Dianne Hesterman

Variable inertia effects of an engine including piston friction and a crank or gudgeon pin offset

Abstract In order to obtain greater accuracy in simulation, more sophisticated models are often required. When it comes to the torsional vibration of reciprocating mechanisms the effect of inertia variation is very important. It has been shown that the inclusion of this variation increases model accuracy for both single-cylinder and multi-cylinder engine torsional vibration predictions. Recent work by the present authors has revealed that piston-to-cylinder friction may modify an engines ‘apparent’ inertia function. Kinematic analysis also shows that the piston side force and the dynamic piston-to-cylinder friction are interdependent. This has implications for engine vibration modelling. Most modern engines employ a gudgeon pin offset, and there is a growing interest in pursuing large crank offsets; hence, the effect of these on inertia variation is also of interest. This paper presents the derivation of the inertia function for a single engine mechanism, including both piston-to-cylinder friction and crank or gudgeon pin offset, and investigates the effect of each through predictions. The effect of crank offset on the variable inertia function is also verified by experiment.

The effect of piston friction on engine block dynamics

Abstract When developing vibration models, so as to reduce model complexity, it is typically expected that good prediction accuracy can be achieved by ignoring the complication of friction. In this paper, the significance of friction between the piston and cylinder on engine block dynamics is shown through simulation in both the time and frequency domains. Simulations and experiments indicate that large differences exist between model predictions for the engine block moment if this friction is not accounted for. This is especially true at low crankshaft rotational speeds when dynamic inertia effects are small. Experiments on a motored single cylinder engine at different average rotational speeds confirm the theory and very good tie-up with predictions is obtained. It is expected that these findings will also have implications for the torsional vibration of the engine.

Secondary Inertia Effects in the Torsional Vibration of Reciprocating Engines—A Literature Review

It has been known for some time that the torsional vibration of reciprocating engines and pumps cannot be modelled accurately by representing the reciprocating mechanism by a constant inertia. There have been many publications describing better models than those that use constant inertia and these indicate that the effective inertia of a reciprocating mechanism varies with angular position. The major component of this variation is a twice per revolution cyclic effect—hence the term ‘secondary inertia’. The consequences of this secondary inertia effect can be serious for torsional vibration causing ‘secondary resonance,’ and even instability. This paper contains a review of the current literature on the subject and introduces some recent work by the authors.

Improved model of chatter in grinding, including torsional effects

Abstract To date the effect of torsional vibration on grinding chatter has been largely ignored. Recent research on cylindrical plunge grinding has shown, however, that the torsional characteristics can signi. cantly modify system behaviour and in certain circumstances may stop chatter from occurring. In order to investigate this in more detail, both frequency and time domain solutions have been developed for a cylindrical grinding model. A description of these solution methods and a comparison of their results are presented in this paper. The potential for tailoring the torsional characteristics of the system to improve or eliminate chatter in cylindrical grinding is con. rmed by both models.

Torsional Receptances and Variable Inertia of a Two-Inertia Model of a Universal Joint

This paper presents the derivation of the torsional receptances for a universal joint. The joint is modeled as two inertias connected by a massless cross-piece. The equations of motion and resulting receptances reveal that the universal joint may be represented as a variable inertia. The inertia is a function of misalignment angle and angular position. It is shown that the inertia variation may be approximated by a second order cosine for typical misalignment angles. Systems with variable inertia are known to exhibit non-linear vibration behaviour.