A, 31 (1992) 60-67
New generation of automotive sensors to fulfil the requirements fuel economy and emission control
J. Binder Siemem Automodve
SA, BP 1149, Avenue
du Mimil, 31036 Toulouse Cedex (France)
Abstract The new passenger car regulations for future emission levels and the increasing demands for fuel economy have led to strong efforts to develop new engine-management systems, exhaust-treatment technologies and new automotive sensors. In this paper a new generation of automotive sensors that help to fulfll the requirements for fuel economy and emission control is presented. The demands on these new sensors include the following improvements: (i) better resolution, higher accuracy and better long-term stability, especially under severe environmental conditions; (ii) good performance at high temperature (e.g., up to 150 “C for rotational speed sensors and 1100 “C for exhaust gas sensors); (iii) short response times for dynamic cylinder-selective measurements; (iv) more efficient compensation methods by measuring different physical parameters. This talk gives an overview on the physical principle, the design, the technology and the application of a new generation of automotive sensors. Representing a potential ‘high-volume’ sensor, we report on an active rotational speed sensor to measure the angular position of the crankshaft. This sensor is based on a newly developed differential Hall IC. The high angular resolution allows more accurate ignition timing as well as ‘misfiring’ detection to be achieved. For the precise control of combustion in future engine-management systems, an ‘air flow sensor’ for cylinder-specific carburatioa, a ‘combustion pressure sensor’ for thermodynamic calculation of the combustion and a ‘fast exhaust gas sensor’ for cylinderselective exhaust-gas analysis can be used. This report will focus on the development of a fast oxygen sensor to measure the oxygen partial pressure of the exhaust gas. As an example of a smart sensor, an ‘alcohol fuel sensor’ for alternative fuel concepts is introduced. This sensor is a key component of a flexible fuel system which can be run on arbitrary mixtures of gasoline and methanol. This sensor measures three physical parameters: the capacitance, the conductivity and the temperature, in order to determine the methanol/gasoline ratio in the fuel rail with high accuracy.
1. Introduction Increasing air pollution and the decrease of crude oil resources, especially in the USA, are forcing the development of new automotive sensors for engine-management systems, exhaust gas systems and alternative fuel concepts. Legislation will define the needs for future automotive systems. As an example, Fig. 1 shows the legislated California hydrocarbon emissions as proposed by the CARB (Californian Air Research Board). Until the year 2000, various periods have been defined where the emission rates of HC, CO and NO, have to be reduced by up to a factor of 10 [l]. In 1998, 3% and in the year 2005 10% of the cars produced in the USA have to be ZEC (zero emission cars). Another driving force is the CAFE (Corporate Average Fuel Economy) regulation, which forces car manufacturers to pay a tax, if the average fuel consumption of their fleet exceeds a defined limit (measured in miles per gallon).
Fig. 1. Legislated California HC emissions vs. durability milage with an assumed linear catalyst deterioration.
Concerning future automotive systems, the following features have to be taken into account: (i) The OBD2 regulation (On Board Diagnostic, version 2) demands a diagnosis of all components of the engine-management system with depollution function.
Q 1992 - Elsevier Sequoia. All rights reserved
(ii) Future engine-management systems will include a cylinder-selective measurement of key parameters like intake air, combustion pressure and the oxygen partial pressure within the exhaust gas. The principle of integrated sensor systems in future automotive engines is shown in Fig. 2. (iii) A further approach is the development of alternative fuel concepts. Alcohol fuels like methanol may be able to play a major role. This paper gives an overview on three sensors, which might become key components for future systems. An active rotational speed sensor, which is used for measuring the engine speed and the crankshaft position, shows better accuracy and resolution compared to currently used inductive sensors . This allows more accurate ignition timing as well as ‘misfiring’ detection to be carried out, as requested in OBD2. A fast oxygen sensor based on thin-film metal ceramic layers shows a response time that is 20 times lower than that of currently used zirconium oxygen sensors. This feature, as well as the temperature range up to 1000 “C, allows a cylinderselective exhaust-gas analysis to be performed . An important step towards the broad introduction of methanol fuel is the flexible fuel vehicle (FTV) concept. FFVs are able to run on arbitrary mixtures of gasoline and methanol fuel. Since the stoichiometric air/fuel ratios of gasoline and methanol are very different, a dedicated sensor is necessary to determine the actual methanol content of the fuel mixture. This value is needed to correct at least the injection time. The alcohol sensor is built in between the fuel tank and the injection actuators. Its output signal, which is a measure of the alcohol/fuel ratio, is conditioned by the engine control unit (ECU) in order to adjust the injection time (Fig. 3) .
