Abstract
A simulation software (SIMULCO) for road vehicle operating conditions enabling to calculate fuel consumption under hot start conditions had been previously developed. This new research is aimed at taking into account the impact of cold start on fuel consumption increase, in order to reproduce test conditions of the current or draft regulations, but also to analyse this impact under all engine initial temperature and ambient air temperature conditions.
In this modelling study, it is mainly assumed that fuel consumption increase results from mechanical friction increase at engine and power train level, which is due to an increase in oil viscosity. A theoretical analysis of heat flows and viscous frictions enabled to define approximate relations yielding changes in oil temperature and corresponding fuel consumption : dTh/dt = a(C) - [b(N)+h(V)].(Th-Ta) q = q 0 + d(N).[exp(B'/Th n) exp(B'/Th 0n)]
Where t = time, Th = engine oil temperature, Ta = ambient air temperature, q = mass fuel flow, C
= engine torque, N = engine rotation speed, a, b, h and d are functions, B' and n are constants, the zero subscript corresponding to the thermal equilibrium.
The functions of engine torque, engine rotation speed and vehicle speed involved in these relations were determined from theoretical considerations and tests performed on a petrol engine passenger car using a chassis dynamometer. Comparing the measurement results to the calculation results for oil temperature and fuel consumption demonstrated that this approach is adequate considering the objectives set : the deviations recorded amount to a few percents on the ECE15 urban cycle or under steady speed conditions.