Afton Chemical presents Study of Effects of Deposit Formation on GDI Injector and Engine Performance

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Study of Effects of Deposit Formation on GDI Injector and Engine Performance Jon Pilbeam Afton Chemical, Bracknell, UK Alex Thomson Afton Chemical, Bracknell, UK Amrit B. Sahu University of Birmingham, Birmingham, UK Haoye Liu University of Birmingham, Birmingham, UK Hongming Xu University of Birmingham, Birmingham, UK

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2020-01-2099 Published 09/15/2020

ABSTRACT

INTRODUCTION

Gasoline Direct Injection (GDI) vehicles now make up the majority of European new car sales and a significant share of the existing car parc.

Ten GDI fuel injectors, with differing levels of coking were taken from engine testing and consumer vehicles and compared using a range of imaging and engine tests.

Despite delivering measurable engine efficiency benefits, GDI fuel systems are not without issues. Fuel injectors are susceptible to the formation of deposits in and around the injector nozzles holes. It is widely reported that these deposits can affect engine performance and that different fuels can alleviate the buildup of those deposits.

At the time of writing, a new GDI engine test is being developed by the Co-ordinating European Council (CEC) to be used by the fuel and fuel additive industry. One such test was used to precondition six of the injectors in this study. It was found that injectors run in this test without additives experienced significant deposit formation. The use of fuel with Deposit Control Additives (DCA) was found to reverse that effect.

This project aims to understand the underlying mechanisms of how deposit formation ultimately leads to a reduction in vehicle performance.

This study builds significantly on an earlier paper (SAE 2019-28-2392), adding data from more injectors and single cylinder engine testing. This additional content brings new insights and helps to put that earlier work into greater context.

The CEC test is a well-controlled steady state test, which allowed a straightforward comparison of the different injectors. Despite only being a steady state test, the test point of 2000 rpm / 56Nm was considered realistic and one that occurs regularly in real driving and vehicle emissions testing.

GDI engines have been mass produced since the early 2000’s. The position of the injector inside the combustion chamber has enabled car manufacturers to realize different combustion modes and engine architectures in a drive to reduce fuel consumption and emissions.

In comparison with earlier Port Fuel Injector (PFI) systems, the physical location of the GDI injector brings it into a more severe physical environment. Direct exposure to combustion brings higher absolute temperatures, larger thermal gradients, higher pressures (fuel and combustion chamber) and combustion byproducts. Even with well refined and stable fuel components, this environment readily leads to the formation of deposits in and around the nozzle holes on the injector.

Although there is a self-cleaning effect as fuel flows through the injector, any residual fuel remaining in and around the nozzle tip will contribute to deposit formation [1].

A successful and cost-effective solution is the use of low deposit forming fuels, either by using fuel components which do not degrade readily or using Deposit Control Additive (DCA).

OBJECTIVES OF THE STUDY The first objective of the study was to confirm understanding of how DCA restore performance of engines with pre-existing deposits or prevent performance degradation in ‘clean’ engines. From existing literature [1,3,4,5,6,8,9,11] we assumed that deposit formation adversely affected fuel injector characteristics, which then affects combustion and vehicle performance.

This project sought to understand as many elements in that chain as possible and so confirm or correct our assumptions.

A second objective of this project was to understand how similar the deposits formed in this new test are to deposits from vehicles in the market. The CEC VW DISI test method is a highly accelerated engine test intended to form deposits quickly. Whilst accelerated tests are desirable for rapid and efficient product development, there is a concern that the fundamental mechanisms of deposit formation can diverge from those in consumer vehicles. Creating such deposits risks developing irrelevant DCA with limited consumer and environmental benefits.

Individual techniques used in the comparison work are not novel, having been established and used before by several authors [1,4,5,6]. This study combined a number of those techniques together and showed them to be useful in identifying and understanding differences between dirty and coked injectors and CEC test and ‘real’ injectors.

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METHODOLOGY EXPERIMENT OVERVIEW There are six separate methodologies used in this study which are summarized below. The details of each methodology are described in six separate methodology sections: 1. A full-length VW DISI (Volkswagen Direct Injection Spark Ignition) test engine test to provide injectors with varying levels of deposits.

2. Analysis of the ‘face’ and ‘nozzle’ deposits on each injector, where possible:

a. Deposit Topology by Scanning Electron Microscope (SEM) b. Deposit Composition by Energydispersive X-ray Spectroscopy (EDX)

3. Mass injection rate test of all the injectors.

4. Imaging and characterization of injector spray:

a. Entire spray (Macroscopic)

b. Near to the tip (Microscopic)

5. Combustion imaging in a single cylinder optical research engine.

6. Accurate efficiency and emissions measurement in a single cylinder thermal research engine.

Source of Injectors with Varying Deposit Levels At the time of writing a new CEC test method for fuels is being developed and is named VW DISI. The VW DISI test method (Table 1) is based on a VW in-house test using an inline 4-cylinder VW EA111 BLG engine.

This engine was introduced to the market in 2004 and manufactured until 2012. It was novel in being an early example of mass market downsized engine technology and for having both a mechanical and exhaust driven superchargers. It uses Bosch Fuel Injection Equipment with side mounted sixhole injectors and a maximum fuel pressure of 172 bar.

From SEM imaging the injector nozzle holes were estimated to be 150 microns in diameter with a depth of 130 microns.

There was a concave shaped ‘stepped area’ machined into the surface surrounding the nozzle hole exit with a diameter of 250 microns.

Although there are a wide variety of GDI injector designs and mounting locations in the existing vehicle car parc this combination of side mounted, sixhole injectors is commonly seen. The specification for the engine is shown in Table 2. The test is steady state i.e. maintains the same engine speed and torque throughout. It is primarily designed as a Dirty Up / Clean Up (DU/CU). Dirty Up (DU) builds a significant amount of deposit in and around the injector spray holes using an un-additised reference fuel.

