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METAL-FORMING TECHNOLOGY Lightweighting of IC engines

Jan 23, 2020

Lightweighting of IC engines is another means of reducing fuel consumption and emissions. Attempts are being made to make the entire IC engine out of sheet metal with a view to making it lighter. This article discusses the technology-based alternatives to reduce emissions. - P P Date, Professor, Dept of Mechanical Engg, IIT-Bombay, ppdate@iitb.ac.in

Use of alternate fuels like CNG, light-weight construction of the vehicle bodies and structures, are among the major steps towards reducing emissions. Lightweighting of IC engines is another means of reducing fuel consumption and emissions. Captive research across the world aims at lighter engines. In the ensuing era of electric vehicles, in the Indian context, the adoption of total electric mobility will have to pass through a long transition phase. During this phase, hybrid vehicles would have to be used. A hybrid vehicle running on electric power would have to carry the IC engine around and therefore, the IC engine better be light. This will enable the hybrid electric vehicle to run farther for a single charge. In addition, a light-weight engine will be inherently fuel-efficient. As of now, batteries are the heaviest components of an electric vehicle.

Attempts are being made to make the entire IC engine out of sheet metal with a view to making it lighter. A light weight engine leads to spinoffs as all downstream components like the engine cradle etc can be made lighter. The advantage of replacing cast and forged parts by sheet metal parts is that the environment–unfriendly manufacturing routes would be dispensed with. Moreover, parts like piston and connecting rod contribute to reciprocating masses, which in turn influence the ‘piston slap’ and hence, the engine noise. With lighter reciprocating components, the engine would be much quieter in addition to being more fuel-efficient and environment-friendly. With this in mind, the article presents manufacturing of light-weight piston and light-weight sheet metal gears integral with the shaft. The author has been working on other parts (like the cylinder block, connecting rod, crankshaft, etc, as well), some of which were well received by the international audience.

The different shapes of an automotive piston made by different processes are shown in Figure 1(a). It is understood that features related to combustion will differ from one OEM to the other. The features of the piston considered for this work are shown in Figure 1(b) and the values assigned to each feature are given in Table 1.

Manufacture of the piston from sheet metal

This section describes the manufacture of a piston of 50 mm diameter using 6000 series aluminium sheet metal. The service parameters used were peak pressure of 60 bar, piston diameter of 50 mm, brake power of 5.5 kW at 8000 rpm and bore to stroke ratio equal to one. The aluminium sheet having tensile strength 275 MPa, a thermal conductivity of 152 W/m-K and coefficient of thermal expansion is 25 μm/m-°C in the temperature range of 20.0 - 300 °C was used for the calculations.

Initially, the service conditions of the existing piston were used as benchmarks to calculate stresses at the service temperatures as performance criteria for the sheet metal piston. The piston was re-designed for manufacture using sheet metal. Drawing, redrawing, ironing, punching and hole flanging were the major operations to be performed. The sequence of operations leading to the final sheet metal piston is shown in Figure 2.

The as-drawn cups after first and second draw are seen to be practically free from earing and wrinkling as observed in Figure 2. The corner radius of the piston was achieved during bottoming as required for engine application. The flanged hole for the piston pin was achieved without a circumferential crack (or any other defect) over the circumference of the flanged hole over the bearing length. Using the processing route, circularity within a tolerance close to 13 microns was obtained. The dimensions of the piston were within a 50 micron tolerance limit.

Process simulations were carried out using the AUTOFORM R7 software to design the drawing tools. Specially designed tools were manufactured to deform the sheet metal blank into drawn and ironed cups using the 'Electropneumatics' hydraulic press. Further, a tool was designed and manufactured to make a pair of diametrically opposite flanged holes on the cylindrical surface. This tool had the facility of locating the drawn cup and making the flanged hole consistently at a fixed distance from the crown of the piston.

Distribution of thickness on the wall of the cup from the simulation agreed well with that obtained from the experiment. Finally, approximately 24% of weight reduction was achieved, compared to the existing design.

Manufacture of gear integral with the shaft

Lightweight transmission components enable reduction in inertia forces. This influences favourably the starting torque and that needed for stopping the vehicle. Making the gears integral with the shaft enables precise positioning of the gears on a single shaft. Flow lines are favourably oriented and work hardening during forming adds to the in-service strength of the gears, besides eliminating the need to assemble the gears on the shaft using keys.

It is understood that the gears and shafts made from sheet metal will be hollow and will therefore, not have the same torque transmitting capability as that of the solid gears and shafts. However, the specific torque transmission capability using hollow shaft and gears is higher than that of the solid gears and shafts.

The models of the shaft and the gear integral with the shaft, and the model of the die (made in two halves) to make the gear using the hydroforming process are shown in Figure 3 (a-c). The entire process was designed based on simulations using AutoForm Hydro 2016 software. It was a two-step procedure (HS1 and HS2 in Table 2), wherein the first step pertained to forming the teeth partly followed by inter-stage annealing and complete formation of the teeth. The deformation of various features was monitored using the Strain Non-uniformity Index (SNI). The maximum SNI was found to be lower than the critical SNI when inter-stage annealing was performed.

It was found that the thickness of the gear obtained by hydroforming process was greater than or at least equal to the initial wall thickness of the tube. This was by virtue of compression of the tube during forming (axial feeding) and adequate lubrication to enable smooth flow of the material into the shape of the gear teeth.

The hollow gear teeth would have good torque transmission capability, which can be enhanced by filling a (virtually) incompressible medium like oil into it, which would get pressurised in service and enhance torque transmitting capability, especially the specific torque transmitting capacity.

Such a gear can be used in the transmission of force and motion for low and constant torque requirement. For instance, in case of hybrid cars, the torque needed to run the generator that would charge the batteries would be independent of the terrain, and hence, such a gear transmission might be useful. Tooth profile change durig torque transmission can be avoided using a pressurised incompressible medium filled into the tube (and sealed to prevent leakage), and tooth deflection reduced using adequate wall thickness of the tube.

Summary

Some innovative concepts and thoughts on using sheet metal for lightweighting of IC engine components have been presented in this article. There are various merits of lightweighting IC engines (especially the reciprocating masses) and usage of sheet metal is among the viable means of doing so.

Image Gallery

  • Figure 1(a): Features of pistons - Possible manufacturing processes and piston shapes

  • Figure 1(b): Features of pistons - Design features used for the present design

  • Table 1: Design values assigned to each design parameter / design feature

  • Figure 2: The complete processing route starting with the sheet metal blank to the final piston

  • Figure 3(a): Scheme for hydroforming hollow gears integral with the shaft - Hollow shaft

  • Figure 3(b): Scheme for hydroforming hollow gears integral with the shaft - Tool assembled

  • Figure 3(c): Scheme for hydroforming hollow gears integral with the shaft - Tool with the tube inside for hydroforming

  • Table 2: The SNI encountered in the two steps

  • P P Date

    Professor, Dept of Mechanical Engg

    IIT-Bombay

    ppdate@iitb.ac.in

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