Re: fuel consumption question
Why do I even try . . . ? I'll answer again, because Silvertip is giving this board incorrect info. First the truckers he describes did in fact re-gear their trucks for efficiency and performance. But they did not do it for throttle position because DIESELS DO NOT HAVE A THROTTLE.
Then there is this brilliant observation: "You seem so convinced that open throttle is most efficient but you don't prove it. If that was true, all of us should be driving with our foot on the floor." To which I ask: Why in hell would you do that? If the engine in your car is capable of 280 bhp and yet it only needs 61 bhp to cruise at 72 MPH (I made that up), why in the flyin' flip would you "command" 280? Which is precisely what your foot on the floor would be, a command for full power.
Believe it or not Sr. Silvertongue, you have once again come close to proving my point. Imagine that you have two engines in your car, one that was designed to make 61 bhp and another one that was designed to make 219 for reserve power. When we cruise at 72 MPH above we could just shut off the 219 bhp engine because we don't need it. But the little 61 bhp engine would in fact be operating at WOT.
Now, to be fair, if you did this on purpose, you would set the engine up to run at say 3200 RPM and also optimise the cam etc for exactly that RPM and you would then run it at WOT (at least very close), and what you would get is dramatically improved MPG. So in this case, yes, I would run that car with my foot on the floor for the little cruise engine. But when I wanted to pull a grade, I would flip a little switch to start my 219 bhp engine and I would get my 280 bhp available back under my control. And if anybody is paying attention, what I sorta described is cylinder cutout strategy that basically turns a V8 in to a V6 and then a V4 so that you have basically 3 or more little engines that you can combine together so you can run at a wider throttle setting for the horsepower required to get the job done. Instead of a much more closed throttle, that makes the air pump that your engine is work harder to deliver smaller amounts of power . . .
The winky guy is getting really tired, so, since I have been challenged for proof by our Sr. Engineer, the following is provided for your perusal. Oh and before I do that, again for clarity, we are talking about ENGINE efficiency in these more focused discussions and the good doctor keeps throwing in hull efficiency to muddy up the discussion, and the only point I have been trying to make from the beginning is that if you have two boats identically set up, and you are looking for the best fuel efficiency at an efficient hull speed, the smaller of the two engines that can get the job done will provide better MPG due to a wider open throttle for the amount of work being done (work being the cruise). This is contrary to the commonly held belief that the larger engine will be "loafing" so it will get better economy, but it is the truth in a controlled environement within reasonable restrictions like a 4.3 compared to a 5.7, or a 5.7 compared to a 8.1. Or a 3.0 compared to a 4.3. Not a 4 stroke weed-whacker engine running at 27,000 RPM vs. a 7.4 MPI at 2500 RPM in a 3500 lb boat . . . And BTW, I never recommended that anyone buy a small engine in their boat. If you are gonna tow stuff and you want to go fast, find a 2000 model year 454 MPI 385 bhp. That's what I want!
This is from a Toyota site explaining the benefits of Hybrids (guess what, it is throttling losses often called pumping losses) :
Pumping Loss
The major cause of loss of efficiency at low power is "pumping loss". How is an Otto cycle engine designed for a peak power of 108 hp persuaded to run at an output of, say, 10 hp? The answer is that the flow of air into the cylinders is restricted by closing a "throttle" valve. This forces the engine to drag the air through a narrow opening, creating a partial vacuum in the inlet manifold. As the air entering the cylinder during the intake stroke is below atmospheric pressure, there is less of it. A smaller amount of fuel is injected and the resulting smaller fuel/air "charge" causes the engine to run at low power, as desired. But, as well as having this intended effect, maintaining the partial vacuum in the inlet manifold wastes energy. As the piston moves down during the intake stroke, normal pressure below it and a partial vacuum above cause drag on the crankshaft's rotation.
This does also reduce power output, as desired, but at the expense of wasted fuel, which is an undesired side affect. Note that cars suffer from pumping losses even at highway speeds. The throttle is really only opened right up when accelerating or climbing hills.
Interestingly, diesel engines do not have this problem because
there is no throttle. Low power is achieved by simple injecting less fuel.
This is one of the reasons why diesel engines achieve higher efficiency. This technique cannot easily be used by petrol engines because the burn temperature becomes too high and damages the cylinder.
This is from a site describing a new Peugeot/BMW engine:
In a conventional engine the engine power output is controlled by means of a throttle butterfly. The required amount of air to be drawn into the engine is regulated by the position of the throttle butterfly; however, at certain engine speeds and throttle butterfly positions, the incoming
air has to squeeze past the partially closed or closed butterfly, reducing the air flow into the engine.
Overcoming this resistance reduces potential power and overall engine efficiency and also increases fuel consumption.
