Originally Posted By: SonofJoe
I posted this first on Brother Gokhan's recent 'Thick vs Thin' troll-fest but it might be more usefully reposted here as it's to do with fuel economy...
Just thought I'd add something else based on the numbers in SR5's Valvoline table (if some kind person could cut and paste the table into this thread, it would be very helpful) ...
Most folks 'get' why low viscosity oils might potentially be good for fuel economy. However what they may not realise is there are two parts to this arguement and one isn't always obvious.
In the Valvoline table, it shows a 10W30 and a 5W30 both with nominally the same KV100 (10.9 & 11.0 cst). Once these oils are up to operational temperature (for the sake of argument, say that's 100°C), both oils will give essentially the same fuel economy.
That the 5W30 gives better FE than a 10W30 isn't always obvious because the W rating is generally considered to be all about low temperature startability. The 10W30 is designed to pump at -30°C (-22°F) and crank at -25°C (-13°F). A 5W30 is designed to go even lower at -35°C (pump) and -30°C (crank). [actually both the Valvoline oil should work a few °C below the formal limits]. So on the surface, the W rating has naff all to do with better fuel economy!
However for a fixed KV100 (or oil weight), dropping the W rating has the side effect of dropping the oil's KV40. In the Valvoline table, in moving from the 10W30 to the 5W30, the KV40 of the oil drops from 70.5 cst to 62.8 cst. If you go even lower to a 'typical' start-up temperature (say 10°C for the UK), the 10W30 has a KV10 of 351.5 cst while the 5W30 has a KV10 of 271.7 cst. Now a 30% higher oil KV does not worsen fuel economy by 30%! I've forgotten most of the fluid dynamics I learnt at uni but I do recall that viscosity is a minor factor in Reynolds Number and Prandtl Number. However lower oil viscosity, as the oil warms-up from say 10°C to 100°C, should yield a couple percent fuel economy benefit.
Now if you contrast the 5W30 with the 5W20 in the Valvoline table, not only does the KV100 drop (from 11.0 cst to 8.6 cst) but the KV40 also drops (from 62.8 cst to 48.9 cst). The 'start-up' KV10s also drop (from 271.7 and 217.5 cst).
So here's the thing.... if you drop an oil weight (but not the W rating) you get two fuel economy savings; one during start-up/warm-up and one when your oil's up to operational temperature. If you just drop a W rating (but keep the weight the same) you only get one saving (during start-up/warm-up).
Now contrast the 5W20 with the 0W20 in the Valvoline table. The KV100's are nominally the same (8.6 vs 8.7), the KV40 drops but not by much (48.9 to 45.2) while the calculated KV10 drops from 217.5 to 181.2). This is very interesting. Given the way thermo-dynamics works, the RATE at which the temperature of your oil increases should be fastest between 10°C and 40°C, slower between 40°C and 70°C and slower still between 70°C and 100°C (after which it notionally reaches steady state). If this is the case, on an time-aggregated basis, I would not expect the 0W20 to give much of a fuel economy benefit over and above what you see with the 5W20 during the warm-up phase and no benefit at operational temperature.
So in relative terms, oil weight probably trumps W-rating in delivering tangible fuel economy.
Which is interesting because lower W-rating usually comes with more problems; more expensive base oil, more VII, more Ashless and higher Noack). Just dropping an oil weight does exactly the reverse.
So 10W20? Wherefore art thou??
Originally Posted By: SonofJoe
I posted this first on Brother Gokhan's recent 'Thick vs Thin' troll-fest but it might be more usefully reposted here as it's to do with fuel economy...
Just thought I'd add something else based on the numbers in SR5's Valvoline table (if some kind person could cut and paste the table into this thread, it would be very helpful) ...
Most folks 'get' why low viscosity oils might potentially be good for fuel economy. However what they may not realise is there are two parts to this arguement and one isn't always obvious.
In the Valvoline table, it shows a 10W30 and a 5W30 both with nominally the same KV100 (10.9 & 11.0 cst). Once these oils are up to operational temperature (for the sake of argument, say that's 100°C), both oils will give essentially the same fuel economy.
That the 5W30 gives better FE than a 10W30 isn't always obvious because the W rating is generally considered to be all about low temperature startability. The 10W30 is designed to pump at -30°C (-22°F) and crank at -25°C (-13°F). A 5W30 is designed to go even lower at -35°C (pump) and -30°C (crank). [actually both the Valvoline oil should work a few °C below the formal limits]. So on the surface, the W rating has naff all to do with better fuel economy!
However for a fixed KV100 (or oil weight), dropping the W rating has the side effect of dropping the oil's KV40. In the Valvoline table, in moving from the 10W30 to the 5W30, the KV40 of the oil drops from 70.5 cst to 62.8 cst. If you go even lower to a 'typical' start-up temperature (say 10°C for the UK), the 10W30 has a KV10 of 351.5 cst while the 5W30 has a KV10 of 271.7 cst. Now a 30% higher oil KV does not worsen fuel economy by 30%! I've forgotten most of the fluid dynamics I learnt at uni but I do recall that viscosity is a minor factor in Reynolds Number and Prandtl Number. However lower oil viscosity, as the oil warms-up from say 10°C to 100°C, should yield a couple percent fuel economy benefit.
Now if you contrast the 5W30 with the 5W20 in the Valvoline table, not only does the KV100 drop (from 11.0 cst to 8.6 cst) but the KV40 also drops (from 62.8 cst to 48.9 cst). The 'start-up' KV10s also drop (from 271.7 and 217.5 cst).
So here's the thing.... if you drop an oil weight (but not the W rating) you get two fuel economy savings; one during start-up/warm-up and one when your oil's up to operational temperature. If you just drop a W rating (but keep the weight the same) you only get one saving (during start-up/warm-up).
Now contrast the 5W20 with the 0W20 in the Valvoline table. The KV100's are nominally the same (8.6 vs 8.7), the KV40 drops but not by much (48.9 to 45.2) while the calculated KV10 drops from 217.5 to 181.2). This is very interesting. Given the way thermo-dynamics works, the RATE at which the temperature of your oil increases should be fastest between 10°C and 40°C, slower between 40°C and 70°C and slower still between 70°C and 100°C (after which it notionally reaches steady state). If this is the case, on an time-aggregated basis, I would not expect the 0W20 to give much of a fuel economy benefit over and above what you see with the 5W20 during the warm-up phase and no benefit at operational temperature.
So in relative terms, oil weight probably trumps W-rating in delivering tangible fuel economy.
Which is interesting because lower W-rating usually comes with more problems; more expensive base oil, more VII, more Ashless and higher Noack). Just dropping an oil weight does exactly the reverse.
So 10W20? Wherefore art thou??
10w20= sae20?