Shade tree cylinder deactivation?

[roflwaffle]

I've seen this idea tossed around the web, supposedly w/ some success w/ V engines. I'd love to see it done w/ my V6 sedan, since it would allow me to cruise at a relatively slow speed where possible (~45-55mph) w/o being in an undesirable portion of my engine's BSFC map. The downside to doing it this to a vehicle not designed to close the cylinders, aside from the possibility of undesirable vibration, would be too much cooling of the deactivated bank from the continuous air at whatever the outside temperature was.

My question is, would jerrying a system similar to Toyota's TVIS in order to close the intake runners to the deactivated cylinders result in the bank staying hot enough from the exhaust to avoid significant wear, or is trapping the hot exhaust from the last power stroke charge in the cylinder needed for that?

 

[mattsooty]

I am currently working on a V8 cylinder deactivation application in which 2 cylinders of each bank are deactivated, not a full bank.

This allows the catalysts to remain hot & also neatly fits into every other cylinder within the firing order.

A point to note is that the deac cylinders have both inlet & exh valves disabled and the cylinder is then run as a type of 'gas spring' with no gas exchange occuring, with a corresponding reduction in pumping loss benefits.

Im not sure what you mean regarding increased wear due to lower temps? Surely wear has more to do with lubrication than operating temperature within the combustion chamber?

MS

 

[BrianPetersen]

On V8's with normal 90-degree crankshafts, it's normal to de-activate 2 cylinders on each bank, and if the correct cylinders are selected (the outer pair on one bank and the inner pair on the other one, with Chevrolet's firing order and crank pattern), it achieves an even firing order.

On V6 engines, it's a different story, because in order to achieve an even firing order, you have to de-activate all cylinders on one bank. But, the Honda VCM system does this, and doesn't appear to have problems. Keep in mind that the coolant still circulates through the entire block.

Closing off intake runners won't result in the (almost) complete elimination of pumping losses, which is what mechanically de-activating both the intake and exhaust valves achieves. It doesn't matter whether you trap air or exhaust in there; both will act equally as a gas spring.

 

[roflwaffle]

I was planning on closing off the intake runners on the deactivated bank in order to minimize cooling of the pistons/rings and hopefully any potential for increased wear/blow by on the deactivated bank.

I don't mind much pumping losses associated w/ moving air in and out of the cylinder, which is associated w/ volumetric efficiency, correct? What's killing me is part throttle wrt the BSFC map. I figure that even w/o completely closing a bank of cylinders, the jump from ~1/4 to ~1/2 throttle on average should be enough to get me a good ~8-14mpg at lower cruising speeds. Granted, ideally it would be best to see no energy expended moving air in and out of the deactivated bank/exhaust, but compared to low load operation I don't think this is as much of a concern in terms of efficiency, at least in my case.

Ideally, if I had decent off the shelf gearing available, I could just go that route in terms of increasing load on the engine, but w/o anything decent available I would have to go w/ a custom gear set, which probably means lots of cash. In terms of simplicity, this seems to be the easiest way to increase load, and decrease BSFC.

 

[roflwaffle]

I should also add that if cooling the pistons/rings on the deactivated bank wouldn't increase wear, I wouldn't bother in the first place. So I suppose a better question is how much does cooling the piston/rings increase wear? For instance, would having an electric motor turn a cold engine result in more wear than if the engine were operating at the same speed due to differences in clearances?

 

[ivymike]

allowing the piston/rings on the deac. bank cool off won't increase wear. turning off fueling&firing will reduce wear in those cylinders. Letting the piston cool a bit while keeping the block relatively warm will loosen up the piston-cylinder fit a bit vs. high-load operation. shouldn't be a concern.

 

[roflwaffle]

Good to know!

 

[patprimmer]

However if the ring seal is not very good, those cylinders will load up with oil and I suspect be prone to smoking and misfire after prolonged running in deactivated mode.

I think (don't know for sure) that Honda cycles through which cylinders are deactivated to keep the plugs clear and wear evenly shared across the engine.

I have no idea what others do.

Regards

eng-tips, by professional engineers for professional engineers
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

 

[roflwaffle]

Good point! Maybe I could set up a timer to alternate between which banks were deactivated after I figure out how long it would take for the cylinder to get oiled up to the point where it would cause trouble.

