1 

# Memo

To: Docket for the Tier 3 Vehicle Emission and Fuel Standards Program

From: Joseph McDonald, U.S. EPA-OTAQ-ASD

CC: Paul Machiele, Ed Nam, Mic hael Olechiw, U .S. EPA-OTAQ-ASD

Date: April 4, 2013

Re: Exhaust emissions from light-duty vehicles with gasoline direct injection.

The attached report provides a summary of the PM emissions of GDI and PFI light-duty vehicles and trucks certified to Federal Tier 2 and California LEV II standards along with a summary of research and development currently underway to reduce GDI PM emissions.

# Exhaust Emissions from Light-duty Vehicles with Gasoline Direct Injection

Joseph McDonald and David Hawkins U.S. EPA – Office of Transportation and Air Quality



### ABSTRACT

In response to new regulations on greenhouse gas emissions and CAFE, EPA expects that manufacturers of light duty vehicles will increase the penetration of gasoline direct injection and turbochar ged gasoline direct injection engines in the United States. Recent studies have raised concerns regardin g the emissions of particulate matter from vehicles equipped with gasoline direct injection engines. This report provid es a summary of the PM emissions of GDI and PFI light-duty vehicles and trucks certified to Federal Tier 2 and Califor nia LEV II standards along with a summary of research and development currently underway to reduce GDI PM emissions.



### INTRODUCTION

Light-duty vehicle and truck engines using gaso line direct injection (GDI) fuel systems in lieu of port fuel injection (PFI) has been steadily increasing in United States. This has been occurring in part due to increased stringency of CO2 and fuel economy standards, and the increas ed efficiency of GDI over PFI engines. 1,2 There are also some market advantages for GDI engines relative to traditional port-fuel-injected (PFI) engines for light-duty applicatio ns that include improvements in drivability and vehicle carrying or towing capacity due to the higher available to rque and availability of high torque over a broader range of engine speeds. Recent testing by the Ca lifornia Air Resources Board indi cated that particulate matter (PM) emissions for first generation GDI vehicles were signi ficantly higher than PFI vehicles when comparing vehicles certified to comparable LEV II emission standards. 3 This report provides a summary of the results of recent FTP and US06 emission tests of vehicles at U.S. EPA – NVFEL and comparable emission test data obtained from the EPA VERIFY emissions certification database. This report also provides a summary of the current state of emissions mitigation strategies currently under development for GDI light-duty vehicle and truck applications.



### BACKGROUND

WHAT IS GDI?

Stoichiometric gasoline direct injection (G DI), or Spark Ignition Direct injection (S IDI), engines inject fuel at high pressure directly into the combustion chamber as opposed to injecting f uel directly upstream of the in take valve as with previous PFI engine fuel systems. GDI requires changes to the inject or design, an additional high pressure fuel pump, new fuel rails or cylinder-cast passages to handle the higher fuel pressures and changes to the cylinder head and piston crown design. Direct injection of the fuel into the cylinder improves cooling of the air /fuel charge within the cylinder, which allo ws for higher compression ratios and increased thermodynamic e fficiency without the onset of combustion knock. Recent injector design advances, improved electronic engine manag ement systems and the introduction of multiple injection events per cylinder firing cycle promote better mixing of the air and fuel, enhance combustion rates, increase residual exhaust gas tolerance and improve cold start emissions. GDI engines achieve higher power density and match well with other technologies, such as boosting (turbocharging and supercharging), engine downsizing, and variable valvetrain designs.

There two basic configurations for packaging of the fuel inje ctor and sparkplug within the cylinder heads of GDI engines currently in production: walled-guided injection (sometimes referred to as side-mounted injection) and spray-guided injection (sometimes referred to as centrally mounted injectio n). A comparison of basic la yout of the two systems is shown in Figure 1. Figure 2 and Figure 3 show more detailed examples of wall-guid ed and spray-guided systems, respectively, including the use of multi-hole fuel injectors.Figure 1: Comparison of the Mercedes-Benz CLS 350 CGI spray-guided GDI combustion system to a wall-guided combustion system. 4Figure 2: Solid-model of the wall-guided GDI com bustion system of the Ford Ecoboost 3.5L engine. Adapted from Yi et al. 5

