If it's the same engine as in BigDog (I assume that it is), it's a 15 HP go-kart engine.
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What gets me in general with stuff like this is that much of the time the R&D is interesting and may bear fruit eventually, but in the here and now you can literally field other options that a robotic mule can't match. If you want to field a machine with ICE powertrain to help carry gear, you can use something like the legacy M274 or do what they are already doing and militarize 4-wheelers or gators. In my old Cav units, movement-to-contact, probing, and then falling back to provide flank and rear security while heavier combat elements engage was the order of the day so something like a 4-legged mule would be a liability (too slow, too specialized, can't match wheeled/tracked vehicles at all). If you don't want engine noise and high speed isn't necessary (matching grunt footspeed), then it's infinitely easier to either use literal mules or horses, or just...add a few more grunts to help hump that shit.