[Alan Carroll]

I was playing with the power chart that comes in the Lycoming O-360 users manual, and noticed an interesting thing that maybe someone who understands engines can explain. I expected that the power output would always be the same at the same density altitude, since the air density is the same. However, this turns out to not be true. Consider the following examples at 6000' density altitude:

1. Pressure alt. 4000', inlet temp. 76?F, density alt. 6021'. Available MP is 24.8" (std. pressure minus 1" of loss through induction system). Following instructions on chart I get 156 hp.

2. Pressure alt. 8000', inlet temp 1?F, density alt. 6022'. Available MP is 21.3" (same assumptions as above). The chart now says I have 146 hp, even though the air density is unchanged.

Kevin Horton's spreadsheet for the O-360 gives about the same answers, 156 and 144 hp respectively. I believe the spreadsheet is based directly on the chart. The IO-360 chart shows a similar difference.

These examples are fairly extreme, but not unrealistic for Wisconsin summer vs. winter. In practical terms the hp differences are fairly small, but they could be enough to make a difference when testing airframe modifications designed to increase TAS (especially if the test happen at different times of year).

 

[hevansrv7a]

a quick guess

Available MP depends on pressure, not density. That's what the P in MP stands for. Available oxygen, however, depends on density (assuming equal water vapor content, etc.).

Kevin's and many other similar formula's use MP and RPM, disregarding DA.

Kevin?

 

[plehrke]

when testing airframe modifications designed to increase TAS (especially if the test happen at different times of year).

Remember TAS is a function of Temp and density altitude as well. If you put in your numbers and you have an indicated airspeed of150 kts both will give a TAS of 164 kts.
Check it out
http://www.paragonair.com/public/aircraft/calc_TAS.html

Your question though is that you have more power. That is why most curves show performance with several % power curves vs density altitude. You will get a curves that looks like this.

 

[hevansrv7a]

A thought about testing

...

These examples are fairly extreme, but not unrealistic for Wisconsin summer vs. winter. In practical terms the hp differences are fairly small, but they could be enough to make a difference when testing airframe modifications designed to increase TAS (especially if the test happen at different times of year).

If you are testing airframe mods, BHP is perhaps a little less important than THP. Yes, I know we don't have a way to measure THP. But perhaps now we do. Follow the last link below to see.

Your BHP varies with MP and RPM but it also varies with altitude (higher gives greater BHP) and with mixture. Lastly, although it may not be huge, the propulsive efficiency (in an RV, mostly comprised of propeller efficiency) will vary with altitude, CAS and RPM.

For observing drag reduction, most pilots just use equal density altitude at full throttle, best power and a 3 or 4 way GPS series of observations. That's good, but perhaps it suffers from the problems you've noted. A good look at the drag curve with the new methods I described may help.

BTW: if you use equal RPM, equal MP and equal DA will you then get equal BHP?

 

[Steve Brown]

Convert to kelvins

I think power output should be inversely proportional to the inlet temperature in kelvins
kelvins = deg C + 273

Full power should be at 15C standard temperature, so can be normalized to 288k

 

[AlexPeterson]

snip

BTW: if you use equal RPM, equal MP and equal DA will you then get equal BHP?
[/SIZE][/FONT]

Almost. DA is directly related to how many molecules there are per cubic foot (only the oxygen molecules matter, however, but their percentage is the same). MP is how much of this particular density of air is going in per charge. If the DA is the same, the prop should be "biting" the same amount of air.

But, the lower the exhaust pressure, the more work that will be extracted from the engine with those three variables held constant.

To answer the question, I think with those three variables constant, more power will be generated at higher pressure altitudes.

There may be an effect from a reduction of the pressure inside the engine as well, but it isn't immediately obvious to me the mechanism, or which way it takes things.

 

[Alan Carroll]

Available oxygen, however, depends on density (assuming equal water vapor content, etc.).

DA is directly related to how many molecules there are per cubic foot (only the oxygen molecules matter, however, but their percentage is the same). MP is how much of this particular density of air is going in per charge.

This gets to the essence of my question. I would expect that the engine power depends on how many molecules of oxygen enter the engine during each cycle. This number depends on both temperature and pressure, not just manifold pressure alone. Multiple combinations of temp and pressure can give the same density. However, the chart indicates that they don't give the same HP.

To answer the question, I think with those three variables constant, more power will be generated at higher pressure altitudes.

Makes sense, except this is the opposite of what the chart says. On a high, colder than standard day (8000' PA, 6000' DA) the chart says you'll get 146 hp. On a low, warmer than standard day (4000' PA, 6000' DA) the chart says 156 hp.

I realize that DA also has an effect on drag and therefore TAS, but for any given DA an increase in available hp should cause an increase in TAS. The chart doesn't say whether its giving BHP or THP. It does use Kelvin as the basis for the temp. comparison (although it expresses it in ?F which is a bit strange).

