Originally Posted by 
JDNSW
				
			 
			I am aware of them, and others, but through the history of diesel engines, they have never been as popular as four strokes - except possibly in ships. What I  don't know is why they have ceased to be popular as they were. Is it really because of the inability to meet emission standards? My guess is that rather than that, it is because the advantages they had over four strokes have disappeared with the ubiquitous turbocharger. These advantages can never have been all that great in practice, because the majority of manufacturers never made a successful one. 
As far as petrol engines go, yes it is emissions that have killed them - although for a lot of applications they were already going, as a more prosperous society was no longer prepared to put up with the drawbacks that seem to be inherent in them, for example in lawnmowers in this country, where, while still available, and usually cheaper, they sell a far smaller proportion than they used to. (Although I think it is correct to say that they are still in widespread use where light weight is the dominant concern, such as small outboard motors and brush cutters.)
John