How long for a hot start

[Sue Bon]

4 minutes ago, ArtVandelay said:

Since we’re talking about hot starts, oil is hot, and all internal parts are covered with oil, so I don’t see any reason to limit starting RPMs, staying within reason of course. If you ever do a flooded start you’ll exceed that 1100 by a lot.
I think higher RPMs help stabilize the engine since your pumping more fuel and cooling air and therefore cooling the fuel lines. I’m not talking 2000+, but 1200 seems enough. I give it a few seconds to settle down and then reduce power to 1000.

Yes, I agree. The thing about the hot start for me is that my throttle is relatively open, almost half way. As soon as the engine catches and I go to full rich I have to be mindful to pull back on the throttle to not let the rpms go wild. I was taught 1100 but I'm not at all saying I meet that number every time. With a cold start, the throttle is not as open so it's easier to coax the engine to the 1000-1200 sweet spot.

[Guest]

3 hours ago, M20F said:

Full throttle, no mixture, fire, insert mixture, jiggle fuel pump a bit after mixture insertion if it starts to die.  Has yet to fail me unless I get a little slow on the uptake (miss the mixture or the pump if needed).  
 
When in doubt though just flood it.  Most folks wait far too long to flood and kill their battery/starter.  If you don’t get it by say third time, flood it. 
 
@carusoam is correct that outside temp has a big impact.  When I was basing up north it was a slightly different plan in the winter.  Now that I am in Dixie pretty much hot start whenever it flies and I need to retire. 

The starter and battery are the least of the concerns.  The entire time people are grinding away trying to start their engine the cam and lifters get no oil.

[Pinecone]

1 hour ago, Alex M said:

I always thought this was more related to Oil Temperature rather than pressure. You don't want to run the engine at a high RPM if it's cold and the oil is super thick as it's harder to get that oil to coat all surfaces, better to run at 900-1000rpm until the Oil Temps warm up. I think that same principle would apply with starting at a high RPM. 

FYI my 252 POH says 1000 - 1200 for warmup.

[jaylw314]

13 hours ago, Vance Harral said:

Due respect to Don, but the only hot start in that video is the first one, which he describes as "kind of a dirty start".  Every subsequent start he demonstrates is uninteresting, because the problems that cause hot starts require a couple of minutes or so to develop.  Shutting down the engine for 30 seconds and then restarting doesn't give enough time for the problems jaylw314 describes just above to actually develop.  Those only occur after the heat from the cylinders has a little while to "cook" the fuel in the lines into vapor.  The way I teach it, if you shut down at the pump, fuel up, and restart right away, it's generally not a hot start.  If you also need to pee and do it quickly, still not really a hot start.  If you get a cup of coffee, chat up the desk staff at the FBO, etc., you're looking at a genuine hot start.

I suspect Don was just trying to demonstrate the technique a few times in a short video.  He doesn't strike me as the kind of guy who films himself all the time like young people these days.  He might have even thought the camera would run out of film quickly!   I'm just kidding, if you ever read this, Don 

It's interesting the technique still works even just a few seconds after shutdown, it suggests there's enough dribbling from the injectors immediately afterwards.

1 hour ago, Sue Bon said:

Yes, I agree. The thing about the hot start for me is that my throttle is relatively open, almost half way. As soon as the engine catches and I go to full rich I have to be mindful to pull back on the throttle to not let the rpms go wild. I was taught 1100 but I'm not at all saying I meet that number every time. With a cold start, the throttle is not as open so it's easier to coax the engine to the 1000-1200 sweet spot.

It's probably less important if the engine ran earlier, since it had a chance to throw oil everywhere inside.  Still, I think most here would agree it is good form not to let the engine go HAM on startup  

FWIW, once it catches, the motor will continue spinning for 5-6 seconds after catching, even after it runs out of fuel.  As long as you push the mixture in before the prop stops completely, the motor will continue to run, you generally don't have to flail at the mixture control.  The only gotcha with a hot start is that there may be enough vapor in the lines that it take a couple seconds for fuel to actually get to the injectors, but if the motor does stop, now you know you just burned off all the fuel in the intake and can do a normal cold start.  

