I too was raised that way only it was 4\40 around here. I also prefer driving with the windows down, sunroof open and radio cranked up somewhat.

I don't know about others but it's always refreshing the way you show the numbers and put things back in perspective. However, don't let my wife hear you say those things. That hundred bucks can help supplement the expense of a new toy for me...I mean my 2 sons!

It really does put things back in perspective. I wonder what it would be like if we could get a good portion of America to try half as much as some of us.

FWIW I drove with the are on this morning and averaged 50mpg for my commute. This is the highest I have ever gotten on a single trip or tank in my 80some days of ownership!!!

 

rBurt-

have you had the dealer change any of the HVAC settings on your car? ie AUTO and recirc

 

I recently discovered if you run the AC on HIGH for two minutes and switch to Auto and ECO the temp is very comfortable for the entire trip. It can take the ECO awhile to drop the temp on hot days. I have seen the same MPG drop using the AC as posted above.


Bill

 

One can compute some numbers here. According to the TCH Manual, the electrical A/C compressor draws up to 6.1 kW of electrical power. It's driven by a variable-speed, 3-phase, AC motor fed via its own inverter from the high-voltage NiMH battery. The variable-speed feature means that its speed (and so power usage) can be set based on the user's cooling demand, and independently of the engine speed. This means that it can be much more efficient than a belt-driven ICE compressor. Let's consider three scenarios. I'll assume that the car is travelling at a constant 60 miles per hour, achieving an FE of say 40 mpg, and compute the effect of the A/C compressor (ignoring the electrical power drawn by the radiator and passenger compartment fans) at full power, at half power, and at one-quarter power:

1. At full power (6.1 kW) — In 1 hour, 6.1 kWh of electricity is consumed. This equals 6.1x3.6 = 22 MJ (megajoules) of energy used. Don't forget that this electrical energy must be replenished by the ICE — it doesn't come for free! Assuming that the ICE's efficiency is say 33%, the ICE has to use an additional 66 MJ of energy per hour from the gasoline. Gasoline has about 121 MJ of energy per galUS. This means that the A/C at full power uses at least an additional 66/121 = 0.55 galUS per hour. At 60 miles per hour and 40 mpg, the ICE was using 1.5 galUS per hour. It's now using 1.5+0.55 = 2.05 galUS/h, and the FE drops from 40 mpg to 60/2.05 = 29.3 mpg, an FE drop of 10.7 mpg.
2. At half power (3.0 kW) — In 1 hour, 3.0 kWh of electricity is consumed. This equals 3.0x3.6 = 10.8 MJ (megajoules) of energy used. Assuming that the ICE's efficiency is say 33%, the ICE has to use at least an additional 32.4 MJ of energy per hour from the gasoline, representing an additional gasoline usage of 32.4/121 = 0.27 galUS per hour. It's now using 1.5+0.27 = 1.77 galUS/h, and the FE drops from 40 mpg to 60/1.77 = 33.9 mpg, an FE drop of 6.1 mpg.
3. At one-quarter power (1.5 kW) — In 1 hour, 1.5 kWh of electricity is consumed. This equals 1.5x3.6 = 5.4 MJ (megajoules) of energy used. Assuming that the ICE's efficiency is say 33%, the ICE has to use at least an additional 16.2 MJ of energy per hour from the gasoline, representing an additional gasoline usage of 16.2/121 = 0.13 galUS per hour. It's now using 1.5+0.13 = 1.63 galUS/h, and the FE drops from 40 mpg to 60/1.63 = 36.8 mpg, an FE drop of 3.2 mpg.

So, the FE drop depends on your current FE (i.e., your speed) as well as on the amount of A/C power you're using, and my estimates show that it can vary over a wide range. The drop will be worse if your current FE is high, as in the examples above. If your current FE is poor (say, you're travelling faster), the FE drop caused by the A/C will be less. The important figure is not the mpg, but the extra galUS/h used by the A/C.

Stan

 

Good analysis. But you are showing the problem with the US miles per gallon metric rather than the more reasonable gallons per mile, since we drive a certain number of miles and consume gas rather than driving a certain number of gallons! (The rest of world is better this way with liters per 100 kilometers).

