Just placed an order for a '04 Civic LX

It only applies if the ambient temperature is optimal. Not when the
temperature is near zero. At that temperature gas does not vaporize as
well in the cold, and the computer will run the engine richer until it
is warm.

Is it true more HP = higher mileage? Does a turbo/super charged engine
get better mileage or just more power?


Don't forget the car has to move through the denser air. It's probably
negligible.

 

Of course it does! Just the richer mixture needed for the first
quarter hour while the engine is getting up to temperature will use
more gas.

In really cold climates you might as well. In Cold Lake Alberta I
remember drivers used to remove the fans from their air-cooled
Volkswagen engines in a desperate attempt to get some heat into the
cylinders. It's for sure your thermostat on a water cooled engine
doesn't open very much at 30 below! I know I had a piece of cardboard
covering the entire rad on my car between December and February.

Maybe where YOU live!

After the admixture of more gas

No, lower milage. You don't get something for nothing.

## OK, who stopped payment on your reality check?

 

It was once common practice up here in Canada to put a piece of cardboard
in front of the rad to cut down on air movement so as to help the engine
keep its temperature, even on cars with electric fans.

The very newest cars appear to have cooling systems that are efficient
enough not to need this. However, diesel highway tractors still regularly
employ rad blankets.

Do you mean "reformulated" fuel as defined by the State of California? Or
"reformulated" as in having more aromatics than summer gas?

 

Don't forget that people tend to spend much more time in slow or stalled
traffic in the winter than in summer. Even if the engine is running at peak
temperature and efficiency, you're still burning gas while sitting in that
traffic jam at 0 mph.

 

More horsepower, maybe, on a car that doesn't measure the mass of air
passing through it, *and* only during open loop operation. But your mileage
is toast during open loop.

You're not going to have _both_. You won't have either on any car with a
oxygen sensor.

 

if the temperature drops 20-30 degrees F your tire pressure will also decrease
3-4 lbs hence higher rolling resistance

 

Theoretical thermodynamics does indicate that the lower the starting temperature
of the working substance (the fuel-air mixture in this case), then the higher
the efficiency. Some might recall that the theoretical maximum efficiency of an
ideal engine is (Th - Tc) / Th x 100%, where Th is the peak temperature in the
cycle and Tc is the low temperature in the cycle, and all temperatures are
measured on an absolute temperature scale such as Kelvin or Rankine.

Also, you're right that colder air is desirable from the standpoint of
increasing density and thus being able to pack more air into each cylinder. As
I'm sure many people here are aware, many turbo-charged and supercharged engines
have an intercooler precisely for the purpose of lowering air temperature to
increase density.

But as several here have pointed out, practical considerations preclude applying
these rules in their entirety to the typical automobile.

First, even in a spark ignition engine, the temperature of the fuel-air mixture
will affect the efficiency of ignition; the efficient burning of the mixture;
and thus the efficient expansion of the gases. It's not just the initial
"explosion" that drives the piston; it's a (preferably steady) expansion of
burning gases. The fuel air mixture ideally burns through part of the piston's
downward stroke. In sum, the lower the air temperature, the less able the
fuel-air mixture is able to combust efficiently.

Second, the viscosity of the lubricating oil is higher in winter, and so moving
the pistons will take more energy, and thus more fuel.

Third, just because the car eventually becomes warmed up in the winter time,
this does not mean that maintaining the steady state warmed up temperature will
require less fuel than in, say, the summer time. The engine (including the
radiator cooling system and anti-freeze) is still "battling" the ambient losses
to the surroundings. The anti-freeze coolant's purpose in general is to cool,
but it also maintains more uniform temperatures of the engine block and lube
oil. I would argue the coolant's function in winter emphasizes uniform
temperatures more than cooling, so the higher winter ambient losses still mean
more fuel has to be burned to maintain the whole engine at a certain, optimal
temperature. Coolant temperature itself is an input to modern engine controls;
it does help determine how much fuel will be burned. The average coolant
temperature will be lower in winter, because the temperature difference between
it and the outside air (where it's dumping its heat) is on average higher. The
higher temperature difference means the coolant loses heat more rapidly in
winter than in summer. What replaces this heat? More combustion heat,
translating to higher fuel demand.

Fourth, and this is arguably a negligible effect, tire pressure lowers at lower
temperatures. I estimate with some back of the envelope, Pv = nRT calculations
that it falls about 2.5 psi when temperatures fall from around 70 F to 25 F. Yet
I understand such a decrease in pressure will decrease fuel mileage by a few
percent, at most. So the car loses maybe 1 mpg at most from not monitoring tire
pressure.

 

I am surprised at how quickly my 2003 Civic Hybrid comes up to operating
temperature. It's easy to tell when it gets there because the automatic
climate control suddenly increases the fan speed when it decides there's
enough heat to spare for the occupants ;-). My digital gauge seems to
start at two bars, even with ambient temps around 20f. I don't idle long
for it to warm up. I figure it is designed for engine-stop when warm,
so extensive idling seems contrary to the plan. I drive easily on level
streets for about 3-5 minutes before the temp is at four bars and warm
air is blowing.

I recall a 73 Mazda RX-2 taking 10 miles before the temp gauge was normal,
and the heater was blowing warm air.

I realize a quick warm up is helpful for better mileage, but what about
the bulk of the time, when it is already warm? It seems to me that if
the Honda heats up that quickly, it must be tossing a a lot of waste
heat out the radiator under normal driving conditions.

 

In general, both.

The efficiency of a gasoline engine cycle (for the most part, an Otto cycle)
depends mostly on the compression ratio. While the starting pressure of the
working fluid is higher when turbo/super charging, the fact that more fuel can
be combusted on each stroke means a much higher final pressure occurs. So in
general, the efficiency is higher.

