14 ATPL Training Gas Turbine Engines #14 Thrust Augmentation

Summary

This lesson explores thrust augmentation methods used in turbojet and turboprop engines, focusing on water injection systems and afterburning. Water injection, often combined with methanol, helps restore or boost engine power at high altitudes or in hot conditions by cooling the air intake or increasing mass flow. Afterburning increases thrust by injecting additional fuel into the exhaust, a technique primarily used in military aircraft. The content highlights the mechanics, benefits, and limitations of these systems, with real-world examples like the Boeing 707 and Concorde demonstrating their application in aviation.

Takeaways

• 😀 Water injection systems help restore thrust in turbojet and turboprop engines under high altitude or high temperature conditions.
• 😀 The use of water injection increases the mass airflow through the engine, compensating for the reduced thrust caused by lower air density.
• 😀 The Boeing 707 and DC-8 carried approximately 300 gallons of water per engine for a 3-minute takeoff and climb.
• 😀 For turbojet engines, water injection restores thrust by cooling the airflow but is limited by the compressor's pressure capacity.
• 😀 Water injection can restore engine power up to about 25°C above ISA (International Standard Atmosphere) conditions.
• 😀 In turboprop engines, water injection increases mass flow through the turbine, boosting propeller thrust without increasing compressor speed.
• 😀 Water-methanol mixtures are often used in turboprop engines to prevent freezing and regulate turbine gas temperature.
• 😀 The ratio of water to methanol in the injection fluid is crucial for maintaining optimal engine performance without exceeding temperature limits.
• 😀 Afterburners inject additional fuel into the exhaust to utilize unburned oxygen, significantly increasing thrust in military aircraft and Concorde.
• 😀 Afterburners lead to higher fuel consumption, so they are typically used for short periods during takeoff or combat situations.
• 😀 Afterburner operation is managed by adjusting the variable area propelling nozzle to accommodate the increased gas volume.

Q & A

What is the primary function of a water injection system in turbojet and turbopropeller engines?

The primary function of a water injection system is to restore or boost the thrust of a turbojet engine and increase the shaft horsepower of a turbopropeller engine, especially in high altitude or high ambient temperature conditions. It does this by cooling the airflow passing through the engine.

How does high altitude or high ambient temperature affect engine performance?

At high altitudes or high ambient temperatures, the density of the air decreases, which reduces the mass of airflow through the engine. This results in a reduction of thrust in turbojet engines and a reduction in shaft horsepower in turboprop engines.

What is the typical water flow rate during takeoff for the Boeing 707 and DC-8 water injection systems?

The Boeing 707 and DC-8 used to carry approximately 300 gallons of water injection fluid per engine, with a flow rate of 100 gallons per minute during takeoff and climb.

What are the limitations of using a water injection system for thrust restoration?

The thrust restoration is limited to a temperature increase of approximately 25°C above ISA (International Standard Atmosphere) because increasing compressor speed to boost airflow can cause the compressor casing to split due to excessive pressure.

How does a water-methanol mixture benefit the water injection system?

A water-methanol mixture helps prevent freezing by using methanol as an antifreeze agent. Additionally, methanol burns, raising the turbine gas temperature back to its original level, counteracting the cooling effect of water injection.

What happens when water is injected into the combustion chamber of a turboprop engine?

Injecting water into the combustion chamber increases the mass flow through the turbine, which is absorbed by increasing the pitch of the propeller, thus increasing the thrust produced without exceeding the normal maximum rotational speed of the engine.

Why is it important to control the proportion of methanol in the water-methanol mixture?

The proportion of methanol in the mixture must be carefully controlled to ensure that the turbine gas temperature does not exceed safe limits. A typical ratio is 40% methanol to 60% water by volume.

What is the principle behind thrust augmentation using afterburning?

Afterburning works by injecting fuel into the jet pipe between the turbine and the propelling nozzle, where it is ignited using unburnt oxygen from the exhaust gases. This combustion increases the temperature and velocity of the gas stream, leading to an increase in thrust.

How does the use of afterburning affect fuel consumption?

Afterburning significantly increases fuel consumption because extra fuel is burned to increase the temperature and energy of the exhaust gases. Its use is therefore limited to short periods, especially in military aircraft or during critical phases like takeoff or combat.

What role does the variable area propelling nozzle play in afterburning?

The variable area propelling nozzle adjusts its size automatically to accommodate the increased volume of gases produced during afterburning. This ensures the correct pressure ratio across the turbine (P3 to P6), maintaining efficient engine performance.