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DOI 10.5162/sensor2013/P1.

A Low-cost Capacitive Fuel-level and Quality Sensor

for Automotive Applications

Dr.-Ing. Volker Skwarek , Dipl.-Ing. Torsten Eggers

Hella Fahrzeugkomponenten GmbH, Dortmunder Str. 5, 28199 Bremen, Germany

Due to internationally significant differences in fuel ingredients and the resulting fuel quality, modern
direct injection gasoline and diesel engines need to be able to adapt their point of optimal operation
according to the used type off fuel. This is necessary to achieve optimum fuel efficiency, exhaust gas
minimization and protection of exhaust gas after-treatment components. Therefore it is important to
understand at least minimal qualitative properties of the fuel in the cars tank. In future fuel scenarios
and emissions regulation standards this is no longer possible with software based virtual sensors
based on lambda and knock-sensor read-out only, because the control is delayed.
The conventional fuel level sensors based on float and potentiometers - nowadays encapsulated only deliver fuel level information at relatively high cost. They cannot provide fuel quality, temperature
and mixture information and feature a passive (resistive) output only. In most saddle fuel tank
configurations two are used to get accurate fuel level information to estimate remaining mileage.
With a capacitive sensor based on a specific printed circuit board design it is possible to measure five
characteristic properties of fuel level and quality with only one sensor structure: fuel level, major
components, fuel layering by decomposition, tank inclination and fuel movement.
Basic principles of the measurement system proposed are a combination of capacitance
measurement and impedance spectroscopy. In this paper the concept of this new sensor and first
results of simulation and measurements are presented.
Key words: level sensor, fuel, capacitive, fuel quality, impedance spectroscopy

Automotive fuel consistency (= normative
purity) varies internationally significantly.
Furthermore the higher the potential shortage of
oil the more alternative fuel ingredients such as
ethanol are added. Unfortunately as well the
optimum point of operation of the engine has to
be adapted to the fuel as parts with permanent
contact with potentially aggressive fuel may be
One of these parts to be protected is the
currently most popularly used fuel level sensing
principle: Floats in the fuel swimming on the
surface. With a usually metal rod connected to
a potentiometer its resistance is a measure for
the fuel level. But some fuel components
increase corrosion of the metal whereas more
precious - and more expensive - materials have
to be used, making this sensing principle more
expensive. Because there is no additional
functionality within the conventional sensing
principle the price to performance ratio
becomes unbalanced. This is even worse in up-

AMA Conferences 2013 - SENSOR 2013, OPTO 2013, IRS 2 2013

to date fuel tank designs, because in most

cases two fuel level sensor, in rare cases like
four-well drive cars even four sensors, have to
be used due to the saddle-like geometry.

Fig. 1 Complete fuel tank (saddle configuration)

with two fuel supply modules [Source: Delphi].

In each of the two separate volumes in Fig. 1

one fuel supply module is mounted, both of
them equipped with a float based fuel level


DOI 10.5162/sensor2013/P1.4

Capacitive sensor element

The capacitive sensor element consists
principally of etched stacked capacitors on a
standard printed circuit board for determining
the fuel level itself. As the fuel consistency does
not need to be homogenous in the tank it might
be complicated to determine an exact level with
a deviation of less than 2% within a potential
mixture of different fuel components. For that
reason reference elements are integrated into
the capacitive structures to obtain the local
values for the dielectric constant [see Tab. 1]
used for compensation of neighbored level
measuring stacks.
Fig. 2 Fuel supply module with fuel pump etc. and
fuel level sensor (in front) [Source: Delphi].

Tab. 1: Dielectric constant of media in fuel system

(for measurements at low field strength, at room
temperature with e.g. 100 kHz to 1 MHz frequency)

Several new challenges have to be solved with

an innovative fuel level sensor in order to rebalance the price to performance (functionality)


r [-]





Using no metal in contact with fuel



Using no moving parts



Determining at least basic fuel components

such as conventional fuel, water, ethanol
content of gasoline fuel, biodiesel content of
diesel fuel





Optionally, detect quality relevant fuel

properties like e.g. evaporation behavior
without using a complex method like e.g.
near infrared absorption spectroscopy used
in dedicated in-line fuel quality sensors
currently under development [1,2]

Introduce diagnostic features, power-down

and digital bus interface

Furthermore, with a specially designed

differential sub structure of these capacitive
stacks the inclination of the fuel can be
measurement. And a fast changing inclination is
a sign of fluid sloshing; therefore the
measurements can be integrated over a period
of the movement.

