Integrated Front and Rear HVAC Unit
2014-01-0690
Published 04/01/2014
Kevin Cheung
Honda R & D Americas Inc.
Erich Becker
Delphi Automotive
CITATION: Cheung, K. and Becker, E., "Integrated Front and Rear HVAC Unit," SAE Technical Paper 2014-01-0690, 2014,
doi:10.4271/2014-01-0690.
Copyright © 2014 SAE International
Abstract
Vehicles with a large cabin volume incorporate two HVAC units
to provide comfort to the front and rear cabin. Each HVAC unit
can generate independent airflow volume, temperature, and
airflow direction. A new HVAC unit was developed to achieve
the performance and functionality of two HVAC units. A unique
HVAC construction was used to achieve independent front and
rear airflow volume, temperature, and airflow direction
distribution. This integrated front and rear HVAC unit provides
additional packaging space for other vehicle components and
reduces the overall number of HVAC system components.
Introduction
Vehicles equipped with dual HVAC units provide independent
airflow to the front cabin (driver and passenger) and rear cabin
(2nd and 3rd row passengers). Figure 1 shows an example of
independent temperature zones in a 3 row passenger vehicle.
The front HVAC unit is packaged inside the instrument panel
area. The rear HVAC unit is located in the console box area or
the rear quarter panel area of the rear cabin. To transport
refrigerant and coolant from the engine compartment to the
rear HVAC unit additional piping is needed.
An integrated HVAC unit provides the same performance and
function as two separate HVAC units to meet the customer's
expectation. Additionally, it would reduce the required
packaging space in the vehicle and provides extra storage
space. The integrated HVAC unit would improve upon the dual
HVAC unit by reducing the cost and weight of the HVAC unit by
reducing the number of components.
HVAC Functionality
The integrated front and rear HVAC unit would have tri-zone
independent temperature functionality (Driver, Front
Passenger, Rear Passengers).
The driver and front passenger airflow distribution are VENT,
BI-LEVEL, HEAT, HEAT-DEFROST, and DEFROST. The rear
passenger airflow distribution is VENT, BI-LEVEL, and HEAT.
The airflow distribution is controlled by damper doors and
driven by actuators.
The front and rear airflow volume can be independent of each
other. All HVAC unit settings can be manually set by the
customer or automatically controlled by the climate control CPU.
Method
The integrated HVAC unit has two blower fans packaged
upstream of the evaporator to provide independent front and
rear airflow (figure 2 shows the respective blower fan
orientation). The front blower fan intakes either outside (or
fresh) air or re-circulated cabin air (recirc). The rear blower fan
only intakes re-circulated cabin air; it is positioned in the front
lower center section of the HVAC unit.
Figure 1. Tri-zone independent temperature in a 3 row vehicle.
Figure 2. Integrated front and rear HVAC iso-view. The location for the
front and rear blower, and each air outlet are labeled.
To provide independent airflow volume to the front and rear
cabin, the evaporator, which cools passing air, is partitioned
into a front and rear zone [1]. The evaporator core is
approximately 50% larger compared to a single airflow core
due to the increased capacity from dual airflows. Figure 3
shows the cross section through the center of the main HVAC
case; the front and rear evaporator zones are indicated.
Figure 4. Vehicle center cross section. HVAC unit, Instrument panel,
console, and ducts in vehicles package.
Figure 4 shows the integrated HVAC unit packaged inside the
vehicle constraints.
The air conditioning refrigerant loop for the integrated front and
rear HVAC unit functions in a typical A/C system loop. A benefit
for the integrated HVAC is the refrigerant (and coolant) does
not need to be transported to the rear HVAC unit. This reduces
the effect of refrigerant oil trapped in the refrigerant lines.
Figure 5. Rear piping comparison between integrated HVAC unit and
dual HVAC unit system.
Performance Target
Figure 3. Integrated HVAC center cross section. Evaporator, heater
core, damper door, and rear blower fan components are labeled.
The heater core is also partitioned into a front and rear zone
(figure 3). The core is approximately 60% larger compared to
the previous single airflow heater core due to the increased
capacity from dual airflows.
The integrated HVAC unit is packaged behind the instrument
panel. The target was to maintain the instrument panel size,
and not intrude on other surrounding vehicle components
-since the HVAC unit is not visible to the customer.
However with a larger evaporator, a larger heater core, and
combining the front and rear blower fan, the integrated unit has
a larger volume than a single HVAC unit. It is then necessary to
optimize the package size and meet the target performance.
The integrated HVAC system performance target is similar to
the dual HVAC unit system (Table 1):
Since two separate airflows pass through a single evaporator,
the independent airflows affect the temperature distribution
through the evaporator core. There are conditions when airflow
will pass through one zone of the evaporator and no airflow
pass through the other. If the airflow does not flow through the
entire heat exchanger, uneven heat distribution creates
regional hot and cold spots that can cause the evaporator to
freeze. A frozen evaporator can cause issues within the HVAC
system, including odor, inability to reach target cabin
temperature, and shortened compressor life.
To prevent the evaporator from freezing, the following items
were implemented:
• HVAC Architecture, front and rear zone orientation
• The evaporator was angled from horizontal.
• The evaporator was divided to separate the front and rear
airflow.
• Two condensate drain paths were installed.
• The evaporator sensor position was optimized.
Table 1. Performance target comparison between old Dual HVAC unit
system and new Integrated HVAC unit system.
Figure 6. Evaporator split. Evaporator partition seal is shown.
Figure 7. Cross section through Evaporator. The front and rear
condensate drain paths are shown.
