ALCo/GE vs EMD diesel engines? You're not talking apples and oranges, but you are talking about lemons and limes. Whether you are talking about an internal combustion engine or an oil fired steam locomotive, black exhaust is generally an indication that you have insufficient air to support complete combustion of the fuel being introduced.

A four-stroke diesel engine does not require a turbocharger. By increasing the amount of air (and oxygen) in a cylinder, the more fuel can be introduced and more resulting horsepower. Since nominal atmospheric air pressure is only about 14 psi, even adding 7 psi potentially represents 50% increase in horsepower. One method is to use a blower (supercharger) to force more air into an engine cylinder on the intake stroke, which early Cummins, Caterpillars, and other engine builders did use. The disadvantage is that the blower is driven by the engine gear train so a portion of the engine's output horsepower must be used.

A turbocharger uses the exhaust gas, otherwise wasted, to drive a turbine which compresses air for the engine. This also substantially increases the thermal efficiency of the engine, converting heat into power. The disadvantage of an exhaust driven turbocharger is "turbo lag", where an increase in rpm is not supported by sufficient combustion air until the turbocharger gets enough exhaust to spool up to the required speed and supply more combustion air. The result is simple – incomplete combustion manifesting itself as black smoke.

To manage turbo lag, several methods have been implemented. The Cooper-Bessemer in a GE 70 ton used an air dash pot which limited how fast the governor rack could increase the fuel, and if not properly adjusted they will smoke when accelerating. On various GE FDL equipped locomotives, GE used a modified throttle schedule often running the engine rpm's at a higher speed than the actual throttle notch so the engine could better respond to a throttle increase. On modern diesels, the computer control uses an input of turbocharger speed and/or turbo boost pressure in calculating how much fuel to introduce, so the software controls the fuel/air mixture to avoid the ire of the EPA. The ALCo 251 series engine is still in production (the last in knew) in India and for marine use, so they have likely converted to EFI and dealt with the turbo lag issue through the engine control computer.

EMD 567/645/710 is an entirely different critter, as these are two-stroke engines. To start and run, a two-stroke engine must hast a positive air box pressure to supply fresh combustion air and to force (scavenge) the remaining exhaust from an engine's cylinder. The 567 and 645 had versions that used a pair of blowers (single on a 6 or 8 cylinder) to supply the needed airbox pressure and combustion air. The disadvantage is that a portion of the engine horsepower must be used to drive the blowers, the advantage is that the combustion air being supplied to the engine is in direct proportion to the engine rpm with almost no delay during a throttle increase. The governor has an adjustment as to how fast it can increase the fuel.

An exhaust driven turbocharger, by itself, won't work on a two-stroke engine. So e EMD uses a hybrid design, acting as a blower and an exhaust driven turbocharger. The turbocharger is driven by the engine gear train, so there is always positive air box pressure. Typically in Run 6 (3/4 throttle) under load, enough exhaust is being generated that the turbocharger "free wheels", outrunning the gear train which is allowed by an over-running clutch in the gear train. At this point, the EMD turbocharger is being driven strictly by exhaust pressure so no horsepower is being used to supply combustion air. A second clutch in the turbine allows it to free wheel and coast to a lower speed as the throttle is lowered. The turbocharged versions can be more easily adapted to maintain higher horsepower output at high altitudes where the atmosphere in thinner.


A turbocharged EMD is at its highest fuel efficiency at Run 6 or better, but at lower throttles it isn't as fuel efficient as the roots blown version, and the turbocharger and drive train (especially the over-running clutch) are a much higher maintenance item than the blowers. For EMD locomotives primarily used in lower throttles, the roots blown engines are preferred, though at high throttles they aren't as fuel efficient as a turbocharged engine.

The desire to keep a turbocharged EMD in Run 6 or better for fuel efficiency has resulted in some interesting technology. IIRC: There was a "Fuel Saver" system that WP and others used, where a selector switch on the engines was set to Lead, Trail 1 or Trail 2, and two mu wires were dedicated to its use. On a leading equipped locomotive, a second selector switch on/near the control stand allowed the engineer to select which of the Trail sets, if any, to isolate. All locomotives in Lead (including any unequipped) always responded to the throttle, the engineer being able to isolate locos set to Train 1, Trail 2 or Trail 1+2, or have all in. When all the trailing power was not required, the engineer could isolate one or more trailing engines allowing the other engines to operate in higher throttles (the goal being Run 6 or better). I'm not sure if the trailing engines were actually isolated or limited to Run 1, and presumably dynamic brakes were always in. In the 1980s, there was a "black box" system out of New Zealand that if the consist was equipped, the engineer would enter a target speed. If the throttle was under Run 6, the equipped trailing units would begin to throttle back to Run 1 one at a time until the engineer was in Run 6 or better. If the target speed could not be maintained, one at a time the trailing units would throttle back up. I'm not aware of anyone widely adopting this system in US or Canada.

The latest variation I've seen on this is an EMD software/wiring option, only required to be on the lead unit. From what I can tell, you manually enter three types of locos into lead computer. The computer intercepts the throttle inputs and controls the lead and mu for trailing locos separately. As I recall, the modified throttle schedule might be something like this for two units of the same type:
Throttle setting – lead unit – trailing unit
Run 1 - Run 1 - Run 1
Run 2 - Run 2 - Run 2
Run 3 - Run 4 - Run 2
Run 4 - Run 6 - Run 2
Run 5 - Run 7 - Run 3
Run 6 - Run 6 - Run 6
Basically the theory is to modulate the lead vs trailing throttle positions to create about the same expected horsepower while keeping when possible as many EMD turbocharged engines in Run 6 or above. I believe UP at least tried this system, but don't know how widespread it was/is used. Other than the setup, and potentially having to suffer from a higher than normal throttle on the lead unit, the system appears to be fairly transparent.