Pratt & Whitney R-4360 Valve Train Development
by Kimble D. McCutcheon
Published 30 Jan 2018; Revised 1 Feb 2018
 Pratt & Whitney R-4360 Cylinder Layout  R-4360 Cooling Airflow |
During the early planning stage, R-4360 designers chose a cylinder layout that was a major departure from prior art. Instead of staggering the cylinder rows or placing them inline, as had been done in previous engines, the designers chose to spiral them around the crankcase and implement ducted side-to-side cooling airflow instead of the fore-to-aft airflow that had been used in most prior air-cooled radials. Another design departure was that the cylinder intake ports were located on the cylinder top, and the exhaust port on the side facing diagonally aft. These innovations were intended to achieve better cylinder cooling by increasing cooling fin area and getting the intake and exhaust plumbing out of the cooling airflow path to the extent possible. Airflow and cooling tests confirmed this to be a viable design. One final departure was the rocker box orientation. Until the R-4360 most engine cylinders had been built with both intake and exhaust push rods either in front or behind the cylinder; the R-4360 cylinder had its intake rocker box and pushrod on the cylinder front and its exhaust rocker box and pushrod on the cylinder rear. This arrangement required a fifth cam ring along with its attendant drive and lubrication complication, so designers must have decided its advantages outweighed its disadvantages. But there was an unintended consequence – the long rocker box "ears" were more flexible than with previous engines, and this caused heretofore unseen cylinder failure modes. The following is a two-part story of the ideas, techniques, tests and people that solved this problem. |
Introduction – The Test Engineers
Most of what we know about R-4360 valve gear development comes from a handful of reports authored by test engineers C. Gordon Beckwith, W.D. Carlson, and Robert E. "Bob" Gorton.
Beckwith came to Pratt & Whitney during the summer of 1939 before his senior year in college and worked in the stock room delivering parts to the engine assembly department. He returned the next summer as a degreed engineer and took a position as a test engineer. He would later lead the very successful JT8D development program.
Gorton got in on the ground floor of the Pratt & Whitney R-2800 vibration problems. While in college, he had a summer job at Pratt & Whitney, but needed support to complete his Master's degree. Gorton did something that was unprecedented for the time – he convinced Pratt & Whitney to finance his Master's study in vibration, and in return, agreed to a work-study program. Pratt & Whitney got its first engineer with actual college training in vibration issues. The relationship was destined to be long and fruitful. Gorton’s diligent testing and instrumentation contributed greatly to getting all of Pratt & Whitney’s reciprocating engines developed. He and his vibration group invented new types of instrumentation to meet the challenge of each new problem. When jet engines arrived, Gorton continued to develop innovative approaches to instrumentation of turbine wheels and other gas turbine components.
Gorton's and Beckwith’s stories are typical of the many bright and innovative test engineers with abiding interests in aircraft propulsion who brought engines like the R-4360 to life.
Background
In September 2010 Bob Wahlgren, an R-4360 enthusiast and AEHS Member, sent specifications for the R-4360-59B, which was used in the Boeing C-97. We were very pleased to receive these because they included details we had not seen before, including valve clearance and timing. I remember thinking at the time that the exhaust valve timing looked strange, but proceeded to add the information to the AEHS website R-4360 page. Please see
http://www.enginehistory.org/Piston/P&W/R-4360/r-4360.shtml
In the years that followed, several website readers again sporadically questioned the exhaust valve timing, but after conferring with Bob and receiving assurances they had come from maintenance manuals of sound provenance, I assured the readers that, as far as we could tell, the values were correct. The issue again surfaced in July 2017 when William Hoddinott, another AEHS Member, sent an email questioning the specifications, leading me to again revisit the issue. According to the data, the exhaust valve opened at 135° before bottom center (BBC), or only 45° after top center (ATC) and closed at 79° ATC, for a duration of 394°!. This made no sense. When the valve presumably opened, the combusting charge was only about 30° past its pressure peak, and not only was the charge still burning, but it had hardly expanded at all and would impose enormous heat and pressure on the exhaust valve, making it very hard to cool and difficult to open. The R-4360 development engineers were probably interested in having the exhaust valve open at about 40° BBC (early) and close at about 40° ATC (late), for a duration of about 280°. I asked myself, "How can the specs be correct and the engine still perform properly?"
I exchanged several emails with Bob and the two of us came up with the idea that published specifications reflected cam timing, not valve timing. I recalled that R-4360 valve gear development had been especially troublesome because of flexibility in the engine components associated with valve motion. Could these early exhaust valve opening specifications be indicative of a scheme using long opening and closing cam ramps to address this flexibility? I also recalled that I had several Pratt & Whitney R-4360 (also called the X-Wasp) development reports on this subject, summaries of which follow.
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