Locomotives and Diesel/Electric Engines

Recently I have been playing a game called Railroad Tycoon II on my PC. If you liked playing with trains as a kid (or adult) and like economic simulations this game is worth a look.

Anyway, while reading through the manual it mentioned (in its historical tidbits) that diesel train locomotives are actually diesel-electric engines. Basically, a diesel engine turns a generator producing electricity for the electric engine which actually moves the train.

  1. Is this really the case?

  2. If it is the case why go to all the trouble and extra expense of essentially two engines? Why not just have the diesel engine move the train?

  3. If the diesel engine is only there to turn a generator how come you see locomotives huffing and puffind as they try to accelerate with a heavy load? You’d think that the engine would spin up the generator and then work at a constant rate after that. The strain of accelerating the train wouldn’t transfer through the electric motor to the generator to the diesel engine (would it?).

Note: I’m not an engineer and I’m sure if this is how locomotives are built then there is probably a very good reason for it. I’m just curious as to what that reason is.

IANARE, but, in general, you are correct in your estimate of their operation.

The engines spin the generators. The electrical load of the generator is dependant of the load on the motors. There is most likely a governor that adjusts the engines power automatically based on demand.

Did you ever notice on an automoble that if it’s idleing and you turn on your headlight and heater blower, the engine speed will vary?

If you attempted to drive the train directly with the engine, you’d have to have some way to couple the power to the driving wheels. Four on the florr with a really big clutch.

Electric power can be easily controlled. Plus on downgrades, the motors act as generators themselves and the power is dumped into big resistors. This provides a braking force without having to actually apply the brakes.

I believe the diesel driven generators also provide the electrical power required for the rest of the trains operation. Someone will correct me if I’m wrong, but passanger cars used to have their own generators on each car, driven by the rolling cars wheels.

Reminder to self: Check spelling!

The generator/electric motor set-up acts as a continuously
variable transmission, extracting the precise amount of torque and horsepower from the diesel needed to drive the traction motors. And you do away with the clutch.
Highly efficient, and mechanically simple.
BTW, a similar arraingement was created by chrysler to power a prototype dodge intrepid-a three cylinder diesel getting about 70mpg

Yes, it would. More power required = more power produced. The engine speed goes up, it spins the generators faster, and makes more electricity, so the electric motor can turn faster. The “car accessories” example was good, but a better one (indeed, the exact same thing) is: ever been around an arc-welding rig with its own generator? It idles slowly, then the welder starts welding (basically a dead short–lots o’ juice going through), and that little engine running the generator screams.

The main reason is the elimination of needing a clutch and gearbox.

…of course they beg another question (one already touched on by enolancooper).

If this is indeed such an efficient setup why aren’t semi-trucks or even cars built this way? Enolancooper suggested that Chrysler already did build such a car with fantastic gas mileage.

Do electric motors have crappy acceleration characteristics that make them undesirable in cars?

Would such a setup be very expensive in a car? If it is wouldn’t that cost be more than offset in fuel savings in the long run?

Is maintaining a combustion/electric engine just too expensive or too much of a hassle to be viable in an automobile?

Both Honda and Toyota do sell a car in the States that is built this way. The Honda Insight(sp) and the Toyota Prius.
I heard somewhere (sorry, no cite) that Honda sells theirs at a loss. And I believe the Insight costs more than a Civic, it’s internal-combustion counterpart, implying that it is more expensive to build a hybred car than a strictly internal-combustion car of the same size.
And you know that the car companies don’t care as much about your cost of operation as they do about their cost to build a car.
I don’t know anything about their relative performance or maintenance costs, BTW.

The Prius and Insight aren’t quite the same principle as a diesel-electric locomotive. In the locomotive, the generator and motors are just a means of transferring and controlling power from the engine to the drive wheels. The Prius and Insight are hybrid-drive vehicles, the gasoline engine and the electric motor are both connected to the drive system and work together when they have to.

I think it breaks down like this. In a regular car, you’ve got an engine that makes something like 150 horsepower, but you almost never need that much. Cruising down the freeway may take only 50 horsepower. That 150hp engine is a lot heavier than a 50hp, so you’re carrying a lot of weight for something that you don’t use very often. A hybrid car stores energy when it can (the electric motor becomes a generator and charges on-board batteries), and uses it when it has to (like accelerating to get on the freeway). But you can size the engine for average need rather than maximum need. If my numbers above are in the right ballpark, you might be able to use just a 60hp engine. Smaller engine, lighter weight (as long as the electric system doesn’t add more weight than you save), better mileage.

And because these cars are designed for maximum efficiency, the designers have added a few more tricks while they were at it. The computer that controls the drive system actually shuts off the engine when you come to a stop. When you press the accelerator again, the electric motor gets you rolling again while the computer restarts the gasoline engine.

Diesel engines generate the most torque at high RPM. Electric motors generate the most torque at rest.

Bizerta’s finally on the right track… (Sorry, couldn’t resist.)

The primary reason electric motors are used is for their enormous torque starting at zero shaft speed and up.

While internal-combustion engines must, by design, be already turning to provide usable power, the electric motor provides full torque from rev one. In the case of a train engine, a direct-driven diesel would have to have a massive clutch or torque-converter assembly, which would allow the engine power to “slip” in relation to the travel speed. And, typically, it would have to slip for upwards of an hour, until the train was travelling fast enough to allow a direct engine linkup.

