[quote="Rich Melvin"]The issue is not related to driver diameter between steam and diesel. The basis for the statement quoted is the difference between the torque and power curves of an electric motor as opposed to a reciprocating engine.

An electric motor develops its maximum torque when it is stalled. A reciprocating engine develops zero torque when it is stalled.

The diesel is a fixed horsepower, variable torque machine. Because the horsepower is fixed, as it goes faster, it's tractive effort drops.
___________________________________________________________

An electric motor produces torque when opposite polarity magnetic fields in its stator coils and rotor coils pull toward each other. Motor torque decreases as the coils move across each other's magnetic fields and produce a counter electromagnetic field (emf). Counter emf from a moving motor armature sends power in the opposite direction from the direction of the motor rotation, cancelling power input to the motor as the motor becomes more of a generator. As speed increases, counter emf increases and net output torque from the motor drops unless more power is fed into the motor. Routing counter emf current across a an air-cooled resistor grid produces dynamic braking.

The fall-off in output torque for the same power input as traction motor speed increases follows the force x speed = power model for constant horsepower. Power loss through electrical conduction inefficiency appears as heat. A diesel-electric locomotive's tractive power cannot exceed the power supplied by its diesel power plant.

A straight electric locomotive's power is limited by its amperage rating.

A steam locomotive's power cannot exceed its boiler pressure multiplied by the steam volume per unit of time that its boiler can produce at that pressure. At its drive machinery's maximum steam consumption, increased reciprocating speed can be traded for reduced steam admitted per stroke at constant drawbar horsepower, until steam flow efficiency losses translate into increased back pressure and reduced net force per piston stroke. For similar boiler-to-valve-to-piston machinery configurations, steam flow limitations take over at about the same piston reciprocation rates, and higher operating speeds require larger driver diameters. Super-power steam designs increased boiler steaming rates while reducing steam flow efficiency losses for more power at higher speeds.

Bruce Manwiller

Eric-

Point well taken.

I was just trying to point out that there was a reason to pair the lead wheelsets on the trucks in the wheelslip circuit, rather than pair the two axles in the same truck. Obviously the WCR is also there to help the situation as is the WS13 and the WS24. Ultimately, you try to keep the wheelslip from happening (sand boxes and traps), then sense a wheelslip when it does happen, and then you have to correct the wheelslip after you sense it.

Ryan

Regarding the comment Ted made earlier about wheelslip in conventional steam locomotives, you will often see that when the locomotive begins to slip, it occurs at the same place in driver rotation on each turn of the driver. The tractive force being applied to the rail varies both with the changing pressure during the expansion of steam in the cylinder and with the mechanical advantage resulting from the changing geometry between the rod and crankpin as the driver turns. On a two cylinder locomotive the force peaks four times per driver revolution, and those peaks are not uniform in magnitude nor are they 90 degrees apart. The highest of the four peaks is the point where slip usually gets initiated.

Preston Cook

I believe this is something that we can see out on today's modern mainlines. I've seen single diesels pull some very long drags down the mainlines. They're pulling the train, just not very fast.

Bob Barcus

Interestingly, you could build a steam locomotive with characterisitics similar to the DieselElectric locomotive The difference between steam power and diesel power from the viewpoint of traction would be minimized if comparing locomotives using similar power transmission methods. If you created a "steam electric" engine where a reciprocating piston steam engine drove an electric generator like a diesel electric, and you used traction motors, the torque characteristics would be very similar.

You could not, however, reverse the roles and use an internal combustion piston to directly power the main rod of a "steam locomotive". On a steam locomotive the piston will exert force as long a steam is being admitted from the boiler to the appropriate side of the piston. But an IC engine must be in a constant state of rotation, owing to the fact that the power stroke is produced by a controlled explosion. To produce power requires air intake, fuel injection, and compression - which is especially hard if the piston rod is stationary........

While the technical aspects of the difference in power production is intrinsically fascinating, it carries a very deep historical significance not yet covered here.

Confronted with the diesel's numerous advantages over steam, the one thing that it can be cited as a diesel deficiency its its high speed power.

Simply, the lack of a mechanical connection from the prime mover to the wheels simply means the prime mover can be roaring @ 20MPH (or less) while the steam engine's power production is directy related to the speed of the locomotive it powers. The diesel gives its all with much regard to speed.

One can see this difference illustrated on page 378 of Ralph P. Johnson's "The Steam Locomotive"-Simmons Boardman 1942 (recently reprinted). The horsepower curves that plot drawbar horsepower in relation to speed show a relatively flat curve for the diesel
(constant horsepower) and a sort of a steeply ascending bell shaped curve for the steam engine.

Johnson shows curves for a hypothetical 4700 HP diesel and a 4700HP steamer that illustrate that until 40 miles per hour, the diesel wins the power battle.

Another way of saying it was given by VP Robert Binkerd, in his "Muzzle Not the Ox" speech of April 25, 1935, in which he said that a diesel retains only 15% of its starting tractive effort @ 80MPH while the NP 4-8-4 cited in the speech retained 1/3 of its starting tractive effort @ 80MPH.

This of course ignores the fact that if you need more horsepower, with a diesel, you just MU it on.

In the 1930's, Baldwin seized upon this obsession with high speed power to the point of myopeia and obsession. Baldwin saw this as a high speed advantage of steam rather than a low speed deficiency. The characteristic of power @ speed, which reached its pinnacle in "Superpower", was seen as an insurmountable obstacle to the replacement of steam in mainline service by the Baldwin management.

Magic bullets such as poppet valves and duplexing, IMHO weren't pursued more heavily, not only because of technical limitations of the time but because they only provided more HIGH SPEED power, while many railroads operate in mountains, so there was "no bang for the buck" under 40MPH, which is the operating envelope of a lot of trains, even today. The diesel was winning in part, because railroads were being forced into carrying durable commodities and other low value density, time insensitive freight-coal, grain and other things where the primary objective was minimizing transport cost, rather than speed.

Had the Lima 4-8-6 been produced, it wouldn't have done anything to address the issue of starting effort and low speed power, nor would the concept engine shown in June 1974 TRAINS or the ACE 3000.

Ironically, at the very time Baldwin was promoting locomotives that would have a power advantage only at higher speeds, the "need for speed" was diminishing, as air travel began to be serious threat to intercity passenger service and high value density, time sensitive freight was being sapped off by trucks and to a lesser extent-air.

Indeed, it was Baldwin's concentration on high speed power, as much as its focus on steam is what killed it. EMD produced a machine that was very much comfortable moving coal from say Ashley PA to Mountaintop-thus relegating the "planes" to history. The reality is that a lot of the time, you have to run full bore at low speed and diesels win. Running 80 MPH requires straight track, flat grades and manicured roadbeds and when the bulk of the business is commodities-the trick is getting it there cheap, not fast.

TYPOS!

I realize its been a year now, (exactly if Im not mistaken) but I did have one other question regarding this topic.

What exactly happens when an engine (steam and diesel) stalls and how hard is it to recover that engine from said stall?