 The need to meet tightening emis-sion regulations is pushing automakers towards smaller, more powerful engines. This would translate into smaller, turbocharged engines that produce as much power as engines double their displacement capacity made a few years ago. While turbo charging is considered the most promising way to seek higher output, many automakers in an effort to develop vehicles with hybrid technology options are moving over to three-cylinder engines and electric motors. Both these require mounts that are different from those seen on vehicles with a conventional setup. The effect of three cylinder engines and electric motors on mounts is significant. It was in the year 2004 that Honda introduced the Accord Hybrid, powered by Honda's third generation IMA technology. With the electric motor filling in the lower rpm torque curve, the Accord hybrid, powered by a 3-litre, V6 engine with variable cylinder management, employed an Active Control Engine Mount (ACM). The ACM used sensors that alert the ECU to direct ACM actuators to vibrate in sync with the engine, to keep the vibration from being transmitted to the passenger compartment. If any vibration would reach the interior, an ANC controller with cabin-mounted microphones (front and rear) would detect the noise and instruct the vehicle's audio system to emit an opposite sound wave through the car's audio speakers, canceling out the noise.
The ACM assumes more importance from the point that to lower emissions and fuel, the V6 motor of the Accord Hybrid is programmed to shut half the cylinders. The variable cylinder management technology takes care of this. VCM monitors throttle position, vehicle speed, engine speed and automatic transmission gear selection along with other factors, determining whether the vehicle is cruising or decelerating. As the cylinders are idled, the system controls ignition timing and turns the torque converter on and off, suppressing the torque-induced jolt caused by switching from three-to six-cylinder operation. Honda engineers chose to deactivate the rear bank because the front bank is in the best position for cooling performance. The rear bank also helps to maintain catalytic converter temperature allowing for optimum emissions performance.
With the requirement of higher performance engines, fuel saving, light weight construction and new drive train concepts, the automotive supplier must deal with many conflicting aims, which need to be resolved during product development. Vibracoustic has developed a new, absorber mount that successfully solves NVH conflicts. The top mounts are used in the chassis to improve driving comfort and so its optimal mount tuning is of primary importance. While noises are better isolated by soft tuning, a harder setting is favourable to dampen vibration. Dr Hendrik Sell, Head of the Acoustic and Vehicle Test Department at Vibracoustic, Germany, explains that the previous problematic acoustical complaints in the chassis often appear late in the development cycle when disturbing noises become noticeable in quiet prototype vehicles. Since top mounts can still be changed in late development phases, they are subsequently adjusted to a softer tuning. Disturbing noises are there by eliminated. This is however done at the expense of driving dynamics, anti-vibration comfort or driving safety.
The Vibracoustic solution involves an integration of additional absorber unit into the new absorber top mount to eliminate interference. For example, in the frequency range of 150-200Hz, which is typical for adjustable dampers. Acoustics and damping can thus be adjusted separately and optimally, and without the efficiency losses. Adds Dr Sell, “For driving safety and anti-vibration comfort, the effect of the chassis dampers is indispensable. Chassis dampers prevent reverberations while driving over single obstacles or uneven roads. Wheel grip is better and driving dynamics are maintained. Disturbing noises can now be eliminated without weakening the required damper effort.
Interestingly, the disturbing noises within the range of 170-200Hz could be eliminated. The upper half of the diagram shows the absolute values of the sound pressure level and the bottom shows the achieved improvements in db (Blue).
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