Pressure Sensors

New Opportunities for Gas Exchange Analysis Using Piezoresistive High-Temperature Abso- lute Pressure Sensors

Dr.-Ing. Andrea Bertola Dipl.-Ing. Andreas Fürholz Dipl.-Ing. Jürg Stadler Dipl.-Ing. Jens Höwing Kistler Instrumente AG, Winterthur, Switzerland

Prof. Dr. Karl Huber Dipl.-Ing. Johann Hauber University of applied sciences, Ingolstadt

Prof. Dr.-Ing. Christoph Gossweiler University of applied sciences, Northwestern Switzerland

Special Print 920-366e-10.08 Contents

1 Abstract...... 3

2 Motivation...... 3

3 Piezoresistive Sensors and Installation...... 6 3.1 Temperature Characteristics of Piezoresistive Sensors...... 7

4 Impact of the Sensor Position on the Measured Pressure...... 7

5 Characterisation of Measuring Accuracy and Influence on the Analytical Results...... 9 5.1 Sensor Temperature Over the Engine Operating Range...... 9 5.2 Accuracy Achieved by Using Piezoresistive Absolute Pressure Sensors...... 9 5.2.1 Pressure Measurement with Piezoresistive Sensors Direct Mounted/in a Cooling Adapter...... 9 5.2.2 Pressure Measurement with Piezoresistive Sensors Installed in a Cooled Switching Adapter...... 10 5.3 Low Pressure Indication with Piezoelectric Sensor and Pneumatic Pressure Measurement (Remote Sensing System)...... 11 5.4 Influence of Absolute Pressure Level on the Result of the Gas Exchange Calculation...... 13

6 Conclusion and Recommendations...... 14

7 References...... 1 4

Appendix: Applied Pressure Sensors and Cooling Adapters...... 15

2 www.kistler.com Pressure Sensors

New Opportunities for Gas Exchange Analysis Using Piezoresistive High-Temperature Absolute Pressure Sensors

Dr.-Ing. Andrea Bertola, Dipl.-Ing. Andreas Fürholz, Dipl.-Ing. Jürg Stadler, Dipl.-Ing. Jens Höwing Kistler Instrumente AG, Winterthur, Switzerland Prof. Dr. Karl Huber, Dipl.-Ing. Johann Hauber, University of applied sciences, Ingolstadt Prof. Dr.-Ing. Christoph Gossweiler, University of applied sciences Northwestern Switzerland

1 Abstract The gas exchange influences to a large extent power, emis- sions and fuel consumption of internal combustion engines. The analysis and optimization of the gas exchange is of primary importance and will become more so with the new homogeneous combustion utilizing high degrees of exhaust gas recirculation (EGR).

Low pressure measurement using piezoresistive absolute pressure sensors has become an important tool for the design and optimization of the gas exchange, for the analysis of process variables and for simulation validation. The advan- tages of an absolute pressure measurement are the high precision (including dynamics) and the ability to resolve the pressure differences between cylinders and single cycles.

Capturing both the dynamic behaviour and an exact pressure level are critical for low pressure calculations, with new mini- aturized piezoresistive pressure sensors installation is possible directly into the cylinder head to enable this. A sensor posi- tion near the valve is most suitable as the effort for modelling the system is reduced. In the exhaust manifold however, a cooling adapter will be necessary unless the sensor is installed directly in the cylinder head.

The utilization of a cooled switching adapter allows a precise zero point correction of the piezoresistive pressure sensor, therefore a reference precision (±1 mbar) can be achieved in any operating condition.

Low pressure indication using a piezoelectric sensor and pneumatic pressure measurement (remote sensing system) is not recommended to evaluate the absolute pressure in the exhaust.

www.kistler.com 3 2 Motivation Low pressure indication is the measurement of low amplitude part of the whole combustion strategy. New homogeneous pressures plotted against the engine crank angle, typically combustion concepts (CAI, HCCI), which combine the prop- in the range of 0 … 5 bar absolute. The primary intention erties of gasoline and diesel engines, are distinguished by a of the low pressure indication is the dynamic measurement strong interaction between gas exchange and the subsequent of very small pressure changes within a few mbar. The par- combustion [1]. These new combustion concepts can be ticular challenge is that simultaneously the dynamic and the realized only with the controlled trapping of exhaust gases absolute pressure level needs to be measured with high pre- during the gas exchange. As a result, the high dependence of cision. Both of these important pressure characteristics form the combustion on the condition of the cylinder charge which the basic fundamental requirements for the simulation and has been set during the gas exchange means that this control optimization of engines. has to be performed precisely.

The main focus of the low pressure indication in engine The work done for the gas exchange, expressed as char- development is the analysis of the gas exchange (Fig. 1). acteristic quantity PMEP, can be determined with today's The direct potential for reducing fuel consumption and CO2 standard cylinder pressure indication systems. This value is emissions is provided by minimizing the gas exchange losses. always available during the testing in the test bench, often This is done in gasoline engines for instance, by implementing as real-time value. In addition to this global information, the variable valve trains, downsizing and dethrottling in stratified test engineer needs a more detailed insight as to the proc- combustion concepts. In addition, the gas exchange plays an esses within the gas exchange. The most important param- important role in the reduction of pollutant emissions and eter which influences the combustion is, besides the charge thus will contribute to help meet future emission regulations. motion, the residual gas fraction of the cylinder charge, The gas exchange has become more and more an integral which influences the ignition, combustion behaviour and combustion stability.

