A chain of causes.
Starting air provides shaft work. Fuel supplies chemical energy. The exciter supplies ignition energy. Light-off adds turbine torque; the starter continues to assist until cut-out.
ENGINE START & SYSTEMS
Engine Start & System Flows
From stillness to self-sustained.
Follow the energy.
HP speed · N2
LP speed · N1
EGT indication
Oil indication
Metered fuel
1 · Motoring2 · Light-off3 · Acceleration4 · Self-sustained5 · Idle region
Shaft response N2 · N1
Thermal response EGT · lagged sensor
All values are illustrative
Read several indications.
Torque → acceleration
Heat + mass flow → temperature
Heat + mass flow → temperature
Dry motoring can produce N2 and oil pressure without combustion. Fuel without ignition does not produce light-off. Once established, combustion can continue after the igniter is switched off.
A generic teaching engine.
The shaft and temperature responses are calculated from a simplified model. Flow paths are schematic, not CFD. Start thresholds, timings and indications are synthetic. This is not an operating procedure or a prediction for any aircraft type.
See the model and its limits ↓What drives the animation and instruments?
Two separate shaft balances
JH ω̇H = τstarter + τHP turbine − τcompressor − τloss,H
JL ω̇L = τLP turbine − τfan − τloss,L
The starter is mechanically connected to the HP spool only. HP and LP turbines receive power from the same combustion model, but the shafts have independent inertias. Reference angular speeds are 1,500 and 500 rad/s; N2 and N1 are percentages of those illustrative references. JH = 20 and JL = 45 kg m².
Starter torque falls with HP speed. Compressor and fan resisting torques grow quadratically with their respective speeds. A simple fuel governor approaches 62% N2. It does not reproduce a real engine control unit.
Heat and indication
Q̇ = ηc ṁf LHV × burn fraction
Pexhaust = Q̇ + Pcompressor − PHP − PLP
Tgas = Tambient + Pexhaust / [(ṁair + ṁf) cp]
The exhaust temperature sensor has a 1 s lag. Oil pressure follows HP speed with a 1.5 s lag. Oil is an indication model: viscosity, bearing temperature and fluid-circuit losses are not resolved.
Teaching controls and bounds
Automatic fuel admission starts at 18% N2; manual fuel is gated at 15%. Starter cut-out is latched at 48%. Ignition is removed at 55% in the normal demonstration; the idle region starts at 60%. These values are invented for this model. Replay resets the latch.
The automatic no-ignition case isolates fuel after 6 s without light-off. Motoring ends after 35 s. Manual mode does not use those timers. Fuel cut-off removes heat release and the rotors coast down. No unburnt-fuel inventory, hot-start limits, purge procedure or restart approval is modelled.
Interpretation
This is a lumped, low-order causal model. It omits component maps, pressure ratios, surge, compressibility, detailed combustor chemistry, windmilling and actual engine schedules. The LP spool remains stationary during dry motoring in this simplified model; real aerodynamic coupling can move it. No thrust, efficiency or aircraft performance is predicted.
Teaching sources: Starting and Ignition Systemsslides 12–18 and 33–35; lubrication; fuel; indications. Independent reference: FAA Aviation Maintenance Technician Handbook—Powerplant, Chapter 5. The engineering diagram, numerical closures and parameter values here are original; no lecture artwork is republished.