Figure 3: Solid-model of a Jaguar spray-guided combustion system. Adapted from Chen et al. 6

Wall-guided systems inject fuel on one side of the combustion chamber and transport an ignitable air-fuel mixture to nearcentrally-located spark plug by relying on guiding of the char ge along a specially curved surface within the piston crown. Spray-guided systems inject the fuel near the center of the combustion chamber and rely primarily on the injector for spray-development and mixing with air. There are advantages and disadvantages to both configurations. Near-centrally mounted spray-guided injection systems allo w a higher degree of charge stratification, have a higher degree of ignition stability over a broader operational range, and allow a more uniform distribution of fuel with mi xture formation that is more independent piston crown geometry. Some disadvantages includ e reduction in the space available for intake and exhaust valves, less time available for mixture preparation and higher temperatures at the injector tip which generally increases the susceptibility to injector coking. Side-mounted, wall-guid ed injection systems allow the us e of larger valve areas; have more time available for mixture preparation and generally have lower injector tip temperatures and thus less susceptibility to injector coking. Disadvantages to wa ll-guided systems include a higher potential for fuel impingement on combustion system surfaces and less ability for charge stratification.

Both wall-guided and spray-guided systems also use manifo ld and port generated tumble and other means of inducing charge motion to improve in-cylinder mixing. More rec ently developed wall-guided and spray-guided injection systems use multi-hole injection nozzles, have maximum injection pressures of approximately 150 to 300 bar and have some capability for multiple injection events, particularly imm ediately following startup and at low engine speeds where air velocity into the cylinder is lower and charge motion is more difficult to induce and maintain. Solenoid injectors have been used in both wall-guided and spray-guided applications. Ou twardly opening piezo-injector s have faster actuation and allow very precise fuel control. Piezo-injectors are also le ss susceptible to the formation of injector tip deposits. Some spray-guided applications requiring more precise fuel control (e .g., lean-stratified combustion modes) use piezo-injectors.WHY USE GDI?

Use of GDI systems with turbocharged engines and air-to-air ch arge air cooling also reduces the fuel octane requirements for knock limited combustion and allows the use of higher comp ression ratios than is typically possible with boosted PFI engines. Recently published data with advanced spray-guided injection systems and more aggressive engine downsizing targeted towards reduced fuel consumption and CO 2 emissions reductions indicate that the potential for reducing CO 2 emissions for turbocharged, downsized GDI engines may be as much as 15 to 30 percent relative to port-fuel-injected engines. 7,8,9,10,11,12 Engine downsizing via use of turbocharging and GDI will be a primary pathway for compliance with the 2012 – 2016 and 2017 – 2025 light-duty vehicle GHG and CAFE regulations. Since 2005, GDI Vehicles from Aud/VW, GM, BMW, Mazda, Ford, Mercedes, Hyund ai/Kia, Porsche, and Jaguar/Rover hav e been introduced in the U.S. Ford’s “Ecoboost” range of downsized, turboc harged, GDI, engines were introduced in the United States on MY2010 vehicles with the initial Ecoboost 3.5L engine. This particular engine used a wall-guided configuration and allowed the substitution of V6 engines into previously V8 engine applications with po wer and torque comparable to much larger displacement V8 engines. 5 The version of the Ford Ecoboost 3.5L engine introduced in the MY2012 Ford F150 pickup truck demonstrated combined-cycle fuel economy improvements of 28 - 30 percent when compared with larger displacement V8 engines in the same chassis having comparable towing capacity. 13 Between MY2010 and MY2014 Ford expanded their range of Ecoboost engines to include 2.0 L engines with wall-guided injection and both 1.1L and 1.6L engines with spray-guided injection.

