Optimizing Acoustic Properties

Complying with noise emission regulations and satisfying owners’ expectations for quiet operation are key areas of attention in acoustic development for drive components.

The acoustic properties and smooth running of an internal combustion engine depend on many parameters and are predetermined to a great degree by the selection of the design for the engine and engine block.

Optimizing the acoustic properties for the engine block structure, such as increasing stiffness at the engine block sidewalls, taking account of the many and varied functional requirements, is an important development target. This is achieved by low noise radiation, avoiding natural frequencies, and damping resonance-inducing vibrations.

The loading on the engine block resulting from the nonuniform progression of torques in the crankshaft due to the free mass forces and moments causes mechanical vibration. Their exciter frequency is in a certain relationship to the rotational speed of the crankshaft, according to the orders of excitation for the free effects of ignition gas and mass. Mechanical vibrations are caused by low exciter orders, are at a low frequency, and are found primarily in the area of the main bearing pedestals and the crankcase.

High-frequency vibrations in the engine block walls are induced by the combustion process itself, and are in part because of pulselike power transmission in the valve actuators and by forces induced at the pistons. The high frequencies are in the audible spectrum and are referred to as acoustic vibrations. A part of the high-frequency acoustic vibrations is radiated from the sidewalls of the engine block.

Low- and high-frequency vibrations exert their effects through the interface of the engine block with the engine mounts in the vehicle. Depending on the type of engine mount used, vibrations and structural noise may be transferred to the vehicle.

To be taken into account in the acoustic optimization of an engine are the following:
- The above-mentioned causes for initiation of structural noise
- The structural noise propagation paths in the cylinder head, cylinders, pistons, wristpins, conrods, and crankshaft
- The design of the engine mounts and their connection to the engine block or to other engine and drive train components
- The structure of the engine block in conjunction with the engine block engineering concept selected

Modem engine block development is undertaken in a closed CAE process chain. The 3-D CAD depiction and networking for the housing structure form the foundation for FEM calculations of strength, stiffness, and dynamic and acoustic properties.

An experimental model analysis at the finished engine block provides additional information on the forms of its own vibrations.

Both experience and the engineering calculation and analysis options available today support the basic claim that noise-optimized engine block design requires the stiffest possible engine block and the stiffest possible combination of engine and transmission.

This is achieved by measures that are independent of the selected engine block design and by exploiting advantages specific to a particular design, such as
- Manufacturing engine block surface structures with reinforced areas and ribs or fins to reduce airborne noise propagation.
- Stiff top plate and a force engagement point for the head bolts that is well below the top surface of the top plate. They minimize deformations at the sealing services and cylinders. The latter is a prerequisite for low piston play and, thus, low piston noise.

- Stiffness at the crankshaft’s main bearing pedestal configuration, which permits only slight bearing play.
- Stiff flanges interfacing with the oil pan and the transmission as a prerequisite for a stiff engine and transmission assembly.

The various engine block designs have differing specific acoustic advantages:
- The closed-deck design has a stiff top surface with benefits, in comparison with the open-deck design, in regard to deformation at the sealing surfaces and cylinders.
- A design comprising upper and lower engine block sections gives a stiff engine and transmission group in comparison to an engine block with side walls extended below the center of the crankshaft in combination with individual main bearing caps. In the latter design, stiffness is increased by joining the individual main bearing caps to form a longitudinal frame.

- In solid aluminum engine blocks, composed of upper and lower sections, gray cast iron components that are cast into the engine block at the main bearing points reduce thermal expansion and, in turn, bearing play.
- Using a cast aluminum oil pan with a flange interfacing with the transmission provides a stiff engine and transmission group.