Altitude, Mixture and Fuel Flow
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Altitude, Mixture and Fuel Flow
Altitude, Mixture and Fuel Flow
Could you experts out there help me in my reasoning?
A normally aspirated piston engine with no altitude compensator for the mixture will, at altitude, have a richer mixture. If climbing at full throttle manifold pressure is decreasing. Likewise, reducing exhaust back pressure is increasing the volumetric efficiency. Which effects are dominant? Does the fuel flow go up or down
Overall, I believe that reducing EHP is dominant and the total fuel flow also goes down, notwithstanding that the mixture is richer.
Notice I have specified full throttle at all heights. I am not comparing range performance at various heights
Thanks, Dick
Could you experts out there help me in my reasoning?
A normally aspirated piston engine with no altitude compensator for the mixture will, at altitude, have a richer mixture. If climbing at full throttle manifold pressure is decreasing. Likewise, reducing exhaust back pressure is increasing the volumetric efficiency. Which effects are dominant? Does the fuel flow go up or down
Overall, I believe that reducing EHP is dominant and the total fuel flow also goes down, notwithstanding that the mixture is richer.
Notice I have specified full throttle at all heights. I am not comparing range performance at various heights
Thanks, Dick
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Under your set of stated circumstances, MP decreases, and fuel flow remains roughly the same. HP can be somewhat recovered in the climb with mixture leaning toward best power, however this also decreases due to lower air density as altitude increases.
This can be varified in an airplane, such as a Bonanza S-35, with the constant flow type of TCM fuel injection.
An airplane engine with a pressure carburetor, it would also be as above.
Float type carburetor...not sure, haven't flown those for a very long time, so will leave comments on these to others who have more experience in same.
Hope that this has been helpful.
This can be varified in an airplane, such as a Bonanza S-35, with the constant flow type of TCM fuel injection.
An airplane engine with a pressure carburetor, it would also be as above.
Float type carburetor...not sure, haven't flown those for a very long time, so will leave comments on these to others who have more experience in same.
Hope that this has been helpful.
Doesn't the reducing exhaust back pressure simply increase the power produced by the same charge? Thus it might make you fly a little faster but it won't decrease your fuel flow.
So the falling MP means that you've got less air in the charge, and, with the quantitative relationship depending on the details of the setup, somewhat less fuel. I can't think of a (first order) effect that increases fuel flow with altitude.
So the falling MP means that you've got less air in the charge, and, with the quantitative relationship depending on the details of the setup, somewhat less fuel. I can't think of a (first order) effect that increases fuel flow with altitude.
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If an aircraft were to climb at constant MAP, the reducing atmospheric pressure would cause exhaust back-pressure to decrease. This would make it easier for the gases to flow through the engine. We would have a constant pressure (the MAP) pushing the mixture into the cylinders and a reducing exhaust back-pressure opposing the outflow of exhaust gas. This would increase the mixture volumetric flow rate through the engine. In addition to this, the constant MAP would maintain constant mixture density. The overall effect would be increases in air mass flow rate, fuel mass flow rate, and power output.
If the same aircraft were to climb with constant throttle angle (full throttle for example) both MAP and exhaust back-pressure would reduce at the same rate. Reducing MAP would reduce the density of the incoming mixture. And because both MAP and back-pressure would reduce at the same rate, there would be little if any increase, and more probably a decrease in volumetric efficiency. Logically the volumetric flow rate must decrease to zero as we climb out of the top of the atmosphere (if we could do so). The overall effect would decreases in air mass flow rate and power output.
To see what would happen to fuel flow rate we must look at what would be going on in the choke tube (I am assuming a float type carb). By accelerating the airflow, the choke tube produces a drop in static pressure. It is this pressure drop that causes fuel to flow through the main jet. The fuel flow rate through the main jet is proportional to the square root of the pressure drop across it. In effect this means the square root of the float chamber pressure minus the pressure at the throat of the choke tube.
The choke tube is a simple venturi, in which the throat is narrower than the inlet. The ratio of air velocity at the throat is inversely proportional to the ratio of the area of the throat to that of the inlet. So if for example the throat area is ¾ that of the inlet then the velocity at the throat is 4/3 of that at the inlet. If we assume that air density is constant then dynamic pressure at the throat, which is proportional to velocity squared would be 16/9 that at the inlet. These ratios are all determined by the geometry of the choke tube, so they do not vary with altitude.
