It is indeed the GE control system and design features of the two designs of engine that affect the speed of “loading up”, rather than inherent features of two and four stroke engines.
The GE control system is set up to slow “loading up” to prevent the production of smoke (and other unfavourable gases) due to turbocharger lag, the turbocharger not supplying sufficient air during the early stages of acceleration. On the other hand, the EMD engine has a turbocharger driven directly from the engine crankshaft until the exhaust gases develop enough power for it to run free. This feature allows earlier “loading up” of the engine.
One more difference. Because the 2 cycle engine gets twice as many “bangs” per crank rev, those “bangs” are about 1/2 as powerful. So, the whole engine design can be much less robust. A good example would be to compare the cross se.ction of an EMD con rod with that of a GE. EMD can get away with a fabricated engine frame vs GE’s cast one, too.
Trust me when I say I’m not playing dumb about this- I am dumb about this. Wouldn’t one downstroke, and one upstroke equal one revolution of the crank, regardless whether it’s a 2-cycle or a 4-cycle?
I believe the argument is that while each rev produces one downstroke, the 2-stroke is producing a bang with every downstroke while the 4-stroke is producing a bang with every other downstroke – kind of like bang-bang-bang vs BANG-nothing-BANG-nothing.
Besides, doesn’t a GE produce a kind of “chug-chug-chug” sound while an EMD produces more of a “brrrrrrr” sound when they start up?
Again, because you get twice the number of power strokes per number of revolutions of the crank shaft, for a given RPM and cylinder size and all other things being equal, a four-stroke will have half the horsepower of the two-stroke. That is why GENERICKLY, a four-stoke is usually heavier and larger than a two stroke. Obviously if the four-stroke runs at twice the PM, has a far more efficient power train (electrical system) and supercharger, and just better design, and is designed for a less rugged environment, you can have a situation where the reverse is true.
I am sure EMD has or will have the technology to allow two-stroke engines to meet future emission requirements.
One possibility is the adoption of the old steam locomotive blow-off valve principle and have exhaust valves built into cylinder walls other than just the valves at the top of the cylinder, overcoming the piston-ring wear that would occur as the rings pass the seated valves throuigh advanced netallurgy and using the latest computer technology with reduncancy to assure accurate valve timing.
The engine structure of a 4-stroke has to be heavier than a 2-stroke engine because higher pressures are developed in the 4-stroke. The compensating factor is that the cooling requirements for a 4-stroker are far less than that of a 2-cycle engine producing the same horsepower. Being less fuel efficient translates into the requirement that the heating value of the excess fuel has to dissappated in the cooling system instead of appearing as useful work.
All else being equal, the 2 stroke diesel looses some power and boost because the intake port opening and closing is controlled by the piston and symetrical around bottom dead center i.e. the ports & exhaust valves open before the piston completes a full power stroke. When the ports and exhaust valves are both open, some of the boost pressure is being blown right through the cylinder in order to scavenge the cylinder. A 4 stroke can use the full power stroke and because it has a seperate exhaust stroke, all of the boost can be used to increase effective compression.
Because it has one power stroke per revolution instead of one every 2 revolutions, a comparable 2 stroke will have more power, but I don’t think the theoretical difference is a factor of 2, that would depend of valve/port timing, compression ratio, boost pressure and turbo size. I suspect the GE’s are running much higher compression and effective boost to make up the difference, thus the need for a beefier bottom end, broken con rods, etc.[:)]
On a very old thread, several years back, our great mentor/teacher/referee, now in Iraq, put a full posting of all EMD engines up to that date. I think I managed to copy it to a floppy and mabe I can locate it.