I have a 1998 Toyota Corolla LE that get's really good gas mileage.
Recently I purchased something called a "Scangauge", which is a
diagnostic tool that connects directly to the vehicle's OBD2
connector. It lets me see in real time several engine parameters at a
glance. I noticed that on warmer days the mileage was much better.
My morning commutes to work nets me around 32 mpg average : that's for
a 23 mile commute that's mostly highway miles, where I NEVER exceed 58
mph. In the afternoon, when it has warmed up considerably, I have hit
averages of between 37 and 40 mpg's, on the same exact route. I have
read that the optimum intake air temp is around 95 to 100 degrees
(f). Has someone found a good way to boost intake air temp on cold
days? I'm thinking in terms of placing a small heating element in the
air plenum, in the ductwork between the grill and the air cleaner
housing. I'd love to hear any ideas!
The improved fuel economy on warmer days is not from higher intake air
temperatures. The warmer the ambient air temperature, the more quickly the
car's computer will reduce the high idle associated with startups, the more
quickly the automatic transmission will upshift, and the more quickly the
emissions controls will go into closed loop operation.
You will get little, if any measurable improvement in fuel economy from
heating the intake air temperature. In fact, cold air tends to be denser
than warm air so the engine burns fuel more efficiently with cool air than
with warm air, which is why people trying to get more power from their
engine run cool air intakes out the front of the car or up on the hood.
"Ray O" <rokigawaATtristarassociatesDOTcom> wrote in
And the faster the oil heats up and the greater the chance it will get
close to 210F, reducing frictional loading on the engine.
By the time the intake air gets into the combustion chamber, it's already
Remember the throttle body is heated with coolant bypassed from the block,
and radiant heat in the intake plenum and runners add their own heat to the
air. Just try to touch the intake runners on a hot engine.
Toyota already does this on many, if not all their models. Even our lowly
Tercel has a factory "cold air" intake.
I would not attribute your increase in mileage with increased intake
temperatures. First, make sure that you are not running the defrost mode
in the morning which will decrease mileage. Colder air is denser than
warm which would equate into higher efficiency in the combustion
process. I have noticed this decrease in mileage affecting small engines
associated with colder temperatures as well. It could be that the engine
displacement being relatively small is very sensitive to loads placed on
it, and in turn uses more fuel. Cold outside air temperatures have a
direct effect on the viscosity of all lubricants, (engine oil,
transmission oil, bearing greases, ect.)usually increasing them, and in
turn, increasing engine loads. Temperature swings will even slightly
affect your rolling resistance associated with tire pressure. Operating
temperatures in cold (winter) conditions can be substantially lower
than during warm (summer) conditions. A head wind or tailwind can be a
factor as well. It's my opinion that the increase in load for the
reasons mentioned increases fuel consumption. I have read no particular
data to back this up. I don't think that this would be as noticeable if
a larger engine was compared. It will be interesting to read others
opinions on this.
Up to a point! If the air is TOO cold, the engine can't get to normal
operating temp and doesn't operate as efficiently. Warm air isn't too
I find that temps between 45-60 degrees gives the optimum performance.
Judging by my Mazda, all winter I could barely crack 25MPG, usually LESS,
but when we had a week of temps ~52-65 degrees, it jumped up to 29 MPG!
The stoichiometric ratio (optimum air/fuel ratio) for gasoline is 14.7:1
by mass. Simply put, colder intake air offers a higher DENSITY of oxygen
(oxygenators) that improve the efficiency (fewer emissions) of the
combustion process which in turn equates to a higher energy output of
the engine. Colder fuel can also increase efficiency by entering the
combustion chamber at a higher density than warmer fuel (racing fuel
cool cans). Colder INTAKE air is always beneficial and the colder the
better (ram air, inter coolers, ect.) unless we are talking absolute
temperatures because it is denser (turbochargers, superchargers, ram air
effect). The modern liquid cooled engine will attempt to reach operating
temperature set by thermostat temperature choice and coolant system
pressure in some instances. Even in extreme cold if the thermostat is
operating closed at times it will open enough to prevent overheating in
the jackets and then cycle again. (This usually happens one cycle every
time we start our cars from dead cold.) Again Hachi, I think it's the
load placed on the small displacement engines due to the increased
viscosity of lubricants when it's cold outside. But I could be all
wrong. IMHO. OBTW, I took the blade off the truck again. ;^)
1. Are you a Physicist or something?!?!
2. Yes, I definitely know the advantages of cold (or cooler) air in the
intake of the engine, and the density of cooler air. I was trying to
figure out a way to wrap tubing from the A/C around the air intake on my
'Hachiroku' to cool the air heading to the plenum. And, I had a 'cold air
intake' on the Tercel. It may not have doen much for HP, but I did notice
that off-the-line starts were a LOT smoother after installing it, and my
fuel economy went from 38MPG over all to 44-45MPG overall.
3. Put that damn blade back on! They're predicting snow all day Thursday!
I'm never going to get my Supra out of storage at this rate!
I think the reduced fuel economy in cold weather is due the longer time it
takes for the engine to reach operating temperature, and because the ECT
will delay upshifts until the coolant has warmed up.
During the summer months, the formula for fuels change, resulting in poorer
The spring and fall, where temps are warm enough for the engine to warm up
quickly, and where summer blend fuels are not in use yet, are when most
people get the bet fuel economy.
In the fuel refining process for seasonal changes, do they not formulate
winter blends to vaporize/atomize easier (for starts) than summer blends
that they try to suppress vaporization/atomization rates? If
vaporization is enhanced for winter blends, would there not be fewer
BTU's and in turn reduce fuel mileage? Usually a fuel with higher
vaporization/atomization rates (more volatility) is refined farther from
the beginning natural product (crude oil) and in turn less efficient
(fewer BTU's)? An example being the available BTU's in fuel oil compared
to gasoline. Interesting Ray. Thanks.
Chemistry is not one of my strong points so I do not really understand the
differences between summer and winter fuel blends. A friend is a chemist
who works for UOP's R&D department formulating fuels, and he has mentioned
that summer fuels burn more cleanly but get slightly less fuel economy, so
that was the basis for my statement. If I think of it, I'll try to remember
to ask him if more refining reduces BTU content next time I see him.
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