HP vs TQ Theory
10-29-2015, 03:24 PM
#81
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Friction increases with load. Load increases with engine speed.
The pistons and connecting rods are reciprocating. Valvetrain is reciprocating. If something is moving in one direction and you have to stop it and accelerate it in the other direction, that takes energy. The faster it is traveling and the more it weighs, the more energy it takes.
The pistons and connecting rods are reciprocating. Valvetrain is reciprocating. If something is moving in one direction and you have to stop it and accelerate it in the other direction, that takes energy. The faster it is traveling and the more it weighs, the more energy it takes.
Far and away, the negative acceleration on the reciprocating parts is calculating the highest drag on power with increasing speed due to the X-squared nature of acceleration vs position. To give some color, I'm calculating at 6800 RPM, it is requiring 211 Ft-Lbs worth of torque just to keep the engine rotating! For comparison, at idle, it is only draining 25 ft-pounds of torque to keep the engine rotating. And those figures are just acceleration-based.
Mentioning the valves reminded me of another drain - having to push open the exhaust valve against cylinder pressure. I'll have to see about adding that. Shouldn't be too hard, since the valve area is known. Should be highest at peak TQ, due to peak VE / peak cylinder pressure(?)
The power it takes for the engine to pump the air it needs increases with rpm.
The power it takes to run the oil pump increases with RPM. The power it takes to run the water pump increases with RPM. Oil windage deflecting off of the rotating assemble robs power and increases with RPM.
The volumetric efficiency of the engine decreases after peak torque.
Those are your players.
For a couple of quick examples:
At peak TQ, I'm calculating 697HP-worth of energy going into the engine as heat from air/fuel, theoretically producing 763 ft-lbs of TQ. Taking out losses from pumping air (167 ft-lbs) and acceleration of rotating parts (149 ft-lbs), TQ is down to 447 and HP is down to 409. At peak TQ, this calculates the engine at 58.7% efficient (409/697)
At peak HP, I'm calculating 951.5-HP worth of energy going into the engine as heat, producing a theoretical 728.4 ft-lbs of TQ. Taking out pumping losses (152.1 ft-lbs) and acceleration of rotating parts (211.1 ft-lbs), TQ is down to 365.2 and HP is down to 472.9. At peak HP, this calculates to the engine being 49.7% efficient (472.9/951.5).
That's my theory, at least. In the real world, there may never be a point of stabilization after peak power because of friction increasing with rpms. But the theory that volumetric efficiency will only fall back to 100% after peak torque, and the power will only fall back to whatever your combination makes at 100% VE, would actually account for the loss of power regardless of increasing air mass.
What do you think of Speedtigger's list? I thought it was pretty complete
10-29-2015, 03:52 PM
#83
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That theory of how the power would stabilize after peak power has been on my mind for a while now, and this discussion seemed like a good place to bring it up.
I am fully aware of the power robbing friction losses, and it should be relatively obvious that you will reach a point of diminishing returns as you continue to increase friction after you stop making power.
I'm not trying to sound like a smartass, and I certainly respect your experience and value your insight.
Friction is absolutely the killer. It's just difficult to model or measure. But even without friction, the horsepower is going to fall after it peaks. And I think the location of this peak is more determined by the decrease in volumetric efficiency than friction.
Because...
If you have more efficient heads, you can use more air mass to a higher rpm, and this would produce more power, allowing you to rev higher before being overcome by friction. You are always fighting friction at every rpm level, it's always been a matter of using more air to create more power. People have built LSx motors to spin in excess of 9000rpms, so regardless of the friction at those revs, it's possible to have efficient enough heads to reach that point... So anyone encountering power drop off before 9k is experiencing a lack of volumetric efficiency, not an over abundance of friction.
I am fully aware of the power robbing friction losses, and it should be relatively obvious that you will reach a point of diminishing returns as you continue to increase friction after you stop making power.
I'm not trying to sound like a smartass, and I certainly respect your experience and value your insight.
