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Active Systems

New age engine mounts and active sound cancellation technologies

Courtesy I-CAR

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This material first appeared in the I-CAR Advantage Online, which is published and distributed free of charge. I-CAR, the Inter-Industry Conference on Auto Collision Repair, is a not-for-profit international training organization that researches and develops quality technical education programs related to collision repair. To learn more about I-CAR, and to subscribe to the free publication, visit www.i-car.com.

Cars are now being produced with active engine mounts and active sound cancellation technologies. Here we take a look at the basics.

Active Engine Mounts

Automotive engine mounts are designed to support the engine and reduce vibrations and force transmitted from the engine to the vehicle structure, and from road surface irregularities. Providing adequate engine support requires a firm engine mount, while controlling vibration and force requires a flexible mount.

With changes in vehicle technology, there are more requirements being placed on engine mount performance. Standard hard rubber, passive mounts provide firm support for the engine. However, the use of hard rubber allows engine vibrations to be easily transmitted to the chassis.

Active engine mount systems overcome these conflicting requirements by combining an engine mount that provides good support properties with an actuator to control engine vibrations, and vibrations from road surface irregularities.

Vehicles that may be equipped with an active engine mount system include, but are not limited to, the 2006 Jaguar XJ, 2007 Hyundai Veracruz, 2007 Lexus RX350, and 2007 Toyota Camry.

The 2006 Jaguar XJ equipped with a 2.7 twin-turbo diesel features electronically controlled active engine mounts, designed to cancel most of the engine vibration at idle. Because diesel engines generate higher levels of vibration than gasoline engines, electronically controlled active engine mounts are used on all 2006 XJ diesels.

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The pictured 2007 Hyundai Veracruz has electronically controlled engine mounts on both diesel and gasoline engine models. The Hyundai engine mount system consists of three passive mounts integrated with active controls, to offset vibrations.

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Some Lexus and Toyota models also use an active engine mount system. The engine mount system used on the 2007 Lexus RX350 and Toyota Camry is called the Active Control Engine Mount (ACM) system. On this system, the rear of the engine is attached to the chassis by a fluid-filled mount to control vibration and noise. The front of the engine is attached to the chassis by an electronically controlled active mount that continuously adapts to the driving conditions.

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The ACM system decreases engine vibration at idling using a vacuum switching valve (VSV). The VSV is controlled by a pulse signal, which is transmitted to the VSV from the electronic control module. The frequency of this pulse signal is matched to the engine speed to decrease engine vibration.

Hydraulic Systems

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Some vehicle models, such as the 2006 Ford Fusion (pictured), Lincoln Zephyr, and Mercury Milan are equipped with passive hydraulic engine mounts. A hydraulic engine mount is commonly called a hydromount. These mounts have two chambers filled with a fluid, typically a glycol mixture. A large orifice valve and a small orifice valve connect the chambers. A hydromount produces variable damping as a result of the transfer of liquid from one chamber to another.

Magneto-Rheological System

One equipment maker is developing yet another type of mount, a magneto-rheological (MR) fluid powertrain mount. Damping properties of the mount are controlled instantly during a variety of road surface conditions by adjusting the state of the fluid. The MR powertrain mount also helps control engine vibrations.

One benefit of the MR powertrain mount is that it is being designed as a direct bolt-in replacement for a passive hydraulic fluid mount.

The MR powertrain mount system works with both front- and rear-wheel-drive engine mounting configurations. An MR powertrain mount system includes the appropriate MR mount(s), sensor(s), an electronic control unit, and the associated wiring.

(See Magnetic Dampers and Breakthrough Technologies for more on magneto-rheological systems being used in suspension dampers.)

Crash Damage Analysis

When analysing damage to an active engine mount system check for visible damage. Inspect hydraulic or electronic engine mounts for fluid leaks, damaged or pinched vacuum hoses, improper connections, or damaged electrical parts.

When analysing damage on an engine mount assembly on a 2007 Lexus RX350, apply a vacuum to the engine mount and verify that there is no change in the needle movement of the vacuum pump gauge. If vacuum pressure remains steady, the engine mount assembly is good. If the vacuum pressure drops or no vacuum pressure exists, the engine mount assembly needs to be replaced.

