We all know you can't run an SI engine very long without some sort of system for recharging the battery, but not all of us are sure to know all the options available. Therefore, I thought I'd open this thread to gather them all together in one place.
1. MetroMPG linked to the
TIGER option. Very cool and sure to see actual production.
2. Then there is the tried and true underdrive pulley. For those not familiar with this option, here's how it works. In round numbers, the pulley on your car's alternator is sized relative to the crankshaft pulley to charge slightly at idle (or perhaps a little above idle), and to produce full rated power by about 1500 engine RPMs. The particulars differ from engine to engine, but that's the general idea.
An underdrive pulley is somewhat larger than the OEM alternator pulley, so slows the rotational speed of the alternator. By slowing the RPMs of the alternator the point at which it begins charging is delayed by several hundred RPMs to as much as a 1000 RPMs or more. That delay removes the alternator's electrical load on the engine at low engine speeds, and hence potentially permits better hypermiling in that same low engine speed range.
As an aside, underdrive pulleys are commonly used on water pumps, as well, and for the same reasons.
3. A somewhat more radical approach, but still very doable, is to move the alternator to a position that turns only when the car is rolling. Here is a photo of a (race) car with the alternator mounted above the transaxle and being driven by means of a conventional 'two pulleys and a belt' setup using a CV joint-mounted drive pulley. Again, sizing the pulleys to start charging at your desired speed is key. Oh, and sorry the photo doesn't show much detail, but I'm confident you get the idea.
4. Field circuit switch. Most (but not all) alternators have a so-called 'field circuit' that uses a little juice from the battery to 'excite' the internal circuits of the alternator to make electricity. Without it the alternator will spin, but does not generate any charge. The wire feeding the juice attaches to a terminal on the alternator housing. Identify this wire and then wire in a dash-mounted toggle switch to cut the field whenever you wish.
There are a few options. Surely you guys can come up with some more!