Dr.-Ing. Andrea BertolaDipl.-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 GossweilerUniversity of applied sciences, Northwestern Switzerland

Special Print920-366e-10.08

Pressure Sensors

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

2 www.kistler.com

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................................................................................................................................................................ 14

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

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1 AbstractThe 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.

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 HöwingKistler Instrumente AG, Winterthur, SwitzerlandProf. Dr. Karl Huber, Dipl.-Ing. Johann Hauber, University of applied sciences, IngolstadtProf. Dr.-Ing. Christoph Gossweiler, University of applied sciences Northwestern Switzerland

4 www.kistler.com

Low PressureIndication

Simulation

Test bench

Functionaldevelopment

Intake/Exhaust

Residualgas

Valves

Combustion chamber

Throttle &actuator

Exhaust gas aftertreatment

Acoustic

Specialtests

Superchargingsystems

Gas exchange Engine parts and systems

CrankcasePumpsMedia

Charge motionEGR systemValves air/exhaustEngine brake

Gas exchange lossesReference measurement

1-D Model calibrationCFD

Combustion optimizationHeat release analysis

Residual gas control (HCCI)

TimingCam profile

Load control (cylinder charge)

Residual gas modelIntake manifold model

2 MotivationLow pressure indication is the measurement of low amplitude pressures plotted against the engine crank angle, typically in the range of 0 … 5 bar absolute. The primary intention of the low pressure indication is the dynamic measurement of very small pressure changes within a few mbar. The par-ticular challenge is that simultaneously the dynamic and the absolute pressure level needs to be measured with high pre-cision. Both of these important pressure characteristics form the basic fundamental requirements for the simulation and optimization of engines.

The main focus of the low pressure indication in engine development is the analysis of the gas exchange (Fig. 1). The direct potential for reducing fuel consumption and CO2 emissions is provided by minimizing the gas exchange losses. This is done in gasoline engines for instance, by implementing variable valve trains, downsizing and dethrottling in stratified combustion concepts. In addition, the gas exchange plays an important role in the reduction of pollutant emissions and thus will contribute to help meet future emission regulations. The gas exchange has become more and more an integral

Fig. 1: Applications utilizing low pressure indication

part of the whole combustion strategy. New homogeneous combustion concepts (CAI, HCCI), which combine the prop-erties of gasoline and diesel engines, are distinguished by a strong interaction between gas exchange and the subsequent combustion [1]. These new combustion concepts can be realized only with the controlled trapping of exhaust gases during the gas exchange. As a result, the high dependence of the combustion on the condition of the cylinder charge which has been set during the gas exchange means that this control has to be performed precisely.

The work done for the gas exchange, expressed as char-acteristic quantity PMEP, can be determined with today's standard cylinder pressure indication systems. This value is always available during the testing in the test bench, often as real-time value. In addition to this global information, the test engineer needs a more detailed insight as to the proc-esses within the gas exchange. The most important param-eter which influences the combustion is, besides the charge motion, the residual gas fraction of the cylinder charge, which influences the ignition, combustion behaviour and combustion stability.

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The residual gas fraction can’t be measured directly, gas exchange analysis be it 0-D or 1-D simulations, are neces-sary for this determination. Simple residual gas models can be used for the fast calculation of the residual gas fraction [2]. These models achieve precise results over wide operat-ing ranges but without considering the dynamics. Cylinder selective information as well as the interpretation of the complicated gas dynamic behaviour, for instance, during the valve overlapping time, can only be obtained by performing detailed gas exchange analysis. The results permit the iden-tification of variables and the customized optimization of the single processes existing during the gas exchange.

The speed of the development process nowadays, requires that the gas exchange analysis is performed at the test bench; the results are used directly in the application of any adjusted operating condition. Computed values, such as the EGR rate are then saved as real data; e.g. pressures or tem-peratures with further test bench data.

Gas exchange analysis requires as an input, in addition to cylinder pressure, the indicated pressures within intake and exhaust systems. The choice of the measuring position for the low pressure sensors is influenced mainly by the acces-sibility to the engine itself. The influence of the sensor posi-tion on the results of the gas exchange analysis depends on the methods used for the computational analysis. 1-D gas exchange analysis considers the running time of the pres-sure wave propagation between the sensor position and the cylinder.

Previous works [3, 4] studied the influence of the sensor 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.

Low pressure indication is the central criteria for ensuring the accuracy of gas exchange simulation calibrations. In 1-D simulations, control points are modelled in accordance with the measuring positions on the engine, there the indicated pressures are compared with the computed pressure curves. The low pressure indication is therefore the reference for the dynamic behaviour and absolute pressure. Differences of the computed pressures will be matched by varying the model parameters e.g. lengths and discharge flow coefficients. The quality of the low pressure indication is therefore a condition for the final accuracy of the simulation [5].

According to the aim of the investigation and in view of the special properties of the measurement technology, the meas-uring positions for the low pressure indication can be varied. Space availability and the high temperature of the exhaust are challenging aspects to the sensor dimensions and the sensor adaptation in general.

