Samuel Morey, or: 1824, with a two-stroke engine.
We spent 4 pages hashing out stuff with 1824 as a POD. Samuel Morey patented a 2-cylinder internal combustion engine in that year, but when his wife, daughter, and mother all three died in the same week, he put all his motors on a boat and sunk them in the middle of a river.
We basically just wondered what would've happened if his family hadn't died, and he'd kept going with his engine development, as well as spreading it to other areas.
On that one, we finally decided that Chickamauga and Vicksburg (there were extensive Rebel fortifications and the Union knew they were there for a long time, hence a long time to build vehicles) were probably the most likely battles to afford it.
We'd worked out unit sizes and commanders, and some dates of usages, though we were still kind of fuzzy on other things.
By the end, we were working out mathematical stuff to see how the things would work. You might want to take a look at it.