In 2010, new engines must comply with lower emissions standards. Engine manufacturers have added diesel particulate filters (DPF) to buses and R-22 refrigerant will be banned for use in new equipment.
“In general, the trend has been toward roof-mounted units, and the 2010 emissions regulations, which require more equipment in the rear of the bus, are increasing the need for these systems,” says Fil MacNeil, Systems Group Leader for New Flyer Industries.
“Roof-mounted units provide many benefits: better weight balance for improved ride quality, lower noise levels for passenger comfort and the additional real estate on the roof provides more flexibility when deciding on the size and capacity of units.”
Steve Johnson, bus HVAC product manager with Thermo King, says that while many are aware of these changes, they may not be aware that the configuration and location of the HVAC unit could be affected.
“In the United States, the most commonly used HVAC in the transit industry is mounted at the rear of the bus above the engine,” he says. “As bus OEMs are forced to find space to accommodate the DPF, they are re-arranging the components in the rear of the bus. The available space for the traditional rear-mount HVAC unit is being reduced or eliminated. The robust, one-piece, compact design is the most reliable configuration available today and is easy to install and service. Will it go away permanently? As of now we don’t know, but we do know that some type of change is inevitable.”
Roof-mounted units are what the industry is seeing more of. “This is not a novel concept,” says Johnson. “Some U.S. transit properties already have experience with roof-mount HVAC, particularly on articulated buses where two cabins have to be cooled. Also, in all markets outside of the United States, roof-mount HVAC is the standard configuration.” He adds, “For the most part, the rear-mount HVAC is preferred in the transit industry, and in fact, more than 80 percent of what Thermo King supplies has been rear-mount over the years.”
Johnson explains how the roof-mount systems are installed. Holes are cut in the roof for the air openings and sealing of the HVAC unit to the roof becomes critical. The refrigerant, water lines and some harnesses have to be routed within the body of the bus to connect the roof-top unit to the compressor in the engine compartment.
To realize the same capacity, the roof-mount unit will be heavier, mainly due to the configuration of the coils and the addition of covers, which are not needed on the rear-mount system. Some roof-mount configurations have separate condenser and evaporator modules that have to be connected after they are on the roof, an additional step in the installation process not required for rear-mounted units.
In addition, some maintenance and repair work has to be done from the roof of the bus instead of from the ground or on a short ladder.
“The roof-mount configuration allows the operator to have a rear window if they choose, and there are more opportunities to manage air distribution, weight distribution and noise because the location of the unit can be anywhere from the back to the front of the bus,” explains Johnson.
Without an air-conditioning unit in the rear, the bus is able to have a rear window, which allows for more natural light so the passengers don’t feel enclosed or uncomfortable when sitting in the upper deck. Also, the driver has a rear window to look out and he or she can keep a closer eye on the passengers. Another convenience for the passengers with the roof-mounted system is that it is quieter and there is less heat as the noise and heat are already going up in the air.
A roof-mounted system accounts for less porpoising. The weight is leveled out more on the bus having the air conditioning unit moved forward. A lot of weight in the rear with the engine and air conditioning system in the back makes the bus porpoise.
Generally, the roof-mount systems are thought of as easier to work on as you can work on the components inside the bus by dropping down the AC filter to work on the system. Some don’t like the position where the blower fans are on the roof as you have to climb up there to work on them.
The other disadvantage is that sometimes the roof height is higher and agencies have to watch their garage clearance.
Johnson adds, “Traditional roof- and rear-mount systems are not the only options — there can be integrated solutions where the components are placed in various parts of the bus and connected.”
“For years, we have been using various combinations of integrated systems based on powertrains and customer requirements,” says MacNeil.
Looking at Compressors
The compressor’s purpose is to withdraw refrigerant vapor from the evaporator and compress the gas to a pressure that will liquefy in the condenser. There are several methods used to seal the compressors to prevent the escape of refrigerant. An open-type compressor is simply bolted together and can be driven by an electric motor. A leakproof seal is required where the crankshaft extends out of the crankcase.
A hermetically sealed compressor has both the electric motor and the compressor in the same airtight housing and they share the same shaft. After assembly, the two halves are welded together to form the airtight seal. This type of configuration eliminates a source of leaks, but it is inaccessible for repair.
“Hermetically sealed scroll compressors are typically used in electric HVAC systems,” says David Knezic, HVAC Technical Lead for New Flyer. “These reduce the amount of maintenance, reduce the refrigerant requirements in other parts of the bus and allow for variable speeds, which provides a more efficient operation.”
Systems with electric and hybrid buses are moving more and more to hermetically sealed systems as opposed to open drive compressors. More recently, this technology has shifted to conventional diesel engine buses as well.
“Creating electric power from an alternator or generator is not a new concept,” says Steve Johnson, bus HVAC product manager with Thermo King.
“The primary obstacle when your alternator or generator is driven by the bus engine is maintaining clean, stable voltage throughout the speed range of the engine.”
Thermo King began operation with this technology in June 2004, in Prague, Czech Republic. The vehicle runs on average, 18 hours a day from the city of Kladno, about 37 miles outside of Prague, into the city. The route operates with both highway and stop-and-go conditions.
Having a compressor that relies on engine speed, a bus that spends a lot of time idling could create hot conditions. One alternative is a large air conditioner with a large compressor to provide enough cooling. Another is an air-conditioning unit that does not rely on the engine for capacity. Electric compressors can provide constant capacity. In Mass Transit magazine’s December 2006/January 2007 issue, Tony Bryant, director, bus maintenance at the Tri-County Metropolitan Transportation District of Oregon (TriMet), talks about the prototype system it was running for an all-electric cooling system.
As he explains it, the system uses a variable-speed electric coolant pump, cooling fans and mixing valves. The system has proven simple, reliable and effective.
With its screw compressor, Thermo King has attained an 80 percent reduction in moving parts as compared to a reciprocating compressor. The reduction in moving parts means a reduction in the chance of breakdowns.
Screw compressors can save gallons of fuel for agencies using reheat. According to Don Nielsen, engineering manager for Thermo King, reheat is a mode of operation that introduces heat into the refrigeration cycle to stabilize the temperature as it reaches set point. It is used to balance the cooling load during light-load conditions and also to keep windows from fogging.
The screw compressor stabilizes the load by unloading some of its pumping ability during these conditions. Reducing the amount of energy used during reheat results in less horsepower required from the engine to drive the compressor.
“We have been using screw compressors for several years,” says Knezic. “Screw compressors have less vibration compared to reciprocating compressors and require less maintenance over the life of the compressor – both of which are important benefits to our customers.”