When view that way, the fuel efficiency drop is independent of your current FE. Using mpg makes it appear that air conditioning is more brutal in our TCH than it would be in a big SUV, but that isn't the case at all!

 

Stan,

Great job on this! Your efforts are very impressive!!

What affect does the ECO mode have on your information? I know the internal tempertures are higher so the overall energy used is less.

I thought I would check on this?

Thanks!


Bill

 

talmy — Yes, I agree that a reciprocal consumption measure like mpg (or km/L) is not a convenient unit for calculational purposes, and in many respects a direct consumption measure like galUS/mile (or L/100 km) is better. But even using a direct consumption measure is not very sensible here. Suppose you're sitting stationary in a parking lot with the A/C on. Neither mpg or gpm would be a meaningful measure in this case, since you're not moving. The galUS goes up steadily from A/C usage while the miles stay constant; but the change in the galUS/mile number depends on the current number of miles in the denominator of the calculation.

The penalty for A/C usage is approximately independent of whether you're moving or not — it is a function of time and not of distance. So, the best way of measuring it is in terms of the additional fuel usage per hour: galUS/h (or L/h).

It's interesting to estimate the length of time for which the NiMH battery can sustain A/C usage before the ICE needs to restart in order to replenish it. The battery consists of 204 cells each of 1.2 V and 6.5 Ah capacity. Its total energy storage is thus 204 x 1.2 x 6.5 = 1 591 Wh. Now, Toyota only allows 40% of this total capacity (the range 40%-80% of full charge) to be used; i.e., 636 Wh. So we can deduce that:

1. At full power (6.1 kW) — The battery can power the A/C for less than 6.3 minutes.
2. At half power (3.0 kW) — The battery can power the A/C for less than 12.7 minutes.
3. At one-quarter power (1.5 kW) — The battery can power the A/C for less than 25.5 minutes.

These numbers seem to be in reasonable agreement with my own experience.

HyCAMBill — The ECO switch, according to Toyota, does have a limiting effect on the amount of electrical power used by the A/C. I haven't tried to quantify this myself, however.

Stan

 

Not sure if I understand all this, or not, but let me ask a simple question (and hope for a simple answer) in a way that I can understand it.

Let's assume you are coasting down a giant mountain (let's say Mt. Everest, assuming it was paved) for your entire trip.
The car isn't using any gas and the coasting regenerates the battery.
If you turn on the AC (ECO or not), the battery powers the AC.
Assuming further that the decrease in stored charge (due to the need of the AC) will be quickly replenished by the regeneration from the coasting... will the gas engine ever turn on?
I know that if the stored charge gets low enough the ICE turns on.

(I know what you're thinking...it's a trick question, since you wont need AC on Mt. Everest, but this is a hypothetical situation, obviously)

 

haroldo — In your scenario, since the rate of regeneration would exceed the rate of electrical energy usage from the NiMH battery, the only reason that I can see for the ICE to need to restart periodically is in order to keep the catalytic converter hot enough to meet government regulations. [Also, possibly, since the transmission's oil pump is driven by the ICE's shaft, there might be a transaxle lubrication issue if prolonged coasting with the ICE "off" occurred. There is, however, likely adequate splash lubrication of the transaxle under these circumstances. Since Mt. Everest is quite steep in places (!), the ICE may actually be spinning in fuel-cut mode on your way down — definitely so if you had to shift to 'B' to get some engine braking — in which case there's no lubrication issue with the transaxle in the first place.] Be sure to report back to us what your experience is!

Stan

 

Thanks!
I know it was an absurd scenario, but it clarified the issue.
The original question (18 posts ago) was the effect of AC on mileage, and to the extent that the battery would be able to keep itself charged (due to some pretty serious coasting and regeneration) the ICE doesn't necessarily come into play.
With 60 miles per hour+ highway driving, the battery usually does a good job of keeping itself reasonably fully charged, so the AC shouldn't effect mileage too much, right?
Thanks!