DCR and E85
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Vizard's data on CR and hp.
http://www.sunyabem.org/images/chart.bmp
The gains are not nearly as much as many people think. However, as another post pointed out, a higher CR allows you to run more cam. If the rest of the combo is up to the task, the higher rpm's allowed by the bigger cam can make substantial hp increases. So, as the other poster said - chose the CR according to the cam (and desired rpm range).
Rich
http://www.sunyabem.org/images/chart.bmp
The gains are not nearly as much as many people think. However, as another post pointed out, a higher CR allows you to run more cam. If the rest of the combo is up to the task, the higher rpm's allowed by the bigger cam can make substantial hp increases. So, as the other poster said - chose the CR according to the cam (and desired rpm range).
Rich
Last edited by rskrause; Oct 16, 2008 at 09:59 PM.
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Joined: Feb 2000
Posts: 1,445
From: Seattle, WA
Agreed... though there's something wonky with that chart (maybe I'm just having trouble interpreting it)... I keep getting a hp gain of 0 going from 10:1 to 12:1... that can't be right.
I do recall the diminishing returns though... almost a logarithmic curve beyond 12:1, but I can't seem to find the chart anywhere (maybe its in a book at home).
Here's an interesting warning about tuning with E85:
Tuning with Class 1 E85 and then running on Class 2 or Class 3 could lead to knock/detonation. Be careful what you put in the tank when tuning.
I do recall the diminishing returns though... almost a logarithmic curve beyond 12:1, but I can't seem to find the chart anywhere (maybe its in a book at home).
Here's an interesting warning about tuning with E85:
The first thing we need to know is that E85, the most common of the ethanol fuel blends, is actually three fuel grades:
Class 1 or "pure" E85 contains 80 to 84 percent ethanol, while the remainder of the blend is commercial-grade (around 85 pump octane) gasoline.
Class 2 or E75 is 75 to 79 percent ethanol, while
Class 3 or E70 is 70 to 74 percent ethanol.
However, all three classes of fuel may be marketed as E85 at various times during the year. While it seems confusing, this is done mainly to offer better cold-starting performance-which is a problem with ethanol fuels. Since straight ethanol has a relatively low Reid vapor pressure (meaning it doesn't like to light off at low temperatures), greater percentages of gasoline are added to the blend for colder weather.
So while E85 is often described as 105 pump octane, its actual rating can vary depending upon the seasonal blend. Naturally, higher gasoline content will tend to lower the pump octane from 105 for "pure" E85 to perhaps 100 for E75...
Class 1 or "pure" E85 contains 80 to 84 percent ethanol, while the remainder of the blend is commercial-grade (around 85 pump octane) gasoline.
Class 2 or E75 is 75 to 79 percent ethanol, while
Class 3 or E70 is 70 to 74 percent ethanol.
However, all three classes of fuel may be marketed as E85 at various times during the year. While it seems confusing, this is done mainly to offer better cold-starting performance-which is a problem with ethanol fuels. Since straight ethanol has a relatively low Reid vapor pressure (meaning it doesn't like to light off at low temperatures), greater percentages of gasoline are added to the blend for colder weather.
So while E85 is often described as 105 pump octane, its actual rating can vary depending upon the seasonal blend. Naturally, higher gasoline content will tend to lower the pump octane from 105 for "pure" E85 to perhaps 100 for E75...
Last edited by Steve in Seattle; Oct 16, 2008 at 10:36 PM.
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Joined: Dec 1969
Posts: 10,745
From: Buffalo, New York
Agreed... though there's something wonky with that chart (maybe I'm just having trouble interpreting it)... I keep getting a hp gain of 0 going from 10:1 to 12:1... that can't be right.
I do recall the diminishing returns though... almost a logarithmic curve beyond 12:1, but I can't seem to find the chart anywhere (maybe its in a book at home).
