Characteristics of the PTC Heater Used in Automotive HVAC Systems

Abstract

The present heating method which uses the cooling system of the internal combustion engine of the vehicle takes a lot of time
to heat the interior air. In order to improve the heating process, auxiliary devices are required. The new innovative techniques
propose as auxiliary heating device positive temperature coefficient heaters. For a better control of the temperature, the
parameters of these devices should be known. This paper deals with a detailed description of the new types of positive temperature
coefficient heaters used in automotive field. The test bench system developed for determining the parameters and characteristics
of the positive temperature coefficient heater is described. The obtained data of the thermal resistance and voltage-current
characteristics are used for controlling the temperature in order to design and control the positive temperature coefficient
heaters properly.

Full text

L.M. Camarinha-Matos, P. Pereira, and L. Ribeiro (Eds.): DoCEIS 2010, IFIP AICT 314, pp. 461–468, 2010.
© IFIP International Federation for Information Processing 2010
Characteristics of the PTC Heater Used
in Automotive HVAC Systems
Radu Musat and Elena Helerea
Department of Electrical Engineering, Faculty of Electrical Engineering and
Computer Science, Transilvania University of Brasov,
29 Eroilor Str., 500036 Brasov, Romania
r_musat@yahoo.com, helerea@unitbv.ro
Abstract. The present heating method which uses the cooling system of the in-
ternal combustion engine of the vehicle takes a lot of time to heat the interior
air. In order to improve the heating process, auxiliary devices are required. The
new innovative techniques propose as auxiliary heating device positive
temperature coefficient heaters. For a better control of the temperature, the pa-
rameters of these devices should be known. This paper deals with a detailed de-
scription of the new types of positive temperature coefficient heaters used in
automotive field. The test bench system developed for determining the parame-
ters and characteristics of the positive temperature coefficient heater is
described. The obtained data of the thermal resistance and voltage-current char-
acteristics are used for controlling the temperature in order to design and con-
trol the positive temperature coefficient heaters properly.
Keywords: PTC heater, HVAC, thermal comfort, vehicle, characteristics.
1 Introduction
The passenger’s thermal comfort inside a vehicle is ensured by using heating, venti-
lating and air conditioning (HVAC) systems.
Modern internal combustion engines (ICE) with fossil fuels offer improved effi-
ciency and dissipate less heat to the cooling system of the engine. On cold days, these
engines produce little heat excess and are unable to provide the amount of heat for
warming the vehicle interior up to a comfortable level [12].
In the automotive field, different technologies are available to provide the heat and
to quickly ensure the passenger’s thermal comfort. The conventional heating system
uses the heat loss produced by ICE, which is transferred to the cooling system and
then used for heating the vehicle interior air. This conventional heating method has
disadvantages: it loses about 60 % of the heat and the interior air temperature in-
creases slowly. To improve the conventional heating method, auxiliary heating de-
vices are required [16].
This paper describes and analyses an auxiliary heating device proposed to be used
in the automotive field. Also, a test bench system is developed for determining the
parameters and characteristics of the positive temperature coefficient (PTC) heater.

462 R. Musat and E. Helerea
The obtained data of the thermal-resistance R = f (T) and the voltage-current U = f (I)
characteristics will be used for controlling the temperature in order to design the PTC
heater for vehicle thermal comfort system.
2 Contribution to Technological Innovation
In the paper an innovative technique is proposed: a PTC heater is used as auxiliary
heating device because of the specific thermal and electrical properties: thermal self-
regulation, high responding time, quickly removing the condensed drops from the
windscreen and improving the visibility through it.
Based on the experimental results described in the paper, a model and simulation
of the PTC heater are proposed. An integration of the PTC model in the vehicle heat-
ing diagram for overall heating process simulation will be made in order to design and
control optimally a PTC heater.
3 State of the Art and Existing Problems
Since the 1990’s, research has been developed regarding the auxiliary heaters that
warm up the interior air as soon as the vehicle is started [18-21]. There have been
difficulties as regards the fact that the auxiliary heaters demand a large quantity of
energy, and consequently they operate only when the engine is running, which in-
creases the fuel consumption and the pollution.
After the significant increase in the fuel prices in 2006, much automotive research
has shown renewed interest in fuel-efficient vehicles. In gasoline-fuelled vehicles,
about 40% of fuel energy is wasted in exhaust heat, while 30% of energy is trans-
ferred through the engine coolant. Because of that, these engines produce little heat
excess, especially in winter time, in city traffic and traffic jams, and they are unable to
warm quickly the passenger compartment to a comfortable level [12].
Various vehicle models from leading manufacturers around the world are now
equipped with PTC auxiliary heaters. In 2007, 65% of all diesel vehicles in Europe
were equipped with an auxiliary heater and this is expected to rise to 90% by 2010. It
is obvious that PTC heaters will not be limited to diesel vehicles but will be also used
in gasoline-powered ones as well [13].
Commonly used PTC materials include high density polyethylene (filled with
graphite), polymeric and titanate ceramic materials (which work by grain boundary
effects). PTC heaters based on barium titanate ceramic (BaTiO3), having high resis-
tance and power-dissipation characteristics, are mostly used in automotive industry
[1], [2], [5], [7], [8].
Several authors have studied the resistance – temperature dependency of PTC heat-
ers [3], [5], [6], [11], [15]. Over the years, various theoretical and experimental mod-
els have been developed to describe the PTC heater effect, e.g. in semiconducting
BaTiO3, such as Heywang (the most accepted model), Jonker and Wang [4], [6], [7].
A comparative analysis between PTC thermistors and heaters sustains the devel-
opment of new heating solutions.

