Turbocharging vs. Supercharging

[broncoman]

Question, I work with baby GM diesels (6V-92 & 8V-92s), in the forums I notice people saying that turbos are removed and roots chargers are put on instead, my question is do the bigger GM 2 strokes not need the roots charger when they have a turbo. I thought that they had to have the roots charger even if they have a turbo, it just blows through.

Thanks for the info.

[wabash1]

super charge a sd70? OH YEA i can see it now. 75 cars of grain Im leaving a siding go to notch 8 ...200mph in 5.3secs conductor loses mirror ( again) railroading at its finest.

same thing only a ge engine put it in the 8th notch fire comes out of the stack 3miles down the road and 5mph. somethings never change

[Anonymous]

Non-turbocharged EMD diesel engines use two Roots-type blowers, gear driven off the crankshaft (only one is provided for the the 6-cylinder engine). The blower is necessary to scavenge exhaust gases from the cylinder: it supplies a large volume of air at a low pressure, about 4 psi. It's not enough pressure to provide a horsepower increase, and these are considered normally-aspirated engines, not supercharged in the sense you would find on a automobile.

If an EMD engine is turbocharged, it has no Roots blowers. The turbocharger supplies air at about 15-18 psi, increasing horsepower by 50% in the same displacement. The turbocharger is gear driven at low rpms; once exhaust flow builds to sufficient volume a clutch disengages the gear train and the turbo is freewheeling.

Alco and GE turbochargers run in the 18-26 psi range, as far as I know.

[Anonymous]

Bronco, the BNSF has been down grading some of their SD40s to SD38s. What they are doing is removing the turbo and putting a roots on. Then the locomotive is used mostly in the yard. There is no advantage to having a turbo on a yard engine. It always appeared to me yard work tended clog up the turbo and increase maintenance.

EMDs SD 90s has the new H Block diesel it is a 4 stroke engine and has two true turbos. No gear drive at low RPMs.

[Anonymous]

Boy you all stole my glory. Being an ex-machinist I couldn't have said it better myself.

[broncoman]

Thanks for the info. With regards to the conversion of sd40-2 to a sd38-2 it looks like UP is also doing this too. Thought I saw a couple of converts in the Roseville yard, Missouri Pac style with 4 stacks. Can anyone confirm.

2 strokes are cool.....twice the work same RPM!

[M636C]

broncoman,

There is perhaps a little more to add to the editor's comments about EMD turbochargers. They are driven from the crankshaft by a train of gears at the generator end of the engine, the same gears as used to drive the roots blowers. The gear ratio is changed because the centrifugal blower turns faster to produce the same pressure than the lobes in the roots blowers.

The gear drive means that at lower power, the turbocharged EMD engine runs exactly like one fitted with the roots blowers. This is what gives the EMD the faster response at lower engine power, and avoids the black smoke on throttle change so familiar with GE (and for those who see them, ALCOs).

The turbocharger doesn't free wheel until fairly high up the power range, about notch 7, by which time quite a bit of power was being used to drive the blower. Once the clutch releases, this power is available for use in the alternator, and this causes the change in the specific fuel consumption figures that we mentioned in the discussion about SD80MACs. Above this point, the EMD is competitive with the GE on fuel consumption, below, with the blower still mechanically driven, the EMD uses more fuel.

For yard switchers, which might not often get into notch 7, a simpler roots blower engine is a better proposition, because it will spend much of its time idling or in low power, where the fuel consumption of blower and turbocharged engines is similar. The clutch for the turbocharger is a relatively high maintenance item, and if the locomotive was frequently going to high power for short periods, the clutch would wear out more quickly. The simple roots blower still gives you a fast throttle response for switching, but a higher fuel consumption for a given power at high power settings (notch 7 and 8).

It is the drive train to the turbocharger that gives turbocharged EMD locomotives their distinctive sound, mentioned by our English member 45144, particularly while they are idling. I think it may be the different gear ratio from the roots blower engines that causes this.

