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Peter, I assume in the Perry-class frigates the Detroit Diesels are for electrical power? While the LM2500 gas turbines are purely for propulsion? Do I understand you correctly that these have 149s have both Roots-type blowers AND turbochargers? Or either but not both? I appreciate you pointing out the "step" in the fuel economy at the point where the turbocharger disengages. That's only obvious when you stop to think about it! It's of interest that EMD has been arranging engines the same way for over 60 years now: blowers or turbocharger on the rear end (turbocharger end), along with the starter motor(s) geared to the flywheel and the auxiliary generator, and water pumps, lube oil scavenging pump, lube oil pressure pump, and governor on the front end (accessory end) -- which is in the rear of the locomotive. So the engine is "backward." But the air compressor can be on either end -- today, as far as I know, it's always on the front end, that is, in the back of the unit, but on F units it was on the rear end of the engine, driven off a flex coupling from the front of the generator. This is a very ugly connection. I foolishly thought it could be unbolted to replace it, but in truth you sort of rip the thing apart piecemeal with the liberal use of cheater bars and oxacytylene torch. To quote from the EMD 645E3 Engine Maintenance Manual: "The turbocharger assembly is primarily used to increase engine horsepower and provide better fuel economy through the utilization of exhaust gases. It has a single stage turbine with a connecting gear train. The connecting gear train is necessary for engine starting, light load operation, and rapid acceleration. Under these conditions there is insufficient exhaust heat energy to drive the turbine fast enough to supply the necessary air for combustion, and the engine is actually driving the turbocharger through the gear train assisted by exhaust gas energy. When the engine approaches full load, the heat energy in the exhaust, which reaches temperatures approaching 1000 degrees F., is sufficient to drive the turbocharger without any help from the engine. At this point, an overrunning clutch in the drive train disengages and the turbocharger drive is mechanically disconnected from the engine gear train." The turbine drives a centrifugal blower which furnishes air to the engine. That air is routed through an aftercooler to decrease its temperature and increase its density before it is routed into the air boxes that run down the sides of the crankcase. The air box is directly behind the upper set of round (rectangular on B and earlier) handhole covers. If you open one and look inside, you see on the other side of the air box the intake ports of the cylinder liner, and you can look inside through those ports at the cylinder wall. By barring the engine over by hand, you can visually inspect every cylinder liner and the piston as it rises past the intake ports. By the way, the part number for a round handhole cover on an SD70MAC is the same part number as on a GP9 -- they've been making the same part for over 50 years. A few other notes on the blowers: they're handed, and weigh 510 lbs each, and a turbocharger weighs 2300 lbs. A 16-645E3 weighs 36,000 lbs not including the turbo, which is quite light for 3000 hp. Air filters for the blowers sit in a big tin box on top of each. The really high maintenance item on a turbocharged EMD has always been the exhaust manifolds, in my experience. I dunno about the GP20-2. All my "old" EMD stuff is here at home, and all the "new" stuff in the office. Sigh.
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