Compression Ratio and Turbo's
Discussion
A quick semi-tech question for all you mechanics out there to answer for me on a dull monday morning.
"Could someone explain why the compression ratio has to be different in a turbo engine as opposed to N/A, why do you not just differ the boost to take into account the difference in compression."
ps. my understanding of the compression ratio is the amount that the fuel:air mixture is compressed immediately prior to ingition.
These sort of threads are where i learn all of my boring motoring knowledge, so don't let me down lads
Greg
"Could someone explain why the compression ratio has to be different in a turbo engine as opposed to N/A, why do you not just differ the boost to take into account the difference in compression."
ps. my understanding of the compression ratio is the amount that the fuel:air mixture is compressed immediately prior to ingition.
These sort of threads are where i learn all of my boring motoring knowledge, so don't let me down lads
Greg
Compression ratio is the ratio of maximum cylinder volume (bottom dead centre) to minimum cylinder volume (top dead centre).
The reason you need to drop compresion ratio when turbocharging is to prevent detonation (fuel/air mixture combusting before the spark). If you were to keep the same compression ratio, in-cylinder pressure would reach the detonation point way before TDC and you would have to have a ridiculously early spark timing.
Does that make sense? I've tried to explain it without resorting to thermodynamics!
>> Edited by dieseljohn on Monday 14th March 13:03
The reason you need to drop compresion ratio when turbocharging is to prevent detonation (fuel/air mixture combusting before the spark). If you were to keep the same compression ratio, in-cylinder pressure would reach the detonation point way before TDC and you would have to have a ridiculously early spark timing.
Does that make sense? I've tried to explain it without resorting to thermodynamics!
>> Edited by dieseljohn on Monday 14th March 13:03
Also by reducing CR and increasing boost a gain in torque can be realised when compared to a higher static CR but a lower boost pressure, the trade off to this is poor off boost performance due to low static CR.
Again all in very simple terms.
>> Edited by Matt_fp on Monday 14th March 13:10
Again all in very simple terms.
>> Edited by Matt_fp on Monday 14th March 13:10
supraman2954 said:
Can anyone confirm if my numbers are correct?
IIRC, the detonation point is around 14:1 (depending on octane count). So a car running with a compression ratio of 10:1 with turbo running at 1.5 BAR will melt the engine head.
14:1 CR? or 14:1 AFR? 14.5:1 AFR is stoch and really too lean for a force inducted car running 1.5bar of boost on a "normal" engine hence combustion chamber temperatures would be very high, detonation would likely occur due to the lean mixture and in all likelyhood you'd end up with a melted piston or two.
10:1CR with 1.5bar of boost may well be absolutely fine, detonation is very dependant on AFR, ignition timing, combustion chamber shape and temperature, piston crown shape/temperature plus several other major factors and a fair few more minor ones. Its really not as simple as "it'll go bang at this point" and just to complicate things further all detonation isn't the same, knock and pre-ignition/detonation aren't the same thing.
Matt
Matt_fp said:
14:1 CR? or 14:1 AFR? .........
10:1CR with 1.5bar of boost may well be absolutely fine, detonation is very dependant on AFR, ignition timing, combustion chamber shape and temperature, piston crown shape/temperature plus several other major factors and a fair few more minor ones. Its really not as simple as "it'll go bang at this point" and just to complicate things further all detonation isn't the same, knock and pre-ignition/detonation aren't the same thing.
Matt
Cheers Matt, that cleared up the issue
I didn’t mean AFR and I certainly don’t want to go there!
So what is the difference between knock and detonation?
Sorry, I didn’t mean to hijack the thread
Put simply detonation is when the mixture reaches a self ignition point (high temperature and pressure) and explodes (i.e very lean mixture in very hot combustion chamber). Leading to a zero time rise in combustion chamber pressure.
Knock is when two flame fronts collide (leading to a high pressure/high temperature point) usualy one being ignited by the spark and one by a hot component within the combustion chamber Or when the spark from the plug ignites the mixture but the mixture burns in an uncontrolled manner i.e rather than a nice equal flame front and gas expansion it goes bang.
The important difference is detonation occurs pre firing and knock occurs during/post firing. Hence why retarding ignition can cure knock (combustion chamber pressure lower)
TBH the terms knock, detonation, pre-ignition, after-burn etc. all see pretty interchangeable these days and are all lumped under the heading of detonation. The definitions of all of the above are fairly well disputed as well!