I became familiar with the Detroit Diesel engines through the 16V-149 engines fitted to US Navy (and Australian) Perry class Frigates. In those, and the engines broncoman mentioned, the roots blower sits down in the vee of the engine, and where used, the turbochargers sit above it. I say turbochagers, because the 16V-149 was basically two V-8s joined together, and had two roots blowers and a total of four turbochagers. This basic arrangement was used on the early Union Pacific experiments with turbocharging GP-9s as well. In Australia, some Krupp-design diesel hydraulic locomotives were rebuilt with Detroit 16V-149 engines, just in case you thought I was getting too far off the subject.

I'm not sure why EMD developed the gear driven turbocharger, the solution adopted by Detroit Diesel certainly works, and appears less complicated. It would reduce engine efficiency at lower power because the blowers would need to draw air in through the stationary turbochargers. The clutch is the additional item that wears out and is a source of additional mantenance and possible failure. But EMD have it working fairly well now.

And now a question - EMD built a few demonstator locomotives as rebuilds from GP9s, I think, called GP20-2 (I don't have my "Guides" with me). At the time, although these locomotives had what looked like a normal turbocharged engine (from the drawings I saw), it was referred to as "supercharged" by some sources. Was this something special, or was it just a normal EMD turbocharged engine?

Peter

[Anonymous]

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.

[Allen Jenkins]

The Seaboard Coast Line Railroad had a problem with EMD GP 40's related to the turbo. Johnny Moore, an engineer who worked the road, whom I met through my church, told me about overheating, probably bearing falure which occures from oil overheating to the point of viscosity loss, starving the bearings,resulting in a costly bit of downtime for turbo failure. This, he asserted, was from using to much throttle during switching moves. I believe he said the turbo freewheels at or by the sixth notch. His respected advice was to use low throttle speed while running locals on a GP40. In perspective the location was Lakeland, FL, which is 197ft above sea level with "hilly" terrain, or ripwrap profile, with two roads diverging ACL/SAL=SCL in the '70's topping the hill from four directions. In the days of first generation power and 150 car trains men would approach Lakeland, the top of the hill, wide open. GP7's and RS3' worked locals, So the GP40's, replaced on road freights by U36B's, became turbo training ground. This, I belive prevented the GP/SD39,49,59's from a future with SCL Ind. The Coast Line removed the turbo from the SD35, calling it an H-15. EMD Leasing(gmemd.com) rates SD38 and SD40 starting TE and max con TE pretty close. Just available HP is 1,000 less than the "turbo." The NS has taken to replacing two GP38's with one SD40 on locals around Rome, GA these days, now that was Different! Even so, you would observe, they work it easy, only using the real power when they need it. So Easy Does It! remember-it's not your dad's GE! Enjoy Your Hobby!

[M636C]

Mark,

Yes the Perry class use four Detroit Diesel 16V-149 engines rated at 1000kW each for auxiliary power. There are plans, both here and in the USN to replace them with Caterpillar 3516 engines, retaining the same alternators.

Since the Perry class has only a single screw, they have two retractable and trainable electric "auxiliary propulsion units" which can be used for manouvring in harbour, or as emergency propulsion (up to maybe 5 knots). That is the only way the diesels get to propel the Perry class.

And yes, the engines each have two roots blowers sitting down in the vee, and four turbochargers, each driven from four cylinder exhausts. These originally sat above the engine (I think) but were moved lower to reduce the engine height, and sit alongside the rocker covers on the outside of the engine now (on the Navy versions). The blowers run all the time, and draw air past the standing turbochargers until the power gets to the level when the turbochargers compress the air fed to the blowers. I think the intercooler is located between the turbochargers and the blowers.

There are versions of the engine sold without the turbochargers, and these rely on the roots blowers to provide the positive pressure. They are rated at a lower power, of course.

Turbocharged versions of the Fairbanks Morse engine had the same arrangement of a turbocharger feeding a roots blower. None were fitted in locomotives in the USA, but a large number of Russian built locomotives fitted with copies of FM engines used a turbo feeding a roots blower.

