How Do Jet Engines Work

Tuesday, April 9, 2013

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How Do Jet Engines Work

How Do Jet Engines Work?

A jet engine works on the principle of Sir Isaac Newtons third law of physics, i.e. for every action there is an equal and opposite re-action. The action of forcing gases out from the rear of the jet engine results in a re-active force in the opposite direction, and is commonly referred to as thrust. This thrust is measured in pounds force (lbf ), kilograms force (kgf ), or Newtons (N). Engines of this type are often referred to as Reaction Engines, a rocket engine being another example. Newtons third law and the action of a jet can be demonstrated in simple terms by inflating a balloon and releasing it, the escaping air propels the balloon in the opposite direction.

Creating thrust takes energy. The energy required is obtained from burning fuels, whether it be in gas or liquid form such as propane, kerosine, diesel or even vegetable oils! This fuel is normally combined with pressurised air to increase the efficiency and power output for a given engine size. This fuel/air mixture is burned in some form of combustion chamber where the resulting hot gases expand creating an increase in pressure inside the combustion chamber. The expanding gases are then used to do useful work. One example of this process is what happens inside the cylinder of a car engine. Air and fuel are drawn into the cylinder by the downward movement of the piston, the piston then moves up and squeezes this mixture which is then ignited. The fuel burns creating a sudden sharp rise in pressure inside the cylinder. This pressure then forces the piston back down producing mechanical work. The piston then moves back up the cylinder to eject the burnt fuel ready for another cycle. This process is commonly referred to as the Suck, Squeeze, Bang, Blow cycle! (SSBB).

Comparison of the Operation of a Typical Jet Engine

and a Four Stroke Internal Combustion Engine

The way a basic Turbojet engine burns its fuel is exactly the same as in a car engine, but instead of burning the fuel in discrete packets, the jet engine continuously sucks, squeezes, bangs and blows all at the same time! Also, instead of using the expanding gases to push on a piston, they are released through the turbine blades which takes some of the energy to drive the compressor, the rest being released to the atmosphere which results in Newtons thrust described above. In a basic turbo jet, the air enters the front intake (suck) and is compressed by the compressor (squeeze), then forced into combustion chambers where fuel is sprayed into them and the mixture is ignited (bang). The gases which form expand rapidly, and are exhausted through the rear of the combustion chambers and out through the nozzle (blow) providing the forward thrust. Just before the gases enter the engine nozzle, they pass through a fan-like set of turbine blades which rotates the engine shaft. This shaft, in turn, rotates the compressor, thereby bringing in a fresh supply of air through the intake. All of these processes are happening at the same time. Engine thrust may be increased by the addition of an afterburner section into which extra fuel is sprayed into the exhausting gases ( which contains surplus hot oxygen ) to give the added thrust.

At this point you may be asking yourself, "what actually makes it work?". When we effectively create a continuous explosion in our combustion chambers, whats to stop that explosion exiting the wrong way out of the compressor as opposed to out of the turbine? What is the physical explanation involved that will drive our engine ( and for that matter ANY jet engine ) the right way? The short answer to this is turbine to compressor Mechanical advantage. For a slightly longer answer, I shall endeavour to explain below what it is and how its used in a jet engine.

Lets start with an experiment. Imagine we have a typical jet engine like the one in the diagram above, that isnt running. We inject a quantity of fuel in to the combustion chamber, ignite it and create a single explosion. If we havent over egged the pudding and the engine is still in one piece, some of the gases from the explosion will have exited out of the compressor intake ( not what we want ), but most of the gases will have exited out of the exhaust. As a result we find that our single explosion has given us a small kick of forward thrust, but additionally and crucially, has given the engines compressor/shaft/turbine assembly a small rotational kick in the direction it would have in normal operation. If our intention was to design and build a one-shot pulse jet then we have succeeded, the compressor/shaft/turbine assemblys rotational kick being a bit redundant from a design point of view and actually detrimental from an efficiency point of view, but comes in handy later on as we shall see! ;o)