GDI GASEOUS CRITERIA EMISSIONS

GDI vehicles sold in the U.S. to date have all used prim arily homogenous charge combustion with near-stoichiometric fueling that allows use of platinum-gro up-metal (PGM) exhaust catalysts for highl y efficient control of NMOG, NOx and CO emissions. The fuel-combustion event, however, is not completely homogeneous. Inhomogeneity, particularly under coldstart conditions, can pose challenges with meeting stringent NMOG emissions standards. Turbocharged GDI engines have an additional challenge of the thermal mass of the turb ocharger serving as a heat sink that can interfere with achieving catalyst light-off shortly after engine ignition during a cold start. Fast catalyst light-off (10 – 20 seconds to li ghtoff temperature from “key-on”) will be necessary for meeting future Federal Tier 3 and California LEV III NMOG and NOx emissions standards. GDI engines do hav e some advantages over PFI engines with respect to emissions of gaseous criteria pollutants. GDI engines are more tolerant of inject ion timing retard used to shorten the time necessary to achieve catalyst light-off after a cold start. 14 Stratified lean or near-stoichiometric cold starts are also possible, which may obviate the need for secondary air injection. The design and capabilit y of the fuel system can also offer additional degrees of freedom with respect to fuel control and calibration of fuel control parameters. Through careful modeling of spray development and mixing, careful development and design of fuel injection targeting and spray pattern, use of multi-hole injectors, use of multiple injections wi th precise timing of injection events and dwell times between injection events and through the use of controllable cam-phasing and careful calibr ation of valve events and ignition timing, manufactures have demonstrated cold-start NMOG emissions and catalyst li ght-off characteristics of bot h naturally aspirated and turbocharged GDI sufficient to meet LEV II SULEV exhaust emissions levels. 5,15 Both naturally aspirated and turbocharged GDI light-duty vehicles have been certified at Bin 4 and below Federal Tier 2 emissions and as low as California SULEV and PZEV. 16,17

GDI PM EMISSIONS

PM emissions from internal combustion engines are complex mixture composed chiefly of:

 Elemental carbon or soot particles formed primarily as pyrolysis products of noncombusted or partially combusted fuel

 Semi-volatile organic compounds from lubricants and fuel that begin primarily in the gas phase and then as the exhaust gases cool are adsorbed and absorb onto other particles or are heterogeneously or homogenously nucleated

 Metal oxides, chiefly from lubricating oil additives

 Sulfate from sulfur in the fuel and lubricating oil

 Nitrate compoundsWith respect to both PFI and GDI stoichiometrically-operated gasoline engines, PM emissions mass is dominated by emissions of elemental and organic carbon. Elemental carbon or soot formation in GDI engines appears to be chiefly from two mechanisms – combustion inhomogeneity and fuel im pingement on the surfaces of the cylinder liner and piston

crown. 14,18,19,20 Soot does not have sufficient residence time within mo nolithic catalysts to diffuse to catalytic surfaces and thus soot and NMOG emissions do not always trend together with changes in engine parameters. Pool fires from fuel films deposited on the piston crown have been demonstrated to be highly correlated with soot emissions but may not produce sufficient hydrocarbon emissions to be a significant contributor to NMOG. 18 Cold piston and other combustion system surfaces also impede in-cylinder fuel vaporization and increase soot-forming diffusion-controlled combustion. Carbon deposits on injector tips can increase PM emissions via deterioration of spray char acteristics and by absorbing fuel hydrocarbons which can then later combus t in a soot-forming, diffusion-limited flame. 21



### SUMMARY EMISSIONS DATA

Figure 4 is a summary of PM emissions for both PFI-equip ped vehicles and GDI-equipped vehicles over the FTP drive cycles. Figure 5 shows PM emissions results for vehicles with a 1 mg/mi compliance margin below the draft Tier 3 PM emissions standard over the FTP. Figure 6 is a summary of PM emissions over the US06 drive cycle. The emissions results in these figures are grouped by engine technology. T he PFI PM data points over the FTP cycle consists primarily of 17 high volume (approximately 80,000 to 370,000 vehicles per year) Tier 2 light duty vehicles and trucks with many repeat PM measurements (6-18) per vehicle. Most of t he vehicles had odometer readings of approximately 20,000 to 40,000 miles. The vehicles were tested at EPA-NVFEL in supp ort of the draft Light-duty Ti er 3 regulations. PM data points for 2 additional PFI (an LDT1 an d LDT4) and 57 GDI vehicles were obt ained from the EPA VERIFY certification database. More than half of the data used from VERIFY co nsisted of vehicle tests co nducted by EPA at the EPA-NVFEL facility supplemented by additional data submitted by vehicl e manufacturers from their own testing. The data from VERIFY consisted primarily of single PM tests although repeat te st results (2-4) were reported for some vehicles. Most of the vehicles were tested with odometer readings of 4,000 to 5,000 miles. All the repo rted tests were conducted using Light-duty Tier 2 certification gasoline (E0).

