Automotive Engineering - June 2022 - 19

ADVANCED HVAC FEATURE
Electric radiant heat for EV cabin comfort
Toyota's 2023 BZ4X EV uses the new radiant
heat system co-developed with Denso.
New HVAC tech helps Toyota tackle the EV heat-vs.-range conundrum.
by Paul Weissler
V
ehicle OEMs have been steadily increasing
the range of their electric vehicles (EV) and
plug-in hybrids (PHEV), but the ranges they
claim publicly are in ideal thermal conditions.
Now comes the hard part: Deliver those same
ratings in real-world weather, in the kind of thermal
extremes that challenge the performance of lithiumion
battery packs.
Although air conditioning use in hot summer weather
typically causes a modest loss of EV range, it's the 3040%
drop of range in cold winter weather when the
cabin heater is deployed that continues to challenge
EVs' ability to fully replace ICE-powered vehicles.
To date, most efforts to improve EV heating performance
have focused on electric seat heaters, some
electric steering wheel heaters and for the most part,
heat pumps built into the HVAC refrigeration system.
But the heat pump performance is limited by the fact
that refrigerant flow slows with low winter temperatures,
so heat pumps must switch to less-efficient resistance
or PTC (positive temperature coefficient)
heaters when ambient temperatures drop below 0°C
(32°F). The Toyota Prius Prime PHEV's A/C refrigeration
system incorporates a liquid-gas separator and
injection of refrigerant gas, a development adapted
AUTOMOTIVE ENGINEERING
from static-mount commercial heat pumps. This technology enables
heat pump operation down to -10°C (14°F).
First use on bZ4X
In a presentation at the 2022 SAE WCX, Toyota detailed its test installation
of an electric radiant heater in a Prius Prime, describing the
system as a method for providing supplementary cabin heat quickly
and with greater efficiency than other methods. This effort may indicate
the Prime, which also has seat and steering wheel heaters, is
being used as a testbed for solving the EV cabin heat-vs.-range conundrum.
The Toyota testing showed use of the radiant heater resulted
in a 5.3% reduction in fuel consumption, and much improved
driver comfort within two minutes of cabin entry.
Radiant heating employs one of the three forms of heat transfer.
When turned on, a radiant heater emits invisible infrared waves a relatively
short distance in a narrow band. The infrared wave band
quickly contacts objects in its path and warms them. Aimed at a driver
in the Prius Prime's cabin, the system warms some of the front
part of the driver's body, working in conjunction with a seat heater
that covers the back of the body from the shoulders down to the
knees. A steering-wheel heater warms the hands and the floor outlet
from the HVAC eventually adds warmth to the driver's knees down to
the feet and provides some heat for the rest of the body.
Seat and steering-wheel heaters represent another type of heat
June 2022 19
TOYOTA
Automotive Engineering - June 2022

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