But air density decreases with increasing altitude, so for a given choke tube geometry the absolute values of these pressure changes will decrease as altitude increases. Suppose for example that for some given choke tube geometry we had 15 psi ambient pressure and a 5 psi pressure drop at the throat at mean sea level. If we were to climb to an altitude where ambient is 5 psi we would not get zero at the throat. The absolute value of the pressure drop would have decreased.
And it is the square root of the absolute pressure difference across the main jet that determines the fuel flow rate. As the absolute value of the pressure drop decreases, so does the fuel flow rate. So fuel flow rate would decrease as altitude increased.
The gradual enrichment of the mixture is caused by the fact that as altitude increases, the density of the air decreases but that of the fuel remains more-or-less constant. So as altitude increases, the air mass flow rate decreases faster than the fuel flow rate.
So as altitude increases with constant full throttle setting fuel mass flow rate, air mass flow rate and power output will all decrease.
If the same aircraft were to climb with constant throttle angle (full throttle for example) both MAP and exhaust back-pressure would reduce at the same rate. Reducing MAP would reduce the density of the incoming mixture. And because both MAP and back-pressure would reduce at the same rate, there would be little if any increase, and more probably a decrease in volumetric efficiency. Logically the volumetric flow rate must decrease to zero as we climb out of the top of the atmosphere (if we could do so). The overall effect would decreases in air mass flow rate and power output.
To see what would happen to fuel flow rate we must look at what would be going on in the choke tube (I am assuming a float type carb). By accelerating the airflow, the choke tube produces a drop in static pressure. It is this pressure drop that causes fuel to flow through the main jet. The fuel flow rate through the main jet is proportional to the square root of the pressure drop across it. In effect this means the square root of the float chamber pressure minus the pressure at the throat of the choke tube.
The choke tube is a simple venturi, in which the throat is narrower than the inlet. The ratio of air velocity at the throat is inversely proportional to the ratio of the area of the throat to that of the inlet. So if for example the throat area is ¾ that of the inlet then the velocity at the throat is 4/3 of that at the inlet. If we assume that air density is constant then dynamic pressure at the throat, which is proportional to velocity squared would be 16/9 that at the inlet. These ratios are all determined by the geometry of the choke tube, so they do not vary with altitude.
But air density decreases with increasing altitude, so for a given choke tube geometry the absolute values of these pressure changes will decrease as altitude increases. Suppose for example that for some given choke tube geometry we had 15 psi ambient pressure and a 5 psi pressure drop at the throat at mean sea level. If we were to climb to an altitude where ambient is 5 psi we would not get zero at the throat. The absolute value of the pressure drop would have decreased.
And it is the square root of the absolute pressure difference across the main jet that determines the fuel flow rate. As the absolute value of the pressure drop decreases, so does the fuel flow rate. So fuel flow rate would decrease as altitude increased.
The gradual enrichment of the mixture is caused by the fact that as altitude increases, the density of the air decreases but that of the fuel remains more-or-less constant. So as altitude increases, the air mass flow rate decreases faster than the fuel flow rate.
So as altitude increases with constant full throttle setting fuel mass flow rate, air mass flow rate and power output will all decrease.
If an aircraft were to climb at constant MAP, the reducing atmospheric pressure would cause exhaust back-pressure to decrease. This would make it easier for the gases to flow through the engine. We would have a constant pressure (the MAP) pushing the mixture into the cylinders and a reducing exhaust back-pressure opposing the outflow of exhaust gas. This would increase the mixture volumetric flow rate through the engine.
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Thank you, Keith et al. As you might guess, this is coming from a weird ATPL exam question. I have a supplementary: Surely (don't call me Shirley) the volume of gas pasing through a piston engine is constant at constant RPM. Or does the fact that there is a time when both inlet and exhaust valves are open (valve overlap) mean that volume flow is susceptible to change of MP or exhaust back presure?
Dick
Edit: Bookworm posted while I was writing this. I think the engine is a fixed displacement pump.
Dick
Edit: Bookworm posted while I was writing this. I think the engine is a fixed displacement pump.
