turbo knowledge anyone?
Discussion
Gadgeroonie said:
here are two compressor maps from 2 very different turbo's
the first one will produce between 70 and 225 hp at 1 bar of boost
the second one will produce between 400 and 1600 hp at 1 bar of boost
quite a difference !
Now you just need to build the Ford V6 to make 1600bhp at 1 abr of boost! the first one will produce between 70 and 225 hp at 1 bar of boost
the second one will produce between 400 and 1600 hp at 1 bar of boost
quite a difference !
Chris.
Turbo's make precisely zero BHP. Well, less than zero if you're being picky 
It very simple, the Turbo with the highest overall efficiency will allow the engine to make the most BMEP (torque) What get's a bit tricky is at what rpm you are conparing that power!
Lets assume 1 bar of boost and 25degC plenum air temp shall we, so on a 2.0 litre engine
Case A: 3500rpm = 0.136kg/sec intake air flow
but at
Case B: 7500rpm = 0.272Kg/sec (obviously double the air flow in case A)
Now the energy to compress that air comes from the exhaust, and negatively affects the engines pressure ratio (more boost = more exhaust manifold pressure = reduced engine pressure ratio)
So, which ever turbo has the best overall efficiency at the airflow (nee speed) you care about most, will be the best (because it will provide the highest plenum density for the lowest back pressure, allowing the engine to ingest more of that compressed air per intake event, and reducing pumping loses as the piston completes the exhaust stroke (and also allowing a leaner mixture (less overfuelling) as the expansion ratio is greater, and a more sdvanced ingnition angle (if detonation limited) as there will be less hot exhaust gas residuals in the cylinder on the next firing stroke.
Combined with all that, the efficiency of the compressor directly affects the air out temp, so a less efficient compressor will heat the air to a greater degree (and take more turbine power to do it) which for a given pressure will reduce charge density (so you can't add as much fuel) and increase the end of compression temp leading to potential detonation phenomina.
As "Power" is Torque x speed, you make the most power at high rpm, hence physically "large" turbo's that are efficient at high mass flows will make more peak power, whereas a smaller frame size device will be more efficient at low rpm and allow you to make a higher torque. For wining races against a clock, power is all that matters (assuming you have a drivetrain capable of efficiently leveraging that power) so big is best!
The key to really HUGE turbo power is when the turbine gets so large and freeflowing that you can actually make more boost than exhaust manifold backpressure (typical road turbo will have a ratio of between 2 and 4 for MAP to EMAP (i.e 1 bar boost = 2 (to4) bar preturbine EMAP) The old 1.5 litre F1 turbo's managed this and hence made massive power at high rpm ;-)
Most people think a turbo "blows air into an engine" but it does not(for most cases, F1 excepted). it simply increases the maximum possible intake charge density. A turbo engines manifold volumetric efficiency is WORSE than for an NA engine (due to the high EMAP)
Turbo's are caught in a vicious circle, where more boost = more back pressure =reduced engine pressure ratio, which then needs more boost, and so it goes on. Most people completely forget about turbo efficiency but it is CRUCIAL to maintaining a decent engine PR.
Lets assume 1 bar of boost and 25degC plenum air temp shall we, so on a 2.0 litre engine
Case A: 3500rpm = 0.136kg/sec intake air flow
but at
Case B: 7500rpm = 0.272Kg/sec (obviously double the air flow in case A)
Now the energy to compress that air comes from the exhaust, and negatively affects the engines pressure ratio (more boost = more exhaust manifold pressure = reduced engine pressure ratio)
So, which ever turbo has the best overall efficiency at the airflow (nee speed) you care about most, will be the best (because it will provide the highest plenum density for the lowest back pressure, allowing the engine to ingest more of that compressed air per intake event, and reducing pumping loses as the piston completes the exhaust stroke (and also allowing a leaner mixture (less overfuelling) as the expansion ratio is greater, and a more sdvanced ingnition angle (if detonation limited) as there will be less hot exhaust gas residuals in the cylinder on the next firing stroke.
Combined with all that, the efficiency of the compressor directly affects the air out temp, so a less efficient compressor will heat the air to a greater degree (and take more turbine power to do it) which for a given pressure will reduce charge density (so you can't add as much fuel) and increase the end of compression temp leading to potential detonation phenomina.
As "Power" is Torque x speed, you make the most power at high rpm, hence physically "large" turbo's that are efficient at high mass flows will make more peak power, whereas a smaller frame size device will be more efficient at low rpm and allow you to make a higher torque. For wining races against a clock, power is all that matters (assuming you have a drivetrain capable of efficiently leveraging that power) so big is best!
The key to really HUGE turbo power is when the turbine gets so large and freeflowing that you can actually make more boost than exhaust manifold backpressure (typical road turbo will have a ratio of between 2 and 4 for MAP to EMAP (i.e 1 bar boost = 2 (to4) bar preturbine EMAP) The old 1.5 litre F1 turbo's managed this and hence made massive power at high rpm ;-)
Most people think a turbo "blows air into an engine" but it does not(for most cases, F1 excepted). it simply increases the maximum possible intake charge density. A turbo engines manifold volumetric efficiency is WORSE than for an NA engine (due to the high EMAP)
Turbo's are caught in a vicious circle, where more boost = more back pressure =reduced engine pressure ratio, which then needs more boost, and so it goes on. Most people completely forget about turbo efficiency but it is CRUCIAL to maintaining a decent engine PR.
Another great post max! So this cycle you talk of where by increasing the Intake presure results in higher exhaust manifold presure and thus less power. Is this why say a LS engine running 16psi of boost might make upwards of 1000bhp, yet at 30psi it might only make an additioanl 3-400bhp?.... And how do you combat this? Dose it simply mean that the turbo is too small for the application (at higher boost presures)? Also how did F1 engines manage to increase the volumetric efficeny of the engines?
Also how dose compound turbo charging effect the EMBP? Would it still be at 2 to 4 time the intake presure?
Also how dose compound turbo charging effect the EMBP? Would it still be at 2 to 4 time the intake presure?
Edited by chuntington101 on Wednesday 17th August 08:40
Gassing Station | Noble | Top of Page | What's New | My Stuff