AC use with minimal FE loss

Discussion in 'Honda Hybrids' started by msantos, Jul 23, 2008.

  1. msantos

    msantos Eco Accelerometrist

    I've often been asked the "AC question" and how to minimize the FE hit of its use in the summer months. This thread is an attempt to leverage the built-in capabilities of the the AC system on the HCH-II not only for driver "comfort" but also to enhance the IPU's thermal management (The IPU is that thing behind the rear seat with an intake vent at the top ;) ).

    Old dog seeking new tricks

    As most of us know, using AC will always have some sort of impact on fuel efficiency. Some would say that the smaller the primary power source (ICE) the more significant the FE hit will be and that is definitely NOT far from the truth - particularly on the HCH-II.

    At first glance many will see this as a biblical truth and for good reason: no matter how we look at AC use, it is a significant loss of energy that will be diverted from your gas tank straight into the thin air.

    So, what else can we do? Never turn the AC on?

    Fortunately for us, the answer is not as simple as black & white and it lies instead in the wide range of shades of gray somewhere between those two extremes.
    Whether we know it or or not, the climate control on the HCH-II is an integral part of the hybrid system and it plays a very significant role in the behavior and relative health of your hybrid specific components.

    How so?

    Without getting too technical, let us just say that the IPU really needs to breathe and that many of the components housed in the IPU enclosure have different temperature tolerances that require some form of active thermal management.

    The primary means of IPU thermal management is quite simple. A fan housed inside the IPU simply draws air from the passenger cabin and channels it through a variety of critical components. The first in the line of this airflow is the battery pack (also the most sensitive) and then all the other modules downstream from it: The BCM, MCM, DC-DC, AC Driver.
    Obviously, as this "cooler" air flows into and over each component it "warms up" and eventually leaves the IPU though a duct in the right rear of the trunk.

    Now, to make things really interesting we could take an inexpensive digital thermometer with a remote probe and we could attach it to the outside shell of the IPU after removing the rear seat back (it is actually quite easy to pull out). Better yet would be to have a multichannel thermal probe and stick each probe to each module's housing along the path of the air flow (as it travels through the IPU enclosure). For this you will have to remove the IPU cover (not recommended for the average person - I mean it !!!).

    You'll be amazed as to "how high" the temperatures can get even with a simple probe setup... and especially if you take into account the individual temperatures thresholds I had published before. Very quickly you'll value the work the IPU fan is doing.

    But still, the IPU fan can only do so much. In a hot day, the fan will be sucking in VERY hot air during the first 5-10 minutes and guess what that does? Yes, the EV assist affinity grows VERY high (hard to keep the assist off) and the regens are minimized (regens cause heat). At this point the whole IPU may even be dropped into the dreaded managed mode which is often accompanied by an instrumented recalibration (SoC drops to 1 bar and then a force regen takes over for a good while).

    So what can we do to avoid all these bad things let alone minimize the FE impact?

    Here are some guidelines acquired from the "collective wisdom fountain":
    1. Before driving off, open the windows and/or doors of the vehicle to let the warm air out. The longer the better of course, but depending on how hot, a good minute or two will help a lot.
    2. Use window tinting or an effective set of sun shades. In fact, when it comes to the HCH-II there's no such thing as a jumbo shade. Buy and use the biggest you can find, I assure you you will never regret it. I mean it.
    3. Drive off with all the windows down and the fan blower on. At this point keep the AC off.
    4. As soon we are gathering speed, close the windows and set the climate control to Auto and the temp setting to the highest possible (31C or 88F). Also set face/upper body vents ON and direct the air flow to the head liner.
    5. As soon as the blower fan begins to slow down lower the climate control temperature by one degree. The fan will speed up and then slow down again. Keep lowering the temp by one degree at a time until you reach a comfortable temperature setting. You'll notice that the comfort zone temperature is much higher than what most of us would set at first. ;)
    6. Whenever possible, turn off the AC outright by pressing the lower right most button until the climate control display reads AC OFF.
    Now, what does this all do?
    • It forces the hotter/stale air out of the car and prevents it from being sucked into the IPU.
    • It forces the hot air out of climate control ducting before running the cooled AC air through it.
    • It coerces the climate control to run off the electric scroll compressor from the very beginning, thus minimizing the FE hit. By making temp changes gradually the power demand from the AC compressor is also fractional.
    • It keeps the pack cool and happy. Remember, the AC driver will produce a lot of heat if the AC is driven at full tilt so we definitely want to avoid heating the IPU too aggressively when we're supposed to be cooling it.
    • By venting it to the headliner, we are forcing the cooler air to travel along the surface of the roof which allows it to travel further. As cooler air is heavier it will easily reach all occupants and even the IPU vent.
    Anything else we can do to make it even better?