Engine Code

BLG

Displacement

4-cyl / 1390cc

Bore

76.5mm

Stroke

75.6mm

Compression Ratio

10:1

Max Power

125KW @ 6000 rpm

Max Torque

220Nm @ 1750-4500 rpm

Engine Management

Bosch Motronic ME 9.5.10

Exhaust Gas Treatment

Catalytic convertor + lambda control

Emission Standard

EU4

Date Introduced DU duration

48 hrs

CU duration

24 hrs

New Injector Conditioning

5 hrs

Primary Test Metric Intended Test DU target Test Cycle Type

Change in Injector Pulse Width (ti_l) +20% ti_l Steady State

Engine Speed Setpoint

2000 rpm

Engine Torque Setpoint

56Nm

May-04

High Volume Application

VW Golf Mk5 GT Bosch six-hole, side mount injectors.

Part # 03C 906 036E

Fuel Injectors

Table 2. VW

BLG Engine Specification (source CEC) LAB DU1

2X Injectors Dirtied Up in CEC engine test

LAB DU2

25°C ± 5°C

2X Injectors Cleaned Up in CEC engine test

LAB CU2

Air temperature Intercooler outlet

35°C ± 5°C

2X KC Injectors Kept Clean in CEC engine test

Intake air temperature

25°C ± 5°C

Coolant temperature engine outlet Fuel temperature at HP pump inlet

90°C ± 2°C

Table 1. CEC

VW DISI Test Overview (source CEC)

LAB CU1

LAB KC1 LAB KC2 CLEAN NEW

New & 72hr aged, cleaned Injectors

CLEAN 72hr

2X Injectors from two different BLG vehicles

Table 3. Naming

REAL 65k REAL 85k

The fuel is then changed after 48hrs to one containing DCA and intended to remove the built-up deposit, known as Clean Up (CU). A second version of the test, Keep Clean (KC) should be straightforward to create in future. KC tests start with a clean injector and use fuel with DCA to prevent deposit build up.

DCA were typical additive chemistries that are commercially available. The purpose of the paper was not a full assessment of different DCA chemistries but to show that the use of additives in a fuel influences deposit formation.

The test uses injector pulse width as a proxy measurement for the formation of injector nozzle deposits. The pulse width is recorded from the engines Electronic Control Unit (ECU), and the main test metric is percentage change in pulse width vs. time. New injectors can be used for the test but require a 5-hour break-in period as recommended by VW. Used injectors can be also used but need to be thoroughly cleaned in an ultrasonic bath beforehand.

The study looked at ten VW BLG engine fuel injectors of the same design, but in different conditions. The injectors are listed in Table 3. The engine test to create the ‘LAB’ injectors was carried out at Drive Technology Centre, Vienna. Prior to testing, all six injectors had been fully run-in with greater than 300hrs of prior engine testing and were then thoroughly ultrasonically cleaned.

The cleaning process was 5hrs in an ultrasonic bath in a mixture of 90% water and 10% high strength detergent at 60°C. The engine test saw a change of 22% increase in injector pulse width with a linear DU profile. This was a typical test result at this laboratory.

Convention for Injectors

in Study

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Each pair of injectors (whether LAB DU, LAB KC or LAB CU) were made up from an inner and outer cylinder to minimise any temperature gradient effects across the engine. At the time of the experiment, the associated reference fuels for the test were not available. The base fuel used for this engine test was Haltermann ‘DISI TF fuel’, a widely used industry fuel which provides consistent, linear DU in this test. This fuel contains no oxygenates and is EN228 compliant. A table with the properties of this fuel can be found in the Appendix.

CLEAN NEW and CLEAN 72hr were brought into the study to act as control / clean reference injectors. They were both cleaned in an ultrasonic bath as described earlier. CLEAN NEW was new from a VW dealer. CLEAN 72hr had been used in one previous VW DISI test lasting 72hrs.

The donor vehicles for the ‘Real’ injectors were picked specifically to have the identical engine and injector design as the VW DISI test (EA111 variant BLG). Both vehicles were from the European market and would have been run on EN228 compliant fuel. One vehicle had been used for 65,000km and the other 85,000k when their injectors were removed.

Analysis of the Nozzle Deposits

Mass Injector Rate Testing Surface

Deposit imaging was carried out on all the injectors by Scanning Electron Microscope (SEM) and Energy Dispersive X-Ray (EDX), carried out at EM Analytical in the UK. SEM allows visualization of the morphology and location of the deposits whilst EDX allows some understanding of the elemental composition of the deposits.

To avoid contaminating the microscope chamber with fuel vapor, all the injectors were placed in a vacuum chamber to remove any volatile material.

It is well documented [1,4,5] that injector deposits can alter spray and flow characteristics of injectors. To be able to link effects on injector performance to engine performance later in the project, objective measures were required. Two types of test were used to produce this objective data: 1.

Mass injection rate Figure 1. Image

of 6 Holes and surface from CLEAN 72hr

Stepped Area

Nozzle Hole

1.

The face of the injector tip. (Figure 1) 2. The ‘stepped’ area around the edge of the nozzle hole (Figure 2)

To be able to get good images of these areas, the injectors had to be orientated at angles inclined away from the ideal for the X-ray detector. This made accurate quantitative analysis of light elements such as carbon very unreliable. So only a qualitative analysis of EDX spectra is reported in this paper.

Figure 2. Image

of Surface, Stepped Area and Nozzle Hole from CLEAN 72hr

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Macro imaging was intended to give overall spray metrics such as length and spray angles. Microscopic imaging was intended to focus on spray just at the nozzle tip, particularly at the end of injection.

It had been noted with injectors from previous VW DISI tests that the deposits on the surface of the injectors were very soft and easy to dislodge. To minimise any loss of deposits during testing, the fuel pressure was set lower than the VW DISI test (86 bar) to 50 bar and minimize the hydraulic shear forces on the deposits. 50 bar was also considered the lowest realistic pressure that might be seen in a vehicle.

The mass injection rate tests measured the fuel mass sprayed per injection (mg/injection) for all the injectors. Each injector was fitted on the top of a rigid mounting plate, and the pressurized fuel was supplied to the injector through an injection pump. The injected fuel was collected in a sealed conical flask. The weight of the flask before/after 250 injections was measured using an accurate weighing scale to calculate the total injected fuel mass.

The total fuel mass was finally divided by the number of injections to obtain the average mass discharge per injection. This was repeated 3 times to give an overall average based across 750 injections.

