Full throttle or not?

[digits]

General question, but as an example, let's consider a continental IO470

At low altitude, during takeoff, one uses full throttle. As you move the throttle, you will see and feel the engine power increase. The last 25% of throttle movement doesn't increase power very much, but it puts more fuel in the engine to cool it down. During climb you reduce power, rpm and eveventually mixture.

At high altitude, eventually you will end up at full throttle for regular cruise. During that time, there is no noticeable fuel flow difference, or MAP or RPM difference if the throttle is fully open or at 75% of travel. Obviously the throttle cable did not get disconnected, so moving the throttle still does something.

Question: what's the difference at high altitude between 75% throttle and full throttle, and which would be better to use? Note that I'm talking about an altitude where there is *no* difference in power or fuel flow. Yet something is still moving in the engine if you move the throttle.

Thoughts?

[Slick Goodlin]

I suppose you could have the butterfly valve in the throttle going over centre with the last little bit, or probably diminishing returns from the geometry of the linkage.

Besides that, all other things being equal I’d take wide open over a bit closed. They say you get best SFC that way since the flow should be at its least turbulent. There’s apparently a crowd that cruises wide open and controls power solely by leaning but I doubt I’d have the guts.

[vanNostrum]

My uneducated guess

As the airplane climbs the airflow through the ventury tube diminishes even with the throttle fully open
Assuming the ventury tube is sized to allow maximum air flow at SL, at a high enough altitude
the reduced amount of air only needs 75% of the ventury tube cross section to achieve maximum flow
therefore the last 25% has no effect

[Colonel]

I like full throttle. I like it a lot.

How I configure the engine (for climb, cruise, descent, whatever):

1) set desired RPM with the prop control, then
2) set desired MP with the throttle (may very well be WOT)
3) set desired GPH fuel flow with the mixture control.

To confirm the above, look at the hottest CHT and ensure it is
below 400F but above 350F. Look at the biggest EGT difference
to confirm all cylinders are doing the same thing. Under 30F
EGT worst spread makes me happy.

I don't think the cylinders really care much about the throttle
position. What they see is the manifold pressure of the air at
the intake port, and the amount of fuel injected into it. They
don't give much of a shit about altitude, AFAIK. They do care
about airspeed and OAT, which distinctly affects cooling.

I used to fly the little four cylinder Lycoming IO-360 in the Mooney
M20J wide open all the time. It wasn't turbocharged, so the MP
was pretty low at altitude. I had to run the RPM up high (2600,
2700 continuously for hours) to develop any power, even at wide
open throttle. Then, lean the mixture and set the fuel flow that
gives you the engine temperatures that you want.

I remember flying non-stop like that from Thunder Bay to Ottawa
in four hours. You don't want to know the route I took.

I just keep the CHT below 400F and above 300F, and the oil below
200F.

PS RPM doesn't hurt an engine. I used to run the engine in Neil's
Pitts to 3,200 RPM. It loved it. What you have to watch out for, is
resonant RPMs - and avoid them.

BMEP isn't really much of a problem with the ridiculous 100LL we
run, combined with the ridiculously low compression ratios.

I think my CBR600RR runs 13:1 compression on pump gas.

NB the above is for FUEL INJECTED engines, which with GAMI
injectors, have reasonable fuel distribution. The same is NOT true
with a carburetor, which may benefit from all sorts of throttle plate
and carb heat flapper shenaigans, to try to even out the very bad
fuel distribution to the cylinders inherent in such a configuration.

Both carbureted and fuel injected engines benefit greatly from
engine monitors which have probes on all cylinders, and log that
value at least once a second. Without data, you are pissing in the
wind.

[digits]

To confirm the above, look at the hottest CHT and ensure it is
below 400F but above 350F. Look at the biggest EGT difference
to confirm all cylinders are doing the same thing. Under 30F
EGT worst spread makes me happy.

Not really related to my original question, but what are your thoughts if you can't get the CHT above 350F?

Last long distance flight around zero degrees C at 10k altitude, leaned and CHT was around 320F. Engine ran smooth though. Couldn't lean it further without power loss/engine roughness.

[Colonel]

Generally, your front CHT's are going to be lower than your rear CHT's
(see above data log) so it's a bit of a balancing act.

As long as your coldest CHT is above 300F, the lead is escaping the
exhaust port in gaseous form. You run it colder than that, you're going
to become an expert on the rope trick, to clean your exhaust valve guides
as per Lyc SI 1425A.

If you have only a single probe CHT, it's going to be on the HOTTEST cylinder
(middle or rear) and will not tell you what the colder front cylinders are seeing.

If you can't get your coldest CHT's above 300F, consider a restrictor kit (aka winter
kit) for your cowl air inlet, to reduce the airflow through the engine. That actually
reduces drag, too, which is how you go fast. Look at the old open cowling on ancient
Mooneys, and how tiny the round air inlets are now on the new cowlings - with bigger
engines!!

Note that Continental exhaust valves are solid and thus run hotter than
the hollow Lycoming sodium-cooled exhaust valves, and as such may
tolerate a slightly lower CHT without lead fouling the exhaust valve guides
than Lycomings. The colder Lycoming exhaust valve stems is better for their
life but act like a moonshine still, condensing the lead gas on them.

I really, really wish I didn't know about this stuff.

[digits]

Lots to consider, thanks. That was 320f on hottest cylinder, but I have an engine monitor and the chts were pretty much the same on each cylinder

[Colonel]

I admire your baffles and seals!

Back on topic ....

It doesn't matter much what angle the throttle is set, at least on a fuel injected engine.

What matters is what RPM, MP and GPH you select. That will result in CHT and oil temps
that you need to keep within limits. And ok BMEP, but no one knows what that is. CHT
and oil temp are pretty good proxies for BMEP. Pilots are all about proxies.

Note that your airspeed is a lot more important to your engine, than the throttle plate angle.

See, the airspeed determines the mass of air that's going to enter (and cool) your cylinder
fins and oil cooler. And ok, the OAT (second order effect) and third order, air density.

Your air-cooled engine really cares about the air cooling, ok? Every time I drop the nose
and speed up to 200 mph on approach, I can see the CHT's sag. This is real. I am not making
this up. See the data log above. If you want to worry about "shock cooling" your engine, worry
more about your airspeed than the angle of the throttle plate.

One of the best ways I know, to "shock cool" an engine, is to level off after a climb with
the engine stinking hot, and immediately accelerate to cruise speed in that cold air aloft
with the cowl flaps left wide open.

Look at the data log above. Note that after touchdown, the engine is "shock heated" and the
CHT's pop up and the oil temp will climb to the highest of the flight, because that engine is HOT
and it now has almost zero airflow through it during taxi, compared to inflight.

I suppose people will tell you that you should never level off in cruise, and you should never
land, either, to avoid "shock cooling" your engine. It's probably a good thing most people
fly turbines, eh?