The FTP and US06 PM data in Figure 4 and Figure 6, re spectively, are graphically represented for each engine technology using "box and whisker" plots. 22,23,24 The two upper and lower "hinges" of the boxplots correspond to the first and third quartiles (the 25th and 75th percentiles) of the PM em issions data for each technolog y. The horizontal line within the boxes represents median PM emissions. The upper vertical line or "whisker" extends from the hinge to the highest value that is within 1.5 * inner quartile range (IQR) of the hinge, where IQR is the distance between the first and third quartiles. 23 The lower "whisker" extends from the hinge to the lowest value within 1.5 * IQR of the hinge.

Median PM emissions for PFI, SG-GDI and WG-GDI vehicles ov er the FTP cycle were approximately 0.7 mg/mi, 2 mg/mi and 4 mg/mi, respectively. The sole vehicle with a fuel syst em consisting of both WG-GDI with a separate PFI system (2013 Subaru BRZ) demonstrated PM emissions comparable to other PFI-equipped vehicles. Most of the SG-GDI vehicles appear to have emissions consis tent with meeting the draft Tier 3 FTP PM standard of 3 mg/mi. Many of the WG-GDI vehicles also demonstrated emissions consistent wi th meeting the draft Tier 3 standards but most WG-GDI vehicles will require additional PM emissions control to co mply with the draft standards. Overall, WG-GDI and SG-GDI vehicles appear to require additional development to achieve PM emissions comparable to Tier 2 and LEV II PFI vehicle. The lowest emitting SG-GDI and WG-GDI vehicles were capable of achieving PM emissions of 0.7 to 1.8 mg/mi over the FTP (Figure 5). The single vehicle combining WG-GDI and PFI technologies had PM emissions comparable to the lowest emitting PFI vehicles over the FTP.

Considerably less PM emissions data were available over the US06 cycle. PM emissions data from PFI vehicles were insufficient to allow direct comparison with GDI vehicles. Some GDI vehicles demonstrated very low PM emissions over the US06 cycle and PM emissions of most of the vehicles were well below the dr aft Tier 3 10 mg/mi LDV/LDT1/LDT2 PM standard. Note that all of the LDT3 and LDT4 GDI v ehicles demonstrated emissions below 10 mg/mi LDV/LDT1/LDT2 PM standard.Figure 4: Summary of PM emissions over the FTP cycle for vehicl es with various engine technologies that have been certified to Federal Tier 2 and California LEV II emissions standards. The draft Tier 3 PM emissions standard is shown for comparison. Note that the median value for the Supercharged SG-GDI vehicles (all from Jaguar/Rover) corresponds to the lower quartile of the boxplot. The PM emissions data for the combined WG-GDI/PFI system were from a single vehicle type (Subaru BRZ). 1 3 7

5



### Draft Tier 3 FTP PM Emissions StandardFigure 5: Summary of PM emissions over the FTP cycle for vehicl es with various engine technologies that have been certified to Federal Tier 2 and California LEV II emissions standards with PM emissions </= 2 mg/mi. The points represent individual composite FTP PM emissions results. The PM emissions data for the combined WG-GDI/PFI system were from a single vehicle type (Subaru BRZ). Each data point represents PM emissions results for one FTP composite test.Figure 6: Summary of PM emissions over the US06 cycle for vehicles with various engine technologies that have been certified to Federal Tier 2 and California LEV II emissions standards. The draft Tier 3 PM emissions standards for LDV/LDT1/LDT2 and LDT3/LDT4 are shown for comparison.