After the SEM/EDX work, all the injectors were then sent to the University of Birmingham in the UK for the remaining parts of the study.

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• Macroscopic (whole spray) • Microscopic (focused on nozzle tip)

SEM images were taken with an accelerating voltage of 10kV, which made the beam more surface sensitive.

EDX analysis was carried out at 25kV for efficient excitation. For each injector, the analysis focused on three areas:

3. Inside the nozzle hole itself. (Figure 2)

2. High speed imaging of the injector spray

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Injector Spray Imaging

Combustion Imaging

High speed light scatter imaging was carried out to record the spray plumes emanating from the GDI injector. The injector was fitted on top of a constant volume vessel with two optical windows.

A high-speed camera was used to capture the injection events at a frame rate of 10,000 fps (frames per second) with 800X800 pixel resolution. The camera was fitted with a lens allowing capture of the spray plumes up to 90mm below the tip of injector.

The thermal and optical engine experiments were performed using single cylinder spray guided GDI engines, both engines have the specification shown in Table 4. The engine was naturally aspirated with a throttle controlling the intake air flow rate. A direct current dynamometer was coupled to the engine to maintain a constant speed (±1 rpm). The VW DISI engine and the single cylinder engines had differences in swept volume, compression ratio and injector position but did have similar bore: stroke ratios. As the research engine testing did not obviously influence any existing deposits (based on flow testing before and afterward) any results from the single cylinder engines are used for comparative purposes to aid fundamental understanding.

The images were processed using analysis software to calculate the penetration lengths, cone angles and homogeneity of the sprays. For a stable estimate of these metrics, 12 injection events were recorded at each condition and the metrics from each event were then averaged. Images were converted to binary images and the boundary of the spray shape could be identified by the software. The plume lengths were calculated automatically by the analysis software, finding the longest point of the image in each of the three zones where the plumes were (Figure 3).

Figure 3. Plume

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Measurement – ‘Config 1’

The operating conditions were matched to the CEC where possible, all tests were conducted with coolant and lubricant oil temperatures maintained at 85±2 °C and intake air temperature at 30±2 °C. The injection timing is 280 CAD (Crank Angle Degrees), BTDC (Before Top Dead Centre) while the ignition timing was 22 CAD BTDC for all injectors. For consistent test conditions with each injector, the throttle and injection duration were altered to maintain an equivalence ratio of lambda one and an IMEP (Indicated Mean Effective Pressure) of 5.5 bar. IMEP is the pressure which, over a single stroke of the engine, produces the same work as the entire cycle. By keeping IMEP constant, any differences in combustion, efficiency and exhaust gas could be attributed to the injectors rather than test conditions.

Rail pressure was lower than the VW DISI engine at 50 bar to minimize deposit loss as described before. Engine speed was also lower, constrained to 1200 rpm, which was not ideal but was the maximum safe engine speed for the optical engine.

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The thermal engine was run at the same settings so results between the two research engines could be correlated with minimum interpretation.

events were recorded for each injector to avoid damaging the quartz piston.

Analysis software was applied to process the images. The 2D flame front propagation speed was calculated to represent the flame speed in the combustion chamber. The average flame front propagation speed in two adjoining images is defined as:

Both engines were run on Haltermann RF-02-03 ethanol free fuel. This is a wellcontrolled vehicle emissions reference fuel that meets the EN228 gasoline specification, see Appendix 3 for table of properties.

Radiusequ = (S / 3.14) ^ 0.5 Flame speed = δRadiusequ / δt

For the combustion imaging in the optical engine, optical access was gained through an extended piston with a quartz window.

By using this with a system of mirrors, images of combustion could be captured looking upwards, towards the top of the combustion chamber.

Emissions Measurements For the thermal engine testing, combustion chamber pressure was measured by a water-cooled pressure transducer (Kistler 6041A) and combustion pressure data were recorded with the resolution of 0.2 CAD. IMEP was calculated from the pressure data. The particulate emissions were measured and analysed by a Cambustion DMS500 fast particulate spectrometer. Gaseous emissions were measured using a Horiba MEXA7100DEGR Gas Analyzer.

The images of the flame propagation were recorded using the high-speed camera at 10,000 fps and imaging was triggered by the crank angle signal at 40 CAD BTDC. A maximum of 5 combustion

Units Engine type

UoB Thermal and Optical Engine

CEC VW DISI Engine

4 stroke, 4 valve

4 stroke, 4 valve

Swept volume

cm³

565.6

1390 / 347.5 per cyl

Bore

mm

90

76.5

Stroke

mm

88.9

75.6

Compression ratio

#: 1

11.5

10.0

Naturally aspirated

Mechanical and exhaust

Center

Side

Spray-guided DI

Air-guided DI

Induction Injector position Fueling type Engine speed

rpm

1200

2000

IMEP

bar

5.5

5.5

Injection duration

ms

3.2

1.6

Fuel rail pressure

bar

50

86

Injector type

Bosch six hole, side moutn injectors. Part # 03C 906 036E

Oil temp

°C

85

90

Coolant temp

°C

85

90

Ignition timing

CAD BTDC

22

22

Start of injection

CAD BTDC

280

280

Table 4. Comparison

of Single Cylinder Research Engines and CEC VW Engine

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RESULTS AND DISCUSSION

SEM & EDX Analysis Summary The following section on deposit analysis combines a qualitative summary of the EDX analysis and SEM images together. In addition, images of individual injectors are in Appendix 1.

1.

Newer injectors show clear signs of roughness inside the nozzle hole and in the stepped area. Differences between the 0hr and 72hr cleaned injectors show that the nozzle hole appears to be eroded / run-in, but the stepped area has not.

Mass Injection Rate Results

3.

The REAL injectors have statistically less flow than the KC and CU injectors which tends to agree with the amount of deposit observed in the SEM images.

Figure 4 shows the flow results across injectors in different conditions, averaging the pairs of CEC LAB injectors. It is logical that the main restriction in flow would come from nozzle holes as they are the smallest diameter of the nozzle.

This appears to agree with SEM images that show injectors with the most deposit (Real 85k, DU and Clean NEW) have the greatest deposit or surface roughness inside the nozzle holes.

