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Here’s the drum on
Here’s
Honda’s 3.5 litre V6
with cylinder
deactivation
By
Leo Simpson
No doubt most readers will have seen the TV commercials for the current model
Honda V6 Accord. The commercial shows a graphic demonstration of the engine’s
three modes whereby it can run on three, four or six cylinders. But while groups of
musicians playing kettledrums might be spectacular, it does not give a clue as to how
it’s done.
10 Silicon Chip
siliconchip.com.au
I
f any normal 6-cylinder engine runs with one or two
cylinders missing, it sounds and feels very sick indeed,
with a major loss of smoothness and pulling power.
So how does the Honda Accord manage to run with
one, two or even with three cylinders out – without loss
of smoothness and power?
Not only does the engine manage to run smoothly in these
three modes, the changes between modes while driving are
imperceptible to the majority of drivers.
Honda is not the only car manufacturer to have an
engine with variable cylinder modes. Chrysler has its
Multi-Displacement System (MDS), Mercedes-Benz has
its Active Cylinder Control and General Motors has Active
Fuel Management.
But in contrast to Honda, these systems are less complex
and apply to V8s rather than a V6.
The Honda V6 uses all six cylinders during start-up,
heavy acceleration and when climbing steep hills. At high
cruising speeds and when climbing modest inclines, the
engine drops into 4-cylinder mode and finally, at light
engine loads, it runs on just one bank of three cylinders.
In 4-cylinder mode, it runs with one cylinder in each bank
deactivated.
Honda uses its i-VTEC (intelligent Variable Valve Timing
& Electronic Lift Control) to shut down the unwanted cylinders. It does this by closing the intake and exhaust valves.
The pistons then continually compress and de-compress the
air trapped in the cylinders and while this might seem like
a power wasting process, it actually reduces the cylinder
pumping losses compared to normal operation.
In fact, Honda claims that pumping losses can be reduced
by up to 65%.
Honda’s i-VTEC is another variant of the VTEC systems
which have been used on its four and six-cylinder engines
for quite a few years. VTEC enables
large increases in volumetric efficiency of an
engine and is
This under-bonnet photo belies the complexity of the
engineering underneath those plastic cowls. In fact, it looks
similar to the previous 3-litre engine which had a simpler
VTEC system and no cylinder deactivation.
an alternative to turbo-charging.
In effect, it enables the benefits of a modest camshaft
profile for smooth low speed running and a high-lift highduration camshaft profile for much higher outputs at high
engine RPM. As a result, Honda’s VTEC petrol engines
are among the most powerful naturally aspirated (ie, not
turbocharged) motors produced worldwide.
The Honda 3.5-litre V6 is a SOHC (single overhead cam)
engine, meaning that it has two camshafts, one for each bank
of three cylinders. Switching between the two cam lobes (on
each camshaft) is controlled by the ECU which continually
monitors engine oil pressure, engine temperature, vehicle
speed, engine speed and throttle position.
At the switch point a solenoid is actuated to control a
spool valve to operate a locking pin which locks the high
RPM cam follower to the low rpm ones. From this point
on, the poppet valves open and close according to the
high-lift profile, which means that the engine’s breathing
is greatly improved.
The switch-over point is variable, between the minimum and maximum point, as determined by engine
load. The switch back from high to low cam lobes is
set to occur at a lower engine speed than the upswitch, to avoid surging if the engine is operating
at or around the switch-over point. (Readers
would know this as “hysteresis”).
The SOHC version of VTEC applies variable valve lift, duration and timing only to
the intake valves but in the 3.5-litre V6 it also
controls valve deactivation via extra hydraulic
controls. At this point, the story becomes more
complicated.
First, consider that the cylinders in the V6 are
numbered from 1 to 6, with the three cylinders on
the rear bank being 1, 2 & 3 and those in the front bank
numbered 4, 5 & 6. As already noted, cylinders 1, 2 & 3 are
Fig.1: This cutaway diagram of the Honda engine gives some idea
of the complexity of the design but it is difficult to make out the
complex double rocker arm system which drives the valves from
the single overhead camshaft (one for each bank).
siliconchip.com.au
January 2009 11
Fig.2 (left): this diagram shows how
synchroniser pistons (red) lock primary
and secondary arms are locked together so
that the camshaft controls the four poppet
valves for each cylinder. At right, Fig.3,
the synchroniser pistons are unlocked
and so the poppet valves are disabled,
de-activating the cylinder.
deactivated in 3-cylinder mode while cylinders 3 & 4 are
deactivated in 4-cylinder mode.
To enable these four cylinders to be deactivated, they
have two types of rocker arm associated with the camshafts
– primary and secondary. The primary rockers follow the
camshafts while the secondary rocker arm compresses the
valve springs.
Synchroniser pistons lock the rocker arms, enabling them
to open or close the valves as required. By the way, there
are four valves per cylinder, two inlet and two exhaust,
making a total of 24 poppet valves.
Fig.2 shows how the primary and secondary rocker arms
are locked together. When the ECU determines that a cylinder is to be deactivated, it reduces the hydraulic pressure
to the primary rocket arm. This slides the synchroniser
piston to the side, towards the secondary rocker arm, to
disengage both the primary and secondary rocker arms,
as shown in Fig.3.
As a result, the camshaft is effectively disconnected from
the rocker arms and the inlet and exhaust valves are held in
the closed position by the valve springs. Thus the cylinder
is sealed and the piston moves up and down to compress
and de-compress the trapped air, as depicted in Fig.4.
