First diesel on our farm was a 1939 TD-6 International. Injection pressure 1500 psi. Move on to my last IDI engine near that horsepower class, a Kubota
L5740. Injection pressure increased all the way to 2000 psi. Remember its the pressure that breaks the fuel into droplets with higher injection pressure resulting in smaller droplets, more surface area exposed to air, and more complete combustion. I remember my dad coming in from the fields after working with that TD-6 and he would be covered with soot. Under extreme load, it made the neatest little soot circles that settled on your clothes. In his 60's dad was diagnosed with a problem most commonly related to heavy smokers, but he never smoked. I remember in his 30's and 40's and that incomplete combustion diesel smoke. My
L5740, now Tier 3 emissions, was similarly a nasty smoker although it didn't make those little smoke rings. I don't have a shop manual for my
L6060 so I can't tell you the injection pressure, but my
M7-171 manual says variable high pressure depending on requirements but relief valve in the fuel rail limiting pressure to 36,000 psi. I will never know how much the injection pressure reduces soot because the DPF takes care of any remaining. Incidentally our first direct injection diesel tractor was a 1963 International 806 with 2000 psi injection pressure, same as the IDI Kubota
L5740. In between there is Caterpillar's HEUI injectors that give about 5,400 psi at low speed low power and 23,500 psi at high speed high power - and anywhere in between based on demand.
All of these changes have been made to get better fuel economy and lower emissions. One of my friends at Cat worked for over a year sitting at a computer terminal tweaking unit injectors to get the best fuel flow and improved fuel economy when Cat got a contract to supply engines for GM trucks. He said they would get atta-boys for every fraction of a percent of fuel economy improvement as it was so important to GMC.
Applications - fluid flow at high pressure with contamination = wear. That has always been a diesel problem even with 1500 psi TD-6 engines, but materials technology was ancient back then. Limitations of improving diesel performance has always been material technology. Even with improving materials, it is easier to reduce injection system wear by filtering. I have a couple injectors sitting on my desk as a reminder of what a customer can do saving money in the short run costing money in the long run. He bought a test machine we had a customer run for 2,500 hours with no problem. Thirty hours later I got a call that the engine wouldn't start. Sent a dealer to check. The customer had replaced our filter with a will fit as one of these "buying it used, will change all the fluids and filters before putting it to work". Only thing he replaced a highly developed filter with a common brand "will fit the same mount" filter. In this case I was not going to let the customer get stuck for his mistake and paid the dealer to replace the injectors - and install the proper fuel filter. But that is how quickly systems can fail using inferior filtration. Incidentally, before selling the machine, I had the dealer perform an injector leakage test to insure the injectors still worked right after 2500 hours (this was implementing common rail in a Tier 3 engine). They were fine. It took only 30 hours using fuel in the Atlanta, Georgia, area with the incorrect filter to take out injectors.