 

[MikeHalloran]

In about 1979, a friend of mine deactivated four cylinders of a Ford V8 by blocking their inlets with shim stock. Inner cylinders on one bank, outer cylinders on the other bank, as discussed.

First surprise: cross- connections in the EGR circuit in the manifold, and in the base of the carburetor, had to be closed.

Second surprise: while the engine was _way_ down on power, it was not proportionally down on fuel consumption.

My friend undid the conversion pretty quickly.

Dynamic cylinder deactivation gives you a better compromise, but I suspect that going direct from a full complement to a half complement of cylinders is going to cause transients that you will eventually find intolerable.

I.e., the technology only became acceptable when the engine control computer existed, and was fully involved in the transitions.

You might be happier just installing a double overdrive transmission.





Mike Halloran
Pembroke Pines, FL, USA

 

[roflwaffle]

I don't expect the increase in fuel efficiency to be proportional to the decrease in power at the time. I figure I can just rig up a couple switches to kill one bank and it's Oxygen sensor so the bank left on won't run rich and pick up a few mpgs. I've plotted the required output at some speeds (40/55/80mph) on flat ground/no wind compared to the peak output here, and I'll probably pick up some decent gains w/ it a bank off during low load situations like cruising at ~40-55mph, but at higher loads there's no point. I've pulled nearly ~40mpg at ~60mph highway if my route involves significant elevation changes where I can cruise up w/ lots of throttle, then cruise down above the DFCO, so that's what I'm hoping for w/ selectively cutting out a bank. I'd love to have a double OD but I dunno how I could rig one up on a FWD automatic trans.

 

[thruthefence]

Back in the 60's,after an unpleasant rod knocking event, a friend of mine "deactivated" one cylinder on his 170 C.I. Ford Falcon 6 cyl. engine by removing the rod & piston, & sealing the crank throw with a bicycle inner tube & a couple of radiator hose clamps. A little above idle it smoothed right out. A little down on power, but after all this was a Ford Falcon, not noted for performance. Sold it that was, too. "Southern Engineering" at it's best.

 

[patprimmer]

I have seen a limp home done by replacing a spun bearing with a block of wood carefully fashioned. Rod and piston reinstalled, and the spark plug removed. Push rods were left in, so the hydrocarbon emissions must have been significant.

Regards

eng-tips, by professional engineers for professional engineers
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

 

[MikeHalloran]

A friend of mine had a piston failure on a Chevy six, and had it replaced in the middle of the night at a service area on the NY Thruway. Remember when you could get service at a service area? It was a _long_ time ago.

While they had the engine open, they discovered someone had fashioned and installed a wooden piston, with a sheet aluminum top screwed on. The wooden piston was _not_ the one that failed. They put it back in.

I last saw the car two years later; it was running fine.



Mike Halloran
Pembroke Pines, FL, USA

 

[thundair]

Would it help to switch an injector to a low ohm resistor? A programed miss if you will, to keep the vibrations out of harmonics.
You could switch every other one every other time.
(what does that mean?)
Anyways you get my point that a little program and a switching circuit would at least stop the fuel flow.
I don't know what the overall advantages would be or if the CPU would start going crazy.
I am still for full power. I just want price/availability to improve without me having to change. TIC

I don't know anything but the people that do.

 

[GregLocock]

You'd see some surprising effects from that, if your engine was running closed loop. It'll richen the other cylinders up to compensate for all the excess air you are pumping, and then throw a code as it recognises a duff injector.

Nice try though



Cheers

Greg Locock

SIGlease see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[patprimmer]

To reduce pumping losses you need to deactivate the valves.

All other things remaining constant and discounting Greg's valid statement, simply cutting fuel will reduce power to the extent that the extra throttle opening to compensate will use more fuel.

The saving does not come from cutting fuel. We need to burn a certain amount of fuel to make a certain amount of power.
We need a certain amount of power to maintain a certain speed.

The only advantage in cutting cylinders comes from reducing pumping losses.

A much better system would be if the redundant cylinders could be disconnected at the crank and therefore also save coasting friction. Even then power would be wasted with overcapacity cooling and lubrication systems unless they could also be reduced.

Variable compression and forced induction on a smaller motor would save more fuel. I know, easier said than done.

I seem to recall that maybe Saab was working on a system where the main bearings could move up and down in the web, so altering the deck clearance and therefore compression.





