Piston Engine Management

[mm_flynn]

I thought separating engine management from when do you reduce power after take off would make sense.
Islander2 is right. Most instructors in the UK haven't got a clue about engine management. In fact I have never met a single one who had a clue.
Easing back on the RPM is going to be better for the engine and bystanders and less harmful on airspeed/climb rate than reducing manifold pressure
Firstly, I have never seen evidence that lower RPM is really better for the engine. I would think it would be, as would most people. But if an engine is running at a given fuel flow, and is set to peak EGT or LOP (when the power generated is proportional to the fuel flow) the power output will be more or less fixed. So, if you drop the RPM with the RPM lever, the CSU sets the prop to a coarser pitch (in order to absorb the available power into the prop) and the stress on the crankshaft and con-rods increases. Now, which is better - lower RPM or lower stresses? I think it's a good question. A lot of engine stresses, particularly crankshaft stresses, are of dynamic origin i.e. are caused by the inertia of various parts rather than by the stresses of combustion. But if something is actually going to break, it will break through stress, not through any velocity of the component involved.
Secondly, one can't separate revs from power output, once in the airway levels. The engine is an air pump. When the inlet valves close, and the combustion starts, that process is isolated from the air pumping side, but any deficiency in the air pump means less air going in, and the need to maintain the right A/F ratio means less air = less fuel = less power. So for max power you need max revs, end of story. Particularly if the MP is down to b*gger-all, at FL160, you want max revs. A fully opened throttle just lets the air in, which is good, but if the engine can't suck, the air won't get in. If you reduce the RPM, you will get less power.

When rich of peak and WOT, setting a lower RPM will generate less horsepower as will setting a lower MP and no change in RPM. If you feel the need to climb at less than 100% power, reducing throttle will reduce fuel cooling and power, reducing RPM will reduce power but keep the WOT excess fuel cooling going and hence provide much better CHT management. Hence my comment RPM is a 'better' way of reducing power than MP when Rich of Peak.

Normally aspirated aircraft are going to be back to full throttle at c. 5000 feet anyhow just to maintain 25 in MP so you might as well keep to full throttle. The Deakin et. al. advice is set up for max continuous power and cruise climb so you get to TOC quickly and minimise the time with high CHT due to reduced cooling airflow in climb vs. cruise configuration.

[drauk]

The Deakin et. al. advice

Their advice is also that higher RPM is better for an engine than lower RPM (at equivalent power), because higher RPM results in lower cylinder pressures. This contradicts the common sense view, that making the engine turn slower means less wear.

[mm_flynn]

True, but when LOP the flame propagation slows and running at a bit slower RPM brings the peak power pulse back where it should be. In the max rpm to 85% max rpm range their data doesn't seem to show any worying increase in maximum pressure.

Specifically, in the TNIO-550 they specify economy cruise at 2200 and 15-15.5 GPH which is close to the 25-30 LOP limit they suggest. It also moves my CHT from 300 (15.5 2500) to about 340 (15.5 2200) which is probably reflective of the increase in pressure.

[Islander2]

In the max rpm to 85% max rpm range their data doesn't seem to show any worying increase in maximum pressure.The test stand data given out by Atkinson, Braly & Deakin at the Advanced Pilot Seminar I attended in Ada, Oklahoma showed a 7% increase (approx) in peak ICP when reducing from 2700 rpm to 2500 rpm at WOT/Full Rich. That certainly could be a worrying increase if the detonation margin has already been reduced by other factors!

Even so, they agree it should be SOP for good-neighbour reasons!

[IO540]

Not disagreeing with any of above.

The main point I was making was that for a given fuel flow, less RPM is going to mean more torque (since HP = torque * rpm).

Now, torque originates from the pressure in the cylinder, directly. So, as Drauk points out, lower RPM means more torque which means higher cylinder pressures.

I too reduce RPM from 2575 to 2500 after takeoff, for lower noise, once terrain clearance issues are out of the way. But this is almost incidental.

[bookworm]

Their advice is also that higher RPM is better for an engine than lower RPM (at equivalent power), because higher RPM results in lower cylinder pressures. This contradicts the common sense view, that making the engine turn slower means less wear.

That's not quite what Deakin says.

From this we can see that there must be a balance between slower/leaner, and faster/richer. For high power, maximum-performance operation, you should run richer mixtures and higher RPMs. For low power, maximum-efficiency operation, you should run leaner mixtures and lower RPMs.

As a rule of thumb, that's very sensible. And sure, we're talking about climb here, so we're likely to be at high power.

The key issue is what is tolerable for the engine in terms of cylinder pressure and cylinder temperature. I can't help thinking that temperature is a very much more important factor than pressure, just because of the way chemistry works. Small increases in temperature can make huge differences in physical and chemical processes.

It's very difficult without instrumentation to decide if a particular operating regime is bad for a particular engine. Even that 7% difference in ICP is difficult to assess -- sure, it could put the engine in a bad operating regime, or it might leave it in a perfectly safe one.

In the same way that everyone wants the lower specific fuel consumption associated with proper leaning provided it does no damage to the engine, I'd like to have the lower SFC associated with lower RPMs with the same proviso.

[bookworm]

BTW in case, like me, any of you hadn't noticed, John Deakin is writing columns again

http://www.avweb.com/news/pelican/

[englishal]

I prefer FADEC myself :)

[mm_flynn]

In the same way that everyone wants the lower specific fuel consumption associated with proper leaning provided it does no damage to the engine, I'd like to have the lower SFC associated with lower RPMs with the same proviso.

I have found the value of lower RPM in economy cruise to be high. In LOP operations so Power is directly propotional to fuel flow, I get 10-12 kts indicated airspeed higher at 15.5 gph 2200 vs. 15.5 gph 2500 - translating into 8% better mpg and still out of the red box (30 LOP).

Thanks for the link BW.

[IO540]

Interesting.

I have done a number of reasonably careful tests and cannot see any impact of varying revs 2200-2500 on the MPG, if I set the fuel flow to be a constant figure.

One would expect the reduced friction at lower rpm to surface somewhere, but it doesn't seem to.

If there is a difference it is less than 1-2 kt.

Maybe I didn't re-lean for peak EGT carefully enough at each new rpm setting. Must try this again sometime.

Piston efficiency varies in a strange way with rpm. Some, apparently, are more efficient at a higher rpm.

[bookworm]

Piston efficiency varies in a strange way with rpm. Some, apparently, are more efficient at a higher rpm.

There are a lot of different factors influencing the effect of RPM choice for a given power:

* frictional losses (Deakin provides numbers)

* pumping losses (high RPM => throttle more closed for same power => engine does more useless work)

* crank angle (direction and magnitude of effect depend on other engine parameters)

* prop efficiency (direction and magnitude of effect depend on all sorts of stuff!)

[mm_flynn]

Some key facts and assumptions
My engine is from Tornando Alley and operated with the recommendation of WOT at all climb and cruise operations.

15.5 gph is about 2 gph less than their 'Go Fast - 85% power mode' so it is well on the LOP side (maybe 150 degrees).

Because (like all of our engines) it has fixed timing and the flame front will be much slower this lean, the peak power pulse will happen much later than optimal. Therefore it is cool, but also probably less effective.


At 2200 RPM the amount of air being pumped per minute is less at the same fuel flow so I am closer to stoichiometric (closer to peak EGT) and the slow RPM means the peak pressure will be more in line with the optimum crank angle. Hence more efficient conversion of fuel to speed.

At least this is the logic I use to explain the efficiency improvement