Diesel electric power for large motor homes and long haul trucks

if the DE system works so efficiently on locomotives why not on the aforementioned type vehicles.

Trains require a metric crapton (yes, that is a technical term) of torque, and electric motors are very good at supplying it without the headaches that come from a purely mechanical drive setup like you have in normal cars (those headaches being trying to get a transmission that is not the size of the train itself, which also will not break under the strain).

In short, your motor home does not weigh 10,000 tons, which is about what the average train weighs, and so it does not need the extra expense and bulk of the electric drive setup.

The reason, btw, that trains are so efficient isn’t because their engines are inherently efficient - they’re not. They get around 18mpg. The efficiency comes from hauling so much stuff - 18mpg is pretty good when you’re hauling 10,000 tons of stuff around. You’d need 50,000 Honda Insights to haul that much weight around, so even though the Insight gets great fuel economy, you end up using more fuel because of how many you have to have to carry your load.

Shadowfax–Do you mean 18 miles per gallon or 18 gallons per mile?

Well, since train fuel efficiency is measured in either gallons per hour, or miles per gallon per ton it’s kind of hard to say. But if you take the railroad advertising number at face value (436mpg) and realize that it’s being measured in mpg per ton, it works out to around 22mpg for the whole train assuming a 10,000 ton train. That’s still fairly unrealistic as that number was doubtless measured on a train that was already up to speed and just cruising, so I penalized them a few mpgs to account for the starting from a standstill fuel burn, and to account for the fact that some of the train’s weight is not cargo, but cars and engines.

It should also be noted that this is assuming a freight train. Passenger trains are much less fuel efficient, especially in the US, because they have to make a lot of stops (the US moreso because Amtrak doesn’t own any track except for the Northeast corridor, and so it has to stop to yield to any freight trains that come along).

“But if you take the railroad advertising number at face value (436mpg) and realize that it’s being measured in mpg per ton, it works out to around 22mpg for the whole train assuming a 10,000 ton train.”

How do you get 22MPG?

436 Miles per gallon per ton is: 436 [miles/(gallons*tons)]. If a train can move 1 ton, 436 miles, on a gallon of fuel, it can also move 436 tons, 1 mile, on that gallon…or 10,000 tons, 0.0436 miles on a gallon.

(Note that 0.0436 MPG is roughly 1/22 mpg (or 22gpm), which is where it seems the discrepancy arises.)

Trains have steel wheels that roll on steel rails. The grades are usually approximately 1%. The engine deflects the air and the mile of freight cars in tow are in the draft. It seems trains have quite a few advantages over automobiles re fuel use/ton mile.


No, what they’re doing is saying that the train gets 22mpg (or something. I can’t be completely certain of what their original number is because I can’t be completely certain what the weight of the train they actually measured was, assuming they measured a real train at all and not some made up statistical average train). Then they’re multiplying it out by how many tons the train is carrying. Or put another way, they’re counting up the tons and then dividing that number by the train’s actual mileage.

In short, it’s BS marketing math (because now they can tell people who want to ship goods that they get 436mpg and won’t THAT save you money!), but knowing what formula they’re using lets us get in the neighborhood of the train’s actual mileage by dividing the number of tons (10,000, which is the high side of the average weight of a freight train - it stands to reason that they’re going to use the heavier number) by their claimed mileage per ton (436).

Passenger trains get around 1.2 to 2 mpg. There is no way a freight train will get less than a passenger train. Once a freight train is up to speed, it stays there for a long time while the passenger train gets on the side track and stops to stay out of the freight’s way. And the freight isn’t stopping in every town on the route.

So why isn’t diesel electric used in 18 wheelers? (back to the original topic)

I work on generators and motors a lot and I can testify to how much these things weigh. Typically, the generator on a genset weighs as much or more than the engine that drives it, even though the generator is physically smaller than the engine, so, having the engine of a truck power the wheels through a generator-motor transmission system amounts to having the weight of a second engine on that tractor-trailer rig.
In the U.S., 18 wheelers are legally limited to 80,000 pounds GVW and adding three or four thousand pounds to the weight of an empty 18 wheeler amounts to three or four thousand pounds of reduced payload.
On a locomotive, the weight of the generator and traction motors doesn’t matter, in fact, locomotives need to be heavy in order to have enough traction with steel wheels on steel rails, to pull a long train.
Diesel electric systems are used on large off highway mining trucks, some which carry a payload equal to five fully loaded on road 18 wheelers.

Generator/motor systems are also common on draglines, where a synchronous 3phase electric motor drives a DC generator which powers the DC bucket motor. High speed elevators in skyscrapers also make extensive use of 3phase motor driving a generator which powers a DC hoist motor.

