ATZ/MTZ
1
2 3
Passenger Compartment Heating
– the Latest Generation
Special reprint from
Auto Technology
Perfection build in
www.beru.com
Passenger Compartment Heating
– the Latest Generation
Fuel-powered passenger compartment heaters are meeting increasing competition from
electric auxiliary heaters. Beru AG has now developed a PTC (Positive Temperature
Coefficient) auxiliary heater that sets new performance and flexibility standards. With its
innovative electronic control system, pioneering design and a host of comfort and convenience-oriented functions, the new PTC auxiliary heater ideally meets the demands that a
modern passenger compartment heat management system must satisfy.
Direct electrical
heating of the air
2
"Buy a fuel-efficient car – and
freeze!" This is how one might
sum up the experience of many
passenger-car drivers who have
to put up with insufficient heat
output. This is due to modern,
economical diesel and petrol
engines, which are much more
efficient thanks to the use of
direct injection. As a result, however, a smaller amount of heat is
lost from the engine and given
off to the coolant. There is therefore less heat available for the
vehicle's heating system. This
development continues unabated.
One way of solving the problem is direct electrical heating of
the air entering the passenger
compartment with a functional
ceramic element with a "Positive
Temperature Coefficient".
Because of its low cost and selfregulating properties, this PTC
thermistor is more suitable than
fuel-powered auxiliary heaters,
which are more expensive, heavier and larger. PTC auxiliary
heaters are normally installed in
the heating system directly after
the coolant heat exchanger. After
the engine has been started, they
can warm up the cold air flow
within a matter of seconds.
PTC heaters are already in
use, but most are not electroni-
cally controlled or can only be
switched in stages. Modern PTC
heaters are incorporated into an
energy management system. Beru
AG, together with its joint-venture partner Beru-Eichenauer in
Kandel (in the Palatinate region
of Germany), has developed an
innovative PTC auxiliary heater
concept with a heat exchanger
which meets the requirements.
The Technology
All the familiar PTC heating systems utilise the self-regulating
properties of the PTC heating
resistors, in accordance with the
AutoTechnology 3/ 2002
R e s e a r c h
a n d
D e v e l o p m e n t
Ig R
Rmax
Rp
Optimum working
range of a PTC
heating element
1
3
min
R
—
b
p
R
+
intrinsic safety principle. If the
temperature at the heat exchanger exceeds a certain limit –
depending on the design – the
resulting sharp rise in electrical
resistance will reduce electrical
power consumption, Figure 1.
The patented Beru heat
exchanger is a versatile design
comprising sheet metal fins made
from a corrosion-resistant aluminium alloy, which are pushed
on to heater rods, Figure 2. The
extremely simple, self-supporting
structure comprises six individual
components that can be assembled by a fully-automated
process. The Beru solution does
not require a frame to clamp the
max
R
—
Figure 2:
Exploded diagram of the
Beru PTC auxiliary heater.
ceramic PTC elements together
and stiffen the structure. This is
particularly advantageous in
terms of pressure loss and overall
weight. At a point typically stipulated in specifications, e.g. 300
kilograms per hour, an excellent
pressure loss figure of 0.13 millibar is achieved.
The PTC heating elements lie
inside square aluminium profile
tubes and are supplied with
power via the contact strip, which
has ceramic insulation. A plastic
assembly frame helps to position
the PTC elements, Figure 3. After
they have been assembled, a controlled pressing method is used to
complete the heater rod. The end
Table 1.
Technical Data
Data
Technial
Power:
1000 W
Number of heating stages:
3
Supply voltage:
13.5 V
Max. inrush current:
< 100 A
Max. temperature:
< 155 °C
Weight:
< 300 g
Degrees of protection:
> IPX4
Typical dimensions:
Lenght:
Lenght:
175 mm
Width:
Width:
125 mm
Thickness:
12 mm
AutoTechnology 3/ 2002
6
Construction of heating
rods
1) Ceramic insulation
2) Contact strip
3) Mounting frame/Mounting rack
4) PTC elements
5) Profile tube
6) Fin
Figure 1:
Typical resistance/temperature
characteristics of PTC heating elements.