In fact, the only locomotive engine built in really large numbers outside the USA was the turbocharged version of the FM engine, which was built right up to the end of the Soviet era in 1990.

Peter

[Allen Jenkins]

I think it's cool to be discussing the Detroit Diesel line in the same context with the EMD two cycle engine, since both share an extrordinary amount of simular traits. two piece crosshead pistons, port type cylinder liners, rack controlled injectors, excluding the heads, the '71 series Detroit cross sectioned resembles a scaled down 710 Series, including two bolt together v8 crankshafts to make the V16. My favorite motors have always been V12's, probably 'cause of the 12VSeries. Detroit Diesel General Motors carried the motto,"Standardization, Is The Key!" So,the inline '6 and 12V71Series share the same 6cyl head, for years. Iv'e seen Uclid dump trucks lift front wheels off the ground, using the motor to"help" dump the load! God, I love this country! PT109(as well as PT73), used three Packard V12 Diesel engines during WWII. Also both the EMD and Detroit Diesel Roots blown engines will not run without the blower, or turbo, attached, since every down stroke is a power stroke, and the motor starves for air. The lobes inside the blower mesh, but do not touch one another, for which there is a gauge made to assure clearence. 'Ole Lefty Scott back at Polk Vo Tech taught us "the nastiest thing in the world is the inside of a Detroit Diesel!" Thanks for the info on the 149 Series in the Perry class Frigates. Iv'e heard of the 110 Series marine Diesel, but not much about the 149. Marine, industrial, power unit, automotive, generator set, MADE IN AMERICA!

[wabash1]

the reason for the sounds like we are taking it easier with the sd40 comparied to the gp-38 is because in notch 3 of the sd40 we are doing the same amount of work in that notch as you would be doing in notch 6-7 with a 38. the gearing and power of a sd40 allows for less throttle at switching than with the good ole gp38. and one other thing being a sd40 is a 6axel unit there are restrictions in spur tracks usually of 5mph. with these units.

[CSSHEGEWISCH]

The conversion of SD40's to SD38's by replacing the turbo with a roots blower is hardly uncommon. BNSF has done it with its SD38P's, although some of those started out as SD35's, UPY SD38's are mostly rebuilt from SD40's and SD40-2's, although some are originally SD38-2's. BRC has done the same with its 560 & 570 series.

In a similar vein, IC rebuilt a number of ex-UP and ex-SR SD24's into SD20's by removing the turbocharger (among other things). Precision National did a similar thing with a batch of SD24's that eventually wound up on C&NW.

[jchnhtfd]

Allen -- good point about turbocharged engines in switching service. The bearings in the turbo take a horrible beating. I've never been sure, though, whether it was so much overheating and thinning of the oil, as a plain ordinary pressure drop when the engine is returned to idle from a high power. At high power, that turbo is really winding! I do know in aircraft engines that the quickest way to kill a turbo is to go from say cruise power to idle without letting the turbo spin down -- takes about three or four minutes to do.

[dekemd]

QUOTE: Originally posted by jchnhtfd

Allen -- good point about turbocharged engines in switching service. The bearings in the turbo take a horrible beating. I've never been sure, though, whether it was so much overheating and thinning of the oil, as a plain ordinary pressure drop when the engine is returned to idle from a high power. At high power, that turbo is really winding! I do know in aircraft engines that the quickest way to kill a turbo is to go from say cruise power to idle without letting the turbo spin down -- takes about three or four minutes to do.

From my understanding of turbos, its the oil pressure drop causing the overheating. The oil pump is belt or chain driven off the engine. As RPMs go up pressure goes up. Turbos are very dependent on oil flow. When you drop the throttle to idle, oil pressure drops tremendously. The turbo is still spinning at high speed when most of the oil goes away. Heat builds up quickly as the bearings wear against each other. The bearings will only take so much of this before they fail.

Derrick

[broncoman]

Mark and/or Peter,

Do the bigger series xV-149 and xV-645, 710 have accessory gear drives at both ends of the engine as well.

