Engine Coolant Temperature Sensor in Automotive Applications

Abstract

Abstract—For safety, comfort, performance, and reliability reasons, modern vehicles keep track of a variety of variables and quantities using sensors and integrated systems. Among those quantities, the temperature is the most frequently measured variable for all of the above reasons. Any change in external or internal temperature triggers the relevant system to act accordingly. In fossil fuel vehicles, the engine temperature is continuously monitored and kept at a certain level to make the engine perform optimally. The primary sensor involved to monitor the engine temperature is known as the Engine Coolant Temperature Sensor, and the temperature is regulated via a liquid substance called engine coolant. This paper focuses on the main characteristics, fabrication, and the way an Engine Coolant Temperature sensor works, with an examination of the Negative Temperature Coefficient (NTC) Thermistor.

Full text

Engine Coolant Temperature Sensor in Automotive
Applications
Ishfaque Ahmed
Department of Automotive Software Engineering
Technische Universit¨
at Chemnitz
Chemnitz, Saxony, Germany
Abstract—For safety, comfort, performance, and reliability
reasons, modern vehicles keep track of a variety of variables
and quantities using sensors and integrated systems. Among
those quantities, the temperature is the most frequently measured
variable for all of the above reasons. Any change in external
or internal temperature triggers the relevant system to act
accordingly. In fossil fuel vehicles, the engine temperature is
continuously monitored and kept at a certain level to make
the engine perform optimally. The primary sensor involved to
monitor the engine temperature is known as the Engine Coolant
Temperature Sensor, and the temperature is regulated via a
liquid substance called engine coolant. This paper focuses on the
main characteristics, fabrication, and the way an Engine Coolant
Temperature sensor works, with an examination of the Negative
Temperature Coefficient (NTC) Thermistor.
Index Terms—ECT sensor, coolant temperature sensor, NTC
Thermistor, thermally sensitive resistor, temperature measure-
ment, ECM
I. INTRODUCTION
Engine Coolant Temperature (ECT) sensor measures the
engine temperature and indicates how much heat the engine is
giving off. The sensor works with the Engine Control Module
(ECM). ECT sensor monitors the engine coolant temperature
continuously and makes sure the engine is running at the
optimum temperature. The resistance of the temperature sensor
(NTC Thermistor) varies with temperature when ECM sends
voltage to the ECT sensor as described in Circuit schematic
Fig.4. ECM uses temperature and resistance variation to mon-
itor temperature changes. The ECM uses reference voltage
to regulate fuel injection, ignition time, control the radiator
fan speed, and update the automotive dashboard’s temperature
gauge. In most cars, the installation place for ECT Sensor
is the thermostat housing. ECM, when required, turns-on
the radiator fan and helps the engine release its heat to the
atmosphere with regulating fan speed, if the coolant to the
radiator has high temperatures. Meanwhile, the cold coolant
received from the radiator absorbs the engine heat. ECT sensor
consists of a sensing element, conductive metal, conductor,
thread, hexagonal corona for a wrench, and electrical connec-
tor, shown in Fig.1.
The Thermistor is the abbreviation of a Thermally Sensitive
Resistor and classified as a ceramic semiconductor. Ther-
mistors have two standard types (1). Positive Temperature
Coefficient (PTC) Thermistor and (2). Negative Temperature
Coefficient (NTC) Thermistors. PTC Thermistor increases
resistance concerning temperature increase while, on the other
hand, NTC Thermistor resistance varies inversely with its
temperature. NTC Thermistor is the primary sensing element
of the ECT sensor, while PTC Thermistors applications are
short circuit current limiters (electrical valves and capacitors)
[1]-[2].
There are four most common types of contact temperature
sensors used in automotive, domestic, industrial, and medical
applications. These temperature sensors are (i) Thermocou-
ples, (ii) Resistive Temperature Detectors (RTDs), (iii) Ther-
mistors, and (iv) Integrated Circuits (ICs) [3]. The operating
range of these sensors varies such that, thermocouples and ICs’
changing parameters are voltage while RTDs and thermistors’
changing parameters are the resistance. However, the usage
of these sensors depends on various environmental variables
such as temperature range [°C], accuracy [±°C], sensitivity
[°C], response time and cost [4].
