In gas engines, does compression ratio stay constant over the RPM range?
- Thread starter RU482
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In a normally aspirated motor yes, CR can be higher related to RPM and boost levels in a turbo or supercharged motor. Engine load has a lot to do with "compressed" motors too..
Well, its a little more complicated than just a y/n answer. It depends on the valve timing hardware, as well as if you mean the actual "dynamic combustion pressure" or simply the mechanical static ratio of the piston stroke calculated with TDC and BDC volume.
Some engines utilize multiple camshafts with more than 2 valves, dynamically adjustable gear or belt drives that can affect cam timing, or lifters (aka "rhoads" or crane "hi-vac" style), and some other specific-engine designs, which vary the valve duration, overlap, and lift RPM dependent. For example, the rhoads type lifters "bleed off" some oil pressure at lower engine speeds, reslting in less overlap and/or duraton, effectively raising an engines low rpm dynamic combustion pressure. So although the static "compression ratio" is constant, there are other confounding variables that affect the engines effective dynamic pressures. Even the intake runners*( and to lesser degree, the exhaust) design can affect dynamic compression pressures, as at certain speeds, a particular tract will be more efficient at filling the cylinder with greater volume of fuel/air (increased volumetric efficiency due to harmonics of length/speed/cross section/surface texture and even type/radius of bends , valve/seat angles) and cause a higher combustion pressure AT THAT SPEED.
That increased "scavenging" effect is also apparent with certain header and exhaust designs, that also cause more spent fuel charge to be removed from engine resulting in relatively lower cylinder pressure (partial vacuum actually) again increaing the amount of new fuel/air mixture drawn into engine, again obviously affecting that cylinders maximum combustion pressure, again at that particular engine speed where the exhaust runner/collector/merger are tuned to take advantage of the flow characteristics.
There are other ways effective cylinder pressures are also affected, ie stoichiometric fuel/air ratio adjustments (FI "fuel curve"), spark timing curve, temperature, rpm, load, but these are more "givens" in the equation than variables.
OK, so I took the long way home, but you asked..!
Some engines utilize multiple camshafts with more than 2 valves, dynamically adjustable gear or belt drives that can affect cam timing, or lifters (aka "rhoads" or crane "hi-vac" style), and some other specific-engine designs, which vary the valve duration, overlap, and lift RPM dependent. For example, the rhoads type lifters "bleed off" some oil pressure at lower engine speeds, reslting in less overlap and/or duraton, effectively raising an engines low rpm dynamic combustion pressure. So although the static "compression ratio" is constant, there are other confounding variables that affect the engines effective dynamic pressures. Even the intake runners*( and to lesser degree, the exhaust) design can affect dynamic compression pressures, as at certain speeds, a particular tract will be more efficient at filling the cylinder with greater volume of fuel/air (increased volumetric efficiency due to harmonics of length/speed/cross section/surface texture and even type/radius of bends , valve/seat angles) and cause a higher combustion pressure AT THAT SPEED.
That increased "scavenging" effect is also apparent with certain header and exhaust designs, that also cause more spent fuel charge to be removed from engine resulting in relatively lower cylinder pressure (partial vacuum actually) again increaing the amount of new fuel/air mixture drawn into engine, again obviously affecting that cylinders maximum combustion pressure, again at that particular engine speed where the exhaust runner/collector/merger are tuned to take advantage of the flow characteristics.
There are other ways effective cylinder pressures are also affected, ie stoichiometric fuel/air ratio adjustments (FI "fuel curve"), spark timing curve, temperature, rpm, load, but these are more "givens" in the equation than variables.
OK, so I took the long way home, but you asked..!
In other words, what Krazefinn said was, static compression ratio never changes, but cylinder pressure can and does.
- Oct 10, 1999
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no, its not more complicated.
the answer is yes, mechanical compression ratio stays the same, always.
In forced induction engines, apparent compression changes with boost applied.