Peter,
We had a GM engineer tell us that although there is a reduction in efficiency with the blower, it had the benefit of keeping the turbo spinning during on the throttle, off the throttle situations since there is a pressure drop on the compressor side of the turbo, turbo lag is reduced. Seems logical, only heard this from him and no where else.

[M636C]

broncoman,

I don't have any data with me on the accessory drives, but my recollection is that the 645 and 710 have all the gear drives, blower/turbo and camshafts at one end (the end the generator connects). As Mark said, there is a driveshaft at the other end for the air compressor, and this drives the radiator fan in older switchers and some exporrt units.

While the 149s are basically two crankcases connected, I think the accessory drives are at one end only, with all the shafts running through with bolted connections. But when I get a chance, I'll check.

Having thought about the EMD Turbo drive and the reasons for its adoption rather than EMD taking the Detroit Diesel solution of a turbo and a roots blower in series, I feel sure that the fuel economy advantages caused the selection of that design.

The cost of diesel was becoming an issue in the late 1950s, and UP were testing diesel locomotives on heavy marine fuel oil (which they used in their gas turbines, and had used in steam locomotives). ALCo were still in business, the 251 was very economical and fairly reliable, and GE were expanding their export business and looking to build locomotives for the US Domestic market. So the same (big) customer who was pressing for more power also wanted better fuel economy, and EMD selected a design that would keep them happy on both counts.

Peter

[UPTRAIN]

The UP has rebuilt about 10 ex MP SD40-2s into "SD38-2s" de-turbocharging them, adding paper air filters, but leaving the SD40-2 style carbody....damn with those bigger radiators they outta cool themselves well!!!!! They are assigned to North Little Rock, Roseville (I think) North Platte, and maybe St. Louis last time I was there 2 were there.

[Allen Jenkins]

It's a heat thing. It all points to the practice of lugging. Now there are factors that are not published about the EMD V-type two cycle Diesel that are not relavant to the application of platform-mounted, power-generation prime movers, now that you mention it, such as peak-torque rpm, which with a compression-ignition engine is at the bottom of the operating range. The whole theory of lugging an engine is dropping below advertised peak-torque rpm speed. After diesel engines drop below peek tor. speed, the governor automaticly reduces fuel injection. On automotive Diesels, this can be noted by drop in tubo speed. At that point, you downshift, and Not before! This is how you save fuel. However, the major principal to be learned is, oil pump speed is directly related to throttle speed. If you need power, graduate it to the work. If by principle, of power application half the distance to the stopping point, the question would be do I need to exceed notch four? Can I accompli***he work without hitting the mark like Napoleon over the Alps? Johnny Moore stongly advocated and taught and evaluated performance in notch 6 or less. Then the heat accumalated would be transfered to the oil flow, as designed. Sudden, quick throttle-ups, raise operating temps., then dropping back to idle speed, (as in branch line daily local service), results in overheating the oil, which breaks down viscosity. O.K., do I get an honorary dergree? Enjoy Your Hobby!

[jchnhtfd]

Allen -- sounds great! And all of that (and more) is why the newest engines (whether from EMD or GE -- and I don't want to get into that!!!) are computer controlled (so is your car, for that matter) -- the designers have gone to a good deal of trouble to figure out what is the best combination of rpm and injector pressure and timing for a given power demand, over the whole power range, for best economy, lowest emissions, and best reliability -- and then equipped the engine with sensors to find out what it's really doing, and adjust things to stay on the best point (and not incidentally, tell the maintenance folks what's going wrong, before it's obvious). Lets the guy in the front office decide how much power he needs, and not worry about the rest of it...

[Anonymous]

Broncoman: re your questions on gear trains. There's one on each end of an EMD engine. It's basically the same thing for 567, 645 and 710, turbocharged or blown, but my notes below are specific to a 645E3:

At the rear end (generator/turbo) end is the camshaft gear train, consisting of a crankshaft gear, two idler gears, and two camshaft drive gears. The turbocharger is driven from a separate gear on the same shaft as the upper (No. 2) idler gear. These are all spur gears.

