Dynamic Cylinder Deactivation

[t vago]

Good. So, we agree that we have a set amount of work being produced by the combustion cycle.

Let's then take away the pumping work from the produced work, and agree that this is the work being made available to do everything asked of the engine. This includes overcoming powertrain and other mechanical losses, overcoming rolling losses, overcoming aerodynamic losses, driving the alternator, driving the engine oil pump, driving any number of other accessories, propelling the vehicle in which the engine is installed, blah-blah-blah... Fine.

Now, we want to maximize this available work as a percentage of the produced work, right?

[christofoo]

By minimizing pumping loss by maximizing manifold pressure. (EDIT: BTW, I'm not impatient. We've been hung up on definitions after all. Thank you for taking the time and bearing the brunt of the explanation work.)

... and now it will be easy for you to show how lean burn improves efficiency...

[t vago]

Originally Posted by christofoo

By minimizing pumping loss by maximizing manifold pressure.

Correct. One way to do this is to deliberately introduce more oxygen than is strictly needed into the cylinder, while keeping constant the amount of fuel being consumed per cycle. The resulting mixture is lean.

Why do we do this? Remember that varying the amount of oxygen required that we vary the intake manifold absolute pressure. Increase the absolute intake manifold pressure, and we increase the amount of oxygen available for combustion. This has the effect of lowering intake manifold vacuum, since we are lowering the difference in pressure between the intake manifold and the exhaust manifold (which we had previously agreed was equivalent to atmospheric pressure).

Lower intake manifold vacuum, and we lower the required amount of pumping work. Since we kept the amount of fuel being consumed a constant, per cycle, we also kept constant the amount of produced work. However, since we agreed that available work is the produced work minus the pumping work, we increased the amount of available work. Right?

[christofoo]

Right. (Next we have to back off a little on fuel since there is now extra work available, resulting in higher FE for a given crankshaft power relative to stoich).

[t vago]

Originally Posted by christofoo

Right. (Next we have to back off a little since there is now extra work available, resulting in higher FE for a given crankshaft power.)

Exactly. "Backing off," here, means that we require less fuel than before.

We need only a set amount of available work to push the vehicle forward at a given constant speed. If we decrease the amount of pumping work while keeping the amount of produced work constant (because we're keeping the amount of fuel per cycle constant), we increase the amount of available work, and ultimately go faster than we did before. Not exactly what we aimed for.

So, we decrease the amount of fuel being combusted per cylinder by an amount that will give us the same amount of available work as before.

By lowering intake manifold, we lower the amount of pumping work being consumed, and we lower the amount of fuel being consumed as well, to propel a vehicle forward at a constant speed. This is considered "lean burn".

Agreed?

[christofoo]

Yep.

[user removed]

You also increased the mass of air in the cylinder, which increases the compression pressure and the post combustion pressure. Less work to get the mass in the cylinder and more work from the mass when it is ignited.
Also in lean burn you get higher temperatures in the cylinder which increases the pressure on the piston.
The downside is NOX emissions, which killed lean burn, but you may see it come back with things like Transonic Combustion Injectors, which heat the fuel to very high temperatures and inject it under very high pressures for ignition without spark on gasoline.
The key is to have the mixture as close as possible to homogenous, meaning every fuel molecule is evenly distrubuted with the necessary oxygen molecules. This also means there will be more than a single injection point. The Mazda SKY-ACTIV uses mulitple injection points to provide detonation free combustion, even on gasoline with 14 to 1 compression ratios.

regards
mech

[t vago]

Originally Posted by christofoo

Yep.

Now, how is DCD different from lean-burn, you ask?

DCD cuts fuel entirely out of one cylinder, for a given firing cycle of an engine. So you have 3 (or more) cylinders, all burning a set amount of fuel, producing a set amount of work, and consuming a set amount of pumping work. You also have one non-firing cylinder, not producing any work whatsoever, but also consuming a set amount of pumping work.

The firing cylinders aren't leaned out at all. DCD does not do that. It only cuts out fuel from one cylinder in a given firing cycle. This is exactly what heihetech described for his wonder device.

[christofoo]

Originally Posted by t vago

Now, how is DCD different from lean-burn, you ask?

DCD cuts fuel entirely out of one cylinder, for a given firing cycle of an engine. So you have 3 (or more) cylinders, all burning a set amount of fuel, producing a set amount of work, and consuming a set amount of pumping work. You also have one non-firing cylinder, not producing any work whatsoever, but also consuming a set amount of pumping work.

The firing cylinders aren't leaned out at all. DCD does not do that. It only cuts out fuel from one cylinder in a given firing cycle. This is exactly what heihetech described for his wonder device.

This is all correct.

Now consider that when we go from 4 firing cylinders to 3, changing nothing else, the 3 are not consuming enough fuel to produce as much work as 4. Correct?

[t vago]

Originally Posted by christofoo

This is all correct.

Now consider that when we go from 4 firing cylinders to 3, changing nothing else, the 3 are not consuming enough fuel to produce as much work as 4.

So, we have a set amount of produced work, now per firing cycle instead of per cylinder. We have a set amount of pumping work being consumed per firing cycle, instead of per cylinder. We produce a set amount of available work per firing cycle, instead of per cylinder.

We shut off one cylinder via DCD. The remaining cylinders are still producing work. You pointed out that the remaining cylinders are producing less work per firing cycle, than before.

So, we need to increase the amount of fuel to the remaining cylinders, so they can provide the amount of available work as before, while also covering the slack of the deadbeat cylinder caused by DCD.

Hm... The other cylinders are already operating at stoich, right? So we must also increase the amount of oxygen going to the running cylinders! Okay, so we lowered intake manifold vacuum by a bit. Good deal! We just lowered the pumping work for all of the cylinders, right?

Waitaminute... We just increased the amount of fuel to the remaining cylinders. We did this to cover the deadbeat cylinder, which is still consuming pumping work. The pumping work per cylinder may be less than before, since we just lowered intake manifold vacuum, but we also increased the work produced by the remaining working cylinders.

Right?