E. PULIDO
2.1. RECUBRIMIENTOS ELECTROLÍTICOS INTRODUCCIÓN
An engine’s emission and fuel-consumption characteristics are very important considera-tions. For that reason, the following demands are placed on the fuel-injection system:
Fuel injection must be precisely timed.
Even small discrepancies have a substan-tial effect on fuel consumption, emission levels and combustion noise (Figures 1 to 4).
It should be possible to vary the injec-tion pressure as independently as possi-ble to suit the demands of all engine oper-ating conditions (e.g. load, speed).
The injection must be reliably terminated.
Uncontrolled “post-injection”
leads to higher emission levels.
The term “injection characteristics” refers to the pattern of the fuel quantity injected into the combustion chamber as a function of time.
Injection duration
One of the main parameters of the injection pattern is the injection duration. This refers
to the period of time that the nozzle is open and allows fuel to flow into the combustion chamber. It is specified in degrees of crank-shaft or camcrank-shaft rotation, or in millisec-onds. Different diesel combustion processes demand different injection durations as illustrated by the following examples (approximate figures at rated power):
direct-injection car engines: 32...38° of crankshaft rotation
indirect-injection car engines: 35...40°
of crankshaft rotation, and
direct-injection commercial-vehicle engines: 25...36 ° of crankshaft rotation.
An injection duration of 30° of crankshaft rotation corresponds to 15° of camshaft rotation. In terms of time at an injection pump speed1)of 2,000 rpm, that is equal to an injection duration of 1.25 ms.
In order to minimize fuel consumption and soot emission, the injection duration must be defined on the basis of the engine operating conditions and the start of injec-tion (Figures 1 and 4).
Basic principles of diesel fuel injection Injection characteristics 55
1) Equal to half the engine speed on four-stroke engines
Start of injection
0.300.20
Specific emission of unburned hydrocarbons (HC) in g/kWh versus start of injection and injection duration
3
Start of injection
0.001
Specific soot emission in g/kWh versus start of injection and injection duration
4
æ
SMK1870EInjection pattern
Depending on the type of use for which the engine is intended, the following injection functions are required (Figure 5):
Pre-injection (1) in order to reduce com-bustion noise and NOXemissions, espe-cially on DI engines
Positive pressure gradient during the main injection phase (3) in order to reduce NOXemissions on engines without exhaust-gas recirculation
Two-stage pressure gradient (4) during the main injection phase in order to reduce NOXand soot emissions on engines with-out exhaust-gas recirculation
Constant high pressure during the main injection phase (3, 7) in order to reduce soot emissions on engines with exhaust-gas recirculation
Post-injection immediately following the main injection phase (8) in order to reduce soot emissions, or
Retarded post-injection (9) of fuel as a reducing agent for an NOX accumulator-type catalytic converter and/or in order to raise the exhaust-gas temperature for regeneration of a particulate filter
Conventional injection pattern
With conventional fuel-injection systems, the pressure is generated continuously throughout the injection cycle by an injec-tion pump. Thus, the speed of the pump has a direct effect on the fuel delivery rate and consequently on injection pressure.
In the case of port-controlled distributor and in-line injection pumps, the injection pattern consists exclusively of a main injec-tion phase, i.e. without pre- or post-injec-tion (Figure 5, Items 5 and 6).
With solenoid-valve controlled distributor injection pumps, pre-injection is also possi-ble (1). On unit injector systems (UIS) for cars, pre-injection is currently controlled by hydromechanical means.
Pressure generation and delivery of the injected fuel quantity are interdependent by virtue of the link between the cam and the injection pump in conventional systems.
This has the following consequences for the injection characteristics:
Injection pressure increases with engine speed and injected fuel quantity (Figure 6)
56 Basic principles of diesel fuel injection Injection characteristics
Fig. 5
Adjustments aimed at low NOXlevels require starts of injection close to TDC at maximum load (en-gines without exhaust-gas recirculation). The fuel delivery point is signifi-cantly in advance of the start of injection and is dependent on the injection system
1 Pre-injection (PI) phase
2Main injection (MI) phase
3 Steep pressure gradient (common-rail system) 4 Two-stage pressure
gradient (UPS with CCRS two-stage solenoid valve) (dual-spring nozzle-holder assemblies can produce a bath-tub needle lift curve [but not pressure gradient]. This re-duces combustion noise but not always soot emission levels.) 5 Gradual pressure
gradient (conven-tional fuel injection) 6 Gradual pressure
drop (in-line and distributor injection pumps)
7 Steep pressure drop (UIS, UPS, slightly less steep with common rail) 8 Advanced
post-injection (PO) 9 Retarded
post-injection ps Peak pressure po Injector opening
pressure b Duration of
combustion for main injection phase v Duration of
combustion for pre-injection phase ZV Ignition lag for main
injection phase without pre-injection
Crankshaft angle of rotation
TDC Crankshaft °
Injection pressure pe
1
b (up to 40…60° cranksh.)