 

[AlexPeterson]

snip



Makes sense, except this is the opposite of what the chart says. On a high, colder than standard day (8000' PA, 6000' DA) the chart says you'll get 146 hp. On a low, warmer than standard day (4000' PA, 6000' DA) the chart says 156 hp.

snip

But, the MAP's are different per your first post. The hypothetical is that MAP, rpm and DA stay the same.

 

[Kevin Horton]

I dug into this question a few years ago, but never really came to a completely satisfactory conclusion.

If you vary the temperature, with everything else unchanged, the air density varies inversely with the absolute temperature. You would think that engine power should also vary in the same way, so that power was directly proportional to air density. But, the Lycoming power charts show the same temperature correction I have seen on every other reciprocating engine power chart - they show the power varying inversely with the square root of the absolute temperature. I did some digging years ago, and this relationship was found during testing of the early aero engines - you can see it in some of the very early NACA reports. They have lots of test data, so the effect is real. This same temperature correction is the SAE standard for automotive spark-ignition engines too. Interestingly enough, the SAE correction for compression ignition engines is a bit different. They show the power varying as (T_std / T) ^ 0.7, whereas for spark-ignition engines it is (T_std / T) ^ 0.5.

Early NACA Reports:

• NACA Report 190, Correcting Horsepower Measurements to a Standard Temperature, published in 1925, presents some test data, and proposes methods to correct engine power to standard conditions. They claim that theoretical calculations of volumetric flow rate through an orifice at a constant pressure drop show that the volumetric flow rate varies with temperature. The combination of the change in volumetric flow rate with temperature, and the change in air density with temperature results in the power varying inversely with the square root of the temperature.
• NACA Technical Report 171, Engine Performance and the Determination of Absolute Ceiling, published in 1924, provides a different explanation. They claim that there is constant exchange of heat from the induction tract to the induction air as it flows from the inlet to the intake valve. Thus the variation in air temperature at the inlet valve is smaller than the variation in atmospheric air temperature.

If you look at predicted power for standard temperature at those altitudes, you'll see that the vast majority of the "discrepancy" is due to the temperature correction. If the power did vary inversely with the temperature the calculated powers for these two cases would be quite similar.

Many folks think that the TAS you achieve will be the same as long as you keep the density altitude the same. They will vary the test altitude to achieve a target density altitude. This would work wonderfully well, if engine power actually varied with air density. But, as the Lycoming power chart tells us, it doesn't.

 

[scsmith]

use the part-throttle power charts for comparison?

Alex is right - the first-order effect on power comes from manifold pressure. So any comparison must begin there. Then, higher Density Altitude means warmer at the same pressure.

So the comparison you want to do can be made with the part-throttle power charts in your Lycoming manual.

Pick one condition as full throttle, standard day. Then for comparison, pick a hot day and find the lower pressure altitude that will give the same density altitude. Since the pressure altitude is lower, you have more manifold pressure, so you close the throttle a bit until the manifold pressure matches. Then compare power.

I did this a while ago and forget the exact result, but IIRC, the lower altitude, part-throttle case will have slightly less power. Some people call this throttling loss, but I think it really comes from the higher exhaust exit pressure for the same manifold pressure, and same induction charge density.

 

[AlexPeterson]

Air viscosity?

The viscosity of air is another variable that might be noticeable. 90F air vs 0F air might make a bit of difference through the induction system and valves. At 0F, the kinematic viscosity is 1.26 (ft^2/s) x 10^-4, while at 90F it is 1.74. It would seem to bias it towards the side of cold having even more power output. I need to ponder this one a bit.

 

[David-aviator]

I was playing with the power chart that comes in the Lycoming O-360 users manual, and noticed an interesting thing that maybe someone who understands engines can explain. I expected that the power output would always be the same at the same density altitude, since the air density is the same. However, this turns out to not be true. Consider the following examples at 6000' density altitude:

1. Pressure alt. 4000', inlet temp. 76?F, density alt. 6021'. Available MP is 24.8" (std. pressure minus 1" of loss through induction system). Following instructions on chart I get 156 hp.

2. Pressure alt. 8000', inlet temp 1?F, density alt. 6022'. Available MP is 21.3" (same assumptions as above). The chart now says I have 146 hp, even though the air density is unchanged.

Kevin Horton's spreadsheet for the O-360 gives about the same answers, 156 and 144 hp respectively. I believe the spreadsheet is based directly on the chart. The IO-360 chart shows a similar difference.

These examples are fairly extreme, but not unrealistic for Wisconsin summer vs. winter. In practical terms the hp differences are fairly small, but they could be enough to make a difference when testing airframe modifications designed to increase TAS (especially if the test happen at different times of year).