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First let me just note how utterly ridiculous it is that pilots in 2008 are learning quick and dirty 1930's engineering to better complete safety critical tasks. There are few things in the world more silly than pilots flying new airplanes into the sides of mountains because they wasted brain cells and time doing what degreed engineers and microcontrollers should be doing far better. That said:
A normally aspirated piston engine with no altitude compensator for the mixture will, at altitude, have a richer mixture. If climbing at full throttle manifold pressure is decreasing. Likewise, reducing exhaust back pressure is increasing the volumetric efficiency. Which effects are dominant? Does the fuel flow go up or down
The exact behavior of fuel flow depends on the engine controls and the references thereof. An engine with no barometric compensation may or may not have fuel flow changes with altitude, but it will certainly have performance and specific fuel consumption changes. The question can't be answered sans handwaving without a knowledge of the controls on a particular engine.
You mention backpressure which is a measurement of limited inferential use. A drop in measured backpressure usually but not always improves net power...cylinder scavenging and the design of the exhaust manifold to achieve same is the key, and these things are weakly linked to static pressure at the exhaust outlet. Backpressure as a measurement is most useful for the mechanic/engineer trying to determine if an exhaust is blocked.
Also, keep in mind that while mass flow between intake/exhaust is similar (changing 1 part in ~12 with the addition of fuel) there is a very large difference in energy between exhaust and intake port flow. Changes in atmospheric pressure at the exhaust outlet are much less dominant than changes in pressure at the intake. Designs tend to settle on greater intake area as a result.
Surely (don't call me Shirley) the volume of gas pasing through a piston engine is constant at constant RPM.
Not even close. Load has a better correlation with mass flow than engine speed. Example at the limit: An engine with a closed throttle pumps a trivial quantity of air, but a propeller in a dive can certainly spin it quite quickly. In this case the engine produces negative net power. The key here is that load depends on factors external to the engine, such as how fast you are going and what pitch your prop is at.
Even in your original example of a climbing aircraft with wide open throttle and constant RPM, the volume flow will change, though much less than the mass flow. What you are have proposed is constant volumetric efficiency, the ratio of the charge actually inducted to the ideal under static conditions. VE isn't constant with changing atmospheric pressure, though I'm afraid any short explanation of why would amount to handwaving. Your concerns about valve events are headed in the correct direction, though overlap isn't at all the only factor.
Undoubtedly a higher exhaust back pressure means the exhaust stroke soaks up energy from the engine leaving it less power at the crank. But why would that change the volumetric flow rate?
As noted, the backpressure term is dubious so I hesitate to make a generalization based on its change. However, a change in exhaust efficiency can/will change mass flow because it is, through valve events, a player in VE. Again, cylinder scavenging is the key. Trying to describe this through backpressure alone is 1930 or even earlier thinking.
Modern high performance engines achieve VE's of ~120 percent. The energy needed to fill that cylinder beyond what atmosphere alone can do comes from the burning of fuel combined with careful design.
A normally aspirated piston engine with no altitude compensator for the mixture will, at altitude, have a richer mixture. If climbing at full throttle manifold pressure is decreasing. Likewise, reducing exhaust back pressure is increasing the volumetric efficiency. Which effects are dominant? Does the fuel flow go up or down
The exact behavior of fuel flow depends on the engine controls and the references thereof. An engine with no barometric compensation may or may not have fuel flow changes with altitude, but it will certainly have performance and specific fuel consumption changes. The question can't be answered sans handwaving without a knowledge of the controls on a particular engine.
You mention backpressure which is a measurement of limited inferential use. A drop in measured backpressure usually but not always improves net power...cylinder scavenging and the design of the exhaust manifold to achieve same is the key, and these things are weakly linked to static pressure at the exhaust outlet. Backpressure as a measurement is most useful for the mechanic/engineer trying to determine if an exhaust is blocked.
Also, keep in mind that while mass flow between intake/exhaust is similar (changing 1 part in ~12 with the addition of fuel) there is a very large difference in energy between exhaust and intake port flow. Changes in atmospheric pressure at the exhaust outlet are much less dominant than changes in pressure at the intake. Designs tend to settle on greater intake area as a result.
Surely (don't call me Shirley) the volume of gas pasing through a piston engine is constant at constant RPM.
Not even close. Load has a better correlation with mass flow than engine speed. Example at the limit: An engine with a closed throttle pumps a trivial quantity of air, but a propeller in a dive can certainly spin it quite quickly. In this case the engine produces negative net power. The key here is that load depends on factors external to the engine, such as how fast you are going and what pitch your prop is at.
