How Jet Engines Work (Turbofan vs Turboprop)

What Is a Jet Engine?#

A jet engine is a propulsion system that produces thrust by accelerating a mass of air rearward, creating forward motion through momentum change and pressure forces. This guide is part of Aviatopia's How Airplanes Fly series.

In modern aviation, “jet engine” typically refers to a gas turbine operating on the open Brayton cycle, where air is continuously compressed, mixed with fuel, ignited, expanded through turbines, and expelled as exhaust. The two most common aircraft variants are the turbofan and the turboprop.

Both share the same thermodynamic core. They differ in how they convert turbine energy into useful propulsive force.

Why It Matters in Aviation#

Engine architecture determines how an aircraft performs and where it is economically viable to operate.

It directly influences:

• Cruise speed and altitude capability
• Fuel consumption and operating cost
• Takeoff and climb performance
• Noise footprint and community impact
• Maintenance complexity and reliability

High-bypass turbofans dominate long-haul and high-altitude airline operations. Turboprops remain highly efficient on short-haul regional routes and in environments where runway length and operating cost are limiting factors.

A clear understanding of propulsion fundamentals also improves context when studying aircraft performance or reading weather and performance data in structured reports such as a METAR.

How It Works#

All modern gas turbine jet engines operate on the open Brayton cycle, which consists of continuous-flow combustion and expansion.

The process follows five functional stages:

The turbine extracts enough energy to drive the compressor and any connected shafts. In turbofans, this includes the large front fan. In turboprops, this energy drives a reduction gearbox and propeller.

The fundamental principle of propulsion is that accelerating a larger mass of air by a moderate velocity change is generally more efficient than accelerating a small mass by a very large velocity change. This concept underpins modern high-bypass turbofan design.

Turbofan Engines#

A turbofan engine generates thrust by moving a large volume of air through a ducted fan, with only part of that air passing through the combustion core.

Two airflow paths exist:

• Core flow – passes through compression, combustion, and turbine stages
• Bypass flow – passes around the core and is accelerated by the fan

The bypass ratio is the ratio of bypass airflow to core airflow. Higher bypass ratios improve propulsive efficiency and reduce noise.

In high-bypass turbofans—common on modern airliners—most thrust comes from the fan-driven bypass air rather than from the hot exhaust stream.

Geared vs Direct-Drive Turbofans#

Some modern engines use a geared architecture, where a reduction gearbox allows the fan to rotate at a different speed than the low-pressure turbine. This improves efficiency and reduces fuel burn at cruise while maintaining structural limits on large fan diameters.

Operational Characteristics#

• Typical cruise: Mach 0.75–0.85
• Optimized for high-altitude operation (e.g., FL300–FL400)
• Low specific fuel consumption at cruise
• Reduced noise compared to earlier low-bypass designs

Turboprop Engines#

A turboprop engine uses a gas turbine core to drive a propeller through a reduction gearbox.

In this configuration:

• The turbine extracts most of the available energy from combustion gases
• That energy turns a shaft
• The shaft drives a propeller
• The propeller produces the majority of thrust

Only a small percentage of thrust comes directly from the exhaust stream.

Because propellers accelerate a large mass of air at relatively low velocity, turboprops achieve high propulsive efficiency at lower airspeeds.

Operational Characteristics#

• Typical cruise: 250–360 knots (approximately Mach 0.4–0.6)
• Optimized for low- to medium-altitude operation
• Excellent short-field performance
• Lower fuel burn on short regional sectors

Turbofan vs Turboprop: Key Differences#

Both are gas turbine engines. The distinction lies in how turbine power is translated into thrust.

For reference, helicopter engines use a closely related configuration called a turboshaft, where turbine energy drives a rotor system rather than a propeller or fan.

Operational Example#

Consider two aircraft operating a 500-nautical-mile sector:

• A narrow-body jet with high-bypass turbofans cruising at Mach 0.78 at FL350
• A regional turboprop cruising at 300 knots at FL200

On short routes, the turboprop may burn less total fuel because its propeller system is highly efficient at moderate speeds. Over longer sectors, the turbofan’s higher cruise speed reduces block time and increases aircraft utilization, which can improve overall network economics.

Airline fleet planning decisions therefore reflect route structure, demand density, fuel price, and scheduling constraints.

Common Misconceptions#

“Turboprops are not jet engines.” They are gas turbine engines. The difference lies in how thrust is produced.

“All turbofan thrust comes from hot exhaust.” In high-bypass designs, most thrust comes from the fan-driven bypass stream.

“Propeller aircraft are outdated.” Turboprops remain economically optimal for many regional operations.

“Jet engines pull aircraft forward by suction.” Thrust results from accelerating air rearward and changing momentum, not from suction.

“Higher exhaust velocity always means higher efficiency.” Propulsive efficiency improves when a large mass flow experiences a moderate velocity increase.

Frequently Asked Questions#

Key Takeaways#

• A jet engine produces thrust by accelerating air rearward.
• Modern aircraft engines operate on the open Brayton cycle.
• Turbofans generate most thrust from fan-driven bypass airflow.
• Turboprops convert turbine energy into propeller-driven thrust.
• Propulsive efficiency improves when large air mass flow is accelerated moderately.
• Turbofans dominate high-speed airline operations.
• Turboprops remain highly efficient for short-haul and regional missions.
• Engine selection reflects route structure, economics, and operational requirements.

A clear grasp of propulsion principles provides essential context for aircraft performance analysis, fuel planning, and operational decision-making across modern aviation. For foundational lift and drag concepts, see How Airplanes Fly.

Sources & References#

• FAA Pilot's Handbook of Aeronautical Knowledge (FAA-H-8083-25B), Chapter 7 — Aircraft systems including turbofan and turboprop engine fundamentals.
• NASA Glenn Research Center — Beginner's Guide to Propulsion — Interactive reference on jet engine principles, thrust, and thermodynamic cycles.
• SKYbrary — Jet Engine — Operational overview of gas turbine engine types and performance.

Related Guides#

• Induced vs Parasite Drag
• Cabin Pressurization Explained
• How Airlines Make Money

Browse Directories#

• Aircraft Families — Compare Airbus and Boeing aircraft types by engine configuration, range, and mission.

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