Monday, March 28, 2011

Jet Engines

Let's start from the very beginning:

There are two types of heat engines  -  External Combustion Engines and Internal Combustion Engines.

An external combustion engine is a steam engine like they have on trains and ships (paddle steamers out on the Mississippi of the Old West; these are cool.).

An internal combustion engine comes in two basic forms:
The Piston Engine - or reciprocating engine
The Jet Engine  -  or Gas Turbine Engine.

The similarities are that they both burn a fuel that transfers the heat energy to air.  The effect on the air is what drives the engine.
This means that, in both cases, AIR is the working fluid.

The differences are that the piston engine has lots of moving parts which means lots of bits to break and lots of friction and lots of power needed to drive each part  -  power that does not get to propel whatever it is that you want to push long.
Jet engines have one moving part (for a 'basic model') and thus it becomes more efficient because less energy is wasted in driving different bits.

The MAIN Difference is that the Piston Engine is an (Modified) 'Otto' Cycle machine and a Gas Turbine is a 'Brayton' Cycle.
What does that mean?
An 'Otto' Cycle is a constant VOLUME engine.
A 'Brayton' Cycle is a constant PRESSURE engine

Now you are going to say that the piston in a piston engine is going up and down so the volume in the cylinder constantly changes.
That's true.
Imagine any very small moment; the piston will appear, in that very small moment, to be in one place.  The volume will appear to be fixed.  The air is getting hooter  -  it is expanding with nowhere to go.  What happens?  The pressure builds up and pushes the piston down to a new "fixed" volume.
So it is the increase in pressure at a fixed volume that drives the piston down  (or across if you are in a Beetle!).

In a jet engine the pressure is held constant (it doesn't work in practice  -  we shall look at that later, 'almost constant' is good).  The volume increases and, in a confined and fixed space, the only way for it to get out of the machine is to increase in velocity.  The increase in velocity on a mass of air means that the momentum is increased.
If you run faster your momentum increase  -  try walking slowly into a wall and then running as fast as you can into that same wall.  See the difference?
Isaac Newton said (Third Law, you know):  To every action there is an equal and opposite reaction.

Increase the momentum of the escaping air and the reaction against the engine increases.

You now have a 'Jet Engine'

No comments:

Post a Comment