### MITIGATION OF PM EMISSIONS IN LIGHT-DUTY GDI APPLICATIONS

A significant amount of research is currently underway to improve the control of PM emissions from GDI-equipped vehicles in advance of new California LEV III and proposed U.S. Federal Tier 3 PM emissions standards as well as European Union Euro 6 solid particle number (SPN) standar ds. The development of technology to reduce PM emissions from GDI engines appears to be following somewhat similar tr ends to the technological deve lopments that reduced soot formation from diesel engines over t he past two decades. GDI PM emissions mitigation strategies include careful attention given to combustion system design (e.g., induction of air-charge motion, piston crown design, injector targeting, higher injection pressure capability, multiple injection capability, reduction in fuel injector production variability) and car eful attention given to calibration of engine parameters (e.g., sp ark timing, camshaft phasing/valve events, fuel injection timing and number of injection events). 21,25,26,27,28,29 Recently, there has also been initial dev elopment of exhaust filtration devices (gasoline particulate filters or GPFs) similar to those curr ently in use with nearly all light-duty and heavy-duty diesel transport applications in North America, Japan and Europe. 30,31

The work by Whitaker et al. identified combustion strategi es for PM control that focused on four modes of vehicle operation: 

### Draft Tier 3 US06 LDV/LDT1/LDT2 PM Emissions Standard Draft Tier 3 US06 LDT3/LDT4 PM Emissions Standard1. Cold start

2. Catalyst heating

3. Cold transient

4. Warm transient/near steady-state cruise

Via careful attention to design and engine parameter calibra tion, soot formation comparable to that of PFI gasoline vehicles over a broad range of operation was demonstrated for a turbocharged WG-GDI engine. 26

COLD START AND CATALYST HEATING

Increased injection pressure to improve f uel atomization coupled with the use of mult iple injections and careful calibration and timing of injection events to prevent fuel impingement ont o the cylinder liner and piston can reduce soot formation immediately following a cold-start key-on event. Berkemeir et al. and Whitaker et al. demonstrated large reductions in soot formation with SG-GDI and WG-GDI systems using three or f our injection events during operation immediately after coldstart. 21,26 Similar multiple injection strategies can be used in conjun ction with ignition timing retard to allow quicker catalyst heatup while avoiding fuel impingement and resultant soot formation.

Current production GDI injection systems typically use maximu m rail pressures of 150 to 300 bar. Increases in fuel injection rail pressure beyond 300 bar to pressures as high as 500 or 1000 bar have been shown to further improve atomization and mixture preparation and to significa ntly reduce both soot formation and PM emissions. 25

COLD TRANSIENT OPERATION

Rapid heating of the piston crown and combustion chamber surfaces to enhance fuel vaporization can reduce soot formation during engine warm-up. This can be accomplished via use of switchable electric coolant pumps or the use of coolant thermal storage devices, both of which are strategies previously used with hybrid electric vehicles. Combustion system surface heating can also be enhan ced via calibration of engine parameters (e.g. multiple injections, injection timing, ignition timing and camshaft phasing).

WARM TRANSIENT/NEAR STEADY-STATE CRUISE

Whitaker et al. found reductions in soot formation durin g warmed-up, higher-speed operation via careful attention to charge motion (e.g., air tumble) and use of up to three in jection events per firing cycle with a WG-GDI system. 26 Berkemeir et al. found improvements via a degree of injecti on timing retard and use of one or two injection events per firingwith an SG-GDI system. 21



### CONCLUSION

PM emissions from current Federal Tier 2 and California L EV II vehicles with SG-GDI and WG-GDI are approximately 2 times and 4-5 times, respectively, higher than PM emissi ons measured from Tier 2 PFI vehicles. Current SG-GDI vehicles demonstrated emissions below the draft Federal Ti er 3 FTP PM emission standards. Both GDI combustion system configurations in their current forms are relative ly recent developments and considerable developmental efforts are underway to further reduce PM emissions. Many of the strategies appear to parallel similar developments (combustion strategies, exhaust f iltration) that have proven effective at redu cing PM emissions of modern diesel engines to very low levels. Recent research has demonstrated t he potential for GDI vehicles to achieve low soot formation comparable to modern Tier 2 and LEV II PFI vehicles over a br oad range of driving conditions. The data collected by the EPA shows that the lowest emitting Tier 2 GDI vehicles had PM emissions approa ching those of PFI vehicles. Future development of GDI PM emissions controls will likely incl ude cold-start and warm-up calibration and continued improvement of GDI combustion systems in order to comply with future light-duty Tier 3 PM emissions standards. PM emissions of GDI equipped, Tier 3-compliant vehicles in th e future should be expected to have significantly lower PM emissions on average and should be comparable or lower than PM emissions from currently available PFI vehicles. The EPA thus believes that both wall-guid ed and spray guided GDI engines can meet proposed Tier 3 PM emissions standards without having to resort to gasoline particulate filters.

