Allen Hazen wrote: ↑Wed Jun 17, 2020 2:40 am
-- I have the impressions that cast frames are a bit more robust and preferred for very high output engines, and that cast frames tend to be a bit heavier than fabricated. Are either of these accurate?
Stronger and heavier for the cast case I think aligns with the conventional wisdom. Nonetheless, those in favour of the cast crankcase would argue that accurate casting allows the metal to be placed exactly where it is needed, and not where it is of little benefit, something that is more difficult to do with the fabricated approach.
Be that as it may, it would appear that satisfactory results and broadly similar weight-to-power ratios are achievable either way, given good design execution and subsequent development.
Whether the relatively small weight differences between the Alco 251 and GE FDL engines stem from the respective choices of crankcase construction, or from the other significant design differences is hard to say.
Although cylinder size is a good starting point for comparison purposes, sometimes the same or cylinder sizes are used for engines whose intended power outputs are quite different. An example that comes to mind is that of the Paxman Ventura and Valenta high-speed engines. Both shared a cylinder size of 7¾ x 8½ inches. The Ventura was designed to match the German high-speed engines of the era, such as the Maybach MD655. The Valenta was of much higher power output, so was much more robust, and as far as I know had a larger bore centres dimension. Basis the Jane’s 1973-74 numbers, the highest output 12-cylinder Ventura engine produced 1500 hp for a dry weight of 11 190 lb and a length of 81 inches. The corresponding Valenta produced 2250 hp for a dry weight of 14 930 lb and a length of 96.75 inches.
In the ocean-going marine heavy medium-speed engine field, it has been customary since the 1990s, and perhaps before then, to categorize engines on bore size alone. The rationale was that the latest engines at least operated with mean effective pressures (mep) at the practicable upper limit, then around 25 bar but higher now, I think, and also at the upper practicable mean piston speed limit, around 10 m/s for heavy fuel and 11 m/s for distillate fuel (gasoil). Thus stroke length and rotational speed were interrelated, and an increase in the former meant a decrease in the latter, more-or-less self-compensating in power output terms. Stroke-to-bore ratios were around 1.2:1, with some variation, with stroke length chosen to achieve maximum piston speed at close to a 50 or 60 Hz alternator synchronous speed, given that the same engines were also used in stationary powerplants. Anyway, the net effect that bore size (along with cylinder count) was a good predictor of the power output range of a given engine type.
Returning to the Alco and GE case, these engines appear to have been aimed at essentially the same target. The GE FDL might have somewhat more development potential than the Alco, but whether that could be attributed to its cast crankcase is I think uncertain. Its articulated connecting rod arrangement may have been another contributing factor.
Applicable I think is the commentary provided in a Diesel Railway Traction 1960 November article which was essentially a précis of a paper by J.C. Rhoads of GE, presented at a Pan-American Railroad Congress in Brasil.
“Some of the more important criteria applying to a locomotive diesel engine were considered by Mr. Rhoads to be: (1) It should be of the V configuration for best locomotive space utilization and maximum maintenance accessibility; (2) It should have maximum h.p. per cylinder to provide the minimum number of cylinders and other engine parts for a given power output; (3) it should have as high a crankshaft rotational speed as possible to minimise engine and transmission weight; (4) Ratings should be based on long-term field experience for optimum life; (5) It should have utmost reliability; (6) It should have minimum fuel and lubricating oil consumption rates; (7) It should be able to idle for long periods without undue distress; and (8) The basic cylinder design should be offered in a multiplicity of cylinder arrangements for maximum h.p. range and maximum interchangeability between engine sizes.
“There is a high percentage of V-engine applications for diesel locomotives in the world. Top cylinder accessibility is excellent and space utilization good. Generally, 8.5 in. or 9-in. bore can be accommodated in a 6 ft. overall width with a 45 deg. V angle. Larger bores can be accommodated only with difficulty; smaller bores do not fully utilize the space. In this connection it is of interest to note that all three of the U.S.A. engines used for locomotives have this 45 deg. V angle and bores in the range just stated.
“Maximum output per cylinder comes from the maximum bore that can be accommodated in a given locomotive cab width in the V configuration. Based on U.S. practice a 9-in. bore appears to be about the maximum obtainable in four-stroke engines, and an 8.5-in bore in two-stroke engines. Output per cylinder ranges from about 140 b.h.p for two-stroke engines to 165 b.h.p. for four-stroke engines.
“Transmissions can be made smaller as engine speed is increased. Some engine design practice outside the U.S. includes speeds up to 1.500 or 1,600 r.p.m., but even at this higher r.p.m. the h.p. per cylinder may not be greater than American practice, since the bore and stroke are less. Consequently, more cylinders may on occasion be required to produce maximum h.p. per locomotive. American practice is to run four-stroke engines in the neighbourhood of 1,000 r.p.m., and two-stroke cycle engines at 800 to 835 r.p.m. Piston speeds are not currently so high as those of modern quick-running engines.”
I think that there is an element of circular argument in the foregoing, but even so the basic premise is valid, and supported by other data, particularly if the upper bore size is moved slightly, say to 10 inches. For example, Lima chose a 9.5 inch bore vee engine, which became the Cockerill CO240. By 1969, the 16-cylinder version provided 4000 cv (3944 hp) at 1050 rev/min for a dry weight of 18 000 kg (39 690 lb). English Electric stayed with its 1930s-origin 10-inch bore for its post-WWII vee engines, widely used in CM-gauge applications. EMD went to a 9 3/16 inch bore with its 645 engine. Sulzer chose a 240 mm (9.45 inch) bore for its late 1950s vee engine.
Thus it could be said that of the options reasonably available to Alco, it made the right choice in doggedly (and perhaps dogleggedly
) pursuing the development of the high-specific output, 9 x 10 ½ inch vee engine.