    • Minimize the assist whenever possible and use a good number of SoC management techniques available in this forum.
    • Avoid full regen braking at 5 bars or more. In fact, if you can glide while regening at 1-3 bars you'll recover far more energy without over-running the pack with peak heat generation.
    Anyhow, enjoy the higher FE and stay cool whenever possible... your pack will thank you. ;)

    If you have any additional tips or comments please don't hesitate to post them... summer does not last for ever, you know. :D
    Last edited: Jul 25, 2008
  2. Right Lane Cruiser

    Right Lane Cruiser Penguin of Notagascar

    That was an excellent read, Manuel! Too bad the Insight doesn't have that electric scroll compressor... I have been airing out the car and do use a highly reflective window shade. I have another one to lay on the cargo bay floor but haven't used that one yet.

    It is rather difficult to select the number of bars of regen when coasting in the Insight...
  3. hansonclan

    hansonclan Member

    Thanks for the tips MSantos. AC is my biggest FE battle here in Phoenix. It's nice to know it can have a positive impact on the pack - now I won't feel quite as guilty when I have to turn the AC on.
  4. xcel

    xcel PZEV, there's nothing like it :) Staff Member

    Hi Manuel:

    ___Excellent overview!

    ___Although not an A/C technique exactly, keeping the windows cracked while a hybrid vehicle is parked can lower internal temps on a warm/hot sunny day by as much as 50 degrees F. There is no worry about someone breaking in because they are going to have to break the window out to get to the locks and such anyway. Besides a more comfortable climate when you first open up both doors and all the windows, the pack will treat you ever so kindly for the period you own and drive it due to lowered internal ambient over the years.

    ___Before going after A/C, I use all the same but use every increasing fan speeds once the driver window and passenger rear window cracked open have done all they can to keep you somewhat comfortable.

    ___And of course cycling the A/C. If you are headed down a long hill where Regen is going t fill the pack and you are going to burn off excess with a spinning ICE (aka Prius), run the A/C while running dow hill. In the non-Hybrids in particular, go ahead and enjoy break rather than throw it away to the brakes if you are coming to a stop anyway ;)

    ___Good Luck

  5. msantos

    msantos Eco Accelerometrist

    Hi Hansonclan, and welcome to CleanMPG !!! :)

    Using AC is always a lossy proposition so the key here is "strategic use of AC" as opposed to just turning ON the AC.

    If I am correct, the heat in Phoenix is also nothing to sneeze at. So, I suspect that you'll need a greater assortment of tricks or even some additional tweaks/measures just to make it bearable. :eek:


  6. msantos

    msantos Eco Accelerometrist

    Hi Wayne;

    Those are excellent points, particularly turning on the AC when going down-hill. In fact, I think when going downhill on an HCH-2 and the fuel consumed is zero we should actually lower the temp to the lowest possible values during the steepest descents.

    By doing this we would be using the belt driven part of the compressor which when activated imposes a fair amount of drag on the ICE.

    Unlike the IMA system where the ICE is assisted by the eletric motor, the HCH-II starts the AC in electric (for small temp changes) and is "assisted" by the belt-driven part for significant temp adjustments. Nice tip indeed.


  7. Mendel Leisk

    Mendel Leisk 2010 Prius (CAN Touring) Staff Member

    When I get behind the wheel and need to reset the vent settings, I typcally hit "Auto", and then the AC button (turning the AC off), so that I've got all the Auto except AC. This is in temparate weather. If it's swelter hot, I do use the AC.

    But if it's just a nice day, I'll crack the windows, and even turn the whole system off. One thing that bugs me about that: when you turn it back on, the system does not remember your previous settings, just reverts to Auto. No big deal, though.

    One thing I've noticed: AutoStop is much more prone to not activating if you have it one of the other modes, particulary involving defog.

    One caution, it is a good idea to use the AC at least every other week. The coolant has an oil in it which need periodic circulation, to avoid seal dry out.
  8. 2008 Civic Hybrid

    2008 Civic Hybrid Serious Hypermiler

    thanks. very informative.