Even in your original example of a climbing aircraft with wide open throttle and constant RPM, the volume flow will change, though much less than the mass flow. What you are have proposed is constant volumetric efficiency, the ratio of the charge actually inducted to the ideal under static conditions. VE isn't constant with changing atmospheric pressure, though I'm afraid any short explanation of why would amount to handwaving. Your concerns about valve events are headed in the correct direction, though overlap isn't at all the only factor.
Undoubtedly a higher exhaust back pressure means the exhaust stroke soaks up energy from the engine leaving it less power at the crank. But why would that change the volumetric flow rate?
As noted, the backpressure term is dubious so I hesitate to make a generalization based on its change. However, a change in exhaust efficiency can/will change mass flow because it is, through valve events, a player in VE. Again, cylinder scavenging is the key. Trying to describe this through backpressure alone is 1930 or even earlier thinking.
Modern high performance engines achieve VE's of ~120 percent. The energy needed to fill that cylinder beyond what atmosphere alone can do comes from the burning of fuel combined with careful design.
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Let's suppose some children were to ram a very large potato into the exhaust pipe of your car. You would probably find it impossible to start the engine because no exhaust gas could escape to atmosphere.. And because the exhaust gas could not get out, little if any mixture could be drawn in. The volumetric efficiency would have reduced to zero. That is an extreme case of course, but it demonstrates the effect.
A less extreme case is that of the wate-gate controlled turbocharger. As altitude increases, the wate gate gradually closes to make the turbine spin faster in order to maintain constant MAP. This makes it harder for the exhaust gas to escape, so the volumetric efficiency and power output gradually decrease.
A less extreme case is that of the wate-gate controlled turbocharger. As altitude increases, the wate gate gradually closes to make the turbine spin faster in order to maintain constant MAP. This makes it harder for the exhaust gas to escape, so the volumetric efficiency and power output gradually decrease.
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OK, so we have to consider all sorts of variables. It is possible, however, to ask if for one engine at fixed RPM or MP a reasonable answer can be deduced Let's try and do that.
Specifically, answer me this: Consider an Otto cycle engine with all valve events at TDC or BDC. On the inlet stroke it inducts one cylinder's worth of volume. On the compression and power strokes nothing goes in or out. On the exhaust stroke one cylinder's worth of volume is expelled. Is the engine not passing one cylinder's worth of volume in each complete cycle? Does that not mean that at constant RPM volume flow is also constant?
And, skiingman, don't confuse mass flow with volume flow.
Dick
I'll come clean. Here is one of the doubtful questions I am trying to analyse to see if we should ask for it to be pulled or not
As altitude increases, if the mixture is not leaned:
A the volume of air entering the carburettor remains constant and the fuel flow decreases
B the volume of air entering the carburettor decreases and the fuel flow decreases
C both the density of air entering the carburettor and the fuel flow decrease
D the volume of air entering the carburettor decreases and the fuel flow increases
Specifically, answer me this: Consider an Otto cycle engine with all valve events at TDC or BDC. On the inlet stroke it inducts one cylinder's worth of volume. On the compression and power strokes nothing goes in or out. On the exhaust stroke one cylinder's worth of volume is expelled. Is the engine not passing one cylinder's worth of volume in each complete cycle? Does that not mean that at constant RPM volume flow is also constant?
And, skiingman, don't confuse mass flow with volume flow.
Dick
I'll come clean. Here is one of the doubtful questions I am trying to analyse to see if we should ask for it to be pulled or not
As altitude increases, if the mixture is not leaned:
A the volume of air entering the carburettor remains constant and the fuel flow decreases
B the volume of air entering the carburettor decreases and the fuel flow decreases
C both the density of air entering the carburettor and the fuel flow decrease
D the volume of air entering the carburettor decreases and the fuel flow increases
Last edited by Dick Whittingham; 27th Oct 2008 at 14:07.
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Consider an Otto cycle engine with all valve events at TDC or BDC.
A particularly useless/non-extant engine, but I'll go along with the exercise.
On the inlet stroke it inducts one cylinder's worth of volume. On the compression and power strokes nothing goes in or out. On the exhaust stroke one cylinder's worth of volume is expelled.
Is the engine not passing one cylinder's worth of volume in each complete cycle?
If the engine is doing meaningful work or even motoring itself, we are adding fuel and burning it rapidly. This combustion results in end gases different than the intake charge, with different volume, pressure, and temperature. At the extremes, we have:
-zero/negative load resulting in very little flow, volume or mass.
-high load resulting in peak flows, volume or mass.
