Mazda Cylinder Deactivation questions

[shooter357]

This has probably been beaten to death but I just watched a you tube Mazda explanation of how it works. It occured to me that there was no mention of the injector being turned off. I guess you have to assume that? Also with both valves being closed during that time isn't the engine working against the compression? Seems to me if it didn't have to do that it would be easier on the engine maybe giving better gas mileage? Of course I'm not an engineer but some things seem like common sense. Oh well my CX-5 seems to work just fine anyway.

 

[N7turbo]

2 of the 4 cylinders are closed off, so they balance each other out. One will be in compression while the other acts like a spring.

 

[yrwei52]

This has probably been beaten to death but I just watched a you tube Mazda explanation of how it works. It occured to me that there was no mention of the injector being turned off. I guess you have to assume that?

Yes, we’d assume the injectors on cylinder #1 and #4 should be programmed to be closed when the cylinder deactivation is on.

Also with both valves being closed during that time isn't the engine working against the compression? Seems to me if it didn't have to do that it would be easier on the engine maybe giving better gas mileage? Of course I'm not an engineer but some things seem like common sense. Oh well my CX-5 seems to work just fine anyway.

I totally agree with you on this. And this’s the major drawback on current cylinder deactivation design as compressing the air in deactivated and sealed cylinders requires energy. The passive moving pistons and other parts with friction also require energy. That’s why the fuel consumption can’t be reduced too much (EPA rating is only 0 ~ 1 MPG gain) when CD is activated. It can never be as efficient as a true 1.25L 2-cylinder engine, and added much more complexity to engine、transmission、and even the exhaust system.

Cylinder deactivation, or variable displacement, seems to be a good idea but only on paper. The major flaw of such idea is you can’t truly deactivate the entire cylinder all the way because the piston and crankshaft connected to those deactivated cylinders still have to move passively with friction which consumes energy. The air inside of the deactivated cylinders with both intake and exhaust valves closed also gets compressed and decompressed which also requires energy. Not to mention the thermal effect changes frequently due to frequent cylinder activation and deactivation, which adds problems for reliability on cylinders which can be deactivated. Even with today’s technology, Mazda simply can’t overcome these flaws inherent from the cylinder deactivation. With complicated CD system from Mazda which definitely would affect engine’s reliability (yes, many components had been redesigned, including the exhaust system to accommodate CD) for only 0 ~ 1 EPA MPG gain on the CX-5, this’s simply a wrong move by Mazda IMO.

Nissan has developed a new variable displacement system which will totally deactivated entire cylinder including piston and crankshaft. This system definitely makes sense to me. But the system is very complicated which may hurt the longevity of the engine, especially those problems from thermal issues. And the reliability data of this new engine isn’t good either after a few years of service.

 

[N7turbo]

And this’s the major drawback on current cylinder deactivation design as compressing the air in deactivated and sealed cylinders requires energy.

Yes, but you are forgetting about the other cylinder which is decompressing and releasing stored energy.

 

[yrwei52]

Yes, but you are forgetting about the other cylinder which is decompressing and releasing stored energy.

Other active cylinders produce power as usual, where the “decompressing and releasing stored energy” coming from?

If you’re talking about “decompressing and releasing stored energy” on the deactivated cylinders, the energy required to compress the air, and the decompressing releasing stored energy won’t be equal as the environment of the “sealed” cylinder isn’t designed to save energy, hence most of it will be lost and there won’t be much saved energy left to compensate the energy required to compress the air for another deactivated cylinder. The cylinder isn’t a balloon.

You can also see this from the real world results from different setups. How much fuel needed to run a 2.5L with cylinder deactivation activated, comparing to a 1.25L 2-cylinder engine? Big difference. This simply indicates the current VD or CD system other than Nissan’s isn’t effective, not to mention other side effects caused by CD such as thermal issues.

 

[N7turbo]

If you’re talking about “decompressing and releasing stored energy” on the deactivated cylinders, the energy required to compress the air, and the decompressing releasing stored energy won’t be equal as the environment of the “sealed” cylinder isn’t designed to save energy,

Yes that is what I'm talking about, and that is where I will disagree. The nature of the engine is that of an air pump. Yes, there are always pumping losses, I just wanted to point out that there is some energy released by the other cylinder as it's springing back. Obviously the system works because there is a fuel economy increase, it's just not as large as we would like. But if you add that to thinner engine and transmission fluids, tire optimization, and other tweaks, they can squeeze a couple more miles per gallon out of these cars.

 

[yrwei52]

Yes that is what I'm talking about, and that is where I will disagree. The nature of the engine is that of an air pump. Yes, there are always pumping losses, I just wanted to point out that there is some energy released by the other cylinder as it's springing back. Obviously the system works because there is a fuel economy increase, it's just not as large as we would like. But if you add that to thinner engine and transmission fluids, tire optimization, and other tweaks, they can squeeze a couple more miles per gallon out of these cars.

Yeh other fuel saving methods I can accept it even though most of them have some side effects. But cylinder deactivation has been the worst on C/P value. We have seen the big failures from GM V8 in 1980’s. Even Honda V6 isn’t much successful. Mazda, with the latest technology and innovation, has tried again to an already fuel efficient 2.5L I4, and we see nothing but problems since 2018 although the percentage may be small. Any problem from CD would be costly based on released safety recalls and TSBs, including engine and transmission. Are these really worth it to have 0 ~ 1 mpg gain on EPA rating?

 

[hsdejong]

While both the intake and exhaust valves of the deactivated cylinders are closed, the engine does experience some compression resistance

But after the compression follows a decompression phase which largely compensates for the energy loss of the compression. What remains are the friction losses of the inactive pistons/cylinders.
What others above already mentioned.

With a petrol price of 2 Euro/liter (8.32 US$/US Gallon) in the Netherlands we try to get as many kilometers out of a liter of petrol as possible!