What is a Dual Mass Flywheel (DMFW) for anyway?

By Brett Elliott

Is a DMFW necessary? Will converting to a solid flywheel hurt anything? Can I just throw my DMFW in a dumpster? What the heck does it do?

In a nutshell, it depends on how the engineers designed the rest of the drivetrain.

When LUK designed the first DMFW in 1985 for BMW’s 525e, they had driver comfort, increased drivetrain longevity, and emissions in mind. As engine output for lower-displacement engines has been ever-increasing over the years, so has the call for reducing the unwanted side-effects from these increases in power… primarily, harmonics. Also, factoring in manufacturers wanting to use thinner and thinner oils for lower fleet MFG averages, calls for even greater harmonic control.

Lets tackle the issue at face value; Did engineers almost 50 years ago ever imagine a factory mass-produced 2.0L four-cylinder engine producing 316HP (via the 2017 Volvo XC90 T6)? For comparison, a 1972 Corvette LT1 5.7L V8 engine produced 255HP. Quite a difference in displacement and output. Those older engines also came equipped with large crankshaft dampeners to assist bottom-end balancing.

By design, a 200HP four-cylinder engine will produce much greater output vibrations than a 200HP V6 or V8. The more cylinders there are, the smoother the engine. Start ramping up the power with a small displacement engine and the intensity of the vibrations increase. Those vibrations are directly transmitted to the crankshaft, transmission components, chassis and driver. Let’s break down the key areas and how the DMFW plays a role:

  • Reduced Crankshaft Vibrations

A DMFW can directly act as a crankshaft dampener, reducing the flexing of the crankshaft. Engineers may design an engine entirely without a front dedicated pulley/dampener if a DMFW is being used. They may also decide to utilize lower-cost materials for the crankshafts construction (cast vs forged) if a DMFW is being used, due to more flexing being eliminated. See the following illustration;

DMF Crankshaft Vibrations

Delete that DMFW and you may wind up causing premature bearing wear, or in a worst-case scenario, a broken crankshaft. If you’re pushing the envelope with power on an engine designed with a DMFW and no other means of dampening and you remove it, be forewarned.

  • Reduced Transmission Vibrations

Concerning the transmission, a DMFW reduces vibrations transmitted to each gear during shifting, along with making the synchronizers job much easier, significantly reducing wear. For the driver, clutch engagement/disengagement is a smoother affair. Anyone that’s driven a car with a DMFW can attest to the ease of clutching. The smooth engagement alleviates a massive amount of shock to the driveline AND crankshaft.

Impact Load after Quick Engagement

Another nice side effect of the smoothing-out of vibrations is more applied power over an engine revolution. Less juttering and more straightforward twisting action… TORQUE!

Transmission Vibrations

  • Reduced Fuel Consumption

While not really related to longevity or reliability, more MPG is always a good thing. The engineering perspective with fuel consumption and DMFWs is again related to vibrations and power. Typically, the lower the RPM the more intense the vibrations. Also, the lower the RPM the less fuel is consumed. If a driver can operate an engine at lower RPMs without feeling the side effects through the steering wheel, they’ll do so.

Dual mass flywheels are and have been the inevitable future. Solid flywheels have worked just fine for ages, but as with anything, there’s always room for improvement. With today’s modern high-displacement engines equipped with dual mass flywheels, be wary of sending the DMFW to the scrapyard, because your engine might be following it.

 

Citations

http://www.schaeffler.com/remotemedien/media/_shared_media/08_media_library/01_publications/schaeffler_2/symposia_1/downloads_11/luk_k7_chap01.pdf

http://www.schaeffler.com/remotemedien/media/_shared_media/08_media_library/01_publications/schaeffler_2/symposia_1/downloads_11/luK_kolloquium_en.pdf

Advertisements

Leave a Reply

Fill in your details below or click an icon to log in:

WordPress.com Logo

You are commenting using your WordPress.com account. Log Out / Change )

Twitter picture

You are commenting using your Twitter account. Log Out / Change )

Facebook photo

You are commenting using your Facebook account. Log Out / Change )

Google+ photo

You are commenting using your Google+ account. Log Out / Change )

Connecting to %s