As each piston rises to full compression -- just before the power stroke -- air trapped in the cylinder fights back, temporarily slowing the crankshaft. When that cylinder fires, the crankshaft rapidly accelerates to a peak speed and then starts to slow again once all the fuel's been burned. This creates a slight oscillation or back-and-forth vibration in the crankshaft, one that can be particularly severe on high-compression diesel engines with a large displacement for a given cylinder count. This statement accurately describes most road-going diesels, as well as many high-performance gas engines.
The ideas behind the dual-mass flywheel have been around for some time, but the mechanism has come to light recently mainly because of advancements in diesel engine deign. Engineers are daily figuring out ways to get more out of each combustion event -- a good thing that creates at least one weird problem in the form of crankshaft oscillation.
A dual-mass flywheel works something like an harmonic balancer and performs much the same function. The DMF has a heavy outer ring -- or drum, in this case -- and a lighter center hub connected to the crankshaft, just like a harmonic balancer, but uses a set of long springs wound around the inside of the drum to connect it and the center hub. Think of the entire assembly as a heavy disc inside a heavier drum. The center disc bolts to the crankshaft and the outer drum connects to the transmission via the clutch.
Plateray Gear DMF
Many heavy-duty DMF units use a planetary-gear center section to keep the drum centered around the drive disc. With this design, the outer drum consists of a pair of large, hollow areas for the springs and a metal ring inward of the springs. This metal ring has inward-facing gear teeth, which mesh with a few smaller -- planetary -- gears mounted into spindles in the drive disc. As the outer drum rotates relative to the inner disc, the planetary gears turn and keep the two in alignment.
Standard Spring Arrangement
The springs can utilize one of two setups. In a basic DMF, a ring with long arms sticking out of either side bolts between the disc and the outer hub. When the crankshaft applies torque or shakes, it applies that torque directly to the ring and its arms. The arms fit between a pair of long springs wrapped around the inside of the drum; one end of the spring hits the ring arm, the other hits a wall-like stop on the inside of the hub. The toque applied from the crankshaft to the outer hub must go through the springs, which absorb the engine's minor back-and-forth vibrations.
Sliding Shoe and Multi-Spring Arrangements
A second and more complex method involves using several smaller springs instead of a few long ones. These springs have different rates, meaning that some will compress faster than others. The springs are separated by sliding "shoes," which fit between each spring and act as a solid stop once the spring fully compresses. Under hard acceleration or high load conditions, all of the springs will compress and the sliding stops will come into contact, essentially turning the assembly into a solid flywheel. Once the load goes away, the springs push the shoes apart and the DMF goes back to normal.
DMF Pros and Cons
The DMF is a boon to any high-compression, low-cylinder count engine -- not just diesels. The DMF's ability to isolate crankshaft oscillations makes for a smoother clutch engagement, longer clutch life and decreased breakage in the transmission and drivetrain. The DMF doesn't have any significant drawbacks, apart from the fact that it's relatively expensive to build, expensive to repair and is one more thing to break. But replacing the DMF is cheaper than replacing a transmission, so it may be worth the money and maintenance if you've got a high-compression engine and do a lot of stop-and-go driving.
- Manual Drivetrains and Axles; Thomas Birch
- Heavy Duty Drivetrains: System and Component Applications; Charles R. Jones
- Photo Credit Comstock/Comstock/Getty Images