Revisiting Lycoming IO-360 hot starts, POH vs Lycoming

[Corky Scott]

I've brought this issue up previously but cannot seem to get it out of
my head. The hot start procedure, as written in the Cessna POH (model
172S) don't make sense to me, and doesn't, in my admitedly limited
experience, work very well either.

Here are the steps as taken directly from the POH (Note, I'm quoting
from the Cessna information handbook used during PPL training and is a
part of the training kit. While Cessna states that it is not actually
the POH, I could find no difference between what I quote below and the
procedure I used in the Cessna POH in the airplane itself)

*********Begin Quote**********

STARTING ENGINE (With Battery)

1. Throttle -- OPEN 1/4 INCH.
2. Mixture -- IDLE CUT OFF.
3. Propeller Area -- CLEAR.
4. Master Switch -- ON.
5. Auxiliary Fuel Pump Switch -- ON.
6. Mixture -- ADVANCE until fuel flow just starts to rise, then return
to IDLE CUT OFF position.
7. Auxiliary Fuel Pump -- OFF.

NOTE

If engine is warm, omit priming procedure of steps 5, 6 and 7 above.

*****End of Quote******

Here's the problem: Engines require fuel, air, compression and
ignition in order to run. Omitting steps 5, 6 and 7 removes fuel from
the equation.

Since the POH specifies that you to stop the engine by pulling out the
mixture control to Idle/Cut off, thus starving the engine of gas, and
you are told NOT to push the mixture in during the hot start, where
does the fuel come from to start the engine?

I thought that was a good question. So I called Textron Lycoming and
spoke with their technical support staff and asked them. They wanted
to know which airplane I was talking about. I told them it was from
the POH of a late model Cessna 172 (there were actually two Cessna
172's I trained in, both POH's give exactly the same starting
procedure). The tech guys said their operator's manual doesn't
describe the hot start that way. They said they recommend pushing the
mixture in to full rich and running the auxiliary boost pump for 3 to
5 seconds, just enough to give the engine something to run on, then
shut off the boost pump and pull the mixture back to idle/cut off and
crank it. Shove the mixture in to full rich when it starts, of
course.

We talked a little bit more about the POH procedure I described to
them and they confirmed that that procedure, as stated, would make it
difficult to start the engine.

Corky Scott

[C J Campbell]

Post by Corky Scott
I've brought this issue up previously but cannot seem to get it out of
my head. The hot start procedure, as written in the Cessna POH (model
172S) don't make sense to me, and doesn't, in my admitedly limited
experience, work very well either.
Here are the steps as taken directly from the POH (Note, I'm quoting
from the Cessna information handbook used during PPL training and is a
part of the training kit. While Cessna states that it is not actually
the POH, I could find no difference between what I quote below and the
procedure I used in the Cessna POH in the airplane itself)
*********Begin Quote**********
STARTING ENGINE (With Battery)
1. Throttle -- OPEN 1/4 INCH.
2. Mixture -- IDLE CUT OFF.
3. Propeller Area -- CLEAR.
4. Master Switch -- ON.
5. Auxiliary Fuel Pump Switch -- ON.
6. Mixture -- ADVANCE until fuel flow just starts to rise, then return
to IDLE CUT OFF position.
7. Auxiliary Fuel Pump -- OFF.
NOTE
If engine is warm, omit priming procedure of steps 5, 6 and 7 above.
*****End of Quote******
Here's the problem: Engines require fuel, air, compression and
ignition in order to run. Omitting steps 5, 6 and 7 removes fuel from
the equation.

The Cessna's engine driven fuel pump does an adequate job of starting the
engine when it is warm. I get a nasty backfire if I try to start a hot
engine in any way other than what the Cessna POH recommends.

[Corky Scott]

On Tue, 27 Jul 2004 08:14:29 -0700, "C J Campbell"

Post by C J Campbell
The Cessna's engine driven fuel pump does an adequate job of starting the
engine when it is warm. I get a nasty backfire if I try to start a hot
engine in any way other than what the Cessna POH recommends.

So the question then is: Where does the fuel come from to start the
engine if the mixture remains at idle/cut off and no priming has
occured?

Sure the engine is cranking and the engine driven fuel pump is
working, but the mixture is at the same position that caused the
engine to stop running when you shut it down. The fuel pump was also
working then, but the engine quit because it starved for gas.

Nothing has changed except that you now want it to run again, but as
far as I can see, there's no gas to make it run because the mixture is
still in the cutting off fuel position.

Yet the engine often DOES start running with this procedure so the
fuel must be coming from somewhere. Where does it come from?

Thanks, Corky Scott

[Jeremy Lew]

You don't use the primer on a hot carbureted engine either (at least the one
on my club's Warrior). I always wondered why too, and read something
recently about there being fuel vapor still in the cylinders. The
explanation stopped there.

