active fuel management operation

[Larsson]

Hi guys,

 

I have a silverado truck with a 5.3L engine and I wonder how the active fuel management works. I know that it uses lifters which can shut down it's cylinder but my question is: why use all of the hardware, like special lifters and stuff,  while it's easier just to shut down a fuel injector instead? Just turn of the voltage to the injectors of 4 cylinders during V4 mode instead. Is there a reason why my simple idea does or does not work?

[14burrito]

Questions answered here:

 

https://gmauthority.com/blog/gm/general-motors-technology/general-motors-propulsion-technology/general-motors-active-fuel-management-cylinder-deactivation-technology/

[M1ck3y]

On 1/18/2022 at 12:59 PM, Larsson said:

Hi guys,

 

I have a silverado truck with a 5.3L engine and I wonder how the active fuel management works. I know that it uses lifters which can shut down it's cylinder but my question is: why use all of the hardware, like special lifters and stuff,  while it's easier just to shut down a fuel injector instead? Just turn of the voltage to the injectors of 4 cylinders during V4 mode instead. Is there a reason why my simple idea does or does not work?

 

When afm actives it shuts down the intake valves by collapsing the lifters (not the exhaust valves). This prevents air from being sucked into the cylinder ie. being put through the compression and exhaust cycle. It saves energy and fuel, making it more efficient since the engine doesn't have to work against that air. As well as enabling accurate o2 reading's in the exhaust, since it's still only combustion byproduct entering the system. Fuel injectors are shut off as well.

 

I imagine there's many other concerns too; perhaps greater vibrational issues, drive ability concerns etc. 

Edited January 24, 2022 by M1ck3y

[Larsson]

so the engine not only collapses the intake lifters of 4 cylinders , it also shuts down the fuel injectors. but doesn't this mean that the cylinder is pulling a vacuum at the intake stroke? and pulling a vacuum like this cost energy, right?

 

perhaps it"s a better idea to use the solution used on a big 4 stroke single cylinder motorbike. I'm talking about the decompression system that is used to lighten up the effort to start such an engine. this system opens the exhaust valve at the compression stroke too.

 

I'm not an engineer ,I'm just trying to figure this out with the help of you guys

[Salsa De Piña]

Both the intake and exhaust valves are deactivated in AFM cylinders.  This allows the cylinders to act as air springs.  Unlike just cutting the fuel, this results in no pumping losses on these cylinders.  There are only small friction losses. 

 

Pumping losses on the active cylinders are also reduced because the throttle is opened up a bit to compensate for the loss of displacement.

[M1ck3y]

21 hours ago, Salsa De Piña said:

Both the intake and exhaust valves are deactivated in AFM cylinders.  This allows the cylinders to act as air springs.  Unlike just cutting the fuel, this results in no pumping losses on these cylinders.  There are only small friction losses. 

 

Pumping losses on the active cylinders are also reduced because the throttle is opened up a bit to compensate for the loss of displacement.

 

Thanks 

Edited January 29, 2022 by M1ck3y

[Larsson]

but a simple decompression system just like on a one cylinder motorbike and a injector deactivation isn't a simple solution?

[Salsa De Piña]

The goal of AFM is to reduce pumping losses.  Say that with me again: Pumping Losses.  Cylinders are always trying to pull in as much air as possible and, except at wide open throttle, they are restricted by throttle plate.  They want more air; the throttle says "No!"  These are pumping losses.  One of the reasons a diesel is more efficient: they throttle fuel, not air.  

 

Imagine we have a large V8 lumbering down the highway.  It's not working very hard.  The cylinders are trying to draw a lot of air in, but they're working against a mostly-closed throttle.  This is manifold vacuum.  Now imagine that we halve the size of the engine.  We still need the same amount of power, but engine is much smaller.  How do we do it?  We allow more air into each cylinder so that each cylinder can create twice the power it was making before.  We do that by opening the throttle.  This reduces pumping losses.

 

AFM effectively halves the size of the engine.  Could we leave the valves open on the deactivated cylinders?  Sure, but then each time air is pulled into or pushed out of one of the deactivated cylinders, we're going to experience pumping losses.  So we keep the valves closed.  The cylinders become air springs.  Every time the air is compressed, it returns that energy on the expansion stroke.  No pumping losses, only small friction and heat losses.

 

All of that to say...

 

The decompression system on a motorcycle is designed to reduce the compression in a cylinder to make it easier to turn over, either to save your leg or the flimsy little twig of metal that is the kick starter (or allows for a smaller starter motor.)  It is not designed to save fuel.  On motorcycles, they're inefficient fuel-wise because unburned fuel is pushed out the exhaust.  If such a system is implemented with fuel cut off to the injector on our trucks, it would be inefficient because air would be needlessly pushed and pulled from the cylinder through the open exhaust valve: pumping losses.

 

[16LT4]

And to add to the above: keeping the exhaust valve open but not the intake would create a lot of negative pressure (suction) on each piston's down-stroke, either fighting the down stroke or sucking in exhaust to that cylinder, only to pump it back out on the compression stroke.  That is a lot of heat into the cylinder without the cooling effect of fuel, so I'd hypothesize that cylinder/ring wear would increase on those cylinders.  With both valves closed, any negative pressure on one cylinder is offset by positive pressure on its opposing cylinder.  Less pumping losses.

 

If there were a simpler, cheaper, and more effective way to deactivate cylinders while still having reliability and longevity, GM would be all over it.  The goal of any business is to maximize profits while also minimizing expenses. 

[Larsson]

Thanks for your reply. Your explanation of the afm system makes a lot of sense. I never had thought about pumping losses before and it's an eyeopener for me. The next step in afm is perhaps downsizing the engine displacement just like ford is doing nowadays with its V6 engines in trucks. But I hope that gm stays true to the mighty V8, which sounds a lot better too

 

thanks guys.

[16LT4]

GM started downsizing in the '70s; most recently in the late '90s when the 5.0 Vortec was replaced with the 4.8 LS, the 5.7 with the 5.3, the 454 "kinda" (brief cohort with the 8.1) with the 6.0.... AFM does downsize the engine, mine is 164 CID have the time I drive it.    Ford's V6 are turbos and will whoop the 5.3, just not in reliability.  17 years of failed chain tensioners and they still can't figure out how to make them last.

 

The Caddy L62 also closed the valves back in the early '80s for the same reasons, but Commodore 64 processors weren't upto the task.

[KARNUT]

I was reading the car and drivers write up on their long term vette. They were marveling over it’s ability to stay in 4cly mode at Hwy speeds. Yup I would be all excited about that too. I bet the exhaust sound  was exciting too. Just what I’d want in a corvette.