Jet Engine Size and Efficiency: Why Bigger Isn't Always Better

Overview

This video explains how airliner propulsion relies on two subsystems, the jet core and the fan, with the fan generating most thrust while the core powers the engine. It discusses why bigger engines are often more efficient up to a point and how exhaust speed affects energy use. It also notes a rough estimate for an optimal engine diameter around 4 meters, just larger than today's largest engines, suggesting engines will continue to grow but not indefinitely.

Key insights

• Two propulsion systems: jet core and fan
• Efficiency depends on exhaust velocity and momentum transfer
• Optimal engine size estimated near 4 meters in diameter
• Engine growth will face tradeoffs like drag and weight

Introduction

The video discusses why jet engine sizes have risen over time and why this increase is not solely due to larger aircraft. Even on smaller airframes like the Airbus A350, engines are larger because bigger turbines and fans can be more efficient up to a limit. The core idea is that engine design balances two propulsion systems: the jet core which provides power and some thrust, and the fan which acts as a large propeller driven by the core and supplies most of the thrust.

Jet Engine Architecture

Modern engines combine a high speed jet core with a large front fan. The core is compact but powerful, while the fan moves a large volume of air at lower speed, delivering most thrust through momentum transfer. This separation allows the engine to generate thrust efficiently without pushing exhaust to extreme speeds that waste energy.

Efficiency and Energy Tradeoffs

The video emphasizes that pure jet exhaust at very high speeds wastes energy because kinetic energy grows with the square of velocity. Doubling exhaust speed increases energy expenditure by a factor of four. Therefore, designers aim to accelerate a lot of air slowly rather than a small amount quickly. The result is greater momentum transfer with lower energy losses, which is more fuel-efficient in normal flight regimes.

The analogy used is like using a machine gun to propel a car: accelerating air to very high speeds is powerful but energetically expensive, whereas moving a lot of air slowly can produce the same thrust with far less energy waste.

Size Tradeoffs and the Ideal Diameter

There is a balance point between too small and too large engines. A rough estimation places the ideal engine diameter around 4 meters, slightly bigger than the largest engines in service today. While engines can continue to grow, they cannot do so forever because larger engines also introduce more drag and heavier structures, limiting practical gains.

Outlook for Jet Engine Evolution

The takeaway is that jet engines will likely increase in size but with diminishing returns, as designers optimize airflow, thrust, drag, and overall efficiency rather than pursuing unbounded growth. The idea of a near-optimal diameter highlights how physics constrains engineering progress in aerospace propulsion.

Conclusion

Understanding the balance between air mass flow, exhaust velocity, and drag sheds light on why engines grow and why they eventually plateau. This framework helps explain current trends in engine sizing and hints at the kinds of future improvements we might expect in jet propulsion.