Why do MPG drop in cold weather - GreenHybrid

clayk · #1 (**permalink**) 11-30-2005, 07:08 AM

As the temperatures have dropped from the 80's to the 30's in the morning, my mileage has dropped by 5 mpg or so. I realize that autostop does not work when the engine is cold and higher revs are required on a cold engine. However, once the engine warms up shouldn't you get the same efficiency at 30 degrees as I did at 80 degrees?

gonavy · #2 (**permalink**) 11-30-2005, 07:18 AM

- the fuel in winter is different in most places- lower energy content per gallon.
- cold air is denser...harder to push outa the way.
- the engine is using substantially more energy just to stay at operating temp, and the driver is not helping by using the heater.

Having said that, the instant FE on a warm engine in winter should not be too much different from one in summer, if the same fuel is used. The killer on a tank basis are the cold startups and fewer autostops.

jdenenberg · #3 (**permalink**) 11-30-2005, 07:19 AM

Clay,

Check your tire air pressure. PV=NRT so the pressure drops as temperature drops. There are other factors, but this one you can deal with.

JND

AZCivic · #4 (**permalink**) 11-30-2005, 07:33 AM

The rolling resistance of the tires goes way up as the temperatures drop. The engine also generally doesn't atomize fuel nearly as well when it's cold. Many Insighters have been able to greatly improve the fuel economy by both partially blocking the radiator and by re-routing the air intake so it draws warm air from around the cylinder head.

clayk · #5 (**permalink**) 11-30-2005, 07:33 AM

I did think of the tires and remedied that issue. I do think the instant FE gauge seems to be the same after the engine is warm. I did not know about the gas mixes being different but had thought of the denser air. I just keep wondering if the weather in the 30's and 40's should make that much difference. That is really not cold like it is in Chicago or further north. The colder weather conicided on the same day I had my first oil change and computer upgrade. The dealer said he would use 0W-20 oil, but did not specify the brand. I wonder if I could be doing better. I need to stop obsessing.

Clay

gonavy · #6 (**permalink**) 11-30-2005, 07:36 AM

obsessing is fine; that's how change happens.

zimbop · #7 (**permalink**) 11-30-2005, 07:40 AM

... also insanity. :-)

Tim · #8 (**permalink**) 11-30-2005, 07:46 AM

Quote:

Originally Posted by clayk

I just keep wondering if the weather in the 30's and 40's should make that much difference.

I posted a 3-year summary here where you can see my seasonal differences. Weather makes a good 5-7 MPG difference for me, and our climate is not that extreme.

I'd be very interested in warm-intake stories. For the owners that re-routed their air intakes - do they experience problems in the summer? Do they switch them back? Any one else have good ideas for warm air intake?

lars-ss · #9 (**permalink**) 11-30-2005, 07:47 AM

Here is my standard response to this problem. Read on at the end for further things of note:

Quote:

Getting low MPG in cold weather is not a problem with hybrid technology, per se. If it was, there would be ZERO drivers achieving EPA numbers in the winter months, and that is not the case.

There are many reasons for cars not achieving max MPG in the colder months...

Car engines must warm up to a certain internal operating temperature before they most efficiently use the fuel. Winter months and colder temps make that transition from cold start to most efficient take a little longer.

Also: If you have a new car, new cars go through what is called a "break-in period" where the metal engine parts are still grinding against each other more rigidly, creating more friction, which also reduces efficiency. Over time, the "extra metal" shaves off and gets lubricated better (expecially if you use Synthetic Oil) and the friction is reduced, which affect MPG numbers.

Winter formula fuels used in most states hold less energy and this also hurts MPG.

Generally speaking, colder weather in many areas means "rain, snow, or slush or ice" on the roads - these things add additional resistance which the tires must push out of the way to contact the road, so they hurt MPG in that way.

Tire pressures also are lowered by the cold weather too. Most people fail to properly inflate their tires all year, but ESPECIALLY in the winter when it is cold and you might have dirty tires and you just dont want to deal with spending time in the cold airing up a dirty tire. Low tire inflation can cause blowouts and adds quite a bit of rolling resistance, which forces the engine to work harder to "push" the car forward, thus using more fuel.

Additional idling uses more fuel and hurts overall MPG, and many people in cold climes by habit or necessity are warming their cars up in the morning. That is good practice to stay warm, but BAD for MPG numbers. While idling in your garage or driveway or carport, it is getting 0 miles to the gallon !!

People who use the defroster in a Hybrid are many times using the A/C compressor to help reduce the humidity in the car and clear the fog. That is a MPG killer too. The compressor can be turned off, but usually by default it comes on.

See this science page for more explanation:

http://tinyurl.com/3s7lo

OK, I understand all that. But I'm tired of losing 2-3 MPG every cold morning commute then only getting maybe 1 MPG of that back by babying the car in the afternoon commute. I'm about 80 miles into this tank, and I have dropped from 55.6 to 51.8 in the last two days. I'm TICKED !!!

I want my HCH to not have the cold engine issue. What is the best way to do that? I have heard much talk about block heaters, oil dipstick heaters, warm air intakes, etc.

I want the DEFINITIVE answer on one thing I can do to significantly reduce the cold morning engine MPG loss effect.

Anyone have that? Any cold weather HCH owners made a good mod?

gonavy · #10 (**permalink**) 11-30-2005, 08:21 AM

look @ the block heater threads.
http://www.greenhybrid.com/discuss/u...ater.4735.html

hopefully rihch will post back with his FE observations pre- and post install?