It seems, from a purely physics point of view, there is a disconnect concerning the HP difference at a density altitude of 6022' as depicted at 4000' and 8000' pressure altitudes.

A given volume of atmosphere at a given density should contain the same number of oxygen molecules no matter the temperature and pressure, why would there be a variance in HP? All the engine knows is oxygen and fuel, i.e., F/A ratio.

Perhaps the engine is not turning at the same rpm at the 2 altitudes.

As it is stated, the chart conclusion regarding HP makes no sense.

 

[Kevin Horton]

It seems, from a purely physics point of view, there is a disconnect concerning the HP difference at a density altitude of 6022' as depicted at 4000' and 8000' pressure altitudes.

A given volume of atmosphere at a given density should contain the same number of oxygen molecules no matter the temperature and pressure, why would there be a variance in HP? All the engine knows is oxygen and fuel, i.e., F/A ratio.

Perhaps the engine is not turning at the same rpm at the 2 altitudes.

As it is stated, the chart conclusion regarding HP makes no sense.

It doesn't make sense to us because we haven't yet identified the reason, but actual dyno tests show that this is the way it works. Actual test results trump theory every time.

 

[David-aviator]

It doesn't make sense to us because we haven't yet identified the reason, but actual dyno tests show that this is the way it works. Actual test results trump theory every time.

Yep, and as Einstein would have concluded, there is an error in the formula to compute density altitude - and I'm not anywhere smart enough to determine what it is.

 

[Alan Carroll]

It doesn't make sense to us because we haven't yet identified the reason, but actual dyno tests show that this is the way it works. Actual test results trump theory every time.

Kevin and others - thanks for the responses. Glad to hear I'm not the only one puzzled by this.

 

[AlexPeterson]

It seems, from a purely physics point of view, there is a disconnect concerning the HP difference at a density altitude of 6022' as depicted at 4000' and 8000' pressure altitudes.

A given volume of atmosphere at a given density should contain the same number of oxygen molecules no matter the temperature and pressure, why would there be a variance in HP? All the engine knows is oxygen and fuel, i.e., F/A ratio.

Perhaps the engine is not turning at the same rpm at the 2 altitudes.

As it is stated, the chart conclusion regarding HP makes no sense.

David - what you say in the first paragraph above is true, but the initial post indicates two different MAP's (24.8/21.3). What I believe we are wondering is the HP difference (than the 156/146 hp discussed in the first post), when MAP, DA and rpm are indeed the same. I don't know what the magnitude of this difference is, but have speculated in an earlier post as to possible reasons. The ratios of the MAP 24.8/21.3=1.16, while the ratios of the HP 156/146=1.07. This large difference in ratios is what seems to be a mystery. At least to me.

 

[Kevin Horton]

There are two different effects going on here:

1. At the same MP, RPM and OAT, the power produced varies with altitude. If the altitude increases, but we keep the same MP, RPM and OAT, the power produced increases. I speculate that the reduced back pressure on the exhaust is a primary factor. Lower pressure at the exhaust leads to better scavenging in the cylinder.
2. At the same MP, RPM and altitude, the power variation with OAT is not as we intuitively expect. We expect that the power should vary directly with the air density, but in fact dyno tests show that the power variation for a change in OAT is only about half as much as we expect.

If you combine the two above puzzlements, the result is that power does not stay the same if you change the temperature but vary the pressure altitude to achieve a constant density altitude.

 

[glenn654]

I'm certainly not in the same league with most of you guys but I have an anecdotal theory about this. I've read somewhere that the optimum temp for fuel atomization is about 70 f. Perhaps you are getting a better fuel/air mixture at the higher temp and a more efficient combustion and thus more power than at the cold temp.

Glenn Wilkinson

 

[David-aviator]

I'm certainly not in the same league with most of you guys but I have an anecdotal theory about this. I've read somewhere that the optimum temp for fuel atomization is about 70 f. Perhaps you are getting a better fuel/air mixture at the higher temp and a more efficient combustion and thus more power than at the cold temp.

Glenn Wilkinson

For sure you are on to something, Glenn.

I was burning 10 gph today at 10-12F OAT and getting the same TAS as I did last summer burning 8 gph.

 

[Andy_RR]

SAE J1349...

SAE J1349 is the standard for correcting measured power to a normalized atmospheric condition. If you take Alan's original data an put it into the (inverse of the) equation, it comes out with 180hp at sea level, 155.7hp at 4000' and 143.0hp at 8000' I suspect that Lycoming has calculated the tables/charts using the SAE standard, rather than a bunch of test data.

On the other hand, I can't tell what is actually happening here. I'm still thinking about it...

 

[ergie63]

Look at Total Heat

Here is my guess...