Friction is absolutely the killer. It's just difficult to model or measure. But even without friction, the horsepower is going to fall after it peaks. And I think the location of this peak is more determined by the decrease in volumetric efficiency than friction.
Because...
If you have more efficient heads, you can use more air mass to a higher rpm, and this would produce more power, allowing you to rev higher before being overcome by friction. You are always fighting friction at every rpm level, it's always been a matter of using more air to create more power. People have built LSx motors to spin in excess of 9000rpms, so regardless of the friction at those revs, it's possible to have efficient enough heads to reach that point... So anyone encountering power drop off before 9k is experiencing a lack of volumetric efficiency, not an over abundance of friction.
10-29-2015, 04:19 PM
#84
What you are missing David is a frame of reference and understanding of the proportions that you are referring to. That is exactly why Darth V8r is doing what he is doing mathematically. He wants to be able to assign actual values to all of these factors.
The thing is, in a imaginary environment where there are no friction losses, pumping losses, windage losses, reciprocating losses or heat losses, the engine will continue to increase in power to nearly infinity as the RPMs raise. The reason is that while the measured volumetric efficiency will go down, it will still continue to take in more fuel and air as it draws harder and harder on the intake tract. Look at the dyno sheet. The engine keeps on taking in more and more air as the RPM increases even though the HP peaks. When you burn more air and more fuel, you make more heat and energy to infinity. That is just physics. The only thing that will stand in the way is if the valve train can no longer remain stable at which point the pump will pump less air and fuel.
You just have to appreciate the amount of power all of those parasitic losses take up from the engines output.
The thing is, in a imaginary environment where there are no friction losses, pumping losses, windage losses, reciprocating losses or heat losses, the engine will continue to increase in power to nearly infinity as the RPMs raise. The reason is that while the measured volumetric efficiency will go down, it will still continue to take in more fuel and air as it draws harder and harder on the intake tract. Look at the dyno sheet. The engine keeps on taking in more and more air as the RPM increases even though the HP peaks. When you burn more air and more fuel, you make more heat and energy to infinity. That is just physics. The only thing that will stand in the way is if the valve train can no longer remain stable at which point the pump will pump less air and fuel.
You just have to appreciate the amount of power all of those parasitic losses take up from the engines output.
10-29-2015, 04:41 PM
#85
TECH Addict
The parasitic losses are huge. I'm not underestimating or discrediting that, at all.
I'm saying that you can account and accept all those losses with a high enough volumetric efficiency.
If you had an infinite VE, you could continue to make power by increasing rpms up until friction welded your piston rings to the cylinder sleeves. And you would still be making power up to the moment the engine seized.
Those losses are happening at every level. It doesn't matter that the friction losses increase with rpms, because so does your volumetric efficiency and capacity to produce power.
Additional VE will always overcome additional friction losses... Up until you start melting **** or reach the natural frequencies of the moving parts. And that's a entirely different discussion about materials and processes, not torque vs horsepower, or VE versus friction.
I'm saying that you can account and accept all those losses with a high enough volumetric efficiency.
If you had an infinite VE, you could continue to make power by increasing rpms up until friction welded your piston rings to the cylinder sleeves. And you would still be making power up to the moment the engine seized.
Those losses are happening at every level. It doesn't matter that the friction losses increase with rpms, because so does your volumetric efficiency and capacity to produce power.
Additional VE will always overcome additional friction losses... Up until you start melting **** or reach the natural frequencies of the moving parts. And that's a entirely different discussion about materials and processes, not torque vs horsepower, or VE versus friction.
10-29-2015, 05:25 PM
#87
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The parasitic losses are huge. I'm not underestimating or discrediting that, at all.
I'm saying that you can account and accept all those losses with a high enough volumetric efficiency.
If you had an infinite VE, you could continue to make power by increasing rpms up until friction welded your piston rings to the cylinder sleeves. And you would still be making power up to the moment the engine seized.
