Ran some tests this weekend showing significant impact of speed and AC on FE for my HCH2.

Conclusion - try to stay below 65mph and dont use AC if you can help it (Duh).

3/15/2008 Update - after a year of tanks and temps going from 20-90F, there seems to be a pretty good relationship between outdoor temp and FE. Looking forward to spring next week.

first pdf has the speed, ac test vs fe

second pdf has the tank FE vs outdoor temps

 

EXCELLENT! Good methodology and analysis.

Have you considered any alternatives to the AC problem? I've been thinking about an ammonia absorption cycle using exhaust heat. Recently I've found a place for the heat exchanger, behind the rear bumper cover.

Bob Wilson

 

Thanks - will be repeating on the Dodge Grand Caravan this week - Wife will LOVE that.

Regarding AC - try not to use it, drive in before full sunrise etc. Thought about vortex spot coolers but they require compressed air - which takes energy...

 

Excellent work, and very revealing too.

However, I do have a little comment/question:

You set the temperature at 58F. This is a rather low initial temperature and does not take into account what the cabin temperature was at the time of the test and the mode the climate control system was working under. Correct?

Here's my point: By setting it at an initial temp of 58F there's a likelihood that you were driving the compressor at its highest load which imposes a measurable impact in to gas engine.
Now, assuming that you had the AC on Auto, the FE impact would likely vary dramatically because when the cabin temp reaches the low target of 58F, it would start to use the electric compressor to keep the temp, and thus impose a negligible load on the gas engine.

Because of this, some of us use the AC on Auto and never set the initial temp so low. Instead, we set the initial temp to a high value and then gradually we lower the setting so as to not impose a load on the gas engine. The gas engine will only power the AC compressor if the temp differential is too big. For small differentials the electric scroll compressor will do all the work, thus minimizing (not eliminating) the impact on FE.

Of course, taking this into account would really complicate things quite a bit, but your study remains a very clear indication of the impact of AC on FE.

Cheers;

MSantos

 

MSantos,

Thanks for the comments.

I thought about the AC settings question, but had somewhat of a time constraint where I could not wait for Auto at xxF degrees to be steady state. My intent was to try to illustrate the impact of FE by using max AC. I thought setting it to 58 (not in AUTO) and once through air would keep the AC load on the engine.

I also wanted to avoid forced regen, so I limited the the consecutive conditions with AC to two. I would have tracked the SOC start/end as well, but had to keep eyes on the road. It did stay between 5-7 bars though.

Certainly, most drivers will not run AC at max settings for extended periods. Maybe these results just show a worst case impact of using AC versus none at all. What suprised me a little was that at "high" speeds, the additional impact of AC load is smaller.

 

You did an excellent analysis! Curiously, I was doing something similar last week - wanting to quantify the cost of air conditioning. I have a loop route convenient to home, 20.2 miles with only two stop signs (which I roll through) and speed limit varies from 35 to 55. Elevation varies from 5050 to 5200, and wind can be a significant factor. On a day when temp was 95F and wind moderate I drove my HCHI 8 mi. out to the start with AC on and thermostat set to 77F, to get the cabin temp stabilized at the start. Drove the loop with AC on (77F), then did it again with AC off. Reset trip A at start and again after the first loop, and noted elapsed time.

Results:

AC on: 76.8 mpg at avg speed 43.3 mph, or .564 gph.
AC off: 85.1 mpg at avg speed 41.8 mph, or .491 gph.

Conclusion: at these speeds and thermostat setting, my AC costs .073 gal/hr, or about 20 cents per hour. Pretty cheap! Can we conclude that setting your thermostat to 58F cost you an extra 50 cents an hour?

Dave

 

Thanks for your results too! I was hoping to run another set of tests soon going from say 30-55mph - your 85mpg at 42mph is impressive! Did you use CC for this?

Thanks also for the 20 cents/hour figure - puts things in perspective. We will be making a 16 hour trip to Disney this fall and I am just fine "paying $3.20" to keep my Wife/kids happy with AC when needed.

I think you might conclude that running the AC at 58F will cost you 50 cents per hour - if doing about 55mph. At 70mph, it seems you are already working the ICE harder and the AC wont cost as much in that case.

 

learned to post an image, meant to earlier this summer...

 

Kudos on the image. Questions about the 65-70 mph mileage.

This is an unexpected result. It is as if something not so good is happening to the HCH at speeds at and above 70 mph but the curve really trends down at 65 mph. Yet curiously the AC curve inflects towards the non-AC at 70 mph. That is also curious.

Now I wish we had an HCH to instrument. Questions to ask:

• ICE rpm vs speed
• ICE power vs rpm (requires torque sensor)
• HCH specific fuel consumption vs rpm
• transaxle temperature vs speed
• tuft testing?

With my 03 Prius, we know there is an MG1 speed limit that causes the mileage to take a nose dive above 68 mph. The limited amount of 04-current Prius data suggests they don't have this problem:

The 03 Prius data is without AC but the modern Prius is with AC.

Bob Wilson

 

Bob, why do you say the 65 and 70 mph are unexpected?
It seems to make sense to me. Isn't overcoming the wind resistance going up (exponentially) as speed increases? I think what spartybrutus' chart is showing is at higher and higher speeds, that the SPEED itself is the greatest hindrance to MPG, and that the AC of (relatively) minimal effect.