Exhaust, turbocharging and supercharging systems
Exhaust, turbocharging and supercharging systems
This section covers exhaust components and the systems that increase manifold pressure above ambient. The exam tests turbine/compressor function, wastegate control and the hazards of overboost.
Core rules and concepts
- Exhaust systems carry hot gas away, drive the turbocharger and may supply cabin heat through a heat exchanger. Leaks are dangerous because carbon monoxide can enter the cabin.
- A turbocharger has a turbine driven by exhaust gas and a compressor that raises intake pressure. A wastegate controls exhaust flow around or through the turbine to limit boost.
- A supercharger is mechanically driven by the engine; a turbocharger is driven by exhaust energy. Both raise manifold pressure, but the turbo has lag and heat-soak issues.
- Critical altitude is the highest altitude at which the system can maintain a selected manifold pressure. Above it, manifold pressure falls unless the wastegate is fully closed.
- Overboost raises cylinder pressure and temperature and can cause detonation or structural damage; the throttle and wastegate must be managed carefully.
Worked example
A turbocharger inlet is at 15 inHg absolute and the compressor outlet is 30 inHg absolute. Pressure ratio = 30 ÷ 15 = 2.0. The compressor must double absolute pressure, which also raises temperature; an intercooler reduces that temperature before the throttle.
Traps
Do not use gauge pressure instead of absolute pressure in pressure-ratio calculations; do not treat the wastegate as a mixture control; and do not ignore exhaust leaks or heat damage.