    I agree with all.

    besides driving off with all windows opened, I will wait till the heat within cabin drops before turning on the aircon.

    Also, I'll put aircon to "recycle" so that the hot air from outside will not be drawn in. Will occasionally open the windows or put to "fresh air" position to draw in outside air if driving long distance. In this case, I could prevent wasting power (fuel) over working aircon compressor.

    I'm able to achieve the indicated MPG with aircon set at 25 deg C, recycle, 60% CITY driving., using all the tips by msantos and tarabell.
  9. Kacey Green

    Kacey Green Well-Known Member

    On the '09 it depends on how you turn it on. If I use the fan-speed to turn it on it remembers everything but A/C on or off.

    If I use the AC button it doesn't remember fan-speed. If I use auto it only remembers the temp.
  10. Mendel Leisk

    Mendel Leisk 2010 Prius (CAN Touring) Staff Member

    "On the '09 it depends on how you turn it on. If I use the fan-speed to turn it on it remembers everything but A/C on or off.

    If I use the AC button it doesn't remember fan-speed. If I use auto it only remembers the temp."

    Kacey, I'll check that out on our '06. Took me a while to get back to this ;)
  11. drimportracing

    drimportracing Pizza driver: 61,000+ deliveries

    If I put the top down I can cut the belt on the A/C....I haven't used A/C in any of my Metros in 4 years. I removed the compressor on one of the cars. The FSP is a different matter, leather seats, black car...good link resurrection. - Dale
  12. psyshack

    psyshack He who posts articles

    A/C use is killing us in the HCHII. It's horrible!

    I can't work on my A/C testing since my SG-1 got stolen. But I can share with you a typical daily grind this time of year. Remember I always get better mpg going in than coming home of the grind. 2 mpg seems to be the norm.

    I always reset trip b for the grind when I'm driving and leave the current tank on trip a. My target speed is 57 mph and I never romp on it unless I have to. Getting on the go pedal even with a full pack is soooooo depressing. I can get 59 to 67 mpg for the trip in with morning temps in the mid 80's and windows down and or cracked. Coming home with the A/C set no lower than 80f in Auto it's like a 10 mpg hit. Plus it seems like its always regening. May only be one bar,,, but there it is. One a 100f degree day we work to keep it no lower that 82f. It's like wft is going on!

    Take today: We rode in together. I had B at 67 mpg when I turned the car over to the wife. She had to do her drive on to her office and her daily rounds. She had knocked it down to 56 mpg. No problem. I get in the damn thing with A/C at auto 84f to start. By the time we get home I have it set to 82f and B is at 53.7 mpg. That SUCKS! I can't even maintain the wifes efforts and she did use the A/C from her office to mine to pick me up.

    Now I can pulse the A/C and keep 56 mpg on the grind. But darn what a pain that is. I wont FAS the car. The CVT ruins that in spades. It fas's worse than the Ranger or any other car or truck I've ever driven in my life. So that tool is left in the bag concerning the HCHII and it's crapstastic CVT.

    It's like the A/C in the HCHII is like a 10 mpg hit min. I just flat out do not understand how this so called high tech wonder car driven close to right fails so bad in the A/C world. Standard hit I've seen from every car I've ever owned is 3 to 5 mpg. Be it a 1.3L Festive to anything else I've ever owned.

    What am I doing wrong?!?!? The car is tinted with 3M CS. We have a shade for the windshield. It even get's parked near tree's at the wifes place of work. ( darn tree sap ) The one recal/crash it had so far made us suffer through was with the A/C on. And that sucked! It starts that stuff a lot it will be history. Took it 10 miles to sort it's self out through the hills on our grind. Kiss mpg by by.

    It seems like if you work to make it run off the pack your screwed. and if you blow it off and don't do a thing the engine labors so much on the mechanical side that your screwed also. Then you throw in the evil CVT,,, God what a hunk of engineering junk. It's like WTF!
  13. msantos

    msantos Eco Accelerometrist

    Hi Jeff;

    AC use is always costly and quite frankly results vary as much as the circumstances dictate. Then results can also vary according to use too. While I am sorry to hear that you are not pleased, I am not sure I can fully understand the appropriateness of the words "killing" and "terrible" either.
    On the account of my experience alone, I have no option but to claim the opposite and I do this without reservation and concern since as far as I have often demonstrated, the system really works very well in most scenarios.

