Automotive Engineering - April 2021 - 31

ELECTRIFICATION FEATURE

Schematic of the
Kia Niro's heat
pump.

passengers by directing air from over the engine to
the vehicle interior. Soon after, engine coolant became
the medium. Using waste energy is smart; wasting
energy is not.
Wilcox's invention was still in use in the 2011 Leaf.
Unfortunately, it decreased the EV's driving range by
30% to 40% in cold weather, as the heat was generated from the primary energy source on board - the
car's battery pack. Nissan's adoption of heat-pump
technology was a revelation. It reduced the load on
the high-voltage battery from approximately 7 kW to
2 kW, for the same amount of heat generated. This
allowed the improved Leaf to go further on a charge
while keeping the occupants comfortable.
Conceptually, heat pumps have been around since
the 1850s. They have been used in buildings since the
1940s. How do they work? In simple terms, a heatpump system heats the cabin using the temperature
delta between a refrigerant and the outside air, creating a heating effect while consuming less power than
a conventional vehicle heater. Essentially in a heat
pump the condenser and evaporator trade places by
adding another expansion valve before the condenser.
The HVAC controller, depending on a request for heat
or air conditioning, calls upon one expansion valve or
the other. A single refrigerant circuit can be used for
both cooling and heating.
A more efficient air-conditioning system is achieved
through optimization of the heat exchanger design
that implements low power consumption, along with
AUTOMOTIVE ENGINEERING

heating and cooling according to set temperatures, and temperature
control when switching between cooler air and heater. In this way the
heat is then produced in the cabin as you try to cool the air outside
the vehicle.
Heat-pump technology saw rapid iterations to improve performance, from Nissan's first automotive use in the early Leaf, to Kia's
tweaks, and then to Denso's further improved heat pump used by
Toyota. These changes came because EV customers wanted more
range in cold weather.

Inside the heat pump
Those readers who aren't HVAC engineers may find value in examining the generic heat pump (see schematic) we use in the ACDC classroom to help repair technicians understand the concept. The system
has two heat exchanger/evaporators (11 and 2). One of them, numbered 2 in the schematic, is used for heating the cabin. The condenser (5) has two jobs as we will explain. The compressor (1) is a typical
high-voltage scroll type and uses an accumulator (9). The accumulator (9) prevents any liquid from entering the compressor.
Two 12-volt solenoid 3-way valves (3 and 6) are fitted before each
expansion valve (4 and 10) with a bypass line to direct the refrigerant
either to the expansion valve or around it. The " 3-way valves, " as
they are referred to by the OEMs, have only two directions. The
switching of the valves will allow the refrigerant to keep going
straight through the same pipe or be redirected to another pipe. It
has only two choices, and only one expansion valve is open at any
one time. There are also subsystems that are needed to support a
heat pump in damp and very cold weather, or a subsystem to make it
perform better. Expect more improvements in the future.
April 2021 31



Automotive Engineering - April 2021

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