The reason the gases exit mostly out of the exhaust which is what we want for forward thrust and also gives us our small rotational kick, is exhaust turbine to intake compressor mechanical advantage. How it works is this: following our explosion, the gases try to go equally in opposite directions through the compressor and turbine wheels, and due to the specific orientation of their blades, also tries to rotate them in opposite directions. If the compressor and turbine wheels were exactly the same size and shape, then we would have the situation where the exhaust gases would exit from both ends equally, generating equal forces in opposite directions resulting in no net thrust. Also, because the rotational forces acting on the compressor and turbine wheels would be equal and opposite, and because they are both connected to the same shaft, the whole compressor/shaft/turbine assembly would remain stationary. But the compressor and turbine wheels are not the same. The turbine blades are generally at a steeper angle than the compressor blades, i.e. their pitch is greater, and the area through which the gases flow through the turbine is generally larger than the compressor. The result of this is that the whole assembly is unbalanced in terms of resistance to the explosion. What this means is that the gases will pass through the turbine more easily giving us our resultant net thrust in one direction, but equally importantly, because of the steeper blade angles of the turbine, the exiting gases give the turbine wheel more torque or turning force in one direction than the compressor wheels turning force in the opposite direction. The net result of these unbalanced torques or turning forces is that the whole compressor/shaft/turbine assembly is given a rotational kick in the direction that favours the turbine. This is the turbine to compressor mechanical advantage mentioned earlier that is employed in jet engines and is key to making them work! ;o)

OK, so we made one explosion, got a short pulse of thrust and spun our compressor/shaft/turbine assembly a bit in the right direction. But hey, why not do this again, immediately following our first explosion with another explosion and then another, etc, in rapid succession, making the engine spin faster and faster? Well, we can do this but we have to wait a bit before we can create another explosion. Our first explosion used up the available oxygen in the combustion chamber and it needs to be refreshed. This is where our now free-wheeling/spinning ( as a result of our mechanical advantage ) compressor comes into play. As it spins, it pulls in fresh air from the outside and eventually replenishes the combustion chamber with a charge of fresh air/oxygen. We can now inject more fuel, create our second explosion and get a second kick of thrust. If we time things right, we can get our second explosion to add to the already spinning compressor/shaft/turbine and make it spin faster than before. We can repeat this process, creating our explosions more frequently as the compressor spins faster and faster, recharging the combustion chamber ever more quickly. Additionally, because of the ever increasing in-rush of air from the compressor, we find there is less and less tendency for our explosions to exit out of the compressor because of the ever increasing pressure barrier coming from that direction. Note also that so far our jet engine is still working discretely, i.e. it is still operating on the SSBB cycle as used in a car engine. Eventually though, there will come a point when our compressor is spinning so fast that it recharges the combustion chamber almost instantaneously, the pressure barrier it creates as a result of the in-rush of air means that our explosions exit fully out through the turbine only, and finally, our explosions are so close together that we have left the discrete SSBB cycle behind and are now experiencing the continuous roar of a typical jet engine! ;o)

Although it is possible in theory to start a jet engine with discrete explosions, it would not be a very practical way to do it but more importantly would more than likely be a very destructive process! Normally the compressor/shaft/turbine is spun up either electrically, or pneumatically to a speed that sees enough in-flow of air from the compressor to make a decent pressure barrier, at which point enough fuel is introduced and burned so that it can take over from the starter motor. This is the point at which the engine can be said to be self-sustaining or idling.

A bit of a long winded explanation but I hope this helps to give a clearer understanding of how things work! ;o) A slightly different and more mathematical approach ( although still employing the mechanical advantage principle ) can be found here courtesy J.S.Denker.

Using a Turbocharger for a Jet Engine

KKK-K26 Turbocharger

A turbocharger is used on internal combustion engines to increase the amount of air and consequently the amount of fuel that can be introduced into the engines cylinders and as a result increases the amount of power that can be produced for a given engine size.