Thermocouples measure the high-temperature range of
about -270°C to +2300°C. The material used in thermocou-
ples are iron, platinum, rhenium, tungsten, copper, chromel
- alumel, and constantan. Thermocouples produce output in
millivolts. Therefore, precision amplification is required for
information processing and error minimizing. The main dis-
advantage of thermocouples is lower sensitivity and accuracy
with compare to thermistors. RTDs are used to measure high
temperatures (-200 to +650). Fabrication material for RTDs
are platinum, nickel, and copper. Compare to a thermistor,
RTDs are less sensitive and have slower response times.
Nevertheless, out of all temperature sensor mentioned above,
NTC Thermistor holds the best place among all the mentioned
temperature sensor due to its high sensitivity (-2°C to -6°C,
at 25°C), accuracy (±0.001°C) fast response time (0.1 to 10
sec.) at a much lower cost with operating range from -50°C
to 250°C. In automobiles, the main applications of the NTC
Thermistor are temperature measurement and monitoring of
cylinder head, exhaust gas, air conditioning system, braking
system, cooling water, and oil [4].
The organization of the paper is as follows. Section II
describes the components of the ECT sensor. Section III
comprises the fabrication and necessary steps, and physical
parameters measurement to manufacturing the ECT sensor and
sensing element (NTC Thermistor). Section IV consists of the
working principle with mathematical equations. Section V de-
scribes the results and discussions. Finally, section VI consists

of the conclusion of the paper. This paper’s objective is to
present a comprehensive study of Engine coolant temperature
sensors, including fabrication and working principles of engine
coolant temperature measurement using NTC Thermistor.
II. COMPONENTS OF ENGINE CO OL AN T TEM PE RATU RE
SEN SO R
The ECT sensor is composed of different components.
These components measure the engine coolant temperature
with the help of an NTC Thermistor or provide a metallic coat
to prevent fluid flow inside the ECT sensor. The appearance
and components of the ECT sensor are shown in Fig.1.
Fig. 1. Components of ECT sensor [5]
A. Terminal
Two-wire terminal or connector acts as an interface, and
it provides connectivity between ECT sensor and ECM, as
shown in Fig.4. Copper or Silver is the material used for
the terminal, due to high electrical conductivity. The two-wire
terminal is the last part of the wire soldered on both sides
of the NTC Thermistor. The two-wire conductive terminal is
encapsulated with a diameter of 13mm thermosetting plastic,
as shown in Fig.3.
B. Hexagonal corona for wrench
A standard wrench place with six equal edges are available
over the ECT sensor for tightening or loosening (apply torque
to turn) the ECT sensor inside or outside the thermostat
housing. The standard Hexagonal size is 19mm, as shown in
Fig.3.
C. Thread
Threads are crafted over the ECT Sensor and work as a
mounting nut for ECT sensor. Threaded mounting provides
easy to install and fix reliable operation in hostile environ-
ments. ECT sensor thread size is M12 x 1.5, as shown in
Table 4.
D. Conductor
Kovar two-wires are soldered to the electrode surface of
NTC Thermistor. These two-wires provide an electrical con-
nection between the vital sensing element (NTC Thermistor)
of ECT sensor and ECM via terminal, as shown in Fig.1.
E. Conductive metal
NTC Thermistor is coated with a high-temperature conduc-
tive metal, which protects the sensing element from the fluid,
as shown in Fig.1. The diameter of the Conductive metal is
8.30mm, as shown in Fig.3.
F. Negative Temperature Coefficient (NTC) Thermistor
NTC Thermistor is high temperature-sensitive metal ox-
ide semiconducting ceramic device used to measure engine
coolant temperature as it is the main element of the ECT
sensor and encapsulated inside the conductive metal shown
in Fig.1.
