Quote:
Originally Posted by bwilson4web
Using your charts, it looks: - Prius - BFSC less than 230 g/kw-h between 2,200-3,400 rpm.
- 6-banger - BSFC less than 240 g/kw-h is between 1,400-2,900 rpm.
- 6-banger narrow - BSFC at 237 g/kw-h, 1,800-2,400 rpm.
Did I misread the charts? |
No, you didn't misread the charts. However it appears you didn't understand what I was talking about. Specifically, you stated that the Atkinson cycle results in a substantial reduction in throttling losses in low power regions, however when comparing it to older engines the change in BSFC compared to load, adjusted for displacement/age of course, is more or less identical. In other words, the Atkinson cycle didn't do a lick for low load throttling losses as far as I can see, adjusted for the difference in overall efficiency/displacement. Granted, Toyota has increased overall engine efficiency, just like they did when comparing the two sixes made about a decade apart in the BSFC map, but all the Atkinson cycle does is allow for a cheap way to reduce the effective displacement, aka a cheaper way of destroking the engine.
Quote:
Originally Posted by bwilson4web
When you get a chance, I'd recommend getting a copy of SAE 2004-01-0064 for this quote, "As a result, the minimum specific fuel consumption of 225g/kWh has been achieved. . . " (pp. 7.) This paper is the source of the first graph and does an excellent job of showing the specific systems in the Prius. More importantly, it shows how the Continuously Variable Transmission keeps the engine at the best BSFC over a very wide, rpm range (the operating range line on that first chart.)
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Send me a copy!
Quote:
Originally Posted by bwilson4web
This operating line is the problem the old 6-banger could never solve with existing transmissions.
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Well, lesse, the old transmission over the same speed range would see BSFC increase about 9% compared to the minimum, specifically a minimum of 237g/kWh and a maximum of 260g/kWh, and the Prius also exhibits a ~9% difference with a minimum of 230g/kWh and a maximum of 245g/kWh. The biggest difference is the more "open" ovals as engine speed increases, which is due to the offset crank and lower friction losses, not the greater expansion ratio AFAIK. Course, like everything either one of us, or possibly both of us, could be wrong, so in short we need more info IMO.
Quote:
Originally Posted by bwilson4web
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A closer comparison would probably be the 2NZ-FE, although I haven't seen a BSFC map for it, and for that matter, since we don't know what the effective displacement of the 1NZ-FXE is, who knows what Otto cycle engine it would be equivalent to in terms of displacement. If we could slap something together with the same effective displacement, offset crank, all that jazz, less the greater expansion ratio of the 1NZ-FXE, then we could see what the difference is. Until then, all we can do is (fun IMO) internetz speculation.
Quote:
Originally Posted by bwilson4web
But the mechanical aspects probably play another important part.
As the expansion ratio increases, the stress on the piston, rod, crank and cylinder head goes up right after ignition. It is equally likely that the 13-to-1 ratio seen with the 1NZ-FXE is a mechanical limitation. They didn't want to add the additional metal needed for a higher expansion ratio that might cause the engine to approach diesel weights.
Bob Wilson
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I've never heard of a greater expansion ratio increasing stress outside of conventional engines. For those it's a given since the CR has to go up too, but if possible, it seems that having ER>CR would lower stress on the engine internals all things being equal.
All things being equal, an engine with a lower CR should see less stress on the internals, and by increasing the expansion ratio they're lowering EGTs/extracting more work over a greater time period, which should also decrease stress compared to extracting more work over the same time period. If anything I think the expansion ratio was dictated by what Toyota could do with the combustion chambers as opposed to the mechanical limits of the block.