At the front end (equipment rack end) is the accessory gear train. The accessory drive gear is on the end of the crankshaft and meshes with and drives the lube oil scavenging pump, main lube oil and piston cooling pump. Through intermediate gears it drives the governor and water pumps.

If the locomotive is equipped with starting motors (that is, it does not use separate windings in the main generator to start it), it has a ring gear bolted to the face of the crankshaft gear. That gear engages the starting motors.

EMD turbochargers have a separate electrically driven soak-back lube oil pump to ensure turbocharger lubrication upon starting and heat dissipation on shutdown.

The two single biggest lubricating problem experienced in locomotives are low pressure and fuel dilution. The first place you look if you have low oil pressure trip-outs on an EMD is the main bearings, because that's the first stop in the lubricating circuit and if pressure is low one or more bearings is probably down into the copper, i.e., worn beyond limits.

Fuel dilution will ruin a locomotive engine faster than anything. I never worked on Alcos much, but people who have told me that fuel dilution coupled with marginal main bearings in the first place is what really hurt the 244 engine. The 244 was highly susceptible to rapid fuel dilution from internal leaks, and that you'd usually first know about a fuel leak when pieces of the engine flew through the hood doors.

[Allen Jenkins]

Wow Broncoman! What a learning experience! Thanks for the question! So much info has been added, and I'm amazed at the memories, and ideas that are born from sharing experience,and learning. Thanks also to Mark, for adding to your credentials, as Trains' Editor. I'm wondering when the oil pump was added to the turbo, or has it always been there? The '40 series, 645 engines have more than proved themselves, Iv'e not heard complaints about the GP40-2 line, although the Southern Railway System rated the GP40 in the same class with the GP30, GP35. This is for the fact of derating power atomatically on long grades. This is consistant today with NS practices. I don't know (but I do listen) about the SD40, but the Geeps' operating manual verifies this. All I see is that a company which owned 700 GP7's shyed away from the 12V645, bought into the U23B, a turbo V12FDL engine. Another oddity is that while the railroads in the South avoided the SD7-9's, (there's one for you What if? modelers, the SD79) the Central of Georgia, and the Bay Line used these, remembering that SD's axle weight rating is less than GP's. I Know what you mean, Wabash 1, the use of idler cars also avoid problems at switches. So the turo blitz, I believe, killed the future of the SD/GP locomotives with 12V engines, which should have replaced the 16V for medium horsepower locomotives.

[M636C]

There is something odd happening here - the index shows much later posts than I can see when I open the topic! The clock shows forum time as 16:33 and the last post visible is the editor's at 08:33!

ALCo addressed the fuel dilution of lubricating oil problem in the 251 engine by moving the fuel pumps to outside the engine, where they sat under rounded covers, each with a little handwheel to screw it into place. This reduced the chance of fuel from small leaks around the pumps getting into the oil sump.

The fuel pumps were driven by rocker arms that contacted cams on the camshaft from below, while the valves were driven by long push rods off the top of the camshaft. This meant the forces on the camshaft completely reversed every revolution, and in the 251F running at 1100 rpm, the camshaft mountings failed by fatigue cracking.

There are no easy answers in locomotive engines, and solving one problem may cause another. But that's a bit off topic, anyway.

The ALCo turbochargers generally failed after a lubrication related bearing failure, but sometimes this was said to be a result of less than adequate checking of the dynamic balance of the turbocharger shaft, resulting in higher vibration levels and bearing damage occurring as a result.

Peter

[Anonymous]

QUOTE: Originally posted by broncoman

Thanks for the info. With regards to the conversion of sd40-2 to a sd38-2 it looks like UP is also doing this too. Thought I saw a couple of converts in the Roseville yard, Missouri Pac style with 4 stacks. Can anyone confirm.

2 strokes are cool.....twice the work same RPM!

Broncoman, you are correct. Roseville has 5 converts assigned to hump duty. I was working there when they came in.

[Anonymous]

Allan: I don't know if the turbo soakback pump was always there, but it was at least there by 1968.