Here's an interesting warning about tuning with E85:
Tuning with Class 1 E85 and then running on Class 2 or Class 3 could lead to knock/detonation. Be careful what you put in the tank when tuning.
I do recall the diminishing returns though... almost a logarithmic curve beyond 12:1, but I can't seem to find the chart anywhere (maybe its in a book at home).
Here's an interesting warning about tuning with E85:
Tuning with Class 1 E85 and then running on Class 2 or Class 3 could lead to knock/detonation. Be careful what you put in the tank when tuning.
Nice point about the different blends!
Rich
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Joined: Feb 2000
Posts: 1,445
From: Seattle, WA
Yeah, the scary part about the three E85 classes is that, well, they call them all E85. Car nuts would probably prefer E75, E80, E85 instead, but I guess they figure the average consumer looking to fuel their E85 engine would balk at using the lower blends.
Do you know if the DCR vs hp chart is specific to gasoline engines? or to any combustion event? I have a feeling curves between fuels would follow the same relationship, but that the hp vs. DCR curves may slope more or less depending on energy density of the fuel. I'm temped to belive a lower energy dense fuel would exhibit even less hp gains with a comparable DCR increase on a gasoline engine (the EtOH gains say 6hp/2% for a DCR increased from 8 to 9, while a gasoline engine may see 7.5hp/2.5% gains for the same change). Every time I've seen this chart (or the graph of %hp gains vs DCR, it was solely for gasoline charts).
The only reason I say this may be the case is that diesel shows a similar hp gain vs. DCR but there compression numbers are approaching 22:1 in the pre-turbo area. On most gasoline charts any DCR over 15:1 is nearly pointless so it seems strange the diesel guys would run so high when it would light off at 18:1 and be quieter to boot. Diesel energy density is higher than gasoline so i'm assuming the minimal gains those DCR's show in gasoline actually become more than insignificant when multiplied by diesels energy advantage... but like I sadi that's all conjecture based on 2 or 3 observations.
If that's the case, I'd wager the Chevy guys running E85 don't need to run on the jagged edge of detonation... DCR increases may return less hp gains than in gasoline (and it's nice to have the octane headroom over pump gas for hot days, etc...). This would be an interesting article for SAE or even just Car Craft Mag. "HP vs. DCR... fuel selection implications"
Do you know if the DCR vs hp chart is specific to gasoline engines? or to any combustion event? I have a feeling curves between fuels would follow the same relationship, but that the hp vs. DCR curves may slope more or less depending on energy density of the fuel. I'm temped to belive a lower energy dense fuel would exhibit even less hp gains with a comparable DCR increase on a gasoline engine (the EtOH gains say 6hp/2% for a DCR increased from 8 to 9, while a gasoline engine may see 7.5hp/2.5% gains for the same change). Every time I've seen this chart (or the graph of %hp gains vs DCR, it was solely for gasoline charts).
The only reason I say this may be the case is that diesel shows a similar hp gain vs. DCR but there compression numbers are approaching 22:1 in the pre-turbo area. On most gasoline charts any DCR over 15:1 is nearly pointless so it seems strange the diesel guys would run so high when it would light off at 18:1 and be quieter to boot. Diesel energy density is higher than gasoline so i'm assuming the minimal gains those DCR's show in gasoline actually become more than insignificant when multiplied by diesels energy advantage... but like I sadi that's all conjecture based on 2 or 3 observations.
If that's the case, I'd wager the Chevy guys running E85 don't need to run on the jagged edge of detonation... DCR increases may return less hp gains than in gasoline (and it's nice to have the octane headroom over pump gas for hot days, etc...). This would be an interesting article for SAE or even just Car Craft Mag. "HP vs. DCR... fuel selection implications"
Moderator
Joined: Dec 1969
Posts: 10,745
From: Buffalo, New York
Yeah, the scary part about the three E85 classes is that, well, they call them all E85. Car nuts would probably prefer E75, E80, E85 instead, but I guess they figure the average consumer looking to fuel their E85 engine would balk at using the lower blends.
