Anyone else notice their fuel economy go for a crap in the winter or is my MAF or something @ fault?
The trip computer, (I know it's not accurate, but should be realative), says I'm using a full liter more of fuel per 100 KM now than in the summer. I just bought the car in June of 09.
The trip said 8.9 liters per 100 km in the summer, (which in actuality was 10 liters per 100 as measured), and now it says 9.9.
That roughly converts to 23 miles per US gal summer and 21 US gal winter.

 

It's the seasonally adjusted gasoline blend; energy density is sacrificed to increase vapor pressure and maintain cold start driveability. I think the EPA quotes about a 5% decrease in fuel economy.

 

yeah my gas mileage goes down the crapper in the winter...sux. Good thing spring around the corner!!!!

 

I noticed this also

 

OMG, cold air is much denser than warm air. So it needs more fuel to keep the AFR at stoich.

 

I notice it also

 

Me three

 

Not quite (although my sarcasm detector doesn't work this early).

Mass of ingested air determines load, so in your case where more air is ingested, more power is produced. To drive at a speed a given load is required. So a reduced air volume is required (less VE) for a given load and ultimate power is increased.

The reduction in fuel economy during the winter is generally a function of fuel composition, cold start enrichment, tire pressure reduction and increased initial oil viscosity; you may be able to throw in increased aerodynamic drag with the denser air, though very minimal.

 

Well, I'm glad it's not just me. I figured cold starts = fuel enrichment, just didn't think it would/should be such an increase.

 

While noticeable, I don't think those stats "go for a crap". (Um, that must be a Canadian phrase.)

In addition to the cold and the winter gas, you're also probably down a few ticks from the 6MT. If I drive like a granny (rare) and keep the RPM's down, and use 6th gear as often as possible... there have been times when I've ticked over 30 mpg for extended periods.

If it were me and I were concerned, I'd take notice for comparison in the Spring, and start taking actual measurements, rather than rely on the trip computer.

 

The thing i hate is that my power steering seems frozen at start up. I let her warm up to operating temp but its just odd...

 

Oxygenated fuel. Starts in Late October and runs thru early March.

Wasn't sure they had the same issues in canada, but I guess this confirms it.

besides that, I hate how my 6MT is so stiff the first couple shifts when it's cold.

Can't wait for warm weather.

 

OT, but OMG yes I share that annoyance. It's even worse with the STS mod, since you're sacrificing leverage for the shorter throw.

 

so you are saying you will get the same gas mileage whether the temperature is 0F or -40 F? I don't think so. It will be much worse at -40 than at 0 because the ECU wants the car to run at 14.7 -15.5 AFR and the only way to do that is to add a little more fuel to prevent it from going lean in the denser air.

(0 °C and 100 kPa), dry air has a density of 1.2754 kg/m3 and at -25C the air density increases to 1.423 kg/m3 which means there is 11% more air per volume at the colder temperature, which means the ECU has to compensate for that extra air

 

Gasoline oxygen content generally doesn't change through the seasons. Although there are a few locations in the southwest where oxygen content varies by season, (there may be a odd-ball pockets where this occurs as fuel requirements are regulated at federal, state and city levels).

Butane is blended into the gasoline pool to increase vapor pressure/ Unfortunately, butane (as with other volatile hydrocarbons) has a short carbon chain and low energy density.

The EPA's site quotes a fuel economy reduction (on average) of 5.3% going from 77F to 20F (primarily fuel factors).

http://www.epa.gov/oms/rfgecon.htm
http://www.epa.gov/otaq/regs/fuels/ostp-3.pdf

 

In the winter, fuel is also wasted in warming the car up. Idling uses up fuel..

 

That wasn't my point, it was that regardless of temperature (and air density) engine load is determined by mass air flow where more air = more power and constant power requirements require constant mass air flows (for a gasoline engine, under stoichiometric operation). So in your example if an engine needs 1 m3 of air at 0 °C for some operation, it only needs 0.90 m3 of at -25 °C (1 kg for both) and consumes 0.068 kg of fuel in both cases.

I think that makes sense, but I am certain it will draw more questions.

EDIT: factors beyond air density change efficiency (fuel economy) at low temperatures (i.e. cold start enrichment, warm-up idling, tire pressure reduction, increased fluid viscosities (oils and greases), increased aerodynamic loads (air density), etc...)

 

YES, I feel the same way! It shifts easier when I let the car warm up.

OP, I get around 22 MPG driving 80% highway 20% city... hate cold weather...

 

Winter time is never a good time for cars

 

OP, I get around 22 MPG driving 80% highway 20% city... hate cold weather...[/quote]


That's about my mix of highway and city as well. So, I guess it's normal. I use 89 octane, by the way.

 

switch to motul 75/90

 

a 3.5 litre engine consumes 3.5 litres of air every two revolutions. when it is colder it still sucks in 3.5 litres of air every two revolutions. But when it is colder there is more air in that 3.5 litres of volume at -40*, than there is in 3.5 litres of volume at 0*. To make up for the denser air more fuel is required.

The way you say it, a car needs less air to run in the cold... the engine does not control how much air it needs. It takes in as much air as the engine displacement. Period. An 8 litre engine sucks in 8 litres of air. The ecu doesn't say, " oh it cold outside i only need 7 litres of air right now so let's just suck in 7 litres of air". it still has to suck in 8 litres of air because that is what the displacement is.

 

No, it doesn't. This would only be true if the engine was able to operate at 100% volumetric efficiency, which would only be attainable at WOT, and is not likely. I would guess the peak VE for the Maxima's VQ35 is ~90%. Regardless, you don't drive around town at constant speeds at WOT.

