Efficiency and MG1 brake

3549 Views 3 Replies 0 Participants Last post by Guest, May 13, 2002

Discussion Starter · May 9, 2002

Ok, After getting burned by Yahoo, I AM taking this discussion here!
You others follow please

Playing with Graham's constant-speed spreadsheet model is
quite enlightening. Enter the engine speeds of 2136, 2349,
2563, 2776, and 2990 for 50, 55, 60, 65, and 70 mph produces:
75.72, 70.67, 65.29, 59.79, and 54.42 for estimated MPG. Those
speeds essentially stop MG1. The resulting MPG values are
approximately 4 mpg higher then the as-delivered spreadsheet.

Interestingly, using the excel solver to maximize MPG by varying
the engine RPM produces essentially identical MPG values, although
it doesn't quite stop MG1.

The idea of "clutching the sun gear" is essentially one and the same
thing. I did not ever intended to describe HOW one puts a brake
to MG1, just that it be mechanically stopped (via an electric control).

There should be no concern of MG2 free-wheeling - one need not
allow a path for any current to flow due to its back-emf, although
it is good to generate whatever power is necessary to run the car
accessories and add/subtract power to handle the small terrain/wind
variations too.

Try as I might, I can't get any better acceleration by stopping MG1 -
it always hurts on that score.

I'm supposing that Graham put actual measured ICE rpms at cruising
speed in his spreadsheet. I wonder if the model is off a bit or whether
Toytota sacrificed efficiency for reduced engine wear and a quieter
cabin.

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Discussion Starter · #2 · May 10, 2002

Re: I wonder if the model is off a bit or whether
Toytota sacrificed efficiency for reduced engine wear and a quieter
cabin.

Or less complexity and less cost.

Anyway, these are interesting results. A sun gear lock, along with Graham's two step transmission might result in a much peppier yet even more efficient vehicle. I think hybrid technology is still in its youth, leaving lots of room for further improvement.

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Discussion Starter · #3 · May 10, 2002

Re: Or less complexity and less cost.

I wasn't talking about the MG1 brake, I was talking about the
software chosen point for ICE rpm at highway speed - if it
actually matches Graham's spreadsheet, they could have higher
efficency by running it a bit faster RPM. I can see no significant
cost or complexity to using different software to set the
highway ICE rpm cruise range.

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Discussion Starter · #4 · May 13, 2002

Harry,
Thanks for trying out the spreadsheet. I hope you're not too disappointed by what I'm about to reveal.
The ICE efficiency figures are calculated from the ICE power only. I have not found a complete engine map for the Prius, only a partial map produced by Argonne using the Japanese car. So, I only have even approximate data for ICE efficiency at the power/rev points that the computer choses to use. What this means is that if you make arbitrary changes to the ICE r.p.m. you are no longer modeling the Prius. I know this makes the spreadsheet a lot less useful. My goal was to model the car as it is and not to facilitate exploration of changes. Frankly, there isn't really enough data available to do the former properly, let alone the latter. I think I'm pushing Excel a bit far here, too.
So, if you increase the ICE r.p.m., you ought to take a guess at what the new efficiency of the ICE will be delivering about the same power at the higher r.p.m. As an example, from the Argonne data, I extrapolate efficiency at 10kW to be 34% with the spin rate at about 1,350 r.p.m. There are a few points in the raw data that fall off the general trend and there happens to be one at 10.4 kW. The recorded ICE spin rate was 2,080 r.p.m., probably due to some transient effect. At this spin rate, the efficiency is only 31.5%. From this I conclude that increasing spin rate above what the computer wants to use degrades ICE efficiency. Above 15 kW, this effect seems to disappear. I wish I had data to quantify this properly!

Graham.

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