At a given density altitude you have the same number of oxygen molecules and will burn the same amount of fuel. True as you expect, but that is not a complete picture of what is happening. That is only part of the equation.

The purpose of burning fuel is to ADD heat. We add this heat very rapidly which causes the pressure in the cylinder to rise rapidly so that a piston can take this pressure differential (one side of the piston vs. the other) and turn it in to mechanical energy.

By definition, everything above absolute zero contains heat (which is why the charts are based on Kelvin). When the intake air is colder, the TOTAL amount of heat in the cylinder is less, therefore the pressure that is developed is less and therefore the amount of mechanical energy that can be extracted from the same amount of burning fuel is less.

 

[Bob Axsom]

Thanks Alan

As you know all of my testing is done at 6,000 ft density altitude. I use the three-way track method specified on the old U.S. Air Race handicap procedure but use the National Test Pilot School spreadsheet supplied by both Kevin Horton and John Huft to determine True Air Speed based on GPS track and Ground Speed over 5 successive, 20 second interval measurements where the GPS ground speed does not vary by more than 1 knot, with wide open throttle and maximum RPM (2720-2730). I lean the mixture to get as close as I can to 1300F as this is quite sensitive and the manifold pressure gauge is not (it seems static at some point between 24 and 25 - ~1/8" on an analog gauge). Variations in mixture do affect the speed but there is a lag and you have to be patient with it. My experience shows me that 1300F EGT on Cylinder #4 is near optimum. Many drag reduction experiments produce very small changes in speed and though my results have been very consistent variations in test conditions could have caused me to delete some good mods (as John Huft has already pointed out to me) but I believe their effect on speed is so small that they are inconsequential.

Bob Axsom

 

[mgomez]

Filling the cylinders matters, too

I think the reason that pressure matters (not just density) is that what you're doing is filling a volume (the cylinders) through an orifice (the intake valve curtain area.)

How fast that happens (and therefore how much fuel and air ends up in the cylinder) depends on delta-P...the pressure across the orifice.

 

[Alan Carroll]

What I think I've learned so far...

Is that power varies linearly with changes in MP, but with the square root of changes temperature (per formula given in the Lycoming chart, expressed in Kelvin or Rankine).

Given different combinations of temperature and pressure that yield the same density altitude, combinations involving warmer temperatures yield slightly more power than those with colder temperatures.

From what I remember of high school chemistry, air density should be affected in equal proportion by changes in either temperature or pressure. So, something in addition to air density is affecting power output. Also it seems that the same thing happens with both carbureted and injected engines.

Available explanations so far include differences in air viscosity, fuel vaporization, heat of burning fuel vs. ambient air, and pressure gradient between intake and cylinder.

Do I have this right?

 

[Kevin Horton]

Do I have this right?

Yeah, I think that is a good summary of the problem. I've spent a bit of time digging through the early NACA reports, as they did a lot of good work. NACA Technical Report 45, from 1919, has the first reference I can find to how engine power varies with temperature. They found that the effect of temperature on power was not as they expected (they expected power to vary with air density). They developed an empirical correction based on dyno tests. This correction is very close to the square root relationship that is found on the Lycoming (and Continental, and Pratt and Whitney) power charts, and in SAE standards. I hope to find some later NACA report that identifies a definitive explanation, but I haven't struck gold yet.

 

[Alan Carroll]

And the answer is... none of the above!

I asked my hangar mate about this; he is a mechanical engineering professor and an expert on engines (he also built and flies an RV-8A). I'll try to paraphrase his answer below; any errors in restating this are purely mine.

His response was that the engine power is governed by mass flow rate of air into the cylinders. This is mostly a function of ambient air density, which is determined by air pressure and temperature (ie density altitude).

However, the mass flow rate is also limited by how fast the air can travel past the "valve curtain", which is the cylindrical area between the open intake valve and the valve seat. The valve curtain is very small relative to the either the intake or the cylinder. Flow through the valve curtain is assumed to be "choked", meaning that air flows through at the speed of sound (air can't travel faster than the speed of sound). The speed of sound increases with the square root of temperature, so warmer air can fill the cylinder faster than cold air.

Getting back to the OP, this means you can't assume the engine will always make exactly the same power at the same density altitude (or at the same MP and RPM).

 

[hevansrv7a]

Speed of Sound "Choking"

At first I did not believe this, but the Wikipedia article says it's so that the temp is the controlling factor, not the density. Of course, to get the same density (mass flow) at higher temp requires starting with lower DA.

According to Will James, in a recent conversation, the air flowing through the induction system only goes about 100 mph. Are we saying that the intake valve is so small that the air can get up to > 700 mph? Apparently so. Else, why does my V-TEC engine have 4 valves, a popular way of getting more power from small engines.

Thanks for settling that!

