Toyota Prius fuel economy has always been about more than its gasoline-electric hybrid system. Low coefficient of drag, high-efficiency charging system, low-rolling-resistance tires, and zero-drag brakes also make measurable contributions. The 2010 model adds ejector-cycle air-conditioning to the list. Even if you have heard of ejector cycle, the design in the Prius not only is the first in an automobile, but one that by virtue of its integrated design is an impressive new engineering development.
In conventional A/C, the energy expended by the compressor to produce and circulate high-pressure liquid refrigerant is merely dispersed as the refrigerant exits the expansion valve, where it depressurizes and expands into heat-absorbing vapor in the evaporator. Engineers long have experimented with devices that, in a cost-effective way, could recover some of the energy of that blow-off of pressure to reduce workload of the compressor that pressurized the refrigerant in the first place. The need to meet new fuel-economy regulations is an incentive, and the ejector is an answer developed by Denso Corp. In the Prius, it reduces compressor power consumption by 11% at 25°C (77°F), 18% at 35°C (95°F), and 24% at 40°C (104°F).
The ejector, a specially shaped pipe with no moving parts, produces a pumping action from the pressurized flow of refrigerant. It has been used in large transport refrigerators and freezer trucks since its introduction in 2003. The first designs (as well as one section of the new ejector-cycle systems) are very similar to the jet pump, a venturi device that uses the normally wasted energy in the return flow from the pressure regulator in fuel systems. The venturi pulls fuel from one tank to another (as fuel in the pump-equipped tank is drawn out) in the "saddlebags" tanks of some SUVs.
Space requirements, including a large gas/liquid separator/accumulator on the low-pressure side, made early ejector-cycle systems unsuitable for automotive A/C applications. In 2008, an ejector system was introduced in the Toyota Land Cruiser, with a liquid reservoir built into the side of the condenser, eliminating the accumulator. However, that was a split application—one evaporator under the dash and a second in the console for a refrigerator, to enable one system to operate at two different temperatures. One temperature is chosen by the driver for cabin comfort, while the other is to keep food cold in the refrigerator.
Although the Land Cruiser system is a precursor of the type used in the Prius, no operating efficiency improvement was quantified, and the considerable space required was available.
In the Prius, the condenser with liquid reservoir continues, but more importantly there are two compact evaporators face-butted together. It is an ingenious package that includes the ejector pipe and a capillary tube that connects one evaporator with the ejector. This integration enables the system to fit into the under-dash area.
At the start of the cycle, operation is conventional. Refrigerant vapor first is pumped by the compressor through the condenser (where it cools to a liquid), then flows to the "expansion valve."
But in the ejector cycle, opening of the expansion valve does not allow refrigerant to vaporize; Denso calls it “flow adjustment valve.” Although pressure drops somewhat as refrigerant passes through, the valve is primarily metering the refrigerant flow according to system cooling requirements. The liquid refrigerant flow then splits, most of it—called "drive flow"—going to the ejector, and some of it—called "suction flow"—into a capillary tube.
Refrigerant flow through the capillary tube continues into and through what is called the "downwind" evaporator, where it vaporizes and produces low-temperature air. Simultaneously at the ejector, a tapered nozzle speeds up the refrigerant "drive flow," creating a low-pressure area around it, which draws in the refrigerant ("suction flow") exiting the downwind evaporator—the jet pump effect.
The drive and suction flows pass into a straight pipe section of the ejector, where they mix. The mixed flow continues into a widening pipe section called the "diffuser." Because the diffuser is wider, the mixed refrigerant flow slows down, so the pressure rises in this section of the ejector—that pressure rise translates to compressor work recovery. The higher-pressure refrigerant continues into and through what is called the "upwind" evaporator, and back to the compressor inlet, where it reduces the work that the compressor must perform.
Although the pressure rise represents work recovery from the compressor outlet, it does raise the vaporization temperature of the refrigerant passing through the upwind evaporator vs. the downwind. So, one compact upwind evaporator would not provide performance equal to the combination component, called a “two-temperature evaporator.”
In the Prius, the two refrigerant flows are engineered to provide complementary A/C cooling from the two evaporators: air cooled in the upwind evaporator is further cooled by the downwind evaporator. Also, Denso has sized the fixed designs of the ejector and capillary tube, and tailored expansion valve operation to optimize efficiency over the ambient temperature range.