Gas exchange Engine parts and systems

Crankcase Gas exchange losses Combustion Low Pressure Special Pumps Reference measurement chamber tests Indication Media

Charge motion 1-D Model calibration Throttle & EGR system Simulation CFD actuator Valves air/exhaust Engine brake Combustion optimization Intake/ Supercharging Heat release analysis Residual Exhaust systems Residual gas control (HCCI) gas Exhaust gas Timing aftertreatment Cam profile Valves Test bench Load control (cylinder charge) Acoustic

Residual gas model Functional Intake manifold model development

Fig. 1: Applications utilizing low pressure indication

4 www.kistler.com The residual gas fraction can’t be measured directly, gas Low pressure indication is the central criteria for ensuring exchange analysis be it 0-D or 1-D simulations, are neces- the accuracy of gas exchange simulation calibrations. In 1-D sary for this determination. Simple residual gas models can simulations, control points are modelled in accordance with be used for the fast calculation of the residual gas fraction the measuring positions on the engine, there the indicated [2]. These models achieve precise results over wide operat- pressures are compared with the computed pressure curves. ing ranges but without considering the dynamics. Cylinder The low pressure indication is therefore the reference for the selective information as well as the interpretation of the dynamic behaviour and absolute pressure. Differences of the complicated gas dynamic behaviour, for instance, during the computed pressures will be matched by varying the model valve overlapping time, can only be obtained by performing parameters e.g. lengths and discharge flow coefficients. The detailed gas exchange analysis. The results permit the iden- quality of the low pressure indication is therefore a condition tification of variables and the customized optimization of the for the final accuracy of the simulation [5]. single processes existing during the gas exchange. According to the aim of the investigation and in view of the The speed of the development process nowadays, requires special properties of the measurement technology, the meas- that the gas exchange analysis is performed at the test uring positions for the low pressure indication can be varied. bench; the results are used directly in the application of any Space availability and the high temperature of the exhaust adjusted operating condition. Computed values, such as the are challenging aspects to the sensor dimensions and the EGR rate are then saved as real data; e.g. pressures or tem- sensor adaptation in general. peratures with further test bench data. High accuracy and process reliability are essential factors Gas exchange analysis requires as an input, in addition to for the general application of any low pressure indication cylinder pressure, the indicated pressures within intake and designated for the optimization of the gas exchange. The exhaust systems. The choice of the measuring position for development of new combustion concepts and the wide the low pressure sensors is influenced mainly by the acces- use of many technologies demand from an Engineer, a sibility to the engine itself. The influence of the sensor posi- deep understanding of the processes governing the internal tion on the results of the gas exchange analysis depends on combustion engine. Low pressure indication provides, in this the methods used for the computational analysis. 1-D gas sense, important measured values with which further detailed exchange analysis considers the running time of the pres- analysis and data processing is possible. sure wave propagation between the sensor position and the cylinder. So the question arises: Is low pressure indication, as a development tool, becoming a Previous works [3, 4] studied the influence of the sensor standard measuring technology in engine development? measuring position on the results of 0-D gas exchange analysis. For measuring positions close to the valve it was determined that it is negligible for passenger car engine speeds. In the same investigation the absolute pressure in the intake and exhaust was identified as a key parameter for the accuracy of the simulation results. The required precision of the absolute indicated pressure in the intake and exhaust was quantified as ±10 mbar.

www.kistler.com 5 3 Piezoresistive Sensors and Installation The maximum operating temperature of piezoresistive sen- The dimensions of the sensor are very important for direct sors for low pressure indication is often lower than that of the mounting into the cylinder head, for this, the compact high- measured media. Long term measurements are only possible temperature sensor Type 4007B is ideally suited. This applica- with special arrangements, such as water cooling the sensor tion occurs mainly in the development of motor sport engines or with the use of a cooled switching adapter. Fig. 2 identi- where concerns for the size and mass of any additional fies a number of installation alternatives for piezoresistive engine mounted hardware are most acute. absolute pressure sensors in the intake and exhaust manifold of an internal combustion engine. Pressure range, compensation technique and thermal proper- ties of the Kistler pressure sensors are shown in Fig. 3.

d

DCE-Sensor Oil filled Sensor a b c Type 4005B Type 4007B Type 4045A Type 4075A

Thread size – M5x0,5 M5x0,5 M14x1,25 M12x1

Measuring 0 ... bar 0 ... 5/10 0 ... 5 0 ... 10 range 2/5/10 Intake Exhaust Max. °C 125 200 140 140 temperature

Analog (+ Type of digital with compensa- – Analog Analog Analog amplifier tion Type 4665)

a Sensor Type 4005B Compensated b Sensor Type 4007B, direct installed in cyl. head temperature °C 0 ... 125 0 ... 180 20 ... 120 20 ... 120 c Sensor Type 4007B, cooling adapter Type 7525A range e f d Sensor Type 4075A, cooling adapter Type 7505 Thermal zero %FSO <1 <1 <0,5 <0,5 e Sensor Type 4045A/4075A/4007B, shift cooled switching adapter Type 7533A f Sensor Type 4045A, cooling adapter Type 7511 Thermal sen- ±% <1 <1 <1 <1 sitivity shift Fig. 2: Different applications of piezoresistive sensors in intake and exhaust Fig. 3: Specification of piezoresistive measuring chains