Because engines have losses, the only way to make your example work is to spin the motor with external power. Even in this case, volume out does not equal volume in, because energy out does not equal energy in. If you could develop a pump to compress a gas 9:1 or so and then decompress it without heating said gas, you'd get to rewrite the Laws of Thermodynamics.
The instances in which I substituted mass flow were because it was a more useful measurement and more relevant to your question IMHO. Engine controls are always designed to reflect this, either by directly measuring mass flow or by measuring volume flow and mass and calculating the resulting mass flow. You say volume flow...volume flow at what location and point in the cycle? It can't be overstated that managing changes in the volume/pressure of a gas at different locations and times in the creation of useful work is the point of the exercise. There are tremendous changes in pressure, temperature, and volume throughout any functioning internal combustion engine including in the intake and exhaust tracts external to the cylinders.
Does that not mean that at constant RPM volume flow is also constant?
No, because the amount of fuel we add has a stronger causal link to load than engine speed, as I explained already. More fuel, more flow. Without fuel, flow might be constant, but intake will not equal exhaust due to heat added in compression.
As altitude increases, if the mixture is not leaned:
A the volume of air entering the carburettor remains constant and the fuel flow decreases
B the volume of air entering the carburettor decreases and the fuel flow decreases
C both the density of air entering the carburettor and the fuel flow decrease
D the volume of air entering the carburettor decreases and the fuel flow increases
The question is inappropriate because the fuel flow term is unique to a given control system and utterly irrelevant in real world operation as every aircraft has a method (automatic, manual, whatever) of managing mixture. It isn't even useful if the mixture control fails...in that case a better indicator of engine health and performance would be provided by EGT.
A particularly useless/non-extant engine, but I'll go along with the exercise.
On the inlet stroke it inducts one cylinder's worth of volume. On the compression and power strokes nothing goes in or out. On the exhaust stroke one cylinder's worth of volume is expelled.
Is the engine not passing one cylinder's worth of volume in each complete cycle?
If the engine is doing meaningful work or even motoring itself, we are adding fuel and burning it rapidly. This combustion results in end gases different than the intake charge, with different volume, pressure, and temperature. At the extremes, we have:
-zero/negative load resulting in very little flow, volume or mass.
-high load resulting in peak flows, volume or mass.
Because engines have losses, the only way to make your example work is to spin the motor with external power. Even in this case, volume out does not equal volume in, because energy out does not equal energy in. If you could develop a pump to compress a gas 9:1 or so and then decompress it without heating said gas, you'd get to rewrite the Laws of Thermodynamics.
The instances in which I substituted mass flow were because it was a more useful measurement and more relevant to your question IMHO. Engine controls are always designed to reflect this, either by directly measuring mass flow or by measuring volume flow and mass and calculating the resulting mass flow. You say volume flow...volume flow at what location and point in the cycle? It can't be overstated that managing changes in the volume/pressure of a gas at different locations and times in the creation of useful work is the point of the exercise. There are tremendous changes in pressure, temperature, and volume throughout any functioning internal combustion engine including in the intake and exhaust tracts external to the cylinders.
Does that not mean that at constant RPM volume flow is also constant?
No, because the amount of fuel we add has a stronger causal link to load than engine speed, as I explained already. More fuel, more flow. Without fuel, flow might be constant, but intake will not equal exhaust due to heat added in compression.
As altitude increases, if the mixture is not leaned:
A the volume of air entering the carburettor remains constant and the fuel flow decreases
B the volume of air entering the carburettor decreases and the fuel flow decreases
C both the density of air entering the carburettor and the fuel flow decrease
D the volume of air entering the carburettor decreases and the fuel flow increases
The question is inappropriate because the fuel flow term is unique to a given control system and utterly irrelevant in real world operation as every aircraft has a method (automatic, manual, whatever) of managing mixture. It isn't even useful if the mixture control fails...in that case a better indicator of engine health and performance would be provided by EGT.
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A less extreme case is that of the wate-gate controlled turbocharger. As altitude increases, the wate gate gradually closes to make the turbine spin faster in order to maintain constant MAP. This makes it harder for the exhaust gas to escape, so the volumetric efficiency and power output gradually decrease.
The B-36 bomber is a perfect example.
The featherweight versions of this rather unique aeroplane had six 28 cylinder engines and each engine had not only a gear driven supercharger, but two turbochargers, and could cruise and deliver atomic weapons from 50,000 feet.
The piston engines on this aeroplane were equipped with Bendix pressure carburetors.
