Technical Question re Forced Induction
Discussion
Ok I understand engines a bit, but something I keep seeing has me confused.
Why do Forced Induction engines use lower compresion pistons than their normally aspirated counterparts, why not just use less boost? I am obviously missing something as if that were the case people would do it.
So what am I not taking into account then.
Why do Forced Induction engines use lower compresion pistons than their normally aspirated counterparts, why not just use less boost? I am obviously missing something as if that were the case people would do it.
So what am I not taking into account then.
Think of the pressures involved.
This is how I have it in my head...
With an NA engine lets say the air/fuel mix in the cylinder is at a pressure of 1 when the piston is at the bottom. And 3 when it's at the top and fully compressing the air/fuel mix.
With a forced induction engine the pressure with the piston at the bottom will already be higher, as teh turbo/supercharger has forced more air in. So say 2 at the bottom and 5 at the top.
This is how I have it in my head...
With an NA engine lets say the air/fuel mix in the cylinder is at a pressure of 1 when the piston is at the bottom. And 3 when it's at the top and fully compressing the air/fuel mix.
With a forced induction engine the pressure with the piston at the bottom will already be higher, as teh turbo/supercharger has forced more air in. So say 2 at the bottom and 5 at the top.
stevieturbo said:
read more.
I'm trying to, hence this post 
Seriously though, my thoughts are the same as Munter's, if a 10:1 compression set up produces 10 times normal atmospheric pressure in the combustiion chamber at TDC then if you were to double the initial pressure to say twice normal atmospheric pressure by means of a compressor of whatever means, then the same set up would produce a compression pressure = to a 20:1 compression ratio on the initial version.So I still dont understand why less boost on the high compression wouldn't work.
PS Steviturbo if you can recommend a good book I would buy it as looking at your vid you obviously know what your doing (or you know someone who does).
Edited by clarenceboddiger on Sunday 3rd August 13:40
clarenceboddiger said:
stevieturbo said:
read more.
I'm trying to, hence this post Seriously though, my thoughts are the same as Munter's, if a 10:1 compression set up produces 10 times normal atmospheric pressure in the combustiion chamber at TDC then if you were to double the initial pressure to say twice normal atmospheric pressure by means of a compressor of whatever means, then the same set up would produce a compression pressure = to a 20:1 compression ratio on the initial version.So I still dont understand why less boost on the high compression wouldn't work.
PS Steviturbo if you can recommend a good book I would buy it as looking at your vid you obviously know what your doing (or you know someone who does).
Edited by clarenceboddiger on Sunday 3rd August 13:40
FI adds more oxygen, to which you can add more fuel, which equals more energy in the equation.
Higher compression and less boost improves off boost performance, but sacrifices peak power. Due to the total energy in the equation.
Lower compression is required due to the temperatures generated during cylinder compression of the mixture.
annodomini2 said:
Because the compression of the cylinder does not add more oxygen to the mixture.
FI adds more oxygen, to which you can add more fuel, which equals more energy in the equation.
Higher compression and less boost improves off boost performance, but sacrifices peak power. Due to the total energy in the equation.
Lower compression is required due to the temperatures generated during cylinder compression of the mixture.
Great explanation, Thanks annodomini2,.FI adds more oxygen, to which you can add more fuel, which equals more energy in the equation.
Higher compression and less boost improves off boost performance, but sacrifices peak power. Due to the total energy in the equation.
Lower compression is required due to the temperatures generated during cylinder compression of the mixture.
You said "Lower compression is required due to the temperatures generated during cylinder compression of the mixture." presumably due to the greater volume of Air/Fuel then?
Julian Edgar's 21st Century Performance is an excellent all rounder.
http://www.amazon.co.uk/21st-Century-Performance-J...
Forced Induction by Graham Bell ( or something like that ), and probably a couple of other books I cant quite remember...
http://www.amazon.co.uk/21st-Century-Performance-J...
Forced Induction by Graham Bell ( or something like that ), and probably a couple of other books I cant quite remember...
The idea of a larger cylinder volume always appeals to me because you can put more air and fuel into it which should equate to greater torque gains due to the longer power stroke/burn. Torque effects bhp to. Go to low on c/r and the car will be ineffecient off boost. Small engines on low c/r must be horrible to drive.
stevieturbo said:
Julian Edgar's 21st Century Performance is an excellent all rounder.
http://www.amazon.co.uk/21st-Century-Performance-J...
Forced Induction by Graham Bell ( or something like that ), and probably a couple of other books I cant quite remember...
Thanks Steve, I will order that up and get into it.http://www.amazon.co.uk/21st-Century-Performance-J...
Forced Induction by Graham Bell ( or something like that ), and probably a couple of other books I cant quite remember...
