The 150 mpg is based on a combination of the all electric 40 miles and some distance on gas. The straight gas mpg is supposed to be about 50. Using the cars momentum to coast instead of capturing it via regenerative braking would increase the mpg by some amount, but I doubt it would be worth it for most people.
A friend of mine just discovered hypermiling. His daily commute is about 60 miles round trip. With his '07 Accord, he measured an increase from 35 to 48mpg, consistently over 3 tanks.
With his success, I thought about it and discovered that by choosing a slightly different route with a more even grade, I could coast the entire way from home to work. (But unfortunately not the other way 'round )
Last edited by Joshua Bretz; 06-04-2008 at 10:21 AM.
The reason that hypermiling works (even on flat ground) is that it is more efficient to run the engine part of the time than all the time. The same could apply to the Volt range extender ICE, when operating in charge-sustaining mode: it should always run at its peak efficiency point (in Joules/Liter) while on, cycling on and off every couple minutes to keep the SOC in the desired range.
This method works even better for power electronics. Advanced PWM controllers will enter a "pulse skipping mode" or "pulse frequency mode" where the power switches are kept off until the power supply output droops out of regulation. In this way, the "idle" frequency can go to zero as the load approaches zero.
The electric drive motor freewheeling loss is going to be very small - there is no "compression braking" effect. However, there is still some power loss in the motor inverter at "idle" that comes out of the battery. I know that this will be better than an ICE, but it could end up mattering a lot for crusing range: if the motor is designed for 140kW peak power, crusing might require less than 1/10th of that power. And, in a normal driving cycle, most of the time is spent at low power. So, light load efficiency could matter just as much as heavy load efficiency.
Last edited by Joshua Bretz; 06-04-2008 at 02:22 PM.
Are you sure its more efficient to cycle an ICE than to run it continuously at an efficiently designed stable RPM ?? If yes, it contradicts what I have been fed for about 35 years. The way I see it, as soon as you turn off, or cycle, you have to slow down due to drag. Then you have to re-start, which is inefficient, then accelerate, which is less efficient, and then the cycle repeats itself. That was my own logical conclusion. Do you have any links to any info which would help me understand how cycling can work better ?? Always open to new ideas. Thanks.
The way I see it, as soon as you turn off, or cycle, you have to slow down due to drag. Then you have to re-start, which is inefficient, then accelerate, which is less efficient, and then the cycle repeats itself.
I'll try and answer from a bunch of different angles.
An engine sucks a lot of fuel just to keep spinning. The less time the engine is on, the less fuel is lost this way.
Imagine two hills next to each other, of the same height. I start at the top of the first hill, heading to just below the top of the other hill. I try several different methods:
1) If I just leave the engine off, and coast down, I'll end up almost at the top of the other hill. I'm not at the top of the other hill because of aero and tire losses. To this point, zero fuel used.
2) If I do the same thing in neutral, I use the amount of fuel that it took to keep the (damn) engine idling the whole time.
3) If I keep the engine in gear, I use more fuel because I'm compression braking the whole time, and then have to make up that lost energy by using more fuel going up the second hill.
4) If I try and keep a constant speed down and up the hill, then I used extra fuel to accelerate to that speed, pressed the brakes to keep from going too fast, and then had to use more gas going up the other hill to make up for both the brake losses and the motor losses.
#4 is how most people drive, because it is safer, and usually faster because you accelerate to the legal limit faster. But it isn't more efficient - in fact, to the extent that the driving differs from #1, it will require extra energy. Now, #1 goes really slow and then fast at the bottom and then slow again, and it's really annoying to everyone to be speeding up and slowing down all the time. But the range extender in the Volt could cycle on and off all it wants, since it is disconnected from the wheels of the car.
The point is to analyze from an energy perspective.
Cruising on level ground doesn't require a lot of power. Maybe 10hp to overcome the aero/tire losses? Assume that the peak efficiency point of the engine is 70hp. That means that running it at 10hp is as if we just left the engine at idle, wasting gas. (For the moment, let's ignore the energy used to start a warm engine) If we cycle the engine on for 10 seconds and run at 70hp, and then off for 60 seconds, the average power is 10hp. But the engine was off for most of the time.
Now running for 10 seconds at 70hp would speed up the car a lot, and coasting for 60 seconds would slow down the car a lot, so this is impractical.
But in the Volt, the battery is absorbing the power difference between the generator and motor, and the car is maintaining a constant speed. It would be pretty reasonable to run the ICE for 100 seconds (the ICE generating 30hp and the electric motor using a constant 10hp) and then turn the ICE off for 200 seconds (the electric motor using a constant 10hp) - the battery SOC would only change by 2.5% each cycle.
As compared to the generator running at 10hp constantly, I would expect the fuel economy in charge-sustaining mode to increase from 50mpg to ~80mpg with a 1/3 ICE duty cycle.
Last edited by Joshua Bretz; 06-05-2008 at 09:38 AM.