Electric cars and cooling drag

[planetaire]

Originally Posted by JulianEdgar

... Just look at a variety of ICE car data to see that - one of the lowest ICE cars I can find for cooling drag proportion is a very powerful car! (Guessing is not a very good research approach.)
...

We don't have the same approach.
I am not interested in comparing plenty of ice car with bev cars.

But in comparing several car that have the two model ice or bev.
For example Ioniq, Up, Golf ...
Tesla for example have only bev model.

I will do math for you :
with my car using it in Bev mode, say one week per year I open totally the front grill, then I have 4% more drag. The year average lost drag for cooling is 0,08%

In ice mode, I think more then 75% time the front grill have to be opened. The average lost drag for cooling is 3%

[Isaac Zachary]

Power isn't always indicative of cooling needs. It's one thing to cruise up a mountain pass in a Ferrari. It's another to do so in a Ford F-350 with a 15,000lb loaded trailer.

Shutters just mean you can be more aerodynamic if where you live is cool enough. There are places in this world that are too hot to properly cool an EV battery. There are other places where you can block off the radiator on just about any vehicle and never have to worry about it.

[Autobahnschleicher]

Originally Posted by Isaac Zachary

Power isn't always indicative of cooling needs. It's one thing to cruise up a mountain pass in a Ferrari. It's another to do so in a Ford F-350 with a 15,000lb loaded trailer.

I'd argue that is very incorrect.
Especialy in EVs the cooling requirement increases faster than the power output for a given drivetrain.

Also when the Ferrari is cruising, it's producing significantly less power than the truck does when towing.
But when you're driving it on the track on a hot summer day, it needs all the cooling it can get.
Cooling requirements are not static, one needs to think less of it as a dumb heatsink, rather it's a thermal management system.
The big deal is that shutters are not an ideal solution, variable air intakes would probably be more efficient as they can reduce the stagnation area significantly when closed.
During winter an EV (or some ICE vehicles) require no cooling airflow at all.
All waste heat is required to keep the drivetrain at operating temperature and the driver comfortable.
Meanwhile during summer at 40°C+ you'll need to run active cooling systems for the battery and driver.

The main issue EVs have with cooling is a low temperature difference between the drivetrain and ambient temperature.
In some situations you even need to cool them below ambient temperature at a high thermal load while stationary.
(charging at a 350 kW charger during a hot summer day)
That requires massive amounts of airflow, strong blowers and heatpumps.
And since heatpumps themselfes produce waste heat, you have even more waste heat to get rid of at a standstill, but a higher temperature difference.

These thermal management systems are rather complex and there certainly is room for improvements.
EVs need a massive range of cooling air from none at all to a ****load due to low temperature differences and the heat load often not scaling with their speed.
Meanwhile ICEs only produce heat when they are running and the heat production scales pretty well with their speed while their coolant temperatures are usualy some 80-90°C, not 20-30°C
Maybe a moveable intake-cone, maybe adjustable NACA ducts or something like that will help them in the near future.

[Autobahnschleicher]

I'd even go so far and say that a high performance EV has a significantly higher requirement of airflow through its radiators than a compareable ICE car despite it producing less waste heat as it has to run a far lower deltaT.
Wich is one reason why the Tesla Model S is thermaly limited in therms of cruise speed despite having not exactly small radiators.
Meanwhile the "electron guzzler" Taycan can maintain high speeds between 350kW chargers.

This likely affects "normal" EVs like the E-Golf or ID-3 far less as they have significantly lower thermal loads due to their less powerfull drivetrains going less hard on their batteries as well as their limited topspeeds

[Isaac Zachary]

I said power doesn't always dictate cooling needs. It's not a so many HP equals so many Cd kind of thing. Not that max power doesn't influence the cooling needs.

There's still some major differences between the truck and sports car. A truck going up a long mountain pass may be at 100% throttle for 20-30 minutes. The car will likely be cycling between 100% and 0% as it brakes and turns. Also the truck is build for longevity. Some sports cars/race cars may sacrifice race track longevity for speed. NASCAR cars have very tiny radiators, for an example. The sports/race car will also be hitting very high speeds pushing more air through the radiator. The truck may have such a load it can't even maintain the speed limit.

[Autobahnschleicher]

There are a few issues:
1. On the track, keeping temperatures in check absolutely is a big deal.
Aftermarket radiators, oilcoolers and so on are very popular modifications for a reason.
And trackdays in summer are absolute hell for the cooling system.
And no, you can't just run it any hotter or you risk blowing up your engine rather quickly.
2. The truck usualy doesn't run at maximum rpm and usualy not quite at WOT, the car on the track does.
3. NASCAR style oval "racing" isn't hard at all on the cooling system, in fact it's so easy on the cooling system due to the high airspeed, they run partial grillblocks.
Going in a straight line at topspeed allows for maximum airflow through the radiators.

On the track, you're at WOT and high in the rev range most of the time. Braking zones are short and you're back on the throttle right after hitting the apex.
Most road vehicles don't realy do well in such situations and overheat, leading to the driver having to take slow cool down laps.

Anyway, my point was:
High cooling air requirement means larger air intakes.
Flaps certainly do help, but even when they are fully closed, you still have additional drag.
The solution are variable crossection air intakes.

[Isaac Zachary]

Originally Posted by Autobahnschleicher

2. The truck usualy doesn't run at maximum rpm and usualy not quite at WOT, the car on the track does.

Have you ever climbed a 7% to 8% for several miles pulling over 30,000lbs with a 200hp engine? You downshift, nearly redline it and keep it floored.

But yes, the tolerances are different between the two. A truck may be redlining at 3,000RPM, the car at 7,000 or 8,000.

[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...

[JulianEdgar]

Originally Posted by planetaire

We don't have the same approach.
I am not interested in comparing plenty of ice car with bev cars.

You are certainly right that we don't have the same approach - glad we have that sorted.

[JulianEdgar]

Re the above discussion of cooling drag of trucks vs cars, etc. Lots of good points, but all of which makes things more complex ie more guesswork needed.

Which is why I don't guess. I look at the published data.

At this stage, the published data shows that cooling drag of BEVs, as a proportion of total Cd, is quite high - and certainly not the zero or near zero that as been claimed by some people here.

As more data on other cars is released, that may change - fine.

Or it might be that, for a variety of reasons, cooling drag of BEVs remains proportionally high.

I don't know - I don't guess and make up theories. I don't have an established position that I then need to defend tooth and nail. My position is based on the research evidence that is published, which compares the cooling drag of BEVs with total drag, as compared to the same with ICE vehicles.

I don't really understand why people have barrows that they push so hard. I don't care whether the cooling drag of BEVs is low or high (proportionally) compared with ICE cars. It is what it is!

In this subforum this occurs again and again - totally covered wheels, 'template' shapes, defining separated / attached flows, etc. People have a position that they fight tooth and nail for, irrespective of what published research - and most often modern research - shows.

It's really weird.