 

[Andy_RR]

inlet valve flow does not choke

flow over the inlet valves does not choke anywhere during the induction stroke. To choke, the pressure ratio across the valve must be less than 0.528 - at WOT (worst case) on most engines, it'd be lucky to be 0.8 at a minimum. I dug up some p-V data I had to confirm this.

I think we're still looking for another theory...

A

 

[mgomez]

Density and pressure both matter

flow over the inlet valves does not choke anywhere during the induction stroke. To choke, the pressure ratio across the valve must be less than 0.528 - at WOT (worst case) on most engines, it'd be lucky to be 0.8 at a minimum. I dug up some p-V data I had to confirm this.

I think we're still looking for another theory...

A

I think we're pretty close...density matters because it's the number of oxygen molecules in the cylinder that counts. Pressure matters because, as Alan's hangar mate says, we're talking about a filling process. The air has to flow through an obstruction (the valve curtain area) to fill a volume (the cylinder's combustion chamber.)

So naturally, the rate at which that happens matters...the faster it fills, the more oxygen there is. There is only a brief period to fill the combustion chamber. That rate depends on the delta-P across the intake valve.

I'm starting to see why density altitude and pressure altitude are not interchangeable.

 

[AlexPeterson]

Shock wave may indeed be present

If you have ever had the air cleaner off from an older vehicle (back when one could actually do this), and rapidly cracked open the throttle, there was a point where you all of a sudden could not hear the growl of the induction as much. I suspect this is where a shock wave was forming somewhere downstream of the throttle, probably at the valves, preventing acoustic energy (noise) from traveling up stream and back to your ear.

Interesting stuff - nothing is ever as simple as we think we understand it to be!

 

[N616LM]

A little compressible flow and thermodynamics

First I will admit that I am Alan's hangar mate. When he first asked me this question (I have been out of engine research for 10 years), I did not remember. So I had to look it up to remind myself.

Anyway, some definitions are in order: For example, if we have a flowing fluid, we can define three pressures at any point in the flowing fluid: the stagnation pressure, p_0, the dynamic pressure, 1/2*density*Velocity^2, and the static pressure, p_s. In equation form

p_0 = 1/2*density*Velocity^2+p_s

One more thing: If the flow is isentropic, which means that the flow is reversible and adiabatic, the stagnation pressure will be the same at every point in the flow. In an isentropic flow, the static pressure and dynamic pressure can change, e.g. an increase in flow speed will result in an increase in dynamic pressure, and with a subsequent decrease in static pressure, but the stagnation pressure will remain constant.

Of course this equation is known to all of us because this is how we get to airspeed. Assume that your pitot tube measures stagnation pressure, you get static pressure from your static pressure measurement, you are in a fluid of know density (air) and we can solve directly for the flow speed, or in the case of our airplanes, the speed of the air relative to the airplane. In other words:

V = (2*(p_0-p_s)/density)^0.5

Getting to the point, the critical pressure ratio mentioned by Andy is correct, for an ideal gas of specific heat ratio of 1.4, the critical pressure ratio is 0.528. What he is missing is that the critical pressure ratio is comparing the flow stagnation pressure to the minimum flow area static pressure. In equation form:

Critical Pressure ratio = p_throat_static/p_stagnation

Thus, when the static pressure at the minimum flow area is 0.528 of the flow stagnation pressure, the flow at the minimum flow area will be at a Mach number of 1, and the flow is considered to be choked. Just as interesting, it does not matter how much we decrease the pressure downstream of the minimum flow area, the static pressure at the minimum flow area will remain the same (assuming we have not changed the stagnation pressure) and the flow velocity will stay the same.

If you are still interested, there is an excellent simulator that you can play with to understand this further. Here is the url: http://www.engapplets.vt.edu/fluids/CDnozzle/index.html

The reason this rocket nozzle simulator is of interest is because in a reciprocating engine, like the rocket nozzle, we are connecting what are effectively two plenums with the intake valve flow area. The upstream plenum is the intake manifold, and the downstream plenum is the cylinder. In the rocket, the upstream plenum is where combustion takes place, and the downstream pressure is the ambient pressure into which the rocket exhausts.

If you play with this simulator, you will find another pressure, defined p_b, which is the static back pressure in the downstream chamber. In the engine case, this would be the measured cylinder pressure. So if you use an area ratio between the upstream plenum and the throat of 8, which in an engine would be the ratio between the intake port area and the valve curtain area, you will see that p_b (or cylinder pressure) does not need to be smaller than approximately 0.9 times the stagnation pressure to cause choking in the throat.

I did some simple calculations today and found that given typical lycoming valve areas, valve lifts, speed of 2700 rpm, choked flow in the valve region was common. Actually this makes sense for numerous pragmatic reasons that aren't relevant here.