History is replete with inventors who have either faced failure many times, or been told accepted wisdom dictates success impossible, yet they continue to advance human inventions. Armchair quarterbacks seem to never make any positive predictions....even in hindsight.
Edison failed 199 times trying to create an electric light...and got it on the 200th.
The water is no heavier than fuel, keeping it liquid is not an insurmounatble problem,
having an engine revolve 50% more certainly is not the premature deathknell of design anymore,
nor is lubrication going to be a concern.
(2 stroke mc engines last twice as long as 4 stroke?)
Furthermore fuelcells are already proven for more than 2 decades at nasa, will be just a matter of time before it gets perfected..
I think being told it wont work is the fuel on inventors...I for one redouble my personal effort when told its not possible. OK, so sometimes I prove that...but then theres that the other outcome...!
Edison failed 199 times trying to create an electric light...and got it on the 200th.
The water is no heavier than fuel, keeping it liquid is not an insurmounatble problem,
having an engine revolve 50% more certainly is not the premature deathknell of design anymore,
nor is lubrication going to be a concern.
(2 stroke mc engines last twice as long as 4 stroke?)
Furthermore fuelcells are already proven for more than 2 decades at nasa, will be just a matter of time before it gets perfected..
I think being told it wont work is the fuel on inventors...I for one redouble my personal effort when told its not possible. OK, so sometimes I prove that...but then theres that the other outcome...!
- Oct 10, 1999
- 31,193
- 12,721
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so what is your point?
besides, 2 stroke engines have many serious differences compared to 4 stroke engines. You can't really compare the two. Each has its good and bad points.
Errr, point is I screwed up and posted reply in wrong thread, above post was meant for another discussion which points were germane to...sorry.
No point salient to this thread, at all! Totally off topic here...duhhh, derailed train of thought. Goin' to bed seein' as I'm already half asleep!)..g'nite all.
wake me when we get there..
No point salient to this thread, at all! Totally off topic here...duhhh, derailed train of thought. Goin' to bed seein' as I'm already half asleep!)..g'nite all.
wake me when we get there..
- Oct 10, 1999
- 31,193
- 12,721
- 136
The answer two your question. Does compression RATIO stay constant over the RPM range.
The answer is yes.
10 to 1 compression is still 10 to 1 compression with 10 pounds of boost or with only atmospheric pressure as the multiplier.
10 to 1 no boost = 14.7 x 10 (14.7 = atmospheric pressure add or take a little depending on your altitude)
10 to 1 ten pounds of boost = 24.7 x 10
Ratio is constant. Yeah there is alot of valve timing tricks , and even more so with camless electronic valve operation. This does not change the ratio.
The answer is yes.
10 to 1 compression is still 10 to 1 compression with 10 pounds of boost or with only atmospheric pressure as the multiplier.
10 to 1 no boost = 14.7 x 10 (14.7 = atmospheric pressure add or take a little depending on your altitude)
10 to 1 ten pounds of boost = 24.7 x 10
Ratio is constant. Yeah there is alot of valve timing tricks , and even more so with camless electronic valve operation. This does not change the ratio.
OrganizedChaos
Diamond Member
- Apr 21, 2002
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static compression ratio won't change.
volumetric efficiancy changes with RPM and is capable of excedding 100% in some engines but i'm not sure if this is considered an increase in effective compression.
edit: thats assuming its a NA engine. if its turbo charged or uses a centrifugul supercharger your effective compression is going to be all over the map depending on boost pressure at any given moment.
edit2: ignore my post and read krazefanns, he explained it better and knows how to spell and use grammer.
volumetric efficiancy changes with RPM and is capable of excedding 100% in some engines but i'm not sure if this is considered an increase in effective compression.
edit: thats assuming its a NA engine. if its turbo charged or uses a centrifugul supercharger your effective compression is going to be all over the map depending on boost pressure at any given moment.
edit2: ignore my post and read krazefanns, he explained it better and knows how to spell and use grammer.
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