With a little fanfare Caterpillar announced last week they’re shipping a diesel engine driven generation set to electric motor hybrid drive bulldozer.  The D7E model is in the upper midsize range with the Cat C9.3 ACERT(TM) diesel engine rated at 235 net horsepower (175 kW) driving a powerful electrical generator that produces AC (alternating current).  Then the current flows through special armored cables and military-grade connectors to a solid-state inverter, that forwards the AC current to the propulsion module. It’s a series type hybrid that has clues for everyone else.

Caterpillar D7E Generator. Click image for a larger view.

Within the propulsion module are two heavy-duty electric motors using the AC current that drives through common gearing into the differential steering system. Power from the steering system is transferred via axles to the mechanical, double-reduction final drives to provide smooth, infinitely variable driving force to the tracks.

Caterpillar D7E Power Train. Click image for a larger view.

Noteworthy is that this system does not use hydraulic pumps and motors for power transitions.  Hydraulic hybrids are much more common than regular folks realize from self-propelled and riding lawn mowers, Bobcat loaders on to large farm machinery and some models of those giant mining trucks.

What Caterpillar has done is engineer the pattern set out in diesel locomotives where a diesel engine to hooked to a generator powers electric motors for the drive wheels.  The cream is in the efficiency numbers.

Caterpillar D7E Graphic Features. Click image for the largest view.

Caterpillar is claiming, and the numbers will likely be borne out by buyers is 10% to 30% lower fuel use per hour that would move at least 10% more material per hour as the blade size is larger than the engine with transmission model.  That should take overall efficiency to an average or better than 25% more work for unit of fuel.  Across the whole of the earth moving industry that would add up to a significant amount of diesel fuel.

Another benefit is the near elimination of dragging one side to steer.  Cat’s brochure claims 50% better steering performance, which in tighter working quarters where there is a lot of directional change has to be better than the 50% number.

For up time situations where dozers see a lot of continuous use, Cat points out that the electric drive model has 60% fewer moving parts.  They expect up 50% longer life from the propulsion unit and the lower track and suspension units should come in lasting 35% to 70% longer.  One could think electric drive is gentler on the tracks and suspension.

Caterplllar D7E Electric Power Train Graphic. Click image for the largest view.

Electric drive brings serious improvements to operational and life expectancy costs.  Add to that there are some things such as the transmission that need lots of lubricating oil, filters and attention, which are simply missing, saving a lot of maintenance costs.  The power train maintenance remaining has doubled time periods before attention in needed.

Cat began its effort to engineer electric hybrids drive back in the 1990s.  At brochure writing there were 18 dozers running at customer’s sites following 50,000 hours of laboratory development and test site work, so far.

The engine runs in a narrow rpm range between 1,500 and 1,800 rpm, instead of between 1,600 rpm and 2,200 rpm in the conventional D7R Series 2. Because it drives a generator instead of a powershift transmission, the D7E’s engine doesn’t need to rev as high or low to regulate speed.  The D7E produces slightly lower power than the Cat 3176 engine in the conventional D7R—however, increased drive train efficiency allows the D7E to deliver better performance from less horsepower, resulting in improved productivity and lower fuel consumption.

Current flows from the generator through special armored cables and military-grade connectors to a solid-state inverter, then to the propulsion module. Within the propulsion module are two heavy-duty electric motors that drive through common gearing into the differential steering system.

Caterpillar D7E Power Flow Block Diagram. Click image for a larger view.

Advanced electronics provide DC (direct) current to power the accessory system. The modular heating and air conditions system, water pump and battery charger are electrically powered for maximum reliability in varied conditions.  The engine has no accessory drive belts, everything is direct or electric drive.

Customer testers are saying things like, “It feels like it has the power of a D8, Never stops. Great looking dozer. Great visibility to the work area.”  The D7E sports a very spacious and comfortable cab.  Setup with a guidance system, it’s a very good finishing machine.

The customers like all the adjustable settings on the controls and arm rests, and like not shifting gears and letting the dozer do the work.  The lighting is “awesome” on this dozer when equipped with a 10 light group package.

The machine is extremely quiet and starts great in cold temperatures. The dozer and steering controls are very responsive. Track speed control is really good when going over a crest and there’s good side-to-side stability.  The electrically controlled hydraulic system is very fast and responsive.  Customers are consistently confirming the 10-30% fuel economy advantage.  In some applications, greater economy benefits are reported.  Just what those applications are is of considerable interest.

Pricing for an initial purchase for a new D7E with electric drive will be approximately 12% less than a larger class D8 and about 20% more than the conventional D7R. That would make the buying minus operating costs savings balance in a 2 ½ year payback compared to a conventional machine.  Cat would do well to try a little harder to drive down the purchase price.

The Cat Dealers are now taking orders for production that began in October 2009.
The dozer is in limited production during the ramp up, with full production capacity due in early 2010.  The conventional D7R Series II will remain available until mid 2010 suggesting that the conventional design is on its way out if the press release writer got the matter correct.

The stepping down from diesel locomotives to huge mining trucks and now the popular midsize bulldozer shows all consumers that the series hybrid can pay off handsomely for fuel and operating costs.  Here Caterpillar is showing with increasing accuracy the benefits that the series hybrid offers in performance and operating costs. Caterpillar dozers while common are by no means the kind of unit volume a popular automobile model would propose.  One should expect that a series hybrid auto could be less costly to buy than the same car with a larger motor, transmission and all the needed support equipment. Price parity should include some battery or capacitor storage and still get price parity.

Cat is also making clear that the next step can have a major impact on oil use, running expense and product lifetime.  Its been known for decades in the railroad business and now in earth moving that series hybrids are very efficient, now its tricking to smaller machines.  Series hybrid light trucks and automobiles should come soon, saving those buyers a lot of fuel and maintenance costs with better performance.

I can assure you of one thing, if this weren’t worthwhile, Caterpillar would not be offering such a machine.  Series hybrid works.


17 Comments so far

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  4. Robert Triebel on March 19, 2010 4:52 AM

    Question: why AC vs. DC generator and drive motors?

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  13. nathan on June 19, 2012 10:57 PM

    correction on on this article it is non solid state inverter it uses three transisters that modulate the three phase AC current being produced by the PM generater it is than rectified through the transister to a 650 DC bus voltage in the inverter than again pulse with modulated to the two drive motors that are linked together turning the same speed and dirrection all the time the steering is done with a conventional hydraulic diff steer pump and motor utalizing three planetay sets in the diff its cool you write about cool stuff but get your information rite

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