Dynamic Cylinder Deactivation

[t vago]

Originally Posted by christofoo

Getting ahead of you for a moment, don't forget there is a reason I'm asking you to compare with lean-burn. You need to be able to explain why a lean-burn engine, having a higher air flow mass rate, gets better FE than than a stoichiometric engine.

Have a little patience, professor. We'll get there, shortly.

[t vago]

Now, (assuming that we're all in agreement about what constitutes pumping work), let's look at the strange critter known as intake manifold vacuum.

All it is, is a pressure differential between atmospheric pressure and the pressure inside the intake manifold, right? There is still an absolute pressure inside the intake manifold, right? And that pressure is still more-or-less different from the absolute pressure inside the exhaust manifold, right?

[christofoo]

Originally Posted by t vago

In reality, no, it isn't a constant pressure process. However, for the sake of this argument, humor me and assume that in an ideal world, the exhaust absolute pressure and the intake absolute pressure are held constant for the intake and exhaust strokes of an ideal Otto cycle engine.

This is right. (I got snagged on a different part of the cycle).

[christofoo]

Originally Posted by t vago

Now, (assuming that we're all in agreement about what constitutes pumping work), let's look at the strange critter known as intake manifold vacuum.

All it is, is a pressure differential between atmospheric pressure and the pressure inside the intake manifold, right? There is still an absolute pressure inside the intake manifold, right? And that pressure is still more-or-less different from the absolute pressure inside the exhaust manifold, right?

Yes.

[t vago]

Now, consider that the exhaust manifold pressure is, for all practical purposes, the same as atmospheric pressure. This means that we can, for all practical purposes, use intake manifold vacuum for this DCD discussion (as well as lean-burn), as intake manifold vacuum is the difference between atmospheric and intake manifold atmospheric pressures. (Please note that I am not addressing exhaust backpressure, take any discussions about exhaust backpressure elsewhere, and let's not muddy up this thread more than it already is.)

Now, it should be obvious that if we raise intake manifold vacuum, we are raising the amount of pump work that the engine is consuming. Similarly, lowering intake manifold vacuum will lower this amount of pumping work. Right?

[christofoo]

Originally Posted by t vago

Now, consider that the exhaust manifold pressure is, for all practical purposes, the same as atmospheric pressure. This means that we can, for all practical purposes, use intake manifold vacuum for this DCD discussion (as well as lean-burn), as intake manifold vacuum is the difference between atmospheric and intake manifold atmospheric pressures. (Please note that I am not addressing exhaust backpressure, take any discussions about exhaust backpressure elsewhere, and let's not muddy up this thread more than it already is.)

Now, it should be obvious that if we raise intake manifold vacuum, we are raising the amount of pump work that the engine is consuming. Similarly, lowering intake manifold vacuum will lower this amount of pumping work. Right?

Affirmative. (Nothing new.)

[t vago]

Now, let's examine why this intake manifold vacuum exists at all. After all, Diesel cycle engines apparently get away without it, so why should it exist here?

Intake manifold vacuum exists as the way for a basic Otto cycle gasoline engine to vary the amount of oxygen being fed into the cylinder for the combustion stroke.

For purposes of this argument, I am not going to address the various strange and wonderful chemical processes associated with combustion, nor any temperature differences associated with combustion that may occur outside of stoich, nor details about catalytic converter operation or O2 sensor operation, or EPA regulations, or any of that. For purposes of this argument, it's not important.

That being said, assume that on the combustion stroke of our hypothetical cylinder here, within this cylinder, that will be some set amount of fuel that will will combine exactly with some set amount of oxygen. If there is more fuel than oxygen present (rich), then there will be unburned fuel at the end of the combustion stroke. Similarly, if there is more oxygen than what is needed by the fuel (lean), then there will be excess oxygen at the end of the combustion stroke. Let's take as an extreme example of lean, the case of no fuel at all.

[christofoo]

Originally Posted by t vago

...
That being said, assume that on the combustion stroke of our hypothetical cylinder here, within this cylinder, that will be some set amount of fuel that will will combine exactly with some set amount of oxygen. If there is more fuel than oxygen present (rich), then there will be unburned fuel at the end of the combustion stroke. Similarly, if there is more oxygen than what is needed by the fuel (lean), then there will be excess oxygen at the end of the combustion stroke. Let's take as an extreme example of lean, the case of no fuel at all.

Agreed all around, except careful what you mean by 'set' amount of fuel. It will be a variable for analysis, fixed for a given type of engine operation (and all other things being equal). (Nah, I'm sure you thought this through, lead on.)

[t vago]

Originally Posted by christofoo

Agreed all around, except careful what you mean by 'set' amount of fuel. It will be a variable for analysis, fixed for a given type of engine operation (and all other things being equal).

Sure. We're getting there, so be patient.

Clarifying things a bit, I am concentrating on a single combustion cycle. I am not introducing any more fuel during this cycle, nor am I taking any fuel away from this cycle. Some set amount of fuel will be combined with some set amount of oxygen, within this cycle. Let's concentrate only on one combustion cycle, and leave steady-state/transient engine operation alone for a little bit longer.

This combustion cycle will produce a set amount of work, because we are combining a set amount of fuel with a set amount of oxygen, right? This is work that the cylinder is producing, right?

[christofoo]

Yes, the work (defined outside of pumping loss) produced by one combustion cycle is dictated by the amount of fuel that completes combustion.