mzso wrote: ↑24 Jul 2024, 19:31
Hoffman900 wrote: ↑24 Jul 2024, 17:36
Electric motors have a torque vs speed graph, with torque decaying with speed. Drag increases with the square of velocity. A modern F1 car has a drag coefficient approaching 1. The decaying torque with speed and the increase in drag with speed are in conflict with each other.
Everyone forgets there is a torque multiplication factor that transmissions provide. Take shaft torque and multiply it by the transmission gear ratio and then the final drive gear ratio, that's the axle torque the wheel sees. You can use this gear multiplication to apply more torque to an axle for a smaller / lighter / less energy demanding motor (ic or electric).
Where are you going with this? Neither of your responses actually addressed what you quoted. So I have no clue what you want to say.
For sure FE doesn't have gearboxes. Only a single gear ratio.
Everyone is sharing "concept" power units without defining power needs.
To start, aero drag increases with the square of velocity. F1 cars with their current levels of downforce have a Cd of near 1. If you express this in horsepower, at 190mph (305kmh) (with tire friction), they're using near 750-800hp to just maintain that speed. If you want to go faster you need exponentially more power. You can express that as force. How do you go faster in road racing? You convert horsepower to downforce. This is why peak trap speeds have been nearly the same for 35-40 years but cars go MUCH faster.
So let's say with today's rule set and aero efficiencies, you need the equivalent of 750bhp(the entire IC power) to have the same trap speed. If express it as torque, say that 750bhp occurs at 10,000rpm, that's 393ft-lbs. Say a F1 car has a 4.5:1 rear gear. So that's 1768ft-lbs of torque at the wheel axle to maintain that vehicle's speed at 190mph. This is the calculation every land speed racer does (I have x frontal area, x Cd, x frictional drag, and the class record is xxx mph, how much power do I need to apply to the axle to get there?)
So now we have established we ned 1768ft-lbs (2397 Nm) of torque to propel a F1 car with the current drag / frontal area / tire friction to 190mph. Let's say for our tire grip and to maximize acceleration, we want 4000Nm of maximal torque (2950ft-lbs). You have now defined the problem statement. So how do you get there with alternative power?
Constant power speed ratio (CSPR) is defined as the maximum speed of the motor divided by the minimum speed at which peak power will be met. Every motor has its own CPSR, but let's define the vehicle CPSR. So let's say I need 4000Nm, and I have an electric motor with 400kw (536hp). Using the relationship of power being the product of speed and torque, the base speed is 400,000w / 4000 Nm = 100 rad/s. If the maximal wheel speed is 200 rad/s, then the CPSR is 200 rad/s divided by 100 rad/s = 2:1. Most of what is being proposed here does not have a CPSR of 2:1, most passenger car EV motors are like 4:1-5:1... so right there we can throw out probably half this thread.
CPSR is gearing blind, but gearing does effect maximal torque (4000 Nm). If we have a 5:1 gear ratio, then we'd need 800Nm and be able to run 1000 rad/s (200*5), you need 9500rpm. If you double the gear ratio to 10:1, the motor only needs 400Nm but twice the speed (19,000rpm). The latter's motor can be nearly half the size and weight, and you use gearing multiplication to reach your maximal torque. It's same as an IC, you use smaller high revving engines that fit better in the car for weight / and aero packaging, and make up for the lack of torque with gearing multiplication. With an EV is the same way, where maybe you don't have as many gears, but more reduction is going to require a bigger transmission.
Yes the 3rd Gen Formula E cars are single speed (with two stage reduction), but the rules dictate the aero which ultimately dictates the total drag, the rule makers back into a rules heavily dictated power unit package, but controlling it via drag. This is the problem the teams are screaming about 2026. They want all the downforce = drag, but that drag plus the car's mass, consumes more power than the PU rules allow them to make and / or sustain, hence the talk of active aero to reduce drag and reducing the car's size to reduce mass... the former is in conflict with extracting maximum performance (reduce downforce) which they all want for speed as well as the spectacle of being "the pinnacle" and the latter is in conflict with safety rules and test technology available to downsize the technology to be able to make that / sustain that power.
So in summary:
* no one here has defined how much power / force you need. I have given a rough estimate above.
* knowing how much force you need, you need to find something that can produce the same force to maintain current performance with the current Cd / tire friction
* knowing how much force you need, you also need to find something that fits. Maybe's a bigger motor but less gearing reduction / less gear ratios, maybe it's a smaller less powerful motor with a large gearing reduction / more gear ratios. How you do that effects CoG, aero packaging, cooling, total weight, and plenty of other performance characteristics.
