27-09-2010, 04:15 PM
Cylinder Deactivation.doc (Size: 540.5 KB / Downloads: 236)
INTRODUCTION
With alternatives to the petrol engine being announced ever so often you could be forgiven for thinking that the old favorite the petrol engine is on its last legs but nothing could be further from the truth and possibilities for developing the petrol engines are endless. One of the most crucial jobs on the agenda is to find ways of reducing fuel consumption, cutting emissions of the green house gas CO2 and also the toxic emissions which threaten air quality. One such fast emerging technology is cylinder deactivation where a number of cylinders are shut down when less is needed to save fuel.
Cylinder deactivation is used to reduce the fuel consumption and emissions of an internal combustion engine during light load operation. In typical light load driving the driver uses only around 30 percent of an engine’s maximum power. In these conditions, the throttle valve is nearly closed and the engine needs to work to draw air. This causes an inefficiency known as pumping loss. Some large capacity engines need to be throttled so much at light load that the cylinder pressure at top dead centre is approximately half that of a small 4 cylinder engine. Low cylinder pressure means low fuel efficiency. The use of cylinder deactivation at light load means there are fewer cylinders drawing air from the intake manifold which works to increase its fluid (air) pressure. Fuel is continuously pumped into each cylinder and combusted even though maximum performance is not required. By shutting down half of an engine's cylinders the amount of fuel being consumed is much less. Between reducing the pumping losses which increase pressure in each operating cylinder and decreasing the amount of fuel being pumped into the cylinders. Fuel consumption can be reduced by 8 to 25 percent in highway conditions.
Cylinder deactivation is achieved by keeping the intake and exhaust valves closed for a particular cylinder. By keeping the intake and exhaust valves closed, it creates an ‘air spring’ in the combustion chamber – the trapped exhaust gases (kept from the previous charge burn) are compressed during the piston’s upstroke and push down on the piston during its down stroke. The compression and decompression of the trapped exhaust gases have an equalizing effect – overall, there is virtually no extra load on the engine. In the latest breed of cylinder deactivation systems, the engine management system is also used to cut fuel delivery to the disabled cylinders. The transition between normal engine operation and cylinder deactivation is also smoothed using changes in ignition timing, cam timing and throttle position. In most instances, cylinder deactivation is applied to relatively large displacement engines that are particularly inefficient at light load. In the case of a V12, up to 6 cylinders can be disabled.
The simple fact is that when you only need small amounts of power such as crawling around town what you really need is a smaller engine. To put it another way an engine performs most efficiently when its working harder so ask it to do the work of an engine half its size and efficiency suffers. Pumping or throttling losses are mostly to blame. Cylinder deactivation is one of the technologies that improve fuel economy, the objective of which is to reduce engine pumping losses under certain vehicle operating conditions.
When a petrol engine is working with the throttle wide open pumping losses are minimal. But at part throttle the engine wastes energy trying to breathe through a restricted airway and the bigger engine, the bigger the problem. Deactivating half the cylinders at part load is much like temporarily fitting a smaller engine.
During World War II, enterprising car owners disconnected a spark plug wire or two in hopes of stretching their precious gasoline ration. Unfortunately, it didn’t improve gas mileage. Nevertheless, Cadillac resurrected the concept out of desperation during the second energy crisis. The “modulated displacement 6.0L V-8- 6-4” introduced in 1981 disabled two, then four cylinders during part-throttle operation to improve the gas mileage of every model in Cadillac’s lineup. A digital dash display reported not only range, average mpg, and instantaneous mpg, but also how many cylinders were operating. Customers enjoyed the mileage boost but not the side effects. Many of them ordered dealers to cure their Cadillacs of the shakes and stumbles even if that meant disconnecting the modulated-displacement system.
Like wide ties, short skirts and $2-per-gallon gas, snoozing cylinders are back. General Motors, the first to show renewed interest in the idea, calls it Displacement on Demand (DoD). DaimlerChrysler, the first manufacturer to hit the U.S. market with a modern cylinder shut-down system calls its approach Multi- Displacement System (MDS). And Honda, who beat everyone to the punch by equipping Japanese-market Inspire models with cylinder deactivation last year, calls the approach Variable Cylinder Management (VCM)
The motivation is the same as before — improved gas mileage. Disabling cylinders finally makes sense because of the strides achieved in electronic power train controls. According to GM, computing power has been increased 50-fold in the past two decades and the memory available for control algorithms is 100 times greater. This time around, manufacturers expect to disable unnecessary cylinders so seamlessly that the driver never knows what’s happening under the hood.
