"Alex Esplin" <[EMAIL PROTECTED]> writes:
> More air means bigger boom. I'm sure we all agree on that by now.
> Bigger boom means less gas for the same amount of power. So when your
> foot is not all the way to the floor you do indeed get better
> efficiency and therefore mileage. More power == more efficient if
> you're not pouring gas out on the ground.
"More air means bigger boom" is true, but it's tempered in gasoline
engines by the fact that they are compressing a fuel/air mixture and
not just air. Compressing air increases its temperature, and high
temperatures can prematurely ignite the mixture, making a big boom
that occurs before the engine is ready for it and possibly blowing the
piston and/or cylinder apart or bending the rods. High-compression
gas engines are more efficient, but they also require high-octane fuel
to avoid pre-detonation.
Turbocharging effectively increases the compression ratio, making
pre-detonation even more likely. Thus, timing has to be pulled and
more fuel added as boost increases beyond atmospheric pressure, which
dramatically reduces efficiency and power below what optimum spark
timing and fuel mixture would bring. This is why turbocharged gas
engines can be tuned to much higher performance on race gas than pump
gas.
To avoid this efficiency problem, gasoline turbo cars don't run any
boost at all during low-load conditions, so they run pretty much like
a non-turbo car would when they're not accelerating or climbing a
steep hill or whatever.
> My Honda Fit, which gets better mileage on the freeway than my old
> Civic did, gets significantly worse mileage around town. Why?
> Because it has less power. Less power == more effort to do the same
> amount of work. Stick anything on my car to increase it's power and
> my in-town mileage will go up (to a point) because it takes less
> effort to do the work.
Yes, underpowered engines are inefficient at certain tasks, and more
efficient at others. All engines will have a power band in which they
are most efficient. If you run with more time in that power band,
you'll have greater efficiency.
> It takes less effort for me to lift 50 pounds than it takes my wife.
> Because my muscles are larger and generate more power than hers, that
> same 50 pounds is less of an obstacle and I can lift it and put it
> back down many more times than she can. I'm sure I could come up with
> more examples, but I hope we're all getting the point.
Bigger muscles are actually far more *inefficient* than smaller ones.
This is why males, who generally have larger muscle mass, burn more
calories at rest than females. In our day of abundant calories, this
is a good thing, since burning more calories means you get less fat on
the same diet. :P Anyway, I see what you mean here, but I don't think
it's a great analogy.
> I spent a good portion of my younger days as a mechanic, both in the
> Army and the civilian world and believe me, I have played around with
> _lots_ of engines. There is a reason that nearly all military diesel
> engines have a turbo attached to them, and that reason is not solely
> for max power. Our ambulance HMMVs (had turbos) when I was in the
> Army had a larger "effective range" than the NA HMMVs did. One last
> time, more power == more efficiency.
As I mentioned earlier, turbo diesels are a very different animal than
turbo gas engines. Gas turbos just don't add efficiency like diesel
turbos do, because gas engines can't take the high compression that
diesel engines do. Gas turbo engines have to run very rich under
boost (or use tricks like water/alcohol injection) to avoid
pre-detonation, and have to avoid boost during cruise via the throttle
and wastegate.
To reiterate, when not in boost (and the engine is not in boost unless
it's accelerating or otherwise under heavy load) a turbocharged gas
engine uses exactly the same amount of gas and air as it would without
a turbo, and thus is pretty much as efficient as it was before. A
turbo diesel, however, can run at much higher compression and probably
runs some boost at cruise. It also doesn't need to worry about
pre-detonation because fuel isn't injected until it's time for
detonation; i.e., the compression stroke only compresses air, not an
air/fuel mixture as in a gas engine.
--Levi
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