Impact of Detergent Type, Detergent Concentration, and Friction Modifiers on PM-PN Emissions in an SI Engine Using EEPS

Abstract

Three TOP TIER^TM gasoline deposit control additives (DCAs) of differing chemistries were tested for their impact on particulate matter emissions in terms of particulate mass (PM) and particle number (PN) at operating conditions representative of road load, cold start, and high load on a 2.0 L, 4-cylinder, gasoline direct injection (GDI) spark ignition (SI) engine. The PM-PN emissions were measured using an Exhaust Emissions Particle Sizer (EEPS). Deposit control additives or detergents are gasoline additives used to prevent and clean combustion chamber and injector deposits in gasoline spark ignition (SI) engines. All three gasoline additives were tested at each operating condition at three different treatment rates. In addition, one of the additives was tested with a fuel-based friction modifier (FM). The results showed that of the treatment rates tested, the lowest allowable concentration (LAC) for all additives requires the least time for the emissions to settle. However, the impact of the gasoline additives on PM-PN emissions is not linear and changes with additive concentration depending on the additive chemistry and operating conditions. The additive with the friction modifier resulted in an increase of over 19% particle number and over 30% particulate mass at the road load operating condition, while the increase at high load was over 27% for particle number and 11% for particle mass.

1. Introduction

The exhaust emissions from internal combustion (IC) engines are regulated globally for pollutants due to their adverse impact on health and the environment. Particulate matter emissions from an IC engine are linked to respiratory health and diseases related to the lungs [1,2]. With the shift in the majority of spark ignition (SI) engines from port fuel injection (PFI) to gasoline direct injection (GDI), particulate matter emissions have been a topic of focus for SI engines [3]. While GDI engines offer fuel conversion efficiency and power benefits over PFI engines [4], the particulate matter emissions from GDI engines are significantly higher than their PFI counterparts [5,6]. The higher particulate matter emission numbers in GDI engines occur primarily due to the impingement of the injected fuel on the piston and cylinder walls.

The majority of countries follow either the United States or the European Union emission regulations for PM-PN emissions. The current US emissions standards (EPA Tier 3) only regulate particulate mass, while the EU emission standards (Euro 6d) include a limit for particulate mass and particle number [7]. There is a reasonably strong correlation between PM-PN values, with previous studies reporting numbers between 2 × 10¹² and 2.5 × 10¹² particles/mg [8,9,10,11]. The current EPA Tier 3 and Euro 6d standards are shown in

Table 1. Particulate matter regulation standards.

Regulation Type	EPA Tier 3 (United States)	Euro 6d (European Union)
Particulate Mass	3 mg/mile (approx. 1.86 mg/km)	4.5 mg/km
Particulate Number	No limit	6 × 1011 particles/km

.

The Euro 6d standards measure solid particles with diameters down to D₅₀ (50% counting efficiency) of 23 nm. However, studies have shown that modern engine technologies generate particles with diameters smaller than 23 nm [3,12,13,14], which can increase the PN value. The upcoming Euro 7 standards will lower the detection threshold to a D₅₀ of 10 nm, thereby capturing a broader range of ultrafine particles in emissions evaluations.

Particulate matter emissions from an SI engine are not only a function of the engine speed and load but also of fuel composition and combustion chamber deposits (CCDs) [15,16]. Deposit control additives (DCAs) or detergents are gasoline additives that aim to reduce the buildup of combustion chamber deposits [16,17]. In gasoline direct injection (GDI) engines, DCAs also help prevent the formation of deposits on injector nozzles [18], which is critical for maintaining the proper spray pattern [3], fuel atomization, and combustion efficiency.

While DCAs mitigate PM-PN emissions, there is an optimum concentration for each additive beyond which the benefits start to diminish. Munroe et al. [16] conducted tests with three TOP TIER^TM (TT) additives and showed that, depending on the DCA chemistry, the particulate mass emissions (in mg/km) are the lowest with either the lowest allowable concentration (LAC) or 1×TT concentration and are the highest at 5×TT concentration. Chapman et al. [19] analyzed the impact of DCAs on stochastic preignition (SPI) in a turbocharged SI engine and concluded that, irrespective of additive chemistry, the SPI rate increases significantly for an additive concentration of 5×TT.

