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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 siliconchip.com.au 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