"The fuel and air required to operate a throttled engine are thus greater than the same engine using a non-throttled technology, such as said valvetronic solution from BMW, because the [variable lift] engine is not having to work against an entire intake system under negative pressure, only the cylinder."

Your second quoting of my text is out of context; read the entire paragraph it was found in.

You also continue to introduce points of argument that strongly resemble red herrings. Perhaps you don't want to understand the answer to your question and simply wish to make conversation.

 

This seemed obvious as of several posts/days ago.

 

Some interesting things about these 2 particular engines haven't been mentioned.

The 6.0 L76 VortecMAX has a comrpession ratio of 9.6:1. The Duramax is at 16.8 AND it has a little thing called a turbocharger. Higher compression engines tend to be more efficient. Turbocharged engines reuse spent energy to make more energy making them more efficient as well.

 

I'll second that.

 

This is immensely frustrating. I'm seriously trying to get a legitimate, complete answer, and no one has yet provided it. It is unfortunate that I can't seem to get any input without being condescended to. If this thread will continue in this manner, I would like to ask that a moderator PLEASE delete it.

I am not trying to challenge anyone to be a dick, I am challenging the knowledge, in order to get a complete understanding. That's it- no disrespect was implied on my behalf- something I cannot say in return, I guess.

One more time...

The throttle does not consume the energy due to the pressure drop across it. Actually, that's not entirely correct- there will be a small energy loss, due to the change in temperature of the air across the throttle, as well as the sound created by the pressure drop. So, that energy is, in fact, lost.

However, in the grand scheme of things, that energy cannot add up to the claim of 8% improvement in fuel economy on a valvetronic engine. At 60mph, it's going to take probably 12 road HP to move a relatively light vehicle down the road, so the engine has to be generated more than that. If the throttle itself were consuming enough energy to increase fuel consumption by 8%, that would mean that several KW of power would have to be dissipated, right there at the throttle. That cannot be the case, because it doesn't give off that much heat.

So there has to be a systemic explanation for the loss of efficiency. Where is that energy ACTUALLY being consumed in a normally-throttled engine, to account for the reduction in fuel mileage? After thinking about it, I suspect that the cam timing has everything to do with it. In a normal throttled engine, the intake valve stays open beyond BDC, so under light throttle, some of the air in the cylinder is getting pushed back out into the intake. I would expect this effect to be more pronounced at the low-density condition of light throttle, for the lower mass should be able to change direction more quickly (you lack the ram-air effect you get at high-density/WOT conditions). I'm guessing that it is this push back of air back into the intake that creates the actual energy loss that accounts for the fuel economy differences. On a Valvetronic motor under light throttle, cam duration, in addition to lift, is going to be really low, so the timing prevents pushing the air back into the manifold.

That's the best I can do for now. I'm thinking of dropping a line to someone at BMW engineering to see if I'm off base.

Todd

 

Wouldn't there a loss factor because of the shape around the throttle, like there would be for a 90* elbow? If I remember right there are loss factor or coefficients for throttle valves.

Its been a while since I done any thermo/fluids stuff, so its not all fresh.

 

You may find all you want to know (and a lot more) about this in Blair's book.
http://www.motolit.com/desandsimoff.html

Like most textbooks, the more good knowledge you have before you pick up the book, the easier it is to understand. Personally, I love it!

 

What is immensely frustrating is that the information regarding the increase in low-load efficiency has been given to you, yet you provide absolutely no empirical or theoretical evidence as to why you condescend upon this information and the members replying to your thread—you simply reject what is logical and supported and insert your own argument about why it is not so. This supersedes any lack of technical knowledge or ignorance to what is being discussed, you simply do not wish to consider any opinion that does not agree with your own, an opinion which—ironically—amounts to: I don't understand this technology.

I told you once, and I will tell you again: a common engine must work harder than the BMW engine because the entire intake plenum and port system is under vacuum at light load. When an intake valve opens, the piston is having to work against the vacuum in the intake manifold while it descends the bore. If you took the mass of fuel and air of such an engine and put it into the same engine, but without a common throttle, the engine would idle higher. This is the part where you solve this: "Where is that energy ACTUALLY being consumed in a normally-throttled engine, to account for the reduction in fuel mileage?" With regard to fuel mileage of normal driving, the variable timing does also play a part in optimization of the VE. Then, you must consider the plethora of other compromises that are no longer required to be made in a variable valve-train engine. However, I suppose I am simply too basic because I try to address the original post before addressing the rest of your ideas.

This concept is so much of a no-brainer, I will not allow you to force me to produce empirical data of this. Before you catapult this thread into another direction attempting to draw congruency with your oven or any other household appliance, I suggest you read the information that has been provided, again.

I will also not allow you to continue your accusation of charging some with condescending attitudes, at least not me. Before you think of your feelings in this very mechanical argument, understand what it means when someone offers information only to have you make absolutely zero attempt at a scientific methodology in falsifying it, you simply say, "I do not think this explanation is correct."

 

I'll just take what you've said and move on. thanks