Rat, I have no problem with Kubota's IDI design although it is an old one they certainly have refined it to the max. I thought that they would have to go to DI injection to meet Tier II emissions but apparently not. DI has lower hydrocarbon emissions due to the fact that the entire combustion chamber is encased within the piston head and the fuel is burned prior to spraying out far enough to contact any metal (requiring a higher pressure injection pump too.) The older IDI design with the relatively low injection pressure of about 3,000psi results in fuel spray contacting the IDI chamber prior to the flame front consuming it all, result is higher emissions. Also, with the single nozzle hole pintle injector, it drips out fuel more.
I see that Kubota has gone to DI with higher pump pressure on their larger 50+hp engines. Of course it is more costly to get it right with Direct Injection due to more expensive piston design, multihole sac-type DI injectors, and the higher pressure pump. But end result is a more complete burn, with no injector drip, and higher combustion chamber pressure for more torque and power at the same compression ratio or lower.
Modern DI design actually have lower compression ratio to decrease NOx formation (less peak temp during combustion event) also the injection timing is better controlled too for less emissions as well.
I really do like the IDI Kubota three cylinders in my L-3130, the
L39 and our ZD-21. They start right up instantly and run exceptionally smoot and are quite powerful for their displacement.
The smallest displacement for Direct Injection diesel engines is about 500cc per cylinder, so these are at thelower limit for a DI design. Reason being is the higher pressure spray must atomize completely prior to contacting the piston, otherwise extremely high temps on piston leads to failure in the long run.
