throttle in that start procedure.  That may be the only way some people can perform the hot start.   I've done it and it worked (sometimes for me).  I was a frustrated with this procedure because I would often run out of battery strength after a start failure (I was new to IO360 hot starts).  My lack of good hot starts may have been incorrect mixture position.  It never worked for me to keep the mixture closed waiting for the engine to catch.  Battery drop off was great in continuing to try some way to start the engine.  Anyway hot starts on an IO360 is a big topic, not just for Mooneys. So thanks again for the info.   My posted method works very well for me now and seems to be easier on the engine (is it really easier on the engine?.....I don't know). I agree that  the full throttle method can work for people too.

[Shadrach]

1 hour ago, skykrawler said:

The wide open throttle/mixture off is really a flooded start.   Which is what happens during a shutdown that is long enough to percolate the fuel out of the divider and lines into the intakes.   It doesn't take much practice to poise your hand on the mixture and shove it in when the engine fires and move left and immediately pull out the throttle.  I quit trying to start a hot IO-360 any other way because it's that reliable.

To be clear, shutting down does not flood the engine.  If we are going to use an analogue for the fuel that boils into the intake after shutdown, it would priming.  No properly rigged Lycoming is flooding itself on shutdown. They will typically start in a few blades provided a ham-fisted pilot has not flooded the engine by unnecessarily priming. 

[Shadrach]

On 8/28/2023 at 1:18 PM, Bobaran said:

Thanks, for the information.   I agree that it is pretty easy to pull back the throttle in that start procedure.  That may be the only way some people can perform the hot start.   I've done it and it worked (sometimes for me).  I was a frustrated with this procedure because I would often run out of battery strength after a start failure (I was new to IO360 hot starts).  My lack of good hot starts may have been incorrect mixture position.  It never worked for me to keep the mixture closed waiting for the engine to catch.  Battery drop off was great in continuing to try some way to start the engine.  Anyway hot starts on an IO360 is a big topic, not just for Mooneys. So thanks again for the info.   My posted method works very well for me now and seems to be easier on the engine (is it really easier on the engine?.....I don't know). I agree that  the full throttle method can work for people too.

There is no magic procedure that works under all circumstances.  There is a simple, methodical, process that starts before shutdown.  That process is best complimented by an understanding of the what is happening under the cowl. It goes something like this:

1) Engine set to 1000rpm at shutdown ICO.

2) At restart, if the engine is hot, fuel was pushed into the manifold. The engine should be treated as if it is primed and ready to start. Hit starter and enrich if/when engine fires.

3) If the engine does not fire or quits before the mixture can be enrichened, there is likely no longer adequate fuel in the intake to support combustion.  A brief 2 count shot of prime should be adequate to deliver a combustible mixture to the intake.

4) Rinse and repeat being conservative with the prime until the engine starts.

It's a rarity that anything beyond #3 is needed.   

[Vance Harral]

Generally agree with everything Ross said, but one extra subtlety to understand is that if the hot engine has effectively primed the cylinders by "boiling" fuel out of the lines from (and in some cases to) the flow divider, then you have "bubbles" (vapor bubbles and/or air) in the lines between the fuel servo and the injectors, which contain no fuel.  As fresh fuel flows from the tanks, through the pumps, and out toward the engine, these bubbles are pushed into the injectors, resulting in a temporarily incombustible mixture due to lack of fuel.  The hope is that when that happens, your engine has developed sufficient momentum to keep turning, until more fuel moves through the lines to again create a combustible mixture.  This is why even a successful hot start involves a certain amount of coughing and sputtering.

When it doesn't work out, and the engine fires but subsequently dies, that doesn't necessarily mean there was anything incorrect about your technique.  The control mechanisms you have (throttle and mixture) operate on the fuel servo, not the intake ports of the cylinders.  So anything you do with throttle and mixture during a hot start is removed in time/space from the immediate problem at the injector ports.  This is one reason having a strong battery and starter help with hot starts: they increase the momentum of the engine during start, making it more likely to keep turning while the system works its way though the bubbles.  I also sometimes turn on the fuel pump if it sounds like the engine is about to die, but it's unclear to me how much that actually helps.

[skykrawler]

When the engine is not running, fuel moving though the lines is strictly a function of available pressure and the throttle and mixture knob position.

Aside from distributing fuel, the fuel divider serves the purpose of shutting off fuel flow to the injectors when the pressure from the servo drops below a certain level required for idle.  It also serves to support idle because the air flow through the servo at idle is too low to regulate pressure.  So once the pressure drops and the divider closes - conceptually the only fuel percolating is in the injector lines.

Whether the fuel line from the servo and the divider itself also percolates and develops enough pressure to open the divider (for very long) is an open question to me.

