You summed up everything I read today. lol Thanks for the clarification Wiki did say that everything from DI and CR is old as dirt being invented around the 20's which is Insane when you think about. Makes you wonder what where discovering now. And why the hell everyone looks to the Prius for the answer there green needs when all you need is a DI engine.
Maybe I need to re-read the article but I don't remember if wiki said that its the mechanical DI system that cant be made for small engines but that makes sense. I think it said in general it cant be done. Wiki said that the mechanical lift pumps are reliable in the sense that it keeps fuel flow in check but its not for making power mostly because it cant keep up irregular speed changes, or high engine speeds. Also Do DI diesels need glow plugs? Because they made it seem that it didn't.
typically you cannot achieve the best thermal efficiency with a direct injection system for a very small motor. that is not to say it is not being done now a days, but it would have to be electrical. most of your smaller engines are infact IDI, unless turbo charged. the reason is because of the design, IDI engines require good quick airflow, rather wise DI systems use slow moving air and fast burning fuel.(back to thermal efficiency) a mechanical system cannot really meter the proper fuel amount on such a small engine, if you think about it most diesel run lean so lean that they are running 50-60:1, now think if your whole Kubota tractor engine is say 23cid that means your burning .46units of fuel, the metering would be to difficult, not to say it couldn't be done, but such small set up could pose problems down the road.
Direct injection systems depending on the set up you can use either glow plugs or grid heaters. grid heaters heat the air in the intake manifold so that it is already warm by the time it gets to your cylinder.
rather wise a glow plug system tries to heat every cylinder, the major problem is you can get bad engine knock, and even then trying to heat 8 cylinders the heat is dissipated to quickly.
there are advantages to both, but i personally like grid heaters as they tend to stay on During the cranking process ensuring a constant flow of warm air into the cylinder area, rather wise diesel and hot glowing metal do not mix
This is crucial as a cold diesel engine literally saps any warmth around it, (like try wrenching on a cold diesel engine and your hand (regardless of gloves), will freeze.) A cold diesel engine usually will not hit the compression temperature for auto ignition of diesel fuel injected. the use of grid heaters and block heaters was necessary a long time ago because the injection systems had static timing, (ratherwise starting with the VP-44 and go newer), they were dynamic, allowing for colder starting operations.
For example there are stories of a 24v(Vp-44) cummins starting down to -20 below zero and some, rather wise a old 12v'er will struggle to even run.
now newer common rails can advance the timing so much that they can achieve the most reliable start because they can inject when the air is the most compressed, but once again you have the basic problem of cold dense air is colder then warmer air.
regardless diesels much use their heating systems to generate heat at idle in the cold, as diesel are cold idlers, meaning that they drop below combustion temp at idle, it can be 100F out and my 99 dodge will idle at 200-250F, now when it is -10F it will idle in the 150-180F, unless i kick in high idle and even then it is only running 250ish. At these temperatures the block is cooling down and absorbing the heat from the motor, without a load the engine will cool down progressively, unlike a gas engine which will overheat if given the chance.
this is why having a working heating system is important for cold weather operation, typically for all 24v trucks it is only needed down below 10-20F, both of my trucks will start down to 10-15F without heaters that is not to say you should always do it.
For IDI engines they must use glow plugs and then the glow plug is tuned on and is located in the prechamber, (where the injector is), rather wise on a DI system the glow plug is located inside the cylinder.