What is the maximum compression ratio a diesel engine will run smoothly ???? Next question after that
will the exhaust be cleaner ( devoid of NOX ???? )
will the exhaust be cleaner ( devoid of NOX ???? )
ta152h0
Turbo or non-turbo?
Compression RATIO is one variable. It can be a cruel mistress.. The trick is to keep the peak cylinder pressures within safe parameters.
NOX is more affected by the fact that diesels run so lean and there is an abundance of O2 in the exhaust as compared to a non-egr gas engine running at stoichiometric.
Something that might make this stuff easier to understand is that when the fuel is "oxidized" in the cylinder several chemical reactions are occurring at the same time. Some dependant on others. The interesting thing about those reactions is that varying the physical conditions in the cylinder can drastically change the resultant gas stream makeup. Suppress one reaction and another one may increase. My point is that what you see in the exhaust is the result of several reactions occurring in "chemical" equilibrium. Some of the parameters that make the engine run efficiently and with good power increase NOX. I think the consensus among those that have to deal with it is that its going to be easier to get rid of NOX via a catalyst in the exhaust vs making the engine not produce it in the first place. Adding EGR has been a step in reducing it to a level where its more practical to get it out with a converter.
I don't know if I am on the right track with the intent of the post. If not you can post more about the results you are after we might be able to better answer the question.
Compression RATIO is one variable. It can be a cruel mistress.. The trick is to keep the peak cylinder pressures within safe parameters.
NOX is more affected by the fact that diesels run so lean and there is an abundance of O2 in the exhaust as compared to a non-egr gas engine running at stoichiometric.
Something that might make this stuff easier to understand is that when the fuel is "oxidized" in the cylinder several chemical reactions are occurring at the same time. Some dependant on others. The interesting thing about those reactions is that varying the physical conditions in the cylinder can drastically change the resultant gas stream makeup. Suppress one reaction and another one may increase. My point is that what you see in the exhaust is the result of several reactions occurring in "chemical" equilibrium. Some of the parameters that make the engine run efficiently and with good power increase NOX. I think the consensus among those that have to deal with it is that its going to be easier to get rid of NOX via a catalyst in the exhaust vs making the engine not produce it in the first place. Adding EGR has been a step in reducing it to a level where its more practical to get it out with a converter.
I don't know if I am on the right track with the intent of the post. If not you can post more about the results you are after we might be able to better answer the question.
I do not know the answer. However, the 6.0L is 18:1 and the 7.3L is 17:1.
It seems to me that if you go much higher then you are going to have have to have some very high powered fuel delivery system to overcome the pressure. This would be more and more expensive to mass produce as the compression ratio goes up. You might also have trouble disapating the additional heat.
If I follow your logic you are thinking that a higher compression ratioed diesel will burn cleaner? Dunno!!??
It seems to me that if you go much higher then you are going to have have to have some very high powered fuel delivery system to overcome the pressure. This would be more and more expensive to mass produce as the compression ratio goes up. You might also have trouble disapating the additional heat.
If I follow your logic you are thinking that a higher compression ratioed diesel will burn cleaner? Dunno!!??
Aren't most of the modern diesels running fuel pressures at the injecter around 20000+ psi? I don't think the fuel getting in there would be the problem I would guess the headgaskets would be the weak point. As a 6.0 owner I am definatly aware of this weakness when boost and cylinder presures get to high. I too have no idea on the effect of NOX emissions or any other byproduct of running higher compression. /ubbthreads/images/graemlins/confused.gif
Turbo. I don't believe we are ever going to see a non-turbo EPA certified power plant used widely on american roads. I just wish the turbo in mine was louder, like a locomotiove turbo spooling up. /ubbthreads/images/graemlins/biggrin.gif
The higher the compression ratio, the better the engine will run and the more efficient it will be.
The only limit is the mechanical design and how much heat the piston tops can take.
Their is less heat rejection to the water jacket with a higher compression ratio.
I am assuming that what your getting at is the higher firing pressures increasing the power pulses and causing the engine to vibrate more.
Thats only a question an engine designer could answer. /ubbthreads/images/graemlins/smile.gif
The only limit is the mechanical design and how much heat the piston tops can take.
Their is less heat rejection to the water jacket with a higher compression ratio.
I am assuming that what your getting at is the higher firing pressures increasing the power pulses and causing the engine to vibrate more.
