Firing Order Selection in Relation to Vibration Aspects

Abstract

Selecting the optimum firing order for a combustion engine is an important decision to make when developing an engine. Typically, the optimum choice is made on the basis of a weighted balance between a number of interrelated engine vibration characteristics, such as: balancing, free forces and free moments, guide force distribution, inner bending moments, bearing load, and torsional and axial vibration behaviour. Also gas dynamic properties making each cylinder do the same amount of work is considered. In addition to the traditional firing sequence selection, implying the same firing interval between cylinder units, also so-called irregular firing orders, characterized by different intervals between the individual firings, are considered. Various methods and strategies can be applied, considering that different engine applications exhibit completely different vibratory behaviour, e.g. marine propulsion plants compared with stationary installations. This paper will give examples relating to various applications for both two-stroke and four-stroke engines. Four-stroke: Realizing that the engine is often an integrated part of a complex system, it is demonstrated in this paper that analysing the whole system in terms of linear vibrations, the optimum firing order is different from the firing order determined with traditional methods. The system analysed here is a genset for marine or stationary application consisting of a 9-cylinder diesel engine, alternator and base frame. All three parts are bolted together as one unit, resiliently mounted on a ship or a land-based foundation. Two-stroke: The traditional 7-cylinder marine propulsion engine firing sequence is modified in order to be able to run safely directly at a two-node torsional resonance without installing a T/V damper. The 12-cylinder stationary engine has been modified so as to reduce installation costs and improve the coupled torsional-axial vibratory behaviour. The 10-cylinder marine engine firing sequence has been modified in order to achieve more favourable structural vibration characteristics.