Due to the limited availability of space and the geometry of a modern intake manifold, size is the main requirement for the For low pressure indication with absolute piezoresistive sen- sensor. The reduced dimensions of miniaturized piezoresis- sors Kistler provides two types of construction. The Direct tive pressure sensors fulfil this requirement and in particular, Chip Exposed principle (DCE) is the new, miniaturized sen- Kistler Type 4005B/Type 4007B (M5) sensors are well suited sor generation Type 4005B/Type 4007B (M5) wherein the for direct installation into the intake manifold or the cylinder semi-conductor measuring element is directly exposed to the head. In addition to such size considerations, temperatures of media and is coated with a special protective film. Oil filled up to 120 °C are possible within the intake, particularly with sensors, Type 4045A (M14) and Type 4075A (M12), utilize high levels of EGR, however, the sensor is capable of with- a similar measuring principle which requires a slightly larger standing these temperatures without additional cooling. package. In this design, the sensor has a thin steel isolation Another benefit of the small diameter (M5) is that the head diaphragm which provides a high resistance to soot and par- of the sensor can be seated flush with the inside surface of ticle emissions. the intake channel. All piezoresistive pressure sensors have thermal effects which In the exhaust, higher temperatures (over 1 000 °C) require are proportional to the full scale output (FSO). Therefore it active sensor cooling. This can be achieved by utilizing dedi- is important to select the appropriate pressure range for the cated cooling adapters or the cooling of the cylinder head. specific application. In the simple cooling adapter (Type 7525A, M14) the sensor housing is cooled but the sensor diaphragm is exposed to the hot gases. The cooled switching adapter (Type 7533A, M14) employs a switching mechanism which is opened by a pneu- matic valve during the time of the measurement only. This maximises the sensor lifetime as well as making a correction of the zero point possible while the engine is still running.

6 www.kistler.com 3.1 Temperature Characteristics of Piezoresistive Sensors 4 Impact of the Sensor Position on the Measured The measuring element of a piezoresistive pressure sensor is a Pressure single crystal silicon wafer into which resistors are implanted The impact of the sensor position on low pressure measure- in a Wheatstone-Bridge configuration. The properties of the ment and therefore calculations of the gas exchange analysis resistors can be influenced by temperature making compen- have been investigated extensively on a V8 spark ignited sation necessary. The temperature behaviour is characterised engine. The high and the low pressure measurements have during manufacture and compensated for using selected been obtained from cylinder 4, which has good measure- resistors (analogue compensation) or digitally corrected utiliz- ment bore accessibility and a representative pressure curve. ing polynomials. The remaining error can be further reduced The disturbance from neighbouring cylinders on the same by performing a zero-point correction on the sensor signal. bank is low, this is due to the angular ignition spacing of Fig. 4 shows the sensor signal output with respect to the 180 °CA. The engine is equipped with variable valve timing applied reference pressure. (cam phasors – intake and exhaust), which was used as an important variable to control the residual gas mass during this investigation. The piezoresistive (PR) absolute pressure sensors (described in chapter 2) were both direct mounted (2) (3) (1) and installed in either a cooling adapter or a cooled switch- ing adapter (Fig. 5 and Fig. 6). The cylinder pressure meas- urement utilized a water cooled M10-sensor Type 6061B mounted flush within the combustion chamber on the intake

Sensor signal side. Both the cooling adapters and the cylinder pressure (1) Calibration curve at Tref (2) Characteristic curve at sensor are cooled by a temperature conditioning unit Kistler T before "Zero-point- A Type 2621. correction" (includes correction"

"Zero-point- sensitivity and offset (o) error) (3) Characteristic curve after "Zero-point- B, Type 4007B correction" at T (α) A direct mounted (α ) Thermal sensitivity error (o) Thermal offset error Remaining error

1 bar (Ambient pressure) Reference pressure [bar] C, PR sensor in approx. Fig. 4: Schematic view of the zero point correction cooled switching 200 mm adapter

The calibration reference curve (1) shows the ideal calibrated A, Type 4007B characteristic of a sensor and therefore each deviation from direct mounted this perfect curve is exhibited as an error. Exposing the sensor to an arbitrary temperature TA produces both a zero-point and a sensitivity error (here shown positive) which gener- ates curve (2). The zero-point error and a certain part of the sensitivity error can be corrected by applying a zero-point correction to TA at ambient pressure at the time of the test. Having done the zero-point correction (3) and assuming that the temperature is stable, the remaining inaccuracy is caused only by the sensitivity error. The achievable accuracy Fig. 5: Measurement locations in the intake system of piezoresistive sensors on a test bed and the optimal zero- point procedure is described in section 4.

www.kistler.com 7 2,5 1, Type 4007B direct mounted p cylinder in cylinder head, rear position 4, PR sensor 2,25 in cooled Exhaust, pos. 2

2 EVC timing IVO timing 2, PR sensor in switching EVO timing cooled switching adapter adapter, frontal 1,75 Exhaust, pos. 3 position Exhaust, pos. 4 1,5 Exhaust, pos. 5 Pressure [bar] 1,25 Exhaust, pos. 1

1 TDC 0,75 270 360 450 540 630 720 Crank Angle [deg CA] 3, PR sensor in cooled switching Fig. 7: Measured pressures in the low pressure phase of the engine adapter, cycle at different positions in the exhaust, operating condi- rear position tion 2 000 1/min, full load. Average over 200 single cycles 5, PR sensor in cooling adapter Measurement position 2 (frontal position in the exhaust bend, see Fig. 6) shows distinctive differences in the gas dynamics at Fig. 6: Measurement locations in the exhaust system high engine load. This measurement position shows, in each case, the largest local peak pressure at the beginning of the gas exchange process. The pressure increase at the frontal meas- At each engine operating point 200 single cycles with a reso- urement position in the bend section of the manifold is caused lution of 0,5 °CA were acquired and averaged. by the redirection of the exhaust gas flow. Measurement position 1 (in cylinder head, exhaust) shows, in each case, One of the objectives was to determine the cyclic pressure the smallest local pressure maximum during the initial exhaust fluctuations at the different measurement positions. This blow down. Due to the small cross sectional area high flow required a correction to the pressure level of the intake velocities are reached and static pressure fractures are as a and exhaust pressure curves after the measurement. As a result of this low. Measurement positions 4 and 5 (on straight reference, the pressure curves of the sensors installed in the duct, distant from valve) show identical pressure curves, even cooled switching adapters were used. By using the switching at high revolutions and high load. adapter, the sensor can be referenced to the known ambient pressure easily and so corrected accurately. The correction of 2,25 sensors installed directly or in cooling adapters was applied during a certain crank angle window when a negligible pres- 2 sure dynamic existed. Therefore, the absolute pressure level

