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, Michael 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 turbocharged gasoline direct injection engines in the 

United States. Recent studies have raised concerns regarding the emissions of particulate matter from vehicles equipped 

with gasoline direct injection engines. This 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. 

INTRODUCTION 

Light-duty vehicle and truck engines using gasoline 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 increased 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 applications that include improvements 

in drivability and vehicle carrying or towing capacity due to the higher available torque and availability of high torque over 

a broader range of engine speeds. Recent testing by the California Air Resources Board indicated that particulate matter 

(PM) emissions for first generation GDI vehicles were significantly 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 (GDI), or Spark Ignition Direct injection (SIDI), engines inject fuel at high pressure

directly into the combustion chamber as opposed to injecting fuel directly upstream of the intake valve as with previous 

PFI engine fuel systems. GDI requires changes to the injector 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 allows 

for higher compression ratios and increased thermodynamic efficiency without the onset of combustion knock. Recent 

injector design advances, improved electronic engine management 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 injector 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 injection). A comparison of basic layout of the two systems is 

shown in Figure 1. Figure 2 and Figure 3 show more detailed examples of wall-guided 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.4

 
Figure 2: Solid-model of the wall-guided GDI combustion 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 charge 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 allow 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 mixture formation that is more 

independent piston crown geometry. Some disadvantages include 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-guided injection systems allow the use 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 wall-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 manifold and port generated tumble and other means of inducing 

charge motion to improve in-cylinder mixing. More recently 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 immediately 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. Outwardly opening piezo-injectors have faster actuation and 

allow very precise fuel control. Piezo-injectors are also less 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 charge air cooling also reduces the fuel octane requirements 

for knock limited combustion and allows the use of higher compression 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 CO2 emissions reductions indicate that the potential for reducing CO2

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, Hyundai/Kia, Porsche, and Jaguar/Rover have been introduced in the U.S. Ford’s 

“Ecoboost” range of downsized, turbocharged, 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 power 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 primarily homogenous charge combustion with near-stoichiometric 

fueling that allows use of platinum-group-metal (PGM) exhaust catalysts for highly 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 turbocharger 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 lightoff 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 have some advantages over PFI engines with respect to emissions of gaseous 

criteria pollutants. GDI engines are more tolerant of injection 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 capability 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 with precise timing of injection events and dwell times between injection events and 

through the use of controllable cam-phasing and careful calibration of valve events and ignition timing, manufactures have 

demonstrated cold-start NMOG emissions and catalyst light-off characteristics of both 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 compounds 
With 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 impingement on the surfaces of the cylinder liner and piston 

crown.14,18,19,20 Soot does not have sufficient residence time within monolithic 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 characteristics and by absorbing 

fuel hydrocarbons which can then later combust in a soot-forming, diffusion-limited flame.21

SUMMARY EMISSIONS DATA 

Figure 4 is a summary of PM emissions for both PFI-equipped 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. The 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 the vehicles had odometer readings of approximately 20,000 to 

40,000 miles. The vehicles were tested at EPA-NVFEL in support of the draft Light-duty Tier 3 regulations. PM data 

points for 2 additional PFI (an LDT1 and LDT4) and 57 GDI vehicles were obtained from the EPA VERIFY certification 

database. More than half of the data used from VERIFY consisted of vehicle tests conducted by EPA at the EPA-NVFEL 

facility supplemented by additional data submitted by vehicle manufacturers from their own testing. The data from 

VERIFY consisted primarily of single PM tests although repeat test 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 reported tests were conducted using 

Light-duty Tier 2 certification gasoline (E0). 

The FTP and US06 PM data in Figure 4 and Figure 6, respectively, 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 emissions data for each technology. 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 over the FTP cycle were approximately 0.7 mg/mi, 2 mg/mi 

and 4 mg/mi, respectively. The sole vehicle with a fuel system 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 consistent with meeting the draft Tier 3 FTP PM standard of 3 mg/mi. Many of the 

WG-GDI vehicles also demonstrated emissions consistent with meeting the draft Tier 3 standards but most WG-GDI 

vehicles will require additional PM emissions control to comply 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 draft Tier 3 10 mg/mi LDV/LDT1/LDT2 PM 

standard. Note that all of the LDT3 and LDT4 GDI vehicles demonstrated emissions below 10 mg/mi LDV/LDT1/LDT2 

PM standard. 
Figure 4: Summary of PM emissions over the FTP 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 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 Standard
Figure 5: Summary of PM emissions over the FTP cycle for vehicles 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) standards. The development of technology to reduce PM emissions 

from GDI engines appears to be following somewhat similar trends to the technological developments that reduced soot 

formation from diesel engines over the 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 careful 

attention given to calibration of engine parameters (e.g., spark 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 development of exhaust filtration devices 

(gasoline particulate filters or GPFs) similar to those currently 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 strategies 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 Standard
1. Cold start 

2. Catalyst heating 

3. Cold transient 

4. Warm transient/near steady-state cruise 

Via careful attention to design and engine parameter calibration, 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 fuel atomization coupled with the use of multiple injections and careful calibration 

and timing of injection events to prevent fuel impingement onto 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 four injection events during operation immediately after coldstart.21,26 Similar multiple injection strategies can be used in conjunction with ignition timing retard to allow quicker catalyst 

heatup while avoiding fuel impingement and resultant soot formation. 

Current production GDI injection systems typically use maximum 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 significantly 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 enhanced 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 during warmed-up, higher-speed operation via careful attention to 

charge motion (e.g., air tumble) and use of up to three injection events per firing cycle with a WG-GDI system.26 

Berkemeir et al. found improvements via a degree of injection 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 LEV II vehicles with SG-GDI and WG-GDI are approximately 2 

times and 4-5 times, respectively, higher than PM emissions measured from Tier 2 PFI vehicles. Current SG-GDI 

vehicles demonstrated emissions below the draft Federal Tier 3 FTP PM emission standards. Both GDI combustion 

system configurations in their current forms are relatively 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 filtration) that have proven effective at reducing PM emissions of modern diesel engines 

to very low levels. Recent research has demonstrated the potential for GDI vehicles to achieve low soot formation 

comparable to modern Tier 2 and LEV II PFI vehicles over a broad range of driving conditions. The data collected by the 

EPA shows that the lowest emitting Tier 2 GDI vehicles had PM emissions approaching those of PFI vehicles. Future 

development of GDI PM emissions controls will likely include 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 the 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-guided and spray guided GDI engines can meet proposed Tier 3 PM emissions 

standards without having to resort to gasoline particulate filters. 
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