Post by Corky Scott
I've brought this issue up previously but cannot seem to get it out of
my head. The hot start procedure, as written in the Cessna POH (model
172S) don't make sense to me, and doesn't, in my admitedly limited
experience, work very well either.
Here are the steps as taken directly from the POH (Note, I'm quoting
from the Cessna information handbook used during PPL training and is a
part of the training kit. While Cessna states that it is not actually
the POH, I could find no difference between what I quote below and the
procedure I used in the Cessna POH in the airplane itself)
*********Begin Quote**********
STARTING ENGINE (With Battery)
1. Throttle -- OPEN 1/4 INCH.
2. Mixture -- IDLE CUT OFF.
3. Propeller Area -- CLEAR.
4. Master Switch -- ON.
5. Auxiliary Fuel Pump Switch -- ON.
6. Mixture -- ADVANCE until fuel flow just starts to rise, then return
to IDLE CUT OFF position.
7. Auxiliary Fuel Pump -- OFF.
NOTE
If engine is warm, omit priming procedure of steps 5, 6 and 7 above.
*****End of Quote******
Here's the problem: Engines require fuel, air, compression and
ignition in order to run. Omitting steps 5, 6 and 7 removes fuel from
the equation.
Since the POH specifies that you to stop the engine by pulling out the
mixture control to Idle/Cut off, thus starving the engine of gas, and
you are told NOT to push the mixture in during the hot start, where
does the fuel come from to start the engine?
I thought that was a good question. So I called Textron Lycoming and
spoke with their technical support staff and asked them. They wanted
to know which airplane I was talking about. I told them it was from
the POH of a late model Cessna 172 (there were actually two Cessna
172's I trained in, both POH's give exactly the same starting
procedure). The tech guys said their operator's manual doesn't
describe the hot start that way. They said they recommend pushing the
mixture in to full rich and running the auxiliary boost pump for 3 to
5 seconds, just enough to give the engine something to run on, then
shut off the boost pump and pull the mixture back to idle/cut off and
crank it. Shove the mixture in to full rich when it starts, of
course.
We talked a little bit more about the POH procedure I described to
them and they confirmed that that procedure, as stated, would make it
difficult to start the engine.
Corky Scott

[Corky Scott]

Post by Jeremy Lew
You don't use the primer on a hot carbureted engine either (at least the one
on my club's Warrior). I always wondered why too, and read something
recently about there being fuel vapor still in the cylinders. The
explanation stopped there.

That depends. What is the carburated engine starting procedure? Do
you leave the mixture at idle/ cut off when cranking for the hot
restart? Or do you move it off the stop to allow some gas to make it
to the cylinders while the engine cranks?

Corky Scott

[Jeremy Lew]

No, mixture is full rich for a hot start, which does seem more logical.
Mixture at cutoff and full throttle for a flooded engine start.

Post by Corky Scott

Post by Jeremy Lew
You don't use the primer on a hot carbureted engine either (at least the one
on my club's Warrior). I always wondered why too, and read something
recently about there being fuel vapor still in the cylinders. The
explanation stopped there.

That depends. What is the carburated engine starting procedure? Do
you leave the mixture at idle/ cut off when cranking for the hot
restart? Or do you move it off the stop to allow some gas to make it
to the cylinders while the engine cranks?
Corky Scott

[Jeremy Lew]

This guy has a theory:
http://www.cardinalflyers.com/l.cgi?b=/pub/ops/startrg.htm

Post by Corky Scott

Post by Jeremy Lew
You don't use the primer on a hot carbureted engine either (at least the one
on my club's Warrior). I always wondered why too, and read something
recently about there being fuel vapor still in the cylinders. The
explanation stopped there.

That depends. What is the carburated engine starting procedure? Do
you leave the mixture at idle/ cut off when cranking for the hot
restart? Or do you move it off the stop to allow some gas to make it
to the cylinders while the engine cranks?
Corky Scott

[Corky Scott]

Post by Jeremy Lew
http://www.cardinalflyers.com/l.cgi?b=/pub/ops/startrg.htm

Thanks Lew. The author (unknown) is writing about the Cessna
Cardinal. The engine and starting/running procedure is virtually
identical to the Cessna 172 I use.

He does have an explanation as to why, after shutting off the engine
using the mixture control, there would still be fuel left for the
engine to run. The mixture control box is on top of the engine, as
are the fuel injection lines. The mixture control controls the box
where the fuel lines disperse to the injection nozzles at each
cylinder. So when you pull the mixture to idle/cut off, you shut off
the fuel going to the distribution box, but the lines are still full
of fuel, at that moment.

The injectors stop injecting because the fuel pressure is shut off, so
the engine quits running for lack of fuel. If the engine is hot,
however, as it inevitably is, and because the fuel lines are on top of
the engine and heat rises, the heat now acts like a hot plate beneath
these fuel lines. With no air blowing across the top of the engine to
keep things cool, the fuel inside the lines now expands and often
boils.