Thats only a question an engine designer could answer. /ubbthreads/images/graemlins/smile.gif
About 20 years ago asked this question of an engineer at Detroit Diesel while he visited our facility (I was working at another GM another division at the time). I recall these facts from our discussion:
1 – The higher the compression ratio the higher a diesel’s thermal efficiency (that’s a fundamental thermodynamic property of the diesel cycle...).
2 – Higher compression ratios result in higher mechanical and thermal stresses in an engine.
3 – There is level of compression where efficiency gains from higher compression are more than offset by higher mechanical frictional losses. The engineer felt that for naturally aspirated engines the this critical CR was in the neighborhood of 24:1. And for turbocharged engines about 18:1 or so.
4 – Higher compression ratios requires closer combustion chamber dimensional tolerances. This means higher engine manufacturing costs. It also means the engine will suffer more rapid loss of compression (thus loss of efficiency) as engine parts wear. Also means shorter engine life.
5 – Certain emissions components (specifically NOX) increase with higher compression ratios.
Bottom line is higher compression ratios are feasible, and in fact are desirable from a thermodynamic standpoint. But for overall cost and reliability the above mentioned ratios are about the practical maximums – at least they were with the materials, lubrication, manufacturing and emission control technologies available 20 years ago. …FB
1 – The higher the compression ratio the higher a diesel’s thermal efficiency (that’s a fundamental thermodynamic property of the diesel cycle...).
2 – Higher compression ratios result in higher mechanical and thermal stresses in an engine.
3 – There is level of compression where efficiency gains from higher compression are more than offset by higher mechanical frictional losses. The engineer felt that for naturally aspirated engines the this critical CR was in the neighborhood of 24:1. And for turbocharged engines about 18:1 or so.
4 – Higher compression ratios requires closer combustion chamber dimensional tolerances. This means higher engine manufacturing costs. It also means the engine will suffer more rapid loss of compression (thus loss of efficiency) as engine parts wear. Also means shorter engine life.
5 – Certain emissions components (specifically NOX) increase with higher compression ratios.
Bottom line is higher compression ratios are feasible, and in fact are desirable from a thermodynamic standpoint. But for overall cost and reliability the above mentioned ratios are about the practical maximums – at least they were with the materials, lubrication, manufacturing and emission control technologies available 20 years ago. …FB
Bob, couple an 18:1 engine with 100 lbs of boost and you have well over a 24:1 effective compression ratio. /ubbthreads/images/graemlins/smile.gif
New computer controls enable retarded timing etc based on boost that allows you to run higher boost (effective compression) and recover lost thermal energy without melting pistons, and wearing out the engine so quickly. /ubbthreads/images/graemlins/cool.gif
New computer controls enable retarded timing etc based on boost that allows you to run higher boost (effective compression) and recover lost thermal energy without melting pistons, and wearing out the engine so quickly. /ubbthreads/images/graemlins/cool.gif
Is the rebuild a "boring out - sleeving the bore operation" ??? Is there enough " cylinder wall thickness " on a PSD to do this ???
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Bob, couple an 18:1 engine with 100 lbs of boost and you have well over a 24:1 effective compression ratio. /ubbthreads/images/graemlins/smile.gif
New computer controls enable retarded timing etc based on boost that allows you to run higher boost (effective compression) and recover lost thermal energy without melting pistons, and wearing out the engine so quickly. /ubbthreads/images/graemlins/cool.gif
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Not exactly... By definition compression ratio is the change in combustion chamber volume from the point where the intake valve is fully closed to the point the piston reaches TDC. Therefore from a thermodynamic perspective, increasing boost does NOT cause an increase an engine's effective compression ratio.
HOWEVER increasing boost does increase combustion air density. This in turn permits more fuel to be burned and thus more power produced. And modern computer controls permit higher boost levels to be run without trashing the engine.
Cool (or more more appropriately "hot") stuff indeed! ...FB
Bob, couple an 18:1 engine with 100 lbs of boost and you have well over a 24:1 effective compression ratio. /ubbthreads/images/graemlins/smile.gif
New computer controls enable retarded timing etc based on boost that allows you to run higher boost (effective compression) and recover lost thermal energy without melting pistons, and wearing out the engine so quickly. /ubbthreads/images/graemlins/cool.gif
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Not exactly... By definition compression ratio is the change in combustion chamber volume from the point where the intake valve is fully closed to the point the piston reaches TDC. Therefore from a thermodynamic perspective, increasing boost does NOT cause an increase an engine's effective compression ratio.