1,75 EVO timing IVO timing EVC timing (given by sensor properties and zero-point adjustment) and Intake, pos. B the pressure dynamic (given by the measurement position) 1,5 Intake, pos. A are independent. The following corrections were made: 1,25

Pressure [bar] 1 Intake p cylinder • Averaging of whole working cycle 0,75 • Reference pressure uses the sensor signal obtained from TDC 0,5 the cooled switching adapter, which was referred to the 270 360 450 540 630 720 ambient pressure before each measurement Crank Angle [deg CA]

Exhaust Fig. 8: Measured pressures in the low pressure phase of the engine • Averaging in a crank angle window when a negligible cycle at different positions in the intake, operating condition dynamic pressure exists (0 ... 270 °CA) 5 000 1/min, full load. Average over 200 single cycles • The reference is the pressure measured by a sensor located in the cooled switching adapter, this in turn, was referred Focussing on the intake, measurement position A (in cylinder to the ambient pressure before each measurement head, close to valve) exhibits distinctive differences in the gas dynamic with respect to the engine revolutions, load and valve Fig. 7 shows the pressure curve during the gas exchange overlapping. The propagation of the pressure wave during the at 2 000 1/min and full load. Following the EVO distinctive intake stroke moves from the valve back into the intake mani- differences in the pressure dynamics are visible in the exhaust fold passing the sensor adjacent to the valve (Position A Fig. manifold (range of 360 °CA). 8) then shortly afterwards reaching the more remote sensor (Position B Fig. 8) with a reduced amplitude. The flow charac- teristics at position B are due to the configuration of the vari- able intake manifold. A conclusion would be that the measure- ment of the pressure at the valve gap is not viable.

8 www.kistler.com 5 Characterisation of Measuring Accuracy and Influence 5.2 Accuracy Achieved by Using Piezoresistive Absolute on the Analytical Results Pressure Sensors High temperatures interacting on a sensor can cause thermal 5.2.1 Pressure Measurement with Piezoresistive Sensors error which leads to reduced overall accuracy. As the accurate Direct Mounted/in a Cooling Adapter measurement of the pressure level is the most critical aspect, Whether sensors are direct mounted or in cooling adapters a procedure for temperature compensation is necessary to the measuring element is always exposed to the exhaust achieve the high requirements necessary to determine the pressure, making it is necessary to stop the engine and refer- pressure level. ence the sensor to ambient pressure. As presented, a change in temperature causes a zero-point and a sensitivity error, To achieve thermal stability of a sensor it is essential that the therefore, in order to reach the high accuracy required, the sensor is cooled adequately. No sensor is able to achieve the sensor must be in the same condition (mainly temperature), required accuracy at temperatures sometimes over 1 000 °C, as it will be during the subsequent measurements, prior to so cooling the sensor is mandatory. The correct, stable cool- applying the zero-point correction. ing of the exhaust pressure sensor will lead to an almost con- stant temperature environment for the pressure sensor during Having installed the sensors into cooling adapters, the zero- the measurements over the entire engine operating range. point correction can be performed accurately, due to well conditioned sensors and a low dependency of the sensor 5.1 Sensor Temperature over the Engine Operating Range temperature on the engine load. When mounting the sensors A characterisation of the temperature in different sensor directly to the manifold, the sensor temperature may change locations was carried out on the V8 gasoline engine. For the according to the engine load (see Fig. 9) and lower the preci- sensor, direct mounted in the intake manifold, this resulted in sion of the zero-point correction. This is more noticeable for temperatures around room temperature. The direct mounted the exhaust pressure measurement with the sensor installed sensor in the intake port of the cylinder head reached directly in the cylinder head, here significant temperature temperatures of 65 °C (2 000 1/min part load) and 90 °C changes may be evident. (2 000 1/min full load) . The higher temperature level can be explained by the heat impact of the cylinder head and the To show the resulting sensor errors which are mostly caused air mass flow. Sensor temperatures at different positions are by thermal effects, an engine load sweep was performed shown in Fig. 9. (Fig. 10). The operating points are chosen in order to succes- sively increase the thermal load into the sensor. All sensors are first conditioned at 2 000 1/min, part load, and then set Intake: sensor in cooling adapter (pos. C) to the ambient pressure. Intake: sensor direct mounted in cylinder head (pos. A) 175 Exhaust: sensor in cooling adapter (pos. 4) Exhaust: sensor direct mounted in cylinder head (pos. 1) 155

135 180 160 Pos 1 115 140 Pos 5 120 95 100 75 80 60

55 Sensor temperature [°C] 40

Sensor temperature [°C] 10 35 0 -10 15 e , , , , -20

–1 –1 –1 –1 Sensor Type 4007B (M5), -30 Pos 1 -40 Engine 000 min 400 min 000 min 000 min full load full load full load part load stop (hot) 2 3 5 2 10 0

Fig. 9: Measured sensor temperature at intake and exhaust positions Absolute pressur error [mbar] -10 -20 Sensor Type 4045A (M14), -30 Pos 5 -40 One of the two sensors installed in the exhaust is located in the manifold (Pos. 4). This sensor, mounted in a cooling , full load , full load , full load , part load –1 –1 –1 adapter reaches a maximum temperature of 80 °C. Using –1

Engine stop (hot) Engine stop (hot) sensors in cooling adapters leads in general, to temperatures 000 min 400 min 000 min 000 min 2 3 5 in the range of 50 … 80 °C across the whole engine operat- 2 ing range. Fig. 10: Absolute pressure error with different piezoresistive sensors mounted in the exhaust in various operating conditions The second sensor is installed directly in the exhaust port in during a measuring campaign. Reference measurement in the cylinder head (Pos. 1), this means that there is no addi- cooled switching adapter in position 4 tional cooling device. A maximum temperature of approxi- mately 170 °C is measured at the same operating points, well below the maximum allowable 200 °C, which means that the thermal error can be easily compensated.

