Jeep Liberty High Fuel Consumption in Winter

Are you driving in snow and using 4WD more often? That would do it.

As I wrote in my question:

I checked it under dry road conditions and similar driving speed and aggressiveness.

Excellent!!!!

Darnit, your answers are killing all of our potential arguments before they even get started.

Thank you SolarEagle.

Lets see what applies to my condition:

1-I never idle longer than the time needed to buckle up.

2-Tire pressure is corrected regularly.

3-We use winter tires (changed in October). This change shows about 5% increase in consumption. The rubber is softer.

4-I checked under dry conditions.

5-Aren't the law of thermodynamic saying that colder air intake will make the combustion engine more efficient?

6-That one that I suspect. Synthetic engine oil doesn't seem to be enough though. Is it worth changing the transmission and differential oil to synthetic? What about the wheel hub and universal joints grease?

7-That is possible that the gasoline affects it. I noticed a difference from one brand to another.

8-I rarely use the defrost and the air blower is also running in the summer. I scrape my windows manually and completely so probably not much effect. We don't have any snow yet either. Note that the thermostatic fan on the radiator runs less in cold weather. That should be a large saving. Nevertheless, the electrical loas is less than 1% of the total engine load in a large vehicule.

9-OK 2% from air drag.

Your answer is very good (GA!) but unfortunately, does not single out what could be the major cause. Do you think that the ones that apply to my case could add up to ~20% increase?

5, No the laws of thermodynamics do not care how dense or oxygen rich the air is that gets sucked into the engine. The laws of thermodynamics say that it will take more energy (fuel) for the mass of your engine to get up to operating temperature when the environment is a cold winter day than a hot summer day. Also it will loose this energy much faster once the engine is turned OFF on a winter day.

Now because there's more oxygen in cold dense air, more fuel can be successfully burned.

You will gain a slight increase in force applied to the piston from the colder air being heated in the fuel burning process but this is not where most of the increased piston force comes from in an internal combustion engine. Most of the mechanical force comes from the dramatic increase in the number of moles of gaseous molecules created by burning the long chain hydrocarbons that was once fuel. This is where answer 7 comes into play. The shorter, more volatile hydrocarbon chains in winter gasoline burn much easier, quicker, and completely than the longer chains found in summer gasoline but at the cost of producing less moles of gas after the combustion process along with less thermal energy released.

Sorry Redfred. "Most of the mechanical force comes from the dramatic increase in the number of moles of gaseous molecules created by burning the long chain hydrocarbons that was once fuel". If you balance the number of moles of gas in the combustion chamber before combustion against the against the number of moles of gas in the reaction products after combustion, it turns out that the numbers are almost exactly the same. I think you may be forgetting all the oxygen molecules which are consumed by reaction with the saturated hydrocarbons and the nitrogen molecules which act as a diluent. Remember also that in the gasoline engine, the fuel is often volatilized by being placed in the combustion chamber before compression, so the gaseous fuel components are compressed along with the air_. Only direct injection into the combustion chamber avoids this loss. In the case of iso-octane, or 2,2,4-trimethylpentane, which is the nominal octane of the "octane number", the reaction is C₈H₁₈ + 17O₂ + 68N₂ goes to 8CO₂ + 9H₂O + 68N₂ or 86 moles of gas going to 85 moles of gas, assuming the nitrogen is an inactive addition, and that the octane is a gas. If the octane is treated as a liquid, the numbers are 85 moles of gas going to 85 moles of gas.

"it turns out that the numbers are almost exactly the same".

They'd better be really close. Otherwise something would either disappear, or appear "out of thin air" during every stroke of every IC engine on the planet.

We don't seem to be losing or gaining anything tangible in the atmosphere, except carbon.

Yep. That makes sense. You're absolutely correct, I forgot that 80% of the gas in air is the typically inert N2. I audited that engine thermodynamics class so long ago.... Wait a minute, you have 34 oxygen atoms on the left side but only 25 oxygen atoms on the right. This is not a properly balanced reaction. Ignoring the 80% nitrogen gas present in air, for the time being. (I won't do that again, and yes I know that it's typically 78%N2 and 21%O2.) This means that 2C₈H₁₈ + 25O₂→16CO₂ + 18H₂O. So of the components that are just part of the chemical reaction there is a change of 27 to 34 moles, a significant but misleading change. Adding the required N2 gives 2C₈H₁₈ + 25O₂+ 100N₂ →16CO₂ + 18H₂O + 100N₂ or 127 moles of gas converts to 134 moles of gas. So the number of gaseous moles increase is about 5 to 6 %. Not an insignificant change but hardly my earlier boast.