HOWEVER increasing boost does increase combustion air density. This in turn permits more fuel to be burned and thus more power produced. And modern computer controls permit higher boost levels to be run without trashing the engine.
Cool (or more more appropriately "hot") stuff indeed! ...FB
That is what I got confused a little bit. I thought compression ratio was a mechanical ratio without the P-v and density variables. You can't " flood " these things and put out the fire like a gasser, can you ????
Throw some stuff. The high efficiency does not come from a high compression ratio. The high efficiency comes from a high EXPANSION ratio. In other words the efficiency comes from maximum temperature drop on the power stroke. That being said the normal way to a high expansion ratio IS a high compression ratio. There are ways (Escape Hybrid as an example) where the expansion ratio is higher than the compression ratio.
Pre-chamber diesels normally have a higher static compression ratio than open chamber engines. Smaller cylinder engines normally have a higher compression ratio than large cylinder engines. The reasons for this have to do with rapid heat loss to the cylinder walls and head at low/cranking speeds. In other words small cylinders and pre-chambers require higher compression ratios to start. This high heat loss continues when these engines are at operating temperature. High cylinder volume to cylinder surface area reduces this heat loss. Big cylinder open chamber engines have better fuel economy because of this lower heat loss.
Most everything said about combustion peak temperatures, flame speed etc. definitely effects the exhaust products produced.
EDWARD
Pre-chamber diesels normally have a higher static compression ratio than open chamber engines. Smaller cylinder engines normally have a higher compression ratio than large cylinder engines. The reasons for this have to do with rapid heat loss to the cylinder walls and head at low/cranking speeds. In other words small cylinders and pre-chambers require higher compression ratios to start. This high heat loss continues when these engines are at operating temperature. High cylinder volume to cylinder surface area reduces this heat loss. Big cylinder open chamber engines have better fuel economy because of this lower heat loss.
Most everything said about combustion peak temperatures, flame speed etc. definitely effects the exhaust products produced.
EDWARD
BOB.
Your referring to the mechanical compression ratio which is fixed.
Effective compression ratio varies based on boost and volumetric effeciency.
If we were spinning the engine freely and we measured the pressures of the gases at TDC compression under changing mechanical compression ratios then compared those ratios to a fixed compression ratio and varied our starting pressures at BDC lets say a 21.5:1 engine has 500 PSI at full compression, and the 17.5:1 engine has 400 but the 17.5:1 engine has 4 lbs of boost then it's full compression pressure would change to be equal to that of that 21.5:1 motor. Netting you an EFFECTIVE ratio of 21.5:1. /ubbthreads/images/graemlins/grin.gif (those numbers are merely for demonstration purposes and are not accurate)
And yes, simply put the efficiency is dicated by your ratio at full compression to your ratio at full expansion. Miller cycles overexpand on the power stroke. Since a gas motor can't run very high mechanical or effective compression ratios, (pinging) this method is necessary.
Diesels can run high mechanical ratios and they can run even higher effective ratios due to turbocharging. With a gas engine you have to lower the mechanical compression ratio if your going to pump boost into it. /ubbthreads/images/graemlins/ooo.gif
Your referring to the mechanical compression ratio which is fixed.
Effective compression ratio varies based on boost and volumetric effeciency.
If we were spinning the engine freely and we measured the pressures of the gases at TDC compression under changing mechanical compression ratios then compared those ratios to a fixed compression ratio and varied our starting pressures at BDC lets say a 21.5:1 engine has 500 PSI at full compression, and the 17.5:1 engine has 400 but the 17.5:1 engine has 4 lbs of boost then it's full compression pressure would change to be equal to that of that 21.5:1 motor. Netting you an EFFECTIVE ratio of 21.5:1. /ubbthreads/images/graemlins/grin.gif (those numbers are merely for demonstration purposes and are not accurate)
And yes, simply put the efficiency is dicated by your ratio at full compression to your ratio at full expansion. Miller cycles overexpand on the power stroke. Since a gas motor can't run very high mechanical or effective compression ratios, (pinging) this method is necessary.
Diesels can run high mechanical ratios and they can run even higher effective ratios due to turbocharging. With a gas engine you have to lower the mechanical compression ratio if your going to pump boost into it. /ubbthreads/images/graemlins/ooo.gif
Hey Armchairengineer (I like that handle!)...you are partially correct here.
