In the previous production of internal combustion engines, there is a requirement for cylinder bores with 1oher cylinder form and surface quality. However, the machining macro shape is not preserved in this way. In the operating condition, complex distortion mechanisms occur that deform the cylinder bore. Gehring Technologies therefore developed a honing process that does not aim to achieve a cylindrical bore shape, but instead already provides a deformation in terms of production technology, so that cylindrical bore shell surfaces are produced under certain operating conditions.
Authors
Dipl.-Ing. (FH) Gerhard Flores is Head of Process Development & Patents at Gehring Technologies GmbH in Ostfildern.
Dipl.-Ing. Andreas Wiens is an employee in the Drives, E-drive department at the Ulm Research Center of Daimler AG.
Dr.-Ing. Michael Lahres is team leader of functional surfaces and machining processes at the Ulm Research Center of Daimler AG.
Dr.-Ing. Hans-Werner Hoffmeister is a member of the Institute Management at the Institute for Machine Tools and Manufacturing Technology at the Technical University of Braunschweig.
Motivation
Currently, the most important target in the development of internal combustion engines for motor vehicles is the reduction of fuel consumption and pollutant emissions. The current focus is on the CO 2 value [g/km]. The reduction of internal engine friction is considered a contribution to the targeted limit values. The optimization of the tribological system between the cylinder bore and the piston ring is particularly promising.
In the previous honing process, cylinder barrels are produced with the smallest possible deviations of a few μm from the ideal cylinder shape. However, these high accuracies are not available to the fired engine, since different deformations act on the cylinder bore and thus exert unfavorable influences on the engine function. In addition to manufacturing deviations, deformation of the cylinder barrel can occur due to assembly forces on the cylinder head and ancillary units. In addition, thermal and dynamic distortions occur which – depending on the motor design – can add up to local form deviations of around 40 to 70 μm. Due to the limited mold filling capacity of the piston rings, there is only an incomplete geometrical approximation to the deformations of the cylinder, so that the unfavorable influence due to blow-by and oil consumption on the function of the engine can become effective.
In order to meet the associated requirement for functionally cylindrical bores, a manufacturing distortion compensation system was developed. This means that distortions are retained by the machining process, which then deform into a largely cylindrical bore after assembly and at a certain operating point of the fired engine. 1 shows the solution approach for compensating cylinder distortions.
Principle of formhoning
In order to define the target contour (nominal shape) to be machined to compensate for the function-related cylinder distortions, an experimental method is used on the one hand, in which the engine block is cylindrically hot-honed under spectacle bracing [1]. Even if the temperature distribution in this process does not correspond to the condition of the fired engine, the result after disassembly of the goggles and cooling of the engine block is a shape that leads to smaller cylinder distortions under functional conditions than after the conventional manufacturing process. Another method is the simulation calculation, which can take into account the cold static, thermal and dynamic influences. The warpage shape obtained in this process is to be inverted for the mold honing process so that the desired nominal shape can be produced as a lead contour. This target shape is defined by R/φ coordinates [2], which describe the free form to be honed sufficiently precisely on eight horizontal section planes with
256 points each.
The aim of formhoning (2) is now to implement this nominal shape in the production process in such a way that any free-form shapes can be produced with harmonic eighth-order shape proportions. In addition to this high-resolution nominal shape that meets the requirements, there is also a demand for a uniform design of the roughness profile in all areas of the shape-honed bore [3]. Thus, the profile formations should be producible with tolerances in all shape-critical features such as neckdowns, embossments and transitions of the free form. This applies both to conventional peak honing and to the complex profile definitions of plateau honing.
Process components
In order to be able to implement formhoning in terms of production technology,3 a large number of innovations are required for controlling the working surfaces of the diamond clay bars. For this purpose, a mold concept was developed that features four honing stones that can be fed independently of each other. The timing of the radial infeed, infeed force and infeed path can be controlled according to the desired shape, so that the working surfaces of the honing tools follow the desired target shape with their kinematics and
thus cut any free form into a cylindrical bore.
Since the four honing stones can only have a limited overall length due to the short-wavelength axial shape characteristics, care must be taken to ensure that a constant contact time of the honing stone working surface is achieved at all points on the shell surface during the shape honing process over the entire bore surface, irrespective of the local shape characteristics. This is a function of the process parameters, including honing time, honing bar length, stroke speed, stroke acceleration, rotational speed, number of honing bars, and bore dimensions.
In addition, in order to get within the range of the usual machining times of < 30 s for cylinder bores in passenger car engines, the cutting speeds are required that lead to the desired self-sharpening in the case of metal-bonded diamond honing stones. At a speed of 400/min, for example, it is possible to produce fifth-order molded parts with a radial feature of 60 μm at infeed frequencies of up to 32 Hz. Higher orders are also possible, since the amplitudes are usually smaller. This independent feed characteristic of the individual cutting bars in terms of dynamics, force and travel is only possible with the use of powerful piezoelectric actuators.