1…5° v
ZV* *ZV: w/o PI: 4…10° cranksh.
with PI: 1… 2° cranksh.
8
2 (up to 36°) 90…180° cranksh.
ps
Injection pressure rises at the start of in-jection but drops again before the end of the injection period (as from the end of the fuel-delivery period) down to the injector closing pressure.
The consequences of this are the following:
Small injected fuel quantities are injected at low pressures, and
The injection pattern is approximately triangular, as is required for good com-bustion in an engine without exhaust-gas recirculation (shallow pressure gradient and therefore quiet combustion).
The determining factor for the stresses to which the components of an injection pump and its drive system are subjected is peak pressure. Peak pressure is also a measure of the quality of fuel atomization in the combustion chamber.
On indirect-injection engines (precombustion or swirl-chamber engines), throttling-pintle nozzles are used which produce a single jet of fuel and determine the shape of the injection pattern. This type of nozzle controls the outlet cross-section as a function of the needle lift.
This produces a gradual increase in pressure and consequently, “quiet combustion”.
Pre-injection
The pressure curve of an engine without pre-injection (Figure 7a) shows only a shallow gradient leading up to TDC in keeping with the compression. The gradient then rises steeply from the start of combustion. That rapid rise in pressure is the cause of the nois-ier combustion encountered on diesel engines without pre-injection.
Pre-injection enables a less abrupt rise in combustion pressure to be achieved. The ignition lag of the main injection quantity is very short. The pattern of combustion is af-fected in such a way that combustion noise, fuel consumption and – depending on the type of combustion – NOXand HC emis-sions are reduced.
Pre-injection involves the injection of a small quantity of fuel (1...4 mm3) in ad-vance of the main injection phase in order to “precondition” the combustion chamber.
This has the following effects:
The ignition lag of the main-injection phase is shortened, and
The combustion pressure gradient is less steep (Figure 7b).
Depending on the timing of the main injec-tion phase and the gap between the pre-injection and main-pre-injection phases, the specific fuel consumption will vary.
Basic principles of diesel fuel injection Injection characteristics 57
Fig. 7
a Without pre-injection b With pre-injection
hPI Needle lift during pre-injection hMI Needle lift during
main injection Fig. 6
1 High engine speeds 2Medium engine
speeds
3 Low engine speeds
Crankshaft angle of rotation TDC
hMI
Valve needle stroke hCombustion pressure pz
hPI b
a,b a
b
Effect of pre-injection on combustion-pressure pattern
7
æ
UMK1587-6EInjection volume me
1 2
3 Injection pressure pe
Injection-pressure curve for conventional fuel injection
6
æ
UMK1722-1EPost-injection
Retarded post-injection
Post-injection can be employed as a means of delivering a measured amount of reducing agent for a particular type of NOXcatalytic converter. The post-injection phase follows the main-injection phase during the ignition or exhaust stroke at a point up to 200° crank-shaft after TDC. It introduces a precisely mea-sured amount of fuel into the exhaust gas.
In contrast with the pre-injection and main-injection phases, the fuel injected is not burned but is merely vaporized by the heat of the exhaust gas. The resulting mix-ture of fuel and exhaust gas is expelled through the exhaust ports into the exhaust-gas system during the exhaust stroke. The fuel in the exhaust gas acts as a reducing agent for nitrogen oxides in suitable NOX
catalytic converters. As a result, the NOX
emission levels are moderately reduced.
Another means of reducing NOXemissions is the NOXaccumulator-type catalytic convert-er (see chaptconvert-er “Emission control systems”).
Retarded post-injection can also be used to raise the exhaust temperature in an oxida-tion-type catalytic converter in order to assist regeneration on the part of a particulate filter.
Retarded post-injection can lead to thinning of the engine oil by the diesel fuel. It is therefore essential that the injection system is designed in consultation with the engine manufacturer.