4.

The Lab DU injectors have the lowest flow, again in line with the amount of deposit visible in the SEM images.

Injection Rate Comparison by Injector 5.0

0.0

0.0 -0.6

2.

Visually, the DU injectors from the CEC test have by far the most deposit of all the injectors.

-4.4

-5.0

-4.7 -7.4

-10.0 -10.3

3.

The injectors which have seen DCA (KC and CU) are significantly cleaner with nozzle holes and as clean as a new run-in injector.

4.

REAL injectors show that the deposit formed is made up of smaller, compacted granular structure that covers the entire injector face.

5.

There is a significant proportion of engine oil components in the deposits of REAL injectors, in conjunction with Carbon and Oxygen.

6.

By comparison all the injectors from the CEC test (DU, CU, KC) had almost no engine oil evident. Most of the deposits were Carbon particularly in the nozzle holes. The tip surface deposits had both Carbon and Oxygen present.

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-13.0

-15.0

-20.0 Lab KC

Lab CU

Clean 72h

Real 65k

Real 85k

Lab DU

Visually comparing still images from injectors with significantly different amounts of deposit does appear to marry correspondingly different spray patterns.

Figure 5-7 compares three injectors of different f low rates and visible deposits.

5.

Most surprising of all were the low flow values for the CLEAN NEW injector which had (8%) less flow than the similar but run-in CLEAN 72hr injector. In the SEM photos of the same injector, the nozzle hole surface has very rough surface compared to other injectors.

mg/inj.

Visual and Chemical Analysis of the Deposits

Clean New

Figure 4. Injection

Rate Comparison

by Injector (7ms pulse width and 50 bar fuel pressure)

Summary of Flow Results 1.

The LAB KC and CU injectors have the highest flow, which is supported by the SEM images which show that their nozzle holes are clear.

2.

The Clean 72h injector has slightly less flow than the KC and CU injectors, despite having no deposits. The SEM images appear to show that the nozzle hole is quite clear, but there is some roughness at the exit which suggests this injector may still require additional running in.

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It is clear from the flow results and SEM imaging that the CLEAN NEW injector has quite different performance to the other injectors as it has not been properly run in. It provides little value as a control or for comparing injector deposits, so any further results are omitted from the remainder of this paper.

For rest of this paper, LAB DU, CU and KC injectors are treated as pairs as the individual injectors are quite similar in flow performance, and pairing gives a more stable average and eases comparison between sets.

The 72hr injector was intended to be the control injector but did not have the highest flow and appeared to need more running in. All further data is presented relative to the LAB KC pair as they had the combined highest mass injection rate.

Figure 5. Injector

LAB KC1 – Well Atomised with Minimal Plume Separation

Figure 5. Injector

LAB DU1 – Poorly Atomised with Clear Plume Separation

Injector Spray Imaging Results Macro spray images provide an overall picture of the spray from which several metrics could be calculated. These metrics were compared to the corresponding SEM images and flow data to try to find any potential correlation and physical explanation.

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Figure 7. Injector

REAL 85k – Some Plume

Separation

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In the images there appears to be three plumes despite being a six-hole injector. This is because two plumes are superimposed on each other. This may introduce some uncertainty about the length of one plume versus another behind or in front of it. Despite this, average plume length was reasonably stable and showed some correlation to the mass injection data (Figure 8).

Analysis of the time history of the images with software was able to see significant differences. Figure 11 compares the flame speed of the different LAB injectors. It shows LAB KC and CU injectors appear to start combustion sooner and had higher peak flames speeds. They also exhibited an initial flare of combustion between -20 and 10 CAD BTDC which LAB DU do not. REAL injectors, as with mass injection rate, ranked between the LAB injectors (Figure 12).

Combustion Imaging Results Visually, it is difficult to discern any difference in combustion images between injectors. It is possible by comparing the highest flowing injector LAB KC2 (Figure 9) and the lowest flowing injector LAB DU2 (Figure 10). The red circle in those figures represents the outer edge of the combustion chamber and it is possible to see that the flame from LAB KC2 propagates more quickly.

There is strong correlation between differences in Injection Mass Rate and Flame Speed at 5 CAD ATDC (Table 5).

This suggests that nozzle deposits and their effect on spray formation and mixture preparation do inhibit combustion and removing them with DCA reverses those effects.

Combustion Speed of LAB Injectors 10.0

8.0

Flame Speed (m/s)

6.0

4.0

2.0

0.0

Change of Average Plume Length vs.

Change in Mass Injection Rate (by injector)

-2.0

10.0%

-4.0

DU

-40

-20

0

20

40

60

0.0%

Crank Angle Degrees

5.0%

KC

REAL 85k 0.0%

KC

REAL 65k

CU

-5.0%

Figure 9. Combustion

Flame of LAB KC2 – 40 microseconds after Ignition

Figure 11. Combustion

CU

DU

Speed of LAB Injectors

-10.0% R2 = 0.26 incl. CLEAN 72h, 0.64 w/o CLEAN 72h

Combustion Speed of REAL vs. LAB Injectors 10.0

CLEAN 72h

-15.0% -12.0%

-10.0%

-8.0%

-6.0%

-4.0%

-2.0%

0.0%

8.0

Flow rate relative to KC KC

CU

CLEAN 72h

REAL 65k

REAL 85k

DU

6.0

Flame Speed (m/s)

Homogeneity measurements showed no correlation at all, regardless of software threshold settings. Overall cone angle was a quite unstable measurement and would rank injectors quite differently with only small changes in the length over which the outside angle was calculated. A typical R² correlation value between injection rate and spray cone angle was 0.4. If this test was repeated, then different test conditions such as fuel temperature would be varied to try to increase differences between injectors.

The exception to this was the CLEAN 72h injector, which had low plume length vs. its mass injection rate. Visually this injector had a well atomized spray, but its relatively short plume length, low mass injection rate and lower ranking in the optical and thermal engine tests suggest it required further running in.

Plume length relative to KC

Spray collapse [8] occurs when the plumes contain a greater ratio of vapour and become drawn to each other to form a cone of well atomized spray. Figure 6 and 7 show less collapse, with separated sprays creating larger voids between the spray plumes. Generally, injectors with more deposits and lower mass injection rates do appear less homogeneous with their plumes remaining separated.