This research study looks at the impact of three TOP TIER^TM additives (the detergents or fuel additives were secured from prominent fuel additive companies and included a mix of PEA, PIBA, and Mannich chemistries. The additive names and chemistries are anonymized as requirements from the Fuels Additives Task Group (FATG), which supplied the additives) on PM-PN emissions in a 2.0 L, 4-cylinder, turbocharged GDI engine. One of the additives was also tested in combination with a friction modifier (FM) to evaluate its impact on the PM-PN emissions. The TSI Engine Exhaust Particle Sizer (EEPS) was used for emissions measurement. The EEPS system detects particles down to a diameter of 6.04 nm. Each additive was evaluated at three different concentrations—LAC, 1×TT, and 3×TT—across three distinct speed load conditions. A 5×TT concentration was excluded from the test matrix because it is not representative of real-world usage and has been shown to significantly increase SPI [19]. The PM-PN data for all additives was compared to the baseline results with unadditized gasoline to determine how the DCAs impact the particle number and particulate mass at each operating condition. The tests were devised to determine how long it took the emissions to settle at the road load condition. Additionally, the observed PM-to-PN ratios were compared to values reported in prior studies.

2. Goals and Motivation

The motivation for this project was to understand the impact of DCA concentrations on particulate mass and particle number emissions in the face of increasingly stringent global emission regulations. The study also aimed at exploring whether fuel-based friction modifiers (FMs)—which are known to reduce friction and engine wear [20,21]—can have an impact on the PM-PN emissions while measuring particles down to a size of 6.04 nm.

Project Goals:

• Develop testing procedures ensuring stability and repeatability of PM-PN emissions using the TSI EEPS system.
• Evaluate the time required for emissions to stabilize at road load.
• Determine the impact of deposit control additives and concentrations on PM-PN emissions.
• Determine the effect fuel-based friction modifiers have on PM-PN emissions.
• Calculate the PN-to-PM ratio at multiple operating conditions with the inclusion of ultrafine particles (particle diameter less than 23 nm).

3. Experimental Setup

3.1. Test Engine

A 2.0 L, 4-cylinder, General Motors LTG engine was used for this study. The GM LTG is a turbocharged engine with gasoline direct injection (GDI), representative of the modern-day SI engines commonly found in mid-sized vehicles. The instrumented engine in the test cell is shown in Figure 1, and the engine specifications are shown in

Table 2. Test engine specifications.

Parameter	Description
Model	GM LTG
Bore (mm)	86
Stroke (mm)	86
Connecting Rod Length (mm)	152.5
Wrist Pin Offset (mm)	0.6
Compression Ratio (-)	9.5
Total Displacement Volume (L)	2
Valve Train Specification	DOHC
Camshaft Phasing Ranges	31° Intake/Exhaust Cam Angle
Intake Charge Delivery	Turbocharged and Intercooled
Fuel Delivery	Direct Injection (Side Mounted Injector)
Engine Control System	MoTeC M142
Ignition System	Coil-On-Plug
Oil Cooling	Block-Mounted Cooler
Coolant	DexCool @ 50/50 Mix with Reverse Osmosis Water

.

3.2. Instrumentation and Controls

The testing for this project was conducted at Michigan Technological University’s (MTU) Advanced Power Systems Research Center (APSRC). The light-duty test cell at the APSRC is equipped with an 8000 RPM-810 Nm General Electric AC dynamometer, which is shown in Figure 1. The brake mean effective pressure (BMEP) was measured using an inline torque meter. The engine was operated at stoichiometric conditions (λ = 1). Lambda (λ) measurement was made using a Bosch LSU4.9 wide range O₂ sensor.

A MoTeC Electronic Control Unit (ECU) and software were used to control engine boundary conditions. Low-speed data was monitored and recorded using National Instruments (NI) Veristand, and combustion data was monitored using CAS 5.7 with an A&D Phoenix chassis.

3.3. Exhaust Particle Emissions Sizer (EEPS)

The Engine Exhaust Particle Sizer (EEPS) was used to measure particulate mass and particle number for this project. The EEPS schematic is shown in Figure 2. The EEPS analyzes the size and number of particles in engine exhaust in real time. The exhaust sample is first diluted in two stages, with the first stage occurring close to the exhaust at approximately 190 °C to prevent condensation and thermophoretic losses. After conditioning, the aerosol is passed through a charging region where particles are given a charge proportional to their size using a unipolar corona charger.