I am now truly sorry, Redfred. My error is compounded by a smidgeon of arrogance, perhaps more. I plead no sleep and too much coffee.

How about this one:

2 C₈H₁₈ + 25 O₂ + 100 N₂ = 16 CO₂ + 18 H₂O + 100 N₂ ,

or 127 moles to 134 moles, a 5% increase. In my role as "Woodpower" I burn wood, mostly a carbohydrate thus

(CH₂O)_n + n O₂ + 4n N₂ = n CO₂ + n H₂O + 4n N₂ , a change of 5 moles to 6 moles, or a 20% increase, still not a major change. My apologies to everyone.

'...3-We use winter tires (changed in October). This change shows about a 5% increase in consumption...'

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If I understand you correctly, your fuel consumption dropped by roughly 5% in October when you changed to winter tires. This is likely a result of more aggressive tread and thicker (even if softer) rubber. While that identified a contributing factor, it does not measure increased rolling resistance due to lower temperature.

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Increased rolling resistance due to tires being less pliable when cold can be substantial and would be in addition to the 5% decrease noticed when switching tires.

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'...5-Aren't the law of thermodynamic saying that colder air intake will make the combustion engine more efficient?...'

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Paradoxically colder air can increase power available, but decreases efficiency, especially in gasoline powered vehicles. One of the reasons efficiency is decreased, has already been mentioned; to energy required to heat up the air and lost through the cylinder wall is not available to drive the piston.

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There is another mechanism of loss which can be very large depending on your driving style, related to colder air intake. Because cold air is more dense, the throttle does not need to be open as far to develop the same amount of power. As a consequence, for the same power output, the throttle is more restrictive and pumping losses are higher with colder air in the intake. In all conditions except wide open throttle, pumping losses are a significant loss in most gasoline powered vehicles.

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The good news is that creating a DIY hot air intake (drawing intake air from around your exhaust manifold) for the winter months is a pretty simple undertaking. Another possibility, if you have a manual transmission, is to use a 'pulse and glide' technique.

"1. More idling

This should be a no-brainer, yet parked idling cars are a common sight in cold weather."

I'm sure that conditions differ greatly in different areas - but driving off before there is any sign of defroster function is suicide around here! I routinely scrape the windows all around with engine started and rear in-glass defroster "On" - and sometimes even have to scrape the inside of the windshield - but it fogs over almost instantly on many, perhaps "most", days during our winter (SE Wisconsin). I also typically have the insides of the two side windows fog over again, eliminating any way to see traffic in order to make an entrance onto a street. Last winter I had the left window's drive mechanism pull off of the bottom of the glass when the window stuck in the "Up" position and the motor tried to move it; that's the third encounter with this problem, in two different brands of vehicle, and I'm aware of multiple other people who have had the same thing happen. Once the mechanism separates, when the glass warms up and thaws its bonding agent, it can bypass the support (returned to its top position, natch), fall down into the bottom of its slot, and give me a really cold shoulder. [TIP: if this happens, and you're able to pull it back up, whittle a couple of wooden wedges to slip into the bottom edge to keep it there!] I've seen the "Drive off immediately" advice for years, and even asked a couple of the identifiable sources how to handle the defroster issue, and gotten no better than a lame, "Uhhhh . . . well, uhhh . . .

Leaving a window down while clearing the car and during the drive helps reducing the humidity released by the melting show and slush on the carpets. Inside fogging is caused by high humidity in the vehicle's air.

Some cars are much better that others with cold winters. In general, Japanese cars don't do well, the Jeep is OK, my Volvo does well. It helps when the car designers face the sub-freezing weather a few months a year.

True - at least when it is possible to put one down! The "Auto" function, where windows make the complete trip after you use the switch momentarily, needs an override; if the window motor is stalling, the only way to stop it is to shut off the ignition. At least with manual windows you can FEEL when there's a problem. The windows have a safety shutoff going UP, in order to prevent strangling children, by law: they don't have it when going DOWN, at least in any I've seen. I've left the door ajar for that purpose, while scraping windows. And, on many cars, another source of interior moisture is snow that fills the plenum from which the heater and defroster draw air. Even if you leave it covered over night, snow will be drawn into it while you scrape (Ask Me How I Know!).

