<VV> Re: Fuel Milage in winter - only minor Corvair

[Jim Burkhard]

Shaun, et al:

Modern cars are smart enough to take the intake air 
temperature into account when figuring out how much fuel to 
flow. They calculate airflow on a mass basis (grams/sec) 
rather than volumetric basis like a carb. This will either 
be done via:
a. an airmeter (a.k.a. mass airflow sensor) which is 
inherently temperature compensated, or
b. a speed density calculation of airflow based on engine 
speed, intake manifold pressure, intake manifold temperature 
  (tadah!), and a factory-calibrated fudgefactor called 
"volumetric efficiency".

Either way, assuming the actual mass airflow hasn't changed, 
the fuel pulse width applied at the injector will stay the 
same as well. Air *volumetric* flow drops, but mass flow 
stays constant and so does injector flow.

There are two minor factors I've thought of that haven't 
been mentioned as a cause for a little bit of economy loss 
at low temperatures. I think they (especially the first I 
mention) are much smaller than extra powertrain, drivetrain, 
and tire friction, or the slower warmup and longer 
enrichment factors people have already mentioned. but, just 
to be really anal, I'll throw it in. Here goes:

When the air gets colder, the charge gets denser. This is 
great if you'd like to make more torque or power (throttle 
fully open), but if you are just trying to run the exact 
same speed & brake load as before (ignore the added load 
friction factors I mentioned in the last paragraph), it 
should actually increase the part throttle "pumping losses" 
a little bit. What happens is that it takes a tiny bit less 
throttle opening to flow the same mass airflow (= same 
power). The driver doesn't realize that the throttle is only 
say 24% open instead of 25% open to hold 70 mph at 2500 
rpm...  Because the throttle is a little more closed , the 
intake manifold pressure will be lower (i.e. vacuum is 
higher) and it takes a tiny bit more negative work to move 
the fresh air past the throttle into the intake manifold. 
It's gonna be a pretty small effect, but I'll bet it could 
be measured on an engine dyno with controlled intake air 
temperature.

There will also be some increased thermal losses to the 
coolant (especially during warmup) and off the block/head 
itself when it's really cold out.  You may be familiar with 
the concept of an superinsulated adiabatic (zero heat loss) 
engine improving fuel economy. This is the oppposite effect.

So add these to the list!  :-)

Jim Burkhard


Shaun wrote:
> I'm guessing that colder, denser air would cause the mixture to richen up in an injected engine...
> Mr. Burkhart?
> SM