Characteristics of the PTC Heater Used in Automotive HVAC Systems 463
4 PTC Thermistors and Heaters
PTC thermistors have specific features: thermal self-regulating and efficient energy
consumption, no danger of fire since there are no glowing parts, no excess tempera-
ture protection required, no smell, no radiation and oxidation, fast thermal response
time, temperature setting from +50 °C to + 320 °C, compact design and long life
service [10].
PTC thermistors have a wide variety of applications, which can be divided into two
distinct categories [22]. The first category utilizes the resistance-temperature or volt-
age-current characteristics of the PTC thermistor. These are known as self heated
applications and include: self-regulating heaters, PTC over-current protectors, time
delay, motor starting etc. The second category comprises zero power or sensing appli-
cations (e.g. temperature sensors and control) [17].
The PTC thermistor has a low value of electrical resistance at low temperatures.
When a voltage is applied to the cold elements, a high current is generated and the
value of resistance rises with the temperature. As soon as the current flows through
the device, the elements warm up by electrical dissipation until a steady state is
reached and the resistive elements have reached their working temperature. This trig-
gers the reduction of the electric current because the electric charge is unable to cross
the boundaries of the tiny crystals at high temperatures. For this reason, the manufac-
turers of auxiliary heaters propose the use of the PTC thermistor principle [18], [19],
[20], [21].
The PTC heater is mounted inside the HVAC system, after the heat exchanger. In
Fig. 1, there is illustrated the position of the PTC heater and the air flow inside the
HVAC system.
The PTC heater is connected to the vehicle electrical system and it is controlled by
the Engine Control Module (ECM), HVAC control panel and relays [9]. In Fig. 2
there is illustrated the PTC heater control diagram.
Fig. 1. HVAC system Fig. 2. PTC heater control diagram

464 R. Musat and E. Helerea
The PTC heater, illustrated in Fig. 3, consists of a moulded plastic plate (1), an
electrical connector (2) and heating resistive elements with fins (3). The PTC heating
resistive elements (Fig. 4) consist of small metallised ceramic plates (1), which are
layered alternately along the unit core with aluminium radiator elements (2). These
layers are held together by spring elements in a frame. The aluminium elements en-
sure the electrical contacts and transfer the heat to the passing heater air flow [14].
Fig. 3. PTC heater components Fig. 4. PTC heating resistive elements
The heating resistive elements are separated in different heating circuits in order to
adapt the heating power to different requirements and they act as positive temperature
coefficient resistors. The maximum value of the temperature of the elements is around
165 °C, if no air flows through the heating system [12].
The heating elements are switched progressively in order to reduce the demand
factor of the generator, to prevent load dumps and ECM problems caused by high
currents.
The PTC heater operates only: (i) at low external temperatures; (ii) when insuffi-
cient heat is supplied by ICE cooling system; (iii) at reduced loads of the generator.
The total electrical power consumption for a PTC heater ranges between 900 W
and 2000 W, depending on the air volume of the vehicle interior. Vehicles equipped
with PTC heater are fitted with a more powerful generator.
5 Experiments and Results
In order to model and simulate the processes which take place in the HVAC system,
data related to the characteristics of the auxiliary heating devices are required. For a
PTC heater, it is important to analyze and measure the temperature – resistance char-
acteristics and the voltage-current characteristic in order to design the PTC heater and
to determine the optimal thermal comfort.
To measure and obtain the temperature-resistance curves, R = f (T), a test bench
was developed. The test bench, illustrated in Fig. 5, consists of a heating camera (type
Carbolite PF/200), a cooling camera (type Derby DK 9620), temperature sensors
(placed inside the heating/cooling camera) and a data acquisition system.
To measure and obtain the voltage-current characteristics U = f (I), a test bench
was also developed. The test bench, illustrated in Fig. 6, consists of a battery (type
Rombat Pilot Diesel Hybrid, 12V / 77Ah), an ammeter, a voltmeter, temperature
sensors (placed on PTC heating elements) and a data acquisition system.
All the acquired data are processed by means of MATLAB tool.

