2 ms·

Your link says that those are "estimated fuel consumptions", but I'm somewhat inclined to doubt the veracity of these estimates: between 120 km/h and 240 km/h w

by NickM 13y ago

Your link says that those are "estimated fuel consumptions", but I'm somewhat inclined to doubt the veracity of these estimates: between 120 km/h and 240 km/h wind drag is going to quadruple, and ICEs get lower efficiency at higher RPM. So if the efficiency falls by less than 50% when you double the speed, then that implies that tire drag and internal friction represent the majority of the force you're overcoming at highway speeds, which is definitely incorrect. Something doesn't add up.

rdl 13y ago
Cars have geared transmissions. Cars designed for the Autobahn (most high end German cars) have gearing optimized for decent cruise range at 120-180 km/h. The engine is sized for peak acceleration (and marketing). It has a turbo (in the 535d; the 550d has 3....), which helps somewhat without adding more fuel consumption, but still, a 300hp engine is pretty big. Basically, to cruise along at 120kph in a reasonable car with reasonable Cd probably is about 20-30hp (yay for mixing metric and imperial units...). Wind is probably 20hp of that. The engine is less efficient at low output (specific output), so you won't get fuel consumption at 10% power which is only 10% of peak fuel consumption. Wind resistance does go up, but your engine also gets to operate more within its power band. With the right gearing, you can stay within the optimal RPMs and be close to the design power level of the engine at a reasonable cruise speed. Empirically, I've driven big heavy cars fast for long distances, and gotten reasonable fuel economy; especially big German cars with big diesel engines. What kills them is stop and go, or when you have a smaller car with, say, 5 gears, and you are running it at redline to keep your desired speed. What is awesome about electric cars is you could get much more efficiency in the motor across a range of power levels, plus regen. What is not at all awesome is the power density of lithium ion batteries.
mturmon 13y ago
...between 120 km/h and 240 km/h wind drag is going to quadruple... Your figures here are an idealization. Real results are not going to match this because air resistance and fuel consumption are not a smooth process. (Some versions of the Tesla S even have active suspension, which really makes things interesting.) I expect you'd need some pretty serious finite element modeling, and engine modeling, to really understand the linkage.

by NickM 13y ago

rdl 13y ago

Cars have geared transmissions. Cars designed for the Autobahn (most high end German cars) have gearing optimized for decent cruise range at 120-180 km/h. The engine is sized for peak acceleration (and marketing). It has a turbo (in the 535d; the 550d has 3....), which helps somewhat without adding more fuel consumption, but still, a 300hp engine is pretty big. Basically, to cruise along at 120kph in a reasonable car with reasonable Cd probably is about 20-30hp (yay for mixing metric and imperial units...). Wind is probably 20hp of that. The engine is less efficient at low output (specific output), so you won't get fuel consumption at 10% power which is only 10% of peak fuel consumption. Wind resistance does go up, but your engine also gets to operate more within its power band. With the right gearing, you can stay within the optimal RPMs and be close to the design power level of the engine at a reasonable cruise speed. Empirically, I've driven big heavy cars fast for long distances, and gotten reasonable fuel economy; especially big German cars with big diesel engines. What kills them is stop and go, or when you have a smaller car with, say, 5 gears, and you are running it at redline to keep your desired speed. What is awesome about electric cars is you could get much more efficiency in the motor across a range of power levels, plus regen. What is not at all awesome is the power density of lithium ion batteries.

mturmon 13y ago

...between 120 km/h and 240 km/h wind drag is going to quadruple... Your figures here are an idealization. Real results are not going to match this because air resistance and fuel consumption are not a smooth process. (Some versions of the Tesla S even have active suspension, which really makes things interesting.) I expect you'd need some pretty serious finite element modeling, and engine modeling, to really understand the linkage.