Combustion stress in compression-ignition engines.

Abstract

South Africa produces alternative fuels from a number of different

sources. The properties of a fuel are known to affect the nature

of combustion in compression-ignition engines significantly, and

have occasionally resulted in engine failures. Combustion analyses

have been conducted on a wide range of fuels and combustion has

been thoroughly quantified. However, the role played by the

different combustion variables in failures was not known. The

result was that it was not possible to predict the implications of

variations in the nature of combustion. There was thus a need to

investigate the relative role of combustion variables in the

failure of engines.

The mechanisms of combustion and engine failure were studied. All

the variables required to determine combustion and engine

durability were measured simultaneously. This research required

the development of a complete engine research facility as well as

specialized transducers. Fast response surface thermocouples were

designed and constructed in order to monitor transient surface

temperatures. Heat transfer rates were then calculated with the

aid of Fourier analysis. Dynamic stresses were monitored by

strain-gauges applied to the engine.

A special high speed data

acquisition system was developed. An existing heat release model

was modified and used to calculate combustion rates. A

comprehensive finite element model was developed to calculate

piston temperatures and stresses. The role of each combustion

variable in stress and durability was investigated by statistical

analysis.

The results successfully identified the causes of combustion

related engine failures. The primary cause of engine failure was found to be thermal loading. The principal cause of any variation

in thermal loading and thus engine durability was maximum cylinder

pressure. The life of the engine was proved to be determined

almost entirely by peak cylinder pressure. The role of the rate

of pressure rise was proved to be insignificant.

All the implications of variations in the nature of combustion can

now be determined accurately. It will thus be possible to optimise

engine modifications and fuel properties before validation by

durability testing.