GreenHybrid - Hybrid Cars - View Single Post - An Unpalatable Fact (with apologies to Al Gore!)

SPL · #**125** (**permalink**) 07-07-2008, 01:11 PM

DesertDog — I apologize for the long delay in responding to your recent posts #123 and 124. I've been away overseas for part of this time, and have been rather busy since I got back. So I haven't had the opportunity to give them the needed careful consideration until now.

Yes, I agree with you that I was wrong not to have taken into account the increased thermodynamic efficiency of the ICE during the recharge phase (3) in my post #29. The higher loading of the engine due to the NiMH battery recharging would indeed have increased its efficiency significantly, as you point out. And, yes, some of Miller's "dots" are indeed misplaced in his efficiency diagrams, and mean that the ICE BSFC numbers that I used need to be revised. By the way, I never claimed that it was impossible to beat Toyota's ECUs in normal driving, only that it seems to me to be unlikely that you can generally do so without resorting to extreme measures, like pulse-and-glide. We have the benefit of knowing what's coming up on the road ahead (which the car doesn't know), but the car has the benefit of having full knowledge of the ICE's and MGs' parameters, and so can optimize its moment-by-moment strategy better than you can.

However, I must disagree with your use of an ICE power of only 9.84 kW during recharging. That number is the one I calculated for the ADDITIONAL (not total) ICE power required during recharging. The total ICE power needed during this phase is thus 6.47 + 9.84 = 16.31 kW. At this power, I read an even better BSFC figure of ~220 g/kWh from Miller's diagrams. Using your example speed of 40 miles per hour, what we need to compare is:

fuel usage for steady-state driving at 40 miles per hour for a given distance (or time) at an ICE power of 6.47 kW and BSFC of 260 g/kWh;
pure EV driving at 40 miles per hour for half this distance (or time), followed by fuel usage for ICE-powered steady-state driving at 40 miles per hour for the other half of the distance (or time), during which the NiMH battery is being recharged, at an ICE power of 16.31 kW and BSFC of 220 g/kWh.

I do not understand why you use 0.8 miles of pure EV followed by 1.5 miles of recharging. I assumed that the battery recharging would be done at the SAME rate as the discharging. This would imply the same distance (or time) for both phases (2) and (3) — each being half of the total. This being so, I compute (using 1 hour for the total time for the sake of simplicity, since it doesn't affect the answer):

1 h (40 miles) @ 6.47 kW => 6.47 kWh => 1682 g of fuel => 2.273 L of fuel => 3.53 L/100 km => 66.6 mpg [as also computed by you] for steady ICE-powered driving;
0.5 h (20 miles) of EV @ 5.85 kW => 2.925 kWh from the battery during the pure-EV phase (no fuel used), followed by 0.5 h (20 miles) of ICE power @ 16.31 kW => 8.155 kWh => 1794 g of fuel => 2.424 L of fuel => 7.53 L/100 km => 31.2 mpg during the recharge phase, giving 3.77 L/100 km => 62.5 mpg for the overall trip of 1 h (40 miles).

But, this is a REDUCTION in FE, not the increase that you claim! Am I missing something? Or was your calculation indeed incorrect? Granted, it's not a great reduction in FE, and the way the numbers fall will depend on the assumptions regarding mechanical-to-electrical-to-chemical efficiencies (and vice versa) in the MGs, inverter/converter electronics, and NiMH battery. Although these calculations are for a Prius, it's interesting to note that the TCH has significantly increased efficiencies, relative to the Prius, in almost all MG and electronics parameters (see the attached very interesting ORNL report — but they don’t measure the NiMH battery). This would seem to confirm my comment in post #58 that the results of my careful test (post #48) show lower losses in my TCH than I had initially guesstimated. I am not aware of any published BSFC data for the TCH's 2.4-L ICE. It's no doubt essentially similar to the Prius' 1.5-L ICE, but without hard data one cannot calculate any numbers for the TCH.

Stan