V8 running on 4 cylinders
Discussion
So how does this work, shutting down 4 cylinders.
The 5.7-litre HEMI -- a V8 engine -- has 16 valves, two per cylinder, in an overhead-valve (OHV) configuration. All 2007 HEMIs use a sequential, multi-port fuel injection system with electronically controlled combustion. It has a distributor-less and coil-on-plugs ignition system.
This is the MDS type engine (which shuts down 4 cylinders when cruising to save fuel).
The 5.7-litre HEMI -- a V8 engine -- has 16 valves, two per cylinder, in an overhead-valve (OHV) configuration. All 2007 HEMIs use a sequential, multi-port fuel injection system with electronically controlled combustion. It has a distributor-less and coil-on-plugs ignition system.
This is the MDS type engine (which shuts down 4 cylinders when cruising to save fuel).
Do you really mean how does it work i.e. how does the system operate or why does it work to save fuel?
The latter is easy. Petrol engines combust most efficiently and require less ignition advance at high throttle openings with properly filled chambers. Low throttle operation is very inefficient so at low load it's better to cut some cylinders and operate the remaining ones at higher throttle openings.
There's still a frictional loss from driving the cut cylinders so it's not as efficient as a four cylinder engine of half the capacity but it's still better than having all eight pots working but each doing very little.
The latter is easy. Petrol engines combust most efficiently and require less ignition advance at high throttle openings with properly filled chambers. Low throttle operation is very inefficient so at low load it's better to cut some cylinders and operate the remaining ones at higher throttle openings.
There's still a frictional loss from driving the cut cylinders so it's not as efficient as a four cylinder engine of half the capacity but it's still better than having all eight pots working but each doing very little.
Displacement on demand (DoD) is a GM technology which has 4 special hydraulic lifters. On a DoD engine there are solenoids that control oil flow to those lifters (effectively turning off the valves, this allows the PCM to turn off those cylinders and not throw off the O2 sensor, it also reduces pumping losses). The PCM also does not fire injectors to those cylinders, however, I believe the plugs still fire. Chrysler's technology is called multi displacement system (or MDS as you said), and it works the same way.
Edited by dblack1 on Friday 7th January 07:09
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