III. FABRICATION OF ENGINE COO LA NT TEMPERATURE
SENSOR
A. Fabrication of NTC Thermistor
NTC Thermistors are very sensitive (typically ten times
more than RTD (Platinum) resistance), small size, and fast
response time ( in milliseconds). Disadvantages include non-
linear nature, they have limited temperature operating range,
and they are prone to self-heating due to the electrical current
sensitivity [6]. NTC Thermistors detect the small change in
temperature very quickly [7]. The most common semiconduc-
tor material for the fabrication of NTC Thermistor is a tran-
sition metal oxide such as Nickel(II) oxide (NiO), Cobalt(II)
oxide (CoO), Manganese(II) oxide (MnO) as shown in Table.
4 [4]-[8]. Metal oxide NTC Thermistors result in a long period
of repeatable temperature measurement [9]. Commercial NTC
Thermistors can be found in a glass bead, disk, rod, washer,
and flake form configuration. These devices can be coated
with resin, glass, or be painted. The protection capsule (coat)
isolates the NTC Thermistor from the cooling fluid. For low
grade ≈300°C temperature applications NTC Thermistor with
nonstoichiometric iron-oxides are used, and is available with
mixed metal-oxide, for ≥300°C applications refractory metal-
oxide is suitable, while zirconia doped with earth-oxide is
recommended for higher temperature applications [10]-[11].
Various chemical composition are prepared in two different
groups for NTC Thermistor manufacturing: (Mn1.62Ni0.72
Co0.57−xSi0.09)O4(0 ≤x≤0.12) and (Mn1.2Ni0.78 Co0.87−x
Cu0.15Six)O4(0 ≤x≤0.15) with applying high temperature
to ball-milled for 24 hours in a using ZrO2(Teflon jar) and
dried at 120°C in an oven for 2 hours, then distilled water and
polyvinyl alcohol (PVA) is mixed into the calcined powders,
and the mix is ground in a mortar. Again, the ground powders
are ball-milled for 24 hours using ZrO2, which form a ceramic,
in the next phase it is flattened using a machine at a pressure
of 750 kg/cm2to make a good compact as shown in Fig.2.
[8].
NTC Thermistors are fabricated from a mixture of high
purity Manganese (Mn), Nickel (Ni), Cobalt (Co), Copper
(Cu), and Silicon (Si) oxides (O) that are transition-metal
oxides powder. Suitable proportions for NTC Thermistors are
shown in given above table 1.
In step 1, a good compact sheet from the material is
prepared and polished in a 3 inches diameter with a 0.5mm

Fig. 2. Fabrication process of NTC Thermistor [4]-[9]
thick layer formed by the electrode (step 2). In step 3, the
compact electrode are sliced into chips (0.75mm x 0.75mm x
0.5mm in size). In step 4, Kovar wires are welded or soldered
to the electrode surface to provide an electrical connection.
Finally, in steps 5 and 6, a chip is coated with resin, glass, or
be painted and formed a final NTC Thermistor [9].
Their resistance designates NTC thermistors at 25 °C,
typically ranging between 1 Ωto 100 kΩ. Modern NTC
Thermistors relatively provide the high accuracy of ±0.01 to
±0.05, are widely available with acceptable accuracy, and are
±1 °C [7]. However, commercially available NTC Thermistor
temperature range varies from up to 1000 °C and depends on
the particular metal oxides used and on the covering. NTC
Thermistor with the most stable temperature measurement has
a restricted range, NTC Thermistors with glass encapsulation
have a range of about -80°C to 300 °C, while NTC Thermistors
with epoxy encapsulation have a temperature limit of about
150 °C, shown in Fig.5 [6].
B. Physical parameters of Engine coolant temperature sensor
The physical parameters, as measured in mm, are necessary
for the standard production of the ECT sensor. These calcu-
lations help to optimize raw material procurement, smooth
production process and minimize waste of resources. These
physical measurements are available in Fig.3 for ECT sensor.
Fig. 3. Physical configuration of ECT sensor [12]
IV. WORKING PRINCIPLE OF ENGINE COO LA NT
TEM PE RATU RE SE NS OR
A. Engine Coolant Temperature Sensor Circuit Schematic
ECT sensor converts coolant temperature into voltage with
the help of ECM. The voltage is higher when the engine
is cool, and the voltage is lower when the engine is hot.