    Second, your complaints about the CVT and the regretful experiences you allude to leave me baffled and puzzled as to why on this Earth you chose to purchase this car to begin with????

    I recall very distinctly "the dislike" you had regarding the CVT tech "years" before buying this poor car, so while I cannot say I am totally surprised by your thoughts and words about this transmission type, I am still saddened that my hopes of change did not come true.

    But still, I will openly state that I am disgusted and deeply annoyed by the constant vilification of the CVT that while not perfect, gives the HCH-II a driving experience that many of us appreciate and enjoy without second thoughts. To me and many other HCH-II owners I've had the pleasure of meeting and sharing daily results with, this transmission is smooth, quiet, and pretty capable of delivering tremendous FE with the ease and silkiness that no other Honda auto transmission can provide.

    Jeff, with all due respect: Please do yourself (and us) a favor and consider selling this poor car as I suspect it will never be able to meet your automotive standards. :(


  14. drimportracing

    drimportracing Pizza driver: 61,000+ deliveries

    Maybe a clinic would improve your experience. I thought I had my car all figured out within the first 4 or 5 months of hypermiling, I've read the links, practiced the techniques and over many thousands of miles just settled for whatever results I logged as good as it got.

    I mean I've delivered in 3 Geo Metros in the last 5 or 6 years, what could anyone tell me about my driving that I didn't already know. I assumed with delivering my results just couldn't get any better, I was mistaken.

    Seftonm spent 10 minutes with me and I picked up a still solid 10% improvement in my FE, He never even drove the Metro. Now I drive it a little faster (mo money) but with better FE than before.

    I'm just saying that sometimes you use the same old bag of tricks and begin to think you've reached the pinnacle of your cars ability when all you need is someone with a different perspective to point out a better way.

    I don't view your post as bashing the HCH as much as I hear your frustration over your results. With all the HCH expert hypermilers here I'm sure someone can walk you through a clinic over the phone or can set up an in person clinic if your willing to travel.

    I still want Wayne to drive my Metro so I can see what real "no holds barred" hypermiling looks like. I think by appearances of my ride he passed on the opportunity last time for fear of doing more pushing than driving. It's the kind of car that when bums walk up to you at the off ramp THEY GIVE YOU MONEY! I still wouldn't sell it for $5K (I paid $500) :p

    Your HCH probably has a lot to show you if you let it. :D - Dale
    Last edited by a moderator: Aug 7, 2009
  15. psyshack

    psyshack He who posts articles

    Had a good morning, 64 mpg. See what the afternoon holds with A/C use. :)
    Was cool enough here I did not have to use the A/C. But did have to fight head winds for the drive home. 58 mpg :(
    Last edited: Aug 7, 2009
  16. psyshack

    psyshack He who posts articles

  17. HCH2007

    HCH2007 Journeyman Hypermiler

    Well said, although I am in a learning process with A/C use and my HCH-II I am very pleased with the function of the CVT as I continue to learn more I like it more, but one thing I don't like was the cost of the transmission fluid when I did the change out, if I had followed the shop manual i.e. change out 4 times drive short distance after each change than empty and refill, it would have been much more expensive, instead I replaced, drove, drained, filtered fluid, refilled drove for 4 times then put the filtered fluid in to drive till next change. :)
  18. Harold

    Harold Well-Known Member

    Most of us only drain and refill. 3 qts. The three drain thing is for CVT that are not behaving or someone mistakenly put in the wrong oil. H
  19. Mendel Leisk

    Mendel Leisk 2010 Prius (CAN Touring) Staff Member

    You have a ways to go yet, grasshopper ;)

    Honda does not recommend that flush procedure at CVT fluid change interval. The documentation is confusing, especially the Owners Manual, which makes unexplained reference to the flush procedure. The only time you need to do a flush is in a case of contamination, say after inadvertantly dumping in motor oil.

    All a CVT fluid change entails is a single drain and fill. As Harold says, 3 US quarts is the sweet spot. Other than checking for leaks, I wouldn't bother trying to check level with the dipstick: the hot check / cold check instruction is out-to-lunch.
  20. deregezen

    deregezen Member

    In my opinion, the polen filter reduce the performance of A/C heavily after a few thousand miles. Frequently cleaning, changing or -better- throwing it away will provide more effective and less consuming A/C.
    Last edited: Jul 20, 2012

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