This page is an attempt to explain the four basic phases in the pulsejet's operational cycle.

1. Ignition
This is the instant that the fuel and air in the pulsejet are ignited.
 

The effect is that a fireball is produced inside the engine which creates a great deal of heat and pressure. The reed valves are held closed by this pressure, effectively leaving the flame and hot gasses only one place to go...

2. Combustion
After ignition, the air and fuel continues to burn and expand in a phase called the combustion phase.
 

During this phase the burning gases expand and travel down the tailpipe, exiting at the rear of the engine. The force of the gases leaving the engine in a rearwards direction creates an equal and opposite force that tries to move the engine forwards -- this is thrust.

3. Intake
Because gases are elastic (they can be compressed and stretched) and because they have mass, the rapidly exiting exhaust gases have a tendency to keep moving -- even after the pressure inside the engine drops below the pressure outside.

This causes a partial vacuum to be created inside the engine.
 

The effect of this vacuum is to draw air and fuel in through the valves at the front of the engine -- which are pushed open by the higher pressure outside the engine.

4. Compression
As mentioned above, gases are elastic -- so now, having been stretched out to create a partial vacuum, some of the hot exhaust gases are now drawn back towards the front of the engine by the vacuum that was created.
 

Once again, because they have momentum, the gases in the tailpipe continue to move even after the pressure inside and outside the engine is equalized. This means that the gases continue heading towards the front of the engine -- towards the fresh charge of air and fuel that has just been drawn in.

Of course, as soon as the pressure inside the engine becomes higher than the air pressure outside, the reed valves slam shut -- stopping the air/fuel mixture from escaping.

This continued movement of the exhaust gases causes the air-fuel mixture to be compressed -- until the hot gases finally travel so far up the pipe that they touch the explosive air/fuel mixture and -- back to step one!

This cycle repeats hundreds of times a second -- producing the characteristic buzzing sound of the pulsejet engine.

What Can Go Wrong?
Given that these are such simple engines it is sometimes difficult to understand why they can be so hard to design and build so that they work.

Here are the effects of a few design mistakes that can affect the engine's ability to run:

	PIPE TOO SHORT If the pipe is too short then the engine won't run because all of the hot gases wil leave the tailpipe. This means there's nothing left to ignite the new air-fuel mixture drawn in during the intake phase.
	PIPE TOO LONG If the pipe is too long then the exhaust gases will "burn out" and cool down too much -- making it impossible for them to ignite the fresh air/fuel mixture. It is worth noting that most engines are far more tollerant of a too-long pipe than a too-short one. If you're designing an engine it always pay to err on the side of making the pipe a little longer than you might think necessary.
	VALVED INTAKE AREA TOO BIG If the valves in the front of the engine are too big then the vacuum necessary to suck some of the hot exhaust gases back to compress and ignite the fresh air/fuel charge will disipate too quickly and the engine will not run.
	VALVED INTAKE AREA TOO SMALL If the valves are too small then not enough air/fuel mixture will be drawn into the engine to provide adequate combustion and the engine will either not run or will run with less than optimal levels of power.

It can be seen from this that there are two critical elements to designing a successful pulsejet engine:

1. the valve area
2. the pipe length

Designing A PulseJet
Armed with the understanding of the basic principles and critical factors it now becomes practical to start designing a pulsejet.

An Overview
Pulsejet engines have a sometimes deserved reputation for being very difficult and problematic to start -- but fortunately this doesn't always have to be the case.

In order to start a pulsejet you need three things:

1. fuel
2. air
3. an ignition source

Not only must you have all three -- but they must also be provided at the correct time and in the right proportions.

The Fuel
Pulsejets can run on a wide range of fuels ranging from LPG/propane through to diesel or kerosene.

For the purposes of small pulsejet engines however, the most common fuel is gasoline of some kind. This can be white-gasoline or low-octane gas from the local pumps. You can use high-octane gasoline but you'll simply be wasting money and possibly getting a little less power at the same time.

