There are other factors that people don’t consider. People tend to let their cars warm up more/idle more. Fluids are thicker and more viscous. Some fluids never get thinned out as much as they would in summer months–your differential or final drive in your transaxle for example, which adds resistance and saps power. But let’s assume the car is fully warmed up:
People drive slower in the bad weather and often in lower gears for longer than normal, which causes RPMs to be higher and efficiency to be less. The resistance added by road slush/snow adds drag. People also spin their tires more in the inclement weather, and driving tends to be a lot more stop-and-go, as people react (sometimes poorly) to road conditions and other drivers. Some people don’t fully clean their cars off, and the added weight of snow and ice as well as the aerodynamics changing can affect mileage. And the air you’re driving through is denser too when it’s cold–which may not add much more resistance, but every little bit hurts.
With both speed-density and mass airflow-based fuel injection, since the computer cannot control the air being input, it must increase the fuel mixture to compensate for more (denser) cold air coming into the engine. While the process is pretty efficient when in closed-loop, you still tend to burn a bit more fuel. Though you do get a bit more engine performance to make up for it, kind of a natural supercharging effect in the winter. Fuel is also less volatile when cold and may not atomize as well or burn as well as warm fuel, even in a hot engine with winter-blend gas, which may itself affect mileage negatively or positively.
@BillRussell, you did not include fuel mixture on your list, as mentioned by @CapriRacer in a previous post. I notice a drop in mileage every year that coincides with cold weather and goes away when the weather warms.
As @oblivion addresses in paragraph 3, a rich fuel condition doesn’t really exist in a warming engine. It is closed-loop controlled by the O2 sensor to stay stoichemetric breathing the cold, dense air. Depending on how far the catalytic convertor is from the engine, there may be extra fuel added short term to more quickly heat up the catalyst. The EPA only allows a very small amount of pollutants during warmup so if dumping extra fuel in is the only way to light off the cats, manufacturers will do that. They don’t want to because it reduces mileage, not CAFE mileage, but real world customer mileage. Nobody wants their customers to get 2/3rds the gas mileage in the winter.
The computer adds a bit more fuel with denser air to keep the equation in balance, but you don’t need to push the pedal quite as much to maintain the same speed.
If all factors other than intake air density remained the same, it’d take exactly the same amount of energy to maintain a given speed. You’d use the same amount of fuel. It’s the changes in drivetrain resistance, rolling resistance, and aerodynamic resistance that affect mileage once the engine is fully warmed up. UNLESS, of course, the ambient temperature is so cold that the engine never fully warms up. In ND, we used to have to put corrugated cardboard in front of the radiator or we could drive all the way to work without the engine ever warming up. Subzero temperatures can dissipate heat faster than the engine could generate it. Been there, done that. I speak from experience.
Seems like the winter fuel blend is being overlooked.
“The US Environmental Protection Agency (EPA) says conventional summer-blend gasoline contains 1.7 percent more energy than winter-blend gas, which is one reason why gas mileage is slightly better in the summer. However, the summer-blend is also more expensive to produce, and that cost is passed on to the motorist.”
If all factors other than intake air density remained the same, it'd take exactly the same amount of energy to maintain a given speed.
If the air density increases because the air is colder, the air has less energy which must be compensated by slightly higher fuel usage. If the air density increases because of higher pressure, the air has more energy and mpg will go up.
If the air density increases, there’s more oxygen available per volume of air. The ECU meters fuel based on the air mass passing by the MAF sensor and the oxygen level out the pipe. If there’s more oxygen available per volume of air, the ECU will compensate with more fuel. The key is that since more volume will be going into the engine, more power will be made and less throttle will be necessary, meaning you’ll break even.
Assume the cruise control is on. The ECU will need to use less throttle to keep the car going a constant speed, because there’s more oxygen per volume of air and it adjusts by adding a bit more fuel.
I severe illustration of the concept is N2O (nitrous oxide) injectors. N2O contains 33% oxygen, 50% more than air’s 22% and enabling more fuel to be added via the injectors for much more power. It also chills the ingoing air as well, increasing its density.
Add extra fuel at the intake ports. But it isn’t because of the air temperature, it’s because of the density.
The energy in the engine comes from the expansion of the gasses caused by the greatly and rapidly increasing activity of the atoms. Temperature is actually a measurement of atomic activity, inertial energy of the atoms. They bang against one another and the inertia is converted to heat energy.