Also FWIW, when I do push the mixture in on a cold or flooded start, I only push it in about 1" from cutoff.  I know from experience I'm going to lean it about there for taxi anyway, and the fuel lines are full.

Edited August 22, 2022 by jaylw314

[Vance Harral]

3 hours ago, PT20J said:

It's called the controlled quantity start. It takes fuel and air in the proper proportions to ignite. The idea is to keep the air constant while controlling the amount of fuel. This is done by opening the throttle somewhat and leaving it fixed during the start.  The exact amount of throttle isn't important (I open it about a quarter inch), but what is important is to not touch it during the start. I start cranking with the mixture in ICO. If there is enough raw fuel in the lines and cylinders (as in a restart soon after shut down) it will start in ICO and then I advance the mixture to keep it running and adjust the throttle to set the idle speed.

The difficulty with this method in horizontally opposed engines that have the fuel injector lines running right across the hot cylinders, is that in the classic hot start scenario, the mixture knob isn't really directly controlling the amount of fuel going into the cylinders.  The presence of vapor in the fuel lines effectively adds high frequency randomization into the mixture control loop.  In any given second, an individual injector line may be delivering liquid fuel or it may be just delivering vapor into the cylinder.  So regardless of whether you're modulating the mixture to try to match fuel to a fixed amount of air, or modulating the throttle to try to match air to the fuel being delivered, it's difficult to maintain a combustible mixture because the fuel delivery isn't stable.

What we hope for is a scenario in which that fuel/vapor variability in the lines is small enough that it doesn't deviate much, such that slowly increasing the throttle or mixture or whatever gets the engine running.  That will be the case if you start just a few minutes after shutting down, or a long time after shutting down.  There is a period of time in between when the liquid fuel/vapor variability is large.  I never have any trouble getting the engine to fire in this condition, the problem comes a few seconds later with keeping it running.  That's when the circus act starts, involving guessing at throttle/mixture settings, running the boost pump, etc.

[Skyland]

Conversely, how about a really cold start - like below freezing.  I recall from years ago if the engine didn't start on the first try moisture drawn into the engine would frost over the plugs and subsequent attempts were nearly futile.  These were carburetor planes.  Can't recall issues with fuel injected but then again I rarely fly in those cold climates now.

[ArtVandelay]

Conversely, how about a really cold start - like below freezing.  I recall from years ago if the engine didn't start on the first try moisture drawn into the engine would frost over the plugs and subsequent attempts were nearly futile.  These were carburetor planes.  Can't recall issues with fuel injected but then again I rarely fly in those cold climates now.

I can’t imagine any frost on the plugs lasting for very long, the heat from the compressed air, low freezing temperature of avgas and the high voltage potential would melt the frost I think, unless we’re talking well below °0F. Not something I plan on testing.

[Ragsf15e]

53 minutes ago, Skyland said:

Conversely, how about a really cold start - like below freezing.  I recall from years ago if the engine didn't start on the first try moisture drawn into the engine would frost over the plugs and subsequent attempts were nearly futile.  These were carburetor planes.  Can't recall issues with fuel injected but then again I rarely fly in those cold climates now.

Ive started pretty cold, like 10f, but engine was preheated.  Longer prime because fuel doesn’t atomize as well in the cold air.  Otherwise no issues.

[Bolter]

3 hours ago, Sue Bon said:

Yes, I agree. The thing about the hot start for me is that my throttle is relatively open, almost half way. As soon as the engine catches and I go to full rich I have to be mindful to pull back on the throttle to not let the rpms go wild. I was taught 1100 but I'm not at all saying I meet that number every time. With a cold start, the throttle is not as open so it's easier to coax the engine to the 1000-1200 sweet spot.

