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MTU Introduces Prototype Underfloor Hybrid Drive Unit for Railcars

The underfloor hybrid drive unit. Click to enlarge.

At this year’s Innotrans rail exhibition in Berlin (23-28 September), the Tognum Group introduced under the MTU brand a prototype of a new underfloor hybrid drive system for railcars. MTU estimates that fuel consumption and carbon-dioxide emissions can be reduced by up to 25% with the hybrid drives, depending on the route.

The hybrid drive combines a MTU Series 1800 six-cylinder inline diesel with a motor/generator unit and energy storage system; electrical output of the unit is 400 kW. The generator will recover kinetic energy during braking which can be used during acceleration and in stop-and-go operation.

The Series 1800 six-cylinder inline engine with generator. Click to enlarge.

Depending on how much power is required by the train driver, the railcar is propelled either solely by electrical power or by a combination of diesel engine and electric motor. In railway stations or in urban areas, this allows trains to operate with low noise and low emissions. Hybrid drive is particularly economical on local transport routes with frequent braking and acceleration, because this results in many charging and discharging cycles.

The drive unit is based on the existing PowerPack 6H 1800, an underfloor diesel-powered motor/generator that delivers maximum power output of 390 kW. MTU developed the engine/generator unit jointly with a system partner.

In addition the hybrid power pack includes SCR exhaust-gas after treatment with a urea tank; this is a prototype that fulfills the emission regulations applicable to Stage IIIB non-road diesel vehicles as of 2012.

The underfloor drive modules could be used to upgrade existing diesel-powered trains as well as in new construction.


Rafael Seidl

I realize this was designed to be suitable for retrofits, but why choose a serial over a parallel hybrid for this type of application (short, lightweight passenger trains plying non-electrified corridors)? With particulate filters and SCR, a parallel hybrid drivetrain would be clean enough while providing more peak power and better fuel economy.

Either type could be modified by adding a pantograph or third rail pickup to take advantage of grid connectivity where available on partially electrified lines.

Note also that the rated power of 400kW (535bhp) is modest compared to US train propulsion systems for two reasons:

1) The FRA (Federal Rail Administration) is focused primarily on passive safety, i.e. surviving crashes. This suits private freight operators but burdens passenger services with high train weight and fuel costs.

The rest of the world is focused primarily on active safety, i.e. avoiding crashes through advanced signaling, positive train control, track surveillance etc. (as appropriate). This suits state-owned railroads that must prioritize passenger service.

2) DMU (diesel multiple unit) designs eliminate the traditional locomotive in favor of self-propelled cars with coordinated traction management. The ones at the ends feature a driver cab to facilitate direction reversal at the end points of single-track lines. A typical DMU train would therefore feature at least two small diesel engines.


How did you figure that this is serial arrangement? This news says tehre is a motor/generator attached to ICE, so it's suggesting parallel hybrid.

fred schumacher

This unit could be used today to completely redesign tractor-trailer truck morphology. There are two components:

1) serial diesel hybrid mounted underfloor; no mechanical transmission; hubmotors in every axle, tractor and trailer, making every axle a driver.

2) reduce total frontal area by lowering the height of the whole rig through taking advantage of the box beam nature of a shipping container, eliminating the need for a separate I-beam trailer frame, and placing the trailer wheels in a trailing position rather than underneath the trailer.

This allows for diesel-electric drive advantages of high electric motor torque and dynamic braking (no need for jake brakes), and the reduction of frontal area by about 25 square feet, greatly improving aerodynamics. The trailer would run less than a foot off the pavement and would be raised to dock height with hydraulics. There are issues dealing with bridge loadings that would need to be resolved.

Such a redesigned tractor trailer should be able to see a doubling in fuel economy.