Matt
Knock is when two flame fronts collide (leading to a high pressure/high temperature point) usualy one being ignited by the spark and one by a hot component within the combustion chamber Or when the spark from the plug ignites the mixture but the mixture burns in an uncontrolled manner i.e rather than a nice equal flame front and gas expansion it goes bang.
The important difference is detonation occurs pre firing and knock occurs during/post firing. Hence why retarding ignition can cure knock (combustion chamber pressure lower)
TBH the terms knock, detonation, pre-ignition, after-burn etc. all see pretty interchangeable these days and are all lumped under the heading of detonation. The definitions of all of the above are fairly well disputed as well!
Matt
On a similar note - Does the Lotus Esprit HC (the nom-asp one that ran for a few years alongside the turbo with more power than the previous NA Esprit), as in 'High Compression', have a high or low compression ratio? (Obviously any abbreviation of 'Low' on a sports car doesn't bode well when it comes to selling it!)
I am doubtlessly massively oversimplifying this, but my thinking goes thus:
If it takes a N/A engine x-psi to start detonating (and x is an absolute due to engine design) then surely altering the CR and bolting a turbo on is a big no no, because with the boost that the turbo produces is surely going to be x+++ hence the power gains to be had by turboing an engine.
am i making myself clear? it makes sense in my head!!!
i know that i am missing something, but what??? please tell me because it is doing my head in
Greg
If it takes a N/A engine x-psi to start detonating (and x is an absolute due to engine design) then surely altering the CR and bolting a turbo on is a big no no, because with the boost that the turbo produces is surely going to be x+++ hence the power gains to be had by turboing an engine.
am i making myself clear? it makes sense in my head!!!
i know that i am missing something, but what??? please tell me because it is doing my head in
Greg
v8thunder said:Yep, well, the 1986 Turbo was running at 8.5:1, the 1987 Turbo HC at 8.0:1, with a carb rather than injection, and the 1988 Turbo was also at 8.0:1, but on twin carbs.
To get more power you have to put more air into the engine. Lowering the compression ratio increases the air volume, whilst simultaneously preventing pre-spark detonation.
So it's likely the 'Esprit HC' actually had lowered compression then?
greg_D said:
I am doubtlessly massively oversimplifying this, but my thinking goes thus:
If it takes a N/A engine x-psi to start detonating (and x is an absolute due to engine design) then surely altering the CR and bolting a turbo on is a big no no, because with the boost that the turbo produces is surely going to be x+++ hence the power gains to be had by turboing an engine.
am i making myself clear? it makes sense in my head!!!
i know that i am missing something, but what??? please tell me because it is doing my head in
Greg
No, x is variable depending on the compression ratio, igintion timimg and quality of the fuel.
The problem is the amount of heat in the amount of air you are whacking in. Obviously the compression of the turbo jacks the temperature up and then the compression jacks it up again. You can minimise this by using an intercooler but in order to cool it sufficiently, you need a large dT (temp diff across cooler). 1.5 bar is quite a lot of boost to be running (about 22 PSI) I reckon you would need to have a big look at your intercooler size.
Any questions, give me a shout.
Any questions, give me a shout.
love machine said:Do intercoolers actually minimise the temperature increase due to compression, or just lower it? I'd imagine that if you're near to detonation, the temperature's going to be pretty high...so if there are alot of hot gases going through the system, how much of a difference do intercoolers actually make?
The problem is the amount of heat in the amount of air you are whacking in. Obviously the compression of the turbo jacks the temperature up and then the compression jacks it up again. You can minimise this by using an intercooler but in order to cool it sufficiently, you need a large dT (temp diff across cooler). 1.5 bar is quite a lot of boost to be running (about 22 PSI) I reckon you would need to have a big look at your intercooler size.
Any questions, give me a shout.
mine is 8:2:1 for copen the boost is 15 psi upto 4500rpm and 5 psi after.i get max torque is 81 lbs at 2500 to 3000rpm.max bhp is 83=125 bhp per litre.so i can guess it is high pressure turbo and it is cooled by intercooler.when i took it to power engineering they said it is one the most responsive turbos they had seen.
quite a bit of confusion here.
An engine burns fuel/air mixture by compression the charge and igniting it. That is straight forward enough. The volume of air is greatly affected by the pressure and temperature of the air - i.e. the density of the air.