Do you know if the DCR vs hp chart is specific to gasoline engines? or to any combustion event? I have a feeling curves between fuels would follow the same relationship, but that the hp vs. DCR curves may slope more or less depending on energy density of the fuel. I'm temped to belive a lower energy dense fuel would exhibit even less hp gains with a comparable DCR increase on a gasoline engine (the EtOH gains say 6hp/2% for a DCR increased from 8 to 9, while a gasoline engine may see 7.5hp/2.5% gains for the same change). Every time I've seen this chart (or the graph of %hp gains vs DCR, it was solely for gasoline charts).
The only reason I say this may be the case is that diesel shows a similar hp gain vs. DCR but there compression numbers are approaching 22:1 in the pre-turbo area. On most gasoline charts any DCR over 15:1 is nearly pointless so it seems strange the diesel guys would run so high when it would light off at 18:1 and be quieter to boot. Diesel energy density is higher than gasoline so i'm assuming the minimal gains those DCR's show in gasoline actually become more than insignificant when multiplied by diesels energy advantage... but like I sadi that's all conjecture based on 2 or 3 observations.
If that's the case, I'd wager the Chevy guys running E85 don't need to run on the jagged edge of detonation... DCR increases may return less hp gains than in gasoline (and it's nice to have the octane headroom over pump gas for hot days, etc...). This would be an interesting article for SAE or even just Car Craft Mag. "HP vs. DCR... fuel selection implications"
Do you know if the DCR vs hp chart is specific to gasoline engines? or to any combustion event? I have a feeling curves between fuels would follow the same relationship, but that the hp vs. DCR curves may slope more or less depending on energy density of the fuel. I'm temped to belive a lower energy dense fuel would exhibit even less hp gains with a comparable DCR increase on a gasoline engine (the EtOH gains say 6hp/2% for a DCR increased from 8 to 9, while a gasoline engine may see 7.5hp/2.5% gains for the same change). Every time I've seen this chart (or the graph of %hp gains vs DCR, it was solely for gasoline charts).
The only reason I say this may be the case is that diesel shows a similar hp gain vs. DCR but there compression numbers are approaching 22:1 in the pre-turbo area. On most gasoline charts any DCR over 15:1 is nearly pointless so it seems strange the diesel guys would run so high when it would light off at 18:1 and be quieter to boot. Diesel energy density is higher than gasoline so i'm assuming the minimal gains those DCR's show in gasoline actually become more than insignificant when multiplied by diesels energy advantage... but like I sadi that's all conjecture based on 2 or 3 observations.
If that's the case, I'd wager the Chevy guys running E85 don't need to run on the jagged edge of detonation... DCR increases may return less hp gains than in gasoline (and it's nice to have the octane headroom over pump gas for hot days, etc...). This would be an interesting article for SAE or even just Car Craft Mag. "HP vs. DCR... fuel selection implications"
The book I am thinking of also has a big section on diesels. You got me curious and I will dig it out and look for some answers. Problem is, I can understand only bout 50% of the math in the book. When I dig it out I will also post the title - good reading though difficult. Bok is old, but the principles are still true.
Rich
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Joined: Feb 2000
Posts: 1,445
From: Seattle, WA
Well I tried running through two engineering texts I have and they really didn't go deep enough into this topic to be much help. So, I did what I REALLY didn't want to... i broke out the Physical Chemistry text. Uggg...
The issue here is primarily that of the compression stroke (a nearly abiabatic compression), followed by the combustion which liberates heat to increase pressure. Basicly its a reversible compression cycle with energy entering as fuel and leaving as heat. The exact details of this combustion are WAY more involved than I'm willing to go.
But there are trends to identify that can shed some light on fuel choices.