Capacitive fuel measurement

Although capacitive level sensing is a wellknown principle [3,4,5], it has never established
in automotive fuel level measurement so far,
probably because of its sensitivity against side
effects like changes in dielectric constant and
conductivity of the fluid. But with increasing
processor power even within sensors and an
compensation of the side effects is possible
with the advantage of measuring more physical
properties but only level.
Precisely, the new approach is able to measure
respectively detect the following properties:

Fuel level

Fuel composition, layering




Fig. 3 Printed circuit board test layout structures;

left to right: level with intermediate references (1),
level calculation by center of gravity calculation (2),
simple level (3), inclination (4), simplified from (1) (5)

To obtain the second most important

information of this sensor, the potential fluid


DOI 10.5162/sensor2013/P1.4

components, the r between different capacitive

electrodes in the capacitor stack will be
measured. With significantly different r for
water, regular fuel, air and alcohols (see Tab. 1)
these ingredients can easily be detected.
dependency of r and tan , these components
can even be sub-divided by performing a
impedance at different frequency points. With
this electrical spectroscopy method the fluid can
be basically examined for different components.

Taking into account all assumptions, the

electrical equivalent network can be simplified
as depicted in Fig.

Sensor design
The complete electrical equivalent network of
arrangement and the surrounding fluid are
depicted in Fig. 4.

Fig. 5 Simplified electrical equivalent network of

sensor arrangement and electrolytic fluid to be

Fig. 4 Electrical equivalent network of sensor

arrangement and electrolytic fluid to be analyzed

The network can be simplified taking into

account, that the dielectric insulation 1 (PCB
substrate FR4), the dielectric insulation 2
(molded epoxy polymer) can both be assumed
as ideally non-conducting at low measurement
frequencies, thus Rp2 = . The conductivity of
the backside shielding layer (Cu-plane) can be
set as infinite, thus Rs = 0.

The resulting real-world fuel mixture situation is

more complex than binary mixtures of two pure
fluids. Therefore, an effective calculation of r
based on a physical modeling approach like
e.g. Debye-models is hardly possible.
Sensor manufacturing
The sensor printed circuit board is populated
with all SMDs and afterwards over-molded with
a thin layer of epoxy polymer (EP) by injection
molding to ensure long-term stability of the
electronics and capacitor stack against
chemically aggressive fuels. In a second
molding step, a connector and fixation
structures are built-up from fuel resistant
polyamide thermoplastics (see Fig. 6).
A flexible tooling concept is applied to cope with
different lengths of the sensor for different tank
geometries. The maximum measurement range
is 300mm. Different connectors as well as
fixations can be used in the second molding
step by individual tools.

The electrochemical double-layer capacitance

is neglected (shortcut) in this application,
because of measurement frequencies >1 KHz
compared to the very high capacitance due to
nm range typical double layer thickness.
The pure fuel always shows a specified
electrostatic discharge protection requirements.
On the other hand, fuel conductivity might be up
to 10S/cm due to the penetration of impurities
during fuel refilling. Therefore, the fuel
conductivity cannot be neglected. Additionally,
especially ethanol is hygroscopic: E100 (100%
ethanol fuel) can take-up up to 7% water from
the environment. The conductivity of gasoline,
ethanol mixtures has been studied in detail [6].


Fig. 6 Sensor manufacturing process steps:

1. Sensor PCB with press-fit pins and all SMD
populated (left); 2. Sensor PCB over-molded with
thermoset EP material with stiffening structure
(middle); 3. Completed sensor with molded thermoplastics connector/ fixation element (right)


DOI 10.5162/sensor2013/P1.4

Sensor modeling & simulation

A multi-physics FE-model has been set-up to
optimize the sensor design and to understand
and minimize the sensitivity to manufacturing
process and material tolerances.