www.kistler.com 9 The increased engine load applies a higher thermal load into The short term instability for all sensor types and measur- the sensor causing an increase of the temperature at the ing positions, remain within 0,05 %FSO. This characteristic measuring element. The sensor error is therefore linked to is especially important when considering using sensors with the applied temperature. The biggest increase in tempera- cooled switching adapters. ture, and hence the largest sensor error, occurs at the sensor installed in the cylinder head. Sensor Type Sensor Type Sensor Type EXHAUST In addition to the cited effects of temperature the sensor sta- 4007BA5FS 4045A5V200S 4075A10V200S bility will be evaluated next. Having completed the specified Pressure range 0 ... 5 bar 0 ... 5 bar 0 ... 10 bar load conditions over the engine operating range, the engine Installation with ±20 mbar/ ±20 mbar/ ±30 mbar/ cooling adapter ±0,4 %FSO ±0,4 %FSO ±0,3 %FSO is stopped and the difference in the sensor output, between Total the first last measuring point is determined. To state the error Direct ±45 mbar/ (typical) installation in – – short term instability, the engine is held at a steady operating ±0,9 %FSO cylinder head condition and the change in the maximum sensor errors are obtained. Short-term instability (at ±2,5 mbar/ ±2,5 mbar/ ±2,5 mbar/ same operating ±0,05 %FSO ±0,05 %FSO ±0,03 %FSO Mounted in the intake, the environment is less challenging condition) Insta- as both the ambient and media temperatures are significantly bility Long-term (typical) lower than those that surround the exhaust manifold. Sensor instability ±20 mbar/ ±5 mbar/ ±10 mbar/ (between first errors (Fig. 11), even of those sensors mounted directly in the ±0,4 %FSO ±0,1 %FSO ±0,1 %FSO cylinder head, are less than ±0,2 %FSO and stability is very and last meas- uring point) good also. Less than ±0,05 %FSO difference exists between the readings taken at the first and last measuring points. Fig. 12: Typical total absolute pressure error and instabilities of low This is due to a combination of factors, the extremes in tem- pressure indication. Three piezoresistive sensors installed in perature are less damaging to the sensor and the additional the exhaust cooling provided by the charge media help to provide a stable diaphragm temperature. 5.2.2 Pressure Measurement with Piezoresistive Sensors Installed in a Cooled Switching Adapter INTAKE Sensor Type 4007BA5FS A cooled switching adapter has the feature whereby, a pneu- matically controlled valve provides switching between ambi- Pressure range 0 ... 5 bar ent and exhaust pressures. The use of a cooled switching Installation with cooling ±5 mbar/ adapter enables a precise and flexible zero-point adjustment adapter ±0,1 %FSO Total error of the piezoresistive pressure sensor referenced to ambient (typical) Direct installation in cylinder ±10 mbar/ pressure at any time. The adjustment can be made while the head ±0,2 %FSO engine is running under the same thermal load as the follow- Short-term instability (at ±2,5 mbar/ same operating condition) ±0,05 %FSO ing measurement will take place. Instability Long-term instability (typical) ±2,5 mbar/ (between first and last In addition, the sensor installed in the cooled switching ±0,05 %FSO measuring point) adapter, has reduced exposure to extreme conditions like thermal load and soot contamination. Fig. 11: Typical total absolute pressure error and instabilities of low pressure indication. Sensor type 4007BA5FS installed in the intake Using an established measuring procedure in addition to reg- ular use of the cooled switching adapter, as shown in Fig. 13, high process reliability can be achieved. In each case where On the exhaust side (Fig. 12), the sensor errors are greater a verification is made prior to every measuring point, even due to more dynamic temperature environment, relative to the smallest thermal zero-point error can be measured and the intake. The sensors that are installed in cooling adapters therefore corrected, ensuring the most accurate scaling. have errors of less than ±0,4 %FSO. The sensor installed in the cylinder head, because of the elevated temperature With this procedure for zero-point adjustment done, a refer- levels, displays the highest errors, up to ±0,9 %FSO. The dif- ence accuracy of ±1 mbar can be achieved at every single ference in accuracy therefore, is not dependent on the sensor operating point. It should be noted that the short term stabil- type but on the quality and stability of the sensor cooling. ity during an engine test point is not corrected (Fig. 14).

The difference in the sensor output between the first and last measuring points can be attributed to the sensor type. Oil filled sensors (Type 4045A/Type 4075A) show a very small change, while DCE-Sensor (Type 4007B) exhibits a more noticeable instability.

10 www.kistler.com Status cooling Ambient Exhaust Ambient Exhaust Ambient switching (not switched) (switched) (not switched) (switched) (not switched) adapter

Protection of Warm sensor Zero-point Protection of Purpose Measurement sensor (approx. 60 s) correction sensor

Engine status Any Operating point stabilized Any

Time

Fig. 13: Procedure for zero point correction of the sensor in the cooling switching adapter

