How does this kind of engine save fuel?

[hungdynasty]

quote:

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5.7-Liter HEMI® Multi Displacement Engine

Yes; HEMI®, as in hemispheric combustion chambers, and the legendary engine of Daytona Beach. This engine generates up to 340 horsepower at 5,000 RPM, and 390 pounds-feet of torque at 4,000 RPM. The Multi-Displacement System (MDS) acutally deactivates 4 of the 8 cylinders during light acceleration or cruising. So when the semi's are only a distant memory behind you, MDS means up to 20 percent improvement in fuel economy. The original HEMI beams with pride. HEMI is a registered trademark of DaimlerChrysler Corporation.

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I think the quote above says that this engine will shut down 4 of 8 fuel supplies in some situations to save fuel.
My question is: with a certain amount of fuel supply , even the engine shut down 4 cylinders, physically the remaining 4 operating cylinders still need to operate not only themselves but also the other 4 shut cylinders. To me it makes no difference in work with an engine operating all 8 cylinders with the same certain amount of fuel supply. Do I make sense to you?

OK, I know those high paid engineers in Chrysler would not make a Multi Displacement Engine which does not save fuel. Can anyone explain this to me if you have any ideas? I'd appreciate it.

 

[Al]

You are seeing the reason that the Big 3 of the U.S. is losing money and Honda/Toyota are making money.

 

[javacontour]

Sure,

You only need so many horsepower to keep a moving car moving to overcome friction and drag. It sounds to me is as if you believe 100% of the horsepower produced is what is required to keep the car moving at a steady speed on level pavement. This is typically not true, and certainly not true for a 5.7L V8 in a 3500# car.

Yet it takes much more horsepower to accelerate.

So, when you are under hard acceleration, all 8 cylinders are providing power. While crusing, then engine produces more horsepower than is needed to keep the vehicle traveling at that speed, so if you deactivate the extra cylinders, then you reduce the horsepower produced and the amount of fuel burned.

Other ways to do this with more granularity is with a variable boost turbo. When power is needed, the vanes can be pitched to provide more boost, when less power is needed, at cruise, then boost can be reduced. (Yes, I know the throttle does this as well.)

A turbo is just another type of variable displacement, even without all the extras on the turbo. When the turbo is providing boost, it is the equivalent of having a larger displacement engine since more air and fuel is being shoved into the cylinder.

So MDS is just another way to have power when you want it, and reduce fuel consumption when you don't need maximum power.

 

[Gary Allan]

It's kind of a waste if you ask me. Just don't buy a Hemi if you want to save fuel. It still sucks even with the MDS system. The last thing the world needs is a fuel squeezing V8 that gets 15 city and 19 hwy.

How much worse would it be and how much less would it cost without the system (R&D, etc.)?? Probably enough to pay for a good bit of fuel.

 

[rusty 63 model]

When the V-8 is running on 4 cylinders, it does not use the same amount of fuel. It is cut in half, along with the power. It takes relatively little power to keep a vehicle running at 65 mph, hence shutting off the cylinders. Todays sophisticated computer controls, and electronically controlled fuel injectors and variable valve timings allow for this type of contols. Here is an explanation from Consumer Guide:

"In all variable-displacement engines, the intake and exhaust valves of the deactivated cylinders are kept closed by using computers and advanced hydraulics. According to Nick Richards at GM Powertrain, "This reduces pumping losses and helps ensure clean tailpipe emissions." GM calls their variable-displacement system Displacement on Demand (DOD). It is available on V8 versions of the Chevy Trailblazer, Buick Rainier, and GMC Envoy. In 2006, DOD availability will expand to include some V8-powered versions of Chevy and Pontiac cars and other V8-powered SUVs.

Chrysler also offers variable-cylinder displacement in the passenger-car version of its Hemi V8 engine. Called Multi-Displacement System (MDS) it is found in the Chrysler 300, Dodge Charger, and Dodge Magnum.

Honda offers variable-displacement on the V6 engine offered in the top trim levels of its Odyssey minivan. Because this V6 operates in 3-cylinder mode at times, Honda had to come up with unique electronic engine mounts to quell vibrations and employ active noise cancellation to reduce engine noise."

Heres the link:

http://auto.consumerguide.com/Articles/index.cfm/act/trendarticles/article/TA_May_2005.html

Note for Al: Notice that Honda also has a variable displacement offering. So in this case the Big 3 bash is unwarranted.

