Long ago the injectors themselves were extremely simple. On the old indirect injection engines the "injector" was just a simple nozzle that squirted some fuel into a precombustion chamber an the chamber took care of atomization through very high swirl of the air. Some of the pumps were just a row of little pumping chambers the same configuration as the engine while others were called "distributor pumps" because they were round and had fuel lines snaking off to the cylinders in firing sequence like a distributor.
After that came direct injection, which everyone has gone to. Atomization is caused by fine holes in the injector tip and high injection pressure. The early systems used the same pump as the old IDI engines with a few changes to handle higher pressures. The pump is driven by the gear train to create the pressure and heavy wall tubing to deliver the pressurized fuel to the nozzles. Those nozzles are what people are thinking of when they talk about "pop testing" the injectors. The pump could force fuel, but it took resistance at the nozzle to create the proper pressure. The pop test would verify that pressure and you could make adjustments by shimming the spring. All of the fuel lines had to be the same length so that pump metering and timing adjustments would be consistent across all cylinders, so on these engines you'll often see a mess of fuel lines coiled around for no apparent reason. The internals of the pump could adjust timing based on engine speed using a mechanical governor.
After than came unit injectors. A mechanical rack would move a scroll wheel inside the injector to move a spill port to adjust metering and the top plunger would be pressed by the cam to create pressure. The tip was essentially the same as the old "pop test" injectors. These were fun because one injector rack could stick and hold the engine at full fuel, causing a runaway. Detroits were infamous for this.
At that same time were numerous oddball electronic control systems popping up that would have mechanical pumps with various parts moved by motors, etc. Cat's PEEC engines were the worst to deal with.
Next were electronically controlled unit injectors. Essentially the same thing, but with a solenoid to control the spill port. Mack did their own thing with a unit pump which was just the pump separated from the actual injector tip. Same thing, but different arrangement of parts. EUIs can control timing and metering pretty accurately with just one solenoid, so it's a cheap system with good performance, which is why it lasted a long time, and common rail has only recently replaced it.
Cat came up with HEUI which uses oil pressure to power a small pressure intensifier built into the top of the injector. It eliminated the cam from injector operation, but didn't require the super high pressures of common rail systems to be pumped around the engines. Ford Powerstrokes used the same system. But the downside if that it's pretty sensitive to oil and coolant quality. Because it was so different, it frequently got misdiagnosed and lots of injectors and pumps got replaced for no reason over the years.
The latest and greatest is common rail. Amusingly, I have an Audell's book from the '30s that mentions common rail, so it's not actually all that new. It uses a simple high pressure pump that feeds fuel into a pipe that feeds all of the injectors. That pipe is the rail. The injectors are really nothing more that very high pressure on/off solenoid valves with atomizer tips screwed on. It wasn't until pretty recently that manufacturers could make the internals last at very high pressures long enough to be suitable for industrial use. Hino jumped the gun a bit and their warranty must have spent billions on injector claims.
The DD15 has s cross between common rail and pressure boosting. Using common rail fuel to power a HEUI-ish injector that doubles(?) the pressure.
Cummins had a weird CAPS system for a while in the days of electronic unit injectors that was a cross between an old distributor pump and common rail. There was a large accumulator mounted on top of the fuel pump, an old distributor arrangement at the back of the pump, a simple high pressure pump body at the core, and a very expensive metering solenoid hidden above the distributor and below the accumulator. The injectors were reduced back to just old pop-off nozzles. It worked great, but the solenoid needed a diode to dissipate power and allow it to shut quickly and that diode caused all kinds of odd problems. Another case of parts being replaced frequently for no good reason. And it ended up being more expensive than the common rail system that replaced it. But the core of the common rail pump they use today it the same as the core of this pump, so it more or less lives on.
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