if you are interested. A lot depends on the length of your regular commute; VW TDI engines take a while to warm up and fuel efficiency is much worse when they are cold. I notice my Oregon cold weather mileage has dropped a lot more on my daily commute (10 miles each way) than on a once a month 60 mile each way drive to a work site on the coast. For the 10 mile trip, the engine is cold for the first 3 or 4 miles and you can watch the instantaneous mileage on the MFD start to climb as the engine warms up. HTH,
I believe the main reason is that since the air is denser at lower temperatures, the computer in your car has to direct the fuel injectors to provide more fuel to compensate for the denser air. Your Oxygen sensors are constantly monitoring the combustion process and force the injectors to maintain the correct air/fuel ratio of 14.7:1. Other reasons are explained above, namely, the oxygenated fuel and the thicker fluids in your engine, transmission, power steering system, and rear differential for rear-wheel-drive cars. For 4X4s, add on the transfer case and the front differential. Finally, if you live where it snows a lot (like me) there is the added resistance of the snow and slush that your tires have to push through.
Like bentheswift says, denser air + more fuel = more power, so any impact from this should be easily compensated for my easing off the throttle a bit and still making the same pace and using the same fuel.
My money for chief culprit, as others have pointed out, is the time taken for the engine to warm up. I've just switched to a diesel from a petrol engined car and am stunned by how long it takes to warm up at this time of year. (& I know from experience that until the engine is warmed up, the ECU is programmed to run pretty rich)
WHat I have no idea about is am I better off just tip-toeing around for 15 minutes while the car warms up - or should I just give it some welly (within reason, of course) to try and warm it up quicker and get out of this "running rich" phase as soon as possible..
(I know I Know - it's bound to be the boring option..)
..someone in another thread (ok, sorry to be so vague) stated that engine warm-up was a thing of the 70's-mid 90s..cars now can deal with cold engine climates without any warm-up needed (well, non-extreme climates)..
..hopefully that sage will chime-in here.. but i (personally) work off the theory of crank-&-go..i don't even use the accelerator the 1st couple of blocks, but within 10 blocks my engine is heating up (on the guage, as well as in my car cabin)..
..i think pre-heating/warming your engine is mostly a waste..the engine will warm faster while moving anyhow..so if you're waiting for a warm interior (unless you REQUIRE it to be warm when you sit down), it's faster to get on the road..
I'm not too sure about diesels, but certainly with gasoline engines, you require a richer fuel mixture before the engine gets up to operating temperature. Since, when it's cold, the engine takes significantly longer to reach operating temperature, you're running a richer fuel mixture and burning more fuel.
Also, just keep in mind in general that coolant temperature is not the same as oil temperature. My Miata is an excellent example, where the coolant temperature rises really very quickly, but oil temperature (and I am using the oil PRESSURE gauge to make inferences about oil temperature, assuming that higher idle oil pressure is due to higher viscosity and therefore colder oil) takes many more miles to get to operating temperature / pressure. I would not start flogging your car just because the coolant gauge has reached a normal operating point.
I mostly just start and go. My main vehicles stay outside all the time so they are going to be bound by whatever the outdoor temps are. The colder it is, the harder to start and the longer it takes to warm up. If I have the baby joining for a trip I tend to start and let it run at least 5 minutes, just so it isn't cold for long once I get going. Usually start it, strap him in, then get going for example.
These vehicles on average take a few miles or so for the transmission to warm up enough for the overdrive/lockup to kick in. Until then they will run higher RPMs that fluctuate with changes to engine demand, therefore using more fuel. Fortunately I live in the country, so driving at 55 tends to warm up a car a lot faster than someone living in the city and going only a mile or 3 to get to work. Often cars in this situation never properly get to warm up to operating temperature.
I do have a block heater on my newer van, but I think I have used it once, maybe twice when the temps dipped below -10F. It does help though, noticeable difference in warm up time. Most of my winter temps here average around 15-20F, and that doesn't really require this.
I also agree about oil temps. Oil pressure does always seem to be higher when cold and stay higher longer than it would in the summer. My older van will even kick on the low pressure light occasionally on a hot day due to the lower oil viscosity, and that's even using a heavier weight oil during the summer!
Those of you that keep them in a heated or semi heated garage will have at least half as many incidents with this sort of thing. Most of us don't have heated garages at work for example. LOL