Turtle n Peeps wrote:
Wes Tausend wrote:
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On the other hand, I believe that a turboed engine is slightly more efficient than a NA (normally aspirated) motor for the same hp and/or equivalent displacement. The reasoning is that the end exhaust is slightly cooler on the turbo, meaning it derives more power (more is used to do work) out of the same fuel that must be equally burned to provide identical propulsion of a pair of trucks. One of the pair is assumed to be turboed, the other NA in this imaginary test.
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You would be incorrect. BSFC is quite a bit higher with a supercharged engine.
Hi, Turtle,
We've had this discussion before although I always welcome your posts and comments. I know you've had a lot of experience with race engines.
I believe you, and your author are out of context about this type turbo application. You will note that the sole reason given by your source to the contrary is the air/fuel mix
must be rich to prevent detonation.
QUOTE:
"Well, we can get a range of BSFCs for engines. For example, in our last post I used 0.65 as a safe number for a turbocharged engine. Most turboโd engines run between 0.6 and 0.65 BSFC while supercharged cars have a BSFC between 0.55 and 0.6, and naturally aspirated engines use only 0.45 to 0.5 Lbs/Hp*Hr. These are only approximations, but you can clearly see the difference between naturally aspirated engines and turbocharged engines. Turbocharged engines usually require more fuel to keep detonation at bay due to the increased temperature and pressure of the intake air". I think the BSFC efficiency is worthy of further debate in the
overall 3.5L EB design. Like I said, I'd like to
try it. From what I've read here, there are some indications that the towing efficiency is up to 1 mpg better using a turbo rather than larger NA displacement in this case.
That the
turbo air/fuel mix must be rich to prevent detonation certainly
usually true of non-intercooled carburation or indirect gasoline fuel injection. However, even in these cases, water/alcohol injection can be used to almost entirely mitigate the detonation and both rich mix and water/alcohol are only used best during high boost. The water/alcohol can also be used in turboed diesel engines to reduce exhaust gas temps. During racing, alternate EGT cooling can also be achieved by cold nitrous or, I believe, generous overfueling.
The one thing that is a game changer for Ford is the use of direct injection (DI), and under-piston oil spray cooling along with good intercooling. The combustion air/fuel mix can be run closer to stoic 14.7:1 because under conditions of high boost, the injection(s) can be delayed to any point beyond TDC (after top dead center). The pure air mix cannot detonate early, or pre-ignite, without the fuel being added. EGR can also be added before fuel, further reducing combustion temps. After TDC, the chance of detonation is less of a serious problem. This is nearly the sole reason DI is used in high tune diesel engines also.
None of this gasoline technology was commonly available to your author (good article by the way) on turboed race engines when he wrote the piece. In addition, he is basically just parroting earlier "safe" tech understandings without offering much on, what would be considered, exotic modification for efficiency. Ford has done something different here. Never before has any manufacturer actually combined all the previous knowledge into actual production, certainly not the last great turbo app OEM offering, the stupendous 3.8L Buick Grand National.
One of the reasons straight supercharging can be more efficient than a turbo is that the boost level, therefore the "planned" fuel curve, is well predetermined. Turbos are less predictable at any given RPM and only a very fast control system could possibly adjust parameters quick enough to prevent detonation. Fords computer is that good, it's a new game.
Which brings us finally to the question of whether a cooler downtube exhaust gas has really raised efficiency. The exhaust gas is hotter before it enters the turbo (even if we have to best wrap it with insulation ala Corvair). There is no doubt that more heat has been subtracted from the combustion process. But has it actually turned more heat into better torque propulsion? Or has the precious heat escaped by radiation from an extremely hot turbo feed, or intercooler loss? I don't really know. If the additional subtracted heat has been used to reduce intake pumping losses, then yes, the engine has used the fuel more efficiently to achieve the improved end torque we are looking for.
I think my Excursion might roll at low 3.5L boost cheaper than the V-10 internal losses at moderate highway speeds. I wouldn't expect a lot of difference. But just maybe. That is really the crux of this thread.
Wes
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