HVAC Architecture
Construction
Condensate will freeze when the refrigerant capacity
significantly exceeds the air loading of the system. The climate
control system allows the customer to turn the rear zone off
which results in a significant capacity inequality [3]. The single
evaporator heat exchanger architecture continues to flow low
pressure, low temperature refrigerant through the tube plates
that span the front and rear zones. The design inherently
protects against rear evaporator zone icing by positioning front
zone which constantly generates condensate on the bottom of
the evaporator (figure 3). This leaves the rear zone relatively
free of condensate and any potential pooling that would result
in ice formation.
Separator and Cooling Performance Verification
Traditional evaporator air centers contain louvers to enhance
heat transfer. In this application the open louver would permit
air exchange between the front and rear zones. The evaporator
tube plates are orientated 90 degrees to the HVAC wall that
defines the front and rear air zones and therefore could not be
utilized for a sealing surface. To overcome this obstacle Delphi
applied a patented manufacturing process to close louvers at a
defined position within the evaporator [4]. The closed louver
coupled with effective sealing at the inlet and out let face of the
evaporator separates the front and rear air zones.
Evaporator Separator
An evaporator separator divides the front and rear airflow
passing through the evaporator. The separator prevents airflow
intended for the front to pass into the rear and vice-versa [2]. It
consists of a partition seal (figure 6), exterior of the evaporator,
and an air side divider.
The separator splits the evaporator (and heater core) into
areas approximately proportional to the target front and rear
cabin airflow volumes (figure 6).
The separator also provides for independent front and rear
condensate drain paths (figure 7 and 10). Front and rear drain
paths are essential because each section accumulates a
different level of condensate due to their unique airflow
volumes and intake air humidity.
Figure 8. Total Cooling Performance and Test Conditions.
Table 2. Front and Rear Cabin Cooling Performance and Airflow Target
and Results
Large amounts of condensate pooled in any area of the
evaporator can increase the probability for the water to freeze
on the evaporator.
With the evaporator separator providing a rear drain path, a
rear drain is incorporated in the upstream rear airflow path
(figure 10). Similar to the front drain, the condensate is drained
outside of the vehicle.
The targets were met by the integrated HVAC unit design.
Angled Evaporator
Condensate forms on the evaporator when humid air (drawn
from either outside or inside the vehicle cabin) contacts the
cold evaporator surface. Fresh intake air will have more
humidity than recirculated cabin air.
Freeze and Water Splash Verification
A new test was developed to test the front and rear zone, so
the evaporator does not freeze at various air intake
conditions, compressor speeds, and various front and rear
airflow volumes (e.g. high airflow to the front zone, low or no
airflow to the rear zone).
Table 3. Evaporator Freeze Test Condition
The conditions where the front air intake is outside warm humid
air (>25°C, >60%RH) and rear air intake is dryer re-circulated
air (>20°C, >40%RH), the evaporator has different relative
humidity levels. Thus, the partitioned evaporator can have
different condensate levels in the different zones.
Figure 9. Comparison between evaporator upright and angled
evaporator inside the HVAC unit case.
Figure 11. Thermistor location in relation to Evaporator front and rear
zone.
Figure 10. Front and rear condensate drain water path.
By positioning the evaporator at an angle, more condensate
drains from the evaporator. It is also more difficult for
condensate to pool in one concentrated spot (figure 9) [1].
It was observed that the rear drain path was necessary in high
humidity and high cooling load conditions to remove
condensate from the evaporator rear zone (figure 10).
No frost was observed on the surface of the evaporator.
Based on the same test condition, the thermistor was tested at
different evaporator locations to determine which location was
optimal to detect the lowest evaporator temperature. (figure
11). The thermistor value is sent to the A/C climate controller,
so logic is used to disengage the compressor to prevent frost
on the evaporator.
Vehicle orientations were simulated on a system bench at
extreme air loading conditions to determine an evaporator
angle of 30 degrees effectively managed condensate.
Using the integrated HVAC unit resulted in a 20% reduction
both in cost and in weight and a volume increase of 2.5 times
within the console space (figure 4).
The integrated was packaged in a space that did not affect the
instrument panel size and thus does not affect the customer's
interior space. In order to achieve overall interior package,
there was extensive collaboration between the design of all
surrounding components.
Bench and vehicle tests were performed using the same
conditions as a dual unit HVAC system. The integrated HVAC
system performance was met and confirmed to be the similar
to a dual HVAC system. A new test was developed to confirm
evaporator freezing and it was confirmed no evaporator freeze
occurs with the integrated HVAC unit architecture.
Figure 12. Water splash verification on prototype part.
No water splash was detected in testing.
Airflow Path through Integrated HVAC Unit
With the angled evaporator and heater core layout, the VENT
outlet paths are shown in figure 13.
Figure 14. Production Integrated HVAC unit during vehicle assembly
for mass production.
References
1. Kanemaru Junichi, Kakizaki Shinji, “Integrated front and
rear hvac system” U.S. Patent 20100304654, Dec 2, 2010.
2. Kanemaru Junichi, Kakizaki Shinji, “Seal and drain
structure for a front and rear integrated hvac system” U.S.
Patent 20100273411, Oct 28, 2012.
Figure 13. Front and Rear airflow path inside Integrated HVAC unit.
Front and Rear VENT mode airflow path shown.
Summary/Conclusions
The integrated front and rear HVAC unit met the heating,
cooling, and airflow performance targets for a vehicle with
three rows of passengers. By meeting these targets, it is
applied to a mass production vehicle (figure 14).
3. Kanemaru Junichi, Kakizaki Shinji, Yelles Daniel,
“Evaporator frost prevention control logic for front and rear
integrated HVAC system” U.S. Patent 8187063, May 29,
2012.
4. Gerstung Kirsten A., Schmidt David G., Hunt Terry J.,
Franco Martin J. “Evaporator having separate air flow
paths and method of manufacturing the same” U.S. Patent
2013123144, Aug 22, 2013.
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