Electric shock

In a move that has taken the construction equipment industry by surprise, 395 Caterpillar unveiled at 404 ConExpo its first bulldozer powered by an AC electric drive. Cat said that the D7E offers "significant productivity increases and reduced owning and operating costs". Backed-up by over 100 patents, the mid-range machine will be introduced in selected markets during 2009.

"From an environmental sustainability perspective, the D7E uses substantially less fuel per hour - dramatically reducing the amount of carbon dioxide emissions," said Caterpillar Group President Stu Levenick.

The 27tonne D7E is powered by a Cat C9 engine producing 175kW. Compared to the D7R Series II, the D7E aims to deliver 10% greater productivity and 10% lower lifetime operating costs.

In the D7E power train, the diesel engine drives a generator to produce electricity that ultimately powers two AC electric drive motors, which are connected to a differential steering system. A traditional mechanical transmission is not needed, because the variable speed electric motors serve the function of a continuously variable transmission. The electric drive train has 60% fewer moving parts compared to previous D7s. The electric system also provides power to auxiliary components so that no engine belts are needed. The C9 engine, complete with ACERT Technology, meets US Tier 3/European Stage IIIA emissions standards, and the tractor has been designed with Tier 4/Stage IIIB in mind.

It is worth noting that the diesel-electric drive concept has been used in crawler machines as far back as the 1940s but that this machine benefits from the latest AC motor technology, whereas previous units have used earlier generation DC motors, which are less efficient.

The new D7E has other new features, including an exclusive centre-post cab and SystemOne low drive undercarriage, which offers 35-70% lower owning and operating costs. Other key figures include 35% more visibility and 50% less noise. Cat claimed the concept offers more efficient gains than if it were applied to excavators. However, a Cat spokesperson said it "was possible for other machines - we are just getting started".

Specification

Manufacturer: Caterpillar
Model: D7E dozer
Operating weight: 27tonnes Power: 179kW C9 engine is used to power two AC electric drives

Special features: SystemOne undercarriage to lower owning and operating costs; AC drive to reduce costs by 10%; Improved visibility from the cab; Noise lowered by 50%