Electric cars and cooling drag

[Isaac Zachary]

Originally Posted by Autobahnschleicher

A 200 hp truck engine is going to produce less waste heat than let's say a 400 hp sports car engine.
Especialy since truck engines are likely diesels...

Are you just trying to disagree with everything I say? So apparently you truely believe there's some perfect correlation between power and CdA.

[redpoint5]

I keep thinking it would be neat if cooling needs were sufficiently low, that radiating the heat could be passive through the skin of the vehicle.

I've got a PC with no cooling fans, but instead the case is used to dissipate the heat.

[Autobahnschleicher]

Originally Posted by Isaac Zachary

apparently you truely believe there's some perfect correlation between power and CdA.

Yes and no.
There is a correlation between cooling air demand and cooling drag as well as a corellation between power and cooling demand.
It's not perfectly proportional and depends on the engine type a lot, but more powerfull engines of a given type have a tendency of requiring more cooling, resulting in a higher cooling air demand, wich causes more drag.
In addition to that a lower deltaT means more cooling air is required for a given thermal load.

[JulianEdgar]

Originally Posted by Autobahnschleicher

Yes and no.
There is a correlation between cooling air demand and cooling drag as well as a corellation between power and cooling demand.
It's not perfectly proportional and depends on the engine type a lot, but more powerfull engines of a given type have a tendency of requiring more cooling, resulting in a higher cooling air demand, wich causes more drag.
In addition to that a lower deltaT means more cooling air is required for a given thermal load.

This is theoretically correct but it is incorrect in practice.

There is no correlation between engine power and cooling drag (as a proportion of overall Cd) for ICE cars for example.

The ICE car that I have data on with the second-lowest cooling drag proportion is also a very powerful car; whereas the data I have on the car with the lowest cooling drag proportion is a very low power car.

So more depends on car design than engine power.

(This is also why people saying that my use of the Tesla Model S and Taycan for high proportional cooling drag in BEVs is no good, because they're powerful cars, are missing a key point.)

[Autobahnschleicher]

Originally Posted by JulianEdgar

The ICE car that I have data on with the second-lowest cooling drag proportion is also a very powerful car; whereas the data I have on the car with the lowest cooling drag proportion is a very low power car.

Yes, but that's kind of cheating as it uses its cooling system exaust as air curtains for the front wheels.

Originally Posted by JulianEdgar

So more depends on car design than engine power.

Partialy correct, cooling system design indeed makes a major difference in drag.
However for a given type of design, drag increases with airflow demand.
And you can only realisticly feed two air curtains, there is only so much air they need.
If it was a little smaller, it might even have net negative drag.

Originally Posted by JulianEdgar

This is also why people saying that my use of the Tesla Model S and Taycan for high proportional cooling drag in BEVs is no good, because they're powerful cars, are missing a key point.)

The Model S also has issues because of its undersized cooling system while the Taycan does not.
And the Taycan carries a drag penalty for it, making it a known "electron guzzler", but it can make up for it in higher cruising and charge speed.

[freebeard]

Originally Posted by Julian Edgar

So more depends on car design than engine power.

Case in point: the 1947 Volkhart-Sagitta versus the KdF-Wagen Typ 60 aka Beetle. Chassis and engine are identical, only the body shape and it's material (steel==>aluminum) change. Cd went from ~041 to 0.21 or less depending on the tunnel. Thirty-three post thread here.

[JulianEdgar]

Originally Posted by Autobahnschleicher

Yes, but that's kind of cheating as it uses its cooling system exaust as air curtains for the front wheels.

How is that "cheating"? It uses a design of cooling system that gives very low drag. And it's a powerful car.

There is no demonstrated correlation in real cars between engine power and proportional cooling drag Cd. (For a given car, there might be, but not for cars in general.)

A good example of where theory and practice do not match.

[JulianEdgar]

Originally Posted by freebeard

Case in point: the 1947 Volkhart-Sagitta versus the KdF-Wagen Typ 60 aka Beetle. Chassis and engine are identical, only the body shape and it's material (steel==>aluminum) change. Cd went from ~041 to 0.21 or less depending on the tunnel. Thirty-three post thread here.

Huh?

We have no idea of the proportional cooling drag of the 1947 Volkhart-Sagitta versus the KdF-Wagen Type 60.

(Or if you do have that data, do you want to cite it?)

[Isaac Zachary]

Apparently the Aptera is going to have 0% cooling drag.

[Autobahnschleicher]

Originally Posted by JulianEdgar

How is that "cheating"? It uses a design of cooling system that gives very low drag. And it's a powerful car.

It is a very efficient design indeed, but the cooling system on its own isn't that low drag, it manipulates airfoil around the front wheels to reduce drag somewhere else.
It's a genius design, but sadly it doesn't scale up very well as manipulating airflow around the front wheels can't reduce the drag much further.

Originally Posted by JulianEdgar

There is no demonstrated correlation in real cars between engine power and proportional cooling drag Cd. (For a given car, there might be, but not for cars in general.)

While this is partialy true, tricks like the 996 don't scale up very well for high cooling air demand vehicles such as the Taycan wich needs a lot more air due to a far lower deltaT.
An example for an ICE with high airflow demands over its radiators would be the Veyron.
I can't find any data on its exact cooling drag percentage, but the 10 radiators are often cited as the culprit for the 0.36-0.41 cd.