jawnn, I assume you still have the same constraints as in your
earlier thread?
400 pounds, 14% slope? What is the average percentage of time where you will be climbing hills, compared the time when you'll be going downhill/level?
A no-battery situation is probably not feasible. This is because the electric motor you will need is on the order of 1500 watts (2 HP.) Small DC alternators/generators are on the order of 50% efficient, so, you would need at
least 4 horsepower to push the bike up hill when using this approach. So, you're looking at approximately a 160cc Honda or equivalent. Note that this doesn't address the controller efficiency or the DC motor efficiency. (A good controller is about 90% efficient - a good motor about 80% efficient. 90% times 80% = 72%, so the SYSTEM efficiency is on the order of 36%. Meaning that you would
actually need 4200 watts from a motor, or approximately 5.6 HP...
) On the plus side, no power is needed when going downhill, and maybe 2HP when on the flat. (those efficiency losses hurt.)
So, let's move away from your motor - generator - motor, no battery option. We'll add batteries to supplement a smaller gasoline motor when going uphill and accelerating. Generator voltage will drop as the load increases, to the point that the battery voltage is higher than the generator voltage, and it (the battery) is thus providing the bulk of the current when climbing, supplementing the power.
A smart controller will take the power generated as you go downhill, and pump it back into the battery (about 25% of it, anyway, when accounting for efficiency losses) So, the motor would need to replace about 75% of the power used in hill climbing. If you have a series of hills, where you go up, then down hill, you are looking at a 50% 'duty cycle.' (actually probably somewhat higher than this, as you can go downhill faster than you went up hill...) But, we assume 50% for the moment. This means that your generator will provide 100% of it's rated output when going uphill, and at least 75% going downhill (to recharge the battery) But, with the generator at about 50% efficiency, and the charger at about 70% efficient, only about 35 % of the power from the motor actually gets stored in the battery. To recharge the 75% of the power used from the battery when climbing in the same amount of time as when climbing, the motor would need to run at about the same power level as that needed for climbing.
Lets assume that the motor/generator set can supply half the power for the climb. (and, you have a 3 HP motor spinning the generator.) The battery supplies the other half. And, you climb for 5 minutes, then descend for 5 minutes. So, the battery loses 3750 watt-minutes of power during the climb. When descending you regain about 940 watt-minutes by regenerative action. You would need 2810 watt minutes from the MG set and the charger. The 3 HP motor-generator-charger has the capacity to restore almost 3920 watt-minutes, so, the motor would need to run at 100% for a little more than 3 and a half minutes on the way back down.
However, since you're
probably going downhill faster than you were able to climb, a 3 HP motor is what you would need for the worst-case scenario of a 'sawtooth' landscape, where you spend half the time climbing, and the other half descending.
If you intersperse level areas into the equation, the motor power requirements are reduced, so, you could
probably get away with the 2.5 HP Honda.
However, alternators are are fairly heavy, and you're adding a gas motor to the mix. Plus, you'll need some batteries as well. Granted, not as much as a full electric solution, but they need to be thrown into the weight mix as well...
So, overall, this hybrid motored bike approach doesn't appear to be as efficient as a gas motor, directly driving the bike... (with a 2.5HP honda, coupled through a CVT, you would need 100% of the rated power during the climb, nothing on the descent, and about 50% available power when cruising on the flat.)
***
Hybrid
auto systems are feasible, because as power goes up, so do efficiencies. And, a Prius, for instance, stores quite a bit of energy in it's batteries. Enough for 15-20 minutes of driving around. A 200 KW system is over 93% efficient, for instance. A 25KW generator system, similar to what a hybrid car would need, is probably on the 80% efficiency level. This larger storage capacity, coupled with higher efficiencies, tip the scales, so that it can be more efficient than directly driving the wheels with a larger motor.
