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 Procedure
kfz-tech.de/PDM16
It's entirely justified that the 'M' and 'HM' ('High-Performance Center Sphere') direct injection systems were highly praised by the engineers of the time, but the pre-chamber, representing indirect injection,
clearly held the dominant market position. Why, when it can't score points in terms of fuel consumption and performance?
We've already mentioned what a misbehaving child the diesel engine of that era was, loud and uneducated. This is due to its self-ignition, where, due to the high temperatures, the fuel often begins to burn
aleady at the edges during injection. However, this requires a very finely atomized nozzle or a device that allows the fuel to burn in layers.
The former is what we see today with common rail, where even raindrops are highly dispersed. If this isn't the case, combustion is delayed for a moment because oxygen doesn't reach all parts of the
concentrate quickly enough, resulting in what's known as ignition delay. The best example of this is the knocking of a diesel engine after a cold start, which used to be much more severe than it is today.
kfz-tech.de/PDM17
So what do you do? You create a secondary combustion chamber and absorb the initial combustion force there. The pressure then flows through holes into the main combustion chamber. It's important to
remember that Daimler-Benz also chose the passenger car as an object of desire for the diesel engine early on. And after all, it hasn't only made it into coupes and convertibles.
1936 Mercedes 260 D, 33 kW (45 hp) at 3000 rpm
kfz-tech.de/PDM18
However, the first engine from 1936 was miles away from this. A beast of a machine, one that could only be expected of the taxi industry due to its reduced wear during cold starts and its improved fuel
consumption. With such an aiml in mind for a diesel engine for the luxury class, one inevitably has to flirt with the pre-chamber, even if this process hinders rapid combustion and thus the joy of performance.
Oil Motor 138 2.545 cm3 (90 mm * 100 mm), 20 : 1, R4, OHV, 33 kW (45 hp) 3000 rpm, 4-speed, 1./2. unsynchronized, approx. 95 km/h, 1936-40, approx.
2.000.
kfz-tech.de/PDM19
The swirl chamber, discovered primarily by VW starting in 1976, is better at this. Here, as with direct injection, swirl comes into play to an even greater extent. During the compression stroke, air enters the
chamber. Since the connecting duct from the main combustion chamber opens tangentially, it is set into a rotating motion, justifying the name 'swirl chamber'.
This is also where the injection nozzle and the glow plug end. The nozzle has always had this name, even though it is strictly speaking a pressure-controlled valve. The difference from the nozzle of a direct
injection engine is important: it is a pintle nozzle, or more precisely, a throttle pintle nozzle. It is a jet that fills the space of the swirl chamber, but with an attempt to distribute it fairly evenly over the injection
time.
Below you can see the pintle nozzle of the direct injection engine. Here, the nozzle needle doesn't extend all the way to the outside, but ends in a small extension of the nozzle containing tiny blind holes of
perhaps 0.1 to 0.2 mm in diameter. The nozzle opening pressure is also higher, at approximately 175 bar compared to approximately 130 bar for the pintle nozzle. The pintle nozzle therefore creates a sharp jet
that hits the wall of a more or less confined space in the piston, where it burns off in layers, if possible.
The image below, of course, doesn't show a piston from the vehicles discussed here, but rather from a smaller diesel locomotive. However, the image does show one thing very clearly: the effect of perforated
nozzles, in this case with six spray nozzles.
kfz-tech.de/PDM20
Which brings us back to the M-process. It goes back to Siegfried Meurer and is based on the sphere he invented, located in the center of the piston and open at the top. Single- or two-hole nozzles spray
tangentially onto the piston wall, which, combined with strong air movement, causes the fuel to burn in layers.
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