What you're seeing in the NS tables is that GP40s begin derating at 22 mph to around 2200 hp at 11-12 mph, to power-match with SD40s, via the PF17 Performance Control Module. As far as I know, all GP40s are so equipped. That's because their Minimum Continuous Speed is 22 mph, whereas an SD40's MCS is 12 mph (assuming the common 62:15 gearing). Otherwise, if you were running from an SD40 and you start going upgrade, you'd look at your ammeter as speed dropped into the teens and assume everything was cool, while back at the trailing GP40, the traction motors are melting. You wouldn't be able to have that horsepower from the GP40 anyway, except for a very brief period of time, so the PF17 module is a nice feature to have.

(For those just coming in here, Minimum Continuous Speed is the speed at which a D.C.-drive locomotive can exert full horsepower without exceeding the temperature limits of the traction motors. A.C. locomotives can go to a full stall without overheating their motors.)

I wi***hat 50 years ago we (Trains) hadn't obsessed over the so-called light weight of the SD7 and SD9. It turns out that very few SD7s or 9s were truly light weight -- you can usually tell them because they have a half of a normal fuel tank. Milwaukee Road and CB&Q had some, the Q going so far as to put a little "s" on the SD9 plate these units carried to signify short fuel tank. I don't know what the Q needed lightweight SDs for, but the Milwaukee Road, with what was probably the most eclectic system the world has ever seen, seemed to have a need for at least a few of everything. Who else ran string of M.U.d SW1s in everyday road service? Those must have sounded very cool.

Most railroads seemed to try to ADD weight to their SD7s and 9s to make them as heavy as possible to get maximum tractive effort. I saw a lot of them with scrap freight-car wheels stacked in the back end for extra ballast. Some of them were in the nearly 400,000 lb. range, which is SD70MAC territory. Of course, you only got maximum tractive effort up to 6 mph or so, depending on whether you bought standard 62:15 gearing or the slower-speed 65:12 gearing, but if you're the DM&IR or the B&LE, that's OK.

Most railroads didn't buy SD7s or 9s because they correctly couldn't figure out what on earth they needed them for. About all they were really good for was extremely heavy tonnage on a line where speed was inconsequential. Sure, they'd lug half of the yard inventory up the hill, but who would want to tie up their main line with a train going 4 mph? Thus, you see the Southern Pacific, with a raft of heavy-grade, heavy-train, one-train a day lines all over Oregon and California, going for a huge fleet of SD9s, and the Santa Fe, with practically nothing of the sort, staying away from them. You don't see the D&RGW, the other heavy-grade railroad, buying them in large quantities because their steep, heavy-train branch lines were few in number and fairly short -- or had such light rail and tight curves you didn't want a six-axle truck anywhere near them. Thus the D&RGW remains a four-axle road until the SD45 appears, and even then remains mostly a four-axle road until the coal boom of the 70s makes it need SD40T-2s.

Today, the reverse is true, and four-motor locomotives aren't built anymore. You can practically sum up the last 50 years like this: until the 1960s, you haven't got enough engine to need six traction motors. After the 1980s, you've got too much engine for four traction motors. Only from about 1965 to 1980 did you really have to choose exactly which one you wanted; after that, you were better off in almost all cases with six motors.

Your last question about 12-cylinder vs. 16 is interesting. I would posit that what killed the medium-horsepower locomotive was deregulation. Once that hits in 1980, loose-car traffic, branch lines, and yards have to begin going away, and with them, the need for new examples of these locomotives. You can handle what remains with downgraded main line power.

Turbos didn't kill 12-cylinder engines, I don't think. Few railroads went for 12-cylinder four-axle locomotives after 1965, because you can put all of the 2000 hp a normally aspirated 16-cylinder engine develops (the GP38) into four axles with the wheel-slip technology of the time at a reasonable speed, 10 mph. If you put a 12-cylinder normally aspirated engine into the same package, you're saving very little money -- not more than 5% of the purchase price of the locomotive -- while giving up 25% of your tractive effort. So that variation was almost never made in the 1960s except a handful of GP28s.

