Engine combustion process monitoring and cylinder head gasket pressure sensor

This article introduces Electronic control of the combustion process of the engine and its significance, the principle of the cylinder head gasket pressure sensor and the pressure sensor integrated in the cylinder head gasket. Electronic control of the combustion process of the engine and its significance In order to optimize the adjustment of injection parameters and ignition parameters, the engine management system needs to collect accurate combustion process data. This involves multiple electronic control projects of the engine combustion process, such as fuel dosing, ignition timing, injection timing, EGR, boost pressure, gasoline engine lean burn, cylinder fuel cut-off and methanol blending electronic control. These projects rarely use sensors to monitor directly, but only monitor the combustion process indirectly by monitoring torque, knock, etc. However, experiments have shown that through appropriate sensor technology, engine electronic control can be optimized, even in mass-produced cars. In recent years, research on sensors that directly monitor combustion process parameters has mainly focused on the following sensors: flame front sensor, ion gap sensor, cylinder pressure sensor, optical flame sensor. Among them, the information provided by the cylinder pressure sensor is the most meaningful and abundant, including: compression pressure, combustion starting point, cylinder pressure peak and the crankshaft position when it occurs, mean effective pressure change rate and the change rate of crankshaft position when cylinder pressure peak occurs, indicated power, heat release rate, knock detected by cylinder, misfire detected by cylinder, etc. The cylinder pressure signal can not only be used to realize electronic control of the combustion process through electronic control such as fuel dosing, ignition timing, injection timing, EGR, boost pressure, gasoline engine lean burn, cylinder fuel cut-off and methanol blending, but also can be used for misfire fault diagnosis.

A typical example of electronic control of the combustion process in gasoline engines is the "MBT = Minimum spark timing for the Best Torque" control. The crankshaft position corresponding to the peak of the cylinder pressure can be monitored using this control and the peak is kept at a certain crankshaft position by adjusting the ignition advance angle. It is worth noting that knock control has a higher priority than MBT. Knock control and MBT control are combined to determine the best ignition advance angle. A similar effect is also achieved for monitoring the combustion process in diesel engines . Principle of cylinder head gasket pressure sensor There are various solutions and sensors available for monitoring cylinder pressure. The key is that the construction of the sensor is suitable for general engines without modifying the cylinder head. In addition, the construction and sensitivity characteristic curve of the sensor have the following characteristics: temperature close to the combustion chamber wall; tightening torque of the spark plug; good durability even when the surrounding is full of oil and combustion gases. In the early years, some people tried to attach strain gauges to the cylinder head bolts to monitor the cylinder pressure, but this was due to the difficulty in distinguishing the contribution of each cylinder pressure to the sensor signal. Another approach is the cylinder pressure sensor of the spark plug type for gasoline engines and the glow plug type for diesel engines. The basic construction of this sensor is now almost complete, in which a piezoelectric element is used as the sensor element; however, the operation of this cylinder pressure sensor depends on the indirect detection of the movement of the sealing gap. In contrast, the pressure sensor integrated in the cylinder head gasket has a great advantage because it can directly detect the cylinder pressure. In order to eliminate the influence of the engine structure, Erling Klinger, Dettingen/Erms and Kistler Instruments in Winterthur, Switzerland are developing a cylinder head gasket with an integrated cylinder pressure sensor. This cylinder head gasket with pressure sensor places high demands on the piezoelectric sensor itself. In particular, it is a great challenge to make the sensor very small while ensuring the highest possible signal gain and insensitivity to lateral influences. In 20 years of research work, new crystal compounds have been tested and new crystal growth processes have been developed. Now there is a family of crystals available. These crystals, called "Piezo Stars", have been grown at Kisler since 1998 and processed into sensor elements. The properties of these Piezo Star crystals are: high piezoelectric sensitivity (five times higher than quartz); high stability of important properties such as sensitivity, linearity and lack of hysteresis over temperature and time; no twinning, i.e., no change of polarity of the individual crystal regions when subjected to mechanical loads at high temperatures; reproducible growth of the crystals on an industrial scale; and prototypes have been produced and tested in practice in high-grade piezoelectric sensors. Piezoelectric pressure sensors are tested in spark plugs and glow plugs, and the first samples of miniaturized test elements with a diameter of 3 mm have been successfully produced, which can be screwed directly into the sealing gap, see Figures 2 and 3. The most severe working condition is the sensor diaphragm, which is subjected to high mechanical and thermal loads under special installation conditions. Therefore, in addition to determining the test accuracy, the sensor diaphragm usually also determines the service life of the sensor.

Pressure sensor integrated into the cylinder head gasket

In principle, the signal quality is mainly influenced by the position of the sensor. A well-sealed sensor arranged on the combustion chamber is the most favorable for combustion pressure. Modern engine designs, with their multi-valve technology and complex and expensive cylinder head channels, leave little space for sensors. The metal laminated cylinder head gasket, as a functional element arranged close to the combustion chamber, is an alternative installation location for the pressure sensor, which avoids drilling holes in the engine structure specifically for the sensor. The goals of integrating the pressure sensor into the cylinder head gasket are:

- To obtain a pressure signal of a quality level that meets the requirements of the dynamometer diagram
- To achieve the smallest possible installation thickness, i.e. no sensor element
- To achieve the lowest possible sensitivity to lateral influences, i.e. the lowest possible thermal and mechanical coupling of the sensor to the gasket
- To achieve maintenance friendliness, i.e. the sensor can be installed and removed without disassembling the cylinder head
- To achieve a service life of all components and the entire system that is greater than or equal to the service life of the engine.

Due to the limited size of the sensor (diameter 3.6 mm, including the sensor housing), only a larger installation thickness is required. In order to keep the compression ratio of the engine constant, the height of the engine cylinder block is reduced by a point, which is equivalent to the difference in thickness of the cylinder head gasket used in series production.

Conclusion and Outlook

The test results show that this gasket with pressure sensor can achieve more accurate control of the combustion method - this is mainly used in diesel engines with common rail injection or pump nozzle injection. The components of the injection unit must have processing tolerances, and the resulting deviations in the full-load injection quantity of each cylinder can be compensated by the engine electronic control system. The "safety distance" of the full-load peak pressure from the mechanical load limit can be significantly reduced. The pressure curve can be used to monitor the combustion process cylinder by cylinder: engine wear or faults in the injection device, exhaust gas recirculation or boost pressure closed-loop control will be immediately recognized based on the atypical peak pressure.

This concept is basically suitable for collecting cylinder pressures through the cylinder head gasket. Even if its application is initially limited to operation on the test bench, it has already revealed the possibility of being used in series production engines. Prerequisites for future series production engines are extensive, long-term system tests and further reductions in the installation thickness, for example by combining the sealing and housing functions and reducing the influence of transverse forces. For the realization of future high-tech engines, development potentials must also be further investigated, such as:

- Use of the system for improved uniform pressure distribution in the cylinders
- Improvement of the on-board fault diagnosis OBD
- Potential of the system for optimizing the combustion process precisely in accordance with emission requirements.