These shelters are very popular around here in the winter. This is my neighbor's. The shelter prevents ice and snow accumulation and even eliminate frost scraping in the fall. They are temporary structures removed in the spring. The cost is around $500 and last 5-10 years. Most people here don't use the garage for their car. I don't know why. I like my garage as the car is not so cold in the morning.

Good choice at home, but not at work, with a wide-open employee parking lot. And it needed to remain open so that plow trucks could clear it (if every employee had one of the shelters, there would still be aisles that needed clearing after every significant snowfall. Shelters would reduce the total number of possible spaces, though we had sufficient area for current head count). No way in hell that the company would build a structure and roof the whole place!

On a separate note: at home I'd need to have two of them if I wanted my car under one, even though hers might sit home 4 - 5 days of the week, while mine needed to be driven AT LEAST 5 days a week. I'm explaining, because I'm sure that no one ELSE has ever experienced this issue . . . Right?

At least you have a car of your own. I had a car of my own, but since I also had a work vehicle filled with all my "toys", this was manifestly unfair, so we got rid of the second cqr. We wouldn't dream of covering my old piece of junk . . . Right?

They make them for two cars wide by two long!

Nine good reasons for living Florida, too!

GA for a classic answer.

GA! This is another 3% "confirmed", With the 2% from air drag we have ~15% left to explain.

Actually, with the 5% from the winter tires, we have accounted for ~10% of the 20% increase.

We are getting there... Thank you all!

The winter months normally require more use of the defroster. Jeeps automatically run the AC on the defroster loop to help in removing moisture from the inside of the windshield.

Increased load from the AC compressor will use more gas.

The winter gas info provided by another post is also very important.

Thank you Pete. This is good to know.

I almost never need to use the defrost before January. So this does not affects the present measurements.

I haven't used the heated seats either.

Poor atomization of fuel plays a part too.

Good article.

Thermostat stuck open. Replace with 176 degree F thermostat.

On a general basis, fuel consumption goes up in cold weather. The engine just isn't as efficient whether it is a small 4 or large 8 cylinder engine. Other aspects you may want to check is the air pressure in the tires and if the thermostat is functioning correctly. In cold weather the air pressure will drop significantly in the cold causing increased friction. A poorly working thermostat failed in the open position will continuously make the engine run cool thus causing an even grater loss of mpg. Checking the pressure of the tires is easy. I run my tires 5-8 psi above the recommended pressure to get better mpg numbers. An easy check for the thermostat is to run the car when it is cold outside and check the air temperature coming off of the heater. If it feels cool or cold get the thermostat changed. If the temperature is still running cool or cold, you may need to change the heat exchanger, or have the radiator fluid checked for air in the system. Check for air in the radiator fluid system first though. Changing the heat exchanger is expensive.

The engine temperature gage reaches the middle position within ~two minutes of starting (and driving on small streets) when the outside temperature is around freezing.

Is it fast enough for a 6 cylinders?

your engine is not reaching to operating temperature in winter. check thermostate and its opening.

Thank you.

I have owned it for two years. I noticed it last year.

The previous owner cannot be reached. The vehicle was very well treated and used as a second vehicle in southern Ontario where the climate is warmer. He might not even have noticed.

With the IR thermometer, I should see a temperature difference between the radiator hose and the engine / heater box. Do you know what I should read after a minute of idle when the ambient is near freezing?

I have already checked for codes and there is nothing to report.

If it is fully warmed up it should read what the thermostat is opening at within a degree or two. If the temp seems to check out I would suspect one of the temp sensors. A high end scanner should be able to tell you on the fly sensor readings, so that might a place to start. If the mechanic doesn't have one that will do that check around for one that does.

Scantily attired pump attendants are shivering and spilling fuel during refueling.

Not here. In North America we usually have self served gas pumps. The pumps accuracy is checked by the government.

The gas station owners make more money selling cigarettes and drinks. They basically sell gas to attract you in their store.

Try changing the thermostat, If it's running too cold this will affect your consumption, a faulty stat will also affect warm-up times..

Bazzer

When the rear brake drums are colder their reduced diameter may result in brake dragging.

Do you notice a higher brake pedal in cold weather?

No, I haven't noticed that.

Shouldn't the brake system adapt?

