A few auto companies are switching to turbo charged 4 cylinder engines
to replace a naturally aspirated V6's.
Hyundai is a great example.
In the new Santa Fe SUV and mid sized sonata, engine choices include
a naturally aspirated 2.4 liter 4 cylinder
at around 200hp or a 260-260hp turbocharged 4-cyclinder.
My question is about fuel economy. How come highway fuel economy
figures are -worse- for the turbocharged engines. Common sense tells
me that the smaller engine (displacement) would be more or at least
equivalently efficient when producing the same horsepower to keep the
vehicle moving at a specific highway speed.
The turbo charger should have little effect, with good engine design,
at low engine loads it shouldn't reduce overall fuel efficiency.
I'm not sure where you see "low load" but on mine the 4 banger is
2500 rpm (60-65mph). Which means turbo is spooling.
Where you gonna get fuel savings on freeway with the turbo beats me.
It's lighter, yes so you have lower mass to push around in town
but during freeway slog it's all about combatting the drag and physics
And having turbo helps to dodge an occasional idiot tailgating you
Dodging idiots is not a very fuel saving activity.
here ya go
"displacement" increases with a turbo/supercharger - you can
physically cram more oxygen molecules into the engine with a turbo
than you could achieve with a NA engine operating at 100% VE and
therefore you can burn more fuel. Therefore, as a rough analogy, you
could think of a one liter engine running with 7.3 psi boost
(handwaving, 1/2 atm) as being roughly equivalent to a 1.5 liter
engine. That analogy breaks down when you look at it too closely for
various reasons (heating of intake charge being the most obvious) but
for a handwavy, back of the envelope explanation, hopefully you get
the idea. Savings, if any, come from the lighter weight of the
I have never owned a turbocharged car, but have driven several.
On one of them, a Passat, the mileage indicator on the dash promised
pretty good mileage as long as you didnt put your foot through the
accelerator...So cruising at road speed wasnt so bad. It was the
acceleration phase that seemed to eat your lunch.
True, partially true, or just BS?
The only thing that makes a turbo special is that it's capable of stuffing
more air into each cylinder than a "naturally aspirated" engine, which
needs to /suck/ the air instead. More air means more gas. More gas means
more consumption. Light throttle means less air stuffed in; heavy throttle
means more air stuffed in.
A turbo allows a smaller engine to pretend it's a bigger one, since the
only difference between smaller and bigger is the amount of air they can
combust at once.
Steady-state cruise requires a small throttle opening, and allows a lean
mixture, with low power-output. During such an engine state, a turbo would
have little to do. If it continued to stuff air into the engine, it would
simply result in increased fuel-usage as the computer adjusted fuel-
delivery to keep the mixture correct. So the turbo is told to shut up for a
But, put your foot down, and you'll get the "pretend" big engine effect,
with resulting big-engine gas mileage.
The whole point of modern CAFE-driven turbos is that the designers can
program the engine to operate as a smaller engine at light-throttle, and as
a larger engine as heavier throttle. The problem with small, turbo'ed
engines is that fuel-economy becomes completely a function of the
sophistication of the computer's programming, and the necessary compromises
between power, driveability, and fuel-economy. I think that if a modern
CAFE-turbo engine delivers worse mileage on the highway than it does in the
city, then the culprit is the programming.
you're making stuff up again tegger. and you're letting your insane
obsession with emissions legislation get in the way of your ability to
comment objectively. thermodynamics determine efficiency - you can't do
more work without a penalty.
it's not "the most obvious" if you know what you're talking about. the
biggest "break down" is that there's no free lunch. you can't drive a
turbocharger [and get it to do work], without putting energy in, and
that comes via the motor.
thermodynamically, an engine [and turbocharger] converts heat to
mechanical energy. if too much energy is being taken out of the hot
exhaust gas by an engine running too efficiently, then it can't do much
work at the turbo stage. thus, with turbo engines, you have to run them
with impaired efficiency so that you get enough thermal energy out to
drive the turbo. /then/ the turbocharger has its own losses, but
they're incidental - the thermodynamics are much more fundamental.
turbos don't offer you savings - they offer a relatively cheap way to
get a 3" deposit out of a 2" pipe - if you do it right.
You are telling that you can make the exhaust gases somehow
at the temperature of the mixture ingested? That's about the only way
you would have the exhaust mixture unusable for running the turbo.
Wow, a car would have an absolutely cool (literally) exhaust hardware.
Think of how durable and lighter that would make the exhaust piping.
Plus no heat losses from the engine which is like 100% internal
combustion efficiency. Wow.
Your engineering capability somehow never ceases to amaze me.
and why is it that some people, when confronted by something that
doesn't make sense for them, presume that it's bullshit rather than
their actually standing on the threshold of new knowledge? you'd be on
the threshold of learning something truly spectacular from the world of
thermodynamics if you didn't have this affliction.
if you want the turbo to do actual work, you need to supply heat energy.
the fact that the majority of civilization's energy is derived from
some form of heat engine should tip you off to this fact.
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