Electric sports cars, why such low top speeds?
Discussion
I was watching this video about the electric Delorean prototype. The film says the electric motor is 250hp and the car can do 0-60 in under 5 seconds, which sounds quite good. Then they go on to say it has a top speed of 'over 100mph', which isn't impressive at all. With a conventional 250hp I would expect top speeds of 140-150mph.
I've seen this with other electric cars too, for example the Tesla Roadster has even more power but has a top speed of 125mph. What is it that makes these cars so slow compared to their piston powered contemporaries?
I've seen this with other electric cars too, for example the Tesla Roadster has even more power but has a top speed of 125mph. What is it that makes these cars so slow compared to their piston powered contemporaries?
Simply because most of the current crop of EV's don't have multispeed gearboxes. Hence, you have to have a suitable low ratio in order to pass the required gradability and kerb mounting tests, but this "low" gear means your emachine is well beyond its base speed (the rpm at which it makes peak power) as the vehicle speed reaches the higher values. Using tricks like "field weakening" you can extend the "constant power" region of the Emachine significantly, but thermal and demag limits (and inverter capability) soon become issues in a practical system.
As a further issue, most battery systems do not store much energy (compared to say 65l of gasoline) so their range at high speed, where power demands are increasing with at least the square of the vehicles speed, is very very poor. It makes sense in these cases to limit what the system can output.
As a further issue, most battery systems do not store much energy (compared to say 65l of gasoline) so their range at high speed, where power demands are increasing with at least the square of the vehicles speed, is very very poor. It makes sense in these cases to limit what the system can output.
NISaxoVTR said:
Interesting, so in the future can we expect to see the introduction of gearboxes in these cars or will we just have increasingly powerful motors to pull a longer gear? Does a sports ev with multiple gears currently exist?
not sure...Tesla was originally to have a two speed box, but they failed to get one to work as they wanted apparently, this is why the original performance spec was not achieved.
I guess the next bit is EV's are all about economy, so adding a gearbox (weight/drag/etc) kind of goes against the grain...
High top speeds are about pure sustained power output, dissipated by aero drag. So the real reason surely has to be battery capacity, and motor temeperature. No point winding it out for a headline number if it utterly kills the battery in a minute or three and the motor overheats.
Transient high outputs for acceleration no problem, it's only 6-10secs at a time of course.
There's a huge difference between having a potential output of XXX Kw and holding that output for any length of time. Remember that most of the battery packs are pretty small, 20-30KwH or thereabouts (equivalent to roughly a gallon of petrol - but > half a tonne heavier).
Transient high outputs for acceleration no problem, it's only 6-10secs at a time of course.
There's a huge difference between having a potential output of XXX Kw and holding that output for any length of time. Remember that most of the battery packs are pretty small, 20-30KwH or thereabouts (equivalent to roughly a gallon of petrol - but > half a tonne heavier).
Weight isn't the be-all or end all of economy in an pure EV, thanks to the ability to use regenerative braking (it does have some effect, as overall round trip efficiency from chemical energy in battery to KE in the vehicles mass and back to CE in the battery is only typically a maximum of ~65%).
The issue with multispeed transmissions is cost. EV's are expensive because the battery systems, inverter systems, and Emachines are expensive (even more so at the low volumes that currently EVs sell in) so saving money on a simple transmission system is usually a high priority for EV developers. Added to which, once you have to "change gear" you have to have a level of motor and torque control comenserate with speed matching and inertia cancelling during the shift event.
The issue with multispeed transmissions is cost. EV's are expensive because the battery systems, inverter systems, and Emachines are expensive (even more so at the low volumes that currently EVs sell in) so saving money on a simple transmission system is usually a high priority for EV developers. Added to which, once you have to "change gear" you have to have a level of motor and torque control comenserate with speed matching and inertia cancelling during the shift event.
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