Finally, getting back to the second OP (is that original point?), Alan should stop trying to beat me worse than he already does. Do not, I repeat, do not hangar with a tail dragger racer if you have a nose dragger with ordinary N numbers. It is a lesson in humility.

 

[Alan Carroll]

First I will admit that I am Alan's hangar mate. When he first asked me this question (I have been out of engine research for 10 years), I did not remember. So I had to look it up to remind myself....

Jay - thanks for jumping in on this.

 

[glenn654]

What'd he say?


Glenn Wilkinson

 

[Andy_RR]

Getting to the point, the critical pressure ratio mentioned by Andy is correct, for an ideal gas of specific heat ratio of 1.4, the critical pressure ratio is 0.528. What he is missing is that the critical pressure ratio is comparing the flow stagnation pressure to the minimum flow area static pressure. In equation form:

Critical Pressure ratio = p_throat_static/p_stagnation

The flow stagnation pressure (your term) is approximately manifold absolute pressure (MAP), certainly not a million miles from it. The minimum flow area static pressure (again, your term) is approximately cylinder pressure, or if anything higher than this.

At sea level, MAP is around 29.something" - say 30" for round figures and worst case. Cylinder pressure data at WOT will show you that it falls no lower than 24-ish" at any point during the induction stroke - say this is 23.5" worst case. This gives you a minimum possible pressure ratio across the intake valve of 23.5/30 = 0.783 - this is nowhere near critical pressure ratio for air (0.528) so sonic flow is extremely unlikely!

I don't think I've missed anything here at all.

 

[Andy_RR]

...So if you use an area ratio between the upstream plenum and the throat of 8, which in an engine would be the ratio between the intake port area and the valve curtain area, you will see that p_b (or cylinder pressure) does not need to be smaller than approximately 0.9 times the stagnation pressure to cause choking in the throat.

Sorry, this makes no sense. If cylinder pressure is 0.9*atmospheric pressure, then your lowest possible (static) pressure ratio you can achieve is by definition 0.9 - i.e. not critical or even near it. See my previous example.

BTW, if valve flow was choked under any circumstance, then engine power would be proportional to only manifold pressure - which we know to not be the case. To the contrary, it is actually approximately proportional to manifold pressure * engine speed - otherwise known as the speed-density calculation and a common way to meter airflow by many electronic engine controllers.

 

[Alan Carroll]

Comparing different pressures?

The flow stagnation pressure (your term) is approximately manifold absolute pressure (MAP), certainly not a million miles from it. The minimum flow area static pressure (again, your term) is approximately cylinder pressure, or if anything higher than this.

At sea level, MAP is around 29.something" - say 30" for round figures and worst case. Cylinder pressure data at WOT will show you that it falls no lower than 24-ish" at any point during the induction stroke - say this is 23.5" worst case. This gives you a minimum possible pressure ratio across the intake valve of 23.5/30 = 0.783 - this is nowhere near critical pressure ratio for air (0.528) so sonic flow is extremely unlikely!
.

I'm just a geologist and definitely not qualified to take sides on this (the nice thing about rocks is that they hardly ever move).

However, it appears that N616LM and Andy_RR are defining the "critical pressure ratio" differently. As I read the post, N616LM says its the ratio of the static pressure in the narrowest part of the system (at the valve) to the stagnation pressure. Stagnation pressure=static pressure+dynamic pressure, and is the same everywhere in the system. Where the air is moving slowly the static pressure is relatively high and dynamic pressure is low, but where the air speeds up the static pressure is low and the dynamic pressure is high.

In contrast Andy_RR appears to be saying the critical pressure ratio is based on the static pressure in the cylinder vs. static pressure in the intake. Is this right?

 

[Andy_RR]

Stagnation pressure=static pressure+dynamic pressure, and is the same everywhere in the system. Where the air is moving slowly the static pressure is relatively high and dynamic pressure is low, but where the air speeds up the static pressure is low and the dynamic pressure is high.

It depends on what the word stagnation means to you. To me, it means no movement - i.e. the pressure of the fluid where there is no movement, therefore dynamic pressure = 0

http://en.wikipedia.org/wiki/Stagnation_pressure

In a plenum volume (intake or cylinder), absolute pressure approximates stagnation pressure of the fluid.

 

[hevansrv7a]

Is this too simplisitic?

Let's assume for the sake of argument that Will James is correct (his data is based on actual testing) that the speed of the air in the intake is limited to about 100 mph regardless of the speed of the RV because that is all the air the engine can pump.

There is a knowable cross section area to the intake (although I don't know what it is). Similarly, there is a knowable cross section area to the valve opening after subtracting for the stem. At any given moment, only one piston is in the intake phase. For the speed of the air going past the valve to be near-sonic, the ratio of the areas must be around 7:1 (700 mph for speed of sound, more or less). If the ratio is higher, then it is trying to go faster than sound. If it is lower, it isn't.