There is more mass of 'air' (oxygen is what is important) per unit volume, but at constant loads the engine requires a constant mass flow. In other words if you normally drive at 55 mph at 20% throttle in the warm, you may only need 17% throttle at colder temperatures.

No and sorta-kinda. The engine needs the same mass of air, but as you pointed out, the volume for a given mass at different temperatures varies. And the engine can control the air intake with the DBY throttle, but this is generally a function of user input through the accelerator pedal.

See above.

The engine knows the incoming air mass flow through the mass air flow sensor (MAF). At a given engine speed this is varied by throttling (restricting) the incoming air flow from nothing to peak VE at that engine speed (VE varies as a function of engine speed).

 

Why? Is its viscosity at low temperatures less than other 75/85 or 75/90 fluids?

 

What is VE? VE varies with RPM?

 

As a senior in mechanical engineering who just took a course in IC engines, I have to say that 2slow is correct. Knight, what I think you're not seeing is that in any situation, the car will adjust according to what it's required to do. When the air is more dense, the engine is able to make a more complete combustion, making more power (work/cycle). Because of this, the throttle does not need to be open as wide as it would when the air is less dense, and therefore nearly the same amount of fuel is used, even though more fuel is required for the same mass of air. Your general theory is correct in that the car needs to maintain stoichiometric A/F ratio, but you have to look at the big picture, especially when discussing something over such a broad period of time as miles per gallon of fuel.

Volumetric efficiency. It's a number that basically compares the actual airflow in when compared to the maximum possible. When knight said our engines take in 3.5L of air every two cycles, he was assuming 100% volumetric efficiency, which actually doesn't happen often.

 

You know, I never had this stiff shifter issues until this winter, and I just assumed it was because of the STS mod and the loss of leverage.

But now that you bring up gear oil, I did swap in MT-90 last Spring, which would make this my first winter with MT-90 (and the STS mod). Since everything is fine once it's warmed up, the STS is not the culprit, it's the MT-90 that's making things stiff when cold.

I'm probably not going to do a damn thing about it (except wait for winter to end), but it does explain things. Thanks.

OK now, you and 2slow can get back to arguing about mechanical engineering stuff that 90% of us don't understand. Carry on.

 

Exactly, and good choice on mechanical engineering.

I pulled this out of my head, and I think it's correct (although I hate mixing variables and units, or using units as variables):

 

thats your problem...switch to 92-93

i get between 23-25 (havent had the car in warm weather yet)

B

 

FWIW, I use 93 octane gas and average 24mpg. I see almost no variation between warmer weather and colder weather.

 

You are correct.

To put it simply,

Cruising at 60mph will give you roughly the same fuel consumption in 0 degrees as it will in 50 degrees, assuming the fuel composition stays the same.

The only difference between the two is that in the winter, the VOLUME of air consumed is less, because the mass/volume is higher. In the summer, the VOLUME of air consumed is higher, because the mass/volume is lower.

The reason the change in volume doesn't change fuel economy is because the mass of the air is what determines the amount of fuel needed to keep it at 14.7 and how much power is produced per a given volume.

This is why our cars have a MASS air sensor, not a volume air sensor (flow rate sensor).

Oh and before anyone says it, the car DOES NOT run in richer until the engine reaches operating temperature. I have dis-proven this with my wideband o2 sensor.

I can go and start my car right now in the freezing cold and within 10 seconds, the AFR will hover around 14.4-15.0.

I'm not sure, but this could possibly mean that the car does not run in open loop until operating temperature either. I know that at WOT in open loop, my car runs closer to 12.9-13.2 AFR at 1200rpm.

This behavior may be because I let the car sit for 10 seconds or so with the key on the "ON" position before I actually start the car. This allows the heated o2 sensors to get a small "head start" on the warm up process. (IDK, only reason I can think of)

 

i get around 24 MPG
in warmer weather i use to get 26 to 28 MPG

I guess with the heater blasting pretty much throughout my 3 hour combined daily commute, 24MPG isn't actually that bad....


I have an auto-tragic btw

 

Well put.

Holding the car in 'ON' prior to starting should decrease the time spent in open loop after start-up.

For start-up, there are generally fuel additions made based upon IAT and/or ECT feedback; colder temperatures mean less vaporized/volatilized fuel available for combustion, so more is used. Though I don't know the period for which this applies, and it could be very short as start-up emissions are of great concern to (manufacturer) engine calibrators.

My gut feeling is the car is still in open loop, but the fuel trims for cold starts (based on IAT) only last for a second or two; perhaps until the engine reaches a specific speed threshold to signify successful start-up.

Where do you live?

 

it happens to everyone it also depends on your driving if you gas it all the time

 

wow you guys get good das mileage in the winter or year round for that matter. i notice in the winter in which i will do 70% around town 30% hiway my mpg from the computer (i know thats not accurate) states 20.9. Using the computer in the summer as well it gets to 24.?. i havent actually taken a full take and did the math by the tank size but..i do know when i got the car in 05 it did get better gas mileage. not by a ton but better. Just to note my last tuneup was prob 35k ago and i actully cleaned my air cleaner today since it gets dirt and junk thrown up from the roads in the winter.

 

20mpg in cold weather is pretty normal
my rx-8 gets about 16mpg

 

I've had my car just a week now and I think I'm getting about 18mpg, but a lot of city driving.

 

Aw, man. I noticed no one mentioned that to you in your goodbye thread, and good too, because it would have been tacky to dis your new ride the way some people did. However, now that you bring it up... that rotary engine is not exactly fuel efficient. But you probably knew that going into it.

Worth it? I think so. That Mazda is a unique and respectable design, mileage notwithstanding.

 

oh yea, freaking fantastic engine, 8500rpm redline everyday.
no mean to highjack op's thread.

 

Only 8500 rpm? Oh, look at those pistons.



Hijack complete.