Dual Mass Flywheel (DMF)

Internal combustion engine operation

The vast majority of road vehicles are equipped with internal combustion engines. Due to the working principle of the internal combustion engine, there are torsional vibrations generated at the crankshaft. The combustion process generates an extremely rapid rise of pressure inside the cylinder, during the power stroke, which results in a torque output with peaks. The pressure generated in the cylinders applies a force on the top of the piston, which is transmitted through the connecting rod and allows the crankshaft to turn. The pulsating torque generated by the cylinders causes the vibrations at the crankshaft.

Engine speed amplitude at idle (low) speed

Image: Engine speed amplitude at idle (low) speed

On a reciprocating piston engine the pressure gradient in the cylinder during the four cycles produces an uneven torque on the crankshaft. The pulsating torque generated at the crankshaft makes the engine speed to be also pulsating. For example, if we measure the idle speed of an engine, with a sampling time of 100 ms or less, we can see that the engine speed in not constant at around 975 rpm but rapidly oscillating between 925 and 1050 rpm.

All these rotational vibrations are transmitted further into the drivetrain and can affect the durability of its components. These vibrations can create gear rattle, body boom and tip-in/tip-out vibrations in the driveline which produce considerable noise and loss in driving comfort.

Simple flywheel

On each power cycle, the combustion of the air-fuel mixture greatly accelerates the crankshaft. During the other three cycles (intake, compression and exhaust), the crankshaft decelerates sometimes strongly and sometimes less strongly. To enable the engine to mainly run smoothly at lower speeds a centrifugal mass, the flywheel, smooths out these rotational speed irregularities to a certain extent.

A 4-cylinder four cycle internal combustion engine has a firing interval of 180°. For example, if the 4-cylinder engine runs at 3000 rpm, there are 6000 ignitions per minute, which corresponds to 100 ignitions per second. The engine rotational speed irregularities are therefore very slight.

The lower the engine speeds the clearer the engine rotational speed irregularities appear in the form of torsional vibrations. At 1200 rpm, there are approximately 40 ignitions per second, which means that a power cycle only occurs every 25 milliseconds. The engine rotational speed irregularities and therefore the torsional vibrations are very marked in this engine speed range.

If these torsional vibrations are transferred to the gearbox, without being damped, resonance vibrations arise in the gearbox and in the drivetrain. In turn, these resonance vibrations cause boom and humming noises or gear rattle. Also, higher resonance vibrations can damage the components in the gearbox and drivetrain in the long term. Without appropriate damping of torsional vibrations, the driving comfort at low engine speeds is unacceptable and low engine speed fuel saving driving is also not practical.

The reduction of the rotational vibrations of the crankshaft can be achieved by using a flywheel. A flywheel is a mechanical component designed to store rotational energy (kinetic energy). Flywheels resist changes in rotational speed due to their moment of inertia. The amount of energy stored in a flywheel is proportional to the square of its rotational speed and its mass.

\[E = \frac{J \cdot \omega^{2}}{2}\]

where:

E [J] – kinetic energy stored in the flywheel
J [kg·m2] – flywheel moment of inertia
ω [rad/s] – flywheel angular velocity

The higher the inertia or the angular velocity of the flywheel, the higher the stored energy.

1.3 JTD 16v Multijet engine

Image: 1.3 JTD 16v Multijet engine
Credit: Fiat

In the internal combustion engine case, the flywheel is attached at the end of the crankshaft. How it works:

  • during the power stroke of the engine, the flywheel stores the kinetic energy
  • during the intake, compression and exhaust strokes, the flywheel releases the kinetic energy

This way, the spikes in torque are dampened during the power stroke and distributed through the whole cycle of the engine. This effect applies for all the cylinders of the engines. The higher the number of cylinders in an engine, the smoother the output torque/power.

Engine torque during a 4-stroke cycle

Image: Engine torque during a 4-stroke cycle

The type of the engine (diesel/petrol), the number of cylinders, the engine cubic capacity and the specific power [kW/L] of the engine has significant impact on the rotational vibrations of the crankshaft. For example, high capacity atmospheric, petrol/gasoline engines, have low torque at low speed. Also its moving parts, pistons, connecting rods, crankshaft have higher mass, which means higher inertia thus more manageable spikes in rotational speed. These factors combined makes the output torque ripples (oscillations) manageable with a standard flywheel.

Downsizing & Downspeeding

A major task of the automotive industry in recent years has been to reduce consumption and CO2. One effective measure for achieving this goal is to exploit even lower engine speeds for driving. Torque is increased to achieve this without losing power. Doing so allows the engine to run only very slightly above idle speed and therefore in an extremely consumption-efficient range. One challenge is to achieve adequate powertrain isolation even for these low engine speeds and thus provide drivers with their usual level of comfort.

The rapid development of vehicle technology over the last few decades has brought ever higher performance engines paralleled by an increased demand for driver comfort. Weight-saving vehicle concepts and wind tunnel-optimised bodies now allow other sources of noise to be perceptible to the driver. In addition, lean concepts, extremely low-speed engines and new generation gearboxes using light oils contribute to this.

In order to improve fuel consumption and reduce exhaust gas emissions, recent engine development strategies included engine downsizing and downspeeding.

Downsizing for V6 to L4

Image: Downsizing for V6 to L4

Effect of number of cylinders on speed oscillation

Image: Effect of number of cylinders on speed oscillation
Credit: Schaeffler

  • downsizing means that the total engine volumetric capacity is reduced by reducing the number of cylinders (e.g. from 6 cylinders to 4 cylinders), but maintaining the torque/power output (usually using intake air boosting, variable valve lift technologies, direct fuel injection, etc.)
  • downspeeding means that the peak engine torque is obtained at lower engine speed (e.g. from 2500 rpm to 1500 rpm), achieved for example by using dual-stage turbocharging, electric air compressors, etc.

With other words, downsizing and downspeeding are processes whereby the speed / load operating point is shifted to a more efficient region through the reduction of engine capacity whilst maintaining the full load performance via pressure charging.

Ford engine downsizing torque output

Image: Ford engine downsizing torque output
Credit: Ford

The combination of smaller inertia of the moving components with the higher torque at low engine speeds generates higher rotational vibrations at the crankshaft. Also, with more stringent CO2 and exhaust gas emissions being impose worldwide, more engine downsizing and down speeding strategies are being put in place by internal combustion engines manufacturers. The side effect of this strategy is that more vibrations are generated at the crankshaft which are being transferred in the driveline.

Engine downsizing trend

Image: Engine downsizing trend
Credit: Global Insight & Honeywell

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