Fig. 2. Integrated
sensor systems in future engines.
Fig. 3. Application rail.
of an alcohol
sensor in a fuel
--------- Evaluation logic
Fig. 4. Principle
of an active rotational
2. Active rotational speed and crankshaft
speed sensor for engine position measurement
For ignition timing, modem engine-management systems use a rotational speed sensor to measure the engine speed as well as the crankshaft position. This sensor gives a periodic output signal as an ‘image’ of the teeth of the crankshaft fly wheel. Currently, inductive sensors are most commonly used. A new generation of active sensors is presented here, where the basic sensor device is a differential Hall IC element (TLE 4920 G, Siemens) as shown in Fig. 4. The sensor has to meet the following requirements concerning environmental and electrical influences. 2.1. Environmental infruence parameters The operating temperature range is defmed from - 40 to + 160 “C. The environment is characterized by oil, fuel, brake fluid and salt-spray pollutions and vibrations up to 4Og.Influence of the fly wheel have to taken into account: airYgaptolerances are between 0 and 1.5 mm. The sensor has to be
insensitive to radial vibrations. In addition, the wheel shows material deviations that will influence the magnetic flux. The speed range of the flywheel is specified from 30 to 8000 rpm. Radial vibration
2.2. Electrical influence parameters These are variations in the supply voltage between 5 and 24 V, EM1 pulses up to f 100 V, and reverse polarity of the supply. Figure 5 shows a cross-sectional view of the active sensor. The sensor is magnetically biased by a permanent magnet. The two Hall cells of the IC each detect the magnetic field. The difference between the two output signals is amplified, highpass filtered and fed into a Schmitt trigger in order to be converted into a digital signal. The output stage consists of an open collector transistor which drives a pull-up resistor in the electronic control unit. The amplitude of the output voltage is equivalent to the supply voltage and the frequency is equivalent to the frequency of the passing teeth of the gear. Figure 6 shows the output signal of an active sensor compared with the signal of an inductive one. The active sensor, in comparison to the inductive type sensor, shows the followingfeatures: (i) High accuracy of the absolute signal phase of k0.4 degree crank angle, which is shown in Fig. 7. Figure 8 illustrates the phase angle of the output signal corresponding to the position of the fly wheel. (ii) For misfiringdetection, a high tooth-to-tooth reproducibility is necessary. The achieved value is 0.05 degree crank angle. (iii) As shown in Fig. 7, the phase offset can be specsed to f0.4 degree within a wide range of temperature, air gap and rotational speeds down to idle speed. Due to this, there is no need for
to an inductive sensor.
I:;] -1 .o -40
., ,I 0
*iJ 100 Temwratun
Fig. 7. Phase offset of the active rotational function of the temperature.
speed sensor as a
Fig. 8. Phase angle of the output signal corresponding to the position of the fly wheel for an active rotational speed sensor.
Fig. 5. Cross section of an active rotational
compensation of the speed dependence of the phase shift, as is the case for inductive sensors. (iv) The output signal waveform of the active sensor is rectangular; the amplitude is independent of the rotational speed, which saves some costs
Fig. 9. Application
-l--L1 P (02)
= 1 bar
Fig. 11. Prototype technology.
of a fast binary oxygen sensor in thin-film
Fig. 10. Response
time of an SrTiOI oxygen sensor at 950 ‘C.
for further conditioning of the sensor signal in the ECU (see Fig. 6). In order to fulfd the high requirements on accuracy and reproducl%ility, extremely low tolerances in the production process have to be ensured. For instance, the positioning of the Hall IC element on the PCB has to be within f0.25 mm, which is tested in the production process by an optical inspection procedure. To guarantee a temperature range from -40 to + 150 “C, a low-stress potting material (epoxy resin), which will suppress disturbing piezoresistive effects on the Hkll IC, has been developed. 3. Fast oxygen sensors systems
for engine-management analysis
The application of a three-way catalytic converter is only efficient if the engine is driven within a small range around the stoichiometric air/fuel ratio (A=l).
Fig. 12. Conductivity of Nb-doped concentrations at T-1000 “C!.
