The truck engine powers the compressor, usually through a serpentine belt, so there is no additional engine to maintain or fuel required.
"In modern under-hood compressor systems, the engine speed is regulated directly proportional to air need. This saves both fuel and engine wear and cuts down noise and exhaust emissions," Hutchinson points out. "Advanced under-hood technology scheduled to be released later this year will even control the truck engine by turning it off when air is not being used, and then automatically restarting it when air is used again. This will reduce truck engine idling time, ultimately improving fuel economy, decreasing engine operating hours, lowering truck emissions, reducing noise levels and cutting life-cycle costs."
The units are also more protected from external conditions. "Deck-mounted systems are not shielded from the elements," says Hutchinson.
Under-deck systems provide greater capacity. "Under-deck systems can usually provide more air, but there can be challenges mounting the systems. There are typically no truck-specific kits and universal kits often need fabrication," Hutchinson notes. "Also, under decks tie up a PTO port that may be required for other uses. Finally, under-deck compressors are typically for medium-duty trucks, whereas under-hood systems are used on smaller trucks."
Today's engine designs further complicate mounting. "The drawback of under-chassis or under-hood compressors is the complex engine compartments with the Tier IV emissions. It creates less space and it is a little harder to mount everything," notes Durfur. "With the Tier IV regulations for trucks, the majority of the under-chassis area has been taken just with the aftertreatment controls. So the advantage of above deck is that nothing above deck has really changed.
"We are focused on above-the-deck compressors because that is where we believe everything is going to go in the future," he adds. "No matter how much the engine changes impact the under chassis or under hood, we will be able to service that truck with our compressor."
Noise can also be a factor. "[Under-hood compressors] run at a very high engine speed, usually 2,100 to 2,300 rpm," says Kokot. "It is going to be a lot louder than an under-deck or hydraulically-driven unit that runs at close to half those rpms."
Sorting out airend choices
"Rotary screw compressors and reciprocating piston compressors each have unique benefits," says Worman. "More contractors are turning to rotary screw compressors because they are ideal for jobs that require a continuous supply of air on demand. These units are compact, lighter in weight than reciprocating piston compressors and are designed to run for extended periods of time, making them more efficient."
Although rotary screw compressors don't require a reservoir tank, IMT recommends utilizing a tank to help dissipate heat and remove moisture from the air. "In addition, a reservoir tank provides contractors with a quick burst of air without the need to power up the compressor," Worman explains.
"Rotary screw compressors are not designed for short, intermittent job needs. For those applications, a reciprocating compressor is the best option," advises Worman. "Reciprocating piston compressors are more forgiving and aren't as susceptible to moisture or particulate contamination. They are also about 50% cheaper than a rotary screw unit when total system costs - including the unit itself, auxiliary hydraulic cooler and larger hydraulic oil reservoir - are figured in."
"The disadvantage [of a rotary screw compressor] is that it needs to run longer to maintain some heat," says Dufur. "If you don't run it longer - 30 minutes to an hour - it is going to be less efficient and maybe have some oil carryover. In that application, you want to pursue more of a reciprocating type that cycles on and off for lower demand. You are going to build air from the compressor and store it in the receiver tank. It is allowed to cycle on and off as you draw that air."