 

[hungdynasty]

quote:

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Originally posted by rusty 63 model:
When the V-8 is running on 4 cylinders, it does not use the same amount of fuel. It is cut in half, along with the power. It takes relatively little power to keep a vehicle running at 65 mph, hence shutting off the cylinders. Todays sophisticated computer controls, and electronically controlled fuel injectors and variable valve timings allow for this type of contols. Here is an explanation from Consumer Guide:

"In all variable-displacement engines, the intake and exhaust valves of the deactivated cylinders are kept closed by using computers and advanced hydraulics. According to Nick Richards at GM Powertrain, "This reduces pumping losses and helps ensure clean tailpipe emissions." GM calls their variable-displacement system Displacement on Demand (DOD). It is available on V8 versions of the Chevy Trailblazer, Buick Rainier, and GMC Envoy. In 2006, DOD availability will expand to include some V8-powered versions of Chevy and Pontiac cars and other V8-powered SUVs.

Chrysler also offers variable-cylinder displacement in the passenger-car version of its Hemi V8 engine. Called Multi-Displacement System (MDS) it is found in the Chrysler 300, Dodge Charger, and Dodge Magnum.

Honda offers variable-displacement on the V6 engine offered in the top trim levels of its Odyssey minivan. Because this V6 operates in 3-cylinder mode at times, Honda had to come up with unique electronic engine mounts to quell vibrations and employ active noise cancellation to reduce engine noise."

Heres the link:

http://auto.consumerguide.com/Articles/index.cfm/act/trendarticles/article/TA_May_2005.html

Note for Al: Notice that Honda also has a variable displacement offering. So in this case the Big 3 bash is unwarranted.

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Thanks rusty, now this makes more sense to me.

 

[hungdynasty]

First thank you all for your replies.
Now we find out that under deactivated mode, all the valves are closed on deactivated cylinders. Suppose all these valves are disconnected(??) from the cam shafts, then it makes sense that the engine would save some(very little) fuel because it doesn't need to operate these valves. However, the pistons in those 4 deactivated cylinders are still connected to the main shaft, which means the 4 operating cylinders still need to operate these 4 deactivated pistons. Other than the own mass of the engine, it still would never be like a 4 cylinder engine, I would say not even close. So how much fuel is saved? Chrysler mention it's up to 20 % but I really doubt it.

 

[javacontour]

Of course, Your Mileage May Vary, LOL.

It depends how much time is spent with the four cylinders deactivated.

To me, on the surface, the "up to" 20% savings makes sense.

Remember what "up to" means. That means the maximum you can expect to save is 20%, not a 20% savings all the time.

Up to 20% includes 0% savings.

But back to making sense.

You mentioned one aspect, the friction losses from the four pistons still travelling in the cylinders even without burning fuel. So the engine will not be as efficient as a 4 cylinder engine of exactly 1/2 displacement. Figure you don't get rid of the mass of then engine either, so you have to still carry it around, and that even with 4 cylinders running, at cruise there is probably an excess of horsepower.

So you really cannot get to 50% savings for a number of reasons. Perhaps there are even more than I've enumerated here.

 

[JHZR2]

I agree with Gary, regarding fuel savings...

but regarding this engine, when the cylinders have been deactivated, you do not have any P-V work going on on them, there is no compression, etc., to speak of. As a result, there is some incrementally smaller amount of work required. You still have momentum effects of the cylinders and valves moving, but it is a lot less.

Now, that said, it takes, say, 65 hp to move a 3500 lb vehicle 60 MPH. It takes that same amount whether being produced by a 4 cylinder engine or an eight cylinder engine. By deactivating four cylinders, you essectially have a 4-cyl engine, with lower total power output, and operating at a higher loading point, which is theoretically more efficient. The net result? 1, 2 MPG? What economy does ford and chevy make in their equivalent power and engine size trucks?

JMH

 

[XS650]

quote:

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Originally posted by JHZR2:
I agree with Gary, regarding fuel savings...

but regarding this engine, when the cylinders have been deactivated, you do not have any P-V work going on on them, there is no compression, etc., to speak of.

JMH

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I believe there is compression, but no pumping loss in the shut off cylinders. The cylinders act like gas springs, not soaking up much energy. The real gains are from moving the combustion process to a more efficient point on the BSFC map, like you said. And reducing pumping losses.

