What critical feature did the 1860 Lenoir atmospheric engine lack compared to later designs?

It did not compress the fuel-air mixture before ignition.

The Lenoir engine, while being the first commercially successful internal combustion engine, operated on a principle closer to external combustion technologies like the steam engine. It drew in the air and illuminating gas mixture, ignited it via an electric spark, and then relied on the subsequent expansion of the combusted gases to push the piston. Crucially, this design omitted the step of intentionally compressing the fuel-air mixture before the ignition event occurred. This lack of pre-compression, which the later Otto cycle embraced, meant the Lenoir engine wasted a significant amount of potential energy, resulting in inherent inefficiency due to its atmospheric operation.