2012 Jeep Grand Cherokee SRT8: Eco Mode
September 24, 2012
Drive our longterm 2012 Jeep Grand Cherokee a few times and you might notice this small 'Eco" indicator appear in the instrument cluster display from time to time.
Tow with it and you won't need to look for the indicator. You can hear when it goes into Eco mode.
Eco mode in this vehicle signifies when the big V8 goes into four-cylinder mode. It works like this -- the lifters of half of its cylinders are deactivated (oil pressure is re-routed away from them via a solenoid, so the lifters essentially 'collapse' when the cam lobes act on them). This keeps their valves closed, turning those four cylinders into air springs. Basically the same approach to cylinder deactivation taken by Mercedes-Benz and, later, GM.
Cylinder deactivation works best during light-load operation because there's a lot of pumping losses when you try to draw air in against a tightly-closed throttle. By knocking out half the cylinders, the throttle is opened for the four active cylinders, reducing pumping loss and hence improving fuel efficiency.
As you might imagine, four cylinders operating at high load have a very different sound signature than eight cylinders at light load. Chrysler engineers had to fit a doohickey to the exhaust, an electronically actuated flap, in order to manage the sound situation.
And that takes us, full circle, back to my earlier observation: when towing on the freeway, the switch to four-cylinder mode in the GC is much more prominent than when not towing, onaccounta the higher load on the engine. It makes a somewhat boomy sound in four-cyl mode, and you can easily pick out the transition even when the radio's on.
Anyway, the point is, don't be alarmed if your GC occassionally gets boomy when towing. All it means is you're saving some fuel. Which leads us nicely to: what was the observed fuel economy during this ~1000-mile towing trip? Good question. Stay tuned.
Jason Kavanagh, Engineering Editor