ECT sensor consists of a two-wire circuit (NTC Thermistor)
immersed in coolant and measures the temperature, typically
supplied with a voltage of +5V. The onboard Engine Control
Module (ECM) uses the signal of ECT sensor as a correction
factor when calculating ignition and injection duration. The
ECT sensor and ECM circuit schematic shown in Fig.4. Never-
theless, the two-wire NTC Thermistor negates the resistivity of
NTC Thermistor and provides accuracy as ±0.01 to ±0.05°C
[10].
Fig. 4. ECT sensor and ECM circuit schematic [12]
B. Electrical Characteristics
Electrical properties are always present in metallic solids
due to the existence and free movement of electrons. Typically,
this is a substantial reason for higher conductivity. On the
other hand, in non-metallic solids (electronic semiconductors),
electrical properties exist due to the electrons’ movement or
ionic movements (ionic conductivity). Generally, electrons and
ions movement take place simultaneously. Electronic semicon-
ductors are used as a material for NTC Thermistor because
of ionic conduction and electrons movement achieved by

chemical changes. Generally, electrical conductivity is proven
by Eq. (1) [4]:
σ=neµ (1)
where nis the number of current carriers, eis their charge,
and µis the mobility. While the number of current carriers is
constant in metallic solids but the mobility decreases gradu-
ally with temperature due to collisions of electron-phonon.
Consequently, increasing temperature causes a reduction in
conductivity [4].
Oxides are electrical insulators, and a mix of oxides have
transitional electronic states that change the ceramic to a
semiconductor [6]. When the temperature increase, resistance
decrease, and vice versa. Sensitive resistance is exponential to
temperature, as shown in the Arrhenius Eq. (2) [4]:
ρ(T) = ρ∞exp Ea
kT (2)
where ρ∞is the resistance of material at infinite temperature,
Eais the activation energy required for the electrical conduc-
tion, kis the Boltzmann constant, and Tis the temperature (in
Kelvin). Alternatively, Eq. (2) is rewritten for NTC Thermistor
with fixed dimension and resistivity, as shown in Eq. (3) [4]:
R(T) = Aexp B
T(3)
where A=R∞is the resistant of material at infinite
temperature (i.e., 1
T= 0) and Bis the constant value of
NTC Thermistor, expressed in K, which is committed by the
activation energy qwith Boltzmann’s constant relationship [8]:
B=q
k(4)
Hence, distinctness of Eq. (3) shows the sensitivity coefficient.
The sensitivity is the fractional change in resistance for a 1°C
and sensitivity approximately equal to the α= 3.85 ×10−3°C
as shown in Eq. (5) [4]-[6]:
α=1
R·dR
dT =dln(R)
dT =−B
T2(5)
Therefore, Eq. (5) shows the temperature sensitivity decreases
in case of temperature increment. Large B-Values and resistiv-
ity require for high-temperature applications. Otherwise, NTC
Thermistor shows little sensitivity changes in temperature.
The large Bvalues for high-temperature applications can be
calculated from resistance measurements as follows in Eq. (6)
[4]:
B=
ln R1
R2
1
T1−1
T2(6)
where R1and R2are the resistance at the temperatuers T1
and T2respectively. Most, NTC Thermistor commercially
manufacturers specify Bvalues as a standard temperature
between between 25°C and 100°C.
Hereafter, the approximation of temperature relationship
with NTC Thermistor resistance and modeling of NTC Ther-
mistor is elaborated with Eq. (7) [13]:
RT=R0exp 1−B1
T−1
T0 (7)
where RTis the resistance at temperature T,R0is resistance
at temperature T0,Bis a constant value for NTC Thermistor
material, and Tis the NTC Thermistor temperature in degrees
Kelvin. While the temperature coefficient of resistance is
available in Eq. (4) [13]. The resistance properties of NTC
Thermistors are negative and nonlinear, as shown in Eq. (6)
[10].
