Operation of pulse detonation engine

Pulse detonation engine

The pulse detonation engine (PDE) marks a new approach towards non-continuous combustion in jet engines. They are light, easy to manufacture, and promises higher fuel efficiency compared even to turbofan engines. With the aid of latest design techniques and high pulse frequencies, the drawbacks of the early designs promise to be overcome. To date, no practical PDE engine has been put into production, but several test bed engines have been built by Pratt & Whitney and General Electric that have proven the basic concept. Extensive research work is also carried out in different NASA centers. In theory, the design can produce an engine with the efficiency far surpassing gas turbine with almost no moving parts. These systems should be put to use in the near future.

All regular jet engines operate on the deflagration of fuel, that is, the rapid but relatively gentle subsonic combustion of fuel. The pulse detonation engine is a concept currently in active development to create a jet engine that operates on the supersonic detonation of fuel.

Pulse detonation engines (PDEs) are an extension of pulse-jet engines. They share many similarities including the operating Humphrey’s cycle. However, there is one important difference between them, namely, PDEs detonate, rather than deflagrate, their fuel. Detonation of fuel is a sudden and violent supersonic combustion of fuel that results in immense pressure which in turn used as thrust.

Combustion process in PDE resembles perfectly constant volume combustion, while combustion process in valveless and valved pulsejets is approximated only as a constant volume process. Others identify combustion in PDE as neither constant volume nor constant pressure. Details of test results of both PDE and pulsejet are given in. PDE achieves higher specific impulse in comparison with pulsejet engines for same operating static conditions. The main objective of PDE is to provide an efficient engine that is primarily used for high-speed (about Mach 5) civilian transport as well as many military applications including supersonic vehicles, cruise missiles, UAVs, SSTO launchers, and rockets. However, their noise and the drop in efficiency at higher Mach number imply that pure PDEs will likely not to be used often for large-scale applications.

The single flight of an aircraft powered by a pulse detonation engine took place at the Mojave Air & Space Port on 31 January 2008. The aircraft selected for the flight was a heavily modified Scaled Composites Long-EZ, named Borealis.

The engine consisted of four tubes producing pulse detonations at a frequency of 80 Hz, creating up to 890 Newton of thrust and using a refined octane as fuel. A small rocket system was used to facilitate the liftoff of the Long-EZ, but the PDE operated under its own power for 10 s at an altitude of approximately 100 ft (30 m). This demonstration showed that a PDE can be integrated into an aircraft frame without experiencing structural problems from the 195 to 200 dB detonation waves. Pulse detonation engines use intermittent detonation waves to generate thrust.

Unlike the pulsejet, combustion in PDE is supersonic, effectively an explosion instead of burning, and the shock wave of the combustion front inside the fuel serves the purpose of shutters of valved pulsejet.

I. Answer the questions:

1. What are the advantages of the PDE?
2. What do all regular jet engines operate on?
3. What is the main difference between PDEs and pulse-jet engines?
4. What is the main objective of PDE?
5. What is the combustion in PDE unlike the pulse-jet?

II. Find the English equivalents:

1. непрерывное сгорание
2. новейшие конструкторские технологии
3. высокая эффективность расхода топлива
4. высокая частота импульсов
5. недостатки (недоработки) устраняются
6. намного превышающая эффективность
7. постоянный объём сгорания
8. по сравнению с чем-либо
9. для широкого применения
10. облегчить взлёт
11. пульсирующие взрывные волны
12. клапаны двигателя

III. Make up a sentence with the phrase “the drawbacks promise to be overcome”.

IV. Say some definitions of the words from the text to make your group mates guess.

Operation of pulse detonation engine

A detonation propagating from the closed end of the tube is followed by an unsteady expansion wave (called Taylor wave), whose role is to bring the flow to rest near the closed end of the tube. When the shock wave reaches the rear of engine and exits, the combustion products are ejected in “one go”, the pressure inside the engine suddenly drops, and the air is pulled in front of the engine to start the next cycle.

PDE operation is not determined by the acoustics of the system and can be directly controlled. PDEs typically operate at a frequency of 50–100 Hz, which means that a typical cycle time is on the order of 10–20 ms. Since PDE produces a higher specific thrust than a comparable ramjet engines at low supersonic speeds, it is suitable for use as part of a multi-stage propulsion system. Single-tube supersonic impulse PDE with straight detonation has higher performance than ideal ramjet engines for flight Mach number up to 1.35.

The PDE can provide static thrust for a ramjet or scramjet engine or operate in combination with turbofan systems.

PDE can be classified as:

• Pure (standalone)

• Combined cycles

• Hybrid turbomachinery cycles

Pure PDE as the name implies consists of an array of detonation tubes, an inlet, and a nozzle. The applications of pure PDEs are mainly military, as they are light, easy to manufacture, and have higher performance around Mach 1 than current engine technologies. Their noise and the drop in efficiency at higher Mach number imply that pure PDEs will likely not to be used often for large-scale applications.

Combined-cycle PDEs may provide the most exciting possibilities for aviation. Adding a PDE to the flowpath of a ramjet or scramjet would make an engine capable of operating efficiently as high as Mach 5.5. These engines would seem initially suitable for high altitude, high-speed aircraft.

Hybrid PDE’s make use of detontative combustion in place of constant pressure combustion, usually in combination with turbomachinery. Hybrid engines are of two types, one in which the PDE replaces the conventional combustor, but has several disadvantages among which providing high temperature flow into turbine blades. An alternate approach involves arranging the PDE combustor in the bypass duct of turbofan engines and mixing the PDE exhaust with the turbine exhaust in a mixer located aft of the turbine, which resembles a mixed turbofan arrangements. In other researches of hybrid turbofan-PDE: The central core engine would still turn the large fan in front, but the bypass would flow into a ring of PDEs in an unmixed turbofan arrangement. The bypass air enters pulse detonation tubes that surround the standard combustion chamber. The tubes are then cyclically detonated; one detonates while the others fill with air or are primed with fuel. This combination promises to require simpler engine mechanisms and yield higher thrust with lower fuel consumption as examined by GE.

Hybrid PDEs will allow commercial aircraft powered by subsonic gas turbines to be faster, more efficient, and more environmentally friendly. Similarly, hybrid supersonic gas turbines can also be used in military applications. Generally, hybrid PDEs will deliver the same thrust of a turbofan engine but with less fuel consumption. Moreover, they would produce significantly more thrust without requiring additional fuel.