Of the three different analysis software calculations applied to quantify the sprays – plume (penetration) length, cone angle and homogeneity – none correlated well with mass injection rate.

Figure 8. Change

of Average Plume Length vs. Change in Mass Injection Rate (by injector)

4.0

2.0

0.0

-2.0

-4.0 -40

-20

0

20

40

60

0.0%

Crank Angle Degrees KC

Figure 10. Combustion

Flame of LAB DU2 – 40 microseconds after Ignition

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DU

Figure 12. Combustion

65k REAL

85k REAL

Speed of REAL vs.

LAB Injectors

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Combustion and Emissions Results from Thermal Engine All the data presented in this section is shown relative to a change in the mass injection rate data. Mass injection rate was shown to be the most significant objective representation of deposit formation, and the intention is to understand how combustion varies accordingly. Table 5 shows the correlation between combustion, emissions and engine settings vs. injection rate across the different injector sets.

Variable (relative to injection rate)

R-Square Value

Flame speed 5 CAD ATDC

0.97

MFB10_90

0.84

Peak cylinder pressure

0.91

Ignition delay

0.86

Fuel consumption

0.64

Injector pulse width

0.89

Carbon monoxide

0.70

Unburnt hydrocarbon

0.74

Particulate number

0.80

Nitrogen oxides

0.72

Table 5. Correlation

of engine and emissions changes to injection rate The LAB KC injectors had the highest mass injection rates, so are used as the reference point for any relative changes.

The single cylinder thermal engine testing provided accurate combustion chamber pressure vs. Crank Angle Degrees (CAD).

This data was used to calculate Heat Release Rate (HRR), which in turn was used to calculate Mass Fraction Burnt (MFB).

The Indicated Mean Effective Pressure (IMEP) is also calculated from cylinder pressure and is kept constant to ensure the engine produces the same amount of work regardless of which injector is used. It was kept constant by modulating the throttle and fuel injection duration.

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The throttle and fuel injection were also trimmed to maintain consistent exhaust oxygen content equivalent targeting lambda 1 air: fuel ratio.

From the optical engine testing, it was observed that peak flame speed was inhibited in proportion with the loss of mass injection rate. This agrees with the combustion duration, shown in Table 5 and described by MFB10_90. This is the number of CAD for 10% of fuel to 90% of the fuel injected to be combusted. The injectors with more deposit have a longer MFB10_90 duration and lower peak cylinder pressure.

As described earlier with optical engine testing, injectors with less deposit, appeared to have an initial ‘burn’, which was not seen in injectors with deposits.

This seems to agree with a ignition delay, which is defined as the time for 10% MFB from the spark plug firing. Table 5 shows a general trend of ignition delay increasing with the mass injection rate – likely due to poor mixture preparation.

Fuel Consumption was calculated by mass balance of the carbon containing compounds measured by the exhaust gas analyser. Using the hydrogen to carbon ratio of the fuel, the carbon could be back calculated to a total mass of fuel consumed.

This is common practice in vehicle emission testing. The fuel consumption showed a trend of increase relative to increasing mass injection rate (Fig. 46).

Table 5 shows the relative increase in injector pulse width correlates well to loss of injection rate. During mass injection rate tests, it was seen that fuel mass increased linearly with the amount of time that the injector was open. If the nozzle holes are constricted by deposit, it is logical that to maintain the air: fuel ratio of lambda 1, the injectors would have to be held open for longer.

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As described in the test methodology, the gaseous and particulate emissions were measured during thermal engine testing.

Apart from NOx, Table 5 shows there is an overall increase in all the emissions relative to increasing mass injection rate.

The fact that different injectors show differing amounts of increases in CO and HC indicates that combustion by-products are not formed consistently from injectors with different levels and locations of deposits.

Particulate number shows a strong correlation to reduction in mass injection rate. This is despite particulates being linked to injector surface deposits [9] and the LAB DU injectors having lost most of their face deposit (see Discussion). This would suggest that the nozzle deposits alone were enough to affect the spray pattern, combustion and subsequent particulate formation.

In contrast to the other emissions, NOx decreases as mass injection rate decreases.

The speed of combustion and peak cylinder pressures suggest that the HRR is lower in injectors with greater loss of mass injection rate and therefore the peak temperatures are lower. This agrees with the lower NOx values seen as it is generally accepted that more NOx is formed as combustion temperatures increase [10].

Surface Deposit Loss After initial assessment of the spray image data, it became clear that there was visible loss of surface deposits. Injector tip images were then requested after each stage of the remainder of the testing. It was observed that loss of surface deposits was seen on all the injectors (except CLEAN 72h) to some extent.

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Flow checks between tests showed that mass injection rates were stable, the only exception may be CLEAN NEW. That injector saw increasing injection rate during the project and could be running itself in as it was being used in different tests.

As nozzle deposits are considered the main reason for loss of mass injection rate, this leads to the conclusion that nozzle deposits remained intact throughout the study.

Although surface deposits, particularly in the LAB injectors were lost after the imaging tests, significant differences in combustion were still evident between the injectors.

Other papers [9,11] have reported that surface deposits can lead to undesired combustion and excessive particulate number. That may still be true for injectors from this test but even with just nozzle deposits remaining, injector spray can be altered sufficiently to cause significant decrease in combustion quality.

Deposit Analysis When looking at SEM image, it is unsurprising that the LAB DU injectors could have such a pronounced effect on the spray. There are large amounts of deposit in the stepped area and on the injector tip surface. It is interesting to note however, that the REAL 85k injectors achieve similar distortions of the spray pattern with much less material in the stepped area.

This confirms that nozzle hole deposits alone can impact the breakup and atomization sufficiently without any spray impingement from deposits in the stepped area. This is also seen by the engine results correlating well to the loss of mass injection rate alone.

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Overall, the morphology and location of the deposits are similar between LAB DIRTY and REAL injectors. The LAB DIRTY injectors have more surface and stepped area deposits and the asperities which make up the deposit are spherical and relatively large in diameter.