Compression Factor
This factor describes how resistant a gas is to compression. An ideal gas has a Z of 1, while real gases just above 0psi have Z values less than 1, making it easier to compress than ideal, but by ~250+ bar the Z curve becomes >1 meaning it becomes more difficult to compress than an ideal gas (increasing pumping losses). The smaller the Z value on the compression stroke, the less pumping losses you'll have. Z value curves (Z vs P) vary with each unique substance, but on the whole, the larger the molecule, and the smaller the ionic/dipole charge the smaller the Z value will be intially, but it will transition over Z=1 at a lower pressure at which point the Z is higher for such substances.
This suggestes that as far as pumping losses on the compression stroke, Diesel<Gasoline<<Ethanol (small weight AND polar dipole). The other benefit here is that unburnt fuel after combustion would make the diesel less compressable during the start of the power stroke, imparting more force on the piston top sooner than gasoline and even more than ethanol.
It should be noted that the total resistance to compression includes a factor for the total number of moles used. A mole of Diesel is 150% the weight of gasoline and about 370% that of EtOH. For a fixed cylinder size with the same weight of fuel (diesel and gas both use ~14#air:1#fuel), diesel would have say 0.66 Moles diesel, or 1 mole gasoline, while EtOH would have 5.5 moles (using a AFR of 6). As you can see, the total resistance (pumping losses) are nearly 6 times greater for EtOH than gasoline. This is just the fuel of course, but the air is the same and dilutes the effect by about 95%... so the differences aren't much, but they are still significant enough to measure based on the chart I'm looking at in this text.
What does all of this have to do with compression ratio? Well, it seems the higher the pre-combustion pressure the lower Z drops, allowing greater gains (or more correctly, fewer pumping losses) from a lower Z. Beyond ~80 to 100 atm (~1400 psi) the benfits start to reverse, but this is way less than you'd see in any engine pre-combustion. The lowest Z value reached is directly related to molecualr weight and neutral charges... diesel can reach very efficient compression compared to gasoline and WAY more than EtOH.
It should be noted, that after complete combustion, all 3 have simlar Z values and molar volumes and the post-combustion pressures make for very high Z values, increasing the expanding exhaust's resistance to compression (and hense moving the piston instead). The high temperatures post combustion also translate to much higher Z values as well.
Kirchhoff's Law (Yes, the same guy as Kirchhoff's Electrical Circuit Laws... smart guy)
This is where things get a bit hazy, and where all the expensive R&D automakers rip into. While most general chemistry courses teach that the heat released by combustion varies by its reactants, they usually reference a standardized table that shows the molar heat of combustion value at a designated temperature. This allows for some reaction math to investigate different combustion reactions, but it that's not the whole story.
See the heat of combustion (enthalpy increases) for a chemical process actually increases with temperature. That means that 1 mole of diesel may have a complete combustion energy of 125,000 BTUs at a starting temp of 220*F, but could have 130,000 BTUs if combusted at 250*F. Those numbers are completely arbitraru though, since finding the exact data would NOT be easy. The two curves showing the enthalpy of products and the enthalpy of reactants can be charted on a graph of enthapy/heat (H) vs Temp (T). When done so they'll look like a parabola centered just above the origin (T=0, H=0) with the products curve being translated vertically some amount and the slope also increasing... this allows the difference between the curves (the net heat released) to get wider and wider apart as temperature rises.
While combustion will rise chamber temps significantly, the compression stroke itself actually does a great job of this as well. The hotter the initial temps, the more efficient the energy produced from combustion. This is likely one reason emmissions has increased engine temps in an attempt to get a cleaner burn. In reality, the temperature increase also brings with it BSFC gains (lower BSFC), but it also increases the likelyhood of detonation... especially at max engine VE (which is about the max torque and max heat generated per ignition event).
