Crankshaft Assembly Design, Mechanics and Loading
Together with both the piston pins and the crankshaft’s crank pins, the connecting rod in reciprocating piston engines converts oscillating piston motion into rotary crankshaft motion. Running smoothness is a universally important criterion for the design
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Crankshaft Assembly Design, Mechanics and Loading Eduard K¨ohler, Eckhart Schopf, and Uwe Mohr
8.1
Designs and Mechanical Properties of Crankshaft Assemblies
8.1.1
Function and Requirements of Crankshaft Assemblies
Together with both the piston pins and the crankshaft’s crank pins, the connecting rod in reciprocating piston engines converts oscillating piston motion into rotary crankshaft motion. Running smoothness is a universally important criterion for the design of crankshaft assemblies. High speed has priority in gasoline engines, thus making a minimum of moving masses an absolute imperative. The emphasis shifts somewhat for diesel engines. Firing pressures can be twice as high as in gasoline engines and continue to increase as the size increases. Thus, controlling the effects of the gas force is the primary challenge. In conjunction with direct injection and one or two-stage exhaust gas turbocharging and intercooling, car diesel engines now attain the volumetric power outputs of gasoline engines. Moreover, cuts in fuel consumption (reduction of CO2 emission), strict emission laws and lightweight and increasingly compact designs that do not sacrifice reliability are currently driving engine development. However, in principle, steadily increasing ignition and injection pressures result in ‘‘harsher’’ combustion. This inevitably creates more problems with acoustics and vibrations as demands for comfort increase. Now common in diesel cars, multiple injections, optimized vibration damping, camshaft drives shifted to the flywheel-side, dual-mass flywheels and partial encapsulations serve to improve modern diesel engines’ acoustic and vibration performance. Not least, the steadily increasing amplitudes of the gas torque characteristic make using the engine mounting and the entire power train to control crankshaft assembly vibrations, improve mass balancing and reduce the excitation of vibrations even more important. In Europe, the diesel engine has evolved from merely being the primary
commercial vehicle engine to also being a frequently used car engine. A modern diesel engine’s tremendous mechanical stress on the crankshaft assembly also has to be accommodated. Crankshaft assembly components require a structural design optimized for structural strength, stiffness and mass. Knowledge of locally present fatigue limits of materials that affect components has not quite kept pace with the simulation of loading conditions, which has become quite precise in the meantime. This reveals a potential weak point in fatigue strength simulations in the limit range. Thus, even more precise measurement of the practical impact of technological influences and quality fluctuations in manufacturing will be essential in the future.
8.1.2
Crankshaft Assembly Forces
The literature includes numerous studies devoted to the crankshaft assembly of a reciprocating piston engine (c.f. [8-1, 8-2]). Varying with the crank angle j, the piston force FK(j)acts on the crankshaft assembly on the piston side. According to Fig. 8-1, this ensues
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