Cross-Section Through a Typical Turbocharger

The turbochargers compressor provides the pressurised air for the engines cylinders. The compressor wheel is driven by a turbine wheel via an interconnecting shaft. The turbine wheel is driven by the exhaust gases produced by the engine. The whole compressor/shaft/turbine rotating assembly is exactly the same setup as in a typical turbojet.

Flow Diagram for a Turbocharger in Normal Use

So, fortunately for us, a turbocharger already has two of the three major the elements that we need to build a turbojet, i.e. a compressor section and turbine section. The only difference between the turbocharger and a real commercial turbojet are the designs of the compressor and turbine wheels. In a commercial turbojet the wheels are designed to work axially which means that the gases flow through the wheels along their axes of rotation.

Commercial Engine with Axial Wheels and Gas Flow

In a turbocharger, the wheels are designed to work radially that is, the gases exit the compressor and enter the turbine in a radial direction, i.e. at right angles to their axis of rotation, which is the reason for the snail shell-like shape to the housings. The reason for this is efficiency, radial compressors and turbines work more efficiently below a certain size, above this size axial compressors and turbines are used, but this is not an issue for us apart from one of design compactness.

Turbocharger with Radial Wheels and Gas Flow

The third element that we need to build our jet engine, requires us to build some form of suitable combustion chamber. A turbocharger, when bolted to an engine is almost behaving like a jet engine already, it provides compressed air to the engines combustion chamber where fuel is burned, the resulting gases then being forced out of the chamber by the piston which spins the turbine wheel and hence driving the compressor. When we introduce a jet engine style combustion chamber we effectively replace the engine and its cylinders for the burning of our fuel, turning the discrete suck, squeeze, bang, blow cycle into a continuous one as in a real turbojet. The combustion chamber will essentially be a large can into which the fuel is sprayed and burned. The air from the turbochargers compressor is fed in, fuel is added, burned and the resulting hot expanding gases exit the combustion chamber through a pipe connected to the inlet of the turbochargers turbine thereby completing the loop. Combustion chambers have been constructed using a variety of basic materials, built up from tubular steel or from modified fire extinguishers using mild or sometimes stainless steel for durability.

Because of the inherent design of the turbocharger ( radial inflow wheels as opposed to the more normal axial flow wheels ) and the fact that on most DIY jet engines we are using it as is, the combustion chamber needs to be constructed outside of the turbocharger as a separate unit. This leads to the construction of a jet engine that is bulkier, heavier and far less efficient, thrust for thrust, than their more streamlined commercial brethren ( both full-size and model jets ) but is the price we have to pay in order to reduce complexity and cost to achieve a real working jet.

Getting Started

Ok, so we know that we can use a turbocharger to build a jet engine, but how do we go about choosing the right turbo? What sort of thrust levels can we expect? How do I go about designing my combustion chamber? What other bits and pieces do we need to make it work such oil and fuel systems? Because of the numerous types of turbocharger out there and the varying levels of access to parts and materials that builders encounter, but moreover the fact that There Is More Than One Way To Do It.., there is no one definitive set of plans to go by. Instead, what is presented below are links to a set of guidelines ( Rules of Thumb ) in Adobe Acrobat Reader format (.pdf) that have been an invaluable aid to myself and others to help get to grips with these problems and come up with working solutions. They were originally drawn up by Australian John Wallis a veteran DIY gas turbine builder and long time member of the DIYGasturbines Yahoo group. The Rules of Thumb are reproduced here with his permission. You can see examples of Johns (Racketmotorman) projects on the DIYGasturbines group and on Nick Haddocks website ( see Links below ).

The diagram below shows a typical layout of a DIY gas turbine and gives the names of the various parts ( click on diagram for a more detailed version ).

