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Old 09-05-2016, 03:33 AM   #1 (permalink)
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BSFC turbocharged vehicles

I was wondering n/a vehicles runs best at 80 percent load at around maybe 2000-2500 rpm. How about turbocharged gasoline engines? What if lets say at low RPM boost kicks in i believe entering into boost will require a rich mixture so will probably kill effeciency? so what is the "general rule" on these engines? and how about diesel? VGT engines turbo kicks in so early and from what i understand diesel is effecient when in boost? is this correct?

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Old 09-05-2016, 04:41 AM   #2 (permalink)
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I think turbocharged vehicles vary much much more and you would have to study the stock tune to get a feel for this.

Good rule of thumb is the highest load where it's still stoichiometric is the most efficient load. Some turbo engines can stay stoichiometric under boost (newer ones), while others run rich quickly. You can get much more mpg by leaning out the fueling under boost.

On a diesel it's hard to tell because combustion efficiency is much more variable.

With VVT it is possible to trade torque for efficiency. You can delay the intake valve to get "simulate Atkinson cycle", which decreases VE and allows you to open the throttle fully while not inducting as much air. I don't think this is a common setting since I imagine response would typically be prioritized, but I have no idea.
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Old 09-05-2016, 04:55 AM   #3 (permalink)
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I see so it will vary model to model thanks
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Old 09-05-2016, 04:56 AM   #4 (permalink)
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oh and im reading lots of articles mentioning how turbocharged engines are "effecient" due to the volumetric effeciency i think? is this true since there is a posibility of a more rich mixture?
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Old 09-05-2016, 05:24 AM   #5 (permalink)
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Volumetric efficiency increases the more air you can get in to the cylinder, so yes, forced induction increases volumetric efficiency.
Unfortunately, to keep combustion chamber temps down and prevent detonation, gasoline engines usually pump extra fuel in that goes unburnt. The only real way (that I am aware of) to prevent the need for this is to inject the fuel immediately before it's to be ignited (direct injection). So, yes, some modern engines can be turbocharged and have higher compression ratios and are thus can be more efficient.
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Old 09-06-2016, 12:06 AM   #6 (permalink)
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Diesels run more efficiently under boost. The higher air mass in the engine absorbs more combustion heat and thus keeps it from being absorbed by the cylinder and combustion chamber. The extra heat energy absorbed by the air can then be extracted in the power stroke.

Keep in mind that the benefit from this is a diminishing return and beyond a certain point frictional bearing losses from the increased pressure will overwhelm any gains.
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Old 09-12-2016, 10:45 PM   #7 (permalink)
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I see so it still highly depends on lots of factors then
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Old 01-05-2017, 03:01 PM   #8 (permalink)
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MOST turbocharged vehicles run slightly rich under boost, even if the ecu doesnt "see" it that way, its just a way to keep the combustion process happy with added air and temperature. and most factory cars, like my cruze 1.4, with tiny hairdryer sized turbos, are almost always under boost. my car is at 0 psi at 55 ish(which is still technically in boost) and it just goes up with more speed. not entirely an efficient design
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Old 01-05-2017, 03:12 PM   #9 (permalink)
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When sizing gasoline fuel injectors you figure 0.6 pounds per horsepower per hour.
For N/A it's 0.5 pounds per hp per hour.
Turbo rotory engines can be around 0.7 pounds, well you know.
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Old 02-02-2017, 09:06 PM   #10 (permalink)
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I was wondering that myself. Got a new Fiesta ST. Right now, calculated ~30-31 mpg average, mostly highway. been cold, I mean it is winter! I bet I can get it up in the mid 30's. But its of course not the best choice for mileage.

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