Now here's the interesting part. In complete combustion, the products of diesel, gasoline, and EtOH will all be the same... CO2 and H2O. The reactants however will vary by stoichometry and molar heat of combustion. From the values in my text:
EtOH: 5.5 moles x 1400kJ/mol = 7700 kJ
Octane: 1 mole x 5500kJ/mol = 5500kJ
Diesel (C12): 0.66 mole x 5550kJ/mol= 3663 kJ
So in our hypothetical cyclinder volume we'd see a measurable heat of combustion difference (difference between the numbers above and the much higher, yet equal reactants). This gives diesel the heat generation advantage we'd expect with its higher energy density, but it also says that the advantage of increasing the inital temperature (via increasing DCR) would more pronounced in diesel fuel than in gasoline, and even more so than ethanol.
All this points to measurable gains for increasing DCR in diesel and even gasoline to a point, but in EtOH the hp gains vs DCR would be even smaller (less bang for your buck as it may be). Obviously the hp gains from higher DCR is limited in gasoline due to knock, but teh Visard and other NASCAR data shows that even at higher octane, premium pump gas can get you pretty close to ideal.
Increasing temperatures also seems to be good for BSFC in the combustion event, but detrimental in the compression stroke (higher temps increases Z values)... so the temperature increase is more of a balancing act when you look at max VE conditions (but less so at part throttle when compression stroke losses are much less due to starting in a vaccum).
I still believe the C6R running on E85 mentioned earlier is probably running a poitn or two high compression than we could run on premium pump gas, but to think that it would be finding hp gains by upping the DCR from even racegas levels seems unlikely.
I don't think they had to mess with DCR to get E85 to perform.
Disclaimer: I hated P.Chem. I'm probably missing a few points in here but I think that's the overall thermodynamics pertaining to DCR... except for the really gritty combustion changes at high pressures (the stuff engine makers spedn big bucks to investigate). EtOH may behave much differently in the combustion event that longer hydrocarbons (i.e. shockwave propigation), but that's WAY beyond the scope here and unlikely to change any apprecable amount due solely to DCR (once the pressure spike from combustion happens, the few extra psi added by 1 point fo DCR is minimal).
The issue here is primarily that of the compression stroke (a nearly abiabatic compression), followed by the combustion which liberates heat to increase pressure. Basicly its a reversible compression cycle with energy entering as fuel and leaving as heat. The exact details of this combustion are WAY more involved than I'm willing to go.
But there are trends to identify that can shed some light on fuel choices.
Compression Factor
This factor describes how resistant a gas is to compression. An ideal gas has a Z of 1, while real gases just above 0psi have Z values less than 1, making it easier to compress than ideal, but by ~250+ bar the Z curve becomes >1 meaning it becomes more difficult to compress than an ideal gas (increasing pumping losses). The smaller the Z value on the compression stroke, the less pumping losses you'll have. Z value curves (Z vs P) vary with each unique substance, but on the whole, the larger the molecule, and the smaller the ionic/dipole charge the smaller the Z value will be intially, but it will transition over Z=1 at a lower pressure at which point the Z is higher for such substances.
This suggestes that as far as pumping losses on the compression stroke, Diesel<Gasoline<<Ethanol (small weight AND polar dipole). The other benefit here is that unburnt fuel after combustion would make the diesel less compressable during the start of the power stroke, imparting more force on the piston top sooner than gasoline and even more than ethanol.
It should be noted that the total resistance to compression includes a factor for the total number of moles used. A mole of Diesel is 150% the weight of gasoline and about 370% that of EtOH. For a fixed cylinder size with the same weight of fuel (diesel and gas both use ~14#air:1#fuel), diesel would have say 0.66 Moles diesel, or 1 mole gasoline, while EtOH would have 5.5 moles (using a AFR of 6). As you can see, the total resistance (pumping losses) are nearly 6 times greater for EtOH than gasoline. This is just the fuel of course, but the air is the same and dilutes the effect by about 95%... so the differences aren't much, but they are still significant enough to measure based on the chart I'm looking at in this text.