As boost increases, so also does the overall compression that air undergoes as it passes thru the engine/turbocharger system.
HOWEVER it’s only the mechanical compression (actually the mechanical expansion…) taking place in the cylinder that results in mechanical energy showing up at the crank.
Increasing compression/expansion external to the cylinder (i.e. thru the turbocharger) affects combustion air density, and therefore fuel burn, maximum combustion temperature and ultimately power output. But it does not add to the compression ratio of an engine from a thermodynamic cycle standpoint.
There is however one exception to the above: Turbocompounding. Here the turbocharger is geared to the crankshaft thru an overrunning clutch and a bunch of gears. Under load, a portion of the mechanical energy extracted from the exhaust gases by the turbo is fed thru the clutch and gears back to the crank. This extracted energy adds slightly to the engine’s output. Therefore with turbocompounding a portion of the expansion occurring thru the turbo exhaust turbine DOES effectively add to the overall expansion ratio thru the engine.
Turbocompounding was successfully employed in some large WWII era aircraft engines. Then Mack (and probably others….) experimented with it briefly on diesels in the mid-70’s. But apparently they learned the small gain power/efficiency apparently are not worth the added cost and complexity. ...FB
As boost increases, so also does the overall compression that air undergoes as it passes thru the engine/turbocharger system.
HOWEVER it’s only the mechanical compression (actually the mechanical expansion…) taking place in the cylinder that results in mechanical energy showing up at the crank.
Increasing compression/expansion external to the cylinder (i.e. thru the turbocharger) affects combustion air density, and therefore fuel burn, maximum combustion temperature and ultimately power output. But it does not add to the compression ratio of an engine from a thermodynamic cycle standpoint.
There is however one exception to the above: Turbocompounding. Here the turbocharger is geared to the crankshaft thru an overrunning clutch and a bunch of gears. Under load, a portion of the mechanical energy extracted from the exhaust gases by the turbo is fed thru the clutch and gears back to the crank. This extracted energy adds slightly to the engine’s output. Therefore with turbocompounding a portion of the expansion occurring thru the turbo exhaust turbine DOES effectively add to the overall expansion ratio thru the engine.
Turbocompounding was successfully employed in some large WWII era aircraft engines. Then Mack (and probably others….) experimented with it briefly on diesels in the mid-70’s. But apparently they learned the small gain power/efficiency apparently are not worth the added cost and complexity. ...FB
Is compression ratio a representation of mean effective pressure and vice versa, if so then I have to agree with Armchair that the compression ratio will vary as a function of boost.
Back to the high compression ratio thing, I have one of those turbo diesel Lincolns (1984) it has a BMW 2.4L TD that has a 24:1 comp. ratio with 13 psi on top of that. Great running little engine, max governed rpm 5400. It did have an egr device that would induce eg at idle. (when nox is at it's highest)
Back to the high compression ratio thing, I have one of those turbo diesel Lincolns (1984) it has a BMW 2.4L TD that has a 24:1 comp. ratio with 13 psi on top of that. Great running little engine, max governed rpm 5400. It did have an egr device that would induce eg at idle. (when nox is at it's highest)
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Is compression ratio a representation of mean effective pressure and vice versa, if so then I have to agree with Armchair that the compression ratio will vary as a function of boost.
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Nope! Once again, by definition compression ratio is the ratio of cylinder volume when the intake valve becomes fully closed compression to the cylinder volume at TDC.
The keyword here is VOLUME. Nowhere does pressure enter the CR calculation. (Though by measuring pressures at the two points mentioned above one can approximately determine an engine's CR).
Therefore an engine's compression ratio is unaffected by boost level .
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Mean effective pressure (more properly BRAKE mean effective pressure or BMEP) is another animal entirely. It is measure of engine efficiency relating engine output to engine displacement.
Specifically BMEP is the average effective pressure in the cylinder through a full cycle from intake to exhaust. Consequently BMEP takes into account the overall volumetric efficiency of an engine design in addition to the peak pressures developed (e.g. manifold friction losses, pressure losses thru the valves, etc).
Therefore BMEP DOES indeed vary with boost level - the higher the boost the greater an engine's BMEP. ...FB
Is compression ratio a representation of mean effective pressure and vice versa, if so then I have to agree with Armchair that the compression ratio will vary as a function of boost.
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Nope! Once again, by definition compression ratio is the ratio of cylinder volume when the intake valve becomes fully closed compression to the cylinder volume at TDC.