Henry Gibson

For a much simpler and cheaper version of a hybrid rail vehicle see the "Parry People Mover" with engine, hydraulic transmission and large slow flywheel. In a series hybrid with battery, the engine can be a much more efficient and smaller engine that usually runs at its most efficient speed or not at all. The intent is to have it behave much like the standard electro-diesel that runs on electricity alone if there is a third rail or catenary.

A Capstone microturbine or more would have sufficient efficiency for this application with less maintenance costs. Someday Capstone might get their purchase costs of a one moving part machine down to the costs of a thousand moving parts engine. A ZEBRA battery would make this a very flexible car that could go even a hundred miles beyond the catenary without starting the engine. The price is much to high right now for ZEBRA batteries for long distances. ..HG..

Rafael Seidl

@ yaross -

I don't see a driveshaft or transmission anywhere. A serial hybrid system can use the final drive motors as generators when braking.

But yes, the press release isn't really all that clear on what type of hybrid architecture this is.

@ Henry Gibson -

diesel efficiency: up to 45%
microturbine efficiency: around 25%

Gas turbines have many advantages once you get to the multi-megawatt scale. For small powerplants, they are suboptimal unless you have a very low cost source of hydrocarbon fuel.


Series hybrid units are interesting because they could be built to use overhead electric power lines where and when available.

As rail lines are progressively electrified, (within cities and towns) the ICE genset would be used less thereby reducing GHG, noise and fuel consumption.


I'm not sure why, but heavy diesel-electric trains are, and have always been, series electric drive. The diesel engine turns a generator, and the wheels are driven by electric motors. They are more like axle motors than hub motors, though, since trains don't have differentials or steering. I'd guess that there were difficulties in achieving direct mechanical power transmission and gearing across all the drive axles in the locomotive. Since the wheels are steel, you need all all the wheels to be driven so you can get some traction. As it is, they still have systems to spray sand on the track for extra traction when they are climbing mountain grades.

Something tells me that even if they could achieve power transmission, gear reduction, and a suitable range of gear ratios in order to make it a parallel hybrid, the power transmission equipment would take up too much space, and have lubrication problems. The gears wouldn't be turning that fast, but the forces put on them would be huge. It would be tough to find a suitable lubricant. Oh, yes, don't forget that all this gearing and drive mechanism would still need reverse gearing too!

Plus when you need to start moving a multi-million-pound load, from a dead stop, the 0-RPM torque of electric motors is unbeatable. Now to translate this to light rail, some of the concerns are different- the weight and traction concerns are reduced- but you still have the issues of needing to achieve direct power drive across several axles. The cost and complexity of power distribution, gear reduction, and the range of gearing needed, would add up in a serious way if you had to put it in every car. (Has direct drive from an ICE ever been attempted in a rail application at all?) And as we always say, series has advantages in applications that do a lot of starting and stopping relative to the amount of high speed cruising.

Interesting thought exercise one way or another.


@Wes & Rafael,

Actually, diesel locomotives haven't always been diesel-electric. In Germany, almost all diesel locomotives during the 60's and 70's were diesel-hydraulic, built by Krauss-Maffai. Something like a torque-converter/hydraulic coupling connected the engine to the wheels with driveshafts and gears, essentially a one-speed automatic transmission. These were ubiquitious in Germany, and exported around the world. Some were tested for an extended period of time in the US (D&RGW, and others). I remember that the advantage was some greter degree of efficiency, which was of particular importance in Europe. American railroads have always put greater emphasis on minimum servicing and maximum hands-off reliability, and this is where the test of the German locos fell short.

But diesel-electric/hybrids like the one described above undoubtedly are more efficient than a straight diesel electric, and avoid the challenges of a mechanical drive like Rafael suggests. But perhaps mechanical drive of some sort will come back, because it is more efficient. Controlling traction and slippage would be quite a challenge, though.

The best part of this is passenger comfort !
When was in Scotland last year, most of the trains had diesel motors under each car that rattled even when stopped at stations. For someone who grew up with smooth electric trains, this ruins the train experience.

Hybrids rule! (at least until we go to all electric)

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