Particulate Formation in GDI Engines

The need to comply with stringent emission regulations while improving fuel economy and reducing criteria pollutant emissions from transportation presents a major challenge in the design of gasoline Direct Injection (DI) engines because of the adverse effects of ultrafine Particulate Number (PN) emissions on human health and other environmental concerns. With upcoming advances in vehicle electrification, it may be the case that electric vehicles completely replace all current vehicles powered by internal combustion engines ensuring zero emissions. In the meantime, Gasoline Direct Injection (GDI) engines have become the primary mode of transportation using gasoline as they offer better fuel economy while also providing low CO2 emissions. However, GDI engines tend to produce relatively high PN emissions when compared to conventional Port Fuel Injection (PFI) engines, largely because of challenges associated with in-cylinder liquid fuel injection.\ua0Cold-starts, transients, and high load operation generate a disproportionate share of PNemissions from GDI engines over a certification cycle. The mechanisms of PN formation during these stages must therefore be understood to identify solutions that reduce overall PN emissions in order to comply with increasingly strict emissions standards.This work presents experimental studies on particulate emissions from a naturally aspirated single cylinder metal gasoline engine run in a homogeneous configuration. The engine was adapted to enable operation in both DI and PFI modes. In PFI mode, injection was performed through a custom inlet manifold about 50 cm from the cylinder head to maximize the homogeneity of the fuel-air mixture. The metal head was eventually modified by incorporating an endoscope that made it possible to visualize the combustion process inside the cylinder. The experimental campaigns were structured to systematically isolate and clarify PN formation mechanisms. Tests were initially performed in steady state mode to obtain preliminary insights and to screen operating conditions before\ua0conducting transient tests. Particulate emissions were measured and correlated with theimages obtained through endoscope visualization where possible.Key objectives of these studies were to find ways of reducing PN formation by increasing combustion stability. It was found that by avoiding conditions that cause wall wetting with liquid fuel, PN emissions can be substantially reduced during both steady state operation and transients. Warming the coolant and injecting fuel at later timings reduced PN emissions during warmup and cold transient conditions. Additionally, experiments using fuel blends with different oxygenate contents showed that the chemical composition of the fuel strongly influences particulate formation under steady state and transient conditions, and that this effect is load-dependent.Overall, the results obtained in this work indicate that wall wetting is the dominant cause of particulate formation inside the cylinder and that fuel-wall interactions involving the piston, cylinder walls, and valves during fuel injection account for a significant proportion of PN emissions in the engine raw exhaust

Particulate Formation in Gasoline Direct Injection Engines

Gasoline direct injection (GDI) engines are facing a great challenge because of the need to comply with increasingly stringent emission regulations while improving fuel economy. GDI engines are popularly known for their high fuel efficiency (by the standards of\ua0gasoline engines) and low emissions, enabling higher compression ratios and thus increased volumetric efficiency. Unfortunately, GDI engines tend to produce higher particulate number (PN) emissions than conventional port fuel injection (PFI) engines, mainly due to the challenges of in-cylinder liquid fuel injection. Cold starts, transients, and high loads account for a disproportionately high share of all PN emissions from GDI engines over a certification cycle. Understanding the mechanisms of PN formation during these\ua0stages is necessary for the further market penetration of GDI under the constraint of tighter emission standards. This knowledge becomes especially important when in future particles with sizes smaller than 10 nm are measured and legislated.This work presents experimental investigation of particulate emissions from a naturally aspirated single cylinder metal gasoline engine operated in a homogeneous configuration. The engine was modified to be capable of operating using DI, PFI, or both simultaneously. PFI was configured with a custom inlet manifold to inject about 50 cm upstream of cylinder head, forming a more homogeneous fuel-air mixture than would otherwise be possible. The experimental campaigns were structured to systematically isolate and study different PN formation mechanisms. Mixing quality was improved\ua0 ubstantially by using a small amount of upstream injection together with direct injection and could be controlled by varying the mass split between the direct and upstream injectors. It was found that using a small upstream injection when operating in GDI mode could reduce PN emissions by up to a factor of 10 while only modestly increasing fuel consumption.The chemical composition of the fuel could also strongly affect particulate emissions. Therefore, to find alternative ways of reducing PN emissions, experiments were conducted using a gasoline engine with fuel blends containing renewable oxygenates – either 10% (v/v) ethanol (EtOH) or 22% (v/v) ethyl tert-butyl ether (ETBE). It was observed that PN emissions was reduced using oxygenated fuels at low load for both PFI and DI operation, but not at higher loads where PN increased instead. Measurements of solid PN (SPN) emissions revealed that more soot was formed at high load along with an increase in emissions of volatile organic compounds (VOC).PN measurements were conducted using a DMS500 fast particle spectrometer supplied by Cambustion. In addition, solid particulate measurements were performed by passing exhaust samples through a thermodenuder and a catalyst to remove most of the volatile organic compounds (VOCs) from the raw emissions. The results indicated that wall-wetting is the dominant particulate formation mechanism inside the cylinder: fuel-wall interactions with the piston, cylinder walls, and valves during the fuel injection period account for a significant fraction of the PN content of raw exhaust

Soot Sources in Warm-Up Conditions in a GDI Engine

Gasoline direct injection (GDI) engines usually emit higher levels of particulates in warm-up conditions of a driving cycle. Thus, sources of soot formation in these conditions were investigated by measuring particulate numbers (PN) emitted from a single-cylinder GDI engine and their sizes. The combustion was also visualized using an endoscope connected to a high-speed camera. Engine coolant and oil temperatures were varied between 15 and 90oC to mimic warm-up conditions. In addition, effects of delaying the start of ignition (SOI) on the emissions in these conditions were examined. Coolant and oil temperatures were varied individually to identify which factor has most effect on PN emissions. While coolant temperature strongly influenced PN with cold oil, the oil temperature insignificantly affected PN at low coolant temperature. These findings indicate that PN emissions are heavily dependent on the engine block\u27s temperature, which is dominated by the coolant. SOI plays a significant role in PN formation because it influences the wall film thickness on the piston top. In the experimental warm-up conditions, injecting fuel at a later SOI was found to decrease PN emissions. Visualization showed no occurrence of diffusion flames at late SOI timings due to the associated reduction in interaction between liquid fuel and the piston. The integrated luminescence from combustion images was found to correlate closely with