[Ecky]

Originally Posted by swineone

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.

Probably not. The turbo has a lifespan, and being in boost more frequently might increase piston ring wear somewhat, so maybe it'll start burning oil a little sooner. I don't have a good reference to go by for small turbo VW engine lifespan though.

Originally Posted by swineone

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).

Even under boost? That's surprising, but suggests much lower economy losses under boost.

Originally Posted by swineone

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.

Same as my car, approximately. The Insight stock engine was 10.8:1. That was 20 year old port injected tech though.

Mazda's SkyActiv direct injected engines are 13-14:1, but they're on the upper end of what's typical.

OTOH, higher compression does show diminishing returns. I think at ~10:1 it's something like 2.5% efficiency for an additional point of compression, but less going from 12:1 to 13:1.

Originally Posted by swineone

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.

I would think so, yes. There are some other confounding factors though.

For example with my K24 engine (10.5:1 compression), I have to pull timing at WOT below 3000rpm, or I get knock, even with 93 octane fuel. Undoubtedly timing is being pulled with cylinder pressures literally twice as high in a turbo engine. Some of those efficiency gains are being lost due to not being able to use MBT ignition timing.

Turbo boost is also not *entirely* free, it's just far less costly than a supercharger. The exhaust gases leaving the cylinder have to push through a turbine, and that steals a bit of energy from the piston trying to evacuate the gases from the cylinder.

Originally Posted by swineone

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.

With my Insight in perfect functioning order, with some small aero mods, before my engine swap, I was able to achieve around 100mpg @ 50mph, in good weather, which is around 43km/L. Even with the much bigger and less efficient ~240hp 2.4L K series engine under the hood, I'm still seeing ~26km/L at lower highway speeds.

I'm certain there's some more you can squeeze out of your car. I'll be looking forward to seeing what you come up with!