What does all of this have to do with compression ratio? Well, it seems the higher the pre-combustion pressure the lower Z drops, allowing greater gains (or more correctly, fewer pumping losses) from a lower Z. Beyond ~80 to 100 atm (~1400 psi) the benfits start to reverse, but this is way less than you'd see in any engine pre-combustion. The lowest Z value reached is directly related to molecualr weight and neutral charges... diesel can reach very efficient compression compared to gasoline and WAY more than EtOH.
It should be noted, that after complete combustion, all 3 have simlar Z values and molar volumes and the post-combustion pressures make for very high Z values, increasing the expanding exhaust's resistance to compression (and hense moving the piston instead). The high temperatures post combustion also translate to much higher Z values as well.
Kirchhoff's Law (Yes, the same guy as Kirchhoff's Electrical Circuit Laws... smart guy)
This is where things get a bit hazy, and where all the expensive R&D automakers rip into. While most general chemistry courses teach that the heat released by combustion varies by its reactants, they usually reference a standardized table that shows the molar heat of combustion value at a designated temperature. This allows for some reaction math to investigate different combustion reactions, but it that's not the whole story.
See the heat of combustion (enthalpy increases) for a chemical process actually increases with temperature. That means that 1 mole of diesel may have a complete combustion energy of 125,000 BTUs at a starting temp of 220*F, but could have 130,000 BTUs if combusted at 250*F. Those numbers are completely arbitraru though, since finding the exact data would NOT be easy. The two curves showing the enthalpy of products and the enthalpy of reactants can be charted on a graph of enthapy/heat (H) vs Temp (T). When done so they'll look like a parabola centered just above the origin (T=0, H=0) with the products curve being translated vertically some amount and the slope also increasing... this allows the difference between the curves (the net heat released) to get wider and wider apart as temperature rises.
While combustion will rise chamber temps significantly, the compression stroke itself actually does a great job of this as well. The hotter the initial temps, the more efficient the energy produced from combustion. This is likely one reason emmissions has increased engine temps in an attempt to get a cleaner burn. In reality, the temperature increase also brings with it BSFC gains (lower BSFC), but it also increases the likelyhood of detonation... especially at max engine VE (which is about the max torque and max heat generated per ignition event).
Now here's the interesting part. In complete combustion, the products of diesel, gasoline, and EtOH will all be the same... CO2 and H2O. The reactants however will vary by stoichometry and molar heat of combustion. From the values in my text:
EtOH: 5.5 moles x 1400kJ/mol = 7700 kJ
Octane: 1 mole x 5500kJ/mol = 5500kJ
Diesel (C12): 0.66 mole x 5550kJ/mol= 3663 kJ
So in our hypothetical cyclinder volume we'd see a measurable heat of combustion difference (difference between the numbers above and the much higher, yet equal reactants). This gives diesel the heat generation advantage we'd expect with its higher energy density, but it also says that the advantage of increasing the inital temperature (via increasing DCR) would more pronounced in diesel fuel than in gasoline, and even more so than ethanol.
All this points to measurable gains for increasing DCR in diesel and even gasoline to a point, but in EtOH the hp gains vs DCR would be even smaller (less bang for your buck as it may be). Obviously the hp gains from higher DCR is limited in gasoline due to knock, but teh Visard and other NASCAR data shows that even at higher octane, premium pump gas can get you pretty close to ideal.
Increasing temperatures also seems to be good for BSFC in the combustion event, but detrimental in the compression stroke (higher temps increases Z values)... so the temperature increase is more of a balancing act when you look at max VE conditions (but less so at part throttle when compression stroke losses are much less due to starting in a vaccum).
I still believe the C6R running on E85 mentioned earlier is probably running a poitn or two high compression than we could run on premium pump gas, but to think that it would be finding hp gains by upping the DCR from even racegas levels seems unlikely.
I don't think they had to mess with DCR to get E85 to perform.
