In all commercial helicopters, and by and large all helicopters period, there are two main engine types. There are piston engine, and turbo shaft engines. You’ll often see either type mentioned in literature, but what’s the difference?
In a piston engine, the engine inside the helicopter is similar to one that is in your car. These use a reciprocating set of pistons to drive a drive shaft, which is turn is connected to the gearbox and the rotor head. Many training helicopters use reciprocating engines because they are fairly inexpensive to operate, and relatively simple compared to turbines. However, they produce much less power than turbine engines, and thus, are unsuitable for most heavy applications.
In a turbo shaft, or turbine, engine, the gay turbine is made up of a compressor, a combustion chamber, the turbine itself, and an accessory gearbox. The compressor draws filtered air into the combustion chamber, and compresses it. This air is directed into the main combustion chamber through discharge tubes, where atomizes jet fuel is injected into it. The fuel air mixture is ignited, and allowed to expand. This combustion gay is then forced through a series of wheels, like a windmill’s’s blades, which forces them to turn.
The spinning of these wheels provides power to both the engine’s shaft, which turns the main rotor head(s), and an accessory gearbox, which is often used to generate electrical power for the aircraft in flight. Power is provided to the two rotor systems though the accessory gearbox power output shaft, which sticks out of the turbine unit and connects to the main rotor shaft, which sticks out of the turbine unit and connects to the main rotors gear assembly. The turbine’s turbine wheels also provide power to the compressor on the engine itself, thus allowing the process to continue operating.
The exhaust gas is then, finally, expelled through an exhaust outlets. The temperature of the gas is measured at different locations throughout the process, depending on the manufacturer. But a few common terms are the inter-turbine temperature (ITT). which measures the temperature inside the spinning turbines; the Exhaust Gas Temperature (EGT), which measures temperature inside the exhaust pipe; and the Turbine Outlet Temperature (TOT), which measures temperature at exactly the point where the exhaust gas exits the turbine arrays.
Most helicopters have a single turbine; however, many are twin-turbine aircraft, especially in high-reliability situations and where more power is needed. Turbines, unlike the piston engines in training and personal aircraft, require Aviation grade kerosene, also known as JET-A fuel, to operate properly, whereas most piston engines for aviation use operate on 100LL fuel, a type of AVGAS that has a low lead content. Neither is suitable to operate on automotive petroleum or diesel, although JET-A and Diesel Fuel have many similarities. Regardless of the type of engine your helicopter uses, make sure you match it up with the proper fuel in order to avoid catastrophic failures and potential fires.