The keyword here is VOLUME. Nowhere does pressure enter the CR calculation. (Though by measuring pressures at the two points mentioned above one can approximately determine an engine's CR).
Therefore an engine's compression ratio is unaffected by boost level .
----
Mean effective pressure (more properly BRAKE mean effective pressure or BMEP) is another animal entirely. It is measure of engine efficiency relating engine output to engine displacement.
Specifically BMEP is the average effective pressure in the cylinder through a full cycle from intake to exhaust. Consequently BMEP takes into account the overall volumetric efficiency of an engine design in addition to the peak pressures developed (e.g. manifold friction losses, pressure losses thru the valves, etc).
Therefore BMEP DOES indeed vary with boost level - the higher the boost the greater an engine's BMEP. ...FB
Compression ratio is the mechanical swept volume of the piston plus cylinder head volume (?) from bottom of crank throw to top dead center ( nothing to do with what materials are being compressed ). The total mass compressed is variable with how much boost you provide plus how much fuel mass ( fluids are incompressible ) the injectors " inject ".....correct ???? /ubbthreads/images/graemlins/biggrin.gif /ubbthreads/images/graemlins/biggrin.gif
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Compression ratio is the mechanical swept volume of the piston plus cylinder head volume (?) from bottom of crank throw to top dead center (nothing to do with what materials are being compressed).
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Just about.... Compression cannot begin until the intake valve is closed. And since the intake valve doesn't close completely until slightly after BDC, a bit less than the full cylinder swept volume is utilized for compression, for calculation of CR.
But for conceptual purposes your statement above is right on!
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The total mass compressed is variable with how much boost you provide plus how much fuel mass (fluids are incompressible) the injectors " inject ".....correct ???? /ubbthreads/images/graemlins/biggrin.gif /ubbthreads/images/graemlins/biggrin.gif
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Correct indeed! ...FB
Compression ratio is the mechanical swept volume of the piston plus cylinder head volume (?) from bottom of crank throw to top dead center (nothing to do with what materials are being compressed).
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Just about.... Compression cannot begin until the intake valve is closed. And since the intake valve doesn't close completely until slightly after BDC, a bit less than the full cylinder swept volume is utilized for compression, for calculation of CR.
But for conceptual purposes your statement above is right on!
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The total mass compressed is variable with how much boost you provide plus how much fuel mass (fluids are incompressible) the injectors " inject ".....correct ???? /ubbthreads/images/graemlins/biggrin.gif /ubbthreads/images/graemlins/biggrin.gif
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Correct indeed! ...FB
Count sarnit Bob!
If a gas engine runs the same mechanical compression ratio as a diesel, at idle the throttle plate is closed and only about 1/3rd the amount of pressure gets into the cylinder.
This reduces the EFFECTIVE or true compression ratio.
This main efficiency driver in the engine is the difference between the high and low temperatures in the engine which is a direct function of high and low pressures.
Go to the extreme for a sec what if my starting pressure was "0" psi versus 14 psi. What is my compression ratio going to be?
Im not going to be able to generate any pressure or heat which means that I am certainly not going to drop much temperature when I add heat at TDC.
High and low pressures, high and low temperatures, max compression ratio plus maximum starting pressure gives you the best efficiency.
Any engine (gas or diesel) has it's best BSFC at full throttle because thats when the effective compression ratio is the highest.
Turbocharging compresses air for free outside the cylinder and increases effective compression netting you a more efficient engine. /ubbthreads/images/graemlins/biggrin.gif
If a gas engine runs the same mechanical compression ratio as a diesel, at idle the throttle plate is closed and only about 1/3rd the amount of pressure gets into the cylinder.
This reduces the EFFECTIVE or true compression ratio.
This main efficiency driver in the engine is the difference between the high and low temperatures in the engine which is a direct function of high and low pressures.
Go to the extreme for a sec what if my starting pressure was "0" psi versus 14 psi. What is my compression ratio going to be?
Im not going to be able to generate any pressure or heat which means that I am certainly not going to drop much temperature when I add heat at TDC.
High and low pressures, high and low temperatures, max compression ratio plus maximum starting pressure gives you the best efficiency.
Any engine (gas or diesel) has it's best BSFC at full throttle because thats when the effective compression ratio is the highest.
Turbocharging compresses air for free outside the cylinder and increases effective compression netting you a more efficient engine. /ubbthreads/images/graemlins/biggrin.gif
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