High accuracy and process reliability are essential factors for the general application of any low pressure indication designated for the optimization of the gas exchange. The development of new combustion concepts and the wide use of many technologies demand from an Engineer, a deep understanding of the processes governing the internal combustion engine. Low pressure indication provides, in this sense, important measured values with which further detailed analysis and data processing is possible.

So the question arises:Is low pressure indication, as a development tool, becoming a standard measuring technology in engine development?

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DCE-Sensor Oil filled Sensor

Type 4005B Type 4007B Type 4045A Type 4075A

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

Measuring range

bar 0 ... 5/10 0 ... 50 ...

2/5/100 ... 10

Max. temperature

°C 125 200 140 140

Type of compensa-tion

– Analog

Analog (+ digital with amplifier

Type 4665)

Analog Analog

Compensated temperature range

°C 0 ... 125 0 ... 180 20 ... 120 20 ... 120

Thermal zero shift

%FSO <1 <1 <0,5 <0,5

Thermal sen-sitivity shift

±% <1 <1 <1 <1

3 Piezoresistive Sensors and InstallationThe maximum operating temperature of piezoresistive sen-sors for low pressure indication is often lower than that of the measured media. Long term measurements are only possible with special arrangements, such as water cooling the sensor or with the use of a cooled switching adapter. Fig. 2 identi-fies a number of installation alternatives for piezoresistive absolute pressure sensors in the intake and exhaust manifold of an internal combustion engine.

Fig. 2: Different applications of piezoresistive sensors in intake and exhaust

Due to the limited availability of space and the geometry of a modern intake manifold, size is the main requirement for the sensor. The reduced dimensions of miniaturized piezoresis-tive pressure sensors fulfil this requirement and in particular, Kistler Type 4005B/Type 4007B (M5) sensors are well suited for direct installation into the intake manifold or the cylinder head. In addition to such size considerations, temperatures of up to 120 °C are possible within the intake, particularly with high levels of EGR, however, the sensor is capable of with-standing these temperatures without additional cooling. Another benefit of the small diameter (M5) is that the head of the sensor can be seated flush with the inside surface of the intake channel.

In the exhaust, higher temperatures (over 1 000 °C) require active sensor cooling. This can be achieved by utilizing dedi-cated cooling adapters or the cooling of the cylinder head. 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.

Sensor Type 4005BSensor Type 4007B, direct installed in cyl. headSensor Type 4007B, cooling adapter Type 7525ASensor Type 4075A, cooling adapter Type 7505Sensor Type 4045A/4075A/4007B,cooled switching adapter Type 7533ASensor Type 4045A, cooling adapter Type 7511

ExhaustIntake

abcd

f

e

a b c

d

fe

The dimensions of the sensor are very important for direct mounting into the cylinder head, for this, the compact high-temperature sensor Type 4007B is ideally suited. This applica-tion occurs mainly in the development of motor sport engines where concerns for the size and mass of any additional engine mounted hardware are most acute.

Pressure range, compensation technique and thermal proper-ties of the Kistler pressure sensors are shown in Fig. 3.

Fig. 3: Specification of piezoresistive measuring chains

For low pressure indication with absolute piezoresistive sen-sors Kistler provides two types of construction. The Direct Chip Exposed principle (DCE) is the new, miniaturized sen-sor generation Type 4005B/Type 4007B (M5) wherein the semi-conductor measuring element is directly exposed to the media and is coated with a special protective film. Oil filled sensors, Type 4045A (M14) and Type 4075A (M12), utilize a similar measuring principle which requires a slightly larger package. In this design, the sensor has a thin steel isolation diaphragm which provides a high resistance to soot and par-ticle emissions.

All piezoresistive pressure sensors have thermal effects which are proportional to the full scale output (FSO). Therefore it is important to select the appropriate pressure range for the specific application.

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3.1 Temperature Characteristics of Piezoresistive SensorsThe measuring element of a piezoresistive pressure sensor is a single crystal silicon wafer into which resistors are implanted in a Wheatstone-Bridge configuration. The properties of the resistors can be influenced by temperature making compen-sation necessary. The temperature behaviour is characterised during manufacture and compensated for using selected resistors (analogue compensation) or digitally corrected utiliz-ing polynomials. The remaining error can be further reduced by performing a zero-point correction on the sensor signal. Fig. 4 shows the sensor signal output with respect to the applied reference pressure.

Fig. 4: Schematic view of the zero point correction

Reference pressure [bar]1 bar

(Ambient pressure)

Remaining error

(2)

(1) Calibration curve at Tref

(2)

(3)

(α)

(o)

(3) (1)

(α)

(o)

Sens

or s

igna

l

"Zer

o-po

int-

corr

ectio

n"

Characteristic curve at TA before "Zero-point-correction" (includes sensitivity and offset error)Characteristic curveafter "Zero-point-correction" at TA

Thermal sensitivityerrorThermal offset error

4 Impact of the Sensor Position on the Measured Pressure

The impact of the sensor position on low pressure measure-ment and therefore calculations of the gas exchange analysis have been investigated extensively on a V8 spark ignited engine. The high and the low pressure measurements have been obtained from cylinder 4, which has good measure-ment bore accessibility and a representative pressure curve. The disturbance from neighbouring cylinders on the same bank is low, this is due to the angular ignition spacing of 180 °CA. The engine is equipped with variable valve timing (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 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 side. Both the cooling adapters and the cylinder pressure sensor are cooled by a temperature conditioning unit Kistler Type 2621.

