Design and Analysis Aspects of Medium and Heavy-Duty Engine Crankcase

Abstract

The crankcase is the base part to which all important parts of an engine are assembled inside and on to it. The structure though is apparently stationary, it transmits or receives highly fluctuating loads at high frequency, from the piston, crankshaft and different pumps and gears inside it, and through bolt holes, its walls, ribs and surfaces mating with the other components. It enables the piston movement and wears out over the period of its maintenance life. The subject on crankcase is vast and an attempt is made in this chapter to describe the design and development of this basic part, with sufficient peer references. To construct a crank case of a heavy-duty diesel engine, materials of choice are grey cast (GJL), vermicular (CJL) and ductile irons (GJS). The physical and mechanical characteristics of grey, vermicular and ductile cast irons are tabled for use in various calculations. The strength of the material, basically affected by alloying elements and their effect on phase transformation and material properties is given with reference to the basic iron carbon equilibrium diagram. The resulting material properties specific to application like shock, vibration, fatigue and heat transfer are given. The cylinder liner may be integral with the crankcase or separate depending on the philosophy of design and application. The liners are classified as dry liner and wet liner; the latter can be either having a stop at the top or at the middle. The wear of the liner especially by the high contact pressure of the rings at the top and bottom dead centres is controlled by providing sufficient oil film thickness without much carryover past the piston to the combustion chamber. The surface is carefully honed where the type of honing is a choice after balancing the cost and required performance. The liner thickness is designed by not only considering the strength but also stiffness against cavitation. The design of a liner, in general, ponders over the failure modes like liner fillet cracking, bore distortions, bore polishing and, in case of wet liner, cavitation. The functionalities of the bays like crankcase top deck, between the cylinders, crankcase bottom and main bearing cap, crankcase front end and crankcase rear end are taken care of while designing the crankcase. While laying out the top deck, the following important parameters are studied: gasket sealing, crankcase top stiffness, cylinder head bolts and cone of compression, deck cooling by CFD analysis, brinelling and indentation, fretting, oil-hole management, and bore distortion. Similarly, while designing bays between the cylinders, the parameters to be considered are coolant heat transfer, flow velocity, cavitation, crankcase ventilation, piston secondary motion and NVH. The important parameters while planning the bottom and main bearing caps are the assembly aspects, strength against firing and inertia loads, high cycle fatigue of crankcase and main bearing caps, main and side bolt fatigue, fretting at crankcase at the bearing cap interface and NVH aspects. The front end is constructed by considering the mounting components, and NVH by carrying out modal analysis; the bearing crush, radial loads, sealing methods, main and side bolt placement are some of the important aspects borne in mind. On the flywheel end of the crankcase, locating the flywheel housing, oil seal housing design as well as NVH are the aspects to take care. The design is validated experimentally at the hydro-pulsation rig. Finally, the success of the design is very dependent on the quality of production.