Piston kinematics

Piston kinematics determines the law for cylinder volume variation and fundamental internal aerodynamics characteristics. Piston kinematics is a determining factor for combustion, performance and pollutants generation. As examples:

	a)	The piston max speed affects cylinder filling and fine-scale turbulence generation. Fine scale turbulence determines mixture homogeneity and combustion speed, which allows determining ignition timing and/or compression ratio, and as a consequence, indicated efficiency.
	b)	The piston stop duration at TDC gives more or less time to combustion before expansion. This has a direct impact on ignition advance, gases-to-wall heat transfers, knock sensitivity and efficiency. On the other hand, depending on piston stop duration at TDC, intake-exhaust valves opening overlap will not have the same effect on gases scavenging, cylinder filling and engine torque and power.

Piston kinematics also determines intake charge pulsing, flow smoothness, and pumping losses. It has also consequences on the engine design, such as piston to valve clearance, intake and exhaust cam profiles and valve timing. Piston kinematics also determines crankshaft torque irregularities resulting from gases pressure forces and inertia forces, with direct consequences on vibrations transmitted by the engine to the vehicle.

Most VCR designs present a piston kinematics which is not strictly identical to that of a conventional engine. For example, multilinks rod-crank mechanisms present a particular piston kinematics that varies depending on the Compression Ratio (asymmetrical piston kinematics). Some multilinks rod-crank mechanisms may also present a near-to-sinusoidal motion which is not favorable to cylinder filling at low speeds and fine-scale turbulence.

Engines which present an asymmetric piston kinematics between TDC-to-BDC-stroke and BDC-to-TDC-stroke cannot be properly balanced and their crankshaft torque irregularities are increased.

As another example, bearings mounted on eccentrics or articulated cylinder head VCR engines operate as if their piston pin offset or crankshat offset was variable. On conventional engines, such offsets permit setting the piston slap timing (and resulting noise emissions, friction losses and cylinder wear) and balancing the force applied by the piston to the two sides of the cylinder.

As can be noticed, unconventional piston kinematics can lead to unexpected behaviours and defects related to internal aerodynamics and engine components noise emissions, durability, and mechanical efficiency.

Conclusion

To avoid unexpected results and to make the most of engineers’ know-how related to combustion, performance, pollutants generation, engine design and engine balancing, piston kinematics of future VCR engines must remain identical to that of conventional engines.

(see: The MCE-5 technology response to VCR engines piston kinematics requirements)