maxbc wrote:
I don’t think the engine health has anything to do with that. If anything it means the engine is “pulling” too much, so shouldn’t be a problem.
Exactly. You need to look at it differently whether it is NA or TN/TC/SC .
IN all cases, a WOT condition will derive the minimum pressure loss through the throttle, but it will never be zero. I have not read what looks like a good AOPA mag article, (thx for posting) but 1" or around is normal. This will be zero with a stopped engine, of course, but once you have airflow, there will be some pressure loss, as with any fluid going through a pipe.
On NA it is suction form the cylinders that causes the airflow, so there must be lower MP than atmospheric press or else the flow would not occur.
On TN/TC/SC the compressor will increase pressure vs atmospheric, and at WOT (depending on the control system on each aircraft, not that easy, some are manually controlled and you can only only be WOT when at altitude), there may be higher pressure than atmospheric both upstream and downstream of the throttle, and it is the compressor that “pulls” air from the induction air filter and atmosphere, rather than the cylinders.
There will still be a pressure loss from atmospheric as air flows through the air filter and associated ducting , so that pressure at the compressor inlet will be a bit lower than atmospheric. This pressure loss will be amplified by the turbo, which . in electronic terms, works mostly as an amplifier of whatever comes in.
At high speed, say 160KTAS, and 8000ft, a quite optimal NA cruising altitude, a forward-pointing inlet will see almost 1" dynamic pressure which, when added to atmospeheric pressure of about 22" is a significant power boost. Hence a lot of aircraft are designed with such type of inlet (Mooneys M20J’s famously have an openable ram air inlet) .
If TC, then the turbocharger will amplify such dynamic pressure causing a significant boost.
On aircraft we have sensors for atmospheric static pressure (altitude) , dynamic pressure (IAS) , and on some, engine upper deck pressure (or compressor outlet upstream of the throttle) as well as MP (downstream of the throttle). A little graph below of our last take-off for your perusal:
Ok, so in fact just 1.4 inches of difference is not so bad.Thx for the article, I’ll give it a read when less busy… probably this night before to Zzzzz.
greg_mp wrote:
You meant that on a normal day you don’t have such a loss without the turbo kicking in
That means that I see practically the same as any normally aspirated engine if the Turbo is not operated. So on hot conditions MP can be a lot lower than with engine shut down, several inches. During takeoff I think I get one to two inches more due to ram air.
Theoretically the manifold pressure at full throttle should be the same as when the engine was at rest (actual atmospheric pressure), but in reality it will be somewhat less. With full throttle on the ground the manifold pressure on a naturally aspirated engine will be approximately 2 to 3 inches below what the gauge indicated with the engine at rest. This difference is due to air drag across the throttle and through the induction system. In flight, the intake ram effect will reduce this drop to approximately one inch on well-designed systems.
From here:
Book: Manifold Pressure – Aero Resources Inc
I gave it a full review and have to say it is quite a good article on this topic!
An engine with a convoluted air intake tract will present a greater resistance to intake air flow, and therefore will have greater pressure drop between ambient pressure and measured MP.
A normally aspirated (non-turbo) engine in good condition will have lower full throttle manifold pressure than the same engine if it were malfunctioning. The malfunctioning engine will conversely have a higher full throttle manifold pressure, because it is incrementally closer to an engine that is shut down completely, when the engine has the highest possible (ambient) manifold pressure.
You meant that on a normal day you don’t have such a loss without the turbo kicking in (but tubing still here)?
greg_mp wrote:
you can’t really have this information as any loss is compensated by the turbo
Yes I do, it’s a manually operated installation, and I do a typical takeoff without turbo. Of course if I need it (hot, high, heavy) I use it. But the tubing for air is a bit special on my engine, and that is why the manual states explicitely that a loss of MP is normal.
However, there is always a loss in MP against ambient pressure, already due to the fact that the engine sucks in on intake. But it shouldn’t be so noticeable.
I don’t think the engine health has anything to do with that. If anything it means the engine is “pulling” too much, so shouldn’t be a problem.
Even at full throttle there might remain a slight narrowing in the throttle which causes a small depressurization. There might be another narrowing upstream (in the intake), the air filter may create drag too. This might also simply be the pressure gradient itself, which creates the airflow into the engine, although it seems a bit high.
As a data point I remember on a hot day (in flight) the MAP barely reached recommended cruise setting of 24.7" at full throttle (while atmospheric pressure even on hot days is pretty much always > 29").
Still standing, FT on the brakes.
What I’d like to understand is the part of natural losses of the induction system and what could be attributed to the engine wear. As you have a TN engine, you can’t really have this information as any loss is compensated by the turbo…
Still standing or including ram air? Mine has to have more than an inch less on hold, but with ram air it gets a bit more. However my case is a bit special (turbonormalized engine).