While the temperature differences that we’re discussing here are sufficient to affect the air density and thus the amount of oxygen entering the chambers, it is way below sufficient to affect the combustion of the fuel, way too insufficient to affect the expansion of the gasses once the heat from the sparkplug starts the combustion process. Remember that the temperatures in the cylinders reach 1500F, in some cases 2500F, all from the combustion process. The engine in general is operating at temperatures of over 200F.
As a matter of fact, preignition (fuel self igniting from the residual heat of the cylinder plus the heat created by compressing the fuel) is a problem with an engine at full operating temperature rather than lost power due to too-cool intake air. Systems such as EGR systems (now being replaced with variable valve timing) had to be created to control the preignition problem. If the heat gets too high, preignition can happen and the engine can also produce excessive NOx. Nitrogen molecules in the air can bond with oxygen atoms in the air. The EPA says that ain’t good.
One factor that applies to my Prius- and any other car with a stop-start technology is that, in cold weather and while cold- instead of shutting off at a stop light it will run to heat the interior. And it is not running to get the engine to operating temperature; I can turn off the interior heat and no matter how cold it will shut off at a stop light.
the same mountainbike In ND, we used to have to put corrugated cardboard in front of the radiator or we could drive all the way to work without the engine ever warming up.
Yeah and then you might get a freak warm day, forget about the blocked radiator, and, well…
Been there, done that. I speak from experience.
Well now I personally have no direct experience with the boneheaded negligence mentioned above, nosirreee. Just sayin…
Speak about crappy motors that refuse to warm up; the two stroke SAAB we had came with curtains that snapped on over the grill. After the engine warmed up sufficiently, you had to stop and take them off. If you drove too fast or the air temp dropped a little, it was back on again. 40 np and in the winter, I don’t think it got 20 mpg. It loved to foul plugs too if you failed to put the curtains on or ran the heater too early and the car too slow so you always carrying several extra plugs. The worse winter motor, , correction the worse motor I ever had in a car during cold weather. Playing with curtains and replacing plugs on the side of the road was a regular ritual in cold weather with this POS motor.
There was one good thing about those 2 stroke SAABs. They were fun to take out in traffic and rev them to the moon while keeping a subtle eye on other motorists or pedestrians.
Some of the reactions to a cloud of smoke and the sound of a chainsaw buzzing through a tree trunk were pretty funny.
Wow, auto-owner, you really need to take your meds…
We were all having a pretty good discussion here. I’m sorry you disapprove of the way I adapted to the north country. For the record, I never did forget about the cardboard. It’s a normal part of living in North Dakota. Perhaps if you actually did have some experience living in subzero temperatures, you’d feel differently. Just sayin…
If you think you’ve contributed anything, well, you haven’t. Have a nice evening.
Re; increased friction losses in the winter due to increased viscosity…
It should be self regulated to some extent. When cold, the increased viscosity causes more friction, which warms the oil, decreasing the viscosity, decreasing the friction. Sounds like negative feedback with low gain, with net result that the transmission/differential tends to equalize the internal temperature.
It seems to me that most if not all of these temperature related MPG issues are solvable by some engineering. Things like heating the incoming air, or decreasing air flow around the transmission/differential. It also seems to be a neglected area in the manufacturer’s never ending quest for better MPG numbers, as mandated by the Feds.
Or do the measurements the Feds do to check MPG numbers not take into consideration lower air temps in the winter? So the manufactures could care less if the MPG numbers drop in cold weather?
Well,look on the brightside,more and more vehicles are coming with shutters over the airflow to the cooling system(I hope they are cold adaptive)Seems like it takes a gallon of gas to move My V-6 a mile from a cold start
I do remember having a MG1100 while living in mid NH. To start it, I’d run the starter with the choke fully closed till the battery started to get low, go away for 15 minutes or so, then try again after the battery recovered a bit, and it would usually start. Lousy weber carburators had to be filled with gas as most of it condensed on the intake manifolds.
The old Rolls Royces didn’t have a thermostat inside the cooling system.
Instead, they had a thermostat that activated a servo motor which opened and closed shutters on the outside of the radiator grill, in order to control the air flow.
I don’t know exactly when they abandoned that design, but if you see any old RRs at a classic car show, more than likely they will have this somewhat unorthodox approach to allowing the engine to get up to normal operating temperature.
I think we’ll start seeing a lot of radiator shutters in the future, not just for better control of engine temperature, but also to minimize air drag during high speed cruising.
Aircraft use adjustable cowl flaps, open and you get maximum air flow through the engine cowl during take off and climb when the engine is at full power and the airspeed is low. Closed, and you get minimum drag during cruise when the airspeed is high and the engine is running at reduced power.