Fig. 5: Measurement locations in the intake system

A, Type 4007Bdirect mounted

C, PR sensor in cooled switching adapter

approx. 200 mm

B, Type 4007Bdirect mounted

The calibration reference curve (1) shows the ideal calibrated characteristic of a sensor and therefore each deviation from 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 of piezoresistive sensors on a test bed and the optimal zero-point procedure is described in section 4.

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Fig. 6: Measurement locations in the exhaust system

1, Type 4007B direct mounted in cylinder head, rear position

2, PR sensor in cooled switching adapter, frontal position

4, PR sensor in cooled switching adapter

3, PR sensor in cooled switching adapter, rear position

5, PR sensor in cooling adapter

At each engine operating point 200 single cycles with a reso-lution of 0,5 °CA were acquired and averaged.

One of the objectives was to determine the cyclic pressure fluctuations at the different measurement positions. This required a correction to the pressure level of the intake and exhaust pressure curves after the measurement. As a reference, the pressure curves of the sensors installed in the cooled switching adapters were used. By using the switching adapter, the sensor can be referenced to the known ambient pressure easily and so corrected accurately. The correction of sensors installed directly or in cooling adapters was applied during a certain crank angle window when a negligible pres-sure dynamic existed. Therefore, the absolute pressure level (given by sensor properties and zero-point adjustment) and the pressure dynamic (given by the measurement position) are independent. The following corrections were made:

IntakeAveraging of whole working cycle•Reference pressure uses the sensor signal obtained from •the cooled switching adapter, which was referred to the ambient pressure before each measurement

ExhaustAveraging in a crank angle window when a negligible •dynamic pressure exists (0 ... 270 °CA)The reference is the pressure measured by a sensor located •in the cooled switching adapter, this in turn, was referred to the ambient pressure before each measurement

Fig. 7 shows the pressure curve during the gas exchange at 2 000 1/min and full load. Following the EVO distinctive differences in the pressure dynamics are visible in the exhaust manifold (range of 360 °CA).

Fig. 7: Measured pressures in the low pressure phase of the engine cycle at different positions in the exhaust, operating condi- tion 2 000 1/min, full load. Average over 200 single cycles

Fig. 8: Measured pressures in the low pressure phase of the engine cycle at different positions in the intake, operating condition 5 000 1/min, full load. Average over 200 single cycles

Measurement position 2 (frontal position in the exhaust bend, see Fig. 6) shows distinctive differences in the gas dynamics at 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-urement position in the bend section of the manifold is caused by the redirection of the exhaust gas flow. Measurement position 1 (in cylinder head, exhaust) shows, in each case, the smallest local pressure maximum during the initial exhaust blow down. Due to the small cross sectional area high flow velocities are reached and static pressure fractures are as a result of this low. Measurement positions 4 and 5 (on straight duct, distant from valve) show identical pressure curves, even at high revolutions and high load.

Focussing on the intake, measurement position A (in cylinder head, close to valve) exhibits distinctive differences in the gas dynamic with respect to the engine revolutions, load and valve overlapping. The propagation of the pressure wave during the intake stroke moves from the valve back into the intake mani-fold passing the sensor adjacent to the valve (Position A Fig. 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.

Exhaust,pos. 2

Exhaust, pos. 3

Exhaust, pos. 1

p cylinder

Exhaust, pos. 5

Exhaust, pos. 4

0,75

1

1,25

1,5

1,75

2

2,25

2,5

270 360 450 540 630 720Crank Angle [deg CA]

Pres

sure

[ba

r]

EVO

tim

ing

IVO

tim

ing

EVC

tim

ing

TDC

p cylinder

Intake,pos. B

Intake,pos. A

0,5

0,75

1

1,25

1,5

1,75

2

2,25

270 360 450 540 630 720

Crank Angle [deg CA]

Pres

sure

[ba

r]

EVO

tim

ing

IVO

tim

ing

EVC

tim

ing

TDC

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5 Characterisation of Measuring Accuracy and Influence on the Analytical Results

High temperatures interacting on a sensor can cause thermal error which leads to reduced overall accuracy. As the accurate measurement of the pressure level is the most critical aspect, a procedure for temperature compensation is necessary to achieve the high requirements necessary to determine the pressure level.

To achieve thermal stability of a sensor it is essential that the sensor is cooled adequately. No sensor is able to achieve the required accuracy at temperatures sometimes over 1 000 °C, so cooling the sensor is mandatory. The correct, stable cool-ing of the exhaust pressure sensor will lead to an almost con-stant temperature environment for the pressure sensor during the measurements over the entire engine operating range.

