88 Auto Heating and Air Conditioning Copyright by Goodheart-Willcox Co., Inc. the cylinder. When the piston begins to move upward, the intake reed valve closes and a reed valve connected to the outlet opens. Th e upward movement of the piston drives the refrigerant out through the open reed valve. Th is cycle repeats every time the piston moves. Some piston compressors resemble small engines, with the pistons at right angles to the crankshaft . Th ese are called radial compressors. Many modern vehicles use axial compressor designs, with the pistons parallel to the crankshaft . Th e pistons are moved back and forth by the action of an off -center place called a swash or wobble plate. A few compressors are rotary vane or scroll types. Th ey use a rotating action, similar to an engine oil pump to move the refrigerant. A magnetic clutch allows the compressor to be connected and disconnected from the drive belt. Th e compressor clutch is oft en cycled on and off to control evaporator pressure. Most compressors are equipped with a high pressure relief valve. If the compressor outlet pressure becomes too high, the valve opens to relieve pressure. Hybrid Compressors On most new hybrid vehicles, the compressor is driven by a high-voltage electric motor instead of being belt driven from the engine. A few older hybrids used a belt-driven com- pressor. An advantage of the motor-driven compressor is that it can be driven at just the right speed for maximum effi - ciency and cooling. Motor speed is controlled by the vehicle’s ECM, based on temperature and pressure sensor inputs. Muffl er To reduce noise caused by the compressed refriger- ant leaving the compressor, a muffl er may be used. When used, the muffl er is located at the compressor outlet, as in Figure 5-13. Like an exhaust system muffl er, it consists of a series of baffl es that reverse the refrigerant fl ow, causing the sound impulses to cancel each other out. In some cases, the muffl er is incorporated as part of the hose assembly. To save weight, many modern refrigeration systems do not use a muffl er. Condenser Like the evaporator, the condenser is a heat exchanger. Its job is to transfer heat from the refrigerant to the outside air. Like the evaporator, it is one or more tubes coiled to save space. Th e coils are surrounded with fl at fi ns for better heat transfer. Th e condenser is always installed at the front of the vehicle, ahead of the radiator. A typical condenser installa- tion is shown in Figure 5-14. Receiver-drier Some older vehicles use a receiver-drier, located between the condenser and expansion valve. Th e receiver-drier holds extra refrigerant, and separates the liquid and vaporized Figure 5-12. A typical axial compressor. The compressor is equipped with a magnetic clutch and pulley assembly. Figure 5-13. When a muffler is used, it is installed on the out- let of the compressor to reduce compressor pulsation and noise. Some systems do not use a muffler. Other systems have two small mufflers on the compressor inlet and outlet lines. (General Motors) Muffler Figure 5-14. The condenser is installed ahead of the radiator where air can be pulled through by a fan or forced through by vehicle movement. (Modine)