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