Turbocharger Pressure Ratio and Charge-Air Temp
Why boost is a gauge number and why it needs an intercooler.
Example
You enter
- Target boost (psi, gauge) 15
- Ambient pressure (psia, 14.7 at sea level) 14.7
- Compressor inlet air temp (°F) 80
- Compressor isentropic efficiency (%) 70
You get
- Pressure ratio 2.02
- T out f 251.58
- Temp rise f 171.58
Checked against published worked examples from: Garrett Motion.
Details, formula, and sources
ScopePR = (ambient_abs + boost) / ambient_abs, so the ambient must be added before dividing -- 15 psi at sea level (14.7 psia) is PR 2.02, but the same 15 psi a mile high (12.2 psia) is PR 2.23, a hotter outlet for the identical boost. Compressing air heats it: T_out = T_in x [1 + (PR^0.2857 - 1)/efficiency], so 15 psi at 80 F inlet and 70% efficiency reaches 252 F -- a 172 F rise from compression alone, why an intercooler is mandatory on a serious build. Reports the compressor-outlet (not manifold) temperature. A planning estimate; the compressor map and engine build govern.
Formula or tablePR = (ambient_psia + boost_psi) / ambient_psia; T_out = T_in x [1 + (PR^0.2857 - 1) / (efficiency/100)] with temperatures absolute (Rankine); temp_rise = T_out - T_in.
Source and editionTurbocharger pressure-ratio and charge-air-temperature model (compressor-map sizing; ideal-gas adiabatic compression with gamma = 1.4), first-principles, by name; the compressor map and the engine build govern.
Free accessThe pressure-ratio and adiabatic-temperature relations are first-principles thermodynamics; boost, ambient, inlet temp, and compressor efficiency come from the application and the compressor map.
What governsEstimate. AHJ and licensed professional govern.
API field namesboost_psi, ambient_psia, inlet_temp_f, compressor_eff_pct, pr, t_out_f, temp_rise_f
- Gauge boost PR uses absolute pressures; add the ambient to the gauge boost before dividingcompressor-map sizing
- Heat of compression T_out ~ T_in x PR^0.2857 adjusted by efficiency; a modest PR adds over 100 Fadiabatic compression (gamma = 1.4)
- No intercooler reports the compressor-outlet temperature, not the post-intercooler manifold temperaturescope of this tile