### REFERENCES

1 

### “Light-Duty Vehicle Greenhouse Gas Emission Standard s and Corporate Average Fuel Economy Standards; Final Rule,” U.S. Federal Register, Federa l Register, Vol. 75, No. 88, p. 25324- 25728, May 7, 2010.

2 

### “2017 and Later Model Year Light-Duty Vehicle Gr eenhouse Gas Emissions and Corporate Average Fuel Economy Standards,” Federal Register, Vol. 77, No. 199, p. 62624-63200, October 15, 2012.

3 

### “Particulate Emissions for LEV II Light-Duty Gasoline Direct Injecti on Vehicles,” Zhang, S., McMahon, W., SAE Technical Paper No. 2012-01-0442.

4 

### Green Car Congress, “Mercedes-Benz Premiers New Ga soline Direct Injection Sy stem for More Power and Lower Fuel Consumption,” February 24, 2006. Accessed on the Internet on February 20, 2013 at the following URL: http://www.greencarcongress.co m/2006/02/mercedesbenz_pr.html

.

5 

### “Development and Optimization of the Ford 3.5L V6 EcoBoost Combustion System,” Yi,J., Wooldridge, S., Coulson, G., Hilditch, J. Iyer, C.O., Moilanen, P., Pa paioannou, G., Reiche, D. Shelby, M., VanDerWege, B., Weaver, C. Xu, Z., Davis, G., Hinds, B. Schame l, A. SAE Technical Paper No. 2009-01-1494, 2009, Docket EPA-HQ-OAR-2009-0472-2860.

6 

### “A study of mixture preparation and PM emissions using a direct injection engine fuelled with stoichiometric gasoline/ethanol blends,” Chen, L., Stone, R., Richardson, D. Fuel (2012) 96 p. 120-130, Elsevier Ltd.

7 

### “GDI Turbo – The Next Steps” Kapus, P.E., Fraidl, G. K., Prevedel, K., Fuerhapter , A. JSAE Technical Paper No. 20075355, 2007 JSAE Annual Congress Proceedings.

8 

### “Sabre: a cost-effective engine technology combination for high efficiency, high performance and low CO2 emissions,” Turner, J.W.G., Pearson, R.J., Curtis, R ., Holland, B. Low Carbon Vehicles 2009: Institution of Mechanical Engineers (IMechE) conference proceedings, May 2009.

9 

### “Development of a Turbocharged Direct Injection Downsizing Demons trator Engine,” Lumsden, G., OudeNijeweme, D., Fraser, N., Blaxill, H. SAE Technical Paper No. 2009-01-1503.

10 

### “Water Cooled Exhaust Manifold and Full Load EGR Technology Applied to a Downsized Direct Injection Spark Ignition Engine,” Taylor, J., Fraser, N., Wieske, P. SAE Technical Paper No. 2010-0356.

11 

### “EBDI® - Application of a Fully Fl exible High BMEP Downsized Spark I gnited Engine,” Cruff, L. Kaiser, M., Krause, S., Harris, R., Krueger, U., Williams, M. SAE Technical Paper No. 2010-01-0587.

12 

### “Doing More with Less - The Fuel Economy Benefits of Cooled EGR on a Direct Injected Spark Ignited Boosted Engine,” Kaiser, M., Krueger, U., Harris, R., Cruff, L. SAE Te chnical Paper No. 2010-01-0589.

13 

### “Fuel Economy Guide – Model Year 2012,” U.S. Depart ment of Energy and U.S. Environmental Protection Agency, May 22, 2012.

14 

### “Measures to Reduce Particulate Emissions from Gasoline Engines,” Paper No. 2011-01-1219, SAE International Journal of Engines (2011) 4 (1) pg. 1498-1512.

15 

### “Multi-Stage Injection Control fo r Super Ultra Low Emission Vehicles ,” Kawasumi, I., Yasui, Y. IEEE International Conference on Industrial Tec hnology, Mumbai, India, December 15-17, 2006.16 

### “DriveClean – A buying guide for clean and efficient vehicles,” California Air Resources Board, accessed on the Internet on 2/22/2013 at the following URL: http://www.driveclean.ca.gov/searchresu lts_by_smog.php?smog_slider_value=9&x=11&y=10

.

