pulse jet engine

A pulsejet engine is a type of jet engine in which combustion occurs in Pulses.

Air is compressed and combusted intermittently instead of continuously.

They have better power to weight ratio than other engines.

Pulsejet engines are a lightweight form of jet propulsion, but usually have a poor compression ratio, and hence give a low specific impulse.

Types:

Valved

valve less

valve-less pulsejetIn these, the exhaust mechanism is controlled by its geometry itself.

Following are the steps followed:

1.Initial air + fuel=ignition

2.Increase in pressure , exhaust-1.

3.After exhaust-1,pressure decrease which draws more air in form of returning flame and more air + fuel + compression

4.Ignition of compressed air-fuel mixture.

5.Power strokes starts from here, exhaust-2 with more power than exhaust-1.

Due to the geometry of valve-less engine , the exhaust happens on both intake and exhaust sides but more propulsion would be in longer tube i.e., exhaust pipe.

Valved pulse jet

Valved pulsejet engines use a mechanical valve to control the flow of expanding exhaust, forcing the hot gas to go out of the back of the engine through the tailpipe only, and allow fresh air and more fuel to enter through the intake.

Steps involved:

First part of the cycle: air flows through the intake (1), and is mixed with fuel (2).

Second part: the valve (3) is closed and the ignited fuel-air mix (4) propels the craft.

Lenoir cycle:• The Lenoir cycle is an idealized thermodynamic

cycle often used to model a pulse-jet engine.

• Comprises of 3 cycles:

▫ Heat added at constant volume.

▫ Adiabatic Expansion.

▫ Exhaust of the hot gasses at a constant pressure.

• Thrust can be directly calibrated on the basis that

the cycle is completed over two working strokes.

Efficiency of Lenoir cycle

1-2:isochoric heat addition

2-3:isentropic expansion

3-1:isobaric heat rejection

comparing Lenoir cycle and Otto cycle

The absence of any compression process in the design leads to lower thermal efficiency than the more well known Otto cycle and Diesel cycle.

And also the power obtained in the combustion phase is effectively used for exhaust which in case of other cycles the compression process uses some power obtained from the combustion phase eventually increasing the work input.

But overall thermal efficiency is less in Lenoir cycle compared to other cycles as the heat addition is done in the absence of high pressure(without compression phase).