Hello, I’ve been reading about electric cars, and this got me thinking. Why does Tesla and all the other electric cars use direct drive…? I would think having a ICE RWD setup, but with the ICE replaced with an electric motor would work just fine. Are there factors here that would make such a transition impossible (i.e too much torque)?
Partly the torque limitations but mostly that more than one gear isn’t needed for performance.
I mean for efficiency. When going at 80 mph in a direct drive type, the motors are spinning so much faster, hence wasting more electricity. Not to mention that a higher top speed could be achieved, and since you could shift gears, it would allow for speed variation. Just a thought I had
Adding gears means adding a transmission, and that is additional cost.
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What I’m saying is taking an already existing gasoline car, adding a battery, and swapping the engine out for a decent sized motor. Take a Camaro for example. It might not be the best design ever, but its a start. I’d personally want gears if I ever owned an electric car.
Conversions are more likely to use the existing gearbox than purpose built electrics. Have to try to find the pilot episode but there was a show years ago where they converted a 1967 Camaro to electric motor and batteries but left the 4spd manual alone. So it’s doable.
Gm was developing a EV conversion based on the Chevy Bolt that in the case of the show truck used a Production gm transmission but haven’t heard about anything past a concept.
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Tesla’s don’t have direct drive. A motor that has the torque to directly drive the wheels would be stupid-heavy. They use a single ratio two stage gear reduction with the motor spinning about 8X the axle speed.
Unlike gasoline engines, electric motors remain very efficient when spinning at high rpm under a light load. This low load efficiency can be improved even more when the variable frequency drive reduces the voltage delivered to the motor under low torque conditions.
The nearly flat efficiency curve of an electric motor is why you can just about double the range of an electric car by cruising at around 30 mph vs 70 mph. Driving a gas powered car that slow may result in better gas mileage, but it sure won’t double the 70 mph gas mileage, unless you use extreme techniques like “pulse and glide”, accelerating at near full throttle where the gasoline engine is efficient and then shutting the engine off and coasting for a mile or so and then repeating.
Ok, I always thought Teslas had direct drive. Either way, with 3-4 gears, you could achieve a higher top speed, which would make electric cars probably go faster than gasoline cars. I mean, just imagine an electric motor with a gearbox. There would be so much potential (from what I see), not to mention an electric car with a gearbox would be more… fun I guess you could say (in my opinion)
Not true. Electric motor efficiency is still very good at high rpms.
The Tesla Roadster HAD a 2 speed transmission. It was a huge reliability problem so they dropped it. ICE’s have transmissions because they don’t develop much torque at low rpms, electric motors do develop high torque at low rpms so there is no need for multiple gears.
And there is an efficiency loss every time a gear is involved. Notice none of the EV makers use a multiple gear transmission.
No. Top speed is limited by how much horsepower can be applied to oppose the wind resistance. A 100 hp electric motor will have a similar top speed to a 100 hp ICE if each were geared to reach max HP at the rated max speed.
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Yes, true, but the major downfall of performance electric cars is their lower top speed (than an ICE with similar amounts of power). Take the Model S for example. If that car had just 2 gears, it could go so much faster, and wouldn’t top out at 155 mph. What I’m trying to say is, they could get a higher top speed (similar to an ICE with a similar amount of power) if they geared the electric motors up.
There’s no value in making the S going over 155, now is there? Certainly nothing that would compensate for the great cost of developing and installing a reliable transmission. Many high performance cars are limited to 155 or so because tires become a major problem, too.
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true, but I’m saying for a performance car. If you’re going for the world speed record, you’ll need more than one gear
And for what reason do you need a 155 MPH vehicle on public streets and highways in the first place ?
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It was just a suggestion for a high performance variant, not saying for the power to be used on public streets or highways, like I said, just a suggestion
This is your reason? Or are you joking?
When I come up with an ‘obvious thing to do’ that anyone with a BS in that topic would know, I assume they’ve thought it through, and my obvious idea is actually a bad idea.
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It was a joke, I believe this discussion is over now since I have my answer. Thank you for replying
It was a good question, don’t get me wrong. And Tesla already has the highest-performing sedans available, I imagine they don’t see the need to push it further.
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We’ve recently discussed this very issue in another thread in this forum, and I learned something new in the process: Where an ICE gets peak performance at a relatively low rate of RPM, an electric motor tends to be most efficient at a high rate of RPM.
So, mating a replacement electric motor to a transmission that is designed for an ICE would not work well, because the electric motor would continuously function in its least efficient RPM range, particularly if the transmission is an automatic, but also with a manual transmission, in which the lower gears are not low enough.
Here is a link to that discussion: For the luddites in modern time
The “KillaCycle”, an electric drag bike that managed to do ¼ mile: 7.89 seconds @ 270.36 km/h (167.99 mph), did shift gears, but not with a two speed transmission. It left the starting line with its two motors in series, each motor getting all of the amps the battery delivered, (around 2000 amps) but dividing the voltage (374 volts). Amps = torque and voltage equals speed. Then half ways down the track, the motors were connected in parallel, each motor getting all of the battery’s voltage for high speed, but dividing the amps.
Many diesel electric locomotives do this also, have the traction motors in series at low speed and then switch to parallel at high speed.
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I didn’t know that the Tesla Model X has a dual ratio setup until I read this
This is interesting. I thought that they would use a transformer to step up the voltage then modulate using PWM