 

I was thinking of some of the much smaller engines found in other
countries. That is a fast warm-up on the Hybrid though. Are you sure
there isn't some electrical assist to the heater?

I haven't really looked at the engine arrangements on any of the hybrids so
I can't say what might be different. I could envisage a more sophisticated
thermostat - say maybe even some kind of electronic control vs. the rather
crude melted wax+spring which is the norm in regular engines. If the IMA
battery is at full charge it would also be possible to use an electric
heater to add heat to the coolant. I've no idea if those kinds of things
have even been considered - something to find out I guess.

Rgds, George Macdonald

"Just because they're paranoid doesn't mean you're not psychotic" - Who, me??

 

If you ever study thermodynamics you will probably be amazed to find that
the theoretical efficiency of a 4 cycle gasoline engine increases as the
inlet air temperature decreases.

 

What about the surrounding air?

 

what about when fuels temps rise?

150F fuel temp? Think about it.

 

If you ever study thermodynamics you will probably be amazed to find that
As I recall, thermal efficiency of any internal combustion engine depends only
on highest Kelvin temp within the engine and Kelvin temp of exhaust gasses.

Quent

 

I would think this would be the case even--or maybe especially--in a
compression-ignition engine. Why else do diesels have glow plugs?

Ah, an excellent point!

Up here, even with a fully warmed-up car, the oil in the sump cools so
rapidly that it has difficulty burning off water and other contaminants.

Also, when you measure the oil level, you cannot do it during the five
minutes you are at a gas station filling up, as it will register quite low,
much of the oil still attempting to drip off the block walls.

Pulling the dipstick from a fully warm engine at 85F ambient will have the
oil dripping off th stick. At -10F it will not drip at all.

Which are considerable when the difference between block and surrounding
air is 230F in the winter, as opposed to 135F in the summer.

 

It is indeed especially the case in diesel engines.

(I am quoting textbook stuff I used to teach with a bit of my "hands on"
experience thrown in. The mega-year experienced technicians and mechanics can
jump in, and I will be all eyes and ears.)

That's a helluva an observation to back up the contention above.

Same for the following:

Yes. Yet I think my point above could stand some refining. As others were
getting at, there is a question of whether in very cold temperatures the
radiator's function is merely to uniformly warm/cool the engine and oil. In very
cold temperatures, does the anti-freeze actually transfer much heat to ambient?
I'd guess less than in warm temperatures. That is, I guess in really cold
temperatures much of the heat transfer is from engine block to radiator coolant
to other engine components. But in warm temperatures, much more of the heat
transfer is from engine block and lube oil to the radiator coolant which then
dumps the heat to the great outdoors.

 

Turbos allow for smaller engines in a given car, which during most of
their use should get marginally better mileage. Less weight to haul,
and better efficiency. BUT, as a practical matter, it really doesn't
work on the road. The compression ratio of the engine has been
dropped substantially (around 8 to one instead of 9 or 10 to one), so
efficiency is shot too hell even during light duty use. And, if you
push a turbo'd engine, they're set up to inject huge amounts of raw
gas into the engine just to cool the intake charge and prevent
detonation. Something is terribly wrong with the concept of using $2
to $4 a gallon gasoline for engine coolant !!!!

Chrysler used turbos a lot and developed some proficiency at it. They
HAD to. All they had were little engines, and little engine bays to
fit their little engines into.

JM

 

FWIW I agree my comments above are somewhat too "theoretical" and so in fact
somewhat misleading, especially given the practicalities for most conventional
gasoline engines. The biggest practicality is avoiding engine knock. You get at
this below. In sum, gasoline engine knock is the result of, I think it's fair to
say, compression ignition, which is in turn due to too high a compression ratio.
Excessive knock can destroy a gasoline engine, so increasing the compression
ratio in general is not at all practical for conventional gasoline engines.

But more below.

To take this a step further--
As the old timers are no doubt painfully aware, gasoline engine cars designed
for variable compression (VC) ratios are on the road or are nearly on the road.
(Saab leads on this front? I lifted the numbers here from a Saab article on the
web.) At light loads, the compression ratio is high (14:1, according to one web
site) while at lower loads, it's adjusted to something much lower and closer to
what is customary on most non-variable compression cars, but as low as 8:1.
These VC engines are more efficient.

But on these variable compression ratio gasoline engines, to realize the higher
compression ratio at low loads, and thus get the increase in efficiency, a
turbocharger is necessary. So...

Anyway, my first remarks above better pertain to diesel engines and these hybrid
gasoline engines, which certainly is not the focus of this thread. Your point
that any increase in conventional gasoline engine efficiency due to
turbocharging is due to weight considerations, not effects on the compression
ratio, is well-taken.

 

(JM) spake unto the masses in

In the '80s, Ferrari made a tax-friendly version of their then-popular 308.

It was called the 208. 308 and 208 being, of course, the cylinder
displacement in cubic centimeters.

208cc x 8 = 1664cc was under the 2000cc limit (?) beyond which taxes
increased dramatically. Of course, they had to make up the power
difference. They did this with a turbo. An Italy-only car. The rest of the
world got a 308 with no turbo.

Something's terribly wrong with governments that decide that they will
extort money from you in the interest of social engineering. That $2/gal
gas is really about 90 cents, once you take the government's protection
money out if it.

And boy did those turbos fail regularly! How many of them had oil feed
after shutdown?

 

2.0 being the cylinder displacement, and 8 being the number of cylinders.

512BB. 5 litre, 12 cylinder.

It *used* to be single cylinder displacement...a 365 (times 12) was a 4.4
litre.