In addition to all the other suggestions,

1. there will be a dedicated temperature sensor which tells the ECU when the engine is warm enough to go into closed loop - ie most economical operation - check that it is operating correctly.

2. Your oxygen sensor may be heated by the exhaust gasses alone or it may have a heating element to get it to operating temperature. Either would work fine in summer but in winter perhaps the oxygen sensor is not getting to operating temperature, either because it is heated by the exhaust gasses alone or because its heater wires, or heating element is broken.

You figure a dodgy O2 sensor would raise a fault on the OBD and make a light come on. Right? Brain damaged ECU maybe?

Not getting into closed loop makes a lot of sense. Once in closed loop the O2 sensor (which gives an indirect measure of the air fuel mixture) would keep things stoichiometric in the combustion chamber. The car's brain could be keeping the "choke" pulled out.

If the OP is so inclined they could apply a bit of science and do some data logging of the OBDII port with a suitable adapter and some software running on a PC or schmutt fone. Analysis in front of the fire could follow during winter hibernation.

There's 3 months of cold weather operating data just waiting to be collected and analysed.

The shoes would also contract as well, offsetting any drum contraction.

Hello, I had this exact same problem years ago with a chrysler car. I found that there were two temperature sensors, one for the gauge and one for the computer. Although the gauge always displayed the "normal" range the computer sensor did not.

I used a volt meter to test the sensor and the voltage never increased more than .05 volts.

I replaced the sensor and checked the voltage, it read about 2.5 volts when the engine was warmed up. The total output voltage at max was 4. 8 if I recall.

Hope this helps.

Same problem with an old Mitsubishi Pajero, it was the thermostat, engine doesn't warm up enough in cold weather. Test by blanking some of the radiator off without overheating the engine, more mpg then it's the thermostat, replace it.

I have a 2002 Grand Cherokee and there is a definite seasonality to the mileage. A little digging into the official shop manuals will show that the Incoming Air Temperature (IAT) sensor is used (amongst other things) to make the mixture richer as the temperature drops so expect the poorer mileage, but do check for a stuck thermostat just to be sure.

Those cheap kits that you see advertised for improving the mileage on these vehicles simply put a resistor in parallel with the sensor to "fool" the computer into thinking that the temperature is higher and hence force a leaner mixture, but can cause driveability issues since it puts the computer in an odd place on the program "map".

One possibility that hasn't been discussed is the type of driving you do. It takes longer for the engine to warm up when it is colder out. While the engine is warming up, you are burning more fuel. So if you are habitually driving the car for distances where the engine has just fully warmed in cold weather, you will consume considerably more fuel in cold weather. On the other hand, if you start the car in cold weather and drive long distances, the effect will be considerably less. A related problem can be an air leak in the system that signals the computer that the engine has reached operating temperature. If the computer thinks the engine is below operating temperature, it will provide a richer mixture, even when the engine is warm.

The thermostat works. I will check the running temperature and increase the thermostat temperature if lower than optimal.

One post suggest getting warm air in the intake. I will see what I can safely do without increasing the pumping losses.

I will get a new air filter just in case. I had blown clean this one. It might not be enough.

Do your winter tires have the same circumference as your summer tires?

Slightly bigger.

The winter tire effect is taken into account in the ~5% increase as soon as I changed to them. I don't know if the diameter has an effect but the softer rubbet does.

Larger diameter = less miles registered on odometer.

GA! They are bigger!

+1" on the diameter gives ~5% less Km in the computer for the same distance. (I should have taught about it )

That explains the ~5% increase in consumption when I change the tires.

I still have ~10% of the 20% increase to figure out.

I hope that the temperature for the thermostat and the air intake will cover most of it.

Thank you all!

The majority of the increase in apparent fuel consumption can probably be attributed to factors already mentioned:

1. Increased tire size (real effects + mileage error)

2. Winter blend gas

3. Cold air effects... >% energy lost as heat, increased resistance of colder fluids, and larger pumping losses at the throttle.

4. Effects of cold tires (this is different than #1). The NHTSA uses a linear approximation for passenger cars of a 0.8% increase in rolling resistance per every degree below 25 C. http://www.nhtsa.gov/DOT/NHTSA/NVS/Vehicle%20Research%20&%20Test%20Center%20%28VRTC%29/ca/Tires/811154.pdf

Below is a chart from the EPA website shows rolling resistance is the major resistance at speeds less than around 40 mph.