Cylinder deactivation is used to reduce the fuel consumption and emissions of an internal combustion engine during light load operation. In typical light load driving the driver uses only around 30 percent of an engine’s maximum power. In these conditions, the throttle valve is nearly closed and the engine needs to work to draw air. This causes an inefficiency known as pumping loss. Some large capacity engines need to be throttled so much at light load that the cylinder pressure at top dead centre is approximately half that of a small 4 cylinder engine. Low cylinder pressure means low fuel efficiency. The use of cylinder deactivation at light load means there are fewer cylinders drawing air from the intake manifold which works to increase its fluid (air) pressure. Fuel is continuously pumped into each cylinder and combusted even though maximum performance is not required. By shutting down half of an engine's cylinders the amount of fuel being consumed is much less. Between reducing the pumping losses which increase pressure in each operating cylinder and decreasing the amount of fuel being pumped into the cylinders. Fuel consumption can be reduced by 8 to 25 percent in highway conditions.
Cylinder deactivation is achieved by keeping the intake and exhaust valves closed for a particular cylinder. By keeping the intake and exhaust valves closed, it creates an ‘air spring’ in the combustion chamber – the trapped exhaust gases (kept from the previous charge burn) are compressed during the piston’s upstroke and push down on the piston during its down stroke. The compression and decompression of the trapped exhaust gases have an equalizing effect – overall, there is virtually no extra load on the engine. In the latest breed of cylinder deactivation systems, the engine management system is also used to cut fuel delivery to the disabled cylinders. The transition between normal engine operation and cylinder deactivation is also smoothed using changes in ignition timing, cam timing and throttle position. In most instances, cylinder deactivation is applied to relatively large displacement engines that are particularly inefficient at light load. In the case of a V12, up to 6 cylinders can be disabled.
The simple fact is that when you only need small amounts of power such as crawling around town what you really need is a smaller engine. To put it another way an engine performs most efficiently when its working harder so ask it to do the work of an engine half its size and efficiency suffers. Pumping or throttling losses are mostly to blame. Cylinder deactivation is one of the technologies that improve fuel economy, the objective of which is to reduce engine pumping losses under certain vehicle operating conditions.
When a petrol engine is working with the throttle wide open pumping losses are minimal. But at part throttle the engine wastes energy trying to breathe through a restricted airway and the bigger engine, the bigger the problem. Deactivating half the cylinders at part load is much like temporarily fitting a smaller engine.
During World War II, enterprising car owners disconnected a spark plug wire or two in hopes of stretching their precious gasoline ration. Unfortunately, it didn’t improve gas mileage. Nevertheless, Cadillac resurrected the concept out of desperation during the second energy crisis. The “modulated displacement 6.0L V-8- 6-4” introduced in 1981 disabled two, then four cylinders during part-throttle operation to improve the gas mileage of every model in Cadillac’s lineup. A digital dash display reported not only range, average mpg, and instantaneous mpg, but also how many cylinders were operating. Customers enjoyed the mileage boost but not the side effects. Many of them ordered dealers to cure their Cadillacs of the shakes and stumbles even if that meant disconnecting the modulated-displacement system.
Like wide ties, short skirts and $2-per-gallon gas, snoozing cylinders are back. General Motors, the first to show renewed interest in the idea, calls it Displacement on Demand (DoD). DaimlerChrysler, the first manufacturer to hit the U.S. market with a modern cylinder shut-down system calls its approach Multi- Displacement System (MDS). And Honda, who beat everyone to the punch by equipping Japanese-market Inspire models with cylinder deactivation last year, calls the approach Variable Cylinder Management (VCM)
The motivation is the same as before — improved gas mileage. Disabling cylinders finally makes sense because of the strides achieved in electronic power train controls. According to GM, computing power has been increased 50-fold in the past two decades and the memory available for control algorithms is 100 times greater. This time around, manufacturers expect to disable unnecessary cylinders so seamlessly that the driver never knows what’s happening under the hood.