Fig. 4 shows the piezoelectric infeed system with the rapid feed, the piezo stages and the mold honing tool. With the rapid infeed, the entire piezo infeed train, the infeed rods and the honing bars are first moved until the working surfaces are placed against the bore wall. From this position, the piezo actuators are activated and fed in a locally differentiated manner according to the desired nominal shape. In order to be able to control the honing stones according to the nominal shape, the radial nominal values are stored in a feed matrix (5). For each shape coordinate, a voltage is stored with which the piezo actuators are deflected at the desired position. The stress values are assigned to the nominal shape in individual R/φ coordinates, but also take into account the local transfer behavior in the mold/workpiece system. All honing stones of the form honing tool are fed independently of each other according to this feed matrix, depending on the current location on the bore surface
. If – starting from a cylindrical shape – the non-cylindrical free form is cut into the bore, the final roughness profile must be machined in a further operation. For this purpose, a multi-part honing tool was developed whose individual support strips are designed with individual spring-loaded honing strip segments [4]. These segments are also reduced in length to follow the local shape expressions. It is worth mentioning that the spring constant of the elastic bearing has been designed to be low enough to ensure that a constant radial force keeps the segment strips in contact with the bore wall in the usual shape configurations.
This smoothing operation can be carried out in one operation for peak honing or in two operations for plateau honing in a double-feed die (6) . Both diamond honing stones and ceramic honing stones can be used for the smoothing operation. The entire process chain thus consists of cylindrical pre-honing, then – depending on the depth of the shape – one or two shape honing operations, and finally a subsequent smoothing operation. In order to make the process controllable from a manufacturing point of view, post-process mold shape measurement is necessary. This is done using a pneumatically operating nozzle measuring mandrel (Stotz principle), which measures the bore with a measuring range of 150 μm in 15 s and a repeatability of ≤ 2 μm. This measuring method is insensitive to vibration and dirt influences and can be used in the machining area next to the honing spindles. This determines both the bore shape and the diametral dimension, which is defined as the piston joint dimension of the bore. Feedback controls can be used to stabilize the diametral dimension and nominal shape in accordance with tolerances.
Promising results
The formhoning developed so far delivers free-form shapes that can be reproduced within narrow limits and deviate from the nominal shape by a maximum of ± 3 μm 7 . Statistical tolerance restrictions are not included in this. The total honing allowance during formhoning, starting from the cylindrically pre-honed bore, is at least 0.02 mm in diameter plus the radial forming depth, depending on the shape. Shape tolerancing is achieved by arranging envelope curves at intervals of the desired tolerance around the nominal shape specification. In addition, the amplitudes of the respective order must also be tolerated. By subtracting the actual shape from the nominal shape, the influences of higher orders can be assessed as a detrimental quantity on the function. The processing times for formhoning are around 50 s for commercial vehicle engines and < 30 s for passenger car engines. Piezoelectric infeed has so far proven to be a reliable and accurate technology. In addition to the shape, topographies with plateau structures could also be reliably achieved on the entire shape-honed shell surface. 8 shows such a plateau profile with the measurement points located both in the shape expressions (+15 μm) and in the shape constrictions (-30 μm). The desired tolerances of the surface dimensions Rpk, Rk and Rvk could be achieved to the full extent. Thus, the surface finish is comparable to the conventional honing process of the series.
Outlook
With the developed state of the art of the mold honing process, freeforms with more than 50 μm local shape deviation are also representable. The surface finish and cycle times correspond to the conventional honing process. Functional advantages on the fired engine were demonstrated on the basis of oil consumption and emissions. Since this is a modification of the entire tribological system, the piston ring assemblies must also be adapted to the form-honed raceways in terms of contact pressure and geometric shape. In terms of manufacturing technology, formhoning enables substitution of the honing technique and thus delivers not only functional advantages, but also manufacturing advantages with cost reduction. The current production quantities of molded honing stones are suitable for small-scale production, but they need to be further optimized. This makes formhoning an innovative honing process that makes a significant contribution to increasing the efficiency and reducing the emissions of internal combustion engines. Based on the available findings, the prerequisites for series machining of the crankcases investigated have thus been created.
References
[1] Wiens, A.; Lahres, M.; Hoffmeister, H.-W.; F lores, G.: Production engineering approach to compensate cylinder distortions by formhoning. VDI Reports No. 2109, 2010, pp. 133-145, VDI-Wissensforum GmbH, Düsseldorf.
[2] Flores, G.; Klink, U.; Abeln, T.: Honing of functional shapes in cylinder crankcases. VDI Reports No. 1994, 2008, pp. 79-89, VDI-Wissensforum GmbH, Düsseldorf.
[3] Wiens, A.; Flores, G.; Klink, U.; Abeln, T.: Macroform and Microform Machining of Cylinder Bores by Freeform Honing. Jahrbuch Schleifen, Honen, Läppen und Polieren, Verfahren und Maschinen, 63rd edition, 2007, pp. 329-339, Vulkan-Verlag GmbH, Essen.
[4] Wiens, A.; Lahres, M.; Hoffmeister, H.-W.; F lores, G.: Formhoning of cylinder tracks in crankcases using a piezoelectric formhoning tool. Jahrbuch Schleifen, Honen, Läppen und Polieren, Verfahren und Maschinen, 64th edition, 2009, pp. 265-280, Vulkan-Verlag GmbH, Essen.
