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Hot or Cold

  #1  
Old 06-20-2008, 04:07 PM
Aussie's Avatar
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Question Hot or Cold

I'm sure that I have seen the issue of hybrid performance at different temperatures somewhere else, but it just doesn't seem that clear.

Based on our temperature variations from summer to winter - 100F to 41F.

I understand that in colder weather the vehicle takes longer to warm up, and therefore uses more fuel, BUT, in colder weather the air is more dense, and therefore expands more when combustion takes place.

So isn't the small loss in economy at the start of the trip (because the engine is cold), outweighed by the increase in economy because of the cool air?

And during the summer months 90 - 100F I loose economy because I'm using the AC. - previously owned a Honda Civic Hybrid II.

I'm just not seeing any substantial loss in economy through our winter. The coldest it got to was about 3C - (37.4F). The first tank I put through my new Prius...in mid winter, returned around 52mpg.

Maybe there is some other issue when it gets below freezing, that we just don't see here? Or does the Prius like the hotter weather more than the Honda?

Any comments?
 
  #2  
Old 06-20-2008, 07:02 PM
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Default Re: Hot or Cold

Hi David,

The first thing to do is start collecting your data using the GreenHybrid database. One parameter is the temperature for the amount of fuel recorded. When I first started recording my data, I did both weekday and weekend records to separate commuting from longer distance driving. But this also gave me MPG vs temperature data:


This is the raw data that suggests there is a relationship between temperature and MPG. With some investigation, I found that with my Prius:
  • engine warm-up - takes longer in the cold so more fuel has to be burned before the car will go into fuel efficient, hybrid mode
  • higher drag air - denser, cold air increases the high speed drag, especially at speeds above 30 mph
  • tire and lubricant warm-up - power has to push through colder transaxle lubricants and the tires have greater rolling resistance
Other than these primary effects, no problem.

Bob Wilson
 
  #3  
Old 06-20-2008, 07:42 PM
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Default Re: Hot or Cold

You have to look at what Bob said, I really appreciate his scientific summaries but I have to have it in simple terms so I can wrap my head around it.

Firstly, your low temps are not the same as many of us in the US see, I am usually seeing improvement in MPG figures when the temps climb up to 41 degrees.

My big mileage hit comes when the highs are below freezing...snow. melted snow, ice and freezing rain really draw the heat out of the car, running the heater, resistance of snow on the road, driving on ice for caution rather than mpg and the E-10 the cities love so much in the states factors into it. The cold also dramatically decreases how soon auto stop comes into play, 10=6-8 miles, 30= 3-5 miles, 40 about 1-2 miles, 80 1-2 blocks, the same can be said of regen and assist...lets face it, the car does not like sub freezing temps.

That said, when the highs are in the teens for weeks at a time, I feel good if I get 38-40 mpg. On 60 degree days without the heat or a/c I see 50 in the city....on 80 degree days with the a/c set for 72 I get 44-45 mpg...see the difference?

I have a co-worker with a PriusII, her mileage mirrors mine in the conditions, sometimes I beat her by a bit, sometimes she beats me, rarely is it more than 2mpg difference.
 

Last edited by Bobs Metallic Pearl; 06-20-2008 at 07:52 PM. Reason: add info
  #4  
Old 06-21-2008, 12:42 AM
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Default Re: Hot or Cold

Originally Posted by Aussie View Post
I'm sure that I have seen the issue of hybrid performance at different temperatures somewhere else, but it just doesn't seem that clear.

Based on our temperature variations from summer to winter - 100F to 41F.

I understand that in colder weather the vehicle takes longer to warm up, and therefore uses more fuel, BUT, in colder weather the air is more dense, and therefore expands more when combustion takes place.

So isn't the small loss in economy at the start of the trip (because the engine is cold), outweighed by the increase in economy because of the cool air?

And during the summer months 90 - 100F I loose economy because I'm using the AC. - previously owned a Honda Civic Hybrid II.

I'm just not seeing any substantial loss in economy through our winter. The coldest it got to was about 3C - (37.4F). The first tank I put through my new Prius...in mid winter, returned around 52mpg.

Maybe there is some other issue when it gets below freezing, that we just don't see here? Or does the Prius like the hotter weather more than the Honda?

Any comments?
There is no increase in economy in the cold period. If you didn't use AC you would definitely notice much much higher mpg in the Summer. One of the biggest factors is that any type of lube (engine/wheel bearing/tranny) is thicker/stiffer the colder it gets. My car will coast (engine off) twice as far warm as cold.

The AC is what is throwing off your observations.
 
  #5  
Old 06-21-2008, 08:12 AM
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Default Re: Hot or Cold

Also, they use a different additive in the winter months that supposedly hurts the mileage, I don't know what kind of effect it actually makes though
 
  #6  
Old 06-21-2008, 04:28 PM
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Default Re: Hot or Cold

OK.

So I'm unlikely to see the low's in fuel economy that you guys suffer because we literally never get below freezing. Well maybe sometimes just....overnight. But by the time I'm ready to head off to work, it's 40F or higher.

At the other end of the scale, we do get a lot of 90F and above over summer. So I'm expecting to get a bigger hit on economy than we do during our mild winters.

It seems that since I have purchased the Prius, the average temperature has been hovering around the 55 - 60F range, so according to Bob's chart, this is almost ideal. Which is probably why I hit 52mpg on the first tank.

Thanks to everyone..............
 
  #7  
Old 06-21-2008, 04:56 PM
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Default Re: Hot or Cold

Below freezing winters does not mean it does not get hot in the summer...we see months where the high is below freezing, weeks at a time where the high never gets out of the teens but, come summer, we see months in the high 80's, weeks in the 90's and 100 ish is not unfamiliar territory....My current tank purchased June first started in the 60's & 52mpg, slid to 48 in the 70's and is now just under 44 in the high 80's & HUMID...so, I guess one of us gets the best of both worlds. Oh yeah, I started using the a/c heavily last week....I am now down to 1/4 tank.
 

Last edited by Bobs Metallic Pearl; 06-21-2008 at 04:57 PM. Reason: correct statement.
  #8  
Old 06-22-2008, 04:35 AM
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Default Re: Hot or Cold

So. It's not just the use of the AC that causes a dip in fuel economy, but the incoming air temperature. A double wammy when you use the AC as well.
 
  #9  
Old 06-22-2008, 10:36 AM
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Default Re: Hot or Cold

Well kind of but not exactly, the warmer air temps actually cause the car to run more efficiently to a point (about 80 I have found) after that, the battery's start getting hot (remember, in most hybrids they are stored in or adjacent to the interior).

If the car has been parked in the sun, and the interior is hot, it causes exactly the same reaction in the car as cold, the engine stays on until it can lower the temp enough to bring the battery pack on line and reduce the fan speed.

Me, however they cause a equal decrease in comfort, therefore, it causes me to turn on the a/c more, the larger a/c load takes a toll on the batteries because once the auto stop kicks in, the batteries are powering your compressor at stop.

The more you stop, the more you have to recharge, the more gas you burn etc....so, yes if it is hot for an extended period, a/c is going to burn more gas.

So, I guess what I am saying, in my unscientific observation is the HCHII at least and probability most hybrids are most comfortable when you are, that is when it is warm enough to not use the heat and cool enough not to use the a/c.
 
  #10  
Old 06-24-2008, 02:46 AM
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Default Re: Hot or Cold

Thanks.
 
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