Advanced post-injection
The common-rail fuel-injection system can perform post-injection immediately follow-ing the main-injection phase independently of any post-injection for an NOXcatalytic converter or particulate filter. In this case, the fuel is injected while combustion is still in progress. In that way, soot particles are burned and soot emissions can be re-duced by 20...70 %.
Camshaft-driven injection systems that are capable of post-injection are also under development.
Post-injection and dead volumes Unintended post-injection has a particularly undesirable effect. Post-injection occurs when the nozzle momentarily re-opens after closing and allows “poorly conditioned” fuel to escape into the cylinder at a late stage in the combus-tion process. This fuel is not completely burned, or may not be burned at all, with the result that it is released into the exhaust gas as unburned hydrocarbons. Rapidly closing noz-zles with a sufficiently high closing pressure and a low static pressure in the supply line can prevent this undesirable effect.
Fuel retained in the nozzle on the cylinder side of the needle-seal seats has a similar effect to post-injection. That dead volume runs into the cylinder after the combustion process has finished and also partially es-capes into the exhaust gas. This fuel compo-nent similarly increases the level of un-burned hydrocarbons in the exhaust gas (Figure 8). Sac-less nozzles in which the in-jection orifices are drilled into the needle-seal seats have the smallest dead volume.
58 Basic principles of diesel fuel injection Injection characteristics
Fig. 8
a Injector without blind hole b Injector with
micro-blind hole
1 Engine with 1 l/cylinder 2Engine with
2 l/cylinder
1
0
3
HC emission
2
2 1
0
Injection and blind hole volume of injector g/kWh
mm3
1 2
b a
Effect of injector design on hydrocarbon emissions
8
æ
UMK0800-1ETiming characteristics of fuel-injection systems
Taking as its example a radial-piston distrib-utor injection pump (VP 44), Figure 9 illus-trates how the cam on the cam ring initiates delivery of fuel by the pump and the fuel ul-timately exits from the nozzle. It shows that the pressure and injection patterns vary greatly between the pump and the nozzle and are determined by the characteristics of the components that control injection (cam, pump, high-pressure valve, fuel line and nozzle). For that reason, the fuel-injection system must be precisely matched to the engine.
In all fuel-injection systems in which the pressure is generated by a pump piston (in-line injection pumps, unit injectors and unit pumps) the characteristics are similar.
The common-rail system on the other hand behaves entirely differently.
Detrimental volume in conventional injection systems
The term “detrimental volume” refers to the volume of fuel in the high-pressure side of the fuel-injection system for an individual nozzle. This is made up of the high-pressure side of the fuel-injection pump, the high-pressure fuel lines and the nozzle.
Every time fuel is injected, the detrimental volume is pressurized and depressurized. As a result, compression losses occur and a fuel injection lag is produced. The fuel volume inside the pipes is compressed by the dy-namic processes generated by the pressure wave.
The greater the detrimental volume, the poorer the hydraulic efficiency of the fuel-injection system. A major consideration when developing a fuel-injection system is therefore to keep the detrimental volume as small as possible. The unit injector system has the smallest detrimental volume.
In order to guarantee consistency of con-trol for the benefit of the engine, the detri-mental volume must be equal for all cylin-ders.
Basic principles of diesel fuel injection Injection characteristics 59
Fig. 9
Example of radial-piston distributor injection pump (VP 44) at full load without pre-injection
tL Fuel transit time in pipe Cam pitch Rate of liftLine pressure pump sideInjection rate
hN
Needle lift
hD
Line pressure nozzle sideSolenoid-valve stroke
0
Chain of interaction from cam pitch to injection pattern plotted against camshaft angle
9
æ
UMK0798-1EInjection characteristics of common-rail system
A high-pressure pump generates the fuel-rail pressure independently of the injection cycle. The fuel-rail pressure remains virtu-ally constant for the entire injection cycle (Figure 10). Because of the almost uniform delivery pattern, the high-pressure pump can be significantly smaller and designed for a lower peak drive torque.
Short pipes join the fuel rail to the injec-tors. Since the control unit controls the in-jectors, start of injection and end of injec-tion are infinitely variable in engine applica-tions. Multiple pre- and post-injection phases are possible.
For a given system pressure, the injected fuel quantity is proportional to the length of time that the injector valve is open and en-tirely independent of the engine or pump speed (time-based injection system).
Thus, start of injection, duration and pressure can be individually regulated to suit all engine operating points and optimized to the engine's operating requirements. They are controlled by the crankshaft-position/
time-based system of the electronic diesel control (EDC) system.