Alright, I finally found some solid info on this issue, yay. Below is a link to a paper that tested this exact topic. I didn't read the whole thing, but I'll post quotes of the conclusion.
The effect of engine and transmission oil viscometrics on vehicle fuel consumption
Also note, these tests were done on a diesel engine, not gasoline. Ambient temperature was 25°C.
Quote:
It was found that 495W of heating applied from the start of the drive cycle resulted in a 0.5% reduction in fuel consumption, while 1080W of heating yielded a 0.7% reduction. In order to demonstrate the maximum achievable benefit of heating the current production transmission oil, the oil was preheated to 70°C prior to the start of the cycle resulting in a 1.4% reduction in fuel consumption.
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The fuel saving achieved by using an un*heated, reduced viscosity, transmission oil was found to be 1.1% compared with the baseline, which is greater than was achieved when applying 1080W of heating to the current production oil. Providing the reduced viscosity oil can satisfy durability and other production criteria, its use would be a less complex solution to reducing transmission churning losses and drive cycle fuel consumption than heating the current production oil. Heating the lower viscosity oil during the cycle yielded a further 0.4 % reduction in fuel consumption over the NEDC, giving a total improvement of 1.5% compared with the unheated ‘production’ oil.
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Also of interest is this colder weather testing section:
Quote:
Figure 13 shows that at an ambient temperature of -7C the transmission oil temperature naturally rises by 31C (from -7°C, to 24°C) over the NEDC without any additional heating. The increased warming over that seen at the 25°C tests is most likely attributable to the increased work done to shear the highly viscous oil at the very low temperatures. This increased work is a significant proportion of the extra parasitic load that, when combined with thermodynamic effects on engine combustion, and temperature dependant fueling maps, give a fuel consumption increase of 15% when compared with the results at 25°C. Another significant factor in this increased parasitic load is the increased rolling resistance of the cold tyres at *7°C.
Interestingly, the impact of the additional heating at -7°C is very similar to that observed at 25°C in terms of temperature rise and fuel consumption reduction, with a 0.6% saving observed over the cycle.
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