Later, the GP15 shows up. Basically, this is a SW1500 with a decent ride and a decent cab. My guess is that this model doesn't appear earlier because until that point everyone has plenty of 1200 and 1500 hp switchers with life left in them. At the point when they wear out, the GP15 becomes attractive as sort of a cross between an SW1500 and a GP38, with the flexibility of the latter and the economy of the former. I would very much like to see the purchase price and operating cost difference between a GP15 and a GP38, because the buying decision looks marginal to me. That is, unless EMD found a way to get a lot of the price out, but since there's almost nothing of the GP38 left out, I don't what that would be. A few railroads, notably Santa Fe, went for 12-cylinder turbocharged GP39s, producing 2300 hp, for reasons that remain enigmatic to me. Supposedly Santa Fe bought theirs for "high altitude branch lines," but since they had almost no such thing on the system, I couldn't see that. Besides, they seemed to run pretty much anywhere.

[kenneo]

Mark - interesting comment about the ATSF GP-39's. Bunch of them, bought cheap, on the WPRR/PWRR. Interesting, n'cest pas?

[broncoman]

SP had SD-39's where did they end up running and end up doing (i.e. manifest,hump,intermodel.)?

[M636C]

broncoman,

The SP SD39s were based in Los Angeles for general traffic when new, and didn't normally work far from there (according to Strapac's 1973 SP Annual). On the other hand, the SD38-2s were switchers at Colton from new - an early example of specialising.

I've looked up the Detroit 149 data I have, but it is sales and application design data, and basically shows what the engines look like from the outside. There are some auxiliaries on both ends, apparently the same for "one block" and "two block" engines.

An interesting point is the comparative ratings. The turbocharged and intercooled engines provide "up to 25% more power" than blower engines. The EMD turbo arrangement provides 50% more power, as well as fuel economy.

Peter

[M636C]

Some more assorted comments, one correcting some of my earlier observations on this subject. Having actually checked my 645 blower and turbo engine manuals, I find that the drive gear that drives the turbocharger actually rotates at half the speed of the gears that drive the roots blowers, although there are step up gears inside the turbocharger unit itself.

The distinctive noise of the turbocharger drive train, mentioned by 45144, is caused by a spring flexible drive system between the No 2 idler gear and the actual turbocharger drive gear, which is connected otherwise directly to the No 2 idler gear. The spring drive is to prevent any vibration being transmitted between the engine and the turbocharger.

Either I have misunderstood the editor's comments about the GP28, or he has incorrectly indicated that they used 12 cylinder engines. The GP28 was the blower engine version of the GP35, and used a 16-567D1 engine. It was basically a GP18 fitted with a central air system, and looked like a shorter GP38.

The editor may be thinking of the RS-1325, basically an SW-1200 with a low profile short hood. There were only two built.

As to Santa Fe's GP39s and GP39-2s, there are two reasons for derating diesel engines, altitude and temperature. Santa Fe certainly had branch lines in areas where high temperatures were common. The Santa Fe, however was a big company where the better fuel economy of a GP39 compared to a GP38 would have been noted, and I have heard that EMD did not intend to offer a GP39-2 but Santa Fe insisted they wanted them.

The cost of a GP15-1 could be kept low by trading in a GP9 and reusing the trucks, motors and generator. Many didn't have a central air system, suggesting that every effort was made to keep costs down.

This doesn't explain the GP 15T, with an alternator and a turbocharged EIGHT cylinder engine. This must have been intended for running in road service where operation in higher throttle notches would provide better fuel economy.

Peter

[Anonymous]

Peter: I am wrong on the GP28, you are right, insofar as the engine. I should haul home a few more books so I can verify things before I start typing.

The problem with the GP39-2 theories we're both advancing is we're arguing intent on the one hand, and on the other hand our evidence is practice. Those are two different things!

SP SD39s were principally used as helpers, as far as I saw. For many years, a set worked out of San Luis Opispo. One rarely saw them out of California, as Peter pointed out.