MACS Service Reports - 2015 - AUG2

A/C Compressors & MPG

Refrigerant is drawn through the suction port into these
chambers, which get smaller as the rotor turns. The discharge line port to the condenser is located where the
gas is compressed inside the compressor. The vanes are
sealed against the rotor housing by centrifugal force and
lubricating oil. The oil sump and oil pump are both located on the discharge side, so that the high pressure
forces oil through the oil pump and then onto the base of
the vanes, keeping them sealed against the rotor housing.
See Figures 3 & 4.

Toyota

Using A/C can contribute to more than a 25% fuel economy reduction. In one real world study published in an SAE
white paper, a 2009 Ford Explorer and a 2009 Toyota Camry
were tested at various speeds, both with and without the
A/C turned on as well as with and without the windows
rolled down. At cruising speeds of between 40 and 70 MPH,
both vehicles consumed more energy with the A/C on (windows up of course) compared to driving at the same speeds
with the windows down and A/C off. Ford's variable displacement compressor was running at 100% duty cycle.
Both vehicles were tested again at highway speeds of
around 80 MPH. The Explorer maintained better fuel economy with the A/C off and the windows rolled down compared to having the A/C on and windows up. The Camry,
however, did not see a penalty in fuel consumption numbers
when the A/C was on at speeds up to 70 MPH compared
to A/C off and windows down. This break even observation went away on the Camry at speeds of 80 MPH from
compressor operation due to poor aerodynamics from the
windows being down. Both vehicles exhibited the greatest
fuel economy loss at idle with A/C on versus A/C off.

Scroll and Rotary Vane Compressors - Light is Right!
Compressor downsizing applies to both piston compressors as well as rotary vane and scroll compressors. Both of
these designs are helping with fuel economy via the "light is
right" method. If you've ever seen a compact Asian vehicle's
compressor, they appear so much smaller than conventional
piston style compressors that the clutch and pulley almost
seem to look as big as the compressor itself!
The scroll compressor uses a unique design with two
scrolls; one is fixed, the other is movable, and both are interleaved. The movable spiral is able to ORBIT or oscillate without actually fully rotating. The movable scroll is connected
to the input shaft via a concentric bearing. As the movable
spiral oscillates within the fixed spiral, a number of pockets
are formed between the spirals making a pumping action.
See Figure 2.

Dave Hobbs

Figure 3: This drawing shows the offset axis of the compressor shaft as the vanes
move though the shaft openings to create suction and compression of refrigerant.

Figure 4: Cutaway of a very lightweight compact rotary vane compressor.

Rotary Vane Compressor Tech Tips:
Toyota

During idle, an occasional vane noise from the
compressor may be heard. This happens due to the
time it takes for lubricating oil to circulate through
the A/C system. Occasionally these vanes have
been known to stick. Revving up the engine to 2800
RPMs and rapidly turning on / off the compressor via the HVAC head to engage / disengage the
clutch may sometimes break loose the sticky vane
and allow it to begin pumping again. This fix is
often considered a short term fix as the condition
may return later.

Figure 2: The scroll is also the light weight fuel saving compressor of choice
for electric applications such as HEVs (Hybrid Electric Vehicles) and PHEVs
(Pluggable Hybrid Electric Vehicles) and EVs (Electric Vehicles).

Rotary vane compressors use a rotor with three or four
vanes and a uniquely shaped rotor housing. As the compressor shaft rotates, the vanes and housing form chambers.

August 2015

MACS Service Reports

Table of Contents for the Digital Edition of MACS Service Reports - 2015