Quote:
Originally Posted by RustyLugNut
You claim to be this know all wizard who says they are more capable than NASA and yet you cannot make a simple deduction and calculation correctly. This puts ALL of your claims in doubt.
The video shows nothing about the manifold air pressure which, along with air temperature, would tell you what the mass flow potential is going into the engine. You use this LIE as a gateway to your theory of greater mileage. But your logic is a lie in and of itself. The assumption of 0.8 as your volumetric efficiency, in the Aveo video, can only happen if you have the throttle fully open. The throttle is a variable volumetric efficiency valve! You cannot have 0.8 VE at idle, can you? Only diesels, which have no throttle valves can make that claim. This is why members of this forum work towards lean burn engines so that they may use greater throttle settings and greater VE along with the reduction in fuel used per unit of air. Take the reading of the MAP sensor along with the air temperature sensor and apply the basic gas laws and you can correctly calculate the mass of air used by the engine. Your video evidence does not support your claim.
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Ah, I now understand. Thank you.
Thank you VERY much. That statement was helpful. I think I now see a consistent paradigm here.
Some people think that the throttle plate/valve restricts the engine's air intake, significantly lowering the VE when it is closed or nearly closed and that a VE of 0.8 MUST be at or near at WOT.
This isn't true..
Even a lot of mechanics make this mistake. In previous posts I've tried to explain WHY this miss-conception isn't true but people are still miss-understanding.
I'll try to do explain better... and I'll go looking for references since I sense I have little credibility here and this seems to be a major stumbling block.
Thank you for pointing out this stumbling block.
It is true that if the throttle plate totally blocked off the air intake, the piston would create a high vacuum and no air would get into the cylinder.
This is so obvious that the next natural conclusion is that if the throttle is 10% open, that the cylinder would get only 10% air, 20% open 20% air and so on; but that
logical assumption is wrong!
The truth is that pretty much as soon as the throttle plate cracks open just a little bit, just enough so the engine can idle, the VE goes up to about 0.8.
As the throttle opens up more, the VE goes up more, but only a little more.
And now the REALLY counter-intuitive part,
as engine rpm increases, the VE decreases, even though the throttle is open MORE.
Now I'm talking about 'normal' engine design here, not custom tuned intake manifolds that can actually exceed 100% VE (yep, just PACK that air in even without a turbocharger).
Air pressure and air temperature are important considerations but as far as VE is concerned, they are only 'fine tuning' considerations.
Here are some links to places that can explain volumetric efficiency.
Basic VE explanation
Find your VE with a scan tool
Volumetric Efficiency calculator
Note that this calculator shows the Aveo's VE to be 79.999%
Givens: CFM=62.37, CID=97.64, RPM=2760
Here is a forum discussion about calculating VE
A good experiment to do, to better understand how the throttle restriction and VE interact, would be to take a large syringe (sans needle) and hold it in your left hand with your index finger ready to 'cap' the luer fitting hole (tiny hole where the needle would go.
If you 'cap' the hole and then try to pull the plunger back (with your right hand), you will feel the pull against vacuum
(OK you science guys, I know vacuum is only a relative lack of pressure and that it is the atmospheric pressure that is pushing on the back of the plunger, but work with me here, we're talking to mechanics).
and when you release the plunger it will 'snap' back to the bottom because no air came into the cylinder.
That would be throttle plate 100% covering the engine's air intake, it's not going to run because it isn't getting any air.
So now we take our finger off the orifice and pull the plunger back slowly. As soon as we are done pulling, we instantly cap the orifice...
We now find the plunger doesn't move at all (or maybe only a little bit), the air pressure inside and outside are equal so the cylinder is 100% full of air, even though the air had to come in through a tiny orifice.
This is a principle of fluid dynamics whereby the air will move 'faster' through the orifice, to try to fill the area in the cylinder that becomes available as you pull the plunger out (and the absolute pressure will drop, making a vacuum in the orifice and the cylinder as air moves).
So, when the engine is near idle, the VE is fairly high, even though the throttle plate is near closed.
Next we reset the syringe, plunger to bottom and pull harder/faster and again cap the orifice just as we get done pulling. Ah HA!, the plunger now goes back down the cylinder if we release it!
So the faster we pull the plunger, the LOWER the VE!
Now in real life, we're opening up the throttle as we increase rpm, and valve timing usually allows air to enter the cylinder for a bit after BDC and other factors (like air velocity and intake tuning) help keep VE up, but the point is that VE usually decreases when rpm increases.
I hope I've now convinced you that I do know what VE is and how it works so you will believe that my 'lie' calculation is reasonably accurate and that anyone can duplicate it on their own vehicle.
The next question is... If the throttle plate isn't really restricting air flow, if it doesn't have a significant effect on VE... Why is the throttle plate even there?
It's there because the engine needs the vacuum to lower the 'boiling point' of the gasoline, so vapors will be produced from the volume of liquid being dumped into the engine.
While the throttle plate doesn't significantly affect VE,
it DOES put a significant load on the engine. It takes energy to maintain that vacuum. Remember how much more power you needed to use to pull that syringe plunger back faster?
With my Aveo, I'm 'consuming' about 0.3 gph of fuel just to idle. This is a crazy amount of energy that is wasted, just to help the fuel partially vaporize. If we feed the engine with vapor, we can eliminate the intake manifold vacuum and dramatically lower that parasitic power loss.
We still need a 'fuel regulating valve' of some kind, but it wouldn't have to cause a vacuum. That is how my RV generator is currently set up and, come spring, I'll make a video of it.
But one step at a time. Let's resolve this VE question first.