The gas side wall temperatures of the prechamber in a divided chamber diesel or stratified charge engine have an influence on the combustion process. From this results an important effect on exhaust emissions, fuel economy and engine performance. The insulation characteristic of a prechamber insert should ensure a quick warm-up after cold start on one hand but provide not too high temperatures at full load and maximum engine speed on the other.
Computations were carried out to quantify the effects of various design parameters and materials on temperature, heat flow, stress and deformation and to optimize the design under numerous existing restrictions. An analysis was made of the elasto-plastic behaviour of the insert and the cylinder head, including material creep. For most of the calculations a rotary symmetric model was assumed and solved by the finite element program system MARC. The two-dimensional mesh chosen describes the actual details of the section of the cylinder head, the prechamber insert and the spark plug or fuel injector fairly closely. The gas temperature and the heat transfer coefficient were determined from a separate engine cycle simulation.
Preliminary calculations with a one-dimensional model of a composite wall were made in order to optimize the mesh in regard to accuracy and computing time.
The main cases studied were the following: a prechamber without an insert (base case), an insert insulated by an air gap and being in contact with the cylinder head over a small or larger press fit region, total loss of contact with the cylinder head, a cylindrical insert with and without air gaps, copper alloy and a ceramic as insert material, while stainless steel was used in all other cases.