Very large ones.
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Let's suppose some children were to ram a very large potato into the exhaust pipe of your car. You would probably find it impossible to start the engine because no exhaust gas could escape to atmosphere.. And because the exhaust gas could not get out, little if any mixture could be drawn in. The volumetric efficiency would have reduced to zero. That is an extreme case of course, but it demonstrates the effect
And this is why measuring backpressure is a useful diagnostic tool indeed. In modern cars, the potato is typically a failed and melted catalyst.
And this is why measuring backpressure is a useful diagnostic tool indeed. In modern cars, the potato is typically a failed and melted catalyst.
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skiingman: I know the engine I proposed is not an example of a realistic engine. I am trying to keep things simple so you can understand.
It is simply not true that an engine of this type would not run under its own power. It would run in a most ineficient mode, but it would run.
At the end of the induction stroke there is one cylinder's worth of mixed gases in the engine. At the end of the power stroke on the example engine there is one cylinder's worth of other mixed gases in the engine. They may be hotter, and the pressure may be high, but the volume is still one cylinder's worth. At the end of the exhaust stroke this has been ejected. What happens next outside the engine has no bearing on this example.
As this carefully simplified example shows, your analysis of what would happen at various load conditions is wrong.
Anyway, I have had some useful input from the usual suspects, whose views I respect, so no more posts on this thread from me. Hasta la vista, baby.
Dick
It is simply not true that an engine of this type would not run under its own power. It would run in a most ineficient mode, but it would run.
At the end of the induction stroke there is one cylinder's worth of mixed gases in the engine. At the end of the power stroke on the example engine there is one cylinder's worth of other mixed gases in the engine. They may be hotter, and the pressure may be high, but the volume is still one cylinder's worth. At the end of the exhaust stroke this has been ejected. What happens next outside the engine has no bearing on this example.
As this carefully simplified example shows, your analysis of what would happen at various load conditions is wrong.
Anyway, I have had some useful input from the usual suspects, whose views I respect, so no more posts on this thread from me. Hasta la vista, baby.
Dick
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I'll come clean. Here is one of the doubtful questions I am trying to analyse to see if we should ask for it to be pulled or not
As altitude increases, if the mixture is not leaned:
A the volume of air entering the carburettor remains constant and the fuel flow decreases
B the volume of air entering the carburettor decreases and the fuel flow decreases
C both the density of air entering the carburettor and the fuel flow decrease
D the volume of air entering the carburettor decreases and the fuel flow increases
As altitude increases, if the mixture is not leaned:
A the volume of air entering the carburettor remains constant and the fuel flow decreases
B the volume of air entering the carburettor decreases and the fuel flow decreases
C both the density of air entering the carburettor and the fuel flow decrease
D the volume of air entering the carburettor decreases and the fuel flow increases
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Your own words asked what happened when "exhaust backpressure" falls with increasing altitude, so clearly what happens at or beyond the exhaust port did matter.
Also, in your words: On the exhaust stroke one cylinder's worth of volume is expelled. Volume past the exhaust valve doesn't equal volume past the intake, and it does depend on load. Your background in aerodynamics should be sufficient to understand the implications of compressible flow, local mach number, and temperature of a gas.
Your reduction seems to state that the volume of a cylinder is constant when the piston is at BDC. I would hope that wasn't contested.
In any case, have fun over there.
Also, in your words: On the exhaust stroke one cylinder's worth of volume is expelled. Volume past the exhaust valve doesn't equal volume past the intake, and it does depend on load. Your background in aerodynamics should be sufficient to understand the implications of compressible flow, local mach number, and temperature of a gas.
Your reduction seems to state that the volume of a cylinder is constant when the piston is at BDC. I would hope that wasn't contested.
In any case, have fun over there.
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Compression ratio not infinite
Dick,
I think the incorrect statement in your idealised engine is where you say:
This is not quite true. The exhaust stroke is (inevitably) the same length as the compression stroke, and the cylinder is not "empty". What remains is (some of) the burnt mixture; you will have the volume that corresponds to the cylinder volume at the top of stroke, and the pressure of the gas will be approximately equal to the pressure in the exhaust manifold. To the extent that the pressure in the exhaust manifold is higher than the inlet manifold pressure, the next induction stroke cannot draw a full cylinder of mixture in for the next cycle - in fact no mixture will be drawn until the piston movement has equalised the pressures. That is why the quality (and pressure) of the exhaust system can affect the performance of the engine.
