5 November 2008
Lindertech 2008
Scot Manna – Cylinder Deactivation Systems
By Brian Early
Scot Manna is both an instructor/trainer and a working technician (owner of his own shop, in fact). His seven ASE qualifications include three Master certifications and two Advanced Engine Performance certifications, and he trains for both colleges and independents, such as Snap-On, AutoValue, and Linder Technical Services.
His capabilities have earned him a few “Technician of the Millennium” wins from ACDelco and ASE.
Scot Manna, Cylinder Deactivation Systems. Scot takes
the mystery out of variable displacement systems.
Scot’s topic of choice at Lindertech North 2008 was cylinder deactivation, a fuel-saving feature that has recently been implemented by several automakers as a way to meet ever-increasing economy standards and customer expectations.
Many names, same basic function
These systems go by many names—VCM (variable cylinder management; Honda), DOD/AFM (displacement on demand/active fuel management; General Motors), and MDS (multiple displacement system; Chrysler)—but the basic idea is the same: to shut down unneeded cylinders during periods of light engine load to reduce overall fuel consumption.
Reliable now
Unlike the much-maligned 1981 Cadillac 8-6-4 engine—a little ahead of its time and highly problematic as a result, today’s variable displacement engine designs have so far proven virtually trouble-free. Part of this may be because there aren’t huge numbers of them out there, particularly those with high mileage and years of corrosion.
Like any other automotive system, some will eventually require diagnosis and repair; so having an understanding of how these systems are implemented will prove valuable when they do.
Not just a fuel shut-off
More than simply turning off the fuel to particular cylinders, variable displacement requires that the valves for the deactivated cylinders remain closed, causing the air charge trapped within to act like an air spring, recapturing most of the energy of the piston’s upward motion to greatly reduce the engine’s pumping losses from the disabled cylinders.
There are two primary ways in which this is accomplished, and Scot goes into great detail to describe both mechanical actuation and control strategy.
Money where his mouth is
Scot’s interest in understanding the operation of variable displacement even led him to partially disassemble the engine in his then brand-new Honda Accord Hybrid (which uses a VTEC-based rocker-arm valve deactivation) to perform some tests and shoot some video.
Lifters designed to collapse
In most engines, a collapsed hydraulic lifter is a problem, but in lifter-based variable displacement systems, it’s what allows the valves to remain closed, and is completely intentional.
All of the current systems, whether lifter or rocker arm-based, use oil pressure and control solenoids. Understanding the principals behind the systems’ operation will make diagnosing failures much easier.
They all also involve several other components, such as an electronic throttle (which varies to maintain consistent power output with cylinder loss), the transmission, and possibly even the audio system (Honda uses it for noise cancellation).
Maintenance is key
One of Scot’s most critical points is that good quality, clean oil of the correct viscosity is of vital importance to the proper function and longevity of these systems; a valuable insight that we can pass along to our customers in terms of maintenance recommendations.
Why you should care
These systems are only going to become more widespread, and as the vehicles age and their maintenance suffers, they will eventually require service. A little knowledge and some familiarity now will give you the leg up when it happens.
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