Those losses are happening at every level. It doesn't matter that the friction losses increase with rpms, because so does your volumetric efficiency and capacity to produce power.
Additional VE will always overcome additional friction losses... Up until you start melting **** or reach the natural frequencies of the moving parts. And that's a entirely different discussion about materials and processes, not torque vs horsepower, or VE versus friction.
I'm saying that you can account and accept all those losses with a high enough volumetric efficiency.
If you had an infinite VE, you could continue to make power by increasing rpms up until friction welded your piston rings to the cylinder sleeves. And you would still be making power up to the moment the engine seized.
Those losses are happening at every level. It doesn't matter that the friction losses increase with rpms, because so does your volumetric efficiency and capacity to produce power.
Additional VE will always overcome additional friction losses... Up until you start melting **** or reach the natural frequencies of the moving parts. And that's a entirely different discussion about materials and processes, not torque vs horsepower, or VE versus friction.
10-29-2015, 05:39 PM
#88
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I looked up the Otto cycle efficiency - basically, the equation tells you the most efficient you can make a 4-cycle engine run. The equation is:
1 - 1 / (CR^(1.4 - 1)).
So, on my 11.5:1 CR engine, I get 1 - 1 / (11.5^0.4) = 60.4%
So, any efficiency value above 60.4% isn't possible on my engine. I'm curious to see what happens when I get the valvetrain losses in there. I would expect peak efficiency to be closer to 45%-50% and peak power to be closer to 40%.
You must really like math
Pi, Pi, mathematical Pi
3.14159
etc.
They know pi out to over 30 decimal places. Whatever number of digits gets you to "This'll be the day that I die"
Last edited by Darth_V8r; 10-29-2015 at 06:00 PM.
10-29-2015, 05:52 PM
#89
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Math is akin to poetry once you remove the **** math teachers. Once being "wrong" isn't associated with the negative connotation of failing an assignment or test, you can actually enjoy the mental exercise of trying to figure things out. Stress of judgment is the only thing keeping math from being fun. Figuring things out mathematically can be rewarding and enjoyable if you like puzzles and you aren't mentally incompetent.
10-29-2015, 08:23 PM
#90
Iirc, there have been several "teams" run there engines on test stands with electric motors (complete engines & associated accessories) to get a "real" parasitic number of the complete package.
.
.
10-29-2015, 09:11 PM
#91
On The Tree
Horsepower is a mathematical expression conjured up by marketing dweebs to impress farmers.
Farmers don't know squat about kilowatts, but a horsepower is tangible.....
So tangible a hundred years later we are still using it.
"No no! It sells cars, we can't change it!"
Farmers don't know squat about kilowatts, but a horsepower is tangible.....
So tangible a hundred years later we are still using it.
"No no! It sells cars, we can't change it!"
10-29-2015, 10:23 PM
#92
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Tq ironically enough is in foot pounds. To convert linear to rotational, you divide by 2xPi, which is 3.14158 x 2 which is 6.28316. 33,000 divided by 6.28316 = 5252, hence the formula.
Horsepower was intended to tell the average person how many horses a motor replaced, not anticipating the LS1 putting out 350 horsepower 100 years later.
I know, James Watt was a moron to not anticipate this outcome. By the way, the guy kilowatts are named after.
10-29-2015, 11:00 PM
#93
Second of all, even if it had references, you would need several such research projects to make any short term conclusion to develop any theory.
Finally, and most importantly, the numbers are in VOLUME and not MASS, and therefore, have absolutely no bearing or correlation to what you are asking. You automatically assume the data you are reading is being interpreted properly (as I mentioned a few times, our ability to observe and learn is in our hands, and this is a perfect example of how utilizing output data incorrectly leads to potential problems, it seems to happen more often when you expect or anticipate certain results, when you want to see something that is not there.) I said several times peak power is peak MASS flow. If CFM is increasing, but power is falling, then the air is simply heating and losing mass. It seems like many of you are not clear about the difference between mass and volume, are you so willing to overlook such an important detail?