Fig. 9 Capacitance over normalized level for

gasoline fuel (r =2.2) for the layout #5 (see Fig. 3)

Fig. 7 Complete 3D FE-model to simulate

electrical field distribution and extract capacitance

Utilizing the FE-simulation model (Fig. 7) the

capacitive sensor structure design is optimized
and manufacturing parameter tolerances are
studied (Fig. 8). The capacitances of each stack
element are extracted vs. fluid level for different
fluids (see Fig. 9 and Fig. 10)

Fig. 8 Sensitivity analysis (r =24) for track width

widening (blue), PCB Cu-electrode thickness
variation (red), thickness variation of mold compound
(green). The target design geometry is shown in


Fig. 10 Capacitance over normalized level for

ethanol fuel (r =24.3) for the layout #5 (see Fig. 3)

Sensor Hardware, Software and Calibration

For the level and mixture measurement the
capacitances are multiplexed by analog
multiplexers and converted by a single-channel
capacitance to digital (CDC) application specific
integrated circuit to a digital I C output. The
selection of the capacitances to be converted is
done by a small 8/16 bit microcontroller also
performing the necessary algorithms and
calculations to compensate all influences, e.g.
temperature dependency. In the lab set-up a
mixed-signal programmable system-on-a-chip
(PSOC) is used for the optional fuel quality
measurement by impedance spectroscopy.
The end off line (EOL) in the automated
manufacturing line will use a measurement at
air with defined temperature and humidity and a
fluid dummy measurement. By this method
tolerances introduced by the production
process, tool wear and material (e.g. FR4, mold
compound) can be compensated.


DOI 10.5162/sensor2013/P1.4

Sensor Measurements
Initial measurements could be performed up to
now only because of some instability in early
stage lab measurement set-up respectively
prototype electronics configuration.

neighboring capacitances add additional stray


Fig. 12 Impedance spectra (imaginary part) for

different ethanol concentrations (0.3 mm mold

Fig. 11 Capacitances of basic measurement test

structure for calculation of center of gravity from all
mutual capacitances Cn with n= [1..5]. C6 is the
reference electrode. All capacitances are measured
between numbered electrode and unnumbered
counter electrode.

Using one of the fuel level capacitor

arrangements for impedance spectroscopy one
obtains curves like the one in Fig. 12. The
ethanol concentration can be calculated by e.g.
using a support vector machine algorithm.

The lab prototype printed circuit boards only

contain analog multiplexers with digital SPI
control to switch the mutual capacitance
electrodes to coaxial cable outputs connected
to a LCR-meter or impedance analyzer or the
respective measurement IC (single channel
CDC or complex impedance measurement IC).

Summary and Outlook

The first measurements show deviations
measurements and initial FE based simulations.
The layout of the sensor PCB must be

Tab. 2: Capacitances measured with Agilent LCR

meter 4285A (fM = 150 kHz, UM = 500mVpp, DCBIAS=
0V, open compensation, 2m cable), sensor PCB
completely covered by fluid inside glass container

Parallel to layout optimization a DoE will be setup to perform lab measurements for algorithm
The product idea and production concept
enable the realization of this automotive sensor
at competitive pricing level, which was not
possible in the past. Next step in the
development is the release of feasibility
samples for initial design validation (vibration,
thermal shock etc.) and first customer sampling.














1.537 1.975


[1] A. Lunati, J. Fournel, Innovative on board optical
sensor dedicated to measure water, alcohols and
ethers content in biofuels for Flexfuel engine
optimization, SAE conference paper, 2008-012451 (2008)


1.430 1.815



1.152 1.478



1.199 1.474


The sensor capacitances C1 - C6 do include an

additional offset, because there is neither
perfect shielding nor guarding of the conductors
leading to the measurement electrodes in this
simple measurement set-up. Due to the missing
guarding between the single capacitances,


[2] Euripides-Eureka research project IQfuel,
[3] Patent application DE19916979A1, method for
level measurement and level sensor, 1999


DOI 10.5162/sensor2013/P1.4

[4] Patents US 6,502,460B1, Fluid level measuring

system, Jan. 7, 2003; US 6,497,144B1 &
6,498,566B, Method for measuring fluid level,
Dec. 24, 2002, US 6,823,731B1, Liquid level
sensing assembly and method for measuring
using same, Nov. 30, 2004

Sensoren im Automobil, 79-91(2006), ISBN-13:

[6] D.W. Kirk, Conductivity of gasoline-ethanol-water
mixtures, FUEL, vol.62, December, 1512-1513

[5] H. Steins et al., Selbstkompensierender

kapazitiver Flssigkeitenfllstandssensor,
