1. Field of the Invention
The present invention relates to pressure sensors and in particular to a pressure sensor for use in internal combustion engines.
2. Description of the Related Art
Internal combustion engines generate high pressures inside the cylinder during operation. The pressure generated during combustion is useful for gaining an understanding of the combustion process and also can be used to control the operation of the engine. Measuring the pressure change over time in the combustion chamber can be used as a parameter in the control of spark timing and valve actuation. The advent of computerized engine controls has allowed the use of additional information in the control algorithms. The computer allows for accurate analysis of the change of pressure with high precision. A more affordable pressure sensor is needed for better control. Emission regulations are driving a need to monitor combustion pressure for all cylinders on a vehicle.
A pressure sensor for an engine has to be able to handle high thermal stresses during combustion, chemical attack and vibration from mechanical load cycles. The pressure sensor must be stable and insensitive to temperature changes in a harsh environment.
Various types of sensors have previously been used to measure pressure inside the cylinder. These sensors have suffered from a variety of problems with accuracy, cost and reliability. Quartz pressure sensors have been used for measuring pressure. The quartz sensor undergoes a change in charge when compressed which is directly proportional to the applied force. A diaphragm is placed between the sensor and the environment to be sensed. As the pressure changes, the force on the diaphragm is changed and therefore the force on the sensor. Unfortunately, with large thermal gradients, the outside surface of the diaphragm expands while next to hot combustion gases. The inner surface of the diaphragm being cooler does not expand as much. The net effect is that the diaphragm oil cans or bends away from the sensor toward the higher temperature. When this happens, the sensed force has an inaccurate reading that is less than actual because of the loss of contact between the diaphragm and the quartz sensor. After repeated thermal cycling, the diaphragm welds can break and cause a sensor failure. The diaphragm can also abrade or corrode causing a failure. Quartz pressure sensors are also very expensive and not suited to high volume manufacturing.
A current unmet need exists for an automobile engine cylinder pressure sensor that has improved accuracy, reliability and is low in cost.