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Old 01-13-2015, 05:10 AM   #17 (permalink)
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Originally Posted by Hersbird View Post
What I don't understand is why cars with cylinder deactivation end that deactivation almost as soon as you give it some throttle. I would like to be able to lock in the deactivation and increase throttle and load for a better BSFC. I wonder if it is just to hard on the working cylinders.
It may be an NVH thing. The two engine modes mean that the engine mounts etc. have to be designed to deal with two different "engines" in terms of vibration isolation and damping.

Originally Posted by jeff88 View Post
Also, what makes the red island the best? Why doesn't highest load and lowest RPM produce the best BSFC (and thus MPG)?
The BSFC has a peak wrt to engine speed because the engine is most efficient - due to valve timing and inertial effects in the intake and exhaust tracts - at trapping charge at a particular speed. Either side of that speed there is more wasted work done relative to the charge trapped.

Higher engine speeds always result in lower BSFC due to the effects of friction i.e. engine oil pumping, viscous effects in both bearing oil and the various air flows inside the engine.

An engine designer could optimise the engine for low speed operation, at the expense of maximum power output, or maybe spread out wrt rpm the way the engine pumps air/exhaust and flatten the BSFC plot.
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