A jet engine runs an open Brayton cycle: air is drawn in and isentropically compressed (1→2), fuel is injected and burnt at roughly constant pressure to raise its temperature (2→3), the hot gas then expands isentropically through the turbine and nozzle (3→4) to produce thrust, and it exhausts to ambient conditions before the cycle repeats with fresh air (4→1). The P–V diagram on the left traces exactly this loop in real time, redrawing as you change the compression ratio and fuel flow.
η = 1 − 1 / r^((γ−1)/γ)
r = P₂/P₁ (compression ratio), γ = 1.4 for air
T₂ = T₁·r^((γ−1)/γ), T₄ = T₃/r^((γ−1)/γ)
- Fuel — SAF (sustainable aviation fuel, made from biomass) burns as a "drop-in" replacement with a similar energy density to fossil kerosene, but its lifecycle CO₂ is far lower because the carbon it releases was captured from the atmosphere during feedstock growth.
- Compression ratio — the pressure rise across the compressor stage. Ideal Brayton efficiency depends only on this ratio, not on how hot the flame burns.
- Fuel flow (throttle) — how much fuel is injected into the combustor, which sets the turbine inlet temperature T₃ and therefore the specific work (and thrust) the engine can extract per cycle, within the metallurgical limit of the turbine blades.
- Net specific work — the useful work per unit mass of air, cp·[(T₃−T₄)−(T₂−T₁)]; this is what ultimately drives the fan and produces thrust.
Real-world relevance: SAF's thermodynamic cycle is identical to kerosene's — the efficiency gain from a jet engine comes entirely from compressor and turbine design (compression ratio), while SAF's benefit is almost purely in the carbon accounting, not in how the engine itself performs.