A limited pilot study conducted at VTI shows that engine heating is more fuel efficient via coolant than via lubricant.
Cold starting a motor vehicle results in higher fuel consumption and more emissions than if the engine is started when hot. Engines have traditionally been pre-heated by warming up the coolant, which in turns heats the engine block. A study conducted at VTI examined whether it would be more energy efficient to heat the lubricant rather than the coolant. The hypothesis was that warmer lubricant would lead to less friction in the engine, which would mean less fuel needed to overcome the friction resistance.
“The test results show that the effect of cold starting in terms of increased fuel consumption is reduced most by heating the coolant. The warmer the coolant when the engine is started, the lower the fuel consumption”, says VTI researcher Annelie Carlson.
When the total energy required for cold starts is calculated, i.e., fuel consumption plus the electricity used to heat the engine, the results in terms of whether cold or hot starting is most energy efficient are not clear cut. The reduced fuel consumption that results from preheating is based in part on the electricity consumed to heat the engine.
“One advantage is of course that local emissions are reduced. If we also take into account the fact that Swedish electricity production is based largely on hydroelectric and nuclear power, then engine heating results in lower emissions overall”, says Carlson.
VTI Note 22-2014: Energy efficient use of engine heaters – Evaluating the effect of heating lubricating oil compared to heating cooling water
Authors: Annelie Carlson, Ulf Hammarström, Mikael Bladlund.
Link to report (in Swedish with English summary).