Characteristics of the PTC Heater Used in Automotive HVAC Systems 465
Fig. 5. Test bench scheme implementation
for thermal resistance characteristics
Fig. 6. Measurement scheme for determining
U = f (I) characteristics
To make the experiments, a 1000 W / 13.5 V PTC heater sample was used. This
heater consists of four power stages (two stages of 333 W and two stages of 166 W).
In Fig. 7, there is illustrated the electrical diagram for the PTC heater sample.
Fig. 7. Electrical diagram of the PTC heater sample
In Fig. 8 and Fig. 9, there are illustrated the experimental results representing the
characteristics R = f(T) and U = f(I), for each stage.
Fig. 8. Thermal resistance characteristics Fig. 9. Voltage-current characteristics

466 R. Musat and E. Helerea
As it can be seen in Fig. 8, the obtained data show that the resistance initially de-
creases with the increase in temperature. In the low temperature region, the resistance
curve has a negative temperature coefficient. The minimal values of the resistance
obtained at specific threshold temperatures, Tt and the calculated values for the tem-
perature coefficient of the resistance αR (for the negative and positive slope), for each
stage are given in Table 1. For temperatures greater than Tt, the negative temperature
coefficient is changed to a positive one.
Table 1. The values of the resistance R, the threshold temperatures Tt. and the temperature
coefficient of the resistance αR for each stage.
Stage R [Ω] Tt [°C] αR- [K-1] αR+ [K-1]
1 0.367 120 - 0.0044 + 0.937
2 0.895 110 - 0.0037 + 0.285
3 1.34 130 - 0.0024 + 0.211
4 0.35 130 - 0.0042 + 0.984
The values of the temperature coefficient of resistance require specific thermal
regulating process characteristics of the PTC heater. Based on these coefficients, the
PTC heaters can be properly designed. The thermal resistance characteristic is appro-
priate for preventing the PTC heater overheating.
As it is shown in Fig. 9, the voltage-current characteristics are linear until the PTC
heating elements have reached their working temperature and a steady state is
reached. After these values, the electrical current drastically decreases.
Warming up of the PTC heater depends on the electrical power inside the elements
and can be described with the following relationship:
dt
dT
CTT
tTR
tU
tIIU PTCA⋅+−⋅⋅=⋅ )(
),(
)(
)()(
2
δ
. (1)
where:
U(t) – voltage supply; I(t) – electrical current through the PTC heater; δ – heat dis-
sipation of PTC; T – present temperature of PTC; TA – ambient temperature; CPTC –
heat capacity of PTC; dT/dt – time temperature variation.
Based on the voltage and current data values, the dissipation power – temperature
characteristics for each stage are obtained and illustrated in Fig. 10.
As it can be seen in Fig. 10, there is a maximum dissipation power corresponding
to the threshold temperatures for each PTC heater power stage.
The obtained values of the parameters and the characteristics can be compared with
those from specialty literature. The PTC heater tested above proved to have the same
characteristics with those from a typical PTC thermistor model. The developed test
bench allows testing these characteristics rapidly, efficiently and accurately.
The high responding time is the typical feature of the PTC heater. This is useful
after the vehicle is started: till the engine reaches its operating temperature, the con-
ventional heating system does not supply the necessary heat for the interior air. This
additional heat can practically be accounted to the PTC heater.

Characteristics of the PTC Heater Used in Automotive HVAC Systems 467
Fig. 10. Dissipation power – temperature characteristics for PTC heater sample
6 Conclusion and Future Work
This paper deals with an analysis on auxiliary heating devices used in vehicle thermal
comfort systems. A test bench system was developed for determining the parameters
and characteristics of PTC heater and measurement data were processed.
Based on the experimental results described in the paper, a model and simulation
of the PTC heater was developed. An integration of the PTC model in the vehicle
heating diagram for overall heating process simulation is to be made.
The PTC heaters are necessary in some conditions (e.g. winter time) to increase the
passenger’s thermal comfort. The PTC heaters are ideal for thermal comfort control
due to their capacity of thermal self-regulating and high responding time.
The challenge is that the PTC heater demands a large quantity of electrical current,
so it operates only when the engine is running and thus increases the fuel consump-
tion and the pollution.
New higher capacities of PTC heater would be possible with a 42 V vehicle wiring
system. The transition to 42 V voltages, the decentralisation of power distribution and
the development of more complex energy sources will provide new innovative sys-
tems to supply PTC heaters properly.
Since vehicles are getting more efficient, they free less heat for the heating system;
thus, by including auxiliary devices consuming electric energy, the heating time will
be improved and the engine efficiency increased. The electric energy is taken from
the engine through the generator/battery system, which reduces the overall efficiency
of the vehicle. Consequently, an adequate energy balance and management should be
accomplished.
Increasing the overall efficiency is the main objective in any PTC heater design
and can be achieved by different techniques, such as: improving the technology of
materials, manufacture and control processes.

468 R. Musat and E. Helerea
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