At first glance, Caterpillar's new 336EH hybrid excavator could be mistaken for the non-hybrid 336E. It has the same profile as the existing machine, distinguished by only a small "hybrid" label near the rear. For operators sitting in the cab, the only sign they are running a hybrid excavator is a little green bar on the display console, which details fills and empties as the bucket swings during a work cycle.
As an operator runs it for a few work cycles, he will notice the engine rumbles at a slightly different pitch, with a bit less roar when swinging the boom back to the dig position. At the end of the day, Caterpillar expects users to notice that the excavator consumes much less diesel fuel.
Announced last year and previewed for the press near Cat's Peoria, Ill., headquarters in early April, the 336 EH hydraulic hybrid excavator captures and re-uses energy normally lost in an excavator's work cycle of digging, swinging, dumping and swinging back to dig. The machine's electronically controlled hydraulic system captures swing energy as its rotation slows down, pumping hydraulic fluid into accumulator tanks precharged with nonflammable nitrogen gas.
Then, the hydraulic system releases the fluid back out to aid in the return swing of the upper structure of the machine, allowing the engine to run at a more constant rate. This allows Cat to reduce the load on the engine, resulting in fuel savings and more manageable emissions.
"We've slowed the engine down, from 1,800 rpm to 1,500 rpm, and we've added a larger displacement pump to get the same hydraulic output," said Ken Gray, Caterpillar global project manager for large excavators. "We're now running that engine at its sweet spot."
The machine is powered by the same 300-hp, Interim Tier 4, 9-liter Cat diesel engine as the 336E, but it is running at a lower speed. Combined with the hydraulic hybrid system and an overhauled hydraulic system, the 336EH boasts a 25% improvement in fuel consumption over the 336E, according to Caterpillar's own tests. When measured in tons moved per liter of fuel consumed, the machine can get up to a 50% fuel-efficiency jump.
Caterpillar isn't the first company to make a hydraulic excavator that uses a hybrid power system to capture swing-braking energy. In 2008, Komatsu launched an electric hybrid excavator that used a capacitor to store electrical power generated during swing braking. In 2009, Caterpillar itself introduced the D7E electric hybrid bulldozer, which employed a diesel-electric power train to increase overall fuel efficiency.
However, the 336EH is said to be the first use of a hydraulic hybrid system in an excavator in which energy is stored by pressurizing hydraulic fluid in piston-style hydraulic accumulator tanks. Trash trucks that stop and start frequently also have used hydraulic hybrid systems.
At first, Cat intended to use electricity. "We had an electrical hybrid program running, which is the traditional automotive style and influence," says Gray. "But we also had this Skunk Works going on with these guys who said, 'We have a better way to do this—hydraulically.' "
According to Gray, the hydraulic hybrid system had an incremental cost increase over the non-hybrid, but that increase is about a third that of the electric hybrid system. A typical 336EH retails for $410,000, roughly a 9% increase over the regular 336E. Gray says the machine can pay this back in as little as one year, based on current fuel prices and assuming the excavator is being used in high-productivity applications.