How does Cylinder Deactivation actually save fuel?

[Voltwings]

I am aware of how it "works," but my question is how does it actually save fuel?

A vehicle needs to produce a certain power level to net 0 rotational losses, wind resistance, friction, etc and maintain a constant speed, lets call it 20 hp at 65 mph. 20 hp should have a definite brake specific fuel consumption, and the number of cylinders making "20 hp" should be arbitrary? if it's 1 cylinder making 20 horsepower or 12, 20 hp is 20 hp, and 20 hp worth of fuel is 20 hp worth of fuel? If half the cylinders shut off and we were suddenly using less fuel (and presumably making less power as a result of less fuel), we'd be at a net loss and the vehicle would begin decelerating?

Every article i have found on the topic seems to have been written for, or by, a 5 year old and is more concerned on explaining the deactivation of cylinders rather than fuel savings.

I figure there is probably something very simple i am not taking into consideration, but right now it just doesn't make sense to me.

 

[Black02GT]

Look up Variable Displacement on Wiki:

"Cylinder deactivation is used to reduce the fuel consumption and emissions of an internal combustion engine during light-load operation. In typical light-load driving the driver uses only around 30 percent of an engine’s maximum power. In these conditions, the throttle valve is nearly closed, and the engine needs to work to draw air. This causes an inefficiency known as pumping loss. Some large capacity engines need to be throttled so much at light load that the cylinder pressure at top dead centre is approximately half that of a small 4-cylinder engine. Low cylinder pressure results in lower fuel efficiency. The use of cylinder deactivation at light load means there are fewer cylinders drawing air from the intake manifold, which works to increase its fluid (air) pressure. Operation without variable displacement is wasteful because fuel is continuously pumped into each cylinder and combusted even though maximum performance is not required. By shutting down half of an engine's cylinders, the amount of fuel being consumed is much less. Between reducing the pumping losses, which increases pressure in each operating cylinder, and decreasing the amount of fuel being pumped into the cylinders, fuel consumption can be reduced by 8 to 25 percent in highway conditions.[1][2]

Cylinder deactivation is achieved by keeping the intake and exhaust valves closed for a particular cylinder. By keeping the intake and exhaust valves closed, it creates an "air spring" in the combustion chamber – the trapped exhaust gases (kept from the previous charge burn) are compressed during the piston’s upstroke and push down on the piston during its downstroke. The compression and decompression of the trapped exhaust gases have an equalising effect – overall, there is virtually no extra load on the engine. In the latest breed of cylinder deactivation systems, the engine management system is also used to cut fuel delivery to the disabled cylinders. The transition between normal engine operation and cylinder deactivation is also smoothed, using changes in ignition timing, cam timing and throttle position (thanks to electronic throttle control). In most instances, cylinder deactivation is applied to relatively large displacement engines that are particularly inefficient at light load. In the case of a V12, up to 6 cylinders can be disabled.[1]"

 

[03Sssnake]

 

[Zemedici]

Look up Variable Displacement on Wiki:

"Cylinder deactivation is used to reduce the fuel consumption and emissions of an internal combustion engine during light-load operation. In typical light-load driving the driver uses only around 30 percent of an engine’s maximum power. In these conditions, the throttle valve is nearly closed, and the engine needs to work to draw air. This causes an inefficiency known as pumping loss. Some large capacity engines need to be throttled so much at light load that the cylinder pressure at top dead centre is approximately half that of a small 4-cylinder engine. Low cylinder pressure results in lower fuel efficiency. The use of cylinder deactivation at light load means there are fewer cylinders drawing air from the intake manifold, which works to increase its fluid (air) pressure. Operation without variable displacement is wasteful because fuel is continuously pumped into each cylinder and combusted even though maximum performance is not required. By shutting down half of an engine's cylinders, the amount of fuel being consumed is much less. Between reducing the pumping losses, which increases pressure in each operating cylinder, and decreasing the amount of fuel being pumped into the cylinders, fuel consumption can be reduced by 8 to 25 percent in highway conditions.[1][2]

