Bikes with single ring cranksets and multi speed cassettes tend to experience chains derailling a lot more often than bikes with front derailleurs. The reason being is that when the chain moves from a rear cog to a smaller cog, the chain loses tension. Compounded by bike movement, the chain experiences a wavelike movement resulting in the chain falling off the chainring on the crankset. Without a front derailleur to keep the chain in place, it usually ends up either on your bottom bracker (to the inside) or on your pedals (to the outside).
Short chainstay length and wide rear gear spread (mostly on folding bikes) also result in chain derailment. The distance between the crankset and rear cogs are so short that when the chain is in the biggest or smallest cog, the chain angle as it enters the front chainring is extreme. Look closely and you will see the chain wanting to skip inside or outside.
Long chain lengths especially on recumbents exaggerate the wave motion of the chain. A chain roller of some sort is required not only on the top but also on the lower chain run to prevent the chain from falling off.
We experienced this chain phenomenon on our 8 speed Dahon Custom Helios bike. We had a client who wanted an 8 speed model (11-34 rear cog) with a 50 teeth single chainring on the front crankset. On sudden shifts, the chain would fall off to the outside.
One idea was to put a chainguard. This would prevent the chain from falling to the outside but not to the inside. Also the biggest chainguard available is for a 48 teeth ring.
The best solution we came up with was to use a roller kit used by downhill mountain bikers. It's a urethane roller grooved to guide the chain with bearings on the inside for a smooth turn. To mount it on the frame, we just used a 1" aluminum strip (1/16" thick). We looped a strip around the seat tube to act as a clamp that holds another strip which held the roller. The roller is attached only on one side. Since the roller was attached, our client's chain has not fallen off.
I think the same system will work on recumbents like BikeE which uses the 3x7 rear hub and a single front chainring. An aluminum strip holding the roller can be screwed on to its main beam.
We hope that this idea helps you solve this chain phenomenon. Happy riding !