calculated by the addition of the averaged pressure level and Sensor Type Sensor Type Sensor Type EXHAUST 4007BA5FS 4045A5V200S 4075A10V200S the pressure oscillation around the mean level. Therefore it is important that the signal of the piezoelectric sensor has no Pressure range 0 ... 5 bar 0 ... 5 bar 0 ... 10 bar static component and therefore a mean value of zero. Total Installation with This error can be eliminated by making a error cooling switching zero-point correction The advantage of this method is that a conventional piezore- (typical) adapter sistive pressure sensor can be used, because of the hose Short-term insta- ±2,5 mbar/ length the gas temperature at the sensor is low and the bility (at same ±2,5 mbar/ ±2,5 mbar/ ±0,05 operating condi- ±0,05 %FSO ±0,03 %FSO contamination by soot is less likely. The disadvantage is that %FSO Insta- tion) with a remote sensing system, as it is installed generally, sig- bility nificant measuring errors can occur with the determination of (typical) Long-term insta- bility (between This error can be eliminated by making a the mean pressure level. The error is not related to acoustic first and last zero-point correction phenomena such as pipe oscillations, but is attributed to the measuring point) inflow and outflow of gases through the pressure port. Fig. 14: Typical total absolute pressure error and instabilities of low pressure indication. Three piezoresistive sensors installed in a The pressure oscillation in the intake or exhaust will travel cooled switching adapter in the exhaust through the pressure tap and the hose until it reaches the pressure sensor. Due to this pressure variation within the 5.3 Low Pressure Indication with Piezoelectric Sensor remote sensing system, a temporary non-constant mass and Pneumatic Pressure Measurement (Remote flows in and out of the pressure tap. The typical geometries Sensing System) used for the pressure tap leads to a difference between the Should low pressure indication be attempted utilizing a drag coefficient ζI during inflow and drag coefficient ζO dur- piezoelectric sensor, an additional pressure measurement is ing outflow. This leads to a better emptying of the system necessary to determine the static mean absolute pressure. compared with the inflow, which in turn results in a reduced The pressure measurement consists of a pressure tap or mean pressure level in the remote sensing system. connection point, the connecting hose and a piezoresistive absolute pressure sensor (Fig. 15). The pressure trace can be These effects have been analyzed and demonstrated by Weyer [6] experimentally as well as through simulation.

HP Filter Weyer shows that the error during the determination of the Charge amplifier pressure level in a fluctuating pneumatic system is related to Pressure p the dimensions of the remote sensing system. Of the most influence, is the pressure tap itself where the amplitude and PR sensor frequency of the pressure oscillation have a major effect. One Tube Water cooled Average exception is, when the pressure tap has the same drag coeffi- PE sensor length L cient for inflow as for outflow, this would avoid any error but it means a complex geometry of the pressure tap and cannot Wall static pressure tap be accomplished easily.

For evidence about this effect, Eng [7] performed measure- ments on a single-cylinder diesel engine. He employed a Fig. 15: Low pressure indication with piezoelectric sensor and pneu- remote sensing system which was compared to the results of matic pressure measurement for the acquisition of the mean absolute pressure. The pressure curve results from the addi- a direct mounted piezoresistive sensor located in the cooled tion of the fluctuation with the averaged value switching adapter. The resulting pressure fluctuation and www.kistler.com 11 phase shift of the pneumatic signal are strongly related to the It is evident from the data that the remote sensing system dimensions (diameter and length of the pressure tap, diam- still shows a pressure dynamic, therefore, averaging the signal eter and length of hose and dead volume). It becomes evi- over a complete cycle is mandatory before adding the piezoe- dent therefore, that the error of the remote sensing system, lectric dynamic component. Compared to the averaged direct regarding the determination of the mean pressure level is in piezoresistive pressure measurement the averaged pressure the range of 15 … 20 mbar, which is a rather high number of the remote sensing system is too low. The following errors for this application. A comparison of Weyer's [6] results leads have an influence: to a good correlation. • Direct pressure measurement with piezoresistive pressure The following results, measured on the 8-cylinder engine, sensor in cooled switching adapter: thermal related sensi- include pressure curves from the remote sensing system com- tivity error (small error, reference measurement) pared to a piezoresistive sensor installed in a cooled switching • Remote sensing system with piezoresistive sensor: Error adapter (Fig. 16). The measuring positions 4 and 5 are on related to the pressure tap (inflow and outflow), depend- the same longitudinal position in the exhaust manifold. The ent on hose length. The formation of condensation in the measured pressure traces were corrected in the following hose will cause a dampening effect and has an influence manner: on the dynamics of the signal (considerable error possible) • Direct pressure measurement with piezoelectric sensor: • Direct pressure measurement with piezoresistive pressure thermal related sensitivity error, thermal shock (small error sensor in cooled switching adapter: Zero-point is adjusted in the dynamic pressure) before the measurement according to the ambient pres- sure level In the case of low pressure indication with a piezoelectric sen- • Remote sensing system with piezoresistive sensor (at the sor in combination with a remote sensing system, generally a end of the hose): Zero-point is adjusted to the ambient systematic error of up to 20 mbar can occur. This correlates pressure level during engine stop before the measuring to the error described by Weyer [6]. Therefore this method is campaign not recommended to achieve the best possible accuracy. • Low pressure indication with a piezoelectric sensor and the remote sensing system: the pressure oscillation, measured The difference between the absolute pressure measured using by the piezoelectric sensor and the averaged pneumatic the remote sensing system to that of the direct piezoresistive pressure are added. sensor installed in a cooled switching adapter is shown in Fig. 17 for different operating conditions. It can also be seen It is quite visible, that the remote sensing system indicates that the sensor position has impact due to the differences of clearly reduced pressure amplitudes as well as a phase shift. dynamic pressure at different locations. This effect will become more evident towards higher engine speeds or with a prolongation of the hose.

