Diesel-Electric Trucks

Thursday, May 28th, 2009

A few years ago I assumed that Diesel locomotives were just like Diesel trucks, only larger and on rails. Then I learned that they were in fact Diesel-electrics, using the Diesel engine to generate electricity for electric motors — which eliminates the need for a transmission:

Your car needs a transmission because of the physics of the gasoline engine. First, any engine has a redline — a maximum rpm (revolutions per minute) value above which the engine cannot go without exploding. Second, if you have read How Horsepower Works, then you know that engines have a narrow rpm range where horsepower and torque are at their maximum. For example, an engine might produce its maximum horsepower between 5,200 and 5,500 rpm. The transmission allows the gear ratio between the engine and the drive wheels to change as the car speeds up and slows down. You shift gears so that the engine can stay below the redline and near the rpm band of its best performance (maximum power).

The five- or six-speed transmission on most cars allows them to go 110 mph (177 kph) or faster with an engine-speed range of 500 to 6,000 rpm. The engine on our diesel locomotive has a much smaller speed range. Its idle speed is around 269 rpm, and its maximum speed is only 904 rpm. With a speed range like this, a locomotive would need 20 or 30 gears to make it up to 110 mph (177 kph).

A gearbox like this would be huge (it would have to handle 3,200 horsepower), complicated and inefficient. It would also have to provide power to four sets of wheels, which would add to the complexity.

By going with a hybrid setup, the main diesel engine can run at a constant speed, turning an electrical generator. The generator sends electrical power to a traction motor at each axle, which powers the wheels. The traction motors can produce adequate torque at any speed, from a full stop to 110 mph (177 kph), without needing to change gears.

So, if Diesel locomotives are in fact Diesel-electric, why aren’t Diesel trucks? Well, we’re starting to see some. Oshkosh, for instance, has developed its ProPulse Diesel-electric system, which uses an ultra-capacitor, for the military’s new HEMTT A3 tactical trucks:

ProPulse increases fuel economy up to 40 percent because the diesel engine in the system continuously runs at optimal rpm — significantly more efficient than the maximum rpm usually needed for highway speeds and full payloads. Emissions are also reduced, meeting current and proposed EPA requirements. And the on-board generator offers enough electrical output to power a residential block, airfield, hospital,commandcenter, communications equipment and much more.

There are no batteries to maintain or replace. Less fuel is needed over the life of the truck. The full electrical system lowers torque throughout the drive train, reducing wear and tear on the truck. And the modular design requires fewer spare parts. All of which increase the value of the vehicle by reducing the life-cycle costs.

Propulse allows for a smaller logistics footprint, increasing the mobility and deployability of fighting forces. Lower fuel requirements mean less fuel to transport, while on-board power eliminates the need for cumbersome generators to power field operations.