Disclaimer: I hated P.Chem. I'm probably missing a few points in here but I think that's the overall thermodynamics pertaining to DCR... except for the really gritty combustion changes at high pressures (the stuff engine makers spedn big bucks to investigate). EtOH may behave much differently in the combustion event that longer hydrocarbons (i.e. shockwave propigation), but that's WAY beyond the scope here and unlikely to change any apprecable amount due solely to DCR (once the pressure spike from combustion happens, the few extra psi added by 1 point fo DCR is minimal).
Last edited by Steve in Seattle; Oct 20, 2008 at 06:13 PM.
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Joined: Feb 2000
Posts: 1,445
From: Seattle, WA
A mole of Diesel is 150% the weight of gasoline and about 370% that of EtOH. For a fixed cylinder size with the same weight of fuel (diesel and gas both use ~14#air:1#fuel), diesel would have say 0.66 Moles diesel, or 1 mole gasoline, while EtOH would have 5.5 moles (using a AFR of 6). As you can see, the total resistance (pumping losses) are nearly 6 times greater for EtOH than gasoline. This is just the fuel of course, but the air is the same and dilutes the effect by about 95%... so the differences aren't much, but they are still significant enough to measure based on the chart I'm looking at in this text.
When we (car nuts) normally refer to pumping losses, we're attempting to single out ALL the sources of lost hp, including friction losses associated with piston movement in an engine (which are the largest issue outside the compression stroke). True "pumping losses" are minimal when one of the valves is actually open and the piston is just push/pulling charge in/out of the cylinder compared to the force needed to compress the chamber. It's this force I referring to in this analysis of DCR changes and fuel selection. The others losses are unaffected. It may SEEM like a lot to say diesel is over 6 times more efficient in compression, but the dilution of this affect by the 90% air in the cylinder, and the fact that these losses are but one componant of the parasitic losses in an engine make the differences much smaller (though still measureable).
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Joined: Feb 2000
Posts: 1,445
From: Seattle, WA
Compression Factor
This factor describes how resistant a gas is to compression. An ideal gas has a Z of 1, while real gases just above 0psi have Z values less than 1, making it easier to compress than ideal, but by ~250+ bar the Z curve becomes >1 meaning it becomes more difficult to compress than an ideal gas (increasing pumping losses). ... Z value curves (Z vs P) vary with each unique substance, but on the whole, the larger the molecule, and the smaller the ionic/dipole charge the smaller the Z value will be intially, but it will transition over Z=1 at a lower pressure at which point the Z is higher for such substances.
This factor describes how resistant a gas is to compression. An ideal gas has a Z of 1, while real gases just above 0psi have Z values less than 1, making it easier to compress than ideal, but by ~250+ bar the Z curve becomes >1 meaning it becomes more difficult to compress than an ideal gas (increasing pumping losses). ... Z value curves (Z vs P) vary with each unique substance, but on the whole, the larger the molecule, and the smaller the ionic/dipole charge the smaller the Z value will be intially, but it will transition over Z=1 at a lower pressure at which point the Z is higher for such substances.
8This graph is for air, and you can see how going from 250*K (-23*C / -10*F) to 300*K (27*C, 80*F) you start to lose this "ease of compression" advantage... showing how colder intake temps can reduce pumping losses (in addition to increasing total O2/oxidative content in a cylinder).
It's worth nothing that organic compounds like Methane show similar trends, but even a 2-carbon organic like ethane takes a much steeper dip at room temp than you see for air at even 250K. Organics compress easily until ~200 atmospheres (~3000psi) and by ~300 atmospheres becomes more resistant to compression than diatomic gases.
Diesel gets an advantage here, especially since it gets a lower Z value AND lower number of moles per combustion cycle (given the same hp or displacement)... and of course by having a favorable curve can make temperature increases (to increase BSFC/fuel efficiency) less of a penalty.
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