Hybrid Air Conditioning Systems

By Craig Van Batenburg. In the beginning of hybrid car development the air conditioning systems were R143a systems that shared all the same components as a typical gasoline motor vehicle. The early hybrids were the Honda Insight, Civic Hybrid, Ford Escape, and Classis Prius (2001-2003). All hybrid A/C systems are really climate control. Only the Insight had A/C as an option, and it was a climate control system as well. Every other hybrid to date has factory climate control for some very important reasons.

Some inputs that all hybrid systems look at are A/C request (temperature desired), temperature both inside and outside the vehicle, sunlight, HV battery temperature, intake air at HV battery intake duct, and HV battery SOC (state of charge). On the earlier hybrids, when the correct conditions are met at idle, the hybrid computer will shut down the ICE and continue to run in economy mode (A/C compressor is off) or keep the ICE on and A/C at full maximum. As hybrids have developed improvements over the belt driven Honda, Ford, and early Toyota Prius A/C systems have been introduced. In the older systems it was required that the ICE stays running under Full Max conditions. Having the ICE off equals better fuel economy and lower tail pipe emissions, so changes were in order.

The Prius was redesigned for the 2004 model year. One look under the hood would have you wondering about the A/C or climate control. Gone was the drive belt, and added was an orange cable that snuck into the case of the compressor. What was going on here? The compressor was a new Denso design that required high voltage to operate. This A/C system compressor runs on 201 ac (Not to be confused with AC = Air Conditioning and ac = alternating current). In order for this new style compressor to operate it requires a high voltage source (the new Prius has that) and a computer system tied into the climate control that will signal the 201 volt power inverter to change the 201 volt direct current (dc) to 201 volt ac. So far so good. In ACDC’s, (not to be confused with ac dc, ACDC is my training company, the Automotive Career Development Center! OK, way too many ac, A/C, dc, ACDC. I sure hope you are following this) new 2004 Prius I have logged about 90,000 miles. In the summer heat with the AC on maximum and the car stopped the ICE (Internal Combustion Engine) will stay off for about 6 minutes. During this time the HV battery pack is being depleted. When two bars of NiMH juice show on the dash display the ICE will automatically restart and a generator in the transaxle will perform a fast charge for about 4 minutes and then the ICE will shut down again. This repeats until you run out of gas or start driving. A similar compressor is shared with the Toyota / Lexus twins, the hybrid SUVs cousins, Highlander and RX400h. Re-charging the A/C system with refrigerant is NOT the same as conventional A/C. Caution must be used because of the high voltage cables and special oil is required for the system. Care must be taken not to introduce any other oils into an AC system in the new crop of hybrids.

Honda, not to be outdone by their arch rival Toyota, has equipped the Accord hybrid (now out of production) and the Gen II Civic HEV (2006 – 2009) with a hybrid A/C compressor. I call it a hybrid compressor not because it was fitted to a hybrid but because it used over 144volts ac and a belt. Therefore it is a hybrid by my definition. A mechanical hybrid is a device that has two power supplies to create motion at the same wheel. Someday SAE will define a hybrid, but for now we will use this definition. This hybrid compressor has about 85% of its refrigerant cycling through the belt driven portion of the compressor and the electric drive has its own scroll and delivers the remaining 15%. During most, not all, maximum requests the smaller electric portion can keep the occupants cool enough so that the ICE can shut down during idle stop. As you know one way to increase mileage is to turn off the ICE when not required. The two scrolls can also work together to provide maximum cooling on really hot days.

The AC system in the Ford Escape Hybrid, Mercury Mariner Hybrid and Mazda Tribute HEV is a dual zone type using an off the shelve evaporator to cool the high voltage nickel metal hydride batteries (NiMH) located under the rear deck floor as well as front AC for passengers. This system monitors the HV battery temperature and turns on the A/C as necessary. Because the Ford still uses a belt, the ICE will start to keep the HV battery module cool.

All hybrids, except the Ford Escape HEV (and twins) and the Lexus LS 600h L, use AC indirectly to cool the NiMH batteries by grabbing air from the cabin and with air ducts and a fan draw the cooler air over the NiMH cells, venting the warmer air outside. This keeps the HV battery’s temperature under 80 degrees Fahrenheit most of the time.

The service required to keep the climate control systems operating efficiently in hybrid vehicles are the same as the normal cars you see everyday. Good news for AC shops. Now it is more than sweat we are worried about, it is NiMH battery life.