How Do Jet Engine's Work

How Do Jet Engine's Work?

A jet engine works on the principle of Sir Isaac Newton's 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. Newton's 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 it's 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, what's 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 it's 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 isn't running. We inject a quantity of fuel in to the combustion chamber, ignite it and create a single explosion. If we haven't 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 engine's 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 assembly's 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 wheel's turning force in the opposite direction. The net result of these unbalanced torque's 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