Charged particles then enter an annular sizing chamber with a high-voltage central electrode. Under an electrostatic field, particles migrate toward the outer wall and contact segmented electrometers based on their size and charge. Smaller particles are deflected and deposited earlier, while larger particles travel further. On contact, the particle is discharged, producing a signal from which particle number and size are derived.

Mass estimation requires assumptions about particle density, which can vary due to morphology. Final particle metrics are computed by adjusting for the known dilution ratio (DR).

Figure 3 shows the schematic for the EEPS dilution system. The dilution system was developed according to the SAE J3160 standard [22]. Inconsistencies or deviations in the dilution system can significantly impact the PM-PN emissions measurement. The prominent issues include the following:

• Coagulation;
• Thermophoretic losses;
• Nucleation;
• Impaction losses;
• Diffusion losses;
• Sample line condensation.

Figure 3. EEPS dilution system schematic.

Figure 3. EEPS dilution system schematic.

All issues mentioned above lead to either an overestimation or an underestimation of PM-PN emissions measurement. Additional details on EEPS sense checks and how improvements in the EEPS dilutions system rectified the PM-PN measurements by eliminating nucleation and impaction losses are provided in Appendix A.

4. Methodology

4.1. Operating Conditions

Three operating conditions were run as a part of the study for the baseline as well as each additive tested. The speed load conditions selected were indicative of road load, cold start, and high load conditions for a 2 L turbocharged gasoline SI engine.

• Road Load: 2200 RPM-4.75 bar BMEP;
• Cold Start: 1400 RPM-2.25 bar BMEP;
• High Load: 3000 RPM-16 bar BMEP.

Additional details regarding all three operating conditions are shown in

Table 3. Operating condition details.

Operating Condition	ICL	ECL	Fuel Rail Pressure	SOI	EOI2	Fuel Split Factor	Lambda	Spark	IAT	Coolant Temp.	Oil Temp.
Units	°From LP	°From LP	Bar	°bTDC	°bTDC	--	--	°bTDC	°C	°C	°C
Road Load	20	13	184	277	--	--	1	MBT	30	90	90
Cold Start	10	16	90	240	56	0.6/0.4	1	−12	30	40	40
High Load	11.4	5.2	200	288	--	--	1	Knock Limit	35	90	90

.

All parameters except spark timing, shown in Table 3 for each operating condition, were as per the base calibration for the engine. A split injection strategy was deployed for the cold start condition, where 60% of the fuel was injected in the first injection event and 40% in the second injection event. The spark timing for the cold start was retarded until the COV of IMEP was between 8 and 10%.

4.2. Test Procedure

A procedure was developed to ensure uniformity in testing all additives for PM-PN emissions. The test procedure, which spanned over three days for each test run, can be divided into two parts, the first of which is shown in Figure 4 and the second in Figure 5.

Figure 4 details the first day of testing with a new additive. The combustion chamber was cleaned of any deposits by running port water injection with unadditized Tier 3 premium certification fuel, using a 0.4 water/fuel ratio. This was followed by borescope images of the pistons and cleaning of the injectors using an ultrasonic cleaner to eliminate any deposit remnants from the previous test. Two oil changes, along with an oil filter change, were performed to prevent cross-test contamination. The piston cleaning and fuel flushing points were run at 3000 RPM-10 bar BMEP. A reference point was run at the road load condition to check the repeatability of the EEPS before switching to the additized fuel.

The engine was run at the road load condition until the PM-PN emissions ‘settled’. Details regarding the settling time criteria have been included in the next section. Once the emissions were settled, each test condition was run for two hours. PM-PN data was collected continuously over the settling time and test points and was discretized in 15 min blocks. For example, if data was collected for 120 min, it was converted to eight datasets averaged over 15 min intervals. The first test day was run with the LAC treat rate for the additive in question.

The test procedure for the second and third test days, which involved running the additive at the 1×TT and 3×TT concentrations, respectively, is shown in Figure 5. The major difference in this procedure is that it does not involve the piston cleaning and fuel flushing phase. Every subsequent test day involved running at a higher treat rate of the same additive for which cleaning using water injection and oil changes were not required, as there was no chance of additive cross-contamination. The remainder of the procedure for running 1×TT and 3×TT was the same as running LAC.

