About More Efficient Burning Aircraft Engines

Piston-driven engines were the first generation of internal combustion aircraft engines.

Aircraft engine efficiency has improved dramatically since the Wright Brothers first introduced their human-powered Wright Flyer. Piston engines were developed and then turboprop, turbofan and turbojet engines. Advances in technology allow each of these engine types to gain efficiency with each new generation.

Pistons

The first aircraft were propelled by human power. Propellers were usually geared to leg-operated pedals. Piston engines soon replaced manpower. A key advance in piston engine design was the development of radial engines. Radial engines have pistons arranged in a circle around the crankshaft. As radial engines improved, they achieved a much better power-to-weight ratio than in-line engines that had cylinders lined up in a row along the length of the crankshaft.

Turboprop

As aircraft performance began to peak, a new type of engine was needed to increase speed and altitude. Turboprops use propellers to create thrust, but those propellers are driven by turbines. Fuel is added to compressed air, which rushes toward the back of the engine. This air drives a turbine, which spins the propeller. These engines are more efficient and can reach higher speeds and altitudes than piston-driven engines.

Turbojet

Many military fighters use turbojet engines.

The turbojet is a true jet engine. Air drives through the intakes and is compressed. Fuel is added and ignited. This sends a stream of heated, pressurized exhaust that provides all the engine's thrust. The turbojet is more efficient than the turboprop at higher altitudes and reaches greater speeds. But the turboprop is efficient at lower altitude, because the propeller can create more thrust in the denser atmosphere. As the atmosphere thins at high altitudes, propellers become ineffective.

Fuel

The first fuel for piston-driven aircraft was low-quality gasoline. Engineers discovered that higher octane gasoline burned more efficiently and resulted in better engine performance. Octane levels were raised and a new rating system was designed for fuels that proved better than octane-rated gas. The PN, or performance number, replaced the octane rating for these fuels. Jet engines required a different type of fuel. Kerosene and gasoline were mixed to form JP, or jet propellant. Advances in jet propellant have improved fuel efficiency, but most were geared toward fire safety.

Airframe

Improvements in airframe design have improved fuel efficiency. Better aerodynamics mean that aircraft produce less drag, which works against thrust. If two planes are fitted with identical engines, the more aerodynamic plane will see better engine performance. Another airframe advance is the use of lighter materials in engine and airframe construction. Engines burn less fuel when they carry lighter loads.

References

• Aviation Transport and Environment: "Fuel Efficiency of Commercial Aircraft"
• Pilot Friend: Aircraft Engine Development
• U.S. Centennial of Flight Commision: Aviation Fuel

Resources

• Animated Engines: Jet Propulsion