Inertia forces and balancing

IC engines present 3 main vibrations emissions sources:

	1)	Crankshaft torque variations generated by gases pressure forces;
	2)	Crankshaft torque variations due to acceleration-deceleration of reciprocating parts;
	2)	Unbalanced inertia forces generated by the reciprocating parts (these free forces widely vary depending on the engine architecture: L4, L6, V6, etc...).

Low frequencies vibrations (low engine speeds) are much more penalizing for passengers comfort than high frequencies vibrations (high engine speeds). At low engine speeds, reciprocating parts acceleration-deceleration generate low amplitude crankshaft torque variations compared to that generated by gases pressure:

As can be seen on this example (2 Liters L4 SI engine), low frequencies vibrations are mainly due to gases pressure. As Diesel engines’ pressure gradient and max pressure are higher than those of SI engines, they present low frequency vibrations that are much more difficult to filter.

In the case of SI engines, it is not pressure gradient but cyclic irregularities that penalize passengers comfort at low speeds. This defect is widely reduced on VCR engines: at idle speed, VCR engines’ combustion occurs under lower burnt gases proportion and higher pressure and temperature. This widely reduces cyclic irregularities.

Whatever the technology, VCR engines’ internal inertia forces have to be balanced as far as possible to avoid high second order inertia forces. To reach this objective, asymmetrical piston kinematics have to be avoided (different speed between upward and downward piston stroke). As it is difficult to manage vibrations that vary depending on load and speed, invariable engine balancing rules are required on the entire load speed range and whatever the Compression Ratio.

Depending on VCR engines’ design and kinematics, a same reciprocating mass can result in various friction losses levels.

Conclusion

VCR SI engines pressure gradient is lower than that of Diesel engines. In addition, VCR SI engines present lower cyclic irregularities at idle speed than conventional SI engines: this widely reduces low frequency vibrations due to crankshaft torque variations.

However, VCR designs have to present second order inertia forces as low as possible and balancing rules that never change whatever the Compression Ratio. On the other hand, VCR engines’ design and kinematics must minimize consequences of inertia forces on friction losses.

(see: The MCE-5 technology response to VCR engines’ inertia forces and balancing requirements)