# Efficiency of a jet

1. Oct 2, 2014

### djsourabh

If we make an engine/rotating machine with the use of jets placed around the circumference of a wheel, then how to find its efficiency? This could be something like an Aeolipile (Refer Wikipedia for the same) but without steam.Instead we can use petrol for combustion.Would that make a good engine?

2. Oct 2, 2014

### SteamKing

Staff Emeritus
How would your modern aeolipile improve on a conventional gas turbine, for instance? It seems like the G.T. would be a simpler machine to construct, and it could run at higher speed than the aeolipile.

As always, thermal efficiency = work output / heat input

3. Oct 8, 2014

### stinsonbr

Without any of the math, my guess is it would be very inefficient. Jets consume a lot of fuel. While their thrust can be impressive, when you scale them down to the size of a loaf of bread they become rather ineffective at producing thrust compared to a propeller.

Energy loss might also be an issue. Using the thrust from jets to spin a wheel in order to turn something might mean your output is significantly lower than your input, since the little jets are gas guzzlers.

As an example, I ran a small jet engine in a lab, about half the size of a football (American). We used kerosene as fuel, and it produced about 4 to 5 lbs as max thrust. When we took the same jet and slapped a propeller on the front of it, it produce about 20 lbs. It also went through a gallon of fuel (3.8L) in just over 20 minutes of run time (I'm sure however there are much better designs, by no means was it a state of the art engine).

4. Oct 9, 2014

### djsourabh

Thank you! It's great that you experimented on the idea.So i am getting the feel that Aeolipile is just a toy after all. But still I would like to know the equation of efficiency for the same. I am an electrical Engineer so I am not able to derive the your thermodynamic equations & all.