4.3. Settling Period

At the start of each test day, the engine was operated at road load conditions until particulate mass (PM) and particle number (PN) emissions stabilized—a phase referred to as the ‘settling period’. Since the combustion chamber was cleaned using water injection and the injectors were ultrasonically cleaned prior to each additive test, the system began with minimal combustion chamber deposits (CCDs). This led to significant variability in initial emissions when introducing a new additive or a higher additive treat rate concentration.

The purpose of the settling period was to allow CCDs to re-establish and for PM-PN emissions to reach a steady state representative of real-world deposit accumulation. Data was recorded continuously during this phase and averaged over 15 min intervals.

To define when emissions were considered ‘settled’, a threshold of ±10% variation was applied. Specifically, the PM and PN values in the most recent 15 min interval had to remain within 10% of the values from each of the preceding six 15 min intervals (i.e., 90 min of stable emissions). This approach is illustrated in Figure 6, which plots the percent difference in the final data point against earlier intervals. When this variation stayed within the 10% range for both PM and PN metrics, the emissions were deemed stable, and steady state conditions were confirmed. The 10% value was based on variability in the observed data. The important consideration was to apply the same criteria to all tests for fair comparison. Additional details regarding the settling period are provided in Appendix A.

The threshold for the settling period was 6 h. In the case of a particular test, if the additive did not meet the settling criteria within 6 h, the engine running was transitioned to the cold start operating condition without the settling criteria being met. This approach was adopted to maintain uniformity in the testing by completing each test on the same day. Additionally, it was recognized that for certain additive concentrations, PM-PN emissions may not settle at all. This could be due to fuel–additive interactions, which can lead to an ongoing cycle of deposit buildup and cleanup. Figure 7 shows the variation in particle number, and Figure 8 shows the particulate mass emissions for Additive 2 for the LAC treat rate. The settling time for this test was 195 min. Each data point represents the time at which the data block stopped recording. For example, the data point at 15 min represents the average data from 0 to 15 min.

For all tests, the rate of change in average particulate mass (dPM/dt) and average particle number (dPN/dt) emissions was observed to be the highest at the start of the test, as Figure 7 and Figure 8 indicate. The rates decreased as the test proceeded, indicating a transition towards steady state behavior.

4.4. Test Fluids

Environmental Protection Agency (EPA) Certification Tier III Premium fuel (unadditized) was used as the base fuel for the project. The Tier III fuel is a fuel with a relatively high particulate matter index (PMI), which promotes deposit buildup, and was chosen for this project as a high PMI fuel was expected to better distinguish the effect of DACs.

The piston cleaning, fuel flush points, and baseline tests were run using unadditized EPA Certification Tier III fuel. The additized tests were run with the Tier III fuel mixed with the DCAs at the appropriate concentration. The additives were measured on a precision weighing scale with a resolution of 0.1 g and mixed with the fuel using a pump for 30 min to ensure thorough mixing. For the smallest amount of additive measured (Additive 3 at LAC concentration), the accuracy of the mixture for a measurement error of 0.1 g is over 99%.

Dexos1GEN3 oil (reference # 23-22981 RN006850AAC) was used for all tests for this study.

All fuel and oil used for the project were from the same batch, to ensure that batch-to-batch variation did not impact the results.

4.5. Test Matrix

The test matrix for the project is shown in Figure 9. A baseline was run with the unadditized fuel at the start and end of the test matrix. Each additive was run with three different concentrations in the same order.

• LAC;
• 1×TT;
• 3×TT.

Figure 9. Test matrix.

Figure 9. Test matrix.

A reference check at the road load operating condition was performed using the cleaning fuel before the start of a new additive to ensure that the EEPS output was consistent.

4.6. Density Correction for Particulate Mass Calculation

The EEPS estimates the particulate mass based on the particle number, as previously mentioned. The lower-diameter particles have the same density as carbon. When these small spherical particles aggregate together, the resulting shape is not necessarily a sphere, as shown in Figure 10. Park et al. [23] demonstrated this by examining particulate matter through a transmission electron microscope. The EEPS, however, assigns the particle diameter based on the mobility diameter. This leads to an overestimation of the mass contribution of higher diameter particles and an underestimation of the mass contribution of lower diameter particles.

Considerable research hours have been invested in rectifying the particulate mass contribution of diameters by comparing the EEPS particulate mass output to the reference gravimetric method used in the Federal Test Procedure (FTP) [24,25,26]. This study used an effective density vector based on the fit equation provided by Xue et al. [26] for GDI engines to fit data from Maricq and Xu [24].

