Balancing all the various design objectives keeps engineers very busy, says Howe. What's more, the electronic controls in the new engines have increased in complexity, making it more difficult to interface with the machine's electronic controls.

Marc James, product marketing manager for compressors at Doosan Infracore Portable Power, says the complexity of engine controls is a challenge, but it comes with some positive side effects. Since engines have become more electronic, it allows manufacturers to develop the potential for better diagnostic and troubleshooting features.

Unfortunately, cleaner-burning engines are not necessarily more fuel efficient. In fact, OEMs say that between Tier I and Tier II, fuel efficiency actually went down rather significantly. Engine designers have worked hard to maintain fuel efficiency while reducing emissions, but haven't been able to improve it.

"For manufacturers, cleaner-burning engines have reduced NOx and particulates, but they run hotter and use more fuel," says Marc Leupi, product manager, utility equipment, Wacker Neuson Corp. "Then the challenge becomes how to quiet the unit but keep enough cooling air, because now we have to cool a hotter running engine in the same tight enclosure. The effect is it increases the manufacturing cost. There's a lot more complexity to the equipment, but it's the same product."

Containing noise and fluids
The way compressor and generator enclosures are designed and built has a major affect on the machines' environmental friendliness.