"Fuel injector" covers several very different technologies. A common-rail injector from a modern bakkie, a unit injector from a heavy truck and a mechanical injector from an old generator do the same basic job in completely different ways, and they're tested, repaired and replaced differently too.
This guide covers the diesel systems first, because that's what most of our customers run, then the petrol systems.
New to injectors? Start with Diesel Fuel Injectors: What They Do and Why They Matter.
Diesel injection systems
1. Common rail (CRDi)
Found in: almost every modern diesel bakkie, SUV and passenger car, and most current trucks and industrial engines.
A high-pressure pump fills a shared rail with fuel at about 1,600 to over 2,500 bar. The ECU opens each injector electrically, several times per combustion cycle. Because injection pressure is no longer tied to engine speed, common rail gives full pressure even at low revs, along with precise timing and multiple injections. That makes it quieter, cleaner and more powerful than older systems.
There are two main types of common-rail injector:
- Solenoid injectors use an electromagnet to operate the control valve. They're the most common type: robust, and usually reconditionable.
- Piezo injectors use a crystal stack that expands instantly when voltage is applied. They're faster and more precise, which means more injection events and finer control. They're also more sensitive to fuel quality, and repair options can be more limited.
The main manufacturers of common-rail systems are Bosch, Denso, Delphi and Continental (formerly Siemens VDO).
Key point: every common-rail injector carries a correction code that must be entered into the ECU when it's fitted.
2. Unit injector systems
In a unit injector system, each injector has its own pump built in. There's no separate high-pressure pump or rail. The injector is usually driven by a camshaft lobe and fitted under the rocker cover, so on many engines you won't see any injector pipes at all.
- Electronic unit injector (EUI): mechanically driven, electronically controlled by a solenoid. Common in heavy trucks and earth-moving equipment, including the Detroit Diesel Series 60, Cummins N14 (Celect), and many Volvo, Scania and Caterpillar engines.
- Mechanical unit injector (MUI): mechanically driven and mechanically controlled, with no electronics. Found in older heavy-duty engines such as Detroit Diesel two-strokes and some older Caterpillar engines.
- Pump-nozzle (PD, "Pumpe-Düse"): VW's passenger-car version of the unit injector, used on many 1.9 and 2.0 TDI engines before common rail took over. It produces very high pressure but is noisier than common rail.
- Hydraulically actuated electronic unit injector (HEUI): uses high-pressure engine oil to drive a plunger that intensifies fuel pressure inside the injector. Found on some Caterpillar and Navistar/International engines, including the Ford 7.3 Power Stroke. Engine oil quality directly affects how these injectors perform.
3. Mechanical pump systems (pump-line-nozzle)
Found in: older bakkies and SUVs, tractors, generators, marine and industrial engines.
A mechanical injection pump, either an inline pump or a rotary (distributor) pump, meters and pressurises the fuel and sends it through individual steel pipes to each injector. The injector is a simple spring-loaded nozzle that "pops" open when the fuel pressure is high enough. Pressures are much lower than common rail, and there's little or no electronic control.
These systems are simple, tough and easy to repair in the field, which is why many are still in service. The trade-off is more noise, more smoke, lower efficiency and no way of meeting modern emissions standards.
Petrol injection systems
Port fuel injection (multi-point)
Each cylinder has its own injector, fitted in the intake port just before the intake valve. Fuel sprays onto the back of the valve at low pressure (typically 3–5 bar) and is drawn into the cylinder with the air on the intake stroke. It's simple, reliable and keeps intake valves clean.
Petrol direct injection (GDI / TSI / FSI)
The injector sprays fuel straight into the combustion chamber at high pressure (around 100–350 bar on most systems). This allows more precise control, better efficiency and more power, and it's common on modern turbo petrol engines. The known weakness is carbon build-up on the intake valves, because fuel no longer washes over them. Many newer engines combine port and direct injection to deal with this.
Throttle body injection (single-point)
One or two injectors sit in the throttle body and spray fuel into the intake air for all cylinders. It was an early step from carburettors to fuel injection, and today it's only found on older vehicles.
Quick comparison
| System | Typical pressure | Where it's found | Controlled by |
| Diesel common rail | ~1,600 to 2,500+ bar | Modern bakkies, SUVs, trucks, industrial | ECU (solenoid or piezo) |
| Electronic unit injector (EUI) | Very high, built inside the injector | Heavy trucks, earth-moving | Camshaft + ECU |
| Pump-nozzle (PD) | Very high | Older VW TDI | Camshaft + ECU |
| HEUI | High, intensified by engine oil | Some Cat and Navistar engines | Oil pressure + ECU |
| Mechanical unit injector (MUI) | High | Older heavy-duty engines | Camshaft only |
| Mechanical pump-line-nozzle | Lower | Older vehicles, tractors, generators | Mechanical pump |
| Petrol direct injection | ~100–350 bar | Modern petrol engines | ECU |
| Petrol port injection | ~3–5 bar | Most petrol engines | ECU |
How to tell which system your diesel engine has
- There's a thick steel rail along the engine, with short pipes to each injector and a wiring connector on each injector: common rail.
- You can see no injector pipes, and the injectors are hidden under the rocker cover: unit injectors.
- Steel pipes run from a pump on the side of the engine directly to each injector, and the injectors have no electrical connectors: a mechanical system.
- Still not sure? Check the engine code. It identifies the injection system exactly, and it's what we'll ask for when you need parts.
Why the type matters when you buy injectors
- Common rail: the correct part number, the correction code, and new sealing washers.
- Unit injectors: often need correct setting or calibration when fitted, and some require codes as well.
- Mechanical: opening pressure and spray pattern are set on a test bench.
- For all of them: the injector must match the exact engine and system. Similar-looking injectors from different engines are rarely interchangeable.
Injectors for every system
Turbo Diesel Components supplies injectors and injector components for common-rail, unit injector and mechanical systems across passenger, commercial and industrial applications. Search our catalogue or contact our team with your engine code and injector part number.