Copper is at $3.40/pound, something like that…When you start talking millions of units, there is simply not enough copper. These drives are NOT particularly efficient…A lot of heat is generated and wasted…But they provide the kind of power delivery a locomotive must have: 100% torque at zero RPM…No transmission, no clutch and hopefully no wheel-spin…Locomotives need all the weight they can hang on them for traction…Over-the-road-trucks want PAYLOAD, not dead weight…Trucks are built as lightweight as possible…

DE was devised to eliminate the transmission, which given the size of a train, would then be the biggest and least durable component. I believe this will be the way of the future, but, efficiency for now still is on the side of the direct diesel drive. But soon.

A diesel/generator powered 18 wheeler is feasible. The diesel motor would run at a constant speed and would therefore be efficient. The generator would provide power to 2, 4, or 6 motors that could be mounted in the wheels or on the axles. Using a number of motors keeps them relatively small and lightweight. The configuration of the truck body should not need to be changed and it could haul standard trailers.

Batteries could be added to the system to capture power from “braking” and for extra power for acceleration and hills, but it could be done without batteries to save weight and space. Some truck maker somewhere must be working on fuel efficient trucks and testing out such a system. A fuel efficient truck should be a big seller.

You have to make the cost / benefit equation work…Trucks are tools used to make money, not spend money on advanced technology that offers little or no improvement in the bottom line…

The low rolling resistance of steel wheels rolling on steel rails enables the most efficiency. (Why I keep my tires almost at Maximum pressure.)
One object,hengine, pushes through the air and the cars behind follow in its and each others draft. Wonder how much the wind resistance wouldecrease if the railcars had skirts almosto the rails?
How much loss occurs between the dieselectric generator (now alternator) andrive motors?

The losses show up as waste heat…Locomotives produce a lot of that…

Electric motors that size typically achieve 90 to 95 percent efficiencies. Ninety percent is fairly easy, to reach 95%, you even have to pay attention to bearing and oil seal drag and cooling fan design.
If you have a 90 percent efficient generator powering 90 percent efficient motors, the system efficiency is .9 X .9 = .81 or 81 percent.

Much of the credit for the ton-mile efficiency of railroads needs to be given to the fact that trains are huge. When you double the dimensions of an object, you quadruple it’s frontal area, surface area, but you increase the internal volume by a factor of eight. This scaling effect is why a large ship can cross the ocean and still have room for a huge payload whereas a small skiff with an outboard motor on it probably couldn’t cross the ocean even if it was loaded with nothing but fuel.

I have heard that the latest generation of passenger jets are achieving close to 80 passenger-miles per gallon, while flying at over 500 mph. This is possible mostly because of scaling effect.

It’s interesting to watch the coal trains heading south from Wyoming to Texas…South of Denver, the land begins to rise, a long pull to the top of the Palmer Divide…After that, Raton Pass must be negotiated…Typically, 6 of the biggest locomotives made are needed to accomplish this, speeds falling to 20-25mph, thousands of gallons of fuel being burned…This happens 7 or 8 times a day, everyday, 120 cars each loaded with 100 tons of coal…

Wouldn’t it be a lot easier just to build all the power-plants in Wyoming, right next to the mines and just send the electricity to Texas instead of the coal??

That has kept me puzzled also, Caddyman. Coal in West Virgina and Pennsylvania is shipped to coal fired boilers all across the east when it could be put on the power grid much cheaper from plants near the mines. Common sense seems to go out the window when politicians walk into their offices.

The politicians in Wyoming and West Virgina would have to convince their constituents that the ground pollution and air pollution all concentrated in one area is OK.
And who would want to live near all those transmission lines?

Lots of electrical losses over long distances, so trains are better.

Diesel engines used in heavy trucks are about 50% efficient – that is, half the fuel comes out as heat, half as motive power. For an over-the-road truck, they drive a gear transmission that is better than 95% efficient. A motor-generator set such as used in trains is less efficient than that. Recent city transit buses use an automatic transmission that is much less than 90% efficient plus a motor-generator set and energy storage. They do that because stop-and-go driving with a manual is hard on driver and transmission and a small engine plus energy storage plus conversion losses is still more efficient than a big engine and no energy storage. For trains, the motor generator set is not as efficient as a gearbox, but allows in effect very very low gears. That is important because all the motors in a locomotive are still tiny compared to the load – you might have 10x the horsepower of a heavy truck but 100x or more the load. A big motor-generator set is heavier than a gearbox, but even with a motor generator set they still add ballast to the locomotive to increase traction, so the weight is not an issue like it is for a motorhome, truck, or bus.