Steam Locomotive Power Comparison-To Diesel Eng?

Question

The Diesel is Power from Eng,thru Generator to Wheels?

Steam Eng was ,Raw Power to the Drivers?

Are these staments Correct? Appreciate any Info. as to Power-is and was transfered to Move. Train.

Answer 1

They are properly called "diesel-electric" locomotives.  These days they come in two basic varieties, Alternating Current, AC, and DC, direct current.  The vast majority are of the DC type.

The diesel motor (prime mover) turns an alternator that supplies electricity to the electric traction motors mounted on the locomotive trucks.  To be more detailed in answering, the alternator needs a source of electricity to operate, just as the alternator in your automobile.  In your car, that supply comes from the battery.  In a diesel electric locomotive, that supply (called 'excitation') comes from the companion alternator, also driven by the prime mover.  The prime mover also runs the auxiliary generator that supplies low voltage needs, such as running headlights and other electrical components, as well as charging the batteries.  It puts out 72 volts.

Since the alternator puts out AC current (across a 600 volt field), and the traction motors are DC, the electricity has to be converted from AC to DC.  This is done through a bank of 'rectifiers.'  When supplied to the traction motors they turn and power the drivers by a simple ring and pinion type gearing, usually at a ratio of 62:15 for freight locomotives, higher for passenger locomotives.

Steam engines are a bit simpler by way of design.  Through external combustion, using wood, coal or oil as fuel, water is heated to high temperature and under considerable pressure, ranging anywhere from 170psi to 300psi.  The "steam dome" contains the linkage of mechanisms to allow the engineer to send steam to the locomotive cylinders via the throttle.

The steam flows from there to the "steam chest," right above the cylinders and containing the valve gear necessary to put steam into the cylinders and allow for the exhaust of the expanded steam, which is routed to the "smoke box" and out the stack.

The cylinders move back and forth, supplying power to the side rods and connecting rods of the locomotive's drivers.  The thing that separates them from other types of cylinders and pistons is that they are powered on both strokes in a 'push push' kind of arrangement.

Much of the valve gear is outboard on a steam engine utilizing cranks and eccentrics to operate the valves in the steam chest, their odd looking movements sometimes being referred to as 'monkey gear.'

In general, steam engines have two cylinders, but there were three cylinder types as well, with the third cylinder out of view under the boiler, and connect to the axle of the lead driver by way of a crank mechanism.

There are also "articulated" locomotives that have two engines under one boiler.  One such is the restored UP Challenger Class 3895, with these engines having four cylinders.  But, Baldwin Locomotive Works built a number of 'Triplex' engines that had three engines under one boiler, designed specifically for hauling coal trains ran by the Virginian and Erie railroads, having six cylinders.

The efficiency of converting fuel to energy hands down goes to the diesel electric.  This, coupled with relative ease of maintenance, was the death sentence for the steam giants of the past.

In comparing pulling power, each has an advantage over the other.  The diesel-electrics have a lot of "starting" tractive effort and can get more tonnage moving than a steam locomotive.  But, once moving, a steam engine can pull more tonnage than can a diesel electric.

Staying with a coal hauling example, the old Y3 articulated steam engines of the NW rail road, could drag thousand of tons around all day long, but their top speed was only 20 MPH.  Any more than that they would start tearing up the road way or tearing themselves up, with broken frames the usual result.

Such trains today require a multiple unit engine to handle the same tonnage.

Rule of thumb:  To dertimine the tractive effort of a diesel-electric locomotive, a very close stimate can be had simply by taking 25% of the locomotives overall weight, since all the weight of the locomotive sits on the drivers.  The large steam engines had leading (pony) trucks and trailing trucks for extra support for the weight of the engine, so not all the weight was on the drivers.

Addendum: Planksheer raises a good point.  Too often the term ‘horsepower’ gets interchanged with ‘tractive effort,’ or ‘torque’.  They are not the same thing.  Case en point, a simple comparison between your everyday EMD SD90AC, their equivalent of the GE AC6000, both 6,000 horsepower.  On the territory where I worked they are rated for 4305 tons up to 1.25% grade without getting into “short time” rating, where the train must stop to allow the traction motors to cool before melting, then go some more....

The comparison is made with the 2-8-8-4 articulated single expansion engines built by Baldwin for the Duluth, Messabe and Iron Range.  These were the most powerful steam engines ever built (my apologies to the ‘Big Boy’ enthusiasts.  They were longer by about 4 feet, and the overall weight of the engine was more, but the DM&IR engines developed 140,000 pounds starting tractive effort, slightly more than but comparable to the SD90AC, but more than the Big Boy’s 133,375 pounds).  These steam engines could haul 6,483 tons of train all day long, on grades up to 0.79%.  The DM&IR engines were handling as much as the modern diesel-electric, when factoring in grade and speed up that grade.  But the diesel is going to get into short time or start spinning the wheels, even with todays sophisticated traction control systems.  The steam engine just keeps on chuggin' along.

Here is why steam is superior.  Though the SD90AC’s have a high starting tractive effort, the moment the wheel starts to turn, the power curve (tractive effort) drops off exponentially as speed increases.  The steam engine’s tractive effort remains constant, which is why it can be accurately said a diesel-electric can start more tonnage moving than it can pull, and the steam engines can pull more tonnage than they can start.  The proof is in the pudding.

If your running along at speed on a diesel-electric and ‘crack the throttle,’ you’ll get an increase in prime mover rpm and a few more amps on the meter.  Do that on a steam engine, and you’ll spin the wheels.

Steam is an extremely powerful force.  That is why we have huge naval aircraft carriers that can do 53 MPH.  Steam turbines.  There were steam turbine-electric locomotives as well, but too complicated to be practical, and the Union Pacific gas turbines of the ALCO Century were to loud to be operated anywhere except out on the desert.

The other thing he mentioned that has peaked my curiosity is the weight of his Mikado.  534 tons is 1,068,000 lbs.  The weight of the DM&IR engine referenced above is 695,040 lbs., and the Big Boy tips the scale at 762,000.  Maybe a decimal point was missing?

Answer 2

All I can say is the old timers told me when a steam engine derailed it could bury itself in the ground if it remain running. The diesel would bog down.

Also  told me in some places in the Rocky Mt. they still use steam engine to push diesel freight trains over the mountains.

Answer 3

I won't add much to some of the already excelent answers given, but the first place diesel replaces steam was in yard service where there are heavy cuts of cars to be moved at slow speeds under 10 mph wih 3-5 mph being very common.  A diesel has much more horsepower at start than a steam yard goat.  Steam develops it max hp at higher speeds.  This is why steam lasted longer on the road and was used in pusher service up through the end of most steam in 1960.

Answer 4

Diesel electrics or electric locomotives are generally more powerful.  Our 534 ton Mikado might develope 1200 horsepower, and our RSD-15 ALCO is rated at 2300.

There is no question the steam engine is not as powerful.

And yes, you are using a diesel engine to generate electricity that powers traction motors.  There is nothing about a steam engine that can  be called effecient, wonderful as they are.

Answer 5

Essentially you are correct.  Diesel locomotives are technicially electric locomotives as the diesel engine powers an electric generator which powers the traction motors found within the trucks (where the wheels are housed).  Steam locomotives basically take super-heated water which is converted to steam and transfers this to the cylinders and pistons which accelerates the driving wheels and rods.