The most common characteristic that the combustion engine and the steam engine have is the crank
mechanism. It forms the heart of both the two- and the four stroke engine,
and the most important part of this mechanism is, in fact, the piston. This is where the drive comes from. The rest is simply there to convert the back and forth movement into a rotary movement. This can also be done
using a different mechanism, however, the con-rod and the crankshaft have asserted themselves on the broadest front. The piston in the combustion engine has however, evolved a long way from the one in the steam
With only power- and heat transmission, the tasks of the piston are inadequately described. The time factor plays an important role, because pressure and heat are not developed gradually, sometimes the stress on
the piston is quite abrupt. On the other side, there is the power transfer to the drive-wheels which is marked by large
amounts of inertia force, which can not pass on the
abrupt changes in stress. At least, if there is a jerkiness in the drive train, the friction-clutch or the torque converter do provide a certain amount
of protection from over-straining.
This doesn't help the piston much. It has to transfer almost the entire combustion pressure. The problem is, that due to the crank mechanism, lateral forces also occur, with a strength of up to 10% of the gas forces.
On top of this, in the course of the four strokes, the piston changes sides at relatively irregular intervals, partly under very high pressure, sometimes, in the attempt to overcome this, an offsetting is used. In large engines a crosshead or outrigger, is used to guide the forces in
the direction of the crankshaft center.
The so-called blow-by-gases are those which, instead of pressing the piston down, escape between the piston and the sleeve, and between the sealing elements, into the crankcase. Of course they don't stay there,
on their way back to the combustion chamber they sometimes cause considerable problems. The task of sealing is given to the
piston-rings, which can, partly due to their totally
different material composition, fulfil this task substantially better than the AlSi alloy used for the pistons.
At this point we would like to speak about a critical factor, the gliding- and sealing track of the piston, the cylinder-sleeve surface. Then, also the long-term strain is high. Just for the calculation, we'll assume a mileage
of 100.000 kms driven in mixed traffic conditions amounting to 1500 hours of operation, we'll also assume a very low average of 2000 RPM, the piston speeds travel through the length of the stroke 360 million times!
It's not surprising that a lot of experimenting and altering is still being done as regards the cylinder-sleeve/piston-surface.
In the meantime, now that it is not the wear and tear, but the friction reduction which is paramount. Have a look at the smaller contact surfaces of the modern piston and their coatings. Even the running in period has
been shortened by giving the sleeve surface a limited roughness. Completely without this it probably would not work because too little lubrication would be present. The oil-film is strongly hydrodynamic anyhow, which
causes a problem at the dead-center points.
We'll only go into the inlet- and outlet ports of the two stroke engine, when this engine is reborn as a four wheel drive unit, possibly then, also with valve control. Indeed, the piston has an large influence on the mixture
formation, or to be more precise, on the fuel/air guidance. This is made obvious by the abundance of piston-crown shapes used in the direct injection petrol engine, whereby, one tends to forget, that the piston in a
Diesel direct injection engine, has always taken up a large amount of space in the combustion chamber and because of it's shape, has a great influence on the proceedings, last but not least, for the heat dissipation.
We'll have to, at a later date, come back to the amount of effort put in, e.g., with the help of the lubrication system, to keep the clearly increased stress factor of the piston temperature, within tolerable limits. In this case
one speaks of peaks far higher than 2000°C, which probably occur only locally, are inert and are quite quickly dissipated, otherwise they would not be compatible with the melting temperatures of the piston material,
which with aluminium, lies at 660°C and in Si-alloys is even lower.
Another reason for this material mixture is, that not only during full strain do tons of pressure work on the piston. Indeed, the amount of AlSi through casting and possibly forging, has become less. Not only because
the volume of the piston-shaft has been reduced but because a number of small aids have been cast in or have been shrunken on. With the event of the aluminium engine blocks, the somewhat older strip-inserts
have dissapeared. Then possibly, there are also the ring carriers, some even cooled, and also the usual bushings which hold the gudgeon pin. 07/11