5. Results

5.1. EEPS Reference Checks

The EEPS was checked for repeatability by running the road load condition for 5 min using unadditized fuel before the start of every baseline or additized run. This was performed after the piston cleaning with water injection, which ensured the uniformity of conditions during the reference checks. The total PM-PN emissions for the EEPS reference checks are shown in Figure 11. The same data as a function of diameter is shown in Figure 12.

The data from the EEPS reference checks shows good repeatability for PM-PN emissions. The coefficient of variation (COV) of PN measurements was 5.7%, while the COV of PM measurements was slightly higher at 11.5% due to the high particulate mass value before Additive 2 testing. Figure 12 sheds light on the reason for the high particulate mass value, which is an increase in higher-diameter particles.

The authors believe this is possibly due to the accumulation of large-diameter particles at the exhaust location for the EEPS sampling line. During the piston cleaning procedure using water injection, the sampling line was closed to prevent water from entering the EEPS system. When the line was opened after the piston cleaning and subsequent oil flush, the accumulated high-diameter particles entered the EEPS, increasing the PM count.

Notably, the particle number was not significantly affected, as the number of particles was not large enough to register prominently in the PN plot. However, the increased particle diameter significantly influenced the PM measurement.

To eliminate the effect of infrequent high mass particles, a positively skewed non-negative Weibull distribution was used to predict the mass of particles over a particle diameter of 191.1 nm for all six datasets. This brought the COV of PM measurements down to 5.9%, which is very close to the COV of PN measurements. Additional details have been included in the Appendix.

5.2. Settling Time

As mentioned in Section 4, each test day started with a settling period until the PM-PN emissions reached a steady state according to the PM-PN criteria. Figure 13 shows the settling times for all tests. The baseline shown in the figure is the average of the two baseline runs. The threshold for settling was 6 h, so any test with a 360 min or 6 h settling time did not reach steady state at the road load operating condition.

The results show a trend of settling with additive concentration. For all four additives, the LAC treat rate had the lowest settling time. For Additive 1 and Additive 2, the settling time increased with increasing additive concentration. The PM-PN emissions did not ‘settle’ or reach steady state for 1×TT and 3×TT treat rates for the Additive 3 + Friction Modifier combination, as well as for Additive 1 for a 3×TT treat rate.

It should be noted that a lower settling time does not necessarily translate to lower PM-PN emissions. A test that reached steady state earlier could have higher PM-PN emissions than a test that took more time to settle, as will be evident in the next section.

5.3. PM-PN Emissions

Data was collected throughout the duration of testing and averaged over 15 min intervals. For the results shown in this section, the PM-PN numbers for road load are shown as an average of 90 min (1.5 h) of running, while the numbers for cold start and high load are for the first 15 min of running after the engine reached thermal equilibrium (stable intake air temperature, coolant temperature, and oil temperature), which is explained in Figure 14. This was to reflect real-world scenarios, where vehicles run at the road load condition for long periods, while cold start and high load conditions are run only for short periods.

6. Road Load Condition

The variation in particle number and particulate mass as a function of additive type and additive concentration is shown in Figure 15. The two baselines are shown at the beginning, followed by each additive going from the lowest (LAC) to the highest (3×TT) treat rate. All values reflect the average emissions over a 90 min period, measured after PM-PN emissions stabilized and the engine reached steady state conditions. The dashed line on the plots indicates the average of the two baseline runs and the dotted lines indicate one standard deviation. For cases where the two baseline averages were within 1%, the standard deviation lines have not been included.

The results indicate that all three deposit control additives (excluding the friction modifier) reduced the particulate mass (PM) as compared to the baseline. Except for Additive 1 at the 3×TT concentration, the particle number (PN) for the DCA runs was also lower than the baseline. The interesting thing to note is that the reduction is not linear, and each of the three DCAs has an optimum treatment rate at which the PM-PN emissions were minimized.

• Additive 2 showed the highest PM-PN emissions at the 1×TT treat rate.
• Conversely, Additive 1 and Additive 3 showed the lowest emissions at the 1×TT treat rate.

These results are in agreement with Monroe et al. [16], who showed that the optimum treatment rate is dependent on additive chemistry. The underlying mechanisms for the impact of specific additive chemistries were beyond the scope of this study.