Naming Conventions

Abbreviations - Refer to these if you are unsure of certain terms

Rule of Thumb N.o. 1 - Choosing a Turbo

Rule of Thumb N.o. 2 - Oil Requirements

Rule of Thumb N.o. 3 - Combustion Chambers

Rule of Thumb N.o. 4 - Fuel Requirements

Rule of Thumb N.o. 5 - Ignition

Rule of Thumb N.o. 6 - Starters

Rule of Thumb N.o. 7 - Jet Pipes and Nozzles

Rule of Thumb N.o. 8 - Compressor Flows

Rule of Thumb N.o. 9 - Thrust

Rule of Thumb N.o. 10 - Fuel Consumption

Rule of Thumb N.o. 11 - Freepower Turbines

Rule of Thumb N.o. 12 - Afterburners

Rule of Thumb N.o. 13 - Evaporators

Adobe Acrobat Reader for your particular operating system can be downloaded here.

Engine Designs

The two main design decisions that need to be made are what shape the engine is going to take and the type of fuel that is to be used. The shape of the engine is mainly determined by how the combustion chamber is attached to the turbo. The most efficient ( and common ) way is to have the combustion chamber attached radially to the turbochargers axis, i.e. the output of the combustion chamber is attached directly to the input of the turbine scroll so that it feeds the turbine without any intervening pipe work. This results in the engine taking on an L form ( without the jetpipe ) which is the most efficient, but not necessarily the most compact. Other arrangements that people have used are axially pointing forwards ( as in the diagram above ) or axially pointing backwards, transversely in an X shape, as well as others. These latter forms need extra pipe work giving slightly reduced efficiency. Whichever shape you choose depends on what best suits the mounting of the engine and of course aesthetics! ;o) My original engine design ( see Jet Single below ) has the combustion chamber arranged axially with a 90 degree transfer pipe from the combustor to the turbine inlet. The reason I chose this form is because of an idea I had when I first started out on this project.

Apart from the designs relative compactness, I wondered about the possibility of adding a second turbocharger at a later stage in the manner show in the pictures below ( Twin Jet ). As time went on and I came to understand a bit better about how a jet likes this works I could think of less reasons as to why it wouldnt work in theory. I imagined this sort of design a bit like the inverse of Sir Frank Whittles first engine ( see images below ), where instead there were multiple combustion chambers centred around a single compressor/turbine. The layout lends itself well to the addition of a second turbocharger with minimal extra work with a view to adding more thrust with a small increase in weight. I could simply replace the original small turbo with a larger unit, but larger turbos are harder to come by, are more expensive and would have different operating characteristics which may require a combustion chamber redesign. Using one type of turbo means you can get to know its operating characteristics and build on that knowledge. The idea is that the second turbocharger will share the combustion chamber with the first turbo.

My reasoning behind the twin ( or multiple ) turbo jet is that from the combustion chambers point of view it doesnt care where its air comes from or where its combustion products go to, its job is to burn fuel as efficiently as possible and heat excess air. It is the turbos job to deliver the required air, making use of the exhaust gases as efficiently as possible such that it can sustain its function in providing compressed air, as well as leaving enough energy in the exhaust gases to do useful work. Having said that, there are obvious technical difficulties that will need to be addressed with a twin or multiple turbo setup such as starting, balancing gas flows, etc...

For fuel I decided to go liquid and use Diesel. Propane gas is the first choice for many because it is clean burning, easy to ignite and doesnt require any auxiliary pump to deliver the fuel. The downside is that it is stored in heavy containers that have to be carted around which is more problematic if the engine is to be used in a vehicle, the propane is quickly used up resulting in short run times and is relatively expensive and more trouble to refill ( at least where I live! ). There is also the personal aspect in that, for me, working with gas I feel is inherently more risky and requires greater care. Using a liquid fuel requires the use of a pump and driving motor so is more complicated to set up initially, but the great advantage is that you can use almost any liquid fuel ( NOT petrol, very volatile! ). I decided to use Diesel because it is relatively safe to use ( you can put a blowtorch to a pool of diesel and it wont ignite, you have to atomise and/or heat it before it will light off ) and is readily available at the local fuel station ( half the cost if Red Diesel can be obtained that is used for off road vehicles and generators ).

schematic knowledge

Tuesday, April 9, 2013

How Do Jet Engines Work

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