 

[Matt_S]

quote:

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Originally posted by JHZR2:
I can see the spring analog in order to not require the rest of the valvetrain, etc to have tomove totaly dead weight. However, I was under the impression that there was a 'decompresion solenoid' valve or something to prevent the buildup of pressure... is that not the case?

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There is no such decompression solenoid. You lose some of the benefit if you make the cylinder pump air while there are no combustions events taking place. In addition, they purposely, at least on GM systems, time the deactivation to trap exhaust gasses inside the cylinder, not fresh air. It supposedly (this is according to the service manual) reduces vibration and emissions.

Also, on a V8, at least on GM engines, two cylinders on each bank are deactivated. On the 3900 V6 with DOD, one cylinder on one back and two cylinders on the other bank are deactivated.

 

[oilyriser]

4 cyl = reduced surface to cool the flame

 

[javacontour]

oilyriser,

Huh? Could you expand on that?

Thanks.

 

[jsharp]

As other have pointed out, mostly what is saved is the pumping loss of the deactivated cylinders and a minor amount of the frictional loss depending on how the deactivation is actually done.

One other thing. When I see statements like "At X vehicle speed the engine is producing more power than it needs to maintain that speed" I always wonder - So, why isn't the vehicle accelerating?

 

[Bret Chase]

quote:

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Originally posted by JHZR2:
I can see the spring analog in order to not require the rest of the valvetrain, etc to have tomove totaly dead weight. However, I was under the impression that there was a 'decompresion solenoid' valve or something to prevent the buildup of pressure... is that not the case?

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add a "decompression solenoid" to an MDS/DOD system and you've got a jake brake. a jacobs compression brake uses the cylinder to compress the air, then the exhaust valve opens, dumping out the cyl pressure, so you don't get most of the energy back from the spring effect.

 

[oilyriser]

There's less internal surface area to cool the flame, using four cylinders instead of eight. One big cylinder would be even better, but vibration would be bad.

XS650 told the most important point - fewer cylinders burning the same charge gives higher peak pressures, and better conversion of heat into motion.

The Ultimate would be to have eight separate engines, each making twice the power of the previous one. The first engine would be 1 hp, the second 2 hp, the third 4 hp, and so on, up to 128 hp. You'd vary the total power by activating the engines in a pattern like in a binary number.

-patent pending, pending

 

[javacontour]

Are these MDS engines also throttle by wire?

I made the comment about producing more power.

The more I think about it, the more I believe I want to revise my remarks.

The amount of power is determined by throttle position and the amount of air moved.

So 8 cylinders at 2000 RPM at 5% throttle may not be very efficient when you look at the losses in trying to breath in the air through such a small throttle opening.

Cut that down to 4 cylinders breathing in a quantity of air at a more efficient rate.

I'm an EE, so I'm quickly running out of expertise. But I believe, and someone in the know can check out my instincts, is that power produced at a particular RPM depends upon the throttle position. So 2000 RPMs at WOT is more power than 2000 RPMs at 10% throttle.

Is that right?

I hope Someone weighs in who knows the specifics.

 

[mechtech2]

When in 4 cyl. mode, the engine has less engine vacuum than when cruising in 8 cyl. mode..
It takes power to produce vacuum.
That is the greatest savings.

 

[JeepZJ4.0]

The Hemi system is Drive-by-wire... hence why the AEV Hemi Jeep Wrangler cost so much. The adaption of Drive By Wire system is expensive.

Several Grand Cherokee owners with good driving habits have achieved mid-20s on the highway on road trips. The system works but it all depends on how you drive. If you drive for speed.... don't expect it to work wonders... but even still, any vehicle driven flat out all the time isn't going to be great on mileage. I've made a 4cyl Chrysler Sebring sedan achieve 8mpg...

 

[Matt_S]

quote:

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Originally posted by XS650:
I believe there is compression, but no pumping loss in the shut off cylinders. The cylinders act like gas springs, not soaking up much energy. The real gains are from moving the combustion process to a more efficient point on the BSFC map, like you said. And reducing pumping losses.

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This pretty much hits the mark. Keeping the valves closed allows the gasses trapped in the cylinder to act as a spring, reducing frictional losses from the piston movement and preventing pumping losses as has been mentioned. You also have to consider that there are only half as many combustion events for the same distance traveled while the engine is in DOD mode.