Keeping it cool

Today’s HVAC systems keep passengers cool and keep operators efficient

When operating a motorcoach, one of the major concerns is passenger comfort, and keeping the interior temperature just right. That’s not an easy thing to accomplish when the sun is shining into a bus’s large windows, heating up the air and the inside surfaces.

“Passenger comfort is a major challenge,” said Natalie Van Dyke, transport sales manager–North America, for Bitzer US Inc. “Every motorcoach operator understands if a bus has an interior temperature of 110 degrees on a hot day, you’ll need to cool it down to 80 or lower for a passenger’s comfort…that’s 30 degrees of cooling. That’s a lot. The coach needs an HVAC system design and a compressor capacity that have the ability to do that.”

The AC compressor is a major component of the HVAC system that keeps the inside of a motorcoach cool, and Bitzer Inc., located in Flowery Branch, Georgia,  manufactures the heavy duty piston, scroll and electric transport compressors found in most modern motorcoaches. “If you think of the HVAC system of a bus as the circulatory system of a body,” Van Dyke says, “then the compressor would be considered the ‘heart’ of that body.  It does the heavy lifting to pump fluid throughout the body to keep it operating.”

While the basic air conditioning system design of many coach manufacturers and OEM HVAC system manufacturers has changed little within the last 10 years, improvements have been developed and introduced by some component manufacturers like Bitzer. Exciting opportunities for more efficient cooling, and a reduction in the power draw needed to achieve those cooler temperatures passengers expect, are becoming more widely available to motorcoach customers all the time.


System efficiencies

By many estimates, the HVAC system on a bus or motorcoach can represent as much as 30 percent of the overall power consumption needed to operate the vehicle. This energy consumption is commonly referred to as “parasitic load,” and reducing it can translate directly into lower fuel consumption and therefore lower operating costs for coach owners.  One way HVAC system efficiency can be improved is by adopting an entirely electric system.

“An electric HVAC system does not rely solely on diesel engine operation, so expensive idling time can be greatly decreased,” said Van Dyke.

Most of the currently available electric air conditioning systems are fully contained rooftop units, and these have not yet been widely adopted by North American motorcoach manufacturers.

“That would be the biggest shift potentially happening.  Adaptation away from a pulley-driven reciprocating compressor driven by the engine to a fully electric rooftop system that has an electric compressor installed right next to it, and with no immediate integration to the engine compartment,” said Van Dyke. “That’s a more limited part of the market, but it is happening and available.”

Another, more commonly available way of improving efficiency and reducing power consumption, is by utilizing unloading and pressure actuated blocked suction unloading technology with a traditional reciprocating compressor.

“That means the compressor itself may have a six-cylinder capacity, and when I say cylinders, the compressor has pistons very similar to an engine, but there’s no combustion,” Van Dyke said. “Once you have achieved the desired interior temperature, the point of passenger of comfort, you don’t necessarily have to continue to expend the power to run all six cylinders.”

She said an intelligent system will sense that the major “pull-down,” a big reduction in temperature, has taken place, and it will do what is called “unloading” and run on four cylinders or fewer.

“You’re only going to utilize the power that is necessary in that instance,” said Van Dyke. “A climate control system with a compressor that has unloading, such as the Bitzer, maximizes opportunities for efficiency, so only the power resources necessary at a given point in time are used by the system.

“We are seeing more and more excitement from end users who understand how these efficiencies benefit them.” shared Van Dyke. “Motorcoach manufacturers have been on board for some time, but many end users only recently understand the efficiency differential that exists between compressors with unloading and those without unloading.”



Beyond the changes in efficiency/power, Van Dyke said changes are coming from the Environmental Protection Agency (EPA), which makes it important for operators to have a clear understanding of refrigerants.

“These EPA changes coming in the next few years will necessitate [Green House Gas–GHG] compliance on refrigeration,” she said. “Even though older buses may be utilizing older refrigerants, it’s important to know that as the market shifts, those older refrigerants, such as 407C, which are being discontinued, are going to become more and more expensive for replacement purposes, driving up maintenance costs.

“Understanding the type of refrigerant utilized in a climate control system at the time of a new bus acquisition is a very good consideration going forward, as the vehicle’s cost of ownership stretches well beyond the date of purchase.”


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