Compared to gasoline or diesel engines, a very high compression ratio is required to create ignition of the fuel without spark plugs. An electric starter powerful enough to turn a large diesel engine would be too large to be practical.
Compressed air is an alternative system for starting engines. Compressed air is injected to alternate cylinders, forcing it downward, activating the engine. Air-start valves activate the next cylinder and the rotation continues. Fuel is injected into the cylinders as normal and the air is cut off. Large engines will "blow over" to clear any condensation and check engine functionality.
Finally, the fuel system is engaged over the momentum and the engine runs. Air-start systems are more complex because of additional cylinder heads and extra valves on each cylinder, plus the analog and electrical control systems. This added complexity and costs limit the use of air-starters to very large and expensive reciprocating engines.