This may have been discussed before, but I haven't seen anyone lay this out in one place. Reading up on EVs, range is one of the top concerns if not the top concern. Of course some of this is down to the shorter range of BEVs, but there is a lot of discussion about driving slower with EVs to extend the range, and that isn't as big a concern with ICE cars.
The information is all out there in different places, but I had to dig out the pieces from various places.
Before I started looking for efficiency curves, I suspected the efficiency curves for EVs was narrower than for an ICE.
I found the efficiency curves here:
http://blog.automatic.com/cost-speeding-save-little-time-spend-lot-money/
https://www.teslamotors.com/blog/model-s-efficiency-and-range
This also has some Model S curves:
http://insideevs.com/heres-how-speed-impacts-range-of-the-tesla-model-s/
For ICE cars, the Y-axis is MPG and for the Model S it's miles of range, but they will work for this discussion. The peaks of the curves and width of the peaks is most important. On the ICE/hybrid chart, the Prius gets 50 MPG or better between about 33 mph and 72 mph, with 55 MPG or better between about 35 mph and 65 mph. If you discount the small peak at 60 MPG, the range for top 10% gas mileage is about 40 mph wide.
The BMW 328i, Honda Civic, and Subaru Outback have narrower curves, but the top 10% curves for these cars are:
Outback - 35-70 mph
328i - 40-78 mph
Civic - 35-78 mph
There are a number of Model S curves out there and all of them vary a bit but all of them peak around 20-30 mph. The top 10% range is above about 405 miles, which extends from about 12 mph to about 38 mph. That's not only a less useful speed range for maximum range, but only about 2/3 as wide.
I was wondering why the peak efficiency is so low and narrow for EVs and higher and wider for ICE. ICE cars have transmissions and EVs generally don't. The major productions BEVs: Leaf, BMW i3, and Model S all have 1 speed transmissions. Why is this? Wouldn't a multi-speed transmission help with the efficiency curves on an EV?
Due to the nature of electric motors vs ICE, the answer isn't that simple. Multi-speed transmissions loose some of the energy going through the transmission. With an ICE, which is only about 30% efficient to begin with, the losses aren't very noticeable. Plus an ICE needs a transmission because the range of useable horsepower and torque is pretty narrow band of RPMs. A single speed ICE geared to get started would rip itself apart as it approached highway speeds and one geared for highway speeds would stall out trying to get started. A transmission with multiple gears is needed to keep the engine in that narrow band. ICE cars are getting more and more gears and CVTs to try and squeeze out a little more efficiency.
ICE engines have 0 torque at 0 RPM, which is why they need to idle. Put a load on an ICE that is running too slow and it dies. Electric motors are the opposite, they have maximum torque at 0 RPM. This is the reason there are all those drag race videos out there of P85Ds and P90Ds blowing away fancy sports cars in drag races. A drag race favors the electric motor car. Get up to highway speeds and those fancy sports cars will usually win.
A multi-speed transmission on an EV might be able to boost that efficiency range up a bit, but there are drawbacks, which is probably why it isn't being done. Electric motors can change torque much faster than an ICE. This puts massive loads on a transmission and people who have experimented with transmissions on EVs have often reported the transmission tends to rip itself apart in a very short period of time. Beefier transmissions can be made to handle the beating an electric motor will put on it, but that will add weight and complexity and take up more space.
For designs so far, it's easier to just live with the trade offs. There is talk of transmissions for EVs out there, but so far no idea has made it into production as far as I can tell. Tesla experimented with a two speed transmission for the Roadster, but it had reliability problems. I suspect it ripped itself apart from the sudden torque changes.
As a sort of aside the energy density difference between gasoline and batteries is pretty staggering too. The Model S's battery pack is 96 gallons in volume. That's less than 1 KWH/gallon, less than 30X lower energy density than gasoline, which has 33 KWH/gallon.
Even at 30% efficiency, an ICE is getting close to 10 KWH/gallon out of the car. A 30 MPG car is consuming 1100 KWH/Mi, but since 70% of that energy is being wasted as heat and noise, the actual energy being used to turn the wheels is around 330 WH/Mi. Right in the range you see with a Model S. A 15 gallon tank gets a 30 MPG car 450 miles of range.
If the Model S had a battery that had the energy density of gasoline, it would have a 3168 MWH battery good for about 9600 miles on a charge. Even the most anxious would probably have no range anxiety then!
