HEUI is an evolutionary dead-end, and is an extreme limiting factor to PSD’s making big power. It has no advantages of HPCR. HP Common rail injection is not cheaper than HEUI, as far as I can tell, and no less complex, so that has nothing to do with leaving HEUI behind. HEUI killed itself, for several reasons. Here are as many as I can think of, and I hope that someone more knowlegable and elegant in their descriptions will get on here to help me out:
1.) Timing control (or lack thereof). It is inherently difficult to monitor or control the timing in an HEUI system. The reason is because the entire system works below the pressures necessary to dissolve entrained and unentrained air into solution with the fuel. This is called the “miscibility point” but it is probably not spelled that way. Ha ha. Anyway, the way this works is this… Air bubbles in the fuel are sucked, then pumped into the head rails. The injector sucks a load of fuel in, in preparation to fire. However, in this load of fuel is a bunch of air (simply because diesel is foamy, fizzy, yucky stuff, like soda pop). The plunger drops, and the injection cycle begins, but what happens? Have you ever turned on a hose with a bunch of air in it? How long was the delay before water came out? How long did it take until the water came out in a solid, steady stream, instead of a spurty mess of mist, air, and water? This tiny delay caused by the air means that the timing of each injection event is driven by not only when the PCM tells it to start, but also HOW much air is in the injector, WHERE the air is in the column of diesel inside (are you going to get a late start, a weak middle, or a weak back-end in your injection event?) and also, the total pressure of the injection event is compromised, because air is compressible, and fuel is not.
Add to this the fact that HEUI systems with split shot injectors actually create air by spitting pressurized fuel back into the rails at the pause between “shots” and you can see that there is a real problem here for emissions, efficiency, and power.
HPCR, on the other hand, provides low pressure and high pressure systems, with the high ressure system feeding the injectors WAY above miscibility, so air is no longer a problem.
2,) The number of injection events per powerstroke (or lack thereof). HEUI systems do not lend themselves to multiple injection events per powerstroke. The most ever attained was three, by Ford with the 6-liter, but I think we can all see how well that worked out. That was taken away mere months after it was developed. The reason is because of the difficulties inherent with a hydraulic system. Once you open that valve to the high pressure oil, the even is going to happen, no matter what you do, and so it just happens, bada BING! Split shot injectors just divert the middle of the injection event back into the fuel rail, they do not interrupt the event. Once the hydraulic oil starts to drop the plunger, there ain’t no stopping it, and even if there is, there can be no degree of precision in where it stops, etc, since there is no mechanical, only a hydraulic link, and how much it drops depends on oil quality, viscosity, quantity, HPOP RPM and output, etc. There are so many variables, that nothing can be controlled.
HPCR, on the other hand, has been developed to present as many as 7 high-precision injection events per powerstroke, at the same (or higher) pressures as HEUI can provide, with infinitely more precision. This provides infinite control over emissions, duty cycles, output, fuel economy, etc.
2.) The increased demand for oil at high output. Oil pressures drop off in HEUI systems under high demand. This results in loss of power when you need it most. Hence, expensive dual pump systems, which I know nothing about, but have to assume that they add a large parasitic loss to the engine in their operation.
Anyway, I am going to stop now before I write a novel, but you get the point. HEUI is going away because it is not a very good way to do things, costs (I assume) about the same as HPCR, and performs far worse. HPCR did not kill it. It killed itself.