A general purpose computer model has been developed for analyzing the thermal performance of thermofluid systems. The system thermal behaviour is governed by heat convection and conduction. The model represents a thermofluid system as a thermal network, consisting of several different fluid circuits which are separated by solid walls. The solid walls are represented by hexahedral elements with lumped heat capacitance. The model has the capability to set up and link an equivalent thermal network from input data, using two types of junctions: wall-to-wall and fluid-to-wall. The flow calculations are based on the one-dimensional incompressible flow equation and the heat transfer calculations are based on either forced or natural heat convection for internal and external flows. The heat convection formulae used in the model are in the non-dimensional form which simplifies the program structure.The model has been used to simulate the warm-up process of a four-cylinder spark ignition engine. The engine model consists of four fluid circuits: engine internal gas, underbonnet air, coolant and lubrication oil flows. The engine block is divided into a number of solid wall elements, each wall element having seven temperatures: one for the centre and six for different surfaces. The model has therefore a fairly high-resolution of temperature variation of the whole engine system during warm-up.A parametric study has been carried out to investigate the effect of engine design parameters on the thermal transient response, including engine running conditions, thickness of the combustion chamber, total coolant and lubricant mass. It is shown that the model can be used as an effective design tool for thermal performance optimisation and energy management system.