SrTiO, layers with different
Currently this is solved by a closed-loop system, where a zirconia oxygen A-probe indicates engke operation above or below A= 1. This A-probe is localized behind the exhaust valves in front of the catalytic converter  (see position 2 in Fig. 9). Siemens is currently developing an oxygen sensor , which shows a response time at the transition A< 1 to A> 1 below 10 ms. This feature is shown in Fig. 10. In addition, this sensor can be operated at temperatures up to loo0 “C; this allows the sensor to be mounted near to the exhaust valves of the cylinder or directly into the exhaust manifold, as shown in Fig. 9 (positions 2a and 3).
(4 0. 01 0. ODS 0.006
Fig. 14. Design of the sensor element of the thin-film oxygen sensor. Contacts (2-2, 6-7); heater (l-g); temperature sensor (4-5).
‘: 0 8
Fig. 13. Characteristic curves of (a) a binary A-probe and (b) a ‘proportional’ A-probe in thin-film technology.
This new generation of oxygen sensors allows the oxygen partial pressure of the exhaust gas to be measured cylinder by cylinder. This can be used to optimize the combustion of each individual cylinder. However, a higher microcontroller capacity of the ECU compared to current systems is needed to evaluate the sensor signal. The key element of these sensors is an oxygensensitive thin-Glen layer of.metal oxide (SrTiO, or BaTi03), which is sputtered onto an Al,O, substrate. A prototype of this sensor is shown in Fig. 11.
A characteristic curve of the conductivity of an Nb-doped layer as a function of the oxygen partial pressure at different doping concentrations is shown in Fig. 12. These curves show that, by changing the doping of the layers, the conductance curve can be changed from equivocal to unequivocal behaviour. Two different types of sensors can be defined, depending on their catalytic behaviour. Figure 13(a) shows the output signal as a function of A for a binary A-probe; Fig. 13(b) shows a ‘proportional’ A-probe which can measure in the range h=0.9 up to A= 1.1. This ‘binary’ transfer function is due to the catalytic activity of the sensitive material. The oxygen sensitivity strongly depends on temperature. Below 600 “C, the effect disappears. To reach the accuracy needed, a temperature stability of roughly &25 “C is needed. Temperature stabilization uses a heater and a temperature sensor, both deposited as platinum thick-&n layers. Both types of sensors show response times below 10 ms. A more detailed explanation of the basic technology of the sensor element and the conductivity mechanisms are given elsewhere [3,6]. The strut; ture of the sensor element is shown in Fig. 14 . The oxygen-sensitive SrTiOBlayer is sputtered on an AI,O, substrate by a magneticfield-supported r.f. sputtering process. The thickness of the layer is about 1 pm. ‘&vo meander contact areas are defined by thick-film platinum layers. For compensation purposes, a second sensor element is used as a reference. It is connected with the first to form a half Wheatstone bridge. The
: Fig. 15. Output response time).
,\ ;.1; ‘.. :..~ ......‘.‘; . * . ‘;::;$,I
Fig. 16. Methanol sensor:volumetric consumption for equal energy consumption as a function of the methanol content (source TNO).