On my J, the throttle was opened to just a millimeter or so past the "click" of the gear warning switch.  This applied to cold starts and hot starts.  At this opening, it is easy to modulate and keep the RPM's at a reasonable level without crazy antics. 

In my opinion, there is variability between our engines and planes, but I think that nearly 50% open is too much throttle for hot or cold starts

-dan.  

[ArtVandelay]

On my J, the throttle was opened to just a millimeter or so past the "click" of the gear warning switch.  This applied to cold starts and hot starts.  At this opening, it is easy to modulate and keep the RPM's at a reasonable level without crazy antics. 
In my opinion, there is variability between our engines and planes, but I think that nearly 50% open is too much throttle for hot or cold starts
-dan.  

On my J I use my calibrated index finger to set throttle. There’s confusion with POH, it says 1/4 forward, some take that as 1/4 travel, some take that as 1/4”

The flooded start with full throttle I don’t like due to crazy antics required.

[Sue Bon]

I'm heading out tomorrow on a two-day mission to Gibraltar. During the trip there and back home, I will have fuel stops in very hot places, and for varying amounts of time. I will check for a click and try starting at less throttle than usual. Thanks for the tips!

[ArtVandelay]

I'm heading out tomorrow on a two-day mission to Gibraltar. During the trip there and back home, I will have fuel stops in very hot places, and for varying amounts of time. I will check for a click and try starting at less throttle than usual. Thanks for the tips!

Also try opening your cowl flaps once you’ve put gear down and deployed 1st set of flaps. This will help cool the engine and fuel lines on landing.

[Bolter]

23 minutes ago, ArtVandelay said:

Also try opening your cowl flaps once you’ve put gear down and deployed 1st set of flaps. This will help cool the engine and fuel lines on landing.

That was part of my pre-landing check in the J, because I knew I would be either 1) taxi on ground or 2) making a go-round.  Both best done with cowl flaps wide open.  For some reason, some people have considered that be a controversial thing.   -dan

[jaylw314]

57 minutes ago, Bolter said:

That was part of my pre-landing check in the J, because I knew I would be either 1) taxi on ground or 2) making a go-round.  Both best done with cowl flaps wide open.  For some reason, some people have considered that be a controversial thing.   -dan

Probably has to do with the whole "shock cooling" thing, which wouldn't be an issue anyway that far into your landing anyway.  Although I have to say, there's still plenty of residual heat in the motor, so I imagine this doesn't help prevent the upper compartment from baking after shutdown, but I can't imagine it'd hurt.

[jaylw314]

2 hours ago, Sue Bon said:

I'm heading out tomorrow on a two-day mission to Gibraltar. During the trip there and back home, I will have fuel stops in very hot places, and for varying amounts of time. I will check for a click and try starting at less throttle than usual. Thanks for the tips!

OK, you probably have all of us beat if you're flying through Spain   And here I thought landing in 100F weather was bad!

[PT20J]

8 hours ago, Vance Harral said:

The difficulty with this method in horizontally opposed engines that have the fuel injector lines running right across the hot cylinders, is that in the classic hot start scenario, the mixture knob isn't really directly controlling the amount of fuel going into the cylinders.  The presence of vapor in the fuel lines effectively adds high frequency randomization into the mixture control loop.  In any given second, an individual injector line may be delivering liquid fuel or it may be just delivering vapor into the cylinder.  So regardless of whether you're modulating the mixture to try to match fuel to a fixed amount of air, or modulating the throttle to try to match air to the fuel being delivered, it's difficult to maintain a combustible mixture because the fuel delivery isn't stable.

What we hope for is a scenario in which that fuel/vapor variability in the lines is small enough that it doesn't deviate much, such that slowly increasing the throttle or mixture or whatever gets the engine running.  That will be the case if you start just a few minutes after shutting down, or a long time after shutting down.  There is a period of time in between when the liquid fuel/vapor variability is large.  I never have any trouble getting the engine to fire in this condition, the problem comes a few seconds later with keeping it running.  That's when the circus act starts, involving guessing at throttle/mixture settings, running the boost pump, etc.