5.1 Sensor Temperature over the Engine Operating RangeA characterisation of the temperature in different sensor locations was carried out on the V8 gasoline engine. For the sensor, direct mounted in the intake manifold, this resulted in temperatures around room temperature. The direct mounted sensor in the intake port of the cylinder head reached temperatures of 65 °C (2 000 1/min part load) and 90 °C (2 000 1/min full load) . The higher temperature level can be explained by the heat impact of the cylinder head and the air mass flow. Sensor temperatures at different positions are shown in Fig. 9.

One of the two sensors installed in the exhaust is located in the manifold (Pos. 4). This sensor, mounted in a cooling adapter reaches a maximum temperature of 80 °C. Using sensors in cooling adapters leads in general, to temperatures in the range of 50 … 80 °C across the whole engine operat-ing range.

The second sensor is installed directly in the exhaust port in the cylinder head (Pos. 1), this means that there is no addi-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.

Fig. 9: Measured sensor temperature at intake and exhaust positions

5.2 Accuracy Achieved by Using Piezoresistive Absolute Pressure Sensors

5.2.1 Pressure Measurement with Piezoresistive Sensors Direct Mounted/in a Cooling Adapter

Whether sensors are direct mounted or in cooling adapters the measuring element is always exposed to the exhaust pressure, making it is necessary to stop the engine and refer-ence the sensor to ambient pressure. As presented, a change in temperature causes a zero-point and a sensitivity error, therefore, in order to reach the high accuracy required, the sensor must be in the same condition (mainly temperature), as it will be during the subsequent measurements, prior to applying the zero-point correction.

Having installed the sensors into cooling adapters, the zero-point correction can be performed accurately, due to well conditioned sensors and a low dependency of the sensor temperature on the engine load. When mounting the sensors directly to the manifold, the sensor temperature may change according to the engine load (see Fig. 9) and lower the preci-sion of the zero-point correction. This is more noticeable for the exhaust pressure measurement with the sensor installed directly in the cylinder head, here significant temperature changes may be evident.

To show the resulting sensor errors which are mostly caused by thermal effects, an engine load sweep was performed (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 to the ambient pressure.

Fig. 10: Absolute pressure error with different piezoresistive sensors mounted in the exhaust in various operating conditions during a measuring campaign. Reference measurement in cooled switching adapter in position 4

15

35

55

75

95

115

135

155

175

Engi

nest

op (

hot)

200

0 m

in–1

,pa

rt lo

ad

200

0 m

in–1

,fu

ll lo

ad

340

0 m

in–1

,fu

ll lo

ad

500

0 m

in–1

,fu

ll lo

ad

Sens

or t

empe

ratu

re [

°C]

Intake: sensor in cooling adapter (pos. C)Intake: sensor direct mounted in cylinder head (pos. A)Exhaust: sensor in cooling adapter (pos. 4)Exhaust: sensor direct mounted in cylinder head (pos. 1)

Engi

ne st

op (h

ot)

200

0 m

in–1, p

art l

oad

200

0 m

in–1, f

ull l

oad

340

0 m

in–1, f

ull l

oad

500

0 m

in–1, f

ull l

oad

Engi

ne st

op (h

ot)

-40-30-20-10

010

Sensor Type 4045A (M14),Pos 5

-40-30-20-10

010

Sensor Type 4007B (M5),Pos 1

Abs

olut

e pr

essu

re e

rror

[m

bar]

406080

100120140160180

Pos 1 Pos 5

Sen

sor

tem

pera

ture

[°C

]

10 www.kistler.com

Fig. 11: Typical total absolute pressure error and instabilities of low pressure indication. Sensor type 4007BA5FS installed in the intake

Fig. 12: Typical total absolute pressure error and instabilities of low pressure indication. Three piezoresistive sensors installed in the exhaust

INTAKE Sensor Type 4007BA5FS

Pressure range 0 ... 5 bar

Total error (typical)

Installation with cooling adapter

±5 mbar/ ±0,1 %FSO

Direct installation in cylinder head

±10 mbar/ ±0,2 %FSO

Instability(typical)

Short-term instability (at same operating condition)

±2,5 mbar/ ±0,05 %FSO

Long-term instability (between first and last measuring point)

±2,5 mbar/ ±0,05 %FSO

EXHAUSTSensor Type 4007BA5FS

Sensor Type 4045A5V200S

Sensor Type 4075A10V200S

Pressure range 0 ... 5 bar 0 ... 5 bar 0 ... 10 bar

Total error (typical)

Installation with cooling adapter

±20 mbar/ ±0,4 %FSO

±20 mbar/ ±0,4 %FSO

±30 mbar/ ±0,3 %FSO

Direct installation in cylinder head

±45 mbar/ ±0,9 %FSO

– –

Insta- bility (typical)

Short-term instability (at same operating condition)

±2,5 mbar/ ±0,05 %FSO

±2,5 mbar/ ±0,05 %FSO

±2,5 mbar/ ±0,03 %FSO

Long-term instability (between first and last meas-uring point)

±20 mbar/ ±0,4 %FSO

±5 mbar/ ±0,1 %FSO

±10 mbar/ ±0,1 %FSO

The increased engine load applies a higher thermal load into the sensor causing an increase of the temperature at the measuring element. The sensor error is therefore linked to the applied temperature. The biggest increase in tempera-ture, and hence the largest sensor error, occurs at the sensor installed in the cylinder head.

