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Glow plug with pressure sensor

I'm sure you've seen those odd looking spark-plugs that have more than one connection for the high-tension cable. They are like that so that during the combustion, something can be found out about the situation in the combustion chamber. Particularly now, to fulfil the newest exhaust gas regulations, the age-old saying is still valid: Whatever can be avoided directly during the combustion, does not have to be detoxified afterwards. The raw-emission, i.e., that which is measured directly when leaving the combustion chamber, should already have a particular quality.

Now, what does this have to do with the Diesel engine? Two things. Firstly, up to now, without these special plugs, one was not able to extract the real-time-values from the combustion chamber, and secondly, it seems that the hurdles placed by the future exhaust gas legislation are going to be very high and will make things more difficult. Therefore, the system of measuring the pressure through the glow plug, first developed by Beru, is paticularly relevant.

The glow-plug-sensor technology can be explained relatively quickly. A specific feature is apparently, the movable heating element, which transfers the momentary pressure in the combustion chamber to a steel membrane, whose movement is evaluated by heat-insulated electronics.

High current (heating rod)
Power supply (plus)
Power supply (minus)
Signal cable

In the past, one had to evaluate the acceleration of the crankshaft during the combustion stroke to find out indirectly, what processes were taking place in the combustion chamber. If one cylinder was especially sluggish, it could have possibly been getting too little fuel, the compression may also have been at fault. If one then allowed more fuel to be injected, the situation became worse, particularly as far as the exhaust gas was concerned.

Indeed, whatever is good for the exhaust side, quite often improves the characteristics of the Diesel engine, e.g., the noise development, the performance and, last but not least, the fuel consumption. Practically, every injected drop of Diesel fuel is kept tabs on, to find out just what it has done to influence the overall performance. Perhaps the error-memory will also be given a few tips as far as impending engine wear is concerned, also not bad for the environment.

One now has direct access to to procedures in the combustion chamber. It is now known, e.g., whether the problem is pressure or fuel. Basically, the system can already be working, before the first piston has moved, it can react to, and avoid, miniscule amounts of unnecessary fuel and it can react to varying qualities of fuel. It may even be possible, according to Euro-6, that the expenses of complicated DeNOX systems for the Diesel engine, could be avoided.

It is interesting to have a look at the technical data provided by Beru. It states that the working temperature (probably the electronics) must lie between -40°C and 140°C, thereby , directly next to it, temperatures of more than 1000°C are reached inside of 60 seconds. They also probably didn't consider that the peak pressures in modern Diesel engines can reach a max. of 210 bar.

The output signal is described as 'ratio-metric', this probably means: We are able to master, to a certain degree, the influence on the measuring results caused by the fluctuating power supply. At least the reason for this fluctuation is found in the shape of the heating bar directly next door. Presumably, in this case, there are a few more technical problems than with just spark plugs and sensors … 12/15