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  Compressor 2

How can you compress air? This depends, for example, on the pressure and the flow rate. In our case, the former is high and the latter is low. Flow pumps are elegant and certainly well suited for interior ventilation and charging the engine. But not good enough for the pressures required here.

Anyone who has ever carried out a compression test on the engine knows about the pressures that arise even with a petrol engine. We have the solution, a piston compressor. Is it comparable to an internal combustion engine despite the lack of engine control? Sure, because it also gets hot and has to be cooled with air, coolant and/or oil. It also has pressure circulation lubrication, but no pressure generation of its own. The it takes from the lubrication system of the truck engine.

It is very practical that it is not dependent on the four-stroke principle. As a two-cylinder, like a two-stroke engine, it can wonderfully balance its masses. In addition, the cubic capacity is kept small. 100 to 300 cm³ are enough for one cylinder, 400 to 700 cm³ for two. Three cylinders, as shown in the 'History' chapter, were only available at the railway for a long time.

People used to be proud if you could still help yourself with your own cabin (narrow garage). The demands on the paintwork weren't that high, and in some cases quite respectable results were achieved with wet floors. The pressure generation was the responsibility of a compressor and that was sometimes the problem.

If it was bought too cheaply, i.e. too old, then it mixed the finest units of its oil into the paint-air mixture. If it was too cheap because of just one cylinder, the electric motor was overwhelmed when it restarted, e.g. with 6 bar back pressure. One with two cylinders was much better, but expensive. And of course this compressor had its own splash lubrication, incidentally with a small oil dipstick.

The piston compressor delivers quite unevenly and has to fight against the pressure that is still present in the middle of the stroke with enormous torque. A second piston in the opposite direction of movement compensates for this a little. The problem is better solved by the evenly working screw compressor shown in the previous chapter.

In addition, there is a recent trend towards compressors with only one cylinder. No, it doesn't have a particularly large swept volume, so it delivers less than the one with two cylinders. Whether perhaps the air consumption has decreased due to the electronics, we have to leave this question open here.

The compressor for the air brake should not be confused with the one for the air conditioning system. The difference is particularly clear in the case of touring coaches, although both of course draw torque from the combustion engine at about the same point. Air conditioning compressors can have up to six cylinders, e.g. in a V shape.

One cylinder, air-cooled with connection to the engine oil

Let's stay with the two-cylinder for a moment, as shown at the top. That one is liquid-cooled, except in agriculture today, except in agriculture much more common today. Of course, it is also available air-cooled (picture above). Its cylinder head accommodates fluttering non-return valves, which differ from each other in terms of their installation direction, inlet and outlet. As you can see, you will look in vain for a center bearing of the crankshaft.

No, the drive does not always take place via a belt drive. A renowned company like MAN is proud to only have gear-driven compressors in its program. Nevertheless, the compressor is no longer necessarily connected to the drive, but can be decoupled using a clutch.

Gone are the days when you used a lot of energy to generate compressed air and then let it go when you weren't using it. Stand next to an older truck at a boring traffic light and you'll clearly hear it hiss from time to time. This kind of energy wastage is hopefully a thing of the past, except perhaps in the case of air dryer regeneration.

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