How Does Cylinder Deactivation Work?
I'm sure I read an early story of yours in Wheels magazine about Holden's AFM cylinder deactivation system where the hydraulic lifter basically collapses and thus they do not open the exhaust and intake valves while in AFM mode. This being said, would this create excessive compression on cylinders 1,4,6 & 7 every stroke while in AFM mode? Where does this trapped air in the four combustion chambers escape if the exhaust and inlet valves are closed? I just don't see how this works to save fuel. I look forward to you reply as I have a 2011 Calais V with AFM.
I did once drive one of the first AFM Commodores from Sydney to Melbourne on one tank of fuel for a cover story in Wheels. Made it the whole way, CBD to CBD, but it was close.
I can certainly understand how non-scientifically literate people, from time to time, perform thought experiments in the shower (or wherever), attempting to decompile the physics of thermodynamic systems like AFM, and slowly drive themselves nuts in the process.
Sounds like a fun way to lose your mind.
Your question reminds me of the old chestnut in Ripley’s Believe it or Not: ‘According to aerodynamic theory, bumblebees cannot fly’. The point with this kind of thing is, it’s not the bees having the problem here (because clearly they can fly). There’s a problem with the stated underlying thought process concerning their flight. You’re having Ripley's bumblebee headline problem with the thermodynamics of AFM.
Respectfully, your operational conclusions are flawed because if the system worked according to your hypothesis, fuel consumption would increase as a consequence of AFM activating. It would drag on the engine, and that would be so bad they would cancel the project in the research phase, before productionizing it, and AFM would not exist. But it does because ... bumblebees can fly. Clearly.
I have declared something of a minor jihad on people not understanding technology, mainly because science works. It makes human life infinitely better, and yet people still don’t get it. So: Please forgive me for my long-winded answer.
When AFM is active, the valves are closed as you say. Therefore, the cylinder does not participate in any part of the combustion or pumping process, and (the bit you’re not getting) no net work is done by the cylinders that are deactivated. (This is a practical application of a thing called the first law of thermodynamics, which applies everywhere in the universe and says that the sum of work and energy in an isolated system is constant in the time domain.) There’s a constant interplay between work and energy in the gas volume in the cylinder during AFM activation, and the net effect is zero. Think about it like this:
If the valves shut down with the piston at top dead centre, the engine has to suck the piston down against, effectively, a bit of a vacuum (a well of negative pressure), but then the vacuum assists the piston’s return from bottom dead centre to top dead centre. The work required for expansion in the first half of the revolution is offset by the assistance during the second half - and no net work is done. In other words, the sum of these two effects is zero. The suction required to pull the piston down is paid back (thermodynamically) by sucking the piston back up to TDC for the second half of the rotation.
If the valves close at BDC, it takes work to compress the air in the cylinder on the way to TDC, but this is offset by the assistance from the compression, pushing the piston down between TBC and BDC. Again, no net work is done. In fact, if the valves close in any position, the work done against the gas volume for half of the crank rotation is offset by the energy it acquires and then pays back during the other half of the rotation. The 1st Law is not violated, and we don’t have to worry about the universe ending. So that’s nice. (In any case it’s the second law that’s going to end the universe…)
Effectively AFM just takes the cylinder offline from a thermodynamic standpoint. It nulls it out. Might as well just make it thermodynamically invisible. Obviously deactivating the cylinders is not 100 per cent efficient - because the engine still has to do net work against the internal friction of those rings and bearings, and any losses (like leakage) past the rings or valves. In terms of the first law of thermodynamics (above, in brackets) this just mean the system inside the cylinder isn’t totally isolated. A little energy is lost, but not enough to negate the benefit.
In answer to your specific questions:
- No excessive pressure is created.
- The fuel/air or exhaust gas trapped in the cylinder at the time of AFM activation remains there until AFM deactivates and conventional combustion resumes.
Isn’t science wonderful?
Thank you very much for your detailed reply. I understand it a lot better now.
I am trying to get my head around when I manually crank a V8 around to do valve adjustments etc i can feel how much load is created on the compression stroke and see this as loading up the engine when in AFM mode but the way you have explained it makes it seem very efficient and possible.
Thanks again for the explanation.
No worries Paul.
It’s not ‘possible’. It works. I’d stop short of calling it brilliant, however.
When you do valve adjustments and crank manually the valves are operational. Therefore you are manually compressing the air in the cylinder on every compression stroke. It’s not an isolated system as defined by the 1st Law - you’re providing all of the work, and getting none of the free kick.
In a V8, there are four compression strokes per revolution of the crank, each lasting about half a rev. Therefore during valve adjustment you are always pushing manually against two compression strokes. This is hard work. In AFM mode the engine is getting free assistance from inter-cylinder potential energy’s conversion to work for half of every rev of that cylinder, minus internal friction and other losses. (This is basic physics. You’re an engineer. It’s actually high school physics). See http://en.wikipedia.org/wiki/Variable_displacement
Also, the engine’s doing 20-30 revs per second, or similar, in AFM mode. A bit different to cranking one rev every 10 seconds by hand.