Lecture P7: Ideal Cycle Analysis

I covered Chapter VII of the notes. The important part of this material is understanding the behavior of simple turbojet engine cycles. A necessary complication along the way is expressing the fairly straightforward expressions for thrust and efficiency in terms of principal design parameters, design contraints, and flight conditions--this makes them appear more complicated. We did one PRS question (PRS#1). This question is about at the level that I would like you to understand the behavior of turbojets. You should also know what the principal design parameters and constraints are for these engines (like compressure pressure ratio, and turbine inlet temperature).

Responses to 'Muddiest Part of the Lecture Cards'

(6 respondents)

1) How does a high by-pass ratio make an engine more efficient? (1 student) By providing for a large amount or mass to be given a small increase in velocity (through the fan). This leads to a higher propulsive efficiency.

2) What is the mechanism for an engine stalling-out? (1 student) There is a nice discussion of this in Chapter 5.7 of Prof. Kerrebrock's book Aircraft Engines and Gas Turbines which is available in the library. Once we have had our final lecture (P9) you should have most of the tools necessary to understand the discussion he presents.

3) For all the new variables, can you put up a list of them and what they mean in English and in terms f what we have been using? (1 student) I believe that all of this information is available in the notes, but if you have difficulty extracting it, let me know and I will put together something.

4) Why would you have a leaky compressor? What impact would that have on engine performance? (1 student). A leaky compressor is just an expression for a very low bypass ratio engine. For some military applications the best deisgn solution is an engine with a very low bypass (versus a pure turbojet with no bypass or a turboofan with high bypass).

5) Can you give us a description of the differences between a turbojet, turbofan, SCRAMjet, RAMjet, etc.? I am a little confused. (1 student) A turbojet is an engine where all of the flow into the inlet of the engine passes through all of the compressor stages and through the combustor. In a turbofan engine part of the flow bypasses some of the compressor stages and the combustor (instead, this bypass flow typically goes through a single compression stage -- a fan). A RAMJET has no compressor or turbine, but using ram pressurization from flying fast, to increase the pressure prior to the combustor -- then the flow is expanded in a nozzle (no turbine). A SCRAMJET is a supersonic combustion ramjet. It works the same way a ramjet does, but the flow is not decelerated to subsonic conditions through the inlet and combustor. Rather, it remains supersonic.

6) No mud (1 student).