Of course in a real engine, it is possible to "tune" the exhaust system so that the momentum of the exhaust gas and the resonance in the manifold presents an artificially low pressure at the exhaust valve and thus achieves a much better than expected purge of the cylinder - but now we are confusing the trivial model with reality!
I think the incorrect statement in your idealised engine is where you say:
At the end of the exhaust stroke this has been ejected.
Of course in a real engine, it is possible to "tune" the exhaust system so that the momentum of the exhaust gas and the resonance in the manifold presents an artificially low pressure at the exhaust valve and thus achieves a much better than expected purge of the cylinder - but now we are confusing the trivial model with reality!
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I don't have any useful insight on the question, but a couple of comments:
1) In a *real* engine, there is also an overlap where the intake and exhaust valves are open - which helps purge the exhaust and draw more fresh mixture in.
2) if volume remained constant, the engine would not work! it's the rather large increase in volume as the mixture burns and heats the gasses that gives you the power stroke...
(well, actually, the volume increase is constrained, so that increases the pressure, which drives the piston down and the volume increases.. but you get the idea).
The equation that pops into my mind is pv=nrt.. google then tells me: p=pressure, v=volume, n=number of moles (measure of molecules), R=gas constant, t = temperature.
1) In a *real* engine, there is also an overlap where the intake and exhaust valves are open - which helps purge the exhaust and draw more fresh mixture in.
2) if volume remained constant, the engine would not work! it's the rather large increase in volume as the mixture burns and heats the gasses that gives you the power stroke...
(well, actually, the volume increase is constrained, so that increases the pressure, which drives the piston down and the volume increases.. but you get the idea).
The equation that pops into my mind is pv=nrt.. google then tells me: p=pressure, v=volume, n=number of moles (measure of molecules), R=gas constant, t = temperature.
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Oh dear, I promised not to get into this again, but it was a Mandelson moment.
I agree absolutely that valve overlap changes things. There is then a time in the cycle when the engine is no longer delivering measured volumes of flow but has an additional flow straight through. For all other times the engine is engaged in filling and emptying cylinder's worth of gas. The gas mixture varies in the cycle, the temperature varies and the pressure varies but the one fixed value is cylinder swept volume. RPM determines how often each plug of gas is delivered.
Nothing that happens to the gas mixture inside the engine has any effect on the volume of gas passed through, measured at the inlet and exhaust valves. The gas mixture internally suffers rapid changes of volume,temperature and pressure but volume throughput from in to out is a dimensional factor, depending on bore, stroke and RPM.
Now, this is a pretty pointless argument. It doesn't really matter about volume. If you want more volume, build a bigger engine - there ain't no substitute for cubes. Otherwise, try and make better use of the volume you have by increasing the mass flow by keeping intake pressures up or by reducing losses by keeping exhaust back pressure down.
The only reason this ever came up was because the original ATPL question had "volume" in the options. I think the question is trash, but we are engaged in an internal examination of it to see if we think it valid or not.
Sorry to bug you all, I should never have tried to shift my ATPL problems on to you.
Dick
I agree absolutely that valve overlap changes things. There is then a time in the cycle when the engine is no longer delivering measured volumes of flow but has an additional flow straight through. For all other times the engine is engaged in filling and emptying cylinder's worth of gas. The gas mixture varies in the cycle, the temperature varies and the pressure varies but the one fixed value is cylinder swept volume. RPM determines how often each plug of gas is delivered.
Nothing that happens to the gas mixture inside the engine has any effect on the volume of gas passed through, measured at the inlet and exhaust valves. The gas mixture internally suffers rapid changes of volume,temperature and pressure but volume throughput from in to out is a dimensional factor, depending on bore, stroke and RPM.
Now, this is a pretty pointless argument. It doesn't really matter about volume. If you want more volume, build a bigger engine - there ain't no substitute for cubes. Otherwise, try and make better use of the volume you have by increasing the mass flow by keeping intake pressures up or by reducing losses by keeping exhaust back pressure down.
The only reason this ever came up was because the original ATPL question had "volume" in the options. I think the question is trash, but we are engaged in an internal examination of it to see if we think it valid or not.
Sorry to bug you all, I should never have tried to shift my ATPL problems on to you.
Dick
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Nothing that happens to the gas mixture inside the engine has any effect on the volume of gas passed through, measured at the inlet and exhaust valves.