Can anyone supply a back-of-the-envelope calculation here? I'm going to guess that the ratio is significantly lower than 7.

BTW - I have run my engine to 2900 and the little Continentals at Reno (like the C-150's) run about 4200 rpm. If a 2700 rpm engine were anywhere near mach 1.0 then how are they doing that with a "stock" engine? I had a Honda 65cc bike with only 2 valves that went to an estimated 20,000 rpm in first gear, often. (redline was under 10,000, so the intake was not designed for that). How?

 

[David-aviator]

Let's assume for the sake of argument that Will James is correct (his data is based on actual testing) that the speed of the air in the intake is limited to about 100 mph regardless of the speed of the RV because that is all the air the engine can pump.

There is a knowable cross section area to the intake (although I don't know what it is). Similarly, there is a knowable cross section area to the valve opening after subtracting for the stem. At any given moment, only one piston is in the intake phase. For the speed of the air going past the valve to be near-sonic, the ratio of the areas must be around 7:1 (700 mph for speed of sound, more or less). If the ratio is higher, then it is trying to go faster than sound. If it is lower, it isn't.

Can anyone supply a back-of-the-envelope calculation here? I'm going to guess that the ratio is significantly lower than 7.

BTW - I have run my engine to 2900 and the little Continentals at Reno (like the C-150's) run about 4200 rpm. If a 2700 rpm engine were anywhere near mach 1.0 then how are they doing that with a "stock" engine? I had a Honda 65cc bike with only 2 valves that went to an estimated 20,000 rpm in first gear, often. (redline was under 10,000, so the intake was not designed for that). How?

I don't see how the speed of air entering the combustion chamber is as slow as 100 mph. I think what you are talking about here is the ratio of area within the the combustion chamber as the piston descends (the area is increasing) to the area at the in take valve. That ratio is not a small number, whatever it is, and would seem to be dependent on manifold pressure, density, and the speed of the air entering the combustion chamber. If my observed loss of power at 10F vrs 75F at 8 gph is not caused by a Mach One limit of in take air, what is causing it?

Also, what is going on when LOP? LOP 50F does not work in very cold air, the engine stumbles and acts like it is going to quit. Fuel is not mixing properly with air or not enough of it is entering the combustion chamber.

Your last question about rpm doesn't seem all that complicated, the faster an engine turns the more cycles occur and the more power is created. The limiting factor of speed of air could be the same, it just happens more often over a give period of time.

After some 37 posts here, the thread seems to have taken off into the wilderness with no data to back up departing the known path.

 

[hevansrv7a]

I guess I need to explain it, then

I don't see how the speed of air entering the combustion chamber is as slow as 100 mph. I think what you are talking about here is the ratio of area within the the combustion chamber as the piston descends (the area is increasing) to the area at the in take valve. That ratio is not a small number, whatever it is, and would seem to be dependent on manifold pressure, density, and the speed of the air entering the combustion chamber. If my observed loss of power at 10F vrs 75F at 8 gph is not caused by a Mach One limit of in take air, what is causing it?

Also, what is going on when LOP? LOP 50F does not work in very cold air, the engine stumbles and acts like it is going to quit. Fuel is not mixing properly with air or not enough of it is entering the combustion chamber.

Your last question about rpm doesn't seem all that complicated, the faster an engine turns the more cycles occur and the more power is created. The limiting factor of speed of air could be the same, it just happens more often over a give period of time.

After some 37 posts here, the thread seems to have taken off into the wilderness with no data to back up departing the known path.

OK, if the air entering the induction system (from the outside) is going X mph and the induction system is (obviously) much bigger than the intake valve, then the air flowing through the valve must go faster by roughly the proportion of the sizes. Your interpretation of what I said is not correct. This is simple. A 1/2" garden hose with a tiny nozzle at the end is all the example you need for this. The speed of the water in the hose is smaller than the speed of the water leaving the nozzle and the speed difference must be in proportion to their relative cross-sectional areas. Further, if you want the water going past the nozzle to go faster then the water in the hose must go faster. Assumes equal size nozzle.

Similarly, if the engine intake air is X mph at 2700 then at 4200 the intake air would have to be 1.56 X mph and the increased speed of the air moving through the valve would be higher in proportion. If it is near sonic at 2700, my question remains, how can it be going 56% faster in another engine? If the O-200 running at 4200 is still below sonic, then the IO-360 running at 2700 is way below sonic. If I am wrong about this, what is the explanation? Can air move faster than sonic?

Let's just take this a step further: forget about the X mph. If an O-200 can run 4200 without the air going through the valves going super-sonic then it is well below sonic at 2700. Keep in mind that the F-1 rules keep then engine "stock" so we are not talking about turbo chargers, super chargers, exotic intake systems, etc. To supply the air for that requires the air to move into and through the intake system faster as the rev's go up. Conversely, moving back from 4200 to 2700 means the air is flowing slower.