In addition to the cited effects of temperature the sensor sta-bility will be evaluated next. Having completed the specified load conditions over the engine operating range, the engine is stopped and the difference in the sensor output, between the first last measuring point is determined. To state the short term instability, the engine is held at a steady operating condition and the change in the maximum sensor errors are obtained.

Mounted in the intake, the environment is less challenging as both the ambient and media temperatures are significantly lower than those that surround the exhaust manifold. Sensor errors (Fig. 11), even of those sensors mounted directly in the cylinder head, are less than ±0,2 %FSO and stability is very good also. Less than ±0,05 %FSO difference exists between the readings taken at the first and last measuring points. This is due to a combination of factors, the extremes in tem-perature are less damaging to the sensor and the additional cooling provided by the charge media help to provide a stable diaphragm temperature.

On the exhaust side (Fig. 12), the sensor errors are greater due to more dynamic temperature environment, relative to the intake. The sensors that are installed in cooling adapters have errors of less than ±0,4 %FSO. The sensor installed in the cylinder head, because of the elevated temperature levels, displays the highest errors, up to ±0,9 %FSO. The dif-ference in accuracy therefore, is not dependent on the sensor type but on the quality and stability of the sensor cooling.

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.

The short term instability for all sensor types and measur-ing positions, remain within 0,05 %FSO. This characteristic is especially important when considering using sensors with cooled switching adapters.

5.2.2 Pressure Measurement with Piezoresistive Sensors Installed in a Cooled Switching Adapter

A cooled switching adapter has the feature whereby, a pneu-matically controlled valve provides switching between ambi-ent and exhaust pressures. The use of a cooled switching adapter enables a precise and flexible zero-point adjustment of the piezoresistive pressure sensor referenced to ambient pressure at any time. The adjustment can be made while the engine is running under the same thermal load as the follow-ing measurement will take place.

In addition, the sensor installed in the cooled switching adapter, has reduced exposure to extreme conditions like thermal load and soot contamination.

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 a verification is made prior to every measuring point, even the smallest thermal zero-point error can be measured and therefore corrected, ensuring the most accurate scaling.

With this procedure for zero-point adjustment done, a refer-ence accuracy of ±1 mbar can be achieved at every single operating point. It should be noted that the short term stabil-ity during an engine test point is not corrected (Fig. 14).

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5.3 Low Pressure Indication with Piezoelectric Sensor and Pneumatic Pressure Measurement (Remote Sensing System)

Should low pressure indication be attempted utilizing a piezoelectric sensor, an additional pressure measurement is necessary to determine the static mean absolute pressure. The pressure measurement consists of a pressure tap or connection point, the connecting hose and a piezoresistive absolute pressure sensor (Fig. 15). The pressure trace can be

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 the pressure oscillation around the mean level. Therefore it is important that the signal of the piezoelectric sensor has no static component and therefore a mean value of zero.

The advantage of this method is that a conventional piezore-sistive pressure sensor can be used, because of the hose length the gas temperature at the sensor is low and the contamination by soot is less likely. The disadvantage is that with a remote sensing system, as it is installed generally, sig-nificant measuring errors can occur with the determination of the mean pressure level. The error is not related to acoustic phenomena such as pipe oscillations, but is attributed to the inflow and outflow of gases through the pressure port.

The pressure oscillation in the intake or exhaust will travel through the pressure tap and the hose until it reaches the pressure sensor. Due to this pressure variation within the remote sensing system, a temporary non-constant mass flows in and out of the pressure tap. The typical geometries used for the pressure tap leads to a difference between the drag coefficient ζI during inflow and drag coefficient ζO dur-ing outflow. This leads to a better emptying of the system compared with the inflow, which in turn results in a reduced mean pressure level in the remote sensing system.

These effects have been analyzed and demonstrated by Weyer [6] experimentally as well as through simulation. Weyer shows that the error during the determination of the pressure level in a fluctuating pneumatic system is related to the dimensions of the remote sensing system. Of the most influence, is the pressure tap itself where the amplitude and frequency of the pressure oscillation have a major effect. One exception is, when the pressure tap has the same drag coeffi-cient for inflow as for outflow, this would avoid any error but it means a complex geometry of the pressure tap and cannot be accomplished easily.