C. Self Heating
The electric current must pass through NTC Thermistor to
measure the resistance. Passing current dissipates heat and
increases the temperature, which is known as self-heating
error. It is directly proportionate to the dissipated power
and the thermal resistance between NTC Thermistor and its
proximity, as shown in Eq. (8) [6].
∆Tsh =I2R(T)(ρint +ρext) = V2
R(T)(ρint +ρext)(8)
where Iis the sensing current, Vis the voltage across the
NTC Thermistor, and ρint and ρext are the internal and
external thermal resistance which are associated with the NTC
Thermistor and its surrounding environment. The ρint depends
on the dimension and fabrication material, while the ρext
depends on the thermally conductive medium (velocity and
viscosity if a fluid) in which NTC Thermistor is dipped.
Self-heating can be an issue for high range temperature
measurement. If a fixed sensing current is applied, the power
dissipation (I2R)at low-temperature increases, and if fixed
voltage excitation is applied, the power dissipation at high
range temperatures V2/R becomes an issue. The thermal
resistance for the NTC Thermistor is expressed as dissipation
constant, which is required to increase the NTC Thermistor
temperature 1°C, and expressed in units of mW°C. For the
example NTC Thermistor, 0.4mk error at 0°C in a stirred oil
bath is smaller than self-heating 40mk error at 100mA. Note
that calibration and usage of NTC thermistors in a similar
environment produce similar self-heating errors while in a
different environment may alter due to trouble in the air or
mixed fluids [6].
V. RE SU LTS AN D DISCUSSION
The guidelines presented here are the setup of the testing
environment of the ECT sensor including NTC Thermistor
results. A testing system consists of a coolant temperature
sensor with different temperature parameters. Table 2 consists
of the ECT sensor thermal and electrical properties of the ECT
sensor.
Table 3 describes the mechanical properties of the ECT
sensor. However, these characteristics and specifications of the
ECT sensor can vary according to the automotive applications.

The temperature sensor’s electrical resistance measures
from -100°C up to 250°C in steps of 50°C in all the results.
The results are visible in Fig.5. It is stats that the electrical
resistance of the NTC Thermistor is decreasing exponentially
with increasing temperature. NTC Thermistor with appropri-
ate reference temperature (T0), often 298.15 K(25). NTC
Thermistor material characteristics with typical constant B
values in the range of 2000Kto 6000K[6]. Fig.5 plots the
characteristics of resistance vs. temperature for a range of
commercially available NTC Thermistor as calculated with
Eq. (7). Note, the resistance vs. temperature characteristics
table can vary and mostly available according to product
specification [12].
Fig. 5. Resistance vs. Temperature characteristics [6]
Fig.6 plots the constant values (B), and for NTC Thermistor
of Fig.1, while Fig.7 plots and shows the sensitivity of NTC
Thermistor with range between -0.03°C and -0.05°C at a room
temperature, Eq. (5) refers to sensitivity.
Fig. 6. The Bvalues for the NTC Thermistors [6]
Fig. 7. Sensitivity for the NTC Thermistor [6]
Fig. 8. NTC Thermistor vs. RTD (Platinum) [11]

NTC Thermistor is ten times more sensitive to temperature
than RTD (Platinum) resistance [6], and NTC Thermistor
resistance-temperature (R/T) curve characteristics are non-
linear. Meanwhile, RTD (Platinum) has a slight difference
irrespective of resistance with temperature. Results are
available in Fig.8. While, Fig.9 plots the temperature
accuracy chart.
Fig. 9. Accuracy for the NTC Thermistor [12]
Table 4 summarises the sensitivity Eq. (5), equivalent re-
sistance, and voltage sensitivities for NTC Thermistor at 0°C,
16.67°C, 33.33°C, and 50°C [6].
