The 318 itself had 2 engine variants, from '82 thru '86 they had the B43E Onan rated at 18 HP, and from 1987 thru the end of the run, they came with the Onan P218G also rated at 18 HP. There are slight differences between these motors dimensionally and in their ignition systems. As to why the 318 is so popular over the diesel machines? When new, the diesels were "golly gee whiz" expensive, so they sold in much smaller numbers than the 318. According to serial numbers, there were 898,000 second generation 300 series tractors made, and my rough guess is that the 318 out numbered the other 300 series models by something like 50 to 1
Lily's Dad, I've owned both the 318 and 322 now. I can say that there is very little difference in their capabilities and I suspect that the 322 with the gas Yanmar was likely quite a bit more expensive so I agree with the others that the 318 was the most popular due to cost and capability. It could do everything the 332 and 322 did at less cost, more of them were sold for that reason I'm sure.