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I hope I have learned something after 30 years of general dedication to cars and engines, making my own turbo conversion once, some crazy engine modifications and having succeeded to reach and repeat 4 l/100km or nearly 60mpg (US) with my 40 year old Saab. :-)
Having a small underpowered engine MAY save a lot of fuel, but it's not a guaranteed success. High revs and badly matched fuel system, exhaust and gearing can totaly change the outcome. Adding a turbo to any engine can also go either way...
Regardless of HOW the engine takes care of the induction, it is a process that consumes some of the energy the engine makes. If a turbo is used that energy is taken from the exhaust pulses, that are "free" in theory, but in practise will cause some restrictions to the exhaust flow. Compared to a compressor/supercharger that runs on the crankshaft, the turbo is generally more energy efficient, but even an electric supercharger must get it's energy somewhere, and taking that extra path from the alternator and battery is probably even less efficient than running it directly fron the crankshaft.
Because of that, it may seems like some savings can be done by not running the compressor/supercharger/turbo at all, but if the engine is built to run with forced induction, it will most likely have a lower compression ratio than a naturally aspirated engine of the same maximum power. This means that running a supercharged engine without supercharger will probably result in poorer fuel economy!
-At least this is what I think is true after studying a lot of BSFC-maps and knowing about the importance of a high compression radio to fuel economy.
A turbo/supercharger is simply a good way to pack more power and torque in a smaller engine size. This also means less bearing piston area that causes friction. I guess however that a naturally aspirated engine can reach higher general efficiency by maximising compression ratio and optimizing valve timing. With a diesel it's different, since it always runs with "open throttle" and full actual compression. Here you can have a turbo for peak power and still have a much more efficient engine on light load.
Shutting down a cylinder will increase load on the cylinders still running, giving higher actual compression/combustion pressure and therefore better fuel economy. If the engine is in mid och high rev range the loss of some cycles will probably not give much more vibrations. Running an engine on light load at high revs is however poor driving generally, and I think it would be much better to increase fuel efficiency by proper driving school than having technology that at it's best can do some advanced guessing.
I gues there is an obvious reason that I can do much better with my simple old Saab than with a modern "electronic" car or even a hybrid. -I mostly know exactly what I'm doing in different situations, and most important, I know what I'm planning to do next. The electronics can only make more or less qualified guesses...
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