With the electrics, the diesel generator can run at a steady speed, and the electric motors can grunt n’ strain even at only two or ten or 100 rpm for however long it takes to get everything up to cruising speed.

By the way- the huge “crawler” that drives the Shuttle from the maintenence building out to the launchpad, is also driven this way: huge diesel generators driving truly massive electric motors.

I think some of the smaller shunting locomotives used in freight yards are diesel-hydraulic (still eliminating the clutch problem, I guess.

Also, in reply to an earlier post, maybe large trucks can’t be diesel-electric because they need a clutch/gearbox set-up for safety reasons - assisting the brakes on hills, etc.

Note to self: “PREVIEW, my good man!” Here, have a ) .

A quick visit to the Honda and Toyota websites reveals that Robot Arm is indeed correct. I bow humbly in your general direction.
Incidentally, I got a look at the Prius at the Portland auto show recently and, except for the fact that Toyota still hasn’t figured out the concept of ‘leg room’, I was impressed.

  • if I might venture a guess, weight is part of the problem. For a locomotive, additional weight might actually be an advantage - it’s certainly not a major problem.

OTOH, for trucks & cars, weight is much more of a concern.

Can someone confirm / rip me to shreds on this ?

S. Norman

If the electric engine worked as a generator that could probably work as an engine brake. Large trucks would seem to be the next step if this were an efficient system.

There are bound to be some losses when converting diesel power to electricity, then electric power to motion. Since trucks tend to run near max output for long periods of time, perhaps a ‘direct’ system is more efficient.

Perhaps this diesel-electric system does not scale down well?

Could the diesel-electric system be used with the stirling engine? You could run on battery power until the stirling engine warmed up (Its major disadvantage).

On preview, maybe Spineys on the right track, weight or space.

I remember hearing of a turbine-electric locomotive that was tested for a while. Turbines have some properties that make them tricky to use in a direct drive system, slow to respond to throttle changes, things like that. By hooking it up to a generator, you can run the turbine in its proper operating range and still control power to the drive wheels. Great idea, right? Well, it wasn’t popular with the people who lived near the train tracks. It sounded like a 747 taking off in their backyards.

Almost bought one :slight_smile:

The way it was explained to me is this:

The system does not “tag team” the two motors for high load situations a computer controlled transmission switches between the two but not both.

any time the brakes are applied a “cluch” type mechanism engages first, turning the electric motor in “generator” mode charging the batteries. The load of turning the motor applies braking force, if extra braking force is needed the regular brakes engage.
Once the batteries reach a certain charge, the gas engine shuts down and you drive on electrics for a while.

Batteries get low, gas engine restarts.

Braking is the only time that power is generated, it wouldnt be as efficent to charge the batteries off of an alternator when you could use the cars momentum to generate power.

This combination is why they get better gas mileage in city than freeway, since you dont brake much on the freeway.

At least where I live :smiley:

In response to the hybrid power tractor trailers IANAE but I would imagine that long haul trucks would not be as effective due to the fact that they dont use their brakes much crusing on the freeway. Unless, photovoltaic arrays on the roof for the trailers? Nah.
Smaller in town delivery trucks OTOH I could see benefiting immensely from hybrid power.

As to the OP hmm size maybe?
How much does a train weigh relative to the mass of its locomotive compared to a tractor trailer?

Maybe its overkill/too expensive for the size of the job?

Trains don’t have to stop quickly and or be able to manuver in traffic. Better acceleration in deisels vs deisel electrics?

The Insight has to use both motors at once, because the electric motor just isn’t powerful enough to power the car alone.

I guess tben, you’re referring to the prius. The prius web site (toyota.com/prius ?) talks about how it operates. IIRC, it starts with the electric, then when it gets to a certain speed (around 20mph) , it turns the electric off and switches to gas. When it reaches “highway speeds” it is using both gas and electric.

how stuff works did an article on hybrid cars. If I recall, It said that normal engines waste gas because when you’re crusing at freeway speeds, you’re only using about 10% of the engine’s power. You only need the other 90% when accelerating.

The hybrid cars use less gas because they have a much smaller gas engine. The engine works great for crusing, but it needs help accelerating, so that’s where the electric engine comes in.
Eh, who cares. I would take a viper anyday over one of those hybrid cars…

>>I remember hearing of a turbine-electric locomotive that was tested for a while.<<

In the '50’s and '60’s, numerous gas-turbine-powered freight locomotives were tried in regular service, most notably by Union Pacific. The turbine locos provided far higher horsepower per unit than diesel-electrics of the time, but were ultimately judged unsuccessful, primarily for their high fuel consumption regardless of load.

The French railways, Canadian National (and later VIA) and Amtrak in the USA, have all operated turbine-powered passenger trains. The United Aircraft turbos operated mainly in Canada had severe reliability problems; the French-designed turbos (also used by Amtrak) were better. Unfortunately, these trains were hampered somewhat operationally because they were constructed as fixed consists, without the ability to add or reduce passenger seating according to demand, and in the USA these trains have breen withdrawn from service. Recently, however, Amtrak has commenced the refurbishing of some of their turbos to ad capacity on the Hudson corridorin NY state, and they may be back in service this year.