Thickness:
4
5
Rb
R25
Rmin
25
2
sections of PTC heater rods are
then potted with a sealing compound.
The entire heat exchanger
structure is on the earth (ground)
side of vehicle’s electrical system.
This complies with a motor
industry requirement that no live
parts should be present on the
component surface. Short circuits
caused by contamination or
moisture are thus avoided. The
design data for a typical Beru
PTC auxiliary heater are summarised in Table 1.
The PTC heating
elements lie
inside square
aluminium profile
tubes.
Structure and Method of
Operation
The design of the entire PTC auxiliary heater from Beru is illustrated in Figure 4. Good thermal
and
electrical
connections
between the electronic system
and the heat exchanger are of
particular importance. The heat
exchanger also functions as a
heat sink for the electronics,
which can generate up to 30
watts of heat loss during operation.
A heat conducting plate is
attached to the PTC heater rods,
simultaneously forming the earth
connection. The entire surface of
the plate is glued to the printed
circuit board, thus forming a connection of high mechanical
strength. Below the semiconduc-
3
R e s e a r c h
a n d
D e v e l o p m e n t
Fins
Profile tube
Insulation
UBat.
Contact strip
Ceramic PTC
element
Figure 3:
Cross-sectional
diagram of the
BERU PTC auxiliary heater.
tor switches, the board has copper-clad holes which, in the
reflow process, fill up with solder
and thereby ensure heat flow
through the board. The earth
plate conducts heat into the heat
exchanger structure via its
clamped connection and in this
way transfers the heat losses to
the passenger compartment air.
The adhesive joint is also responsible for electrically insulating
the underside of the board and
the earth plate.
The printed circuit board features other interfaces as well as
the electronic components. A
Figure 4:
The design structure of the PTC
auxiliary heater.
4
particular challenge was to create
high-current connections that
could be assembled by a fullyautomated process. Beru has
adopted a contacting method for
the individual heater rods that
uses plug-in high-current connections. These can be inserted
using surface mounted technology (SMT) and form clamped connections that can safely conduct
currents of up to 50 amperes. For
the positive feed of the total current, which can be up to 150
amperes, a tried and tested system was adopted, whereby the
positive contact is pressed into
the board by means of special
pins. The earth contact is also
pressed into the earth plate in the
form of a pin, and ensures sufficient current flow capacity and
strength thanks to its special
toothed design in the pressed
area. Both contacts protrude
through the upper shell of the
housing and are designed as
screw connections.
A microcontroller (µC) controls the functions and is connected to the vehicle data bus
via the interface. Depending on
customers’ requirements, various interfaces, such as CAN or
LIN data buses or a two-wire
interface, are possible. The
microcontroller receives heating
power commands from the climate control system via the
interface and transmits diagnostic messages which it generates
from system-status data. The
power electronics form another
function block, the core of
which is made up of the power
semiconductors.
These
are
N-channel
MOSFETs, which, as so-called
high-side switches, have combined activation and protective
functions, such as actuation of
the charge pump for generating
the required gate voltage or current limiting. Status information
such as an open or short-circuited load circuit is available as an
output signal for this function
block. To prevent electromagnetic
interference from being generated by the cycling of the relatively high (up to 50 ampere) heater
currents, edge triggering is integrated into the gate activation
system of the N-channel
MOSFETs.
A special measure aimed at
relieving the load on the vehicle's
electrical network is that the individual heater rods are switched
on at staggered intervals to minimise current fluctuations in the
vehicle's main electrical circuit.
Once the engine has been
started, the energy management
system specifies the required
heating power as a signal to the
auxiliary heater. Subject to internal limits, the electronics carry
out this request by generating
equivalent effective current and
activation of the heater rods
using pulse width modulation
(PWM).