reference element is made oxygen insensitive using a passivating layer (Fig. 14, electrodes 2-3 and 6-7). For the control of combustion a A-probe of zirconia oxide (ZrOJ is currently used. These TABLE
signal of the binary oxygen sensor from a cylinder-selective
1. Physical quantities
Conductance Relative permittivity Density Calorik value Stoichiometric ratio Refraction index
on a six-cylinder research
sensors have a typical response time of about 100 ms . They measure a deviation of the original carburation (A = 1) only with a time delay as an average of all cylinders. For a cylinder-selective measurement, these sensors are too slow. Sensors with a response time at 5 ms are needed. The new thin-film oxygen sensors have been tested on a six-cylinder research engine; cylinders 1 and 3 were operated in the lean range (A = l.l), whereas cylinder 2 was operated in the rich range (h-0.9). The output signal of the binary oxygen sensor shown in Fig. 15 clearly indicates that the response time is sufficientlyshort to detect cylinderby-cylinder variations. The mounting position in the exhaust-gas system for these measurements is shown in the photograph of Fig. 15. This result shows that SrTi03 thin-film sensors can evaluate different combustion conditions in
2.05-2.1s 0.70-0.74 43 32
3x 10-s 32 0.79 19.7 15.6
1.4x lo-” 26 0.79 26.8 21.2
5x10-‘0 81 1.00
individual cylinders. Using this feature in a closedloop control, a multipoint injection system is able to control each cylinder. 4. Alcohol fuel sensor for flexible fuel systems Increasing air pollution and the decrease of crude oil resources, especially in the USA, are forcing the development of alternative fuel concepts. Alcohol fuels, particularly methanol, may be able to play a major role in the future. Because of the different stoichiometric air/fuel ratios of gasoline and methanol, the volumetric consumption for equal energy consumption depends on the methanol content. This is shown in Fig. 16. In order to determine the actual methanol content of the fuel mixture, a physical quantity that varies significantly as a function of the methanol content has to be chosen. The relationship must be unequivocal, reproducible and relatively uninfluenced by the composition of the gasoline as well as by additives and pollution. As shown in Table 1, the relative permittivity cr meets the above demands . It varies from approximately 2 for pure gasoline up to 33 for pure methanol, providing an excellent measuring effect. In principle, the process of measurement can be performed by using a simple capacitor. If the capacitor is filled by a medium (methanol/ gasoline), the relative permittivity lr will be a function of the methanol content (Fig. 17). Due to free charges, e.g., ionic components, the specific electrical conductivity of the fuel is not equal to zero. For this reason, the equivalent circuit for a capacitor filled with a fuel mixture is given by a connection in parallel of a capacitance C and a conductance G. Then the measurement principle is based on the complex admittance YZ ]yl = Co(fL+~+ o-%$)1”
where o is the angular frequency, lr the relative permittivity, u the conductivity and C, the capacitance without fuel. Equation (1) suggests that the simple measurement of er causes a considerable error if (a/# is of the same order of magnitude as (WE,)‘. As a consequence, the measuring frequency has to be as high as possible, which is 1 MHz. The Siemens sensor detects three quantities to achieve the demanded high accuracy : (i) Capacitance: a capacitor which is filled with the fuel is connected to an appropriate electronic circuit. Due to the conductivity, there is an error that has to be compensated. (ii) Conductance: a second electrode connected to another electronic circuit carries out the conductance measurement. Using this additional information, one can perform the above-mentioned compensation. (iii) Temperature: the former two measurements allow the permittivity of the fuel to be determined. As pointed out, its strong temperature dependence needs to be compensated (see Fig. 18). The acquisition and processing of the three quantities can be very well implemented by using a microcontroller with an internal set of data arrays. The use of a microcontroller also offers the possibility of an adjustment to other fuel components or other output formats. Figure 19 shows the schematic set-up of the sensor. The two structures for the capacitance and conductance measurement consist of a common housing as the outer electrode and two inner electrodes. These inner electrodes are sealed and insulated by glass soldering. The temperature is measured by an NTC resistor which is positioned at the top of the first inner electrode to provide a good thermal contact to the fuel. A photograph of the sensor is shown in Fig. 20. The output signal of the sensor as a function of the stoichiometric ratio for different alcohol-
Fig. 17. Methanol sensor: relative permittivity the methanol content.
as a function
1. -2odegC T.-OdWC
Fig. 18. Methanol sensor: relative dielectric constant as a function of the methanol content at different temperatures.
2. Main features
Measurement range Accuracy output signal Maximum fuel pressure Maximum pressure drop Influence of aromatic HC
Fig. 19. Schematic sensor.
of the Siemens
of the Siemens methanol
O-100% methanol Error <5% 0.5-4.5 V or PWM digital output 150 psi Cl.5 psi None
gasoline mixtures is shown in Fig. 21. Other output formats such as pulse-width modulated or frequency-modulated signals are possible too. The main features of the sensor are summarized in Table 2.
5. conclusions To meet the requirements of fuel economy and emission control, new system concepts are being developed, which require sensors as key components. In order to reduce costs, only a few sensors should be needed. This requires sensors with new features compared to the ones which are currently being used. The three examples of a new sensor generation presented here show that integration of functions, the use of sensitive materials for sensor applications, as well as the use of a software-based system, can generate new sensor features as a system approach. References
Fig. 20. The Siemens methanol
6.0 5.0 -
Fig. 21. Output signal of the methanol sensor as a function the stoichiometric ratio for different alcohol/fuel mixtures.
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