Crank mechanism 3
Internal combustion is the real game-changer that distinguishes the internal combustion engine from the steam engine. Everything happens within the same chamber. The residual heat from the previous cycle is used for
the next one. If the materials could withstand the conditions, cooling would be unnecessary, a significant advantage in terms of efficiency. Cooling and density are the two main drawbacks of the crank mechanism.
The effects of both phenomena can be observed quite clearly at the top of the piston—from the fire ring, the distance between the top edge and the first piston ring, to the entire ring zone. The various tasks here
are quite difficult. The piston itself does not come into contact with the cylinder wall in this area.
So a large portion of the heat is transferred from the piston crown to the cylinder wall via the top piston ring. A wide contact surface would be advantageous for this. But that causes more unwanted friction. A crowned
surface would, in turn, be better for this purpose, as well as for sealing the combustion chamber towards the crankcase. Oil isn't just taken off the cylinder wall by the oil scraper ring and transported to the crankcase.
That's a real problem caused by conflicting requirements. As the piston moves toward TDC, it should encounter oil to lubricate its sliding surfaces. At the same time, he would have to brush it off on his way to the bottom
dead center so that it doesn't burn during the working cycle and possibly further worsen the exhaust emissions. Furthermore, that would increase the fuel consumption, which is now remarkably low.
So a completely smooth cylinder wall is out of the question. But aren't there chrome-plated cylinders? Probably only in two-stroke engines, which carry their oil supply with them almost all the time. So we roughen up the
cylinder walls after we have just aligned them to the pistons with exceptional dimensional accuracy. Specifically, in such a way that it gradually loop back at an acceptable compromise level during the running-in period.
To further shorten the running-in period, which is typically associated with unfavorable exhaust emissions and fuel consumption, laser technology can be used to create even more precisely defined storage locations for
engine oil. More on this later in this book. The next dilemma is adjusting the spring action of the piston rings. If it's large, the rings seal better, though at the expense of some efficiency. Even at this point, people have
already started chasing after smaller successes.
When aluminum pistons were first introduced into GG cylinders, there were difficulties due to the different coefficients of thermal expansion. In the meantime, this has been resolved by allowing significantly more clearance
in the direction of the piston pin axis. If, as the temperature rose, the piston expanded more than the cylinder, it was forced by a cast-in steel strip to do so in the direction of the greater clearance.
And today? It seems a combination of factors is at work. It's likely that the pistons have a bit more overall clearance when the engine is cold, without causing any rattling. And then, presumably, through the clever distribution
of material, the piston is forced to expand slightly more toward the piston pins. It could be that a certain amount of flexibility in the piston in this direction plays a role here.
And how did they respond to the requirements mentioned in the previous chapter by increasing the pressure in the combustion chamber? Just as differentiated as mentioned above. There is the option of forging the piston
instead of just casting it. At times, such as in the case of trucks, it was built entirely or partially from steel. However, this bolted two-part design (pictured above) is no longer in use, even though steel would, of course, also
offer significantly better sliding and emergency-running properties.
Reinforced aluminum pistons are making a comeback. An integrated ring carrier (image above) has long been an option here. Newer, for instance, are separate bearing bushings for the piston pin. That one, too, may
still be hollow, but it has a greater wall thickness. In addition, there are options for changing the material used for the connecting rod and the piston alloy.
It's unbelievable what you can do with motor oil these days. It has always been possible injection here. But, for example, the precise placement in a circumferential channel in the piston has probably only been done in
more recent times. Please keep in mind that it is in motion. It is also now possible to control combustion more effectively, for example by using multiple fuel injections and even multiple spark plug firings to prevent
pressure spikes.
In any case, today, for example, in court proceedings, when determining the purchase price refund to be paid by VW, 250,000 km is used as the threshold beyond which the car is considered worn out and VW is no longer
required to pay anything. There you can see what a modern crankshaft system is capable of when properly maintained.
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