Cylinder Bank Deactivation - EngineKnowHow

One method to reduce fuel consumption and emissions, especially in engines with a high number of cylinders is cylinder bank deactivation.  Cylinder bank deactivation occurs during part load operation when not all of the cylinders are required to generate the required load.  For example, in a V8 engine which is operating at ≈50% maximum torque one bank could be shut down forcing the four remaining cylinders to generate the load which was previously spread out across the 8 cylinders.  In a petrol engine with cylinder bank deactivation, the throttle opening on the fired bank must be increased to admit the greater mass of required air, reducing pumping losses across the throttle.  The increased air mass, intake velocity and cylinder pressure can also increase combustion efficiency resulting in a net fuel consumption benefit.  Cylinder bank deactivation is generally controlled by the shutdown of fuel injection to the cylinders.

 

Due to the benefits mostly stemming from the reduction in pumping losses across the throttle, cylinder bank deactivation is predominantly seen in high displacement, petrol engines.  Cylinder bank deactivation can be a form of variable cylinder displacement.

 

An Example Engine Capable of Cylinder Bank Deactivation

The plots below are from a petrol, V-engine capable of cylinder bank deactivation via fuel injection shut off.  Each bank had an independent throttle and the engine wasn’t capable of variable valve lift, therefore the valves opened and closed as per normal engine operation.

 

The engine was run in three different modes at a fixed load, BMEP to measure pumping losses,  PMEP and the resulting fuel consumption, BSFC:

Engine Mode Target Load Fuel Supply Throttle Position
All Cylinders Firing ≈ 15% of Max. Power at Engine Speed Banks #1 & 2 Same for both banks
Throttles Matched Only Bank #1 Same for both banks
Unfired Bank, Wide Open Throttle (WOT) Only Bank #1 Bank #1: Target Load

Bank #2: Wide Open Throttle

Reviewing the engine data:

  • When all cylinders a fired as per normal engine operation, less load must be generated by each of the cylinders. As the amount of air required by the engine is shared by both banks, the throttle for each of the two cylinder banks have a low opening angle and therefore a low plenum pressure leading to high pumping losses across the throttles.
  • When fuelling was cut to Bank #2, to generate the same load Bank #1 must compensate by increasing the mass of air delivered to its cylinders, therefore increasing the throttle opening angle (with Bank #2 matching Bank #1), increasing the plenum pressure and reducing the engine’s pumping work (PMEP). As a result, a fuel consumption saving of ≈7% was observed.
  • In the final scenario fuelling was cut to Bank #2 and again, to attain the same load more air was inducted into Bank #1 via an increased throttle angle. However on Bank #2, rather than matching the throttle angles between the banks the throttle on Bank #2 was opened 100%.  As a result, the plenum pressure in Bank #2 is closer to atmospheric pressure, there was a further reduction in PMEP and a fuel saving of approximately 10% was observed.

 

Common disadvantages associated with cylinder bank deactivation include:

 

Noise and Vibration – Engines crankshafts, flywheels, balancing shafts etc. are typically developed for the even distribution of forces on the crankshaft as defined by the engine’s firing order.  Once this firing order is disrupted following cylinder shutdown, noise and vibration issues can occur due to the changed distribution of forces in the engine.

 

Catalytic Converter Temperature – In a multi-bank engine, each bank will typically have a dedicated catalytic converter which operates efficiently above ≈350˚C.  Once combustion doesn’t occur in the cylinders supplying the catalytic converter, the catalytic converter can fall out of its operating temperature range and when combustion reoccurs, high levels of emissions can exit from the vehicle.  This issue is typically resolved by; alternating the firing of cylinders between banks, a valve opening / closing strategy which either limits the amount of fresh air moving though the cylinder or a valve lift system which stops the valves from opening and therefore fresh air moving through the exhaust.

 

In the example provided above, the cylinder deactivation strategy which had Wide Open Throttle on Bank #2 offered the highest potential fuel saving of the three engine operation strategies.  However this strategy had the highest flow of fresh air through the catalytic converter resulting in the fastest time for the catalytic converter to drop out of its operating temperature range.

 

Complexity and Cost – The control of cylinder bank deactivation is controlled by the engine’s management system with typically one ECU required for each bank.  Strategies must be developed for the independent control of each bank or cylinders and ensuring that the shutdown and restarting of cylinders isn’t observed by passengers.  Additionally it needs to be ensured that the lower cylinder pressures don’t result in oil bypassing the piston rings from the sump and entering into the combustion chambers.