The REAL injectors appear to have smoother, more compacted deposits building up largely in the nozzle holes. Whilst they do have deposits in the stepped areas and on the face of the injector they appear as a regular surface coating made up of a smaller scale like deposit. This may come about by the more transient operation of real-world engines building up deposit and then removing it, creating thinner, more stratified layers over time.

Mass Injection Rate and Spray Imaging Visually SEM imaging showed that increasing levels of deposit – whether nozzle or surface – did agree with loss of mass injection rate and spray quality.

Studies [4,5] have noted that deposits always lead to an increase in plume lengths but depending on the injector design this can either increase the cone angle or decrease it.

There was some correlation between plume length and cone angle to mass injection rate and engine testing.

It may have been difficult to find a stronger correlation for two reasons:

Compositionally, the LAB DIRTY deposits in the stepped area and in the nozzle hole are predominantly carbon based with small amounts of oxygen present. The deposits in both REAL injectors also have carbon and oxygen present, but with a higher concentration of oxygen.

1.

There was some surface deposit loss, but it was not possible to understand at what point it occurred in the testing.

It was difficult to converting the spray images into an objective measure and this may be due to the loss of deposit on different injectors at different times.

This suggests that the REAL injector deposits may have been exposed to higher temperatures over a longer period, had more time for deposits to oxidize or been run with fuel containing oxygenates such as ethanol.

2.

It was expected that deposits would decrease atomization of the fuel into smaller droplets. With smaller droplets, the ratio of vapour to liquid increases [8] and the spray plumes tend to collapse into a cone of well atomized fuel. The likelihood of spray collapse increases as fuel temperature rises. For this study fuel was kept at a relatively low temperature of 15°C for safety reasons.

Increasing the fuel temperature may have increased the likelihood of spray collapse and the potential to discriminate more easily between different injectors.

The most significant difference of the REAL injectors is that their deposits have significant amounts of engine oil components evident.

The key difference between CLEAN NEW and CLEAN 72hr was the injection rate.

The new injector had approximately 10% less flow.

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This seems easily explained by the surface finish of the nozzle hole which appears extremely irregular despite having been cleaned in an ultrasonic bath.

The images of the nozzle hole are like images of stainless steel when it has been Electric Discharge Machined (EDM) [7].

The CLEAN 72hr injector appears to have a smoother surface made up of smaller diameter asperities. This erosion or ‘runin’ is consistent with the 10% reduction in injection rate vs. the CLEAN NEW injector.

Overall though it seems clear that going from clean injectors (LAB KC or CU) to those with deposit (LAB DU or REAL) does simultaneously reduces the mass injection rate and distorts the spray pattern. This is encouraging for the CEC test in that to change the injector characteristics in the same manner, suggests that deposits are being formed in similar locations and are of similar topology.

Micro Imaging Results Micro imaging of the injector spray was intended to measure differences specifically at the end of injection. Several studies [1,6,9,11,12] had identified deposit build up as a key cause of PN increase when residual fuel droplets and ligaments were being released from deposits on the surface of the injector.

Based on the original injector SEM images, it was anticipated that injectors with greater surface deposits would be more likely to exhibit a greater proportion of larger droplets.

Surprisingly, when comparing the droplet distributions from different injectors to the mass injection rate, there was no correlation.

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The reason for the lack of difference between injectors during micro imaging is likely to be due to a loss of the face deposits during spray testing, as described above.

The droplet distribution did not correlate with the PN values seen in the thermal engine testing.

Optical and Thermal Engines The optical and thermal engines were of similar design and run with the same operating conditions allowing easy interpretation of differences between injectors.

Loss of mass injection rate and degraded spray pattern are expected to lead to poor mixture preparation. Poor mixture preparation would lead to locally rich and lean areas when combustion starts at the spark plug. The effects of these uneven regions appear to be evidenced by the fact that: 1.

Data from the optical and thermal engines agreed that injectors with more deposit and lower mass injection rates exhibited longer ignition delay and combustion. The physical geometry of the engine dictates that the peak combustion pressure should occur at a specific crankshaft position for maximum leverage. Poor mixture preparation may reduce flame speed and lower heat release rate leading to retarded combustion. This movement of the peak heat release away from optimal crankshaft position leads to a loss of overall engine efficiency.

2.

In addition, poorly mixed areas will not combust completely and lead to greater combustion by-products such as unburnt HC, CO and PN. This correlation shows in the thermal engine data, where increasing engine emissions coincide with lower thermal efficiency.

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CONCLUSIONS For injectors with deposits, worsening engine efficiency and increased fuel consumption result from more fuel and air having to be consumed to maintain the constant engine work output and overcome the two above effects.

This testing simulates the situation that occurs in vehicles and the CEC engine test. The engine’s ECU has a target oxygen concentration which equates to the correct air: fuel ratio, when the engine was being developed by the manufacturer. Injectors with deposits have restricted mass injection rates as shown earlier, which will lead to excess oxygen in the exhaust.

To restore the oxygen concentration the injectors are opened for longer by the ECU. What cannot be determined by the ECU is that the mixture which leads to that average oxygen concentration is the result of locally rich and lean areas combined and therefore the ECU cannot recognize deterioration in spray pattern.

It’s interesting that PN values are high with injectors with nozzle deposits but little surface deposit. This is likely to be due to poor spray break up caused by nozzle deposits, or impingement of the longer spray on the cylinder wall. In either case, removing the deposits with DCA was shown to reduce PN significantly.

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This project aims to understand the underlying mechanisms of how deposit formation ultimately leads to a reduction in vehicle performance.

A second objective of this project was to understand how similar the deposits formed in this new test are to deposits from vehicles in the market.

The first objective of the study was to confirm understanding of how DCA’s restore performance of engines with preexisting deposits or prevent performance degradation in ‘clean’ engines.

• Overall the CEC test produces deposits like those in ‘Real’ vehicles and fuels and DCA developed with this test should provide measurable benefits to consumers.

• The study confirms that injector deposits affect injector flow and spray which in turn affect mixture preparation and then combustion.

• This is based on correlations between LAB and REAL injectors using injector performance, deposit location / composition and effect on engine combustion.