The expanding fuel cannot back up into the mixture control box, that's
been shut off, but the injectors aren't closed, they are just
calibrated orifaces, so the fuel pushes out through them.

THAT'S why there is fuel to restart if you attempt a hot restart
within a certain period, even though you have the mixture control at
Idle/Cut off, and have kept it there since you shut the engine down.

Once you shut the engine down, then, it's a given that the injectors
will bleed out as the latent engine heat cooks the fuel injector
lines. The only question is how much fuel has been pushed out before
you get back in the cockpit and attempt a start.

It could be that the lines from the distribution box have been bled
dry and that the fuel that bled out into the combustion chamber has
evaporated. In that case, attempting to start without a prime may
result in no joy.

With Continentals only, the fuel injection system incorporates a fuel
return line. With these engines, you can run the boost pump for a
minute while the mixture is at idle/cut off, and flow cool fuel
through the injector lines. Then you shut off the boost pump and
start the engine. The fuel line has been pressurized with relatively
cool fuel, so the engine driven fuel pump isn't pushing air (or at
least not as much air) through the injectors.

Here is my understanding of the Lycoming fuel injection system.

There is an engine driven fuel pump that pressurizes the fuel to the
design in-line pressure. This pressurized fuel is then routed to the
mixture and fuel distribution control box. The control box is
controled both by the mixture control and the throttle. The mixture
control varies the mixture from the preset full rich ratio to idle/cut
off, which should be zero pressure.

The throttle controls the fuel pressure at the control box and varies
it from idle pressure to a pressure that allows the engine to develop
full power, depending on the altitude.

So if you lean the mixture significantly at idle, and then advance the
throttle to full, the engine will not/should not respond because the
fuel pressure will not reach full power levels. The air valve will
open, allowing full air flow, but the control box limited fuel
pressure won't allow enough fuel to flow to reach full power. In
fact, because there is now a LOT of air and very little fuel, the
engine will stumble and probably cut out on the now very lean mixture.

I hope that's accurate information.

Corky Scott

[Corky Scott]

On Thu, 29 Jul 2004 11:52:58 -0400, Corky Scott

Post by Corky Scott

Post by Jeremy Lew
http://www.cardinalflyers.com/l.cgi?b=/pub/ops/startrg.htm

Thanks Lew.

Shoot, sorry Jeremy.

Corky Scott

[Paul Sengupta]

Post by Corky Scott
Since the POH specifies that you to stop the engine by pulling out the
mixture control to Idle/Cut off, thus starving the engine of gas, and
you are told NOT to push the mixture in during the hot start, where
does the fuel come from to start the engine?

http://groups.google.co.uk/groups?hl=en&lr=&ie=UTF-8&threadm=bmm3vt%24k8%241%40newstree.wise.edt.ericsson.se&rnum=2&prev=/groups%3Fq%3Dpaul%2Bsengupta%2Bidle%2Bcut%2Boff%26hl%3Den%26lr%3D%26ie%3DUTF-8%26selm%3Dbmm3vt%2524k8%25241%2540newstree.wise.edt.ericsson.se%26rnum%3D2

http://makeashorterlink.com/?D1BE339E8

or here's one I prepared earlier:

http://groups.google.co.uk/groups?q=paul+sengupta+idle+cut+off&hl=en&lr=&ie=UTF-8&selm=cb9fgp%24aep%241%40newstree.wise.edt.ericsson.se&rnum=1

http://makeashorterlink.com/?C5CE259E8

Paul

[Cockpit Colin]

The priming occures at the point where the electric pump is on and you
advance the mixture. The only (obvious?) step missing is that the mixture
must again be advanced to full once the engine fires.

A technique we use is ...

[COLD START ONLY]

MIXTURE FULL RICH
THROTTLE FULL OPEN
PUMP ON UNTIL FUEL FLOW METER MOVES THEN PUMP OFF
THROTTLE 1/2 INCH
MIXTURE ICO
CRANK
ADVANCE MIXTURE WHEN ENGINE FIRES.

[HOT START]

THROTTLE OPEN 1/2 INCH
MIXTURE ICO
CRANK
ADVANCE MIXTURE WHEN ENGINE FIRES

As a rule of thumb, if you prime it when it's warm you're dead :(

[WARM START] (Say 3/4 hour)

Try Hot start but stop cranking every 3 or 4 blades (often it will fire as
you stop cranking for reasons I've never sussed out) - if all else fails a
quick prime followed by a LONG flooded procedure if no joy.


As a little side note - next time you're in a carburated engined aeroplane
try shutting down the engine with Mixture @ ICO and then when it's died,
crank it again with the mags on and the mixture STILL at ICO - many will
fire up - do a few hundred RPM - then die again. Kinda blows the theory that
the reason we shut down an engine via Mixture @ ICO is to make it safer in
the event of an inadvertant live mag.

Cheers,

CC