Dual mass flywheel - 5% FE increase?
 

I was reading about dual mass flywheels today and one of the things I found was that it increases FE by 5% (Polish Wikipedia, which cites this article). Has anyone come across this info? Why would the savings be that big?

For those who don't know, here is a word about DMFs. Normally an engine has a simple, solid flywheel which helps keep the angular velocity of the crankshft constant. But there are still vibrations which need to be smoothed out since they can kill the transmission. This is usually done by some sort of spring setup at the clutch. A diesel engine has enough torque and vibrations at low rpm to still wreck havoc, so this is where the DMF comes in. It is actually two flywheels with a complicated mess of torque and vibration dampening machinery in between. This extends the life of the tranny and syncros, reduces noise and increases FE. The price you pay is pretty high: since it is so complicated, it needs replacing much more often than a normal flywheel or clutch and is much more expensive. About half of all European cars from the last few years have DMFs, and some owners prefer to replace their duals with a low-tech, solid flywheel when their time is up.

I'm thinking they work like very large harmonic balancer. My guess to the efficiency increase comes from absorbing the energy in a spring which is able to return it to the drive line. In a harmonic balancer the energy is absorbed by a hydraulic system or an elastomer damper.

I am guessing due to their cyclic working, the springs fatigue & break. Causing possible damage to the housings. It's like having to have your clutch replaced with expensive parts & lots of labor. Someone wrote that it cost him 1500 pounds-sterling, approx $2500, to have it replaced. Which makes a $900 clutch job look cheap.

Don

Interesting.

My car came stock with a DMF. They all go south eventually and everyone replace them with a SMF, because those lost pretty much forever, and are effectively a lot cheaper. I've never seen an impact on MPG being discussed on TDIClub.com. I highly doubt a 5% difference. That wouldn't go unnoticed.

Ouch
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About to swap in a SMF on my TDI (err)

Here is a link to the source article, google-translated into English:
Dual-mass flywheel - Google Translate
(I can't edit the first post, or I'd have put it there)

More in-depth talk about DMF's in 6 speed swap in to my TDI Golf 4 - page 5.

They might help in constant-state driving, but in traffic a lower-inertia flywheel will be more efficient.

DMF's and heavy flywheels revealed
 

First post....

DMF's are heavy, and a single mass of equivalent size would provide the same benefits. The torque impulse of every single cylinder is harnessed into the rotating assembly. The flywheel is part of this. As the engine rotates (if we were looking at a single cylinder), after firing, it needs to do the work that you are asking of it as well as compress the next air/fuel charge. With a light flywheel, the engine can accelerate and decelerate faster, but it also does this within the four stroke cycle, causing loss of net power (tq). If you don't believe, or even feel slightly skeptical, take the blade off of your lawnmower and try to start it. Typically it will fire, rotate up the compression stroke to the ignition event and change directions, ripping the pull cord from your hands in a painful and violent manner. I hope this helps.

Welcome to ecomodder!

If that were true manufactures would just install single mass wheels, mainly due to cost, but complexity, and failure rate would come it to play as well.

Not quite sure what you mean with the failure rate but...the benefits that you are looking for, mpg, would be present in both a smf and dmf that weigh in at 25-30lbs. The difference for the dmf is that it absorbs shock like a crankshaft balancer. Balancers are dual mass as well, and absorb crankshaft speed variations as well as vibrations. That is the primary benefit of the DMF. They just always seem to be heavier, which leads to fuel efficiency.

As for manufacturers, they surely don't install the dmf for fuel efficiency. It is for the smoother drivetrain benefits. If manufacturers based everything they did on fuel mileage, there wouldn't be forums like this:D