• There are two effects of poor mixture preparation, incomplete combustion and retarded combustion.

• The use of fuel containing DCA causes these effects to be reversed and therefore restore the engine performance.

• Losing surface deposits early in the study left nozzle hole deposits. This was evidenced by flow checks throughout the remainder of the study.

• Remaining nozzle deposits adversely affected mass injection rate, combustion and emissions.

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• The REAL injectors rank consistently somewhere between LAB DU and LAB KC & CU injectors. This indicates that the CEC test creates more deposit than seen with a car of 85k km. By extrapolation it would be equivalent to a vehicle in the same market of 120,000km.

Other interesting observations were: • Injectors can lose face deposits easily.

Any testing that might be influenced by surface deposits should try to take this into account.

• It is likely that surface deposits would affect some aspect of spray, combustion and emissions but a separate study would be required to quantify it.

• New test injectors must be run-in first, ideally for greater than 72h. Alternatively, tests should be run with a run-in injector with deposits, then repeated after ultrasonic cleaning. This would isolate any effects other than deposit formation.

• Spray imaging testing could not discriminate easily between injectors; changes to test conditions may improve this as well.

• Real injector deposits contain engine oil components, whereas the CEC test injectors do not. Additive and fuel companies should validate fuels and DCA in Real World conditions to ensure their chemistry is robust to deposits with engine oil components.

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REFERENCES 1. Xu, H. et al., “Fuel Injector Deposits in Direct-Injection Spark-Ignition Engines,” Progress in Energy and Combustion Science 50:63-80, 2015, https://doi.org/10.1016/j. pecs.2015.02.002.

2. Stainless Steel Grade Datasheets, Atlas Steels, Revised: August 2013.

3. Zhou, J. et al., “Characteristics of Near-Nozzle Spray Development from a Fouled GDI Injector,” Fuel 219, May 2018, https://doi.org/10.1016/j. fuel.2018.01.070.

4. Song, H., Xia, J., Chen, Y., and Huang, Z., “The Effects of Deposits on Spray Behaviors of a Gasoline Direct Injector,” Fuel 180, April 2016.

5. Tawfik, B., Attar, M., and Hutchins, P., “Investigation of Injector Coking Effects on Spray Characteristic and Engine Performance in Gasoline Direct Injection Engines,” Applied Energy 220, March 2018, https://doi.org/10.1016/j. apenergy.2018.03.133.

6. Wang, Y., Zheng, R., Shuai, S., Qin, Y. et al., “The Impact of Fuel Properties from Chinese Market on the Particulate and VOCs Emissions of a PFI and a DIG Engine,” SAE Technical Paper 2016-01-0838, https://doi. org/10.4271/2016-01-0838.

7. Mascaraque-Ramireza, C., and Franco, P., “Experimental Study of Surface Finish During ElectroDischarge Machining of Stainless Steel,” Procedia Engineering 132, 2015, https://doi.org/10.1016/j. proeng.2015.12.547.

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ACKNOWLEDGEMENTS 8. Zhang, G., Xu, M., Zhang, Y., Zhang, M., and Cleary, D.J., “Macroscopic Characterization of Flash-Boiling Multi-Hole Sprays Using Planar Laser Induced Exciplex Fluorescence Technique. Part I. On-Axis Spray Structure,” Atomization and Sprays 22(10):861-878, January 2012, https://doi.org/10.1615/ AtomizSpr.2013006760.

• Dr David Huskisson at EM Analytical for his support with SEM and EDX analysis.

• David Weissenberger, Drive Technology Centre for his knowledge and experience in running CEC engine tests.

• Lutz Reichenbacher at Haltermann Carless for supplying test fuel.

9. Imaoka, Y., Hashizume, Y., Inoue, T., and Shiraishi, T., “A Study of Particulate Emission Formation Mechanism from Injector Tip in Direct-Injection Gasoline Engines,” SAE Int. J. Adv. & Curr. Prac. in Mobility 2(1):376-384, 2020, https:// doi.org/10.4271/2019-01-2244.

10. Doornbos, G., Hemdal, S., and Dahl, D., “Reduction of Fuel Consumption and Engine-out NOx Emissions in a Lean Homogeneous GDI Combustion System, Utilizing Valve Timing and an Advanced Ignition System,” SAE Technical Paper 2015-01-0776, 2015, https://doi.org/10.4271/2015-01-0776.

DEFINITIONS/ABBREVIATIONS BSFC

Brake Specific Fuel Consumption

BTDC

Piston position / Before Top Dead Centre

CAD

Crankshaft Angle Degrees

CEC

Coordinating European Council

CO

Carbon Monoxide

DCA

Deposit Control Additives

EDX

Energy Dispersive X-Ray

ECU

Engine Control Unit / Micro-processor responsible for controlling engine

Fps

Frames per second

GDI

Gasoline Direct Injection

HC

Unburnt Hydrocarbon

HRR

Heat Release Rate

IMEP

Indicated Mean Effective Pressure

MFB

Mass Fraction Burned

NOx

Oxides of Nitrogen

OEM

Original Equipment Manufacturer / Vehicle Manufacturer

PN

Particulate Number

SEM

Scanning Electron Microscope

VW DISI

CEC injector coking test method

11. Leach, F., Knorsch, T., Laidig, C., and Wiese, W., “A Review of the Requirements for Injection Systems and the Effects of Fuel Quality on Particulate Emissions from GDI Engines,” SAE Technical Paper 2018-01-1710, 2018, https://doi. org/10.4271/2018-01-1710.

12. Wen, Y., Wang, Y., Fu, C., Deng, W. et al., “The Impact of Injector Deposits on Spray and Particulate Emission of Advanced Gasoline Direct Injection Vehicle,” SAE Technical Paper 2016-01-2284, 2016, https://doi. org/10.4271/2016-01-2284.

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APPENDIX 1 SEM IMAGES – INJECTOR SURFACE

Figure 11. LAB

KC2

SEM IMAGES – STEPPED AREA

Figure 13. CLEAN

NEW

Figure 16. REAL

• L AB KC. Almost completely clean and clear stepped holes and nozzle holes.

• C LEAN NEW. Clean and clear stepped holes and nozzle holes.

