Engine power related question for the experts

[hevansrv7a]

I have been struggling with the way we estimate BHP. This is just a small piece of the puzzle.

If I understand it correctly, manifold pressure (MP) is a pure pressure measurement just as is the pressure altitude. But, the temperature of the ambient air can vary considerably. Therefore, the density altitude of a given stream of air at a given MP can vary as much. The amount of oxygen in a cubic foot of air is the same for a given density altitude, but not necessarily for a given pressure altitude.

Engines use oxygen. We control the physical mixture of fuel and air (which is roughly 22% oxygen and varies very little). Thus we can use mixture control to get a (for example) best power mixture at any time. My operating assumption is that the ratio of oxygen to fuel (by weight) for best power is pretty constant, especially at high power settings.

The oxygen available thus controls the amount of fuel that can be used beneficially for best power and the density altitude controls/describes the amount of oxygen available. But we keep using MP because we don't have anything better. Or do we?

Well, perhaps not in flight. However, from the comfort of my desktop, I can compensate for the temperature of the incoming ambient air and come to the conclusion that if the temp is lower the oxygen available is higher and put a number to that.

So that means that I can more closely estimate the BHP at a given combination of RPM and MP by compensating for the difference between PA and DA.

A] Is this what the power charts from Superior and Lycoming are talking about? (1% per 15 degrees F or similar)
B] Is this a correct analysis? Is this a correct solution?
C] What if anything is the relationship of this issue to the "fact" that engine power for a given MP x RPM is greater at higher altitude by a factor of roughly 1% per 1,000 feet?

Thanks in advance for your contributions!

[rv6ejguy]

I think this was discussed a few months back and the basic conclusion was that we don't have enough information available to us in flight given the monitoring equipment usually installed. Additionally, we'd need an accurate baseline established on a dyno for our exact engine to even begin to extrapolate hp or % power with any reasonable accuracy.

As a short summary here are some of the things that affect power:

Air/fuel ratio- best power is usually obtained in the 12.5-13.0 to 1 range. We don't have any instrumentation to tell us this in most aircraft. EGT data is skewed by IAT, rpm and power setting so is not an accurate way to set best power AFR except under one set of conditions.

Volumetric efficiency- This varies with rpm although only slightly on most direct drive aircraft engines within the narrow range we generally operate in. Induction or exhaust system changes may affect this to a fairly large degree from standard baseline engine data.

MAP- Most people understand this relationship that higher charge density (given ideal AFRs)= higher power.

RPM- Most people understand that the higher charge mass we can process through the engine in a given amount of time= more work/ more hp. This is offset by falloff of VE and higher frictional losses at some point. RPM can also affect VE in terms of resonant tuning effects from the induction and exhaust systems.

IAT- Intake air temperature. It should be noted that this really should be measured at the intake port, especially on engines with sump heated induction tubes. IAT in this case is affected more at low rpm/ low MAP conditions.

Exhaust back pressure- As altitude increases, exhaust back pressure is reduced. This theoretically reduces charge dilution during valve overlap as well as engine pumping losses. Very difficult to quantify without tests in an altitude type test cell.

Humidity- Higher percentages of water vapor in the charge reduces the amount of fuel and oxygen available and therefore reduces power.

Frictional losses- These will vary with rpm and even to a small degree with oil temperature/ viscosity.

Ignition timing- With fixed timing, hp results may be more predictable. With EI systems potentially varying spark timing with both rpm and MAP, standard assumptions may be invalid.

It could be pretty easy to be off 3-5% in typical operation and a lot more if we got a worst case stack up of parameters in one direction. Again, I'd stress that using data from a standard Lycoming chart could be way off if your engine is a clone and has different intake and exhaust, ported heads and electronic ignition as far as a raw hp figure is concerned. As a % of max hp, calculated values could be somewhat closer but these would be hard to apply to drag/ thrust calculations as true hp is not known.

[breister]

About the only way you can "know" horsepower is to run LOP, and multiply fuel flow (in pounds per hour) times the conversion factor for LOP hp (the smart guys know the number, I've forgotten). That is supposed to be accurate.

You could, if you were very clever, "back into" the max hp by performing timed climbs at a fixed airspeed - first at LOP then at best power. You will need to know your gross weight and the formula for calculating expected climb rate. It will take some algebra, but comparing the two climbs should yield the full-power hp rating relative to the LOP.

[elippse]

You've got to know that any formulaic method of arriving at horsepower will only yield an estimate unless you can accurately measure both torque and rpm. But so what? Do you really need to know exactly how much power your engine is putting out? Sure, if you're an engine designer or modifier and you want to tell people how well your product is doing. Even the manufacturers must only report HP to -0% to +5%. So if you have the MAP and induction temperature and pressure and humidity and rpm and the engine charts and you can estimate your power to 5%, that's great! Why do you need any level of accuracy greater than that?

[hevansrv7a]

I really appreciate these answers and for the most part I agree with them. But, all I was asking was about the difference that ambient temperature makes when MP is equal.

[elippse]

Originally Posted by hevansrv7a

I really appreciate these answers and for the most part I agree with them. But, all I was asking was about the difference that ambient temperature makes when MP is equal.

sqr rt(273.16 + 15 / 273.16 + OAT C?)

[hevansrv7a]

Originally Posted by elippse

sqr rt(273.16 + 15 / 273.16 + OAT C?)

Applied to the HP or to the MP before looking up or computing HP?

[elippse]

To the calculated HP. This formula, which is in the upper-left corner of the engine HP-MAP-RPM curves shows that dropping the temperature 5.2F increases the power 1% due to increased charge density, but decreasing the temperature 5.2F decreases the engine efficiency 0.5%, so the net result is that the engine power increases 1% with a drop of 10.4F.