Cylinder deactivation is achieved by keeping the intake and exhaust valves closed for a particular cylinder. By keeping the intake and exhaust valves closed, it creates an "air spring" in the combustion chamber – the trapped exhaust gases (kept from the previous charge burn) are compressed during the piston’s upstroke and push down on the piston during its downstroke. The compression and decompression of the trapped exhaust gases have an equalising effect – overall, there is virtually no extra load on the engine. In the latest breed of cylinder deactivation systems, the engine management system is also used to cut fuel delivery to the disabled cylinders. The transition between normal engine operation and cylinder deactivation is also smoothed, using changes in ignition timing, cam timing and throttle position (thanks to electronic throttle control). In most instances, cylinder deactivation is applied to relatively large displacement engines that are particularly inefficient at light load. In the case of a V12, up to 6 cylinders can be disabled.[1]"

great post.

Long and short of it is - during routine driving, you do not use NEAR the maximum potential of the engine, and can turn off cylinders directly without any adverse effect on output.

 

[Voltwings]

OK, the pumping losses and cylinder pressure makes sense, i was looking at it too 1 dimensional.

 

[mc01svt]

cylinder deactivation is one of those things that works great for reducing emissions and fuel consumption on a constant load dyno simulated drive cycle but not so much in the real world.

A v8 running on 4 cylinders is an instant reduction of 50% working volume (displacement), however real world fuel savings if any is around 5-10% tops.

its yet another technological gimmick that increases cost, complexity and has higher occurrences of failure with little benefit to the consumer.

for a quick sanity check look at the mpg ratings for the f150 5.0L vs the silverado 5.3L. The 5.3L has cylinder deactivation, the 5.0L does not and they have virtually identical ratings. The trucks are both bricks, similar curb weights and drive ratios.

 

[Pribilof]

Skimmed the previous responses. mc01svt is correct. These technologies, start-stop, included, are intended to raise mpgs on the EPA test. A large portion of that test is idling. Therefore, start-stop, cylinder deactivation, etc. are huge helps on the EPA test but do basically nothing in real world driving. This is also the reason behind smaller engines with turbos, etc. Smaller engine idling is more efficient than a 5.4L engine idling for the same amount of time.

Edit: some reading - https://www.wired.com/2016/07/epa-testing-broken-heres-fix/

 

[jeffh81]

From my personal experience, it dont save jack. My Camaro rarely if ever goes into v4 mode and hits 25 mpg on the highway. My 2013 silverado could not maintain v4 and my current gmc hits 23 on the highway and stays running on 8.

 

[Black02GT]

cylinder deactivation is one of those things that works great for reducing emissions and fuel consumption on a constant load dyno simulated drive cycle but not so much in the real world.

A v8 running on 4 cylinders is an instant reduction of 50% working volume (displacement), however real world fuel savings if any is around 5-10% tops.

its yet another technological gimmick that increases cost, complexity and has higher occurrences of failure with little benefit to the consumer.

for a quick sanity check look at the mpg ratings for the f150 5.0L vs the silverado 5.3L. The 5.3L has cylinder deactivation, the 5.0L does not and they have virtually identical ratings. The trucks are both bricks, similar curb weights and drive ratios.

Skimmed the previous responses. mc01svt is correct. These technologies, start-stop, included, are intended to raise mpgs on the EPA test. A large portion of that test is idling. Therefore, start-stop, cylinder deactivation, etc. are huge helps on the EPA test but do basically nothing in real world driving. This is also the reason behind smaller engines with turbos, etc. Smaller engine idling is more efficient than a 5.4L engine idling for the same amount of time.

Edit: some reading - https://www.wired.com/2016/07/epa-testing-broken-heres-fix/

From my personal experience, it dont save jack. My Camaro rarely if ever goes into v4 mode and hits 25 mpg on the highway. My 2013 silverado could not maintain v4 and my current gmc hits 23 on the highway and stays running on 8.

It's like the stupid start/stop engines pribolf mentioned that shut off at traffics lights. Good concept on paper until you factor in humans. Sitting at a light for one minute and hear the Jeep next to me start up five times because the driver keeps inching forward out of habit/impatience. Probably worst off than an conventional engine. Start/Stop on a non-hybrid is asinine.

 

[apex svt]

great post.

Long and short of it is - during routine driving, you do not use NEAR the maximum potential of the engine, and can turn off cylinders directly without any adverse effect on output.

