Unit Injector & Unit Pump SystemsMagdi K. Khair, Hannu Jääskeläinen

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Abstract: In unit injector and unit pump injection systems, a separate pump serves each engine cylinder. At one time, the unit injector system had the capability to develop the highest injection pressure among all types of injection systems. While advanced, electronically controlled unit injector systems with the capability for multiple injections and rate shaping have been developed, unit injectors are gradually replaced by common rail technology.

Introduction

Unit Injector System

Unit Pump System

Introduction

In unit injector (UI) and unit pump (UP) systems, each engine

cylinder is served by a separate injection pumping element or

injection pump in close proximity to the cylinder. Unit pump (UP)

systems enable short high pressure fuel lines by locating the

pump close to the injector. Combining the pumping element and

the injector into one assembly as in unit injector (UI) systems,

allows these lines to be eliminated altogether. The elimination—or

length reduction—of high pressure fuel lines in the UI/UP

injection systems results in two benefits:

Reduction of line dynamics problems: line dynamics

difficulties in unit injectors/unit pump systems are less

troublesome than in their pump-line-nozzle(P-L-N)

counterparts. The possibility of wave superposition—which

troubled the P-L-N systems by causing after-injections and

contributing to injection delays—is greatly reduced.

However, it should be mentioned that line dynamics

problems generated within the narrow passages of unit

injectors may still modulate the rate of injection [Challen

1999].

Higher injection pressure: the UI system has traditionally

had the highest injection pressure capability among all types

of injection system. In the early 2000s, UI systems had

pressure capabilities of 200 MPa, compared to 160 MPa in

common rail systems. Since then, UI/UP system peak

injection pressures have risen to as high 250 MPa for some

2007 model year applications.

With regards to fuel pressure, it should be noted that common

rail fuel injection system pressures have risen as well and in some

systems have reached or exceeded the pressures available from

UI/UP systems. While there is no technical reason keeping UI/UP

pressures from rising even further, engine manufacturers are

increasingly using common rail systems in applications

traditionally dominated by UI/UP systems. For this reason, UI/UP

systems will likely see little evolution beyond their current peak

pressures of about 250 MPa.

Both the UI and UP systems are driven from the engine camshaft.

In one common mechanical system design, fuel control was

typically achieved by rotation of the pumping element (plunger) in

the same way as is done in P-L-N systems. With the introduction

of electronics to diesel engines, electronic unit injector(EUI)

and electronic unit pump (EUP) systems were developed. These

employ an electromagnetically operated spill valve for fuel

control.

Due to the presence of fuel lines, the unit pump system can be

classified as a variant of the P-L-N injection system. However, the

design of unit pump and unit injector systems is often similar,

making it convenient to discuss these systems together. In fact,

some manufacturers offer their injection systems in both UI and

UP versions (compare Figure 4 and Figure 11).

The commercial application of unit injectors started in the 1930s

on Winton (a GM subsidiary) and GM diesel engines. Winton

continued to supply engines to the Electro-Motive Corporation

(EMC), while GM transferred diesel engine production to its

Detroit Diesel Division. The Detroit Diesel Corporation’s two-

stroke engine line is one of the better known applications of unit

injector technology. From the 1930s to the mid-1980s, Detroit

Diesel used a mechanical unit injector design. In 1985, Detroit

Diesel’s Series 92 two-stroke engine became the first heavy-duty

diesel engine to adopt electronically controlled unit injection[Bara

1990]. Since this introduction of electronic control, unit injectors

continued to evolve to higher levels of sophistication. The

evolution for light-duty and heavy-duty applications followed

different paths.

Possibly the most advanced design of unit injector for light-duty

applications is the PPD injector produced briefly by Volkswagen

Mechatronic (a joint-venture between Volkswagen and Siemens

VDO) starting in 2004 for model year 2006 Euro 4 applications.

This injector used a piezoelectric actuator and was capable of up

to 2 pilot injections and 2 secondary injections in addition to the

main injection event. However, it came at a time when common

rail systems had already taken hold in light-duty applications and

were quickly gaining ground. The PPD injector could not compete

with common rail systems and was phased-out soon after its

launch. Starting in 2007, it was replaced with common rail for

Euro 5 applications. Common rail systems have since become the

preferred choice for light-duty applications and unit injectors are

quickly disappearing from new engine designs.

For heavy-duty applications, electronic unit injectors continued to

evolve. The evolution of some of these designs is described in the

paper on injection systems in HD engines. The pinnacle of heavy-

duty unit injector design is represented by the two-valve designs

of Delphi’s E3 and Caterpillar’s MEUI-C injectors for engines

meeting US EPA 2007 on-road emission standards. While these

advanced unit injector designs have capabilities such as rate

shaping and multiple injections, common rail systems for heavy-

duty applications have evolved to the point were they are

replacing unit injectors in many new engine designs for markets

with the most demanding emission standards. To facilitate this

switch, fuel injection equipment manufacturers have

designed common rail systemsthat can easily be fitted to engine

platforms that were originally designed for unit injector or unit

pump systems and thus avoiding the need for a completely new

engine design.