Quote:
Originally Posted by Ryland
I've gotten 65mpg while driving 70mph just because I had a tail wind, wouldn't this be like having a constant tail wind?
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It depends on your philosophy, the efficiencies of the pieces in the chain, and how you are charging the batteries. The tailwind energy comes from an outside, completely free source. The motor energy comes from your fuel, if you are proposing an IMA-style hybrid.
Here are two possibilities:
1. If all the energy to boost the car comes from the grid, then you are getting the extra umph to get the tailwind effect at very low cost. Definite gain in essentially every respect, including environmental (although in North Dakota, it could be close to a toss up re carbon footprint -- because they are so heavily into coal).
2. At the other extreme would be a system in which the car still idles at stop lights, and in which regen is not used for braking, and in which the batteries are charged by loading the engine more heavily during cruise (at which time the engine operates at better bsfc than it would at cruise, because it is now producing, say, 23 hp of which 8 are used for charging and 15 are used for propelling the car. This one would not plug in, and work like Honda's IMA system.
You'd have to look at the bsfc chart for your engine, but I'd guess that the engine might be 24% efficient at 23 hp, and 22% efficient at 15hp. Assume the generator is 85% efficient, and the controller is 95% (both of these vary with load, etc, but these could be realistic averages), and that you use lithium x batteries that are 95% efficient each way in receiving and delivering charge. Then, for each unit of energy delivered from the engine to the generator there is an amount delivered back to the engine pulley by the motor at a later time. So we start with say 8 hp-hours delivered from engine to generator during one hour of cruising/charging. The energy returned at some later point is 8 x .85(gen) x .95(cont) x .95(bat in) x .95 (bat out) x .95(cont) x .85(mot) = 4.7 hp-hours out. To do better than break even, then, the electric propulsion aid would have to be delivered when the engine would otherwise be operating at 4.7/8 x .24 = 14% efficient. So if you could turn the engine off (disconnect it from the system mechanically too) and used the electric motor for propulsion at low speeds, you gain efficiency. But if you are using say 10 hp from the engine (at .18 effic) and 10 hp boost from the electric motor (at .14effective efficiency) then you'd be better off to put all the load on the engine, and get 20 hp at .24 efficiency.
The IMA system has no belt losses, and the motor-generator is highly efficient (95% in a wide area of speeds and loads). The whole system is well optimized for what it is. But the difference in around-town mileage for the 2010 Hybrid Civic (40 mpg) and the 2010 Prius (50 mpg) highlights the limitation of the IMA system: for it to be effective, the engine really needs to be too small to achieve desired performance levels, so that the engine is operating at high efficiency levels (heavily loaded) most of the time. The the electric motor can make up for the low engine power to provide acceptable performance. If the engine is already large enough to provide acceptable performance, then you loose the opportunities to drive it at high loads (high efficiency levels) because to do so means you are driving too fast.
HOWEVER...
This ignores the possibilities for idle stop and regen. With them, you can gain back some of the losses of all the energy conversions. The effectiveness in doing so depends upon how you are already driving: if you spend a lot of time on the highway, regen is essentially useless. Remember that hybrids like the Prius are automatically implementing pulse and glide effects (in combination with a host of other refinements). If you are already using techniques like pulse and glide, then hybridization is less attractive.
But back to the first case, where you charge from the grid (or better, from solar panels). Then you are getting the energy at a very low price and not using gasoline to provide the energy to charge the batteries.
Re batteries: Lead acid can be helpful for prototyping, because of low initial cost: use them for a while, then recycle them. For long term usage, lithium x have reached the point where they cost less than lead acid, per mile.
Another battery issues: For regen on a car the size of the civic, it would be nice to use up to 200 amps at 72 volts (which the motor can handle periodically). (This enables the car to decelerate roughly as well as it accelerates on almost 20 hp.) This means that the battery pack should be able to charge at 200 amps. Depending on chemistry, that may mean a larger pack than would be dictated by other concerns alone.