Research Academics  
 
   
 

Pressure Measurement in Combustion Engines
 
Contact Person : Dr.-Ing.Ha-Duong Ngo

Introduction

There is a great demand in the automotive industry for high temperature (up to 500°C) pressure sensors that can be operated in combustion engines. By accurately monitoring and controlling the pressure and temperature in the combustion chamber (Fig. 1), the engine efficiency can be raised and the fuel consumption and pollutant emission reduced. Therefore a micromechanical high temperature pressure sensor using a membrane based piezoresistive ß-SiC-on-SOI (SiCOI) sensor chip and a specially designed housing has been developed and fabricated.

 

Combustion Chamber

Fig. 1: Pressure and temperature behaviour in the combustion chamber of a 4-stroke engine.

Sensor Fabrication

For the fabrication of the piezoresistive pressure sensor chips single crystalline, 2 µm thick, n-type ß-SiC films were deposited by CREE Inc., USA, on SIMOX (Separation by IMplanted OXygen) substrates. The sensor chips (4 mm x 4 mm) consist of a circular diaphragm with a center boss and four ß-SiC piezoresistors that are connected in a Wheatstone bridge (Fig. 2). To be operated at high cylinder pressures (up to 200 bar) in combustion engines of automobiles, the sensor chips need to be mounted into a specially designed housing (Fig. 3). When a pressure acts on the bottom of the steel diaphragm the center boss of the sensor chip is deflected upwards and the piezoresistors are in pairs under tensile (on the inside) and compressive (on the outside) longitudinal stress.
  

SensorHousing    

Figure 2: Photograph (top view) of the piezoresistive SiCOI sensor chip.


Figure 3: Schematic of the pressure sensor with the packaged SiCOI sensor chip (black).

Sensor Characterization

a) Static Measurements

Static measurements were performed to determine the sensitivity of the SiCOI sensor. The measurements were performed in the constant current mode with a supply current of 2 mA. Fig. 4 shows the output voltage vs. pressure characteristics for different temperatures. In Fig. 5 the sensitivity of the sensor is plotted versus temperature. At room temperature the sensitivity of the sensor amounts to approximately 0.19 mV/bar. This value decreases to about 0.12 mV/bar at 300°C.

Output

Fig. 4: Output voltage vs. pressure characteristics for different temperatures. The offset voltage is set to zero.

Sensitivity

Fig. 5: Sensitivity of the SiCOI sensor in the temperature range between room temperature and 300°C.

b) Dynamic Measurements

For measuring the dynamic cylinder pressure in the combustion chamber of a gasoline engine (Ford 1.4 liter) the SiCOI sensor was placed into the cylinder head with a M10 thread. The engine was operated at 1500 rpm under different loads. For calibration a quartz pressure transducer from Kistler AG was used as a reference. Fig. 6 shows the output signal (pressure) measured with the SiCOI sensor in dependence of the crank angle and time, respectively.

Dynamic Measurement

Fig. 6: Dynamic cylinder pressure measured with the SiCOI sensor in the combustion chamber of a gasoline engine at 1500 rpm. For comparison the output signal of a quartz pressure transducer from Kistler AG is shown.
  

  Conclusions

With a micromechanical piezoresistive SiCOI pressure sensor static and dynamic measurements have been performed. The SiCOI sensor was tested under static pressures of up to 200 bar in the temperature range between room temperature and 300°C. The dynamic pressure measurements in the combustion chamber of a gasoline engine were performed at 1500 rpm under different loads. The work demonstrates, that the fabricated and characterized SiCOI sensor is best suited for accurately monitoring the cylinder pressure of combustion engines, i.e. the recognition of misfire, above dead center and maximum pressure.