This isn't right. It is an understandable mistake, and I'll try my best to explain why quickly. Volume is not conservative for compressible fluid flows. Many things happen in the engine which effect the volume flow past the intake and exhaust valves.
-The flows at the intake and exhaust ports have local mach numbers certainly over .3, probably >= .8
-Mass is very close to conserved from intake port to exhaust port*, and flow is given by mass flow=density * velocity * area
-Mass flow is related to volume flow by: mass flow=density * volume flowrate
-In compressible flows, density of fluid isn't constant. At mach numbers over .3, we can't accurately assume flows are incompressible.
-For volume flow at intake to equal volume flow at exhaust, you'd have to have two similar gases traveling through two similar ports at similar speeds and temperatures. None of this is the case in a real engine.
*New engines will be net accumulators of carbon deposits, and all engines will have some blowby gases that condense into acids, water, and other compounds in the oil. The remaining blowby gases are conserved by being reintroduced into the intake in most engines but not in aviation.
Otherwise, try and make better use of the volume you have by increasing the mass flow by keeping intake pressures up or by reducing losses by keeping exhaust back pressure down.
This was state of the art thinking at the beginning of the 20th century. Now it's just a vast oversimplification with little practical use. I only point this out because it is exactly the kind of thing that has no business anywhere near a pilot exam.
Mark1234 said:
2) if volume remained constant, the engine would not work! it's the rather large increase in volume as the mixture burns and heats the gasses that gives you the power stroke...
(well, actually, the volume increase is constrained, so that increases the pressure, which drives the piston down and the volume increases.. but you get the idea).
It is interesting to look at cylinder pressure with respect to crank angle.** The textbook definition of Otto cycle gives us a P-V diagram like this. Unfortunately reality is rarely so kind as the textbook. In the real world we have to account for the speed of the flamefront, limitations in rod/stroke ratio, etc, and the result is a battle to efficiently use those hot gases without detonation.
**measuring this in real time was only possible with expensive custom hardware until recently. Now it's part of control strategy on some volume produced automobiles. Instead of directly measuring pressure, you infer it by applying a voltage to the plug and measuring ionization currents through its gap...an elegant solution to the problem of knock compared to the acoustic sensors...
This isn't right. It is an understandable mistake, and I'll try my best to explain why quickly. Volume is not conservative for compressible fluid flows. Many things happen in the engine which effect the volume flow past the intake and exhaust valves.
-The flows at the intake and exhaust ports have local mach numbers certainly over .3, probably >= .8
-Mass is very close to conserved from intake port to exhaust port*, and flow is given by mass flow=density * velocity * area
-Mass flow is related to volume flow by: mass flow=density * volume flowrate
-In compressible flows, density of fluid isn't constant. At mach numbers over .3, we can't accurately assume flows are incompressible.
-For volume flow at intake to equal volume flow at exhaust, you'd have to have two similar gases traveling through two similar ports at similar speeds and temperatures. None of this is the case in a real engine.
*New engines will be net accumulators of carbon deposits, and all engines will have some blowby gases that condense into acids, water, and other compounds in the oil. The remaining blowby gases are conserved by being reintroduced into the intake in most engines but not in aviation.
Otherwise, try and make better use of the volume you have by increasing the mass flow by keeping intake pressures up or by reducing losses by keeping exhaust back pressure down.
This was state of the art thinking at the beginning of the 20th century. Now it's just a vast oversimplification with little practical use. I only point this out because it is exactly the kind of thing that has no business anywhere near a pilot exam.
Mark1234 said:
2) if volume remained constant, the engine would not work! it's the rather large increase in volume as the mixture burns and heats the gasses that gives you the power stroke...
(well, actually, the volume increase is constrained, so that increases the pressure, which drives the piston down and the volume increases.. but you get the idea).
It is interesting to look at cylinder pressure with respect to crank angle.** The textbook definition of Otto cycle gives us a P-V diagram like this. Unfortunately reality is rarely so kind as the textbook. In the real world we have to account for the speed of the flamefront, limitations in rod/stroke ratio, etc, and the result is a battle to efficiently use those hot gases without detonation.
**measuring this in real time was only possible with expensive custom hardware until recently. Now it's part of control strategy on some volume produced automobiles. Instead of directly measuring pressure, you infer it by applying a voltage to the plug and measuring ionization currents through its gap...an elegant solution to the problem of knock compared to the acoustic sensors...