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(a fictitious doubling of every cylinder’sthrow angle) remains centrosymmetric, then the secondorder free inertial forces also disappear. Longitudinally sym-metrical crankshafts of four-stroke inline engines with aneven number of cylinders are also free of all orders ofmoments of inertia.In a V engine, the forces of each single V cylinder pair act onone double crankshaft throw. A ‘‘connecting rod next to con-necting rod’’ configuration, i.e. two connecting rods that areoffset axially but connected to the same crank pin, has becomecommon. Axial connecting rod offset generates a bendingmoment that is dependent on the gas and inertial forces andtransmitted by the crankshaft to the bearing bulkhead walls.This has given rise to significantly more structurally complexconnecting rod designs such as a forked connecting rod and amaster connecting rod with a connected auxiliary connectingrod. Although they eliminate the drawbacks, they are howeverno longer employed for reasons of cost (Fig. 8-4).As Sect. 8.3.3.2 explains in more detail, the conditions in Vengines appearmore complex. A V crankshaft assembly’s firstorder free inertial forces may be fully balanced by counter-weights in the crankshaft when the following condition ismet:d ¼ p
2av (8-12Þ Along with the V angle aV, d denotes the crank pin offset,more precisely, the angle of connecting rod offset (the angleby which a double throw’s crank pins may be offset, i.e.counterrotated). At 908, this angle would be d =08 for a V8engine, d = 1808 for an opposed cylinder engine (a = 1808)and d = +608 for a 608/V6 engine. The + sign signifies anoffset counter to the crankshaft’s direction of rotation.Whenthe angle of connecting rod offset is large, an intermediateweb between the offset crank pins is essential for reasons ofstrength. This entails an increase of overall length thoughsince cylinder bank offset increases by its width at the least(Fig. 8-5).In conjunction with measures to enhance fatigue strength,an intermediate webmay be dispensed with entirely when thecrank pin offset (angle of connecting rod offset) is small. (Thisis called a split-pin design.) Small crank pin offsets areincreasingly being applied to equalize non-uniform ignitionintervals in modular designs. This is primarily found in V6and V10 engines at a V angle of 908 aligned for a V8 engine.Assuming the engine is a four-stroke engine, the angle ofconnecting rod offset is then calculated with the number ofcylinders z as follows:d ¼ 4pz
av (8-13ÞIt follows that d = +308 for the 908/V6 engine and d = –188 forthe 908/V10 engine. (The signs are to be interpreted asexplained above.) A uniform ignition interval may havemore importance depending on the free torsional forces. Abalance shaft (see Sect. 8.3.6) additionally has to be incorpo-rated to fully balance the first order free moment of inertia.Given their regular, i.e. even, numbers of cylinders, V enginesonly have longitudinally symmetrical crankshafts in excep-tional cases. Then, first order free moments of inertia usuallydo not occur. The crank pin offset in a centrosymmetric firstorder star diagram neither produces a first order free inertialforce
nor a first order free moment of inertia. 8.1.3.5 Characterization of Diesel Engine CrankshaftAssembliesThe following characterization of crankshaft assemblies isbased on the predominant working principle of the four-stroke engine. Single cylinder diesel engines (see Sect. 8.3.3.1on the balancing of masses of a single cylinder crankshaftassembly) are important drives for small power sets. Uncon-ventional solutions with combined measures that improverunning smoothness are also applied in small engines [8-13].Conflicting goals exist for four-stroke I2 engines. At auniform ignition interval of 3608, the first and higher orderfree inertial forces add up unfavorably. On the other hand,free moments of inertia do not occur. A crankshaft with athrow angle of 1808 (corresponding to a two-stroke I2 crank-shaft) reverses the conditions. First order free inertial forcesdo not occur but a first order free moment of inertia does. Inlight of the shorter overall length, a uniform ignition intervalis less critical for the excitation of torsional vibrations. TheI2 diesel engine is insignificant as a car engine. The VW EcoPolo [8-14] with an ignition interval of 3608 formerly repre-sented this design (Fig. 8-6). A laterally mounted balanceshaft negatively (counter-) rotating at crankshaft speed fullybalances the first order inertial force. Single-sided configura-tion generates an additional moment around an engine’s longitudinal axis, which however favorably influences thefree inertial torque.Disregarding four-stroke radial aircraft engines and themore recent VWVI5 car engine [8-15], only an inline engineis fundamentally suited for uneven numbers of cylinders.The aforementioned engine types are designed as gasolineengines. The VR design is less suited for the diesel processbecause of the high loads (piston side thrust and bearingload).The four-stroke I3 engine is not particularly popularbecause of its poor comfort due to the freemoments of inertia,particularly those of the first order. When the numbers ofcylinders are uneven, counterweights in the crankshaft can-not manage the balancing alone. Nonetheless, I3 dieselengines have occupied the niche of particularly fuel econo-mizing small car engines for years (e.g. VW Lupo 3L [8-16],DaimlerChrysler Smart [8-17]). The elimination of a balanceshaft is considered better for these small engines. In terms ofperceptibility in the passenger compartment, the 1.5 ordervibrations induced by the gas force become more criticallyapparent than the first order mass actions [8-17].