fuel economy improvements of turbocharged engines

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.
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I'm not sure where you see "low load" but on mine the 4 banger is struggling around 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 is physics.
And having turbo helps to dodge an occasional idiot tailgating you pronto. Dodging idiots is not a very fuel saving activity.
here ya go
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"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 engine.
nate
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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?
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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 while.
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.
--
Tegger

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On 04/09/2012 07:17 PM, Tegger wrote:

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.
--
nomina rutrum rutrum

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On 04/09/2012 02:08 PM, hls wrote:

completely true. the purpose of the turbo is to blow more air in so you can burn more fuel. and that's what you're describing.
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On 04/09/2012 08:06 AM, N8N wrote:

ok so far.

"roughly"??? why not understate it again?

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.

--
nomina rutrum rutrum

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m
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.
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On 04/10/2012 12:11 AM, AD wrote:

no.
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.

you amaze me too. but for a different reason.
--
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