The maximum compressive load is the load cor- responding to the peak gas pressure, and Fig。 1 indicates that it occurs at about 228 crank angle。 Axial component of this load, which is  essentially a static load (where loads at the crank  and  pin ends are the same),  is  the  design  compressive load for the connecting  rod。  This  compressive load acts between the centre of crank end and piston pin end of the connecting rod。 Virtually no load acts on the crank end cap under the compres- sive load。

It should be noted that piston pressure versus crank angle diagram can change with speed。 The actual change will be unique to an engine。 Pressure versus crank angle diagram at different speeds for the engine under consideration was not available。 Therefore, the same diagram was used for different engine speeds。 However, from a plot showing the effect of speed on P – V diagram at constant delivery ratio reported in Ferguson [9], very small change in the peak gas pressure was observed at different speeds, though a change of nearly 10 per cent was observed at lower pressures。 Delivery ratio is the ratio of entering or delivered air mass, to ideal air mass at ambient density。

Load ratio (ratio of minimum to maximum load) at the crank end based on peak compressive load at peak gas pressure and peak tensile load in Fig。 3(a)

618 P S Shenoy and A  Fatemi

Fig。 3  Axial,  normal,  and  resultant   forces  at   the  connecting   rod  ends  at  crank   speed of 5700 r/min。 (a) Forces at the crank end。 (b) Forces at the piston-pin end

is 21。23。 At the piston pin end, based on the same peak compressive load as for the crank end but peak tensile load from Fig。 3(b), load ratio is 22。31。 Therefore, load ratio varies over the length of the connecting rod。 As a result, fatigue testing at different load ratios is often conducted in order to test differ- ent regions of the connecting rod [10]。

It should be noted that the analysis presented assumes that the crank rotates at  a  constant angular velocity。 Therefore, angular  acceleration of the crank is not included in the analysis。 How- ever, when forces at the ends  of  the connecting rod in a similar engine configuration under con- ditions of crankshaft acceleration and   deceleration

(acceleration  of  6000 r/s2 and  deceleration   of 700 r/s2) were compared with forces under constant crankshaft speed, the difference was found to be less than 1 per cent。

3     FE MODELLING OF THE CONNECTING   ROD

In order to capture the structural behaviour of the connecting rod under service operating conditions, quasi-dynamic FEA was performed。 An FE model mesh with about 105 parabolic tetrahedral elements, with uniform global element length of 1。5 mm and local  element  length  of  1 mm  at  locations   with

Dynamic analysis of loads and stresses in connecting  rods 619

chamfers, was used。 As a connecting rod is designed for very long life, stresses are in the elastic range, and as a result linear elastic analysis was conducted。

While performing quasi-dynamic FEA of the con- necting rod, external loads computed from the load analysis discussed in section 2 were applied to both the crank end and the piston pin end of the connect- ing rod。 Many FE models were solved, each model with the applied loads obtained from the load analy- sis at the crank angle of interest。 Therefore, as indi- cated earlier, such analysis is different from a static analysis as the time-varying dynamic nature of the loading represented by load variation at different crank angles is accounted for。 It should also be noted that the dynamic load analysis step was required as a separate step, as input to the stress analysis step using IDEAS。 This is because, although commonly available commercial software are typi- cally capable of providing stresses as output from dynamic input loads (i。e。 gas pressure and inertia) they are not capable of determining the dynamic loads。 Time-varying dynamic loads were determined from multi-body dynamic analysis using ADAMS, based on the crank revolutions per minute, piston gas pressure (which varies with time or crank angle), and mass properties of the connecting rod。 Combining the two analyses (i。e。 multi-body dynamics and FE stress analyses) into a single step requires immense computational power to develop FE models of the entire system, which may become available in the future with the rapidly increasing computing power。