There are a lot of technical terms bandied about where cars are concerned. If you know your Thackeray washers from your Belleville washers, you’re laughing but sometimes, concepts that defy logic pop up. One such is the dual mass flywheel, or DMF. After all, a flywheel is a flywheel, isn’t it? That big iron disc attached to the end of the engine’s crankshaft, that keeps the crank turning between power strokes is simple enough, so why mess with it? Quite! But dual mass flywheels exist; here’s what they’re about.
Start by thinking of a conventional flywheel, as mentioned above. In a four-cylinder engine, each pulse of power corresponds to an ignition (or power) stroke. Virtually all engines have a vibration damper, at the opposite end from the flywheel. This kind of damper has worked for many years but demand led to the need for a device to smooth engines’ power delivery. What demand? The keyword is diesel. As diesel engines became more common in ‘ordinary’ cars, the paying public wanted a smoother drive.
The important point about a dual mass flywheel is that it isn’t there to smooth out crankshaft vibration. Its role is to dampen the ‘torsional spikes’ caused when the power strokes happen. These are especially noticeable in a diesel engine because compression ignition engines have much a higher compression ratio than petrol engines. In short, at each power stroke, the ‘bang’ is bigger – and it can be felt as a result.
In practice, a dual mass flywheel, acting as a power-smoothing device, does more than make the power delivery silky. It also eliminates gear rattle, makes gear-changes easier and is claimed to improve fuel economy. How does it do these things? Read on and learn…
The are two basic kinds of dual mass flywheel. One consists of a primary and secondary flywheel, with torsion springs and cushions. A friction ring between the two elements of the flywheel allows slippage. So, when the torque applied by the engine exceeds the flywheel’s ability to transmit direct drive, slippage occurs momentarily.
The second type of DMF has planetary gears, making it a more complex design. This time, the gears link the primary and secondary elements of the flywheel. When torsional spikes exceed the flywheel’s drive rating, the two elements move in relation to one another, their relative movement limited by springs around the circumference of the unit. Such a DMF can offer improved fuel economy for a very simple reason; because it smooths out the power delivery, the idling rpm can be set much lower. In fact, dual mass flywheels are designed to provide maximum isolation of the frequency below the engine’s operating RPM, usually between 200-400 RPM. They are also most effective during engine startup and shutdown.
DMFs, therefore, sound like a very good idea. The transmission they serve is given a much easier life, both in terms of drive characteristics and gear changing. The same can be said of the driveline’s life – the ‘shunting’ effect of the power delivery through the transmission is eliminated. DMFs are not only for diesel cars either. Some petrol powered cars use them too.
There is, however, a snag. A DMF can do a far better job than the buffer springs in a conventional clutch’s driven plate could ever do. The initial part of the snag is that a more aggressive power delivery, as in a performance car, will give the DMF a hard life. The bottom line is that a DMF, of either kind, is a complicated device that can go wrong. The bad news is that DMFs do wear out and/or fail, and replacing them is a complicated, expensive business. This begs a question: what price refinement?