Metal working – Method of mechanical manufacture – Prime mover or fluid pump making
Reexamination Certificate
2002-08-15
2004-01-27
Cuda-Rosenbaum, I (Department: 3726)
Metal working
Method of mechanical manufacture
Prime mover or fluid pump making
C029S888040, C164S004100, C164S457000
Reexamination Certificate
active
06681485
ABSTRACT:
BACKGROUND OF THE INVENTION
The invention concerns a process for producing pistons for a reciprocating internal combustion engine, especially an Otto engine with direct injection and especially a motor vehicle engine, in which e piston is produced by a casting process.
SUMMARY OF THE INVENTION
In the manufacture of reciprocating internal combustion engines, variations in pistons produced by casting lead to corresponding deviations in the desired piston displacement and the compression ratio &egr;. For example, for &egr;=11, tolerances of 0.5 are normal. In the case of pistons with a smooth surface of the face of the piston that faces the combustion chamber, tolerances from the casting process can be compensated by mechanical finishing. However, this is no longer possible in the case of pistons with a fissured surface, which encompass, for example, depressions and elevations for suitably influencing an injection process in Otto engines with direct injection. In pistons of this type, a significant portion of the surface of the piston that faces the combustion chamber remains as a cast surface. Therefore, the volume in the cylinder that is determined by the surface of the piston facing the combustion chamber is subject to large variations corresponding to the large tolerances in the casting process. However, this has an unfavorable effect on engine function. For example, exhaust gas temperature, pollutant emission, and fuel consumption may rise.
SUMMARY OF THE INVENTION
Therefore, the goal of the present invention is to develop a process of the type referred to above, in which the cited disadvantages are overcome, and the tolerances for piston displacement and compression ratio are reduced to the smallest possible values.
To this end, the invention provides that, at least for the first piston of a casting process batch, the volume V
actual
formed by the surface of the piston that faces the combustion chamber is determined, that this volume V
actual
is compared with the desired nominal volume V
nominal
, and that, based on this comparison, the compression height K
H
is fixed in such a way that its deviation from a set point ensures that, subsequently, V
actual
=V
nominal
within tolerance limits.
This has the advantage of achieving very small variation with respect to the compression ratio, since, for each batch, the piston pin bore is formed accordingly, i.e., at a different place in the piston, so that any deviations that may be present after the casting process due to variations in the volume of the piston defined by the combustion-chamber face are compensated with the compression height.
After a batch of pistons has been cast, it is advantageous to form the piston pin bore in all of the pistons at a predetermined distance K
H
from the surface of the pistons. This is done by machining the pistons, e.g., by milling.
In a preferred embodiment, the volume is determined by immersing the piston with its combustion-chamber face into a liquid cushion and measuring the displacement, which is then used to calculate the volume of the combustion-chamber face.
The pistons produced in the casting process have a fissured surface or depressions and elevations of the face of the piston that faces the combustion chamber. Therefore, a significant portion of the surface of the piston that faces the combustion chamber cannot be suitably mechanically finished, so this surface remains a rough, unfinished cast surface.
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Cuda-Rosenbaum I
Ostrolenk Faber Gerb & Soffen, LLP
Volkswagen AG
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