Engine pressure transducers
Pressure sensors are an integral part of advanced engine testing systems, particularly those used to measure in-cylinder combustion pressures. These provide an invaluable insight into the efficiency of the combustion process and are a vital tool for engine calibration and testing. However, the conditions present in many bespoke race engines can prove problematic for some sensor types, due to factors such as very high combustion pressures and vibrations caused by high rates of valvetrain acceleration. It is important therefore to look at a few key considerations when specifying sensors for high-performance engine testing.
The mounting of cylinder pressure sensors can present engineers with a number of choices and challenges. The easiest course of action, and an especially useful one if an engine is not a purely experimental unit, is to use a sensor integrated into a spark plug or glow plug. The recent appearance on the market of portable combustion analysis systems has opened up the possibility for using plug-based sensors in trackside applications. This is an exciting development and brings a level of analysis to the trackside that was previously available only in the dyno room.
Conversely, if it’s possible to modify a cylinder head to accept sensors then dedicated pressure transducers can be used. While there are only a few different types of plug-based sensor on the market, the range of non-plug based sensors is much wider, presenting a greater choice in terms of sensor specification. These sensors will invariably be mounted in threaded holes into the combustion chamber, and are generally reserved for use on dedicated test engines.
Recent developments have also seen the appearance of a third mounting method – the incorporation of pressure transducers into a multi-layered head gasket. Here, probe-type pressure sensors, complete with signal amplifiers, are built into the structure of a cylinder head gasket, with one sensor per cylinder. This provides a permanent pressure-sensing capability without the need for the structure of the cylinder head to be altered.
Sensors designed for measuring in-cylinder pressures generally use a piezoelectric sensing element. When pressure, force or acceleration is applied to a quartz crystal, a charge is developed across the crystal that is proportional to the force applied. The fundamental difference between these crystal sensors and static-force devices such as strain gauges is that the electric signal generated by the crystal decays rapidly, making such sensors unsuitable for measuring static forces or pressures but useful for dynamic measurements.
The use of piezoelectric crystals in many high-end pressure sensors means that vibration or shocks near the sensor can disrupt the output signal. Obviously this is undesirable, and sensor manufacturers take steps to mitigate the effects.
Different sensors have varying degrees of sensitivity to acceleration (caused by the aforementioned vibrations). For example, certain transducers use water cooling to increase their sensitivity and durability; however, this also increases their sensitivity to acceleration. This means that for pressure-measuring applications in close proximity to components that create shockwaves – notably the intake and exhaust valves of high-rpm race engines – non-cooled sensors with low acceleration sensitivity are used.
There are, however, sensors that can provide the signal resolution of a water-cooled sensor yet still be used in high-acceleration areas. These sensors are designed to compensate for shock- or vibration-induced signal variations. One method of achieving this compensation is by adding a seismic mass and a separate ‘compensation crystal’ of reverse polarity to the sensor. These components are scaled to cancel out exactly the inertial effect of the masses (the end piece and diaphragm that make up the sensor body) which act on the pressure-sensing crystal stack when accelerated.
Pressure sensors are a key tool in engine development, and manufacturers of such devices recognise that motorsport applications put demands on their equipment beyond those presented by most OEM engine manufacturers. To this end, most sensor suppliers will produce sensors specifically for the motorsport industry, and can provide advice on the correct specification to choose.
Written by Lawrence Butcher