One instance I can think of is when 4 cylinder happens if a guy is almost completely off the throttle but not quite full coasting down a small decline,
the engine does not need to make the same amount of horsepower no matter what mode it's in, as it would need to on a flat.
So in that instance I wouldn't think it would need to double the duty cycle. Unless my thinking is wrong which it could be.
When the driver requests 0.0% power from the engine,
AND the ecm determines that the road/tailwind is effectively providing enough power to drive the accessories, then still no,
the engine does not tend to go into V4 mode, because the four cylinders that usually keep all eight cylinders warm (though not to equal effect)
suddenly become the four cylinders that COOL DOWN all eight cylinders,
because the 4 lazy cylinders keep warm exhaust while the 4 working cylinders pass cool intake air through them without combusting anything.
At least that's what my tuner and I noticed during 0.0% TPS events; it'd only Half@$$ during 0.0% TPS if the coolant was warmer than usual (hot summer days).
The ecm tended to use V4 mode at very low load, but NOT ZERO load / DFCO - otherwise the ecm would idle in V4 mode, which'd be tres lame.
Edited to ask Marky Dissod you say the benefit is maybe 2-3MpG.
And don't forget that I also stated that this wanes as the engine ages - not that GM cares about that, CAFE MpG test scores occur when the engine / vehicle are brand new.
If the injector duty cycle is doubled, so same as if all 8 were injecting, how does it save a guy 2-3 mpg? Or any gas?
The same amount of fuel is being injected whether 4 injectors are going or all 8.
Let's say the momentary goal is to produce 30 horsepower to cruise down the highway.
In theory, a certain amount of power is 'wasted' by ANY engine trying to draw air through a partially closed throttle plate.
Just by opening the throttle plate, the engine becomes more efficient - not by leaps and bounds, but it is a consistently measurable effect.
A 2.65L V4 would need to open its throttle wider than a 5.3L V8 to produce that same '30' horsepower.
Because of 'closed loop', the ecm's INITIAL guess is to inject the same amount of fuel (ecm programmed to always guess 'slightly rich' for safety and emissions).
As the O2 sensors make their minor corrections over a few seconds, ecm will find that it consumes SLIGHTLY less fuel in V4 mode ...
especially when the engine is brand new.
As the engine ages, even assuming highest quality parts machining and assembly, the lazy V4 cools and warms and cools and warms over and over
while the fulltime V4 maintains much more temperature discipline - it keeps the lazy V4 warm, but not as consistently as it keeps itself warm.
This leads to two V4s wearing at different rates; not just one set of roller lifters wearing at a different rate, it's cam lobes cam bearings and even crank bearings.
As the one guy I know who had his tuner INCREASE V4 mode duty cycle, I can also attest to increased oil consumption, not just the waning of any MpG benefit.
