Electronic air conditioning -- An overview of systems found in today's hybrid vehicles

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Electronic air conditioning -- An overview of systems found in today's hybrid vehicles


Electronic A/C systems are now appearing in hybrid vehicle applications. You’re more familiar with these systems than you might first realize. You grew up with them. You may even have one in your bedroom window. They are air conditioners — refrigeration units — powered by electricity.

The basics of refrigeration apply whether it is an automotive air conditioner, a window air conditioner or a kitchen refrigerator. Let’s use the Toyota Prius system for our example, since the Prius is the most popular of the hybrids and typical of what you will see in the shop.

Hybrid vehicles have special needs when it comes to heating, ventilation and air conditioning (HVAC) operation. Since an electric motor propels the car, normal belt drives won’t cut it when the engine isn’t running. We need another way to power the compressor.

The 2004 to present Prius compressor is powered by a 200-volt AC (alternating current) electric motor built into the compressor assembly. The electric motor takes the place of the belt-driven pulley found on typical vehicles. Other than the compressor’s AC power source, the A/C operation is the same as Toyota’s other vehicles. NOTE: If there is a problem such as a short circuit or broken wire in the compressor wiring, AC power is turned off.

Incidentally, the heater core in the HVAC system doesn’t get heated coolant soon enough or consistently enough because the engine is not always running. Cabin heat is backed up with an electric heater.

Of course, any power used for heating and air conditioning comes from the car’s high-voltage battery system. That can have a negative effect on fuel economy. To minimize the electrical power draw, the air conditioner compressor’s motor is controlled by an electronic control module (ECM).

The A/C ECM receives inputs from several sensors such as the cabin temperature sensor, the sun load sensor, a humidity sensor (integral with the cabin temperature sensor), a temperature sensor at the evaporator and a position sensor on the out door of the HVAC case. The A/C ECM also receives reports as to whether the wipers are running.

The information is processed and a signal is sent to the car’s high voltage electronic control unit which tells the AC inverter how much power to send to the compressor’s electric motor.

Since the 2004 model year, even the blower motor uses less power by running on a pulsed, duty cycle electrical input.

The duty cycle is controlled by the HVAC control module.

Troubleshooting the Prius air conditioning system is similar to other vehicles’ A/C systems. However, there are some trouble codes that are vehicle specific to hybrids. Always consult a shop manual to be sure. As you know, the preferred way to obtain diagnostic trouble codes (DTCs) is by using a scan tool. Connect yours to the diagnostic link under the dash and follow the instructions.

(NOTE: Check the appropriate Service Manuals for DTC charts.)


There is nothing special about the compressor removal and replacement other than that the high voltage system must be disabled prior to any work.

To access the high-voltage main fuse, you must first remove the service plug grip. You simply remove the floor panel from the rear hatch area. Next, remove the storage well then the subfloor panel. Wearing insulated rubber gloves, remove the service plug grip as you turn the lock lever to the left to expose the fuse which comes out after removing the two bolts.

After removing (and plugging) the high-side and low-side air conditioner hoses from the compressor, disconnect the green, high voltage connector. You must first release the three retaining clips.

Cover the connector contacts with electrical tape. And, it goes without saying that you should be wearing insulated gloves during this procedure.

The A/C compressor and its electric drive motor come out as a single assembly.

A new refrigerant oil was introduced with the 2005 model year. It has a different insulation characteristic than the previous oil and if ND-OIL11 or equivalent is not used electrical current may leak. If that happens, the vehicle may shut down.

The high voltage may also pose some problems when using an A/C service machine. You may want to look into getting equipment that meets the new SAE J2788H “high voltage addendum.”

Now, back to the refrigerator for a cold drink.

As easy as 1-2-3-4-y-f

By now, you have been hearing the buzz about a new refrigerant. It is called HFO-1234yf, or simply R1234yf.
The European Union decided awhile back that R134a would start to be banished by January 2011, so the search began for a replacement.

Carbon dioxide was considered. After all, it is readily available and a familiar compound. Using it would not contribute to global warming because it could be extracted from the gas that already abounds. The down side is that it functions as a refrigerant only under very high pressure.

R152a was considered, but rejected because it is flammable. R1234yf is only slightly flammable — so slight that the EPA is expected to approve it under the SNAP (significant new alternatives policy) and General Motors is reportedly planning to introduce R1234yf in some 2013 vehicles.

R1234yf does not do the kind of environmental harm the R12 and R134a caused. R134yf lingers in the atmosphere only 11 days. For the most part, servicing vehicles with R1234yf will be no different than what you are accustomed to.

Sure, you will need dedicated equipment, so there is a significant cost involved, of anywhere from $3,500 to $7,000. Additionally, the price of R1234yf is expected to be about $100 per pound despite the fact that DuPont reports that it will have no problem supplying enough of the refrigerant.