Regards

eng-tips, by professional engineers for professional engineers
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

 

[roflwaffle]

I'm curious. When you say that in order to reduce pumping losses the valves need to be deactivated, is that assuming a near optimal gear ratio?

To put it another way, lets say that cruising down the highway at 55mph w/ a 3L engine and I'm turning something obscenely high, say 4,000rpm, and BSFC is somewhere in the ~600-800g/kWh range at ~1/8th of peak load. If I go up a hill that requires four times as much energy, assuming I stay in the same gear load will quadruple to ~1/2 of peak load, and according to every BSFC map I've seen, efficiency will increase to ~300g/kWh. No valve deactivation needed, just a change in load.

The same should go for gearing, with possibly greater improvements due to fewer friction losses from the engine turning slower, so I'm wondering how much more of an increase would be seen by, instead of for instance taller gearing, the current incarnations of cylinder deactivation were used. I suppose what I'm trying to say is, given sub-optimal gearing wrt efficiency at a certain load level, how much more would doubling load and shutting down the valves increase efficiency compared to just doubling load, assuming of course the engine continues to run at ~14.7:1? Clearly pumping air in and out of the cylinders requires more energy, but is that difference in energy greater or less than the the difference in pressure between the cylinder and crankcase that hurts efficiency for SI engines at lower loads?

 

[BrianPetersen]

It doesn't work that way, I think you're analyzing it from the wrong direction - although you are right that there is an effect from the gear ratio. More on that later.

Picture the "cycle" that a cylinder is going through, with valves de-activated, and then with the intake runner shut off.

Valves de-activated: There is a certain amount of air in the cylinder at the bottom of the stroke. It gets compressed (taking up mechanical energy). It expands (giving back the mechanical energy). There are no pumping losses whatsoever because nothing is going in and out of the cylinder. Granted, there is a minor loss because of leakage past the piston rings and because of the heat transfer effects, but they're small.

Valves active but intake runner shut off: Start at the top of the stroke this time. Piston goes down with the intake valve open, pulling a vacuum. Intake valve closes. Piston gets sucked back up the cylinder, but there is compression occurring, so the energy you just spent pulling a vacuum on the intake port doesn't get completely recovered. Piston goes back down, pulling a vacuum again - the compression and power strokes do indeed offset each other. But then ... the exhaust valve opens, breaking the vacuum in the cylinder. Consequently, the energy that it took to pull a vacuum on the intake stroke, doesn't get recovered. In effect, this will have (more or less) the SAME losses on the dead cylinder as an active cylinder at idle with the throttle shut!

This is why ALL current production cylinder de-activation systems do it by closing off the valves.

Now, as to that gear ratio question ... you're on the right track with that one. Under certain light load conditions, if your gearbox has the capability to do it, it could be better to go in a tall gear with the engine under load and all cylinders working, than to be in a shorter gear with cylinders de-activated. The trouble is that at slow driving speeds, the power required to run down the road is so little that the required engine revs would be too low for the gearbox to be able to do it, and even if you DID provide such a tall gear ratio, the engine would be lugging and have unpleasant vibration under such conditions.

It's interesting to note that the hybrid versions of the GMT900 are using the 6.0 with cylinder de-activation rather than the 5.3 on the "normal" versions. GM claims that the bigger engine and the hybrid powertrain allows the de-activation to be used more frequently than if they had used the 5.3.

Interesting balancing act, but I'm not entirely sure I buy that, because I've driven trucks that have only slightly less weight than that, and only have 2.4 or 2.7 litres of 4-cylinder power TOTAL, with no extra cylinders to come in, and they were fine.

 

[roflwaffle]

I see what you're saying about having a restriction on the intake. What about scratching the shut-off of the intake runner since that was only because I was worried about the piston/rings getting too cool and wear increasing, which supposedly isn't a big issue? Wouldn't the inactive bank just see more air and fewer throttling losses like the other bank w/ nothing blocking the runner?

My reasoning is that in this case (no cutting off the intake runners), if I cut off a bank, even if three/four cylinders aren't active, since the other three/four cylinders need twice as much air to make twice as much power, and the inactive cylinders will also see twice as much air, the whole engine would see correspondingly less in the way of vacuum, so would it be equivalent in terms of efficiency to doubling the load on the engine according to the BSFC map. Granted, it wouldn't be as nice as cutting the gear ratio in half, since that would result in a doubling of load and slowing the engine, which might reduce friction losses as well, but it seems better than a kick in the head.