Why Modern Airliners Rely on Propellers or Jet Engines: An Examination of Aircraft Propulsion Choices

Aerospace engineers face many challenges when designing aircraft propulsion systems. This article explores the reasons why modern airliners predominantly use propeller or jet engines. It covers the limitations of piston engines, the advantages of jet engines, and how these choices impact the design and performance of commercial aircraft.

The Practical Size Limit of Airplane Engines

There is a physical limit to the size of cylinders in a gasoline aero engine, roughly around 3 liters. This size constraint significantly limits the power that can be generated from a piston engine using such cylinders. To produce high power, piston engines must generate both torque and a high RPM (revolutions per minute). However, the limit for RPM in aero engines is consistently around 3000, which restricts the cylinder size to approximately 3 liters.

To increase power output, engineers add more cylinders. V8 engines can become V12s, radial engines with five cylinders can become seven-cylinder or nine-cylinder variants, and some engines even add a second row of cylinders, resulting in engines with 14 or 18 cylinders. Despite these modifications, the complexity and the presence of sliding reciprocating parts mean that these engines require expensive rebuilds every 2000 to 2500 hours. Even a small Cessna 150-hp engine can cost over $30,000 for a complete rebuild.

The Advantages of Jet Engines

The jet engine is far more scalable and maintenance-friendly compared to piston engines. Jet engines run smoothly and require far less frequent heavy maintenance, often staying on the wing for 10,000 hours or more. The advancement in high-bypass jet engines, which use large fans in front of the engine, has significantly improved engine performance and reduced fuel burn. Additionally, these engines are more efficient at higher speeds and altitudes, where gasoline-powered propellers begin to lose efficiency above approximately 20,000 to 25,000 feet and become less efficient above about 350 mph.

Turbofan engines optimize performance for speeds around 0.8 to 0.85 MACH, which is roughly 500 miles per hour at high altitudes. At these altitudes, piston engines with propellers are less efficient. Jet engines also use kerosene-based fuels, which are less volatile, produce fewer fumes, and are slightly more energy-dense per liter/gallon. This makes the fuel safer to handle and allows for more energy storage in the same volume, improving overall efficiency.

Optimization for Different Flight Conditions

For low-power applications under about 400 hp and at lower altitudes, piston engines still have a place. However, for medium-altitude and medium-speed operations, turbo-prop propellers driven by jet engines are a more efficient choice. For higher speeds and longer distances, jet engine turbofan engines clearly outperform piston engines, securing their dominant role in modern airliners.

The choice of engine type is critical and depends on environmental conditions, flight altitude, and operational requirements. Jet engines, with their inherent scalability and efficiency, are well-suited to the diverse conditions encountered in commercial aviation, making them the preferred choice for modern airliners.

Conclusion

While piston engines have their place in lower-power and lower-altitude applications, the widespread use of propeller or jet engines in modern airliners is driven by their reliability, efficiency, and ability to handle the demands of high-altitude and high-speed operations. This choice underscores the engineering challenges and compromises that must be balanced in the design of commercial aircraft propulsion systems.