In cold conditions where the air temperature is less than 60 degrees F or around 17 degrees C you may find that gasoline isn't sufficiently volatile to ignite reliably when starting an engine. If this is the case then it is recommended that you add some ether -- up to 25 percent. This will significantly increase the ease with which the fuel can be ignited.

Cold can also affect engines that are using propane/LPG because the pressure available from a tank of this gas reduces quite significantly as the temperature drops.

It's worth mentioning at this stage that there are two methods of delivering the fuel to the engine:

	aspiration This is when the fuel is drawn into the engine through an atomizer by the air which enters through the intake. This has the advantage that it is very simple and requires no fuel pump or other ancilliary equipment.
	injection This involves spraying fuel directly into the combustion chamber where it mixes with air that has already passed through the valves. This has the advantage that you can throttle the engine simply by varying the amount of fuel injected -- but it does require the use of a pressurized fuel system such as a bladder or pump.

The Air Supply
Just supplying the engine with fuel is not enough -- you need to force some air into the intake so as to create an explosive mixture inside the engine.

In the case of an injected engine, an air source such as a leaf-blower or vacuum-cleaner with a blow attachment will likely do the job. You don't need a lot of pressure but you do need a reasonable volume of air.

With an aspirated engine you'll need less volume but more pressure. This is because the air has to draw the fuel up the fuel-line and atomize it into a fine spray before it passes through the valves into the engine.

An air-gun driven by a compressor is perfect for the job but if you don't have such luxuries available to you a great substitute is to inflate a car tube or tire without a valve in it -- using a length of flexible plastic tubing slipped over the valve stem to deliver the air to the engine's intake. The flow of air can be controlled by kinking the tubing. Slipping a thinner piece of pipe into the open end of the tubing will give you a narrower and more easily controlled air-jet to spray into the engine. Between starting attempts you can replace the valve and pump the tire up to 40-60 psi using a foot-pump or whatever. This is a great low-cost way to start your engine at the flying field or away from other sources of compressed air.

The Ignition Source
The best ignition source is a spark plug mounted in the combustion zone section of the engine -- but there are alternatives...

The simplest but least effective ignition source is a naked flame situated at the end of the tailpipe. This could be a gas-torch or a spirit burner but you'll find starting an engine using this method to be more difficult than with a sparkplug.

One other technique sometimes used when a sparkplug isn't mounted in the engine itself is a spark-wand. This consists of two wires, separated by an insulator with a spark-gap at the end. It is inserted and energized when starting the engine and quickly withdrawn once it is running.

Putting It All Together
Here's the sequence for starting an aspirated pulsejet:

1. Connect the fuel line and tank. Make sure that the fuel level is no more than an inch or so (20mm) from the engine's fuel jet (the hole where the fuel comes out into the engine).

    

2. Turn on the spark or light your ignition source

    

3. Direct a jet of compressed air over the fuel jet so that it creates a fine spray of fuel droplets that are then blown towards the engine's valves.

Note that it will take some experimentation, practice, and coordination to get this right and you'll probably find that it helps to move the jet of air back and forth a little so as to vary the spray a somewhat.

At this stage the engine should at least pop, bang or burb a little, even if it doesn't immediately burst into life.

If you don't get much activity, try richening the mixture a little by losening the locknut and opening the mixture screw by a quarter of a turn. Repeat this process until things improve. If they don't improve, return the screw to its original position and try closing it a quarter turn at a time in case the mixture is already too rich.

When you've had a little practice and got the mixture setting sorted out then it should be as easy as in the video below of an aspirated PJ8 being started:

The procedure is even simpler for an engine with direct LPG/propane injection.

Thanks to the fact that the fuel is already in the engine's combustion chamber, all we need do is turn on the spark and blow some air into the intake.

If the engine doesn't fire immedately then the gas should be turned up or down until the engine starts. If you still have trouble, try varying the amount of air being blown into the intake.

Once you get the hang of starting a LPG-injected pulsejet its exceptionally easy to do.