Active Noise Cancellation Systems

Vehicle makers use many methods for reducing noise levels inside the passenger compartment. Thick carpeting, cotton padding, foam, and mastics are some of the more traditional items used for this purpose. But these add weight to a vehicle and require extra steps during vehicle assembly. Some vehicle makers use parts made from laminated steel for reducing interior noise. Firewall panels are sometimes made from this material to reduce noise from the engine compartment from entering the passenger compartment. (see New Body Technologies: Laminated Steel and Foams for more on laminated steels.)

Active noise cancellation (ANC) is a relatively new option vehicle makers have for reducing annoying noises heard in the passenger compartment. It employs the same type of technology used for noise-cancelling headphones. Vehicle ANC systems are electronic systems that work by reproducing the same annoying noises through the vehicle sound system. The catch is that the noise is reproduced out of phase with the original. This results in the two noises cancelling each other out. To better understand how this works, let's take a closer look at what sound is and how noise can be silenced with this process.

Sound Waves

Sound is basically a series of pressure waves, created by a vibrating object, that travel through air, water, steel, etc. Pressure waves act similar to waves created on the surface of water when a stone is tossed in, except that they can't be seen. They are a series of positive and negative pressure fluctuations.

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Sound pressure oscillations are identical to the vibration of the sound source. The frequency of sound is how many times the sound pressure waves oscillate during a certain period (see Figure 1), and is normally measured in hertz (Hz) or cycles per second. Musical instruments can be used to demonstrate the relationship between high-frequency and low-frequency sound waves.

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For example, air flowing through a flute vibrates much more frequently than.....

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...the vibrations of a bass guitar string.

Amplitude is the intensity of sound pressure waves or how loud a sound is. Sound wave amplitude is shown by the heights of the illustrated sine waves.

ANC Systems


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An ANC system works by using microphones to detect ambient sounds. The sounds are then converted to electronic information and sent to the ANC unit (computer). Depending on how the ANC unit is programmed, certain frequencies such as human voices may be ignored. Unwanted sound, or noise, is inverted 180 degrees out of phase and sent to the system speakers to be reproduced at the same amplitude level.

How does this make it quieter? A basic analogy may help to explain how this works. Think of a spring-loaded door. It moves when you push on it, and returns to position when the force is removed. Imagine pushing it in a short distance and allowing it to return several times a second. This represents sound waves. Now imagine someone on the other side of the door pushing with the same force and at the exact same times that you are. The door will remain mostly still. The person on the other side of the door represents the inverted sound waves, created by an ANC system, that negates the initial unwanted sound.

Vehicle Systems

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Honda is currently using an ANC system on some 2005-2007 Odyssey and Pilot vehicles with Variable Cylinder Management (VCM). A VCM system disables three of six cylinders during certain driving conditions for improved fuel efficiency. An ANC system is also used on the 2005-2007 Honda Accord Hybrid. The system helps reduce engine noises heard in the passenger compartment that may occur during certain cycles of the VCM. Other road noises may be suppressed as well. The reversed signals are amplified through the two front speakers of the vehicle sound system. The 2005-2007 Acura RL uses the Active Noise CancellationTM system with the Acura/Bose® 10-SpeakerSurround Sound system. It is also used for reducing low-frequency engine noises. The reverse-phase audio signals in this system are amplified through the door speakers and the subwoofer. The systems remain active whether or not the vehicle sound system is on or off.

Repair

ANC system parts are typically located inside the passenger compartment, so the chance of collision damage is minimal. Microphones are generally located on the front and rear areas of the headliner and control units may be located in the centre console behind the audio unit. However, the ANC unit in the 2005 Acura RL is located in the right side of the trunk and is more susceptible to damage. Damaged ANC parts typically require replacement.

A customer complaint of strange noises may be caused by an improper or non-functioning ANC system. Refer to vehicle specific service information for diagnosing problems that may be caused by the ANC system. Troubleshooting may include following a flowchart and checking the system circuits. This is done to determine which parts of the system are not working and may require replacement. There is a technical service bulletin (TSB 06-005) for the ANC system on the 2005 Honda Accord Hybrid regarding a loud humming or booming noise from the front speakers. It provides a simple procedure for determining whether the ANC system is functioning properly. Replacement part numbers and where repair procedures can be located are also provided.

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