The Thai-built Honda Accord uses a 3.5-litre, 200kW-plus SOHC V6 that utilises Honda’s unique, high-tech Variable Cylinder Management (VCM), which means the new Accord can switch between six, four or three cylinders depending on road speed and load conditions.
The system is broadly similar to the one found in the Chrysler 300C but with an additional setting. The system will not only help bring fuel consumption down to a four-cylinder equivalent but will subsequently reduce emissions.
The system works by using all six cylinders during acceleration or high load, and seamlessly switching to four or three cylinders when cruising on the highway or driving at very low speeds (e.g. traffic jams). According to Honda, drivers will not be aware of the automatic switching.
The company says it’s the automotive equivalent of having your cake and eating it too! More details - including fuel consumption, we hope - are expected before February.
Honda released the following regarding the operation of VCM:
How it works
During start-up, acceleration or when climbing hills - any time high power output is required - the engine operates on all six cylinders. During moderate speed cruising and at low engine loads, the system operates just one bank of three cylinders. For moderate acceleration, higher-speed cruising and mild hills, the engine operates on four cylinders.
With three operating modes, the VCM system can finely tailor the working displacement of the engine to match the driving requirements from moment to moment. Since the system automatically closes both the intake and exhaust valves of the cylinders that are not used, pumping losses associated with intake and exhaust are eliminated and fuel economy gets a further boost. The VCM system combines maximum performance and maximum fuel economy - two characteristics that don't typically coexist in conventional engines.
VCM deactivates specific cylinders by using the VTEC (Variable Valve-Timing and Lift Electronic Control) system to close the intake and exhaust valves while simultaneously the Powertrain Control Module cuts fuel to those cylinders. When operating on three cylinders, the rear cylinder bank is shut down. When running on four cylinders, the left and centre cylinders of the front bank operate, and the right and centre cylinders of the rear bank operate.
The spark plugs continue to fire in inactive cylinders to minimize plug temperature loss and prevent fouling induced from incomplete combustion during cylinder re-activation.
The system is electronically controlled, and uses special integrated spool valves that do double duty as rocker-shaft holders in the cylinder heads. Based on commands from the system's electronic control unit, the spool valves selectively direct oil pressure to the rocker arms for specific cylinders. This oil pressure in turn drives synchronizing pistons that connect and disconnect the rocker arms.
The VCM system monitors throttle position, vehicle speed, engine speed, automatic-transmission gear selection and other factors to determine the correct cylinder activation scheme for the operating conditions. In addition, the system determines whether engine oil pressure is suitable for VCM switching and whether catalytic-converter temperature will remain in the proper range. To smooth the transition between activating or deactivating cylinders, the system adjusts ignition timing, drive-by-wire throttle position and turns the torque converter lock-up on and off. As a result, the transition between three-, four-, and six-cylinder operation is unnoticeable.