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Old 03-08-2009, 01:29 AM   #9 (permalink)
Engineering first
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Originally Posted by roflwaffle View Post
. . .
Scaled for torque output, the engine in the Prius exhibits the same difference in BSFC compared to load a V6 engine designed over a decade earlier exhibits.
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?

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.) This operating line is the problem the old 6-banger could never solve with existing transmissions.

Originally Posted by roflwaffle View Post
. . . destroked . . .
In the past, I used "destroked" to mean a mechanical change such as a shorter throw crankshaft to change the piston sweep. I use Atkinson cycle when the gas compression ratio is different from the power stroke gas expansion ratio.

Originally Posted by roflwaffle View Post
. . . The only way to determine how much the higher expansion ratio helps would be to compare the 1NZ-FXE to a version destroked to have the same effective compression ratio.
The Otto version of the same 1NZ-FE is in our Toyota Echo and the early Scion series. The same engine is in the Yaris.

Originally Posted by roflwaffle View Post
. . . Diesels have problems with NOx formation because of CI, not necessarily the higher CR. . . .
Upon further thought, I can somewhat agree. The gas ratios will have a greater impact than duration at high temperature and pressures. Just I remember my chemistry studies and some reactions have non-linear effects based upon pressure and temperature. Wikipedia notes:
Thermal NOx refers to NOx formed through high temperature oxidation of the diatomic nitrogen found in combustion air. The formation rate is primarily a function of temperature and the residence time of nitrogen at that temperature. At high temperatures, usually above 1600C (2900F), molecular nitrogen (N2) and oxygen (O2) in the combustion air disassociate into their atomic states and participate in a series of reactions.
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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Last edited by bwilson4web; 03-08-2009 at 10:01 AM.. Reason: Text clean-up, improve data from charts.
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