To add fuel to the fire of interpretation discussion, even if we could calculate mass flow (via maf sensor for example) within a reasonable resolution (is quite possible) we STILL do not know how much of that airflow is being lost to the exhaust system during overlap. If the camshaft profile and valve events facilitate exhaust gas scavenging, a fraction of all ingested air passes immediately into the exhaust system without participating in the combustion event, and it will show up on our screen as "ingested air" even though we extract no power from it.
Last edited by kingtal0n; 10-29-2015 at 11:16 PM.
10-30-2015, 09:04 AM
#94
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A guy named Janes Watt determined that an average horse can do 33,000 pound feet of work in a minute back when engines were being developed. A linear metric.
Tq ironically enough is in foot pounds. To convert linear to rotational, you divide by 2xPi, which is 3.14158 x 2 which is 6.28316. 33,000 divided by 6.28316 = 5252, hence the formula.
Horsepower was intended to tell the average person how many horses a motor replaced, not anticipating the LS1 putting out 350 horsepower 100 years later.
I know, James Watt was a moron to not anticipate this outcome. By the way, the guy kilowatts are named after.
Tq ironically enough is in foot pounds. To convert linear to rotational, you divide by 2xPi, which is 3.14158 x 2 which is 6.28316. 33,000 divided by 6.28316 = 5252, hence the formula.
Horsepower was intended to tell the average person how many horses a motor replaced, not anticipating the LS1 putting out 350 horsepower 100 years later.
I know, James Watt was a moron to not anticipate this outcome. By the way, the guy kilowatts are named after.
10-30-2015, 09:20 AM
#95
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And according a couple of European friends, there is talk of coming up with a metric horsepower as a unit of work.
10-30-2015, 09:58 AM
#97
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Yes, cfm is a measure of volume, not mass. There is absolutely a distinct difference. But the density of the environment is relatively constant (unless you change elevation or boost), so seeing measured volume increase, with a relatively constant density, you can predict/calculate that mass is increasing with the volume.
Yes, some mass/density is lost to the air expanding as it heats up. We take active measures to try control this with cold air intakes and intercoolers. All in an effort to preserve or maintain air density. More air MASS is more power.
And some is lost to overlap, for sure. We all know overlap effects torque and we find the compromise that fits our needs. We can't measure this lost air mass, but we can, and often do, compensate for it by either doing things to increase the amount of air or decrease the amount of overlap, to maintain the amount of air mass we need for the power we want.
The mass vs volume thing comes up a lot in discussions pertaining to superchargers vs turbos. And the best way I have to describe the difference between mass and volume is, volume is a bucket. It doesn't care if it's filled with bricks or feathers.
We obviously want our buckets filled with bricks, not feathers.
And yes, I understand that the evidence presented in this discussion, or anywhere else for that matter, needs to be taken with a grain of salt. Speedtrigger could have typed that up with numbers he made up in his head. It could be entirely false, for sure. Even if he had references, chances are most sheeple won't cross examine it anyways.
But, the entire reason that this community continues to be a community, is because we allot one another some amount of trust. We generally believe that the people here are, indeed, who they say they are. We trust that the people here own the cars they say they own, and have actually accomplished the things they say they have done. If you don't believe anything you read or see on this site, why the hell would you be part of this community?
So, I apologize if I incorrectly stated that this discussion was a "theory". I did say that I wanted to keep it relatively scientific and asked for where the numbers in the OP's equation came from. But Jesus, I'm not going to start demanding references for dyno results.
Yes, some mass/density is lost to the air expanding as it heats up. We take active measures to try control this with cold air intakes and intercoolers. All in an effort to preserve or maintain air density. More air MASS is more power.
And some is lost to overlap, for sure. We all know overlap effects torque and we find the compromise that fits our needs. We can't measure this lost air mass, but we can, and often do, compensate for it by either doing things to increase the amount of air or decrease the amount of overlap, to maintain the amount of air mass we need for the power we want.
