Designing a pulsejet that produces more power and uses less fuel was the Holy Grail of engineers during the period from 1940-1960.
However, as the turbojet, and later the turbofan, continued to provide increasing levels of power and performance, no serious effort was applied to improving the humble pulsejet.
This doesn’t mean that the pulsejet is a lost cause though, and in recent years there has been something of a renaissance in the design and construction of these engines, driven partly by a global community of enthusiasts.
It’s worth noting that as long ago as 1947, engineers working with pulsejets suggested that there were significant performance gains yet to be made with this technology.
Fuel Consumption
The single biggest problem with pulsejet engines is their horrendous fuel consumption.
Most conventional pulsejets use somewhere between 3.0 and 5.5 pounds of fuel per hour for every pound of thrust generated.
That means a that even a pulsejet operating at the more economical end of this scale and generating 100lbs of thrust would consume 300lbs of fuel per hour. That’s some 50 gallons of regular gasoline.
By comparison, a modern turbofan engine uses as little as 0.34 pounds of fuel per hour for every pound of thrust produced. This means a 100lbs-thrust turbofan would use less than six gallons (just 34lbs) of fuel to accomplish the same task.
I hope you can see now why there are no commercially made pulsejet-powered aircraft around.
Not only would such a craft struggle to get into the air under the weight of its enormous fuel burden, but it would also be incredibly expensive to fly.
So how can we go about reducing the fuel consumption of a pulsejet engine?
The single largest problem is that pulsejets have a very low pressure ratio. In plain-speak, this simply means that it uses fuel very inefficiently because there’s a bloody great hole out the back that makes it hard for the engine to build up any significant amount of pressure inside.
When fuel is ignited in your car’s engine it has already been compressed to about 1/10th its normal volume and 10 times atmospheric pressure. By compressing the air/fuel before it’s ignited, more energy is obtained from that combustion and greater pressures are generated inside the engine’s combustion chamber.
A car engine uses a solid aluminum piston to compress the air/fuel mixture and confine it once combustion has occurred.
Unfortunately, pulsejets don’t have nice solid pistons to compress and confine the air/fuel mixture prior to, or immediately after ignition.
The only thing compressing and confining our air/fuel mixture is the light, relatively
ineffective column of gas in the engine’s tailpipe. Clearly this column of hot gas is going to act like much of a piston.
This is confirmed by measurements made inside a running pulsejet engine which indicate that the fresh air/fuel charge is only compressed by a tiny amount (around 2psi) before it is ignited.
Other measurements indicate that the peak pressure generated inside the engine as the hot burning gases push against the gas column in the tailpipe is a very modest 12-15psi.
Clearly design change that allows an engine to increase the operating pressures that are generated inside it will have a positive effect on both power output and fuel consumption.
Some suggestions for obtaining higher operating pressures have included:
1. multi-point ignition to reduce the time taken to ignite the entire air/fuel load
2. the use of detonation waves to provide pre-ignition compression and much faster ignition of the total air/fuel load
3. increasing the level of turbulence inside the engine so as to promote faster ignition of the total air/fuel load
All of these are valid suggestions but none of them are easily achieved without compromising some other aspect of the engine’s operation.
Multi-point ignition
The faster you can burn the entire air/fuel charge inside the engine, the more power will be produced and the more efficiency will be obtained.
In a perfect engine we’d be trying to obtain what’s known as “constant volume” combustion where the air/fuel charge isn’t allowed to expand at all until it’s been entirely consumed. In such an engine, the internal pressure generated will often be more than ten times that which was present before ignition occurred.
If we have a pulsejet that achieves an internal pre-ignition pressure of just 2psi
The reason for this is simple – the only thing stopping the burning air/fuel from escaping out the rear of a pulsejet engine is the relatively light column of gas in the tailpipe. Being light, this gas actually does a very poor job of stopping the air/fuel from
The most promising of these has been the use of a detonation wave to ignite the air/fuel load.
It is this concept that has spawned a whole new arm of pulsejet research and the development of the pulse detonation engine (PDE).
Unfortunately, PDEs have yet to make it out of the laboratory and into practical application anywhere. Although they do offer the promise of massive improvements in efficiency, they
are complex beasts that require a truckload of support gear in the form of pumps, valve actuators, oxidizer tanks and much more.
Augmentors The simplest way to improve both the static/low-speed thrust and fuel-efficiency of a pulsejet is to add an augmentor to it.
Augmentors increase an engine’s thrust through two basic mechanisms:
1. Increasing the total mass-flow.
Since the amount of thrust generated by an engine is equal to the amount of mass it expels times the amount of acceleration imparted to that mass, there are obvious gains to be had if we can increase the total mass of air that gets ejected out the back (providing we don’t slow down the speed at which it’s ejected.
An augmentor draws cold air into the exhaust through the gap between the engine tailpipe and the augmentor intake – thus adding mass and increasing thrust. Because the hot exhaust gases heat this cold air, it expands and therefore helps maintain the velocity of the exhaust gases.
2. The Bernoulli effect.
The front of an augmentor tube has a curved lip which serves two purposes. The first is to make it easier for cold air to enter the gap between the tailpipe and the augmentor cone, the second is to produce “lift” thanks to the Bernoulli effect. (see the chapter later in this book for an explanation of what the Bernoulli effect is).
As the air passes around this curved lip, an area of low-pressure is formed and which means that atmospheric pressure on the back of that lip effectively applies a forward force on the augmentor – adding to the total thrust produced.
When I added an augmentor to the intake tube of my standard Lockwood engine, the total thrust produced jumped from 57lbs up to around 80lbs – a very worthwhile improvement indeed.