Unless it’s a 5.3 DOD, than it will eat up lifters and sometimes the cam. I’ve had to do a bunch of these. Seems like it’s almost guaranteed to happen 100-150k miles.

 

[jeffh81]

Unless it’s a 5.3 DOD, than it will eat up lifters and sometimes the cam. I’ve had to do a bunch of these. Seems like it’s almost guaranteed to happen 100-150k miles.

And that is why mine is hitting the road.

 

[Zemedici]

Unless it’s a 5.3 DOD, than it will eat up lifters and sometimes the cam. I’ve had to do a bunch of these. Seems like it’s almost guaranteed to happen 100-150k miles.

And this is why we replace MDS lifters when we do a VVT cam, right on the money

 

[Lemmiwinks]

cylinder deactivation is one of those things that works great for reducing emissions and fuel consumption on a constant load dyno simulated drive cycle but not so much in the real world.

A v8 running on 4 cylinders is an instant reduction of 50% working volume (displacement), however real world fuel savings if any is around 5-10% tops.

its yet another technological gimmick that increases cost, complexity and has higher occurrences of failure with little benefit to the consumer.

for a quick sanity check look at the mpg ratings for the f150 5.0L vs the silverado 5.3L. The 5.3L has cylinder deactivation, the 5.0L does not and they have virtually identical ratings. The trucks are both bricks, similar curb weights and drive ratios.

Wouldn't really call a few extra lines of code inside the ECU for higher failure rate.

5-10% is 5-10%.
If you find 5% here, and 5% there and a few in another place all of a sudden you have a vehicle with so much more efficiency it's even noticable.
And getting 5-10% for some extra lines of code inside the ECU (as it's basically just a software function) is no doubt well worth it.

And 5% on 1 car is good.
5% on 10'000 cars, that's where it really shows.

 

[olympic]

I have a couple Honda Odyssey vans with cylinder deactivation. It seems like a sound technology and the vans get good fuel mileage for the size of them. A solid 25% better than the Pontiac vans they replaced. Highest odometer is currently 300,000km (186k) and no problems so far. The only downside I found is it's really hard on the engine oil so I have to change it more often.

 

[mc01svt]

Wouldn't really call a few extra lines of code inside the ECU for higher failure rate.

you are highly mistaken. Active fuel management is not just software implementation. There are physical component changes that introduce complexity in the valve train especially on OHV (pushrod) engines.

The AFM on the Gm 5.3L is a well known problem child. The lifters fail, oil consumption is increased and there are even cases of dead cylinders, loss of compression requiring a total longblock replacement.

Even on DOHC v8s with variable valve timing the cam phasers are high failure items as well.

 

[lilcoop03]

I have an 08 Silverado with DOD. I hate that shit. Whats the best way to deactivate it? Tune? I love my truck and it has been the best vehicle Ive owned. Ive heard about the lifter/cam issues associated with the DOD and Id like to do away with it instead of my whole truck.

 

[AustinSN]

It's been forever, so this might not be entirely correct, but Toyota doesn't use it because it can cause uneven cylinder wear over a lot of miles.

 

[IronSnake]

I have an 08 Silverado with DOD. I hate that shit. Whats the best way to deactivate it? Tune? I love my truck and it has been the best vehicle Ive owned. Ive heard about the lifter/cam issues associated with the DOD and Id like to do away with it instead of my whole truck.

Unfortunately you have to replace the lifters if you want to remove the DOD stuff completely. Go to a conventional timing chain and cover. Retune the truck. Move on with your life.

 

[AAG]

for a quick sanity check look at the mpg ratings for the f150 5.0L vs the silverado 5.3L. The 5.3L has cylinder deactivation, the 5.0L does not and they have virtually identical ratings. The trucks are both bricks, similar curb weights and drive ratios.

The F-150 is pretty disappointing in the mpg dept with the v8. I was honestly expecting better with the 10 speed and aluminum body.

 

[lilcoop03]

Unfortunately you have to replace the lifters if you want to remove the DOD stuff completely. Go to a conventional timing chain and cover. Retune the truck. Move on with your life.

Well that sucks... You would think that it could just be turned off since it only works intermittently anyway.. Guess Ill just keep taking good care of it and making sure to put good oil in it..