Measured Results
The PTC auxiliary heater was
measured in a flow tunnel under
idealised test conditions and
using the following parameters:
• Air temperature at inlet
0 degrees Celsius
• Air mass flow
300 kg/h
• PTC voltage
13.5 volts
• Power requirement
100 %
Figure 5 shows the system’s
dynamic heating performance.
After a few seconds, the air
behind the heater has warmed up
and is available to heat up the
passenger compartment quickly
or de-ice the windows.
It can be seen from the graph
that there is only a slight, electronically controlled, overshoot
in electric power, a circumstance
AutoTechnology 3/ 2002
R e s e a r c h
The new electric PTC auxiliary
heater is a compact but powerful
auxiliary airflow heater that ideally satisfies all the requirements.
The robust, self-supporting structure and the use of aluminium for
the heat exchanger reduce the
weight and facilitate the use of a
joining process that is not sensitive to tolerances. An intelligent
electronic control system using
the intrinsic PTC thermistor principle ensures the required level of
safety and performs at optimum
efficiency. Test runs in the flow
tunnel confirm that the Beru PTC
auxiliary heater heats up the air
quickly and then provides a constant level of heat.
[1] Molt, K.: PTC-Heizung
(PTC heaters). In: ATZ 100
(1998) Issue 7/8.
[2] Goeser, W.; Schütt, T.: Neue
Lösungen verbessern das
Innenraumklima (New solutions
improve the interior climate).
In: ATZ/MTZ Special System
Partners Edition, 2000.
AutoTechnology 3/ 2002
115
1200
1100
T_Air ON
95
1000
85
900
Heating
rod 2
75
800
65
700
55
600
45
500
35
400
25
300
15
200
5
100
-5
0
Power [W]
105
Board
C
T_Air OFF
Power
00
:0
0
01
:2
7
02
:5
3
04
:2
0
05
:4
6
07
:1
3
08
:4
0
10
:0
6
11
:3
3
12
:5
9
14
:2
6
15
:5
2
17
:1
9
Summary
D e v e l o p m e n t
Auxiliary heater: Q_Air = 300 kg/h; T_Air = 0 °C; P = 100%; U_p = 13.5 V
Figure 5:
Performance data
of the PTC auxiliary heater.
Temperature [°C]
which helps to protect the vehicle
electrical network. Thanks to the
effective heat management system, the temperatures of the electronic components – microcontroller and board – also remain
within acceptable limits. In
steady-state conditions, around
95 % of the electric power is
transferred to the air flow, while
the remainder is mainly dissipated via the connecting cables.
a n d
Time [mm:ss]
The authors
Dipl.-Ing.
Hans Houben,
Manager of the
Development
Department at
Beru AG
Dr. Carsten Binder,
Group Leader for
Hardware,
Development
Department of Beru
Electronics GmbH
Dipl.-Ing.
Andreas Hamburger,
Technical Manager at
BERU/Eichenauer
GmbH
Beru AG
BERU Group is a listed public company since
October 1997. The company is the leading manufacturer of diesel cold start systems with an estimated world-wide market share of 40% for glow
plugs. In the field of ignition technology for petrol
engines BERU is one of the four major manufacturers in Europe. BERU sets on the expansion within automotive electronics focusing on comprehensive electronic systems solutions for the automotive industry. The company also produces sensor technology and ignition systems for the oil
and gas burner industry. Almost all OE-manufacturers of automobiles, commercial vehicles and
engines are BERU's customers.
Close co-operation with the customers world-wide
is our success: The BERU sales development shows
in the last millennium, that BERU has a continuous increase in sales. Sales revenues in the business year 2001/2002 increased by 9.6% to 303.1
(276.5) million Euro. The company's headquarters
are located in Ludwigsburg, Germany.
5
Printed in Germany • 03.06.02 • Bestell-Nr. 5 000 001 072
BERU Aktiengesellschaft
Mörikestrasse 155,
D-71636 Ludwigsburg
Postfach 229,
D-71602 Ludwigsburg
Telefon: ++49/7141/132-693
Telefax: ++49/7141/132-220
www.beru.com