NTC Thermistor resistance measurement involves the volt-
age measurement where the accuracy is limited by the volt-
meter resolution or the input-offset voltage. If uncertainty in
voltage measurement is uv, then uncertainty in temperature
measurement is uT=uv
|Sv|=T2
B.uv
IR , where SVis the
voltage sensitivity in Tabl 3, column 5. For example, voltage
measurement of 10 µA for NTC Thermistor and standard
uncertainty at temperature 0°C and 50°C are 0.68mK and
7.4mK, respectively. A combination of T2and the failing NTC
Thermistor resistance leads to uncertainty in the temperature
measurement [6].
NTC Thermistor’s higher resistance and sensitivity leads to
a satisfactory level two-wire resistance measurement and pro-
vides good simplification. However, the lead resistance RL, if
ignored, can become an issue at higher level temperature when
the NTC thermistor resistance is low. The equations shows the
error caused by the lead resistance ∆TL=RL
SR=−T2
B.RL
R,
where SRis the resistance sensitivity for the NTC Thermistor,
as shown in Table 4, column 4.
VI. CONCLUSION
In a nutshell, the engine coolant temperature sensor plays
a significant role in automotive applications. A wide variety
of temperature sensors are available with different materials,
principles, and operating range. However, usage of NTC Ther-
mistor in engine coolant temperature sensor are remarkable for
accurate, fast, reliable, low cost, and ease of application.
ACKNOWLEDGMENT
The author acknowledges that this paper’s worthiness be-
longs purely to existing reference papers and online resources
for the sole purpose of this research report. The writer did
not perform any lab experiments and fieldwork to prove a
scientific theory. Nevertheless, the author also wishes to thank
Dr. Sonia Bradai and Dr. Slim Naifar for their guidance and
encouragement in carrying out this valuable research report.
REFERENCES
[1] W. J. Fleming, “New automotive sensors—a review,” IEEE Sensors
Journal, vol. 8, no. 11, pp. 1900–1921, Nov 2008.
[2] T. Baiatu, “Ptc thermistor and a current limiter device having at least
one ptc thermistor,” Jul. 25 2000, uS Patent 6,094,129.
[3] S. Jagtap, S. Rane, U. Mulik, and D. Amalnerkar, “Thick film ntc
thermistor for wide range of temperature sensing,” Microelectronics
international, 2007.
[4] A. Feteira, “Negative temperature coefficient resistance (ntcr) ceramic
thermistors: an industrial perspective,” Journal of the American Ceramic
Society, vol. 92, no. 5, pp. 967–983, 2009.
[5] H. A. S. A. P. Ltd., “Thermostats, thermoswitches & temperature
senderunits optimum performance,” Accessed on 07.05.2020 17:02,
Available: https://www.hella.com/hella-za/assets/media global/HASA -
Thermo Range Borchure LRes.pdf
[6] D. White, K. Hill, D. del Campo, and C. Garcia Izquierdo, “Guide on
secondary thermometry: Thermistor thermometry,” Bureau International
des Poids et Mesures: Paris, France, 2014.
[7] A. Tong, “Improving the accuracy of temperature measurements,” Sensor
Review, 2001.
[8] K. Park, “Fabrication and electrical properties of mn–ni–co–cu–si oxides
negative temperature coefficient thermistors,” Journal of the American
Ceramic Society, vol. 88, no. 4, pp. 862–866, 2005.
[9] T. Kineri, R. Kogiso, and Y. Kawaguchi, “The characteristics of high
temperature thermistor with new materials,” SAE Transactions, pp. 514–
518, 1989.
[10] P. R. Childs, J. Greenwood, and C. Long, “Review of temperature
measurement,” Review of scientific instruments, vol. 71, no. 8, pp. 2959–
2978, 2000.
[11] G. Lavenuta, “Negative temperature coefficient thermistors,” Sensors-the
Journal of Applied Sensing Technology, vol. 14, no. 5, pp. 46–55, 1997.
[12] D. P. E. Systems, “Coolant temperature sensor part number
12146312,” Accessed on 27.06.2020 13:52, Available: https://pe-
ltd.com/assets/coolant temp.pdf
[13] R. A. Rasmussen, “Application of thermistors to measurements in
moving fluids,” Review of Scientific Instruments, vol. 33, no. 1, pp. 38–
42, 1962.