Finally, I don't see why you think this is thread creep ("wilderness"). The original question was about the effect of temperature on BHP. One answer, proposed by a very knowledgeable expert, was that the limiting factor was the speed of sound as a limit on the speed of air movement and that being variable with air temp. This is my response that that idea, using a reality check. I think either the proposed answer fails the reality check or something else must explain the contradiction. I don't know the answer. All I am saying is that the proposed answer is not sufficient. I also don't know what is going on with your LOP operations but there are many more variables.

 

[glenn654]

Regarding the speed of intake flow, the Sacramento Sky Ranch Engineering Manual says on p 179 "This air charge enters the cylinder with a high velocity, in the range of 120 to 240 feet per second, that is, from about 82 to 164 miles per hour."

This is the best reference I know.


Glenn Wilkinson

 

[ergie63]

Eight is enough?

So if you use an area ratio between the upstream plenum and the throat of 8, which in an engine would be the ratio between the intake port area and the valve curtain area, ...

This is a good post. I had to read this slowly because I'm not an engineer. To paraphrase: the valve is essentially a nozzle where the velocity of the intake air is increased at the expense (loss) of static pressure. In fact, the nozzle exit is so small that the static pressure drops to a minimum which means the airflow is moving at mach 1 and cannot flow any faster.

I'm startled that the nozzle ratio is 8! Sure it heads toward infinity as the valve closes, but 8 times at a minimum?

This must be designed into engines intentionally. I would guess for smooth running purposes. Thinking about performance cams in car engines. Fiddling with the timing, duration and lift of valves changes an engine's personality. The more agressive the cam, the rougher the engine runs. Dragsters being perhaps the most extreme examples. Could it be that "choke" is designed into these engines intentionally so that they run smoothly and don't, perhaps, shatter propellers? Is this why intake plenums appear crude next to high performance auto engines? Is this why there are no (certified) performance exhaust systems?

Interesting.

 

[Andy_RR]

Could it be that "choke" is designed into these engines intentionally so that they run smoothly...

er, no. If I may be blunt, there's a lot of **** flying about in this thread and people are beginning to draw wierd and (IMO) dangerous conclusions about how engines work.

If you want to analyse engines in depth, please pick up a copy of Taylor or Heywood and start studying.

 

[David-aviator]

er, no. If I may be blunt, there's a lot of **** flying about in this thread and people are beginning to draw wierd and (IMO) dangerous conclusions about how engines work.

If you want to analyse engines in depth, please pick up a copy of Taylor or Heywood and start studying.

Both of these gentlemen wrote their books over 20 years ago and the publications are becoming collector items, not necessarily the latest information on engines. There has been much development in fuel delivery, timing and combustion chamber design since 1985. Granted, not much of it has gravitated to the Lycoming engine, but it still is relevant to the discussion.

There may well be some speculation here not grounded in data, but I don't think one will find the answers in studying the subject from a perspective that was current in the 80's.

The basic question posed in the first message here remains unanswered, at least not in a manner everyone will accept. I am certain it will not be found with Taylor or Heywood. They knew less about what we define as density altitude then than we do today.

 

[ergie63]

...beginning to draw wierd and (IMO) dangerous conclusions about how engines work.

Blunt please, rude no. No conclusion was drawn. There is no harm inquiring.

It would be weird to suggest engines are unintentionally designed. It is reasonable to ask whether a design feature which seems reduce an engine's potential serves some other useful and not-quite-so-obvious purpose.

 

[AlexPeterson]

snip
The basic question posed in the first message here remains unanswered, at least not in a manner everyone will accept.

Well, I thought it was. The question in the first post was why different HP's were stated in the chart at the same density altitudes. Answer: different MAP's were used.

snip
I am certain it will not be found with Taylor or Heywood. They knew less about what we define as density altitude then than we do today.

Density altitude's affect on engines has been fairly well understood (within the resolution of questions posed in this thread) for probably 75 years or more. The thermodynamics that govern engine gas flows and combustion have also been understood for a similar period. All the modern electronic controls that have been added to the road fleet are relatively new, but what they do for the efficiency of the combustion is not new knowledge.

The challenge here is that a topic like this takes a whole bunch more studying, typically at a graduate mechanical engineering level, to understand than most of us wish to undertake.

Shove the throttle forward and go fly!

 

[Alan Carroll]

Well, I thought it was. The question in the first post was why different HP's were stated in the chart at the same density altitudes. Answer: different MAP's were used.

Alex - actually my real question, which may not have been clear at first (even to me), was "why does power (at wide open throttle) vary linearly with pressure, but with the square root of temperature?"

I don't think this has been answered in a way that satisfies everyone.