Jet Engines

In general, as wiki says, a jet engine "discharges a fast moving jet of fluid to generate thrust in accordance with Newton's laws of motion" (the 3rd law is for every action there is an equal and opposite reaction). Before we can get into the different types, I figure we should look at how exactly a jet engine works. A normal jet engine takes in a small quantity of fast moving air, compresses it, adds fuel, and ignites the fuel/air mixture, thus expelling the expanding gases out of the back of the turbine. Here's a simple diagram:


Another key aspect of the engine shown in the diagram is the rear turbine. A large portion of the efficiency of the engine is due to the expanding gases rotating the rear turbines as they exit the engine. The turbine in turn drives the compressors and fans (if the engine is a turbofan). This means that after the initial starting of the engine, nothing but the ignition of the fuel/air mixture and it passing out the back is causing the compressors and turbine to spin. In this diagram, you can more clearly see the various stages of the jet engine and how they are mechanically linked. The fan in front of the low-pressure compressor is not a feature on the basic turbojet which we are looking at. We will look at turbofasn when I go over the different types of jet engines.



One of the greatest advantages of jet engines is their simplicity and thus they are designed to be as simple as possible (or at least they were). This can be clearly seen in how the low-pressure and high-pressure compressors are linked to their respective turbines. The low-pressure compressor rotates at a slower speed, so it is driven by turbine that is rotating more slowly due to its larger diameter, thus decreasing the pressure of the air that passes through it. Then the high-pressure compressor is driven faster by a smaller diameter turbine. The differences in speed between the two compressors could be created by gearing, such as with the gears on a bicycle, but this would invite added opportunity for failures since in general, the more moving parts, the greater the probably something will fail. This fundamental principle is the reason why jet engines are so reliable when compared to reciprocating engines (such as in prop planes or cars). While jet engines operate at very high temperatures and RPMs, the stresses are not so great as with rocket engines, where the volatility of the fuels (such as liquid oxygen and nitrogen)and the extreme temperatures are so great as to invite failure despite their simplicity.

As a quick side note, the newest developments in jet engines are actually increasing their complexity in an effort to increase fuel efficiency, such as the use of gearing that I previously stated was generally avoided in the past. But with today's advanced understanding of thermodynamics and material sciences, engineers can add more complexity without increasing the probability of an engine failure too much. I might expand on this some time later.

I figure I should stop here. Certainly everything I've mentioned can be greatly elaborated on. Jet engines are very simple machines and usually I feel like people overthink how they work. That said, while the principles are simple, we can spend endless posts looking at the details about how they are put together, the different types, etc. Probably we'll just look at the types and finish there. That is unless you have other questions. I suppose we'll see.