1990-10-01

An Investigation of Lubricating System Warm-up for the Improvement of Cold Start Efficiency and Emissions of S.I. Automotive Engines 902089

Transient warm-ups of an engine lubrication system are examined using test bench measurements and numerical simulations. It is shown that friction in the bearings is the greatest heat source for oil, but that a major part of the heat received by the oil is transferred to the walls of bearings and passages. It is also determined that the total amount of heat ultimately retained by the oil is a very small part of the engine energy consumption, and that the engine warm-up does not use all the energy made available by combustion heat transfer and friction losses. Within this scope, some systems are examined to properly use available heat and improve lubricant and engine warm-up.
Today, automotive engines are well designed for low consumption and pollutant emissions during fully warmed-up periods. However, emission standards are fixed by test-cycles which begin with cold starts. Furthermore, under city driving conditions, engines operate well below their fully warmed-up optimum fuel consumption and emission levels [1] [2] [3] [4]. Unfortunately, cold start and warm-up of SI engines are very critical periods for fuel consumption and pollutant emissions [5] [6]. It is during these periods that engines emit the largest amount of pollutants (namely carbon monoxide and unbumed hydrocarbons). This increase in pollutant emissions results from both erratic combustion induced by inadequate mixing of the air and fuel, and too rich air-fuel ratios [7]. In addition, under cold conditions, catalytic conversion of the pollutants is inefficient due to the low temperature of the catalytic converter. Finally, since the viscosity of lubricants at low temperatures is very high, frictional effects tend to degrade the system efficiency causing increased fuel consumption.
One way to minimize these problems is to make the warm-up period as short as possible. If we analyze the temperatures of engine parts during the warm-up, it appears that lubricant is generally the last component to reach its equilibrium temperature [8]. Thus, it is relevant to examine the lubricant warm-up process in order to characterize the thermal inertia of the engine system.
Bearing this in mind, the thermal behavior of the lubricant and lubricating system during the warm-up period of an automotive engine has been analyzed. The results are based on both experimental measurements and computer simulation.

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