Have a play around with this:
http://www.rbracing-rsr.com/compression.htm
I'm running around 8psi on a std 9.25:1 v8 chevy, which according to that calculator gives an effective c/r of 14.2:1
http://www.rbracing-rsr.com/compression.htm
I'm running around 8psi on a std 9.25:1 v8 chevy, which according to that calculator gives an effective c/r of 14.2:1
eliot said:
Have a play around with this:
http://www.rbracing-rsr.com/compression.htm
I'm running around 8psi on a std 9.25:1 v8 chevy, which according to that calculator gives an effective c/r of 14.2:1
Interesting, 9:1 static for me and 12psi of boost effective CR of 16.35:1http://www.rbracing-rsr.com/compression.htm
I'm running around 8psi on a std 9.25:1 v8 chevy, which according to that calculator gives an effective c/r of 14.2:1
I think 9:1 is about right for static, 8:1 must be gutless off boost, I guess thats where the old pub know it alls get the "turbos are horrible to drive all gutless and lag then bang and its all over". Modern ECUs' and fueling/ ignition timing allows much higher CR's than were normal 15/20 years ago.
It's also about cams and dynamic compression which could end up being really quite low. The gutless factor is more for 4 pots. It's probably quite hard to make a v8 feel gutless.
On the n/a front I've seen v8's with a stated 16:1 c/r and realised it's that high just to get a decent dynamic compression. Well, that's what I'm guessing at
On the n/a front I've seen v8's with a stated 16:1 c/r and realised it's that high just to get a decent dynamic compression. Well, that's what I'm guessing at
clarenceboddiger said:
Ok I understand engines a bit, but something I keep seeing has me confused.
Why do Forced Induction engines use lower compresion pistons than their normally aspirated counterparts, why not just use less boost? I am obviously missing something as if that were the case people would do it.
So what am I not taking into account then.
Try and look at it this way:Why do Forced Induction engines use lower compresion pistons than their normally aspirated counterparts, why not just use less boost? I am obviously missing something as if that were the case people would do it.
So what am I not taking into account then.
Under normally aspirated use an engine must draw the air in by the vacuum created by the descending piston (lets ignore complications like valve overlap and gas momentum for the moment). This work that has to be done and requires energy. As such the engine will never have 100% volumetric efficency- i.e. it cannot fill the cylinder completely. Modern designs have helped this problem greatly but there is still inefficiency.
In a forced induction engine the charge is pushed into the cylinder. This is more efficient as a greater volume can be crammed in. Because of this a volumetric efficiency far in excess of 100% can be achieved, creating extra power (with added fuel of course). There is a limit to the pressures and temperatures that our fuel will tolerate, hence why we don't have 30:1 compression and produce massive power.
So the reason why turbocharged engines have lower compression is to allow boost to be added before that critical limit of pressure/temperature is reached. Modern turbocharged engines run higher compression than before due to much better cylinder head and piston design, but if you look at a turbo engine making big power (on normal fuel) for it's size you will usually find that it is running a compression of around 8:1, because the engine is more efficient under boost- plain and simple.
ok if you are struggling to understand what the guys above have said dont read an further on in my post as it will mess with your head! lol
i have seen 100% methanol fuel cars running over 12:1 (i think it was 14:1) and still runing about 30psi of boost! they where making over 2500bhp and used no intercoolers to help with weight.
obviously this can only be done because methanol is much more freindy to conmpression and boost (thanks to the very low IAT, cooler flame and high octane).
Chris.
i have seen 100% methanol fuel cars running over 12:1 (i think it was 14:1) and still runing about 30psi of boost! they where making over 2500bhp and used no intercoolers to help with weight.
obviously this can only be done because methanol is much more freindy to conmpression and boost (thanks to the very low IAT, cooler flame and high octane).
Chris.
high compression is not the main reason detonation occurs.......world rally cars run in excess of 12:1 and are very heavily turbo charged, unlimited boost I think, just a 34mm restrictor........detonation is actually related to piston temperature, particularly localised piston temperatures whereby sharp edges get too hot and act like a small spark plug which pre-ignites the charge and causes detonation........this is why all good race pistons feature a high amount of CNC milling in order to remove all sharp edges from critical areas, like the piston intruder/dome and the piscon cut-outs where the valves clear.
also any decent pressure charged engine should feature a decent piston cooling jet, F1 teams have found that if they reduce piston temperature by only 10 degrees, the engine life extends greatly, hence each piston now has 10.....yes 10 per piston! .......piston cooling jets firing a horrendous amount of oil at each piston underside
I was also recently told the piston cooling jets in the JCB land speed car fire 20 litres per minute at each piston.......now thats a lot!