Anyway, many people here probably already know this, but I figured I'd put it out there in case anyone was wondering like I was...
The information is all out there in different places, but I had to dig out the pieces from various places.
Before I started looking for efficiency curves, I suspected the efficiency curves for EVs was narrower than for an ICE.
I found the efficiency curves here:
http://blog.automatic.com/cost-speeding-save-little-time-spend-lot-money/
https://www.teslamotors.com/blog/model-s-efficiency-and-range
This also has some Model S curves:
http://insideevs.com/heres-how-speed-impacts-range-of-the-tesla-model-s/
For ICE cars, the Y-axis is MPG and for the Model S it's miles of range, but they will work for this discussion. The peaks of the curves and width of the peaks is most important. On the ICE/hybrid chart, the Prius gets 50 MPG or better between about 33 mph and 72 mph, with 55 MPG or better between about 35 mph and 65 mph. If you discount the small peak at 60 MPG, the range for top 10% gas mileage is about 40 mph wide.
The BMW 328i, Honda Civic, and Subaru Outback have narrower curves, but the top 10% curves for these cars are:
Outback - 35-70 mph
328i - 40-78 mph
Civic - 35-78 mph
There are a number of Model S curves out there and all of them vary a bit but all of them peak around 20-30 mph. The top 10% range is above about 405 miles, which extends from about 12 mph to about 38 mph. That's not only a less useful speed range for maximum range, but only about 2/3 as wide.
I was wondering why the peak efficiency is so low and narrow for EVs and higher and wider for ICE. ICE cars have transmissions and EVs generally don't. The major productions BEVs: Leaf, BMW i3, and Model S all have 1 speed transmissions. Why is this? Wouldn't a multi-speed transmission help with the efficiency curves on an EV?
Due to the nature of electric motors vs ICE, the answer isn't that simple. Multi-speed transmissions loose some of the energy going through the transmission. With an ICE, which is only about 30% efficient to begin with, the losses aren't very noticeable. Plus an ICE needs a transmission because the range of useable horsepower and torque is pretty narrow band of RPMs. A single speed ICE geared to get started would rip itself apart as it approached highway speeds and one geared for highway speeds would stall out trying to get started. A transmission with multiple gears is needed to keep the engine in that narrow band. ICE cars are getting more and more gears and CVTs to try and squeeze out a little more efficiency.
ICE engines have 0 torque at 0 RPM, which is why they need to idle. Put a load on an ICE that is running too slow and it dies. Electric motors are the opposite, they have maximum torque at 0 RPM. This is the reason there are all those drag race videos out there of P85Ds and P90Ds blowing away fancy sports cars in drag races. A drag race favors the electric motor car. Get up to highway speeds and those fancy sports cars will usually win.
A multi-speed transmission on an EV might be able to boost that efficiency range up a bit, but there are drawbacks, which is probably why it isn't being done. Electric motors can change torque much faster than an ICE. This puts massive loads on a transmission and people who have experimented with transmissions on EVs have often reported the transmission tends to rip itself apart in a very short period of time. Beefier transmissions can be made to handle the beating an electric motor will put on it, but that will add weight and complexity and take up more space.
For designs so far, it's easier to just live with the trade offs. There is talk of transmissions for EVs out there, but so far no idea has made it into production as far as I can tell. Tesla experimented with a two speed transmission for the Roadster, but it had reliability problems. I suspect it ripped itself apart from the sudden torque changes.
As a sort of aside the energy density difference between gasoline and batteries is pretty staggering too. The Model S's battery pack is 96 gallons in volume. That's less than 1 KWH/gallon, less than 30X lower energy density than gasoline, which has 33 KWH/gallon.
Even at 30% efficiency, an ICE is getting close to 10 KWH/gallon out of the car. A 30 MPG car is consuming 1100 KWH/Mi, but since 70% of that energy is being wasted as heat and noise, the actual energy being used to turn the wheels is around 330 WH/Mi. Right in the range you see with a Model S. A 15 gallon tank gets a 30 MPG car 450 miles of range.
If the Model S had a battery that had the energy density of gasoline, it would have a 3168 MWH battery good for about 9600 miles on a charge. Even the most anxious would probably have no range anxiety then!
Anyway, many people here probably already know this, but I figured I'd put it out there in case anyone was wondering like I was...