For evidence about this effect, Eng [7] performed measure-ments on a single-cylinder diesel engine. He employed a remote sensing system which was compared to the results of a direct mounted piezoresistive sensor located in the cooled switching adapter. The resulting pressure fluctuation and

Fig. 14: Typical total absolute pressure error and instabilities of low pressure indication. Three piezoresistive sensors installed in a cooled switching adapter in the exhaust

Ambient(not switched)

Statuscoolingswitchingadapter

Purpose

Enginestatus

Time

Protection ofsensor

Any Operating point stabilized

Warm sensor(approx. 60 s)

Zero-pointcorrection Measurement

Exhaust(switched)

Protection ofsensor

Any

Ambient(not switched)

Exhaust(switched)

Ambient(not switched)

EXHAUSTSensor Type 4007BA5FS

Sensor Type 4045A5V200S

Sensor Type 4075A10V200S

Pressure range 0 ... 5 bar 0 ... 5 bar 0 ... 10 bar

Total error (typical)

Installation with cooling switching adapter

This error can be eliminated by making a zero-point correction

Insta- bility (typical)

Short-term insta-bility (at same operating condi-tion)

±2,5 mbar/ ±0,05 %FSO

±2,5 mbar/ ±0,05 %FSO

±2,5 mbar/ ±0,03 %FSO

Long-term insta-bility (between first and last measuring point)

This error can be eliminated by making a zero-point correction

Fig. 15: Low pressure indication with piezoelectric sensor and pneu- matic pressure measurement for the acquisition of the mean absolute pressure. The pressure curve results from the addi- tion of the fluctuation with the averaged value

HP FilterCharge amplifier

PR sensor

Average

Pressure p

Water cooledPE sensor

Wall static pressure tap

Tubelength L

12 www.kistler.com

phase shift of the pneumatic signal are strongly related to the dimensions (diameter and length of the pressure tap, diam-eter and length of hose and dead volume). It becomes evi-dent therefore, that the error of the remote sensing system, regarding the determination of the mean pressure level is in the range of 15 … 20 mbar, which is a rather high number for this application. A comparison of Weyer's [6] results leads to a good correlation.

The following results, measured on the 8-cylinder engine, include pressure curves from the remote sensing system com-pared to a piezoresistive sensor installed in a cooled switching adapter (Fig. 16). The measuring positions 4 and 5 are on the same longitudinal position in the exhaust manifold. The measured pressure traces were corrected in the following manner:

Direct pressure measurement with piezoresistive pressure •sensor in cooled switching adapter: Zero-point is adjusted before the measurement according to the ambient pres-sure level Remote sensing system with piezoresistive sensor (at the •end of the hose): Zero-point is adjusted to the ambient pressure level during engine stop before the measuring campaign Low pressure indication with a piezoelectric sensor and the •remote sensing system: the pressure oscillation, measured by the piezoelectric sensor and the averaged pneumatic pressure are added.

It is quite visible, that the remote sensing system indicates clearly reduced pressure amplitudes as well as a phase shift. This effect will become more evident towards higher engine speeds or with a prolongation of the hose.

It is evident from the data that the remote sensing system still shows a pressure dynamic, therefore, averaging the signal over a complete cycle is mandatory before adding the piezoe-lectric dynamic component. Compared to the averaged direct piezoresistive pressure measurement the averaged pressure of the remote sensing system is too low. The following errors have an influence:

Direct pressure measurement with piezoresistive pressure •sensor in cooled switching adapter: thermal related sensi-tivity error (small error, reference measurement)Remote sensing system with piezoresistive sensor: Error •related to the pressure tap (inflow and outflow), depend-ent on hose length. The formation of condensation in the hose will cause a dampening effect and has an influence on the dynamics of the signal (considerable error possible)Direct pressure measurement with piezoelectric sensor: •thermal related sensitivity error, thermal shock (small error in the dynamic pressure)

In the case of low pressure indication with a piezoelectric sen-sor in combination with a remote sensing system, generally a systematic error of up to 20 mbar can occur. This correlates to the error described by Weyer [6]. Therefore this method is not recommended to achieve the best possible accuracy.

The difference between the absolute pressure measured using the remote sensing system to that of the direct piezoresistive sensor installed in a cooled switching adapter is shown in Fig. 17 for different operating conditions. It can also be seen that the sensor position has impact due to the differences of dynamic pressure at different locations.

Fig. 16: Pressure curves of direct pressure measurements with piezoresistive (PR) and with piezoelectric (PE) sensor, pneumatic pressure measurement (tube length 0,3 m) with PR sensor. Operating condition 5 000 min–1, full load. Average over 200 single cycles

Direct PRmeas. pos. 4Pegged

direct PEmeas. pos. 5 Pneumatic PR

meas. pos. 5

Direct PEmeas. pos. 5

0,25

0,5

0,75

1

1,25

1,5

1,75

2

2,25

0 90 180 270 360 450 540 630 720

Crank Angle [deg CA]

Abs

olut

e pr

essu

re in

exh

aust

[ba

r]

-0,5

-0,25

0

0,25

0,5

0,75

1

1,25

1,5

Pres

sure

PE

mea

sure

men

t [b

ar]

TDCTDC

Resulting averaged pressure (0 ... 720 °CA):Direct PR measurement 1,361 barPneumatic PR measurement 1,336 bar