No fuel is injected at this time but the spark plugs continue to fire so that they do not cool down, minimising the
possibility of plug misfire or fouling when normal cylinder
operation is restored. When that happens, the relevant
synchroniser pistons slides back into position to engage
both the primary and secondary rocker arms and normal
valve operation resumes.
It is interesting to consider the firing order of the engine
in the three different modes. Normal firing order for the
V6 is 1-4-2-5-3-6 and as already noted, the spark plugs are
driven in the same order whether cylinders are deactivated
or not. Hence, the firing order in 4-cylinder mode is 1-25-6 and in 3-cylinder mode, where one bank of cylinders
is deactivated, the firing order is 4-5-6.
As you can imagine, the 3 and 4-cylinder modes lead
give rougher engine operation than when in 6-cylinder, and
the 4-cylinder mode is particularly rough, due to irregular
firing order (ie, two firing strokes on the front back and two
strokes on the rear bank). However, by restricting cylinder
deactivation to higher speed and lower engine loads, this
12 Silicon Chip
reduction in engine smoothness is minimised.
Active engine mounts
Where the engine potentially becomes very rough is at
the point of cylinder deactivation, whether from six to four
cylinders or for from four to three and back again. Honda’s
VCM (Active Control Mounts) counteracts this. The active control engine mounts are depicted in Fig.5a & 5b.
In effect, each engine mount comprises a linear solenoid
which drives a plunger to control hydraulic fluid inside
the mount. Each solenoid is driven by an audio amplifier
with a signal in anti-phase to the vibration at each mount.
The ACM system operates by using the crankshaft and
Fig.4: when
a cylinder is
deactivated, the
four poppet valves
are disabled and
remain shut. The
gas trapped in
the cylinder is
then repeatedly
compressed and
decompressed as the
crankshaft rotates.
While this takes
power from the
engine, the losses are
less than the pumping
losses associated
with partial throttle
settings.
siliconchip.com.au
Fig.5a: Instead of conventional engine mounts the
Honda 3.5l V6 has “active” mounts each involving
large linear solenoid and an oil damper system.
Fig.6: Honda’s Active Noise Cancellation uses two
microphones within the cabin. The low frequency engine
and road noise signals they pick are processed and
then fed through the car sound system to give a claimed
cancellation figure of –10dB.
is reversed in phase and fed to the sound system amplifiers
and loudspeakers to produce a claimed noise reduction of
10dB – a very significant result.
Conclusion
Fig.5b: the linear solenoid (it has a linear response to a
drive signal) is driven with an audio signal to counteract
unwanted engine vibration when in 3 or 4-cylinder
modes or when changing from one mode to the next.
camshaft position sensors to estimate engine vibration
when a cylinder is deactivated or reactivated and it feeds
an appropriate signal to the solenoids to counteract that
vibration.
At the same time, the transition between the cylinder
modes is smoothed by adjusting the ignition timing, the
drive-by-wire throttle position and by turning the torque
converter lock-up on and off. As a result, the transition
between three, four and six-cylinder operation is unnoticeable.
Noise cancellation
As a final measure to control the perceived noise of the
engine, the Honda V6 Accord uses Active Noise Control
(ANC) which SILICON CHIP readers would know as noise
cancellation. ANC is similar to the Bose Active Noise
Control system used in the current model Honda Legend.
In the case of Honda V6 Accord, the vehicle’s sound system provides noise cancellation and it operates regardless
of whether the radio or CD player is in use.
There are two microphones inside the cabin, one in the
overhead console and one on the rear parcel shelf, to pick
up low frequency engine and road noise. This noise signal
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The 2008 Honda V6 with cylinder deactivation, active
engine mounts and active noise control is a very complex
package. It results in a car with a very powerful but economical engine and one with a very quiet ride.
Power output of the 3.5-litre V6 is 202kW (270 BHP) at
6200 RPM and 339Nm of torque at 5000 RPM, considerably higher than the 177kW and 287Nm of the 3-litre V6
it superseded. Even so, its fuel consumption is reduced
with respect to the previous engine.
Which is all well and good but we should conclude on
a sober note. While the new 3.5 litre V6 is clearly more
efficient, it is installed in a body which is larger and considerably heavier than its predecessor: 1650kg compared
with 1525kg (V6 luxury model in both cases). That’s an
increase of 125kg. The same thing happened when Honda
previously changed models, with the weight for the V6
Luxury model increasing by 71kg. So in two successive
models, Honda has increased the weight of its top Accord
model by almost 200kg. Honda is not alone in this and most
manufacturers continue to increase the weight of their cars
with each model change.
So while engines continue to improve in power output
and specific fuel consumption, due to increasingly complex technology, how much more would fuel economy
improve if weight was not allowed to increase with each
model change?
NOTE: Honda Australia was not willing to release any
of the finer technical details of the operation of this engine
or its control systems for the preparation of this story. All
photographs and diagrams are courtesy of Honda.
Reference:
Development of a 6-Cylinder Gasoline Engine with New
Variable Cylinder Management Technology,
Mikio Fujiwara, Kazuhide Kumagai, Makoto Segawa, Ryuji
and Yuichi Tamura, (Honda R&D Co, Ltd). SAE Technical
Paper Series, 2008 World Congress, Detroit, Michigan,
USA.
January 2009 13
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