A supercharger (exhaust or mechanically driven) compresses the incoming air. Air is normally at an atmospheric pressure of 1 bar (not true up a mountain!). A turbo charger running at 1 bar will have increased the pressure of the incoming air to twice atmospheric, or 2 bar in absolute terms. therefore the air takes up half the volume, so you theoretically get twice the power (as you can put twice as much fuel in and get a bigger charge). The reality with supercharging is that heat is added to the charge. Air-to-air intercoolers and charge coolers are used to lower the temperature, but it cannot simply reduce it to the same level as the ambient air temperature. Thus you have less dense air (effectively undoing the benefit of the compression of the supercharger). The heat is a big contributor to pre-ignition.
Compression ratio on supercharged engines is often reduced to limit the effect of increased temperature and to help reduce in-cylinder pressures. Effectiveness of combustion has as alot to do with cylinder head design, especially squish area and squish clearance. A well designed modern engine can run as much as 10:1 CR with 2.5bar of boost if well designed (plenty of engines out there).
The "old school" of turbocharging (e.g. Cosworth) simply dropped the comp ratio and whacked up the boost, particularly as older ignition and fuelling systems were fairly clunky.
Modern ECUs and ignition systems, and engine designs are able to cope with complex ignition and fuelling maps. For example, the BMW M3 S54 engine runs at 11.5:1 compression ratio, and produces over 100bhp per litre normally aspirated. It is possible to supercharge this at 0.7 bar with a charge cooler with no changes other than fuelling and mapping (no CR reduction).
As a rough guidline:-
Modern engines - 1.5 to 2 bar boost up to 9.5:1
Modern engines - 0.7 to 1.5 bar boost up to 10:1
Modern engines - up to 0.7 bar up to 11.5:1
Modern engines - Normally aspirated up to 12.5:1
Old Engines, take away 1 to 1.5 points on the compresson ratio.
Fuel makes a big difference, and I am assuming unleaded 98. Running leaded fuels, additives such as Toluolene and high octane fuels such as AVGAS will inhibit knock and allow higher boost and/or compression ratios.
Tried to keep it simple.
There are few excellent books on this subject available. One in particular, Forced Induction Tuning (by A. Graham Bell)
An engine burns fuel/air mixture by compression the charge and igniting it. That is straight forward enough. The volume of air is greatly affected by the pressure and temperature of the air - i.e. the density of the air.
A supercharger (exhaust or mechanically driven) compresses the incoming air. Air is normally at an atmospheric pressure of 1 bar (not true up a mountain!). A turbo charger running at 1 bar will have increased the pressure of the incoming air to twice atmospheric, or 2 bar in absolute terms. therefore the air takes up half the volume, so you theoretically get twice the power (as you can put twice as much fuel in and get a bigger charge). The reality with supercharging is that heat is added to the charge. Air-to-air intercoolers and charge coolers are used to lower the temperature, but it cannot simply reduce it to the same level as the ambient air temperature. Thus you have less dense air (effectively undoing the benefit of the compression of the supercharger). The heat is a big contributor to pre-ignition.
Compression ratio on supercharged engines is often reduced to limit the effect of increased temperature and to help reduce in-cylinder pressures. Effectiveness of combustion has as alot to do with cylinder head design, especially squish area and squish clearance. A well designed modern engine can run as much as 10:1 CR with 2.5bar of boost if well designed (plenty of engines out there).
The "old school" of turbocharging (e.g. Cosworth) simply dropped the comp ratio and whacked up the boost, particularly as older ignition and fuelling systems were fairly clunky.
Modern ECUs and ignition systems, and engine designs are able to cope with complex ignition and fuelling maps. For example, the BMW M3 S54 engine runs at 11.5:1 compression ratio, and produces over 100bhp per litre normally aspirated. It is possible to supercharge this at 0.7 bar with a charge cooler with no changes other than fuelling and mapping (no CR reduction).
As a rough guidline:-
Modern engines - 1.5 to 2 bar boost up to 9.5:1
Modern engines - 0.7 to 1.5 bar boost up to 10:1
Modern engines - up to 0.7 bar up to 11.5:1
Modern engines - Normally aspirated up to 12.5:1
Old Engines, take away 1 to 1.5 points on the compresson ratio.
Fuel makes a big difference, and I am assuming unleaded 98. Running leaded fuels, additives such as Toluolene and high octane fuels such as AVGAS will inhibit knock and allow higher boost and/or compression ratios.
Tried to keep it simple.
There are few excellent books on this subject available. One in particular, Forced Induction Tuning (by A. Graham Bell)
Gassing Station | General Gassing [Archive] | Top of Page | What's New | My Stuff