17 

### “Annual Certification Test Results & Data,” U.S. E nvironmental Protection Agency, accessed on the Internet on 2/22/2013 at the following URL: http://www.epa.gov/otaq/crttst.htm

.

18 

### “Piston Fuel Films as a Source of Smoke and Hydro carbon Emissions from a Wall-Controlled Spark-Ignited Direct-Injection Engine,” Drake, M.C ., Fansler, T.D., Solomon, A.S., Szekely., G.A. SAE Technical Paper No. 2003-03-03.

19 

### “Designer diagnostics for developi ng direct-injection gaso line engines,” Fansler, T.D., Drake, M.C. Proceedings of the Second International Confer nce on Optical and Laser Diagnostics, doi:10.1088/1742- 6596/45/1/001, Journal of Physics: Conference Series (2006) 45 p. 1–17.

20 

### “Measurement of Particulate Emissi ons from Gasoline Direct Injection Engines,” Chen, L. PRS Transfer Report, Oxford University, August 2008, accessed on the Internet on 2/22/2013 at the following URL: http://www.academia.edu/197486/Measurement_of_Particul ate_Emissions_from_Gasoline_Direct_Injection_E ngines

.

21 

### “Strategies to Control Particulate Emissions of Gasolin e Direct Injection Engines. ” Berkemeier, O., Grieser, K., Hohenboeken, K., Karvounis, E., Springer, K.M. Proceedings of the FISITA 2012 World Automotive Congress, Lecture Notes in El ectrical Engineering 189, p. 699-714.

22 

### "Some Implementations of the Boxplot". Frigge, M ., Hoaglin, D. C., Iglewicz, B. (February 1989). The American Statistician (1989) 43 (1) p. 50–54. doi:10.2307/2685173. JSTOR 2685173.

23 

### “Exploratory Data Analysis.” T ukey, J.W. (1977). Addison-Wesley.

24 

### "Opening the Box of a Boxplot". Benjamin i, Y. The American Statistician (1988) 42 (4) p. 257–262. doi:10.2307/2685133. JSTOR 2685133.

25 

### “Potential of spray-guided gasoli ne direct injection for reduction of fuel consumption and simultaneous compliance with stricter emissions re gulations.” Schumann, F., Sarikoc, F., Buri, S. Kubach, H., Spicher, U. IMechE International Journal of Engine Research (2013) 14 (1) p. 80-91.

26 

### “Measures to Reduce Particulate Emissions from Gaso line DI Engines.” Whitaker, P., Kapus, P., Ogris, M. Hollerer, P. SAE Internationa l Journal of Engines (2011) 4 (1) Paper No. 2011-01-1219.

27 

### “Effects of Gasoline Direct Injection Engine Operati ng Parameters on Particle Number Emissions.” Energy and Fuels (2012) 26 p. 2014-2027.

28 

### “Effect of the misture preparation on the nanoparticle characteristics of gasoline direct-injection vehicles.” Choi, K., Kim, J., Myung, C.-L., Lee, M., Kwon, S., L ee, Y., Park, S. IMechE Journal of Automobile Engineering (2012) 226 (11) p. 1514-1524.

29 

### “Multi-Stage Injection Control for Super Ultra Low Emission Vehicl es.” Kawasumi, I., Yasui, Y. Proceedings of the 2006 IEEE Internationa l Conference on Industrial Technology, p. 1206-1211.30 

### “Evaluation of a Gasoline Particulate Filter to Redu ce Particle Emissions from a Gasoline Direct Injection Vehicle.” Chan, T.W., Meloche, E., Kubsh, J., Rosenbl att, D., Brezny, R., Rideout, G. SAE International Journal of Fuels and Lubricants (2012) 5 (3) Paper No. 2012-01-1727.

31 

### “Application of Catalyzed Gasoline Particulate Filters to GDI Vehicles.” Richter, J. M. Klingmann, R., Spiess, S., Wong, K.-F. SAE Internationa l Journal of Engines (2012) 5 (3) Paper No. 2012-01-1244.