The reason it quits is that it is running out of fuel. To keep in running, leave the throttle alone and advance then mixture control.

There is a valve in the flow divider that shuts off fuel from the servo to the injectors when the metered fuel pressure drops in order to ensure a clean shutdown. The engine quits when the flow stops. There is a small amount of fuel trapped in the injector lines and it will boil out from the heat of the engine compartment and move to the intakes of the cylinders. This is why the engine will start immediately after shutdown with the mixture in ICO, but it will stop as soon as this small amount of fuel is exhausted, so you must advance the mixture control to keep it running. After a hot engine sits a few minutes, this fuel completely evaporates and now you will need to advance the mixture control during cranking to get fuel flowing allowing the engine to start. 

The engine cannot burn liquid fuel -- it must be vaporized. The fuel comes out of the injectors continuously in a stream about the diameter of a #2 pencil lead. This fuel stream vaporizes when it hits the hot cylinder head components. When the engine is cold, this doesn't happen. Priming injects extra fuel into the cylinder and the heat of compression vaporizes enough of it to allow a start.

Skip

 

[MikeOH]

I've found that, on a hot day, if I'm stopping for less than an hour or so, leaving the oil door open helps quite a bit when it comes time to start up.  A lot of heat will exit from the opening and draw in cool outside air from the cowl flaps.

[carusoam]

11 hours ago, Sue Bon said:

I was just taught not to let the engine rev over 1100 rpm during any start. Apparently it's not good for the camshaft...???

+1 for oil distribution…

Too slow… the engine doesn’t get the splash of oil inside the case…

To fast… dry surfaces can get red hot…

If the engine is started weekly… there is very low chance of having a dry surface on any parts…

So… a good procedure whether the engine has been started recently, hot or cold…

I use 1krpm…

If the start procedure exceeds this value… bring the MP back quickly… while looking at the OilP gauge… if there is no OilP…  that is a long term storage issue… not a hot start thing… 

PP thoughts only, not a mechanic…

Best regards,

-a-

[carusoam]

+1 for opening the oil access door, and park facing the wind… adding cowl flaps can help… the O doesn’t have any to open…

Best regards,

-a-

[Vance Harral]

2 hours ago, PT20J said:

The reason it quits is that it is running out of fuel. To keep in running, leave the throttle alone and advance then mixture control.

Again, advancing the mixture control during a hot start doesn't necessarily introduce a well-controlled amount of fuel into the system.  For one thing, the new supply of liquid fuel that must pass through the still-very-hot lines between the divider and the injectors can itself be pre-vaporized (prior to reaching the injector) by the lingering heat.  For another, heat from the engine doesn't only soak the lines between the divider and the injectors.  Given enough time, it can also soak the lines from the fuel servo to the divider, and the fuel servo itself, such that advancing the mixture doesn't immediately introduce any liquid fuel into the line between the servo and the divider.

In a Continental engine, vaporized lines and orifices in the fuel servo can be cooled by running the boost pump with the mixture set in ICO, to circulate cool fuel from the tanks, through the fuel servo and back through the return lines to the tanks.  In Lycomings, the design of the Bendix RSA system has no return line, hence running the pump with the mixture in ICO doesn't do anything useful.

 

2 hours ago, PT20J said:

There is a valve in the flow divider that shuts off fuel from the servo to the injectors when the metered fuel pressure drops in order to ensure a clean shutdown.

That's news to me, but makes sense, and I guess limits the amount of fuel that can "percolate" into the cylinders as those lines cook.  Thanks for that piece of information, I learned something new.  Doesn't seem to make starts after 10-15 minutes of sitting any easier, though. 

[Ragsf15e]

17 hours ago, PT20J said:

The reason it quits is that it is running out of fuel. To keep in running, leave the throttle alone and advance then mixture control.

