Today, continuous duty applications are dominated in southern California by the spark ignited engine for air quality reasons. Emissions from reciprocating engines using diesel oil as a fuel are difficult to sufficiently control. Reciprocating engines can provide useful heat in the form of hot water and larger engines can provide 15 psig steam. A modest amount of high pressure steam can be generated from the exhaust heat. Reciprocating engines have decent efficiency even in the smaller sizes.
Reciprocating engines are well understood technology with a large existing service infrastructure.
Reciprocating engines start rapidly and deal well with load transients. Reciprocating engines are relatively inexpensive.