Throughout the 20th century, when the utmost fuel, weight and volume efficiencies were required to propel aircrafts, ships and vehicles, an unconventional type of compression ignition engine was used: opposed-piston engines(1).
Two major strengths of this type of engine make them better:
Opposed-piston architecture: With two pistons per cylinder, working in opposite reciprocating action, these engines do not need cylinder heads which are a major contributor to heat losses in conventional engines. Ports in the cylinder walls replace the complex poppet valves and friction-creating valve trains of conventional engines. The intake ports at one end of the cylinder and exhaust ports at the other are activated by the piston motion and enable efficient uniflow air scavenging.
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Two-stroke combustion cycle: A two-stroke engine produces twice as many power strokes per revolution as its four-stroke equivalent. This advantage leads to smaller displacement engines for similar performance, and lower in-cylinder pressure to lower emissions compared to four-stroke conventional engines.
In the past, these advantages were balanced by some well-documented shortcomings of two-stroke engines, which limited their scope of use. High hydrocarbon emissions (due to carburetion and over-scavenging) and excessive oil consumption (due to oil-fuel mixing in spark-ignition engines and port oil ejection in compression ignition, direct fuel injection engines) are difficult issues to tackle in these type of engines.
Sources: (1) J.-P. Pirault, and M. Flint, Opposed Piston Engines: Evolution, Use, and Future Applications, SAE International, 2009