so next time you are chosing an engine for turbo/supercharging, make sure it has piston cooling jets, otherwise expect grief, like detonation, scuffed bores etc
also any decent pressure charged engine should feature a decent piston cooling jet, F1 teams have found that if they reduce piston temperature by only 10 degrees, the engine life extends greatly, hence each piston now has 10.....yes 10 per piston! .......piston cooling jets firing a horrendous amount of oil at each piston underside
I was also recently told the piston cooling jets in the JCB land speed car fire 20 litres per minute at each piston.......now thats a lot!
so next time you are chosing an engine for turbo/supercharging, make sure it has piston cooling jets, otherwise expect grief, like detonation, scuffed bores etc
chuntington101 said:
i have seen 100% methanol fuel cars running over 12:1 (i think it was 14:1) and still runing about 30psi of boost! they where making over 2500bhp and used no intercoolers to help with weight.
obviously this can only be done because methanol is much more freindy to conmpression and boost (thanks to the very low IAT, cooler flame and high octane).
Chris.
Lack of intercooler is not simply for weight reduction, they're simply not required.. obviously this can only be done because methanol is much more freindy to conmpression and boost (thanks to the very low IAT, cooler flame and high octane).
Chris.
knighty said:
high compression is not the main reason detonation occurs.......world rally cars run in excess of 12:1 and are very heavily turbo charged, unlimited boost I think, just a 34mm restrictor........detonation is actually related to piston temperature, particularly localised piston temperatures whereby sharp edges get too hot and act like a small spark plug which pre-ignites the charge and causes detonation........this is why all good race pistons feature a high amount of CNC milling in order to remove all sharp edges from critical areas, like the piston intruder/dome and the piscon cut-outs where the valves clear.
also any decent pressure charged engine should feature a decent piston cooling jet, F1 teams have found that if they reduce piston temperature by only 10 degrees, the engine life extends greatly, hence each piston now has 10.....yes 10 per piston! .......piston cooling jets firing a horrendous amount of oil at each piston underside
I was also recently told the piston cooling jets in the JCB land speed car fire 20 litres per minute at each piston.......now thats a lot!
so next time you are chosing an engine for turbo/supercharging, make sure it has piston cooling jets, otherwise expect grief, like detonation, scuffed bores etc
WRC cars make heavy use of water injection. The restrictor also plays a part in running such high CR, as the engine struggles to ever see normal atmospheric pressure anyway.also any decent pressure charged engine should feature a decent piston cooling jet, F1 teams have found that if they reduce piston temperature by only 10 degrees, the engine life extends greatly, hence each piston now has 10.....yes 10 per piston! .......piston cooling jets firing a horrendous amount of oil at each piston underside
I was also recently told the piston cooling jets in the JCB land speed car fire 20 litres per minute at each piston.......now thats a lot!
so next time you are chosing an engine for turbo/supercharging, make sure it has piston cooling jets, otherwise expect grief, like detonation, scuffed bores etc
As for the oil jets...while nice to have, Ive never run them on my car, and never had any problems. And oil jets will certainly not stop detonation, or scuffed bores.
To be fair, piston squirters do lower piston temperatures. They are so effective that many normally aspirated engines with high bhp per litre outputs now have them.
So if you take a turbocharged scenario where you are on the threshold of detonation due to combustion temperatures/pressures then the squirters will allow you to run more boost with no other changes by dropping the temperatures.
This is well documented and proven. Whilst some engines are exceptionally resistant to detonation it's usually engines which are putting out relatively low bhp per litre or using something more detonation friendly than normal unleaded that don't need it.
As Slinky correctly states, you just don't need intercoolers on forced induction methanol cars. I know of a 2.3l Volvo producing 787hp on methanol with no intercooler!
I'm told one of the biggest problems with Methanol cars is getting the engines up to temperature sufficiently in the first place.
So if you take a turbocharged scenario where you are on the threshold of detonation due to combustion temperatures/pressures then the squirters will allow you to run more boost with no other changes by dropping the temperatures.
This is well documented and proven. Whilst some engines are exceptionally resistant to detonation it's usually engines which are putting out relatively low bhp per litre or using something more detonation friendly than normal unleaded that don't need it.
As Slinky correctly states, you just don't need intercoolers on forced induction methanol cars. I know of a 2.3l Volvo producing 787hp on methanol with no intercooler!
I'm told one of the biggest problems with Methanol cars is getting the engines up to temperature sufficiently in the first place.
bit of a bugger that!Slinky said:
chuntington101 said:
i have seen 100% methanol fuel cars running over 12:1 (i think it was 14:1) and still runing about 30psi of boost! they where making over 2500bhp and used no intercoolers to help with weight.
obviously this can only be done because methanol is much more freindy to conmpression and boost (thanks to the very low IAT, cooler flame and high octane).
Chris.
Lack of intercooler is not simply for weight reduction, they're simply not required.. obviously this can only be done because methanol is much more freindy to conmpression and boost (thanks to the very low IAT, cooler flame and high octane).
Chris.
Cheers
Chris.
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