Techs in the new car dealership shops will be seeing systems with this new refrigerant first, but there should be very little failure. All of the hoses and connections must be designed to prevent leakage. It will probably be several more years before aftermarket technicians will have to deal with it. That leaves ample time to get adequate training and equipment.

Prius ejector A/C system

Beginning with the 2010 Prius, a new type of air conditioning system has hit the road. Called the “ejector cycle system,” it uses an ejector, which has no moving parts, in place of an expansion valve.

The system, designed by Denso, reduces energy consumption. After all, isn’t that what hybrid vehicles are all about? It does this by reducing the demand on the air conditioning compressor by taking advantage of fluid flow dynamics.

The Prius ejector A/C system has two evaporators stacked together so that they would fit inside the blower assembly box behind the dash. One of them actually helps cool the other.


In the ejector cycle, the compressor pumps high-pressure refrigerant vapor to the condenser where it cools and liquefies. It next flows through an expansion valve, but this expansion valve doesn’t make the refrigerant vaporize. The expansion valve only adjusts the flow of the liquid refrigerant.

Next, the refrigerant diverts into two paths with most of it going through the ejector and less going through a capillary tube. The refrigerant in the capillary tube goes into one of the evaporators. Denso calls this the down-wind evaporator.

The large portion of refrigerant goes to the ejector which is essentially a double-walled tube with the inner tube tapering into a nozzle. The tube-inside-a-tube creates a venturi effect that draws refrigerant that is leaving the down-wind evaporator. The venturi creates suction through this “jet pump” effect.

Next, the mixed flow of refrigerant goes into the up-wind evaporator then back to the compressor.

But, since it still has some pressure, the compressor doesn’t have to work quite as hard.

Ejector cycle

A traditional refrigerator draws heat from the air by expanding refrigerant with a valve to cool the air.

Consequently, a large amount of kinetic energy is converted into vortices and is wasted. An ejector cycle prevents vortices by replacing the expansion valve with an injection system.

The ejector cycle also assists in supplying power to the compressor by converting previously dissipated and lost energy into additional pressure force, which remarkably improves cooling efficiency.

Hybrid vehicle safety notes

Hybrid vehicles’ electric motors rely on high voltage battery packs for power. On the Prius, voltage can be as high as 270 volts DC. You should not attempt to service a hybrid vehicle unless you are familiar with its overall operation, including the subsystems.

Some accessories can power up even though the key is turned off and the engine is not running.

Before performing any service, Toyota says:

• Shut off the vehicle. If vehicle has smart key system, disable it and make sure the key fob is 15 feet away from vehicle.

• Disconnect the negative (-) battery cable from the 12-volt auxiliary battery.


• Wear insulated gloves.

• Remove the Service Plug and do not make any repairs for at least five minutes.

• Before touching a high voltage cable (orange) or any cable you cannot identify, use a voltage tester to confirm that the voltage through the cable is 12 volts or less.

• After removing a high voltage cable, be sure to cover the terminal end with rubber or vinyl tape.

• Use insulated tools, when available.

• Do not leave tools or parts (nuts, bolts, etc.) inside the cabin.

• Do not wear metal objects (risk of short circuit).

Information courtesy Toyota Motor Co.

Guidelines for use and handling of HFO-1234yf

HFO-1234yf can be described as being “mildly flammable” as measured by standard methodology. This descriptor is used to characterize the flammability in simplistic terms; however, properties such as minimum ignition energy, heat of combustion, and the burning velocity are assessed in order to arrive at such a descriptor. Measurement of HFO-1234yf flammability properties indicates that a typical static discharge will not have sufficient energy to ignite HFO-1234yf.

1. Refrigerant with air

Fire or explosion may result if vapor-in-air concentrations are within the flammable range and an ignition source of adequate energy level is available. Avoid mixing HFO-1234yf with air, oxygen or other oxidizers at pressures above atmospheric pressure.

2. Cylinder storage

Smoking should not be allowed in storage or handling areas as a general rule. Smoking should be prohibited in storage, handling, and servicing areas where HFO-1234yf is used. Do not store HFO-1234yf cylinders near sources of open flames, ignition sources or at temperatures exceeding 125 degrees Fahrenheit (51 degrees Celsius).

Store cylinders in a cool, well-ventilated area with low risk of fire and out of direct sunlight. Ensure that cylinders are properly strapped into place, avoid dropping, denting or mechanically abusing containers. Protect cylinders from moisture and rusting during storage.

3. Service areas

HFO-1234yf is a heavier-than-air gas. Depending on the quantity released in air, the material can travel a considerable distance to a low-lying ignition point. HFO-1234yf can collect in floor pits. There is potential for asphyxiation in floor pits or confined spaces. Use adequate ventilation in these areas.

Refrigerant charging should be performed away from open flames or high energy ignition sources.

(Excerpted directly from Honeywell “Guidelines for Use and Handling of HFO-1234yf .”)

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