A couple of Wartsila engineers came to town and gave a presentation for CIMarE. One of their slides showed a comparison between their diesel cycle duel fuel engines and their otto cycle duel fuel engine emissions. It showed that CO2 was reduced by 25% of what a conventional engine emits for both of them; NOx was reduced by 25% in the diesel cycle, and by 85% in the otto cycle; SOx was reduced by 95% in the diesel, and eliminated in the otto, and particulates reduced by 37% in the diesel and 99% in the auto. I asked the presenter about the methane slip, he said that the diesel engine doesn't have any and that there is some in the auto. So I guess overall is that the diesel cycle wins the greenhouse gas equation, and the otto cycle engines win the NOx and particulates equations.
They also talked about skip firing, where 1, 2, and 3 cycles can be skipped (staggered over all the cylinders) at progressively lower loads so to cut down on the hydrocarbon emissions. Sort of keeping the load the same by using fewer cylinders, a la the chevy ecotec, I guess.
I wish I could share some of the slides, but they asked me not to.