Fig. 17: Absolute pressure error of the low pressure indication with piezoelectric measurement and pneumatic pressure measure- ment (averaged pressure in window 0 ... 270° CA, see chapter 3). Direct piezoresistive measurement in position 5 as reference

Engi

ne st

op (h

ot)

200

0 m

in–1, p

art l

oad

200

0 m

in–1, f

ull l

oad

340

0 m

in–1, f

ull l

oad

500

0 m

in–1, f

ull l

oad

Engi

ne st

op (h

ot)

-60

-40

-20

0

Position 4

-60

-40

-20

0

Position 2

Abs

olut

e pr

essu

re e

rror

[m

bar]

www.kistler.com 13

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. 19: Measured pressure curves during the gas exchange and computed mass flow rate through the valves. Operating condition 2 000 min–1, IMEP 2 bar, unrestricted operation, exhaust valve lift full, intake valve lift reduced. Average over 200 single cycles

0,2

0,4

0,6

0,8

1

1,2

270 360 450 540 630 720

Crank Angle [deg CA]

Pres

sure

[ba

r]

TDC

Exhaust pressure, pos. 1

p cylinder

Exaust valve lift

Intake pressure, pos. A

Intake valve lift

Exhaust mass flow

Intake mass flow

-20

0

20

40

60

Mas

s flo

w r

ate

[g/s

]

TDC

Exhaust pressure, pos. 1

p cylinder

Exaust valve lift

Intake pressure, pos. A

Intake valve lift

Exhaust mass flow Intake mass flow

-20

0

20

40

60

Mas

s flo

w r

ate

[g/s

]

TDC

0,2

0,4

0,6

0,8

1

1,2

270 360 450 540 630 720

Crank Angle [deg CA]

Pres

sure

[ba

r]

7.5

TDC

-30 -20 -10 0 10 20 300

1

2

3

4

5

6

7

8

9

10

Valve timings: Intake -5°CA /Exhaust -5°CA Intake/ Exhaust series Intake +5°CA /Exhaust +5°CA

Res

idua

l gas

fra

ctio

n [%

]

Delta p exhaust [mbar]

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

5.4 Influence of Absolute Pressure Level on the Result of the Gas Exchange Calculation

Use of low pressure indication for gas exchange optimization shows that a link exists between the measuring task, the measuring position, the selection of low and high pressure instrumentation as well as the analytical process itself. The technology selection should be done with careful considera-tion to the boundary conditions as well as the mission targets. There should be a definition of the quality assurance as well as a confidence check of the measuring results in an early stage.

The gas exchange process is mainly influenced by the pres-sure difference at the valves. Therefore low pressure and in-cylinder indication should be considered complementary. Piezoresistive sensors for intake and exhaust measurements offer an accuracy in the range of ±10 mbar, however, the thermal shock error of the piezoelectric cylinder pressure sen-sor is at least one order higher. The highest uncertainty of the gas exchange measurement is therefore the in-cylinder low pressure signal.

The following two illustrations show the gas exchange of the V-8 engine with a fully variable valve train. The engine oper-ating conditions analysed are 2 000 1/min part load restricted (full valve lift, Fig. 18) and unrestricted (exhaust valve full lift, intake valve part lift, Fig. 19). Representative pressure curves are shown, which are measured with piezoresistive absolute pressure sensors Type 4007B close to the valve at the intake (position A) and exhaust (position 1), as well as the calculated mass flow.

Low pressure indication delivers pressure curves in high reso-lution for all control strategies.

The pressure differences related to the measuring position at the pre exhaust (Fig. 7), have no impact on the global results of the gas exchange calculation. This is because of

the phasing which is considerably before the residual gas relevance range of the valve overlap. The difference in pres-sure dynamic at different measuring positions at the intake (Fig. 8) has just minor effects on the results of the 1-D gas exchange calculation. The reason for this is that this calcula-tion program takes into account the exact position of the sensor and therefore the runtime error of the pressure wave is considered.

An extensive parameter study confirms that primarily, the pressure level and not the sensor position or their adaptation is of central importance for the gas exchange calculation. By increasing valve overlap the sensitivity of the calculated residual gas fraction on the absolute pressure level in the intake and exhaust port increases. In Figure 20, an example is shown on the influence of a different pressure level on the calculated residual gas fraction.

14 www.kistler.com

6 Conclusion and RecommendationsMiniaturised piezoresistive absolute pressure sensors can be placed, due to their size and mass, with minimum restrictions in the manifolds. New high-temperature pressure sensors broaden the application scope, allowing a sensor installation even directly in the cylinder head close to the valve. The con-ditioning of the sensor is still necessary, especially if tempera-tures are high and sustained as in the exhaust manifold. The decision as to which piezoresistive sensor and its adaptation to use has to be considered on a case by case basis.

Measuring Position in the IntakeThe choice of the measuring position is easier in the intake as the temperature of the measuring bore and of the intake gases normally allow direct mounting of the sensor without cooling. The use of a cooled switching adapter in addition to extending the useful life of the sensor, provides a convenient zero point solution in combustion strategies utilizing high levels of EGR.