0 2,25 1,5 Direct PR -20 Pegged meas. pos. 4 2 direct PE 1,25 -40 meas. pos. 5 Pneumatic PR Position 2 1,75 1 meas. pos. 5 -60 1,5 0,75 0 1,25 0,5 -20 1 0,25 -40 Position 4 0,75 0 Absolute pressure error [mbar] -60

0,5 Direct PE -0,25 Pressure PE measurement [bar] Absolute pressure in exhaust [bar] meas. pos. 5 0,25 -0,5 090 180 270 360 450 540 630 720 , full load , full load , full load , part load –1 –1 –1 TDC Crank Angle [deg CA] TDC –1

Resulting averaged pressure (0 ... 720 °CA): Engine stop (hot) Engine stop (hot) 000 min 400 min 000 min 000 min 2 3 5 Direct PR measurement 1,361 bar 2 Pneumatic PR measurement 1,336 bar

Fig. 17: Absolute pressure error of the low pressure indication with Fig. 16: Pressure curves of direct pressure measurements with piezoelectric measurement and pneumatic pressure measure- piezoresistive (PR) and with piezoelectric (PE) sensor, ment (averaged pressure in window 0 ... 270° CA, see pneumatic pressure measurement (tube length 0,3 m) with chapter 3). Direct piezoresistive measurement in position 5 as PR sensor. Operating condition 5 000 min–1, full load. reference Average over 200 single cycles

12 www.kistler.com 5.4 Influence of Absolute Pressure Level on the Result of 60 the Gas Exchange Calculation 40 Exhaust mass flow Intake mass flow Use of low pressure indication for gas exchange optimization 20 shows that a link exists between the measuring task, the measuring position, the selection of low and high pressure 0 instrumentation as well as the analytical process itself. The Mass flow rate [g/s] -20 TDC 1,2 technology selection should be done with careful considera- Exhaust pressure, pos. 1 tion to the boundary conditions as well as the mission targets. 1 There should be a definition of the quality assurance as well Intake pressure, pos. A 0,8 p cylinder as a confidence check of the measuring results in an early 7.5 0,6 Exaust valve lift stage. Pressure [bar] 0,4 TDC Intake valve lift The gas exchange process is mainly influenced by the pres- 0,2 sure difference at the valves. Therefore low pressure and 270 360 450 540 630 720 in-cylinder indication should be considered complementary. Crank Angle [deg CA] Piezoresistive sensors for intake and exhaust measurements offer an accuracy in the range of ±10 mbar, however, the Fig. 19: Measured pressure curves during the gas exchange and computed mass flow rate through the valves. Operating thermal shock error of the piezoelectric cylinder pressure sen- condition 2 000 min–1, IMEP 2 bar, unrestricted operation, sor is at least one order higher. The highest uncertainty of the exhaust valve lift full, intake valve lift reduced. Average over gas exchange measurement is therefore the in-cylinder low 200 single cycles pressure signal. the phasing which is considerably before the residual gas The following two illustrations show the gas exchange of the relevance range of the valve overlap. The difference in pres- V-8 engine with a fully variable valve train. The engine oper- sure dynamic at different measuring positions at the intake ating conditions analysed are 2 000 1/min part load restricted (Fig. 8) has just minor effects on the results of the 1-D gas (full valve lift, Fig. 18) and unrestricted (exhaust valve full lift, exchange calculation. The reason for this is that this calcula- intake valve part lift, Fig. 19). Representative pressure curves tion program takes into account the exact position of the are shown, which are measured with piezoresistive absolute sensor and therefore the runtime error of the pressure wave pressure sensors Type 4007B close to the valve at the intake is considered. (position A) and exhaust (position 1), as well as the calculated mass flow. An extensive parameter study confirms that primarily, the pressure level and not the sensor position or their adaptation Low pressure indication delivers pressure curves in high reso- is of central importance for the gas exchange calculation. lution for all control strategies. By increasing valve overlap the sensitivity of the calculated residual gas fraction on the absolute pressure level in the The pressure differences related to the measuring position intake and exhaust port increases. In Figure 20, an example at the pre exhaust (Fig. 7), have no impact on the global is shown on the influence of a different pressure level on the results of the gas exchange calculation. This is because of calculated residual gas fraction.

60 Intake mass flow 40 Exhaust mass flow 10 Valve timings: 20 9 Intake -5°CA /Exhaust -5°CA 0 Intake/ Exhaust series 8 Intake +5°CA /Exhaust +5°CA Mass flow rate [g/s] -20 TDC 1,2 7 Exhaust pressure, pos. 1 6 1 5

0,8 p cylinder 4 Exaust valve lift Intake valve lift 0,6 3

Pressure [bar] Intake pressure, pos. A 2 0,4 Residual gas fraction [%] 1 0,2 TDC 270 360 450 540 630 720 0 Crank Angle [deg CA] -30 -20 -10 0102030 Delta p exhaust [mbar] Fig. 18: Measured pressure curves during the gas exchange and computed mass flow rate through the valves. Operating condition 2 000 min–1, IMEP 2 bar, restricted operation, full valve lift. Average over 200 single cycles Fig. 20: Computed residual gas fraction in the indicated cylinder for variations of the exhaust pressure of ±30 mbar, three valve timing settings. Operating condition 2 000 min–1, full load

