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Old 07-11-2008, 01:12 PM   #61 (permalink)
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Sounds like a Volt. An onboard generator charges a battery (so it can be setup for optimal output at a said rpm) once the plug in battery has been run low on charge. The car is propelled by an electric motor.


Gearing a motor down to 1000 rpm would be completely doable. In a hilly area you'd have to deal with fluctuating speeds is all. The engine wouldn't stall, it simply wouldn't be able to keep you up to speed.

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Old 07-11-2008, 02:31 PM   #62 (permalink)
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And you can always downshift if it gets too steep. But those red dots are pretty good evidence that slower engine speeds = better mpg, now that I finally figured out what they were
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Old 07-11-2008, 05:09 PM   #63 (permalink)
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Only at cruise. Mid range RPMs are best for acceleration. Many people don't understand this point.
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Old 07-11-2008, 06:36 PM   #64 (permalink)
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could anyone explain the TDI BSFC chart to me?


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Old 07-11-2008, 07:12 PM   #65 (permalink)
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When accelerating, give it almost full throttle and center your shifts on 1750 RPM.

When cruising it is business as usual, highest gear you got with the slowest speed you can stand.

The red dot curve that Daox put together looks suspiciously similar to the red dash curves on that tdi graph. My uneducated guess is that those red dash curves are the torque required for three different speeds (not labeled). And they come tantalizingly close to the "sweet spot" but I don't know if the stock gearing will get you there or not.
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Old 07-14-2008, 03:12 PM   #66 (permalink)
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top line

Quote:
Originally Posted by MetroMPG View Post
cfg, do you know what the line is at the top of your chart with the points marked on it? The original Geo engine chart appears to have a similar line.
Top line is Full-Throttle/Max. BMEP
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Old 07-14-2008, 03:25 PM   #67 (permalink)
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225

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Originally Posted by jonr View Post
Exactly.

It would be nice if the Saturn BSFC chart had one more island - there might be a 225 around 2500 rpm/125 Nm.
The lowest I've seen is about 255,with the Orbital Engine 2-stroke,used in the 1992.GM Ultralite.Anything below that is in the domain of Diesels.
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Old 07-14-2008, 03:34 PM   #68 (permalink)
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gearing

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Originally Posted by pasadena_commut View Post
Ok, let's say you can open the gearbox and replace the gears with any ratios you want. How do you choose those gears for optimal efficiency?

Say you want to optimize for efficient cruising at 65 mph. So for the top gear you look at the BSFC and pick the RPM centered on the lowest fuel use, then 65 mph and that RPM (and tire size) determine the gear ratio. That does not determine the load though, so there is nothing much you can do to pick the location on the BSFC chart along that vertical line for your cruising speed. Is that optimal? Probably not, since a higher gear will increase the load on the motor but move the RPM to the left of the center of the most efficient point, and could lead to better cruising efficiency. Now you want to add a gear for accelerating at that same optimally efficient RPM. How do you choose that acceleration gear? Seems like an even more difficult problem since for a fixed gear, and a given RPM, there will be a fixed speed. It would help if we knew what the path of RPM vs Torque looks like as a function of RPM for a fixed gear ratio.
The best solution they can think up so far,is a twelve-speed tranny.And as you've noticed,they're not available yet.Mercedes is up to 7-speeds,and Toyota has over-taken them with a Lexus 8-speed.Ricardo,in England was putting stacked transmissions in cars so he'd have the "perfect" ratio for any contingency.For us right now,its a matter of getting through the transient environment,up to speed,then conserve momentum.
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Old 07-14-2008, 05:06 PM   #69 (permalink)
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what does "bmap (bar)" stand for on the TDI bsfc chart?

I assume that lower numbers (labeled on chart) stand for higher efficiency?
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Old 07-14-2008, 07:23 PM   #70 (permalink)
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Red dashes??? HAH!!
Behold the power of blue lines!!

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