Measuring Position in the ExhaustAny pressure measurement in the exhaust can be challeng-ing, therefore, when selecting the location for a sensor con-sideration must be given not only to the physical size of the adaptation but perhaps more significantly to the geometry of the manifold. As presented, the dynamic pressure meas-ured at different locations can be influenced by the specific mounting orientation of the sensor related to the flow. In the exhaust manifold a cooling adapter will be necessary unless the sensor can be installed directly in the cylinder head exhaust runner. Piezoresistive absolute pressure sensors (Type 4045A/Type 4075A) with thin steel isolation diaphragms pro-vide a high resistance to soot emissions and have an accept-able lifetime when constant cooling is present.

Absolute Pressure and Zero Point CorrectionStudies on the influence of the absolute pressure on the com-puted residual gas fraction show that a precision of better than ±10 mbar is necessary. 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. High process reliability can be assured by using established measuring techniques in con-junction with the switching adapter. Low pressure indication with a piezoelectric sensor and pneu-matic pressure measurement (remote sensing system) is not recommended for the precise determination of the absolute pressure level in the exhaust.

Modelling and SimulationCompared 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.

7 References[ 1 ] M. Bargende Homogene Kompressionszündung bei Otto- und Dieselmotoren. Anforderungen und Potentiale Symposium IAV Juni 2007 Berlin

[ 2 ] N. Hoppe Erfahrung mit dem Einsatz eines modifizierten Restgasmodells und die Weiterentwicklung zum online-fähigen Optimierungstool Internationales Symposium für Verbrennungs- diagnostik Mai 2006, Baden-Baden

[ 3 ] C. Burkhardt, M. Gnielka, C. Gossweiler, D. Karst, M. Schnepf, J. von Berg, P. Wolfer Ladungswechseloptimierung durch geeignete Kombination von Indiziermesstechnik, Analyse und Simulation 9. Tagung, Der Arbeitsprozess des Verbennungsmotors September 2003, Graz

[ 4 ] A. Wimmer, R. Beran, G. Figer, J. Glaser, P. Prenninger Möglichkeiten der genauen Messung von Ladungswechseldruckverläufen Internationales Symposium für Verbrennungs- diagnostik Mai 2000, Baden-Baden

[ 5 ] H. Alten Der Ladungswechsel im Rennmotor MTZ-Konferenz, Ladungswechsel im Verbrennungsmotor November 2007, Stuttgart

[ 6 ] H. Weyer Bestimmung der zeitlichen Druckmittelwerte in stark fluktuierender Strömung, insbesondere in Turbomaschinen Dissertation RWTH Aachen 1973 DFVLR, Forschungsbericht/ Deutsches Zentrum für Luft- und Raumfahrt 1974

[ 7 ] M. Eng Untersuchung von Sensoren und Messverfahren zur Niederdruckindizierung Diplomarbeit Fachhochschule Nordwestschweiz November 2007

www.kistler.com 15

Low Pressure Measurement in Intake and Exhaust

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 (amplifier)

V mA

0 … 10 4 … 20

0 … 10 4 … 20

0 … 10 4 … 20

0 … 10 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 formeasuring pressures inthe intake system. Verycompact dimensions, versatile, high naturalfrequency. Available asPiezoSmart® sensor ormeasuring chain withamplifier Type 4618A

As for Type 4005B…High-temperaturedesign, digital tempera-ture compensation

Oil-filled pressure sensorwith steel diaphragm.Ideal for measuringpressures in both theintake and exhaustsystem. Available indifferent versions with or without PiezoSmart®, or as measuring chain with amplifier Type 4618A

Oil-filled pressure sensorwith steel diaphragm.Available in differentversions with or withoutPiezoSmart®, or asmeasuring chain withamplifier Type 4618A

Application •Intakepressure •Intakepressure •Exhaustpressurein racing engines

•Intakepressure •Exhaustpressure

•Exhaustpressure

Recommended mounting/adapter

•Directinstallationin intake

•Directinstallationin intake or exhaust (cylinder head) •AdapterType7525A •AdapterType7533A

•Directinstallationin intake •AdapterType7511 •AdapterType7533A

•AdapterType7505 •AdapterType7533A

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

D

LT

D

LT

D

L

T

D

L

T

Cooling Adapters

Technical Data Type 7511 Type 7505B Type 7525A… Type 7533A…

Recommended sensors 4045A… 4075A… in adapter

4075A… 4005B…/4007B… 4005B…/4007B…/ 4045A.../4075A…

Dimensions L mm 12,5 11,8 7 13

T G1/2" M18x1,5 M14x1,25 M14x1,25

Description Damped adapter for applications with high vibration

Compact adapter for sensor Type 4075A

Compact adapter for miniature pressure sen-sors. Damped version available

Switching adapter to reference sensor to ambient pressure

TL

TLT

LL

T

Applied Pressure Sensors and Cooling Adapters

16 www.kistler.com

920-

366e

-10.

08

M

at10

00

©

2008

, Kis

tler

Gru

ppe

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