There is a valve in the flow divider that shuts off fuel from the servo to the injectors when the metered fuel pressure drops in order to ensure a clean shutdown. The engine quits when the flow stops. There is a small amount of fuel trapped in the injector lines and it will boil out from the heat of the engine compartment and move to the intakes of the cylinders. This is why the engine will start immediately after shutdown with the mixture in ICO, but it will stop as soon as this small amount of fuel is exhausted, so you must advance the mixture control to keep it running. After a hot engine sits a few minutes, this fuel completely evaporates and now you will need to advance the mixture control during cranking to get fuel flowing allowing the engine to start. 

The engine cannot burn liquid fuel -- it must be vaporized. The fuel comes out of the injectors continuously in a stream about the diameter of a #2 pencil lead. This fuel stream vaporizes when it hits the hot cylinder head components. When the engine is cold, this doesn't happen. Priming injects extra fuel into the cylinder and the heat of compression vaporizes enough of it to allow a start.

Skip

 

Interesting that you say it won’t start after a few minutes as the fuel is evaporating?

My normal hot start is to leave mixture in cutoff, throttle roughly 1/4”, crank.  It will start.  Obviously you need to apply mixture quickly after it starts to keep it going.  I use that even after several hours.

[PT20J]

2 hours ago, Ragsf15e said:

Interesting that you say it won’t start after a few minutes as the fuel is evaporating?

My normal hot start is to leave mixture in cutoff, throttle roughly 1/4”, crank.  It will start.  Obviously you need to apply mixture quickly after it starts to keep it going.  I use that even after several hours.

The fuel should be all vaporized out of the lines after probably 30 minutes to an hour. If not, then it is getting replenished. This is usually caused by leakage past the mixture valve in the servo. The valve is just a flat plate that rotates to cover or uncover holes in the valve . The surfaces are lapped during manufacture/overhaul but are not designed to be a perfect seal. All that is necessary is that it cuts the fuel flow enough to shut down the engine at ICO. The RSA spec is less than 5 cc/minute leakage. Some are better, some may be right at 5. It's possible that one that has been installed for a time might even have more leakage and you'd never notice unless the engine doesn't shut down cleanly with the mixture control.

After shutdown, the fuel lines from the pump to the servo heat up and form vapor which creates pressure at the servo inlet. This can cause leakage past the mixture plate and up into the flow divider which may be enough to add fuel into the lines to the nozzles. This is also why some engines drip from the sniffle valve. 

My factory rebuilt IO-360-A3B6 came with an AvStar servo rather than an RSA (low bidder). I notice that the AvStar ICO is very tight. I don't get any drip from the sniffle, and after a few minutes sitting, a hot engine will not start in ICO -- I have to advance the mixture control slowly while cranking to get it to start.

Skip

[Shadrach]

On 8/22/2022 at 6:01 AM, M20Doc said:

I believe that many of these hot starting issues are related to condition of the ignition system and the engine. Weak spark plugs, weak and mis-timed magnetos etc, weak battery, weak starter etc.  My IO-720, an engine reputed to have hot starting challenges, starts fine hot or cold.

Agree. I also think some of these problems are caused by owners over complicating a very simple process. 
 

I wish people would stop thinking in terms of hot versus cold and look at it is primed versus unprimed. 
Many “hot start” issues are self induced.

[PT20J]

1 hour ago, Shadrach said:

Agree. I also think some of these problems are caused by owners over complicating a very simple process. 
 

I wish people would stop thinking in terms of hot versus cold and look at it is primed versus unprimed. 
Many “hot start” issues are self induced.

Great point, Ross.

Here’s how the RSA manual describes it. 

 

[Fly Boomer]

On 8/21/2022 at 4:03 PM, gwav8or said:

After a few attempts we used Don Maxwells procedure and got it started.

I think it was Mike Bush who said “if it’s warm, you probably over-primed, while if it’s cool, you probably under-primed”.