ringo wrote:gruntguru wrote:Per wrote:You really need to treat the compressor and turbine as two separate components when you discuss their sizing. The only thing they have in common is their RPM.
. . . . and the massflow. Turbine massflow is always equal to compressor massflow plus fuel flow (about 4 or 5% extra)
. . . . and the pressure ratio.
Turbine PR cannot be a lot different to compressor PR.
Exhaust flow, pressure ratio and temperature dictate the turbine size.
You are probably thinking a proper gas turbine that has constant pressure combustion.
Not at all.
I am thinking firstly about the piston engine and what pressure you can apply to the intake and the exhaust. Running the exhaust pressure significantly higher than the intake creates:
- loss of crankshaft power due to pumping work
- loss of crankshaft power due to poor scavenge and charge dilution
- detonation
- durability issue due to higher temperature
Secondly - thinking about the turbine, you do not want to run the exhaust pressure significantly lower than the intake (certainly not 30% lower PR as in pgfpro's example) because you are throwing away energy that could be recovered by the turbine. For a normal turbocharged application, lower is better. There is no opportunity to recover exhaust energy beyond what is needed to drive the compressor but the piston engine will benefit from any reduction in BP.