Bunker Mulligan "Let us endeavor so to live that when we come to die even the undertaker will be sorry." ~Mark Twain

December 22, 2004

Boring Engineering Stuff

Filed under: Engineering — Bunker @ 10:16 am

I have had a pretty diverse career as a mechanical engineer, and mechanical engineering is as diverse a field as there is. I’ve had to use all the tools I learned (and had to relearn) in school. Calculus, linear algebra, differential equations, thermodynamics, gas dynamics, heat transfer, materials mechanics, power transfer, structures. I am amazed at how all the things Sir Isaac Newton deduced four hundred years ago are still valid, whether it be physics or mathematics. Newtonian Physics are still quite valid and useable here on earth, and most other places and situations in the universe.

At 26 he became a lecturer at Cambridge. Twenty-six. And his brilliance hadn’t really even been exposed at that time. I’ve known some very smart, even brilliant, people. Did any of them even come close to the mind of Newton? I doubt it. He couldn’t explain his thoughts on gravity with conventional mathematics techniques, so he invented Calculus–or discovered it, your choice. Damn.

I’ve been thinking about old Issac the last two weeks as I worked on some spreadsheets to simplify the thermodynamic calculations on turboshaft engines. Every jet engine operates on the premise of converting chemical energy into thermodynamic energy in the form of heat, pressure, and velocity. The basics are the same whether the engine is a turboshaft, turboprop, turbojet, turbofan (turbo as a function of a turbine within the engine), ramjet, scramjet, or pulse jet. There is a stoichiometry common for all with a single fuel, so the air/fuel mixture changes based on the heating value of that fuel. Beyond that, the relationships between the mixture and associated heat, pressure, and velocity are all constant. Newton’s Laws.

The internal geometry of an engine is designed to get the most efficient use of those relationships. That energy is converted to rotational power through a turbine for driving the compressor in all the turbine engines. Turboshaft and turboprop engines also have another turbine which extracts energy to drive a propeller or rotor system. In other types, the energy is converted to high momentum mass flow and nozzle expansion for thrust.

Okay, you’re now bored to tears.

The last two weeks I’ve been involved in correlating two engine test cells. A correlation is more or less a dynamic calibration of the data acquisition system. We take an engine which has been run in a manufacturer’s test cell, and compare specific parameters to those obtained in the candidate cell. We can then certify that the cell is providing accurate and repeatable data. That way we know that every engine we test is performing as indicated. The following is a single sheet of the correlation data comparison:


The lines are the limits we are allowed, and the data points must fall between them (as they do here).

The task can be quite frustrating. The equations, which derive from all that work Issac did those 400 years ago, get complex and involve conversions of data from things like inches of mercury to pounds per square inch gage to inches of water to pounds per square inch absolute. That’s just pressure. Fuel flow must be converted from Hertz (in a flowmeter) to a velocity, modified for specific gravity to calculate mass flow. All the test parameters are similarly confusing when looked at in the form of a spreadsheet equation. You know, twenty sets of parentheses and every math function available. It isn’t something most people want to be responsible for. Thus, this modification of a Dilbert cartoon from one of my compatriots:


Actually, I enjoy the challenge.


  1. Wow, Bunker! That’s fascinating!

    Really, it is.

    Of course, you know what a geek I am. But still, fascinating. The tightness of that correlation is pretty impressive. I’m used to problems with correlation coefficients that are pretty low.

    Now I’m going to ruin your day by making you obsess over whether these data are done (just because I’m feeling evil today). To the eyeball, it looks like there’s a pretty clear upswing to the slope above 10 lb/sec. Are you sure there’s not something going wrong on the high end of the data?


    Comment by UML Guy — December 22, 2004 @ 11:38 am

  2. *BANG*

    That sound you just heard, Bunker, was my brain exploding. Don’t DO that to us, dammit!

    Cripes, trying to make us think…

    Comment by david — December 22, 2004 @ 11:48 am

  3. Yes, there is an upswing–only a geek would notice! I’m trying to get another run in today before the weather runs us out to validate what I think is happening.

    David, think how I feel trying to understand Issac’s immense understanding of the universe!

    Comment by Bunker — December 22, 2004 @ 11:52 am

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