The major takeaway from these results is the pronounced impact of the friction modifier on PM-PN emissions. For all three concentrations of Additive 3 plus the FM, the PM-PN emissions were significantly higher than those of the baseline runs and of Additive 3 without the FM. As illustrated in Figure 13, this was the only additive combination for which emissions did not settle within the 6 h limit at the 1×TT concentration, indicating prolonged emission instability. Comparing the PM-PN numbers for Additive 3 alone with the Additive 3 plus FM combination, the friction modifier appears to be the factor that spikes the PM-PN emissions. The increase in PM-PN emissions with respect to the average of the two baseline runs is quantified in

Table 4. Increase in PM-PN emissions due to the Additive 3 + Friction Modifier combination at the road load condition (compared to the average of the two baseline runs).

Concentration	Particle Number Increase (%)	Particulate Mass Increase (%)
LAC	19.0	30.1
1×TT	62.7	73.6
3×TT	56.2	80.1

.

To ensure the spike in emissions was not erroneous and there was no issue with the EEPS output, the LAC test for the Additive 3 plus FM was run twice. After the first run, when the numbers appeared much higher than the rest of the additives, the EEPS was serviced to eliminate the possibility of an instrument-related error. The repeat test resulted in similarly high numbers, reinforcing the initial findings. This step, combined with the EEPS reference checks before each test and the lower Baseline 2 results at the end of the test matrix, bolstered the observations from the Additive 3 plus FM run.

7. Cold Start Condition

The cold start emission numbers are the average of the first 15 min block after the engine reached thermal stability. Based on empirical data, the time required to reach thermal stability for the cold start and the high load conditions was typically 30 min. So the data shown for the cold start and high load conditions is the average from 30 min to 45 min after transitioning from one load condition to the next.

The cold start emissions as a function of additive type and additive concentration are shown in Figure 16. The differences between the baseline runs and the DCA runs are not stark, even for the Additive 3 plus FM combination.

Compared to the average of the baseline tests, most DCA tests show a slight reduction in the PM-PN emissions at the cold start condition. Unexpectedly, the lowest PM-PN emissions are shown by the Additive 3 plus FM at the LAC treat rate.

8. High Load Condition

The PM-PN emissions for all additive types and additive concentrations at the high load condition are shown in Figure 17. The high load emissions show no particular trend. The common theme is that the 3×TT treat rate reduces the PM-PN emissions as compared to the baseline runs, irrespective of additive chemistry for all DCAs without the FM. The authors believe there could be a unique balance between the deposit creation rate and cleaning ability of the DCA at the high in-cylinder pressures and temperatures, and the 3×TT concentration keeps the combustion chamber clean and minimizes PM-PN emissions at the high load condition.

All treat rates with the friction modifier increase PM-PN emissions with respect to the baseline for the high load condition, similar to the road load condition.

Table 5. Increase in PM-PN emissions due to the Additive 3 + Friction Modifier combination at the high load condition (compared to the average of the two baseline runs).

Concentration	Particle Number Increase (%)	Particulate Mass Increase (%)
LAC	48.5	51.7
1×TT	27.3	10.9
3×TT	39.2	39.4

shows the increase in PM-PN emissions at high load for the Additive 3 + Friction Modifier combination.

9. Particle Number—Particulate Mass Ratio

As stated in the introduction, a multitude of studies have reported correlations between particle number and particulate mass. The numbers are in the range of 2−3.8 × 10¹² particles/mg. The numbers observed in this study are shown in

Table 6. Average particle number to particulate mass ratio for the two baseline runs for all three operating conditions.

Particle Number to Particulate Mass Ratio (Particles/mg)
Road Load (90-min Average)	Cold Start (15-min Average)	High Load (15-min Average)
1.27 × 1013	1.3 × 1013	9.99 × 1012

as an average of the Baseline 1 and Baseline 2 values. The PN to PM ratio is 4 to 5 times higher at all speed load conditions as compared to what has been reported in previous studies. While Table 6 only shows the average ratio for the two baseline runs, the ratio did not change substantially for the additized tests.

The high PN/PM ratio observed in this study could be attributed to the inclusion of ultrafine particles (<23 nm), made possible by the use of an EEPS without a volatile particle remover (VPR), as used by Yamada et al. [11].

