why do diesel trains run on electricity and not diesel engines?

Answer 1

The1 had a good answer.

Diesel engines are designed to produce a huge amount of torque, rather than acceleration, but they do it in a very limited RPM range, not much more than 2,000 to 3,000 RPM maximum.

(Gas engines have a much wider power band, and can go up to 7,500 RPM or higher in racing engines.)

As such, diesel engines in truck applications need a lot of gears to be used efficiently. It's not uncommon to see 13, 15 or more gears in the transmission of a semi-trailer tractor.

Using a mechanical transmission for power transfer with a diesel locomotive would require a huge transmission, and it would be difficult to couple all the driving axles together.

By coupling an electric generator to the diesel engine, the power to the wheels can be easily controlled and transferred to every driving axle, using high-current electric traction motors on each axle.

And multiple locomotives can be managed by a single control system in the lead locomotive, when power & command settings are shared by control cables (MU, or multiple unit), daisy-chained from one locomotive to the next.

Finally, using electric motors to power each axle allows regenerative braking, in addition to the friction brakes, to be applied to each wheel. When braking, a traction motor generates (rather than consumes) electricity, which can be dissipated in dynamic brakes (grids of high-power resistors) on the locomotive. For a single locomotive, that can provide 6 axles of dynamic braking power. For a set of 3 or 4 locomotives coming down a long mountain grade, dynamic brakes can make all the difference in the world.

Answer 2

What are you talking about.
Years ago most Diesel Trains were Diesel Electric where a Diesel Engine drove a Dynamo or Alternator which made electricity to drive one or more electric motors which drove the train. Almost all large Locomotives are of this type
Now many if not most diesel trains use multiple diesel engines spread throughout the train and these use hydraulic transmissions with a torque converter and gearbox.
However there are also ELECTRO DIESELS which have electric motors and run from the third rail and or low voltage overhead wires but also have a small diesel engine typically 600 bhp for moving at reduced speed over non electrified lines.
Class 73 electro diesels have worked in southern england for many years including pulling the Gatwick Express. they have 600 hp as a Diesel but 2550hp from the third rail

Answer 3

Electric motors are much more efficient, but there are diesel hydraulic locomotives out there.

The diesel hydraulics made a brief appearance in the US. The Southern Pacific purchased several for testing back in the early 60s, but they didn't make the grade.

Manufactured by the German firm Kraus Maffei, these hydraulics were plagued by several problems when they were tasked with handling very heavy tonnage and performed poorly. The primary problem was that the drive shafts kept breaking.

As far as efficiency is concerned, "starting tractive effort" is the name of the game, and nothing compares to an electric motor for this purpose. The transmissions were essentially the same as an automatic transmission in an automobile. This meant that, like the auto, the engine had to shift gears.

The engineer on a train must not only control speed, but the configuration of the "slack" present in the train, the goal being to keep it either "stretched" (draft) or "bunched" (buff). When shifting gears with an hydraulic transmission in a locomotive, there is a long moment when power is not being supplied and the configuration of the slack will change. If this happens to excess, then the result can be a train separation as a result of failure in some component of the train's draft gear (couplers, draw bars, end sills, etc.). A derailment can result.

In a diesel electric, "gear changes" are done electrically through switch gear, alternating between series or parallel, as the need requires, and in a very short time.

In addition, diesel hydraulics have more moving parts, which means higher maintenance costs, as well as losing horsepower to move the extra mechanical gear, a problem eleminated with electric traction motors.

And as well, there was no "dynamic brake" available. (See this question below). This was extremely problematic, especially on the SP, since that railroad had much heavy grade territiory, particularly on the "hiil" (the line over Donner Pass), where the ruling grade is 2.4%, and the Siskiyou Line, where grades reached 3.1%, the Cascade Mountains of Oregon, the Siskiyou Mountains of northern California, the Tehachapi mountains of southern California, the Modoc line, on the "Stormy" in Arizona and others.

Answer 4

There are two types of Diesel powered trains.

The first is totally powered by diesel, and can run virtually everywhere in the uk, depending on the type of station thats on the route, as some stations like Victoria, Birminham New Street & London Liverpool Street are not these day allowed diesel trains due to EU rules on diesel emmissions, so they use the Second type called Diesel-Electric trains which use a diesel moter to generate electricty.

Answer 5

They do run on diesel engines,the engine turns a generator and the generator turns electric motors at the wheels.Its an effeciant way of transferring the power of the engine rather than massive transmissions and drive train.

Answer 6

the diesel locomotive purely provides power for the electric traction motors..heating and lighting....... only slight difference is the class 73 which has a small back-up diesel engine but runs off the third rail normally, like a electric multiple unit..

Answer 7

diesel electric trains use diesel to turn electric motors , due to efficiencey.examples in the uk would be a class 159, 170 for dmu stock or class 66 locos. There were diesal hydraulic locos in the uk also but these were not as efficient or powerful.

Answer 8

Diesel Hydraulics are common in Germany. Most Diesel units are hydraulic drive as well as that offers better efficiency with the smaller engines (under about 700hp).

Answer 9

they use a diesel engine to generate the electricity to drive the very powerful electric engines. They generate enough electricity per locomotive to power a small town. You just can't do that efficiently any other way.

Answer 10

diesel engine produce electricity.