High Pressure Compressor output

[byeplane]

Hi,

Refreshing myself on the workings of a High Bypass Engine when it occurred to me I know very little about what the hi-pres compressor is actually doing. For instance on the CFM-56 engine used in the 737-800, 80% of thrust is being generated by the low-pres compressor and turbine assembly (N1). Is the primary function of the hi-pres compressor and turbine assembly to augment the total thrust output or is it also used to pressurise the cabin and various other functions?

[pilot9248]

Please correct me if I'm wrong, but in a high bypass turbofan engine, about 80% of the total thrust are generated by the fan, which is driven by the low pressure turbine. The low pressure compressor, sometimes also referred to as 'booster', is pre-compressing the air that enters the core engine before it is compressed further by the high pressure compressor. The HPC and HPT of a turboshaft engine are often referred to as "gas generator": The HPC compresses the inlet air which is mixed with fuel and then burned inside the combustion chamber. The combustion gas powers the gas generator or HP turbine, which in turn drives the HP compressor. The energy that's left in the combustion gas (which is, obviously, significantly greater than the energy required to power the HPC, because the fan does require a LOT of power, I'm just guessing but it could well be about 20,000 hp in a CFM56) then powers the so called power turbine, or in a two spool engine like the CFM56, the LP turbine which drives the fan. There will still be a little bit of energy left in the exhaust gas, which does add to the total thrust as it exits the tailpipe. Secondary functions of the HPC include driving accessory gearbox(es), supplying bleed air for various functions such as anti-ice, pressurisation etc.

[lomapaseo]

Sounds to me that you got it pretty much right


The HP is part of the suck squeeze bang and blow, providing a major portion of the squeeze.

It truly prepares the bang portion to provide enough blow in the Low speed turbine to drive the giant fan which in the case of your subject engine provides most of the thrust. Whatever is left over can be used for pressurizing other airplane functions

[DevX]

QUOTE: "Whatever is left over can be used for pressurizing other airplane functions."

There is nothing 'left over', the HPC (and IPC in the case of a 3 shaft engine) is simply robbed of air to supply cabin bleed and TAI. The R-R Trent 100 is different in this respect as it doesn't have any cabin bleed functions, only TAI, which is just one of the reasons it's a very fuel efficient engine.

Anyway, to answer the OP's original question, the primary function of the HPC is to supply hot, pressurised combustible air to the combustion chamber, not to augment the thrust supplied by the LP compressor. The small amount of thrust generated from the hot nozzle is a bonus, but never the less taken into account during the design process.

[barit1]

I think we all know enough about piston engines to know that a high-compression engine generates more power than a low-compression one of the same physical size.

Same is true of turbines.