www.kistler.com 13 6 Conclusion and Recommendations 7 References Miniaturised piezoresistive absolute pressure sensors can be [ 1 ] M. Bargende placed, due to their size and mass, with minimum restrictions Homogene Kompressionszündung bei Otto- und in the manifolds. New high-temperature pressure sensors Dieselmotoren. Anforderungen und Potentiale broaden the application scope, allowing a sensor installation Symposium IAV even directly in the cylinder head close to the valve. The con- Juni 2007 Berlin ditioning of the sensor is still necessary, especially if tempera- tures are high and sustained as in the exhaust manifold. The [ 2 ] N. Hoppe decision as to which piezoresistive sensor and its adaptation Erfahrung mit dem Einsatz eines modifizierten to use has to be considered on a case by case basis. Restgasmodells und die Weiterentwicklung zum online-fähigen Optimierungstool Measuring Position in the Intake Internationales Symposium für Verbrennungs- The choice of the measuring position is easier in the intake diagnostik as the temperature of the measuring bore and of the intake Mai 2006, Baden-Baden gases normally allow direct mounting of the sensor without cooling. The use of a cooled switching adapter in addition to [ 3 ] C. Burkhardt, M. Gnielka, C. Gossweiler, D. Karst, extending the useful life of the sensor, provides a convenient M. Schnepf, J. von Berg, P. Wolfer zero point solution in combustion strategies utilizing high Ladungswechseloptimierung durch geeignete levels of EGR. Kombination von Indiziermesstechnik, Analyse und Simulation Measuring Position in the Exhaust 9. Tagung, Any pressure measurement in the exhaust can be challeng- Der Arbeitsprozess des Verbennungsmotors ing, therefore, when selecting the location for a sensor con- September 2003, Graz sideration must be given not only to the physical size of the adaptation but perhaps more significantly to the geometry [ 4 ] A. Wimmer, R. Beran, G. Figer, J. Glaser, of the manifold. As presented, the dynamic pressure meas- P. Prenninger ured at different locations can be influenced by the specific Möglichkeiten der genauen Messung von mounting orientation of the sensor related to the flow. Ladungswechseldruckverläufen In the exhaust manifold a cooling adapter will be necessary Internationales Symposium für Verbrennungs- unless the sensor can be installed directly in the cylinder head diagnostik exhaust runner. Piezoresistive absolute pressure sensors (Type Mai 2000, Baden-Baden 4045A/Type 4075A) with thin steel isolation diaphragms pro- vide a high resistance to soot emissions and have an accept- [ 5 ] H. Alten able lifetime when constant cooling is present. Der Ladungswechsel im Rennmotor MTZ-Konferenz, Ladungswechsel im Absolute Pressure and Zero Point Correction Verbrennungsmotor Studies on the influence of the absolute pressure on the com- November 2007, Stuttgart puted residual gas fraction show that a precision of better than ±10 mbar is necessary. [ 6 ] H. Weyer The utilization of a cooled switching adapter allows a precise Bestimmung der zeitlichen Druckmittelwerte in zero point correction of the piezoresistive pressure sensor, stark fluktuierender Strömung, insbesondere in therefore a reference precision (±1 mbar) can be achieved Turbomaschinen in any operating condition. High process reliability can be Dissertation RWTH Aachen 1973 assured by using established measuring techniques in con- DFVLR, Forschungsbericht/ junction with the switching adapter. Deutsches Zentrum für Luft- und Raumfahrt 1974 Low pressure indication with a piezoelectric sensor and pneu- matic pressure measurement (remote sensing system) is not [ 7 ] M. Eng recommended for the precise determination of the absolute Untersuchung von Sensoren und Messverfahren pressure level in the exhaust. zur Niederdruckindizierung Diplomarbeit Fachhochschule Nordwestschweiz Modelling and Simulation November 2007 Compared to residual gas models that are based on aver- aged pressures, a gas exchange analysis referenced to direct dynamic low pressure measurements provides crank angle resolved data with a high degree of relevance. A sensor mounting position near the valve is more likely to provide the required accuracy for the phasing of pressure at the valve, which has the added benefit of reducing the demand on the model.

14 www.kistler.com Applied Pressure Sensors and Cooling Adapters

Low Pressure Measurement in Intake and Exhaust

T

L T T T L L L

D D D D

Technical Data Type 4005B… Type 4007B… Type 4045A… Type 4075A… Measuring range bar 0 … 5/… 10 1) 0 … 5/… 20 0 … 1/… 2/… 5/… 10 1) 0 … 10 1) Output signal V 0 … 10 0 … 10 0 … 10 0 … 10 (amplifier) mA 4 … 20 4 … 20 4 … 20 4 … 20 Min./Max. temperature °C –40/125 –40/200 0/140 3) 0/140 3) Thermal zero shift ±%FSO <1 (0 … 125 °C) <1 (0 … 180 °C) <0,5 (20 … 120 °C) <0,5 (20 … 120 °C) Thermal sensitivity shift ±% <1 (0 … 125 °C) <1 (0 … 180 °C) <1 (20 … 120 °C) <1 (20 … 120 °C) Linearity and Hysteresis ±%FSO <0,2 <0,2 <0,3 <0,3 Dimensions D/L mm 6,2/4 6,2/4 12/14 9,5/35 T M5x0,5 M5x0,5 M14x1,25 M12x1 Description Miniature sensor ideal for As for Type 4005B… Oil-filled pressure sensor Oil-filled pressure sensor measuring pressures in High-temperature with steel diaphragm. with steel diaphragm. the intake system. Very design, digital tempera- Ideal for measuring Available in different compact dimensions, ture compensation pressures in both the versions with or without versatile, high natural intake and exhaust PiezoSmart®, or as frequency. Available as system. Available in measuring chain with PiezoSmart® sensor or different versions with or amplifier Type 4618A measuring chain with without PiezoSmart®, or amplifier Type 4618A as measuring chain with amplifier Type 4618A Application • Intake pressure • Intake pressure • Intake pressure • Exhaust pressure • Exhaust pressure in • Exhaust pressure racing engines Recommended • Direct installation in • Direct installation in • Direct installation in • Adapter Type 7505 mounting/adapter intake intake or exhaust intake • Adapter Type 7533A (cylinder head) • Adapter Type 7511 • Adapter Type 7525A • Adapter Type 7533A • Adapter Type 7533A

1) other measuring ranges available 2) depends on measuring range 3) other temperature ranges available

Cooling Adapters

T T T T L L L L

Technical Data Type 7511 Type 7505B Type 7525A… Type 7533A… Recommended sensors 4045A… 4075A… 4005B…/4007B… 4005B…/4007B…/ 4075A… in adapter 4045A.../4075A… Dimensions L mm 12,5 11,8 7 13 T G1/2" M18x1,5 M14x1,25 M14x1,25 Description Damped adapter for Compact adapter for Compact adapter for Switching adapter to applications with high sensor Type 4075A miniature pressure sen- reference sensor to vibration sors. Damped version ambient pressure available www.kistler.com 15 Kistler worldwide Europe

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920-366e-10.08 Mat1000 ©2008, Kistler Gruppe [email protected]

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