For a while now I’ve been interested in using turbochargers as stand-alone engines. While there are many sites on the internet documenting how to turn a turbocharger into a jet engine, I have yet to see one that did anything really useful- at least in my opinion. Nearly all of them are only designed to make heat, noise and quickly burn through fuel. Sometimes people try to get some thrust out of them… but rarely does one ever turn into a viable source of power or propulsion.
This is really a shame considering that turbocharger research has been going on for over a century. The technology in modern turbochargers must rival that found in the jet propulsion industry… right? So what if we consider that turbochargers can be hand selected for the purpose of creating an engine. Instead of just getting a turbocharger and throwing some sort of combustor on it, lets think about what we want it to do, and build something useful.
So what can a turbocharger based engine do? We already know they can create heat, but unless that heat is put to use, that isn’t a reason to make a turbo-jet. So not heat. Well, if it isn’t already obvious, thrust would be the next easiest thing to create. Finally, the most useful output would be shaft power. Since producing shaft power is really using thrust to move an impeller (fan), lets look at thrust first.
So far, every turbocharger jet that I have seen is set up the same way: compressor -> combustor -> turbine -> nozzle. Sometimes there’s an afterburner between the turbine and the nozzle. While this works, it is really an inefficient setup. The problem is that people who build these don’t know what all these things do and how they work at the root, and therefore don’t use them to their advantage.
Lets break it down math/engineering-wise: Thrust is created as a reaction force. Specifically, thrust force is equal to the mass-flow of exhaust gases multiplied by the velocity at which they exit the engine. This is apparent if you have ever used a spray nozzle on the end of a garden hose; when there is no nozzle, the water comes out at a low velocity, and the hose sits there– put the nozzle on and you might get the wild comical snake effect as the hose thrashes around. The same amount of water (or possibly even less) comes out with the nozzle on, yet there is obviously more thrust throwing the hose around.
So now that we know that we want lots of mass-flow and the highest velocity we can achieve- how do we get that? Well, people always stick a nozzle on the end of a turbo jet- why do they do that? Well a nozzle trades pressure in exchange for velocity. At the entrance to the nozzle the pressure is high, and at the exit, it is usually very close to atmospheric. I am not an expert in this, but I just found a cool link to a NASA explanation. Careful with that link though, if you aren’t a math person the equations will make blood shoot out your eyes. Using those equations, it is actually possible to get a turbocharger based jet to provide supersonic thrust. YES SUPERSONIC THRUST. That does not mean that your go-cart will break the sound barrier, but it sure will make more noise than you could ever imagine! Of course, the average Joe doesn’t have the technology to make one work, so I doubt we’ll ever see this. Since it is easier to calculate subsonic thrust, I’m just going to use a standard converging nozzle in my calculations.
If you read yet another NASA web page on nozzles, you will discover that the nozzle design is the most important feature of a turbocharger based jet, yet it is usually the part that receives little or no thought at all compared to the remainder of the engine. In case you haven’t figured out from the NASA equations, the nozzle sets the remaining variables for the whole engine. I am going to go ahead and post this article, but in my next article, I am going to crunch some numbers for the nozzle design required for a turbo charger engine to produce 100 lbs of thrust.