• M ainly parent metal with traces of carbon.

• T race lubricant contamination, likely from ultrasonic bath.

Figure 12. LAB

Figure 15. REAL

CU1

• L ab CU. Almost clear stepped holes and nozzle holes.

• M ainly carbon and oxygen along with parent metal.

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65k

• R EAL 65k. Significant deposit on surface but smoother / more regular than LAB DU injectors.

• E DX suggests excessive engine oil consumption vs.REAL 85k, which may contribute to ‘smoothed’ surface.

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85k

Figure 18. LAB

KC2

• R EAL 85k. Significant deposit, but regular and smoothed out across entire injector tip.

• L AB KC. Almost completely clean and clear stepped holes and nozzle holes.

Figure 17. LAB

Figure 19. LAB

DU2

• L AB DU. Ring of deposits grown significantly up and out from tip surface.

• L arge diameter spherical deposits evident.

• Mainly parent metal with some carbon.

CU1

• L AB CU. Some deposit in stepped area, but edge of nozzle hole is clearly evident.

• Mainly parent metal and carbon.

• Mainly Carbon and Oxygen

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SEM IMAGES – NOZZLE HOLES

Figure 25. LAB Figure 20. CLEAN

NEW

• C LEAN NEW. Rough surface, typical of freshly spark eroded stainless steel.

• L ooks like some material ‘peeling’ off surface or left over from spark erosion.

Figure 23. REAL

85k

• R EAL 85k. Stepped hole profile visible, covered in smaller diameter spherical, compacted deposit. Creating a more regular and smoothed surface.

KC2

Figure 27. CLEAN

NEW

• L AB KC. Completely Clean Nozzle Holes with crisp edge to stepped area.

• C LEAN NEW. Looks granular, but this is likely just spark eroded stainless steel.

• Just parent metal evident in EDX.

• T his rough, irregular surface is likely to form a restriction which would lead to the reduced mass injection rate observed.

Figure 26. LAB

Figure 29. REAL

• A lmost equal evidence of carbon and oxygen and elements from engine oil.

Figure 22. REAL

65k

• R EAL 65k. Stepped area almost indistinguishable, smoothly blending nozzle hole to surface deposit.

• L argely elements associated with engine oil present.

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Figure 24. LAB

DU2

CU1

65k

• L AB DU: Substantial face deposits overshadowing the nozzle holes.

• L AB CU. Completely Clean Nozzle Holes with crisp edge to stepped area.

• R EAL 65k. Parent metal and Carbon and Oxygen

• M ainly Carbon and parent metal evident but some traces of copper and Sulphur which may have contributed to significant deposit build up.

• Mainly parent metal with some carbon.

• M uch less engine oil contamination in nozzle holes than stepped area.

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APPENDIX 2 DATA TABLES

Unit

DISI TF Injector Fouling Fuel

Optical & Thermal Engine RF-02-03

RON

-

98.7

100.5

DVPE

KPa

64.5

58.8

n

5

5

5

5

5

5

T10

°C

50

50.6

Value

21.0

20.6

21.1

21.2

22.2

22.5

T50

°C

104.3

92.6

S.E

0.7

1.1

0.9

1.2

1.3

1.1

T90

°C

175.8

161.9

n

300

300

300

300

300

300

FBP

°C

202.5

201.9

Value

32.0

31.9

31.6

31.3

30.1

29.9

Washed Gum

mg

<1

<1

Aromatics

%(v/v)

34

33.8

Olefins

%(v/v)

11.7

4.4

Saturates

%(v/v)

54.2

61.8

Oxygenates

%(v/v)

<0.2

<0.2

ppm

6.1

1.3

Characteristic

Figure 30. REAL

85k

• R EAL 85k. Just carbon, oxygen and parent metal.

• N o trace of engine oil contamination in nozzle hole, similar to 65k REAL. • S ignificant deposits in spray holes. Little islands of deposit starting to grow out of the surface.

Sulfur

Table 4. Test

Fuel Characteristics

In the table 'vale' is the averaged value of repeat tests, S.E. is the standard error of the value at 95% confidence and n is the number of test repeats to get the average value.

LAB KC

LAB CU

CLEAN 72h

REAL 65k

REAL 85k

LAB DU

Value

6.9

7.1

6.3

6.1

5.7

5.2

S.E

0.8

0.5

0.8

0.9

0.5

0.7

Flame Speed m/s @ +5 CAD)

MFB 10_90 (CAD)

S.E

0.8

0.7

1.0

0.8

1.0

0.6

n

300

300

300

300

300

300

Value

10.2

10.1

11.0

11.5

11.2

12.0

S.E

0.3

0.6

0.5

0.7

0.6

0.6

n

300

300

300

300

300

300

Value

263.5

262.1

263.4

275.3

269.3

277.8

S.E

9.0

14.6

11.0

15.8

15.5

13.2

Pmax

Ignition Delay

Fuel Consumption

n

300

300

300

300

300

300

Value

3.1

3.1

3.2

3.2

3.3

3.4

S.E

0.1

0.2

0.1

0.2

0.2

0.2

Injector Pulse Width

n

300

300

300

300

300

300

Value

41.2

45.5

42.6

53.8

51.6

58.1

CO

S.E

0.4

1.1

0.4

1.3

0.5

0.5

n

300

300

300

300

300

300

Value

5.2

5.3

5.5

6.5

6.8

6.5

HC

S.E

0.1

0.3

0.0

0.1

0.2

0.2

n

300

300

300

300

300

300

Value

3.6E+06

3.0E+06

3.4E+06

4.7E+06

5.3E+06

5.7E+06

S.E

3.5E+05

1.1E+05

1.6E+05

3.0E+05

35.E+05

3.0E+05

n

300

300

300

300

300

300

Value

9.3

9.5

9.0

8.5

8.6

8.5

S.E

0.1

0.1

0.1

0.1

0.1

0.1

n

300

300

300

300

300

300

PN

Figure 31. LAB

NOx

DU2

• L AB DU. The deposit surface inside the hole is made up of large diameter asperities.

Table 5. Thermal

Engine Results

• O nly carbon and the parent metal are picked up by EDX, almost no oxygen around.

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