Table 7. Average particle number to particulate mass ratio for the two baseline runs for all three operating conditions, excluding ultrafine particles (<23 nm diameter).

Particle Number to Particle Mass Ratio (Particles/mg)
Road Load (90 min Average)	Cold Start (15 min Average)	High Load (15 min Average)
6.31 × 1012	5.96 × 1012	5.74 × 1012

shows the PN/PM ratio with particles lower than 23 nm excluded in the calculation. With the exclusion of ultrafine particles, the PN/PM ratio is closer to what has been reported in previous studies, although it is still 2 to 3 times higher.

Particles with diameters less than 23 nm are excluded from regulated PN measurements for emissions standards like the Euro 6d, based on the Particle Measurement Program (PMP) protocols that include a VPR to remove semi-volatile particles [22]. These particles contribute significantly to real-world particle numbers and could have a relevance when it comes to respiratory health. The PN-PM correlations will possibly change once new standards are implemented that count particles down to 10 nm. Additional work is required to determine the impact of a VPR on the PN/PM ratio.

10. Conclusions

A 2.0 L, 4-cylinder GDI SI engine was run at road load, cold start, and high load conditions to determine the impact of detergent (fuel additive) type and concentration on the PM-PN emissions. The following conclusions can be drawn based on the results:

• EEPS repeatability: The Exhaust Emissions Particle Sizer (EEPS) showed good repeatability throughout the duration of the project, with a COV of less than 6% for the PM-PN emissions.
• Settling time: The majority of tests achieved emissions stabilization within 6 h of running at road load. The settling time increased with additive concentration. The additive with the friction modifier was the only one for which the PM-PN emissions did not settle for a 1×TT treat rate.
• Road Load: At the road load condition, the three deposit control additives (DCAs) without friction modifiers reduced particulate mass (PM) and particle number (PN) emissions compared to the baseline, but the effect varied nonlinearly with concentration and additive chemistry. The inclusion of the friction modifier caused a substantial increase in the PM-PN emissions for all treat rates. The increase in particle number at the LAC, 1×TT, and 3×TT concentrations of the Additive 3 plus Friction Modifier combination was 19%, 63%, and 56%, respectively, while the increase in particulate matter was 30%, 74%, and 80%, respectively.
• Cold Start: The majority of additive and treat rate combinations displayed a reduction in PM-PN emissions. This includes the Additive 3 plus Friction Modifier combination.
• High Load: No specific trend was observed at the high load condition. For all additives without the FM, the 3×TT treatment rate had lower emissions as compared to the baseline. The Additive 3 plus Friction Modifier combination showed an increase in the PM-PN emissions at the high load condition, with the LAC treat rate showing the highest increase (49% PN emissions, 52% PM emissions).
• PN-to-PM ratios: The measured PN-to-PM ratios were 4–5 times higher than the previous literature values (approximately 2.5 × 10¹² #/mg), primarily due to the inclusion of particles below 23 nm in the EEPS measurements without a volatile particle remover (VPR). The discrepancy is reduced with the exclusion of ultrafine particles in the PN value, although the PN/PM ratios observed in this study are still 2–3 times higher.

The findings from this study have significant practical applications in the development of deposit control additives for use in GDI engines. The conclusions suggest that the optimum concentration for a fuel additive for minimum PM-PN emissions depends on the chemistry and is not necessarily a linear relationship. The data also points to the fact that fuel-based friction modifiers can have an adverse effect on the PM-PN emissions at road load and high load conditions, although additional work on FM chemistry and comparison with oil-based FMs is necessary to bolster this claim. Additionally, the study demonstrates the benefits of the TSI EEPS for the measurement of sub-23 nm particles to help comply with upcoming regulations and provide deeper insights into particle formation trends.

11. Potential Future Work

The prominent finding from this study is the increase in PM-PN emissions from the fuel-based friction modifier. An investigation into the underlying mechanisms behind the observed increase was out of the scope of this study but merits exploration in future studies. The factors impacting particulate matter emissions are detailed by Maricq [3], which include piston and wall wetting, fuel additive properties, volatility, and injector deposit formations. An adverse impact of the FM on these factors could be the reason for the observed increase in PM-PN emissions.

Future work on this subject can aim to isolate the impact of FMs on factors that affect PM-PN emissions. A comparison can also be made between the PM-PN emissions due to different FM chemistries, as well as the difference between fuel-based and oil-based FMs.