Underhood - Service Repair

Search Autoparts/Motorage/Underhood-service-repair/

Hits and misses: Variables of variable displacement engines

Monday, April 1, 2019 - 06:00
Print Article

How about only one out of eight cylinders? That’s true for 2019 and for the last 20 years there have been plenty of other models in your service bays every week that can run on half of their cylinders thanks to the continually advancing technology of variable displacement engine technology. In this article, we’ll pass along information on how the brand new and older systems work plus give some diagnostic tips on what to do when they you get common variable displacement engine complaints like misfires and oil consumption.

Motor Age Magazine Want more ? Enjoy a free subscription to Motor Age magazine to get the latest news in service repair. Click here to start you subscription today.

SAVE 20%

On Automotive Underhood Training Videos, ASE study guides and more.

Variable displacement? The whys and the variations

Figure 1

To meet increasing CAFÉ requirements, OEMs have been downsizing engine sizes and adding technologies such as turbocharging and GDI for years. Another technology seen in your service bay to accomplish the same CAFE goal is variable displacement. A full-size pickup or SUV weighing over 2.5 tons travelling 65 miles per hour only needs around 25 HP to maintain speed on a level surface. In 2005, the Generation IV Vortec 5.3L engine in the GMT 360 platform (Chevrolet Trailblazer and GMC Envoy) were the first engines equipped with Displacement on Demand (DoD) if we don’t include those early Cadillac 4-6-8 engines from the early ’80s. The DoD engine control system has the ability, under certain light load driving conditions, to provide maximum fuel economy and reduce emissions by deactivating four of the eight engine cylinders. Cylinders 1 and 7 on left bank and 4 and 6 on right bank are always the cylinders disabled. GM changed the name to “Active Fuel Management” or AFM shortly after that and other OEMs joined in with their own variable displacement technologies such as Chrysler/Dodge/Jeep with their Multiple Displacement System (MDS) used on Hemi engines (almost identical to GM’s system) and Honda/Acura with their Variable Cylinder Management (VCM) system. Each manufacturer has released subsequent variations with minor changes to their systems over the years. Until the 2019 GM full-size pick-up/SUV model engines (5.3 L84 and 6.2 L87) debuted this year with Dynamic Fuel Management (DFM), there had been one aspect of commonality – the same cylinders were always shut down during reduced displacement operation. With DFM or as Delphi Technologies refers to it, “Dynamic Skip Fire”, almost countless combinations of cylinders activated and deactivated will likely become the new normal for variable displacement technology. Delphi teamed up with the software company Tula in Silicon Valley to come up with an innovative technology that constantly changes which combination of cylinders are deactivated. GM’s DFM utilizing eight separate solenoids (Figure 1) to control all 16 valves can rotate cylinder deactivation patterns as well as run fixed patterns. DFM differs from other systems in its charge trapping strategy with a low-pressure combustion charge trapped in deactivated cylinders, requiring deactivating and activating the intake valve before the exhaust valve. For rotating patterns, which cylinders are being deactivated can change with each subsequent engine cycle.

AFM and MDS mechanical operation

GM and Chrysler/Dodge/Jeep use oil pressure control solenoids to move a spring-loaded pin in both the intake and exhaust lifters essentially allowing the lifter to collapse/shorten on command. The solenoids are housed in common assembly GM refers to as a LOMA (Lifter Oil Manifold Assembly) (Figure 2). On both OEMs these locking pins connect the inner mechanism of the lifter to the outer housing. The inner mechanism interfaces with the pushrod; the outer housing contacts the camshaft lobe through a roller. Thus, the lifter doesn’t lift as far when the cam lobe moves it up therefore the push rod doesn’t push as high and the rocker arm doesn’t open the valves (Figure 3). Although one solenoid controls lifter pin release pressures for both intake and exhaust valves, solenoid activation is timed so the exhaust valve is disabled first. This traps a burnt exhaust charge in the cylinder which contributes to a reduction in oil consumption, noise and vibration levels and exhaust emissions. If all enabling conditions are maintained for variable displacement operation, the PCM calibrations will limit cylinder deactivation to a cycle time of 10 minutes in V4 mode, and then return to V8 mode for one minute. Transitions from 8-4-8 only takes 250 ms. Fuel is disabled while spark remains active to reduce the potential for plug fouling and cylinder wall wash.

Figure 2
Figure 3

Honda/Acura’s VCM system differs in that it uses solenoid-controlled oil pressure circuits to move spring-loaded pins to cause the overhead cam’s rocker arm assemblies to uncouple from the cam lobe followers (Figures 4 and 5).

Figure 4 Figure 5

Electronics and software strategies

All variable displacement systems work via oil pressure control solenoids. The engine management computer’s job is to activate these solenoids while monitoring for current draw and voltage status.  Resistance for each solenoid is around 12 ohms. If, for example, the solenoid’s low-side ground control circuit is NOT commanded to ground the circuit should be at battery voltage. If the solenoid’s low-side ground control is being grounded (to activate the solenoid) the low-side control circuit should be near zero volts. In either state the voltage should be as predicted and if not, a DTC sets for a circuit fault. Nothing new there. The enable criteria for any of these systems to work is lengthy. Basically, there can be no high load or over heat condition nor any DTC setting conditions present for these systems to activate.

Article Categorization
Article Details

< Previous
Next >
blog comments powered by Disqus