The mass vs volume thing comes up a lot in discussions pertaining to superchargers vs turbos. And the best way I have to describe the difference between mass and volume is, volume is a bucket. It doesn't care if it's filled with bricks or feathers.
We obviously want our buckets filled with bricks, not feathers.
And yes, I understand that the evidence presented in this discussion, or anywhere else for that matter, needs to be taken with a grain of salt. Speedtrigger could have typed that up with numbers he made up in his head. It could be entirely false, for sure. Even if he had references, chances are most sheeple won't cross examine it anyways.
But, the entire reason that this community continues to be a community, is because we allot one another some amount of trust. We generally believe that the people here are, indeed, who they say they are. We trust that the people here own the cars they say they own, and have actually accomplished the things they say they have done. If you don't believe anything you read or see on this site, why the hell would you be part of this community?
So, I apologize if I incorrectly stated that this discussion was a "theory". I did say that I wanted to keep it relatively scientific and asked for where the numbers in the OP's equation came from. But Jesus, I'm not going to start demanding references for dyno results.
10-30-2015, 01:39 PM
#98
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Yes, cfm is a measure of volume, not mass. There is absolutely a distinct difference. But the density of the environment is relatively constant (unless you change elevation or boost), so seeing measured volume increase, with a relatively constant density, you can predict/calculate that mass is increasing with the volume.
So, I apologize if I incorrectly stated that this discussion was a "theory". I did say that I wanted to keep it relatively scientific and asked for where the numbers in the OP's equation came from. But Jesus, I'm not going to start demanding references for dyno results.
So, I apologize if I incorrectly stated that this discussion was a "theory". I did say that I wanted to keep it relatively scientific and asked for where the numbers in the OP's equation came from. But Jesus, I'm not going to start demanding references for dyno results.
As far as saying the discussion is theory, I would agree it is theoretical in nature, by design, as opposed to practical application. I don't expect to be able to perfectly model every internal combustion engine out there in MS Excel. I just wanted to understand what causes power to fall off after peak power, and then be able to make some basic changes to the model to see the effect on where the engine makes peak power and peak torque.
Obviously, step 1 is getting the basic variables accounted for. Step 2 is getting the model close enough to reality to see patterns and pattern changes under different conditions. Step 3 is going to be modifying basic variables and seeing if it lines up with known observations. If it does, then maybe we can see what drives power to fall off past peak.
10-30-2015, 02:01 PM
#99
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Like I said in my first response to this discussion, we could really use an equation for cylinder pressure that uses octane rating, compression ratio, air fuel ratio, and cylinder volume as inputs.
I don't know if the btu vs octane rating relationship is linear, but it should be capable of being calculated.
Air fuel ratio and cylinder volume are really only used to find the mass of fuel available for combustion.
A spreadsheet in which you can link each cell to the formula, so you can play with different values for each input would be awesome.
Figuring out the correct formula is why I was so adamant about finding the source of those multipliers that you originally didn't have an explanation for.
I am trying to follow along with this discussion, making the interactive spreadsheet in Excel as we go. But it's my first time seeing or playing with a lot of these formulas, so I need things broken down into layman's terms. It's been a fun process so far, and this discussion has been quite eye opening and helpful.
I don't know if the btu vs octane rating relationship is linear, but it should be capable of being calculated.
Air fuel ratio and cylinder volume are really only used to find the mass of fuel available for combustion.
A spreadsheet in which you can link each cell to the formula, so you can play with different values for each input would be awesome.
Figuring out the correct formula is why I was so adamant about finding the source of those multipliers that you originally didn't have an explanation for.
I am trying to follow along with this discussion, making the interactive spreadsheet in Excel as we go. But it's my first time seeing or playing with a lot of these formulas, so I need things broken down into layman's terms. It's been a fun process so far, and this discussion has been quite eye opening and helpful.
