Engineering Paper Review

Note : These are reviews of papers already in the public domain. 

Centrifugal Compressor Evolution

Below is an overview paper of the technologies that were behind high efficiency centrifugal compressors of today. From a small stint at Dresser-Rand, I had come to interact with both the authors - Mark Kuzdzal and Jim Sorokes and was very inspired by the Rotordynamics department, particularly the mountains of papers and books that were stored in an adjacent library on the all topics compression related. Whenever I would get a chance, I would attend the local ASME Chapter meetings on weekday evenings and soak in as much information as I could get.

The paper introduces readers to an asymptotic compressor polytropic efficiency limit of 90-92% , a trend line that is seen looking at the past 60 years in efficiency advancement. This suggests that the next phases of compressor development will increasingly involve minute, fraction of percentage point advances towards this limit. It might also make the case that engineers getting a deeper knowledge of fluid dynamics and good use of CFD will be extremely important requirements in design skills.

The rest of the paper is an interesting history of the advancements made in aerodynamic components, seal systems and analytic methods. Overall, I liked the paper's conclusions. As the authors so rightly claim, the evolution in manufacturing is what complemented superior analytical techniques towards making complex gas path geometries and high efficiency compressors possible.


Turbocompressor Drive Couplings

Below is a historical ASME design report (circa 1962) which entails J. H Anderson's experience with designing flexible disc couplings for high speed turbocompressors. One might think Anderson was spurred into the act of researching reliable compressor couplings after hearing Prof. Howard Wilson Emmons' (Harvard fame) views and opinions on the matter of compressor coupling reliability.

Nevertheless, he goes on to describe what he thought were the existing problems of geared compressor couplings. Firstly, heavy couplings sitting at the external suction end of a compressor called for large diameter shafts which increased mechanical seal size and wear related leakage. This arrangement meant large overhangs between bearings and the coupling when seen in elevation view. From a vibration standpoint, the setup lowered the critical frequency, but also brought the second and third critical frequencies close to the first, and perhaps dangerously close to run speed. The author cites these inflexible gear couplings having failed in torsional sensitive applications, such as synchronous motor driven applications.

To add to the woes, sleeve gears needed lubrication with some form of heavy oil to prevent gear flank wear as there would be no question that parts wouldn't rub. More importantly, Anderson highlighted how gear couplings had the potential to transmit thrust in some operating cases to the driven end, something you'd much rather not see on compressor drives.

With this background, Anderson describes his experience designing two improved coupling designs to solve the sealing and alignment issues. Notable is the introduction of a flexible, small diameter, quill shaft to reduce seal diameter at the coupling end and a system of thin flexible discs, threaded onto the shaft and with a spacer in between, to take up the movement of the flexible shaft.

While practical experience showed some problems with manufacturing and alignment, Anderson goes on to describe it's evolution to an improved design where the spacer would be eliminated by making the coupling hub itself a spacer, this way reducing the double hinge effect. The paper ends with a critique of the paper from another reader who's unabashedly all for geared couplings, followed by a rebuttal of those points by Anderson in order to prove the flexible disc coupling's superiority for turbocompressors. Very interesting!


Rotordynamics of Pumps Without Fluid Forces

Rotordynamics is hardly a subject that can be elucidated with ease. And few people even attempt to do that because of the myriad factors that go into evaluating a machine. One of those who did try though, was Mr. Malcolm Leader, way back in 1984, in a paper written for the 1st Turbolab Pump Symposium, titled Rotordynamics of Pumps Without Fluid Forces

Mr. Leader was a senior engineer with Monsanto Fibers and Intermediates Company and was someone responsible for auditing machinery and overseeing installations. In the paper, he lays out some conservative concepts and "ground rules" that he claims will never put anyone in trouble, since the concepts have been proven through the experience of many people over the years. 

The concept starts with the idea of bearing and shaft stiffnesses, and how the ratio of those two is a critical factor in determining the modal mass ratio. The modal mass ratio can improve erroneously assumed first critical speeds by correcting the contribution of mass M that goes in the equation wn = root of K / M. Mr. Leader proves that this method of critical speed estimation is very close to actual test results for between bearing systems and equally distributed masses. He also writes that the value of stiffness ratio can help differentiate a "good" from a "noodly" rotordynamic system.

In the next sections, he goes on to diagrammatically show how bearing stiffness, bearing span, rotor diameter and distribution of mounted masses affect the first 3 critical speeds in a rotordynamic system.  Mr. Leader very clearly explains what happens when each of these elements are varied, each time considering the effect of a soft flexible support and a stiff support. 

Although the paper involves no numbers, the qualitative understanding of the concepts in this paper can go a long way in getting a "feel" for what rotor-dynamic analysis involves. For this reason, I've chosen this 1984 paper as an enjoyable and indeed, a very insightful piece of technical literature. 


Progress in Gasoline Direct Injection

I have to make an honorable mention of the following paper by Zhao et al. which beautifully traced the development of the GDI engine back in 1999. 

The paper is remarkable. All known R&D based prototype engines and in-production GDI engines were reviewed through engine schematics, control diagrams and specifications. It reviewed the state of knowledge from more than 370 key publications, many of which as the authors claim, were untranslated into English. 

It gives you a true sense of the amount of work that went into the injection systems in these cars to meet BSFC and emissions targets.   It gives you pointers as to where the GDI engine must focus on additional work and research. I particularly liked the discussion on the operating modes within the engine map and what injection timing does to emissions, and in relevance to PFI and diesel engines. 

After reviewing so many publications on GDI engines, the authors noted an important issue : In many papers, the GDI performance was compared to PFI baselines that are not well defined, thus making it very difficult for the reader to make a direct engineering comparison between GDI and PFI performance. 

The paper wrote : "One extreme example is the comparison of GDI and PFI fuel economy data that was obtained using two different vehicles with two different inertial weights. An example of a more subtle difference is the evaluation of the BSFC reduction resulting from the complete elimination of throttling in a GDI engine, but not noting or subtracting the parasitic loss of a vacuum pump that would have to be added for braking and other functions. A number of published comparisons lie between these two extremes. The readers are cautioned to review all claims of comparative GDI/PFI
data carefully as to the precise test conditions for each, and the degree to which the systems were tested under different conditions or constraints."

Certainly what is noted above is a relevant point for any researcher wanting to get an idea of quick comparisons between any old and new evolving technology. Always ask questions, and take things with a grain of salt.


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