[swineone]

Originally Posted by Ecky

I can't speak for your specific vehicle, but engines under boost have higher cylinder pressures. To prevent detonation (also called "knock"), the typical strategies involve going rich and retarding ignition timing.

Nice observation, which reminds me: are these higher cylinder pressures harmful to the engine? In that case, if I'm trying to balance fuel savings with maintenance cost, trying to make the engine last as long as possible, maybe I should prefer less over more boost? Unless more boost is especially efficient.

Originally Posted by Ecky

Going rich reduces combustion temperatures by dumping in extra fuel which won't be burnt to make power, but instead just absorbs heat and is flushed out the exhaust.

Retarding ignition timing starts combustion later, and effectively dumps part of the useful energy that could have been extracted from combustion, out the exhaust.

If your engine has the proper fuel and spark maps to take advantage of E100's higher knock resistance, it may not need to go rich or pull any timing under boost, but that isn't a given.

I live in Brazil and the absolute majority of cars sold here are flex fuel straight from the factory (and to be clear, my car is one of those). Indeed, you'll find both E100 and E27 on nearly every gas station in the country.

You do remind me, however, that I should perform the same tests on E27 as well. Maybe I'll switch fuels for the next tank to test that.

The results are as shown, though: lambda=1 and closed loop operation under all conditions that I tested (save while in DFCO, evidently, and when switching the car into idle after a pulse).

Originally Posted by Ecky

Turbo engines usually have lower static compression ratios, which makes it so they don't need to dump as much fuel or pull as much timing under boost, but it also makes them less efficient outside of boost.

My car's engine in particular has a 10.5:1 compression ratio. Not sure you'd consider that low; my previous car was NA and had the same ratio.

Still, is it the case that the thermodynamic efficiency increases with boost? I've always thought that, in a way, boost achieves the same effect as increasing the compression ratio. If, at a given moment inside the engine, there is a full 1 bar of boost over atmospheric pressure, then isn't that equivalent, in terms of efficiency, to having a NA engine at WOT with twice the compression ratio?

In that case, and assuming the car is indeed running stoichiometric under all conditions, then I would assume the most efficient operating point would be at max boost.

Originally Posted by Ecky

12-13km/L all-city driving doesn't seem bad at all to me, frankly, but there aren't many (any?) cars sold in the US which can match those figures without hypermiling, that aren't hybrids.

Indeed these are good figures (the local equivalent to EPA figures are 9.6 km/l city and 11.1 km/l highway), and I can get even better figures when conditions help.

For instance, today I took a somewhat larger trip (about 10 km each way), most of which was on the peripheral highway around my city. Recall it's summer here, and I don't use A/C. Because it's Sunday and there were few cars on the road, it was safe to P&G around 50-60 km/h, sometimes going a little higher. Even with a small patch of city driving, a few traffic lights, etc. I was able to achieve 19 km/l according to the trip computer, which after correction should be closer to 17 km/l, but still, it's an excellent figure.

The reason I'm looking for ways to improve is that I've seen people report figures as high as 25-28 km/l (of E27) on the highway, and these people probably aren't taking any heroic measures to save fuel (i.e. they're probably targeting a speed of 100 km/h or more, A/C on, etc.) Now I'm well aware that there are differences on the quality and performance of two engines coming out of the same plant, due to e.g. manufacturing tolerances (and never mind that I haven't yet put 10.000 km on my car, so the engine is probably a little rough still). Also, knowing my fellow countrymen, I wouldn't be surprised if they're "rounding up" the numbers a bit; probably using trip computer values rather than actual measurements at the pump; claiming a record best, once in a lifetime figure is an average; etc. Still, maybe there's some truth to these figures, in which case I have a lot to learn.