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Originally Posted by pasadena_commut
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.
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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.