If I restart within 10 minutes of shutdown sometimes I'll try it closed throttle.  But only once.

[Shadrach]

On 8/28/2023 at 3:36 PM, Vance Harral said:

Generally agree with everything Ross said, but one extra subtlety to understand is that if the hot engine has effectively primed the cylinders by "boiling" fuel out of the lines from (and in some cases to) the flow divider, then you have "bubbles" (vapor bubbles and/or air) in the lines between the fuel servo and the injectors, which contain no fuel.  As fresh fuel flows from the tanks, through the pumps, and out toward the engine, these bubbles are pushed into the injectors, resulting in a temporarily incombustible mixture due to lack of fuel.  The hope is that when that happens, your engine has developed sufficient momentum to keep turning, until more fuel moves through the lines to again create a combustible mixture.  This is why even a successful hot start involves a certain amount of coughing and sputtering.

When it doesn't work out, and the engine fires but subsequently dies, that doesn't necessarily mean there was anything incorrect about your technique.  The control mechanisms you have (throttle and mixture) operate on the fuel servo, not the intake ports of the cylinders.  So anything you do with throttle and mixture during a hot start is removed in time/space from the immediate problem at the injector ports.  This is one reason having a strong battery and starter help with hot starts: they increase the momentum of the engine during start, making it more likely to keep turning while the system works its way though the bubbles.  I also sometimes turn on the fuel pump if it sounds like the engine is about to die, but it's unclear to me how much that actually helps.

Agree with the above. Sometime it just doesn't start after it fires. Being too timid getting the mixture in can make for missed start as well.  I think that a healthy mechanical fuel pump does a decent job of pressurizing the system even at starter RPM.  My engine will start from cold pretty easily in the cooler months just by turning the key with the mixture full rich and no prime.  It takes a four or five blades to go from zero fuel pressure to start. Properly primed on the other hand, is like letting go of the prop on a rubber driven balsa model.

[Shadrach]

On 8/29/2023 at 9:08 AM, skykrawler said:

When the engine is not running, fuel moving though the lines is strictly a function of available pressure and the throttle and mixture knob position.

Aside from distributing fuel, the fuel divider serves the purpose of shutting off fuel flow to the injectors when the pressure from the servo drops below a certain level required for idle.  It also serves to support idle because the air flow through the servo at idle is too low to regulate pressure.  So once the pressure drops and the divider closes - conceptually the only fuel percolating is in the injector lines.

Whether the fuel line from the servo and the divider itself also percolates and develops enough pressure to open the divider (for very long) is an open question to me.

If I restart within 10 minutes of shutdown sometimes I'll try it closed throttle.  But only once.

I do not think that fuel behind the flow divider pushes through. If it did, pressure in the servo would drop. My servo holds pressure long after the fuel from the lines has noisily burbled into the manifold.

[jaylw314]

On 8/24/2023 at 3:41 PM, Bobaran said:

"...fuel in the lines above the motor warms up and pushes out the injectors into the intake manifold, so you have a mildly flooded intake.   Since the throttle is mostly closed, it can hang around for a surprisingly long time, even a couple hours."  

This explains why the hot engine can start even though the mixture is closed or slightly open.  I had no idea of WHY this would work until now.  Thanks Jay.

People have mentioned before that the burbling you can hear after shutdown is some of the fuel seeping out of the fuel nozzles.  Not 100% sure if it's just that, but I suppose if you hear that it'd be a sign if you startup thereafter to try your hot start technique first

[jaylw314]

On 8/26/2023 at 12:37 PM, Vance Harral said:

The first engine start in that video that takes place within the first minute is interesting - the one he calls "kind of a dirty start".  Note that after that first start he "runs it up" and "clears it out", as he does in every subsequent run.

I don't think all the other starts in that video are interesting.  The amount of time the engine is stopped in these subsequent demos is about 20 seconds, 30 seconds, 60 seconds, 5 seconds, and 15 seconds.  He talks about fuel "percolating" in the flow divider during those shutdowns, but he's not not actually waiting long enough for that to happen.  The fresh fuel flowing through the system when he's running the engine and "clearing it out" cools the pump, flow divider, lines, etc.  Once the engine is stopped, it takes several minutes for the hot cylinders to convectively reheat the cooled components and the fuel inside them, and cause the problem we're trying to solve.

In summary: good description of the problem, good discussion of theory, but this circus trick of restarting the "hot" engine just a few seconds after shutting it down isn't very relevant to the hot start problem - it's just indirectly telling you not to let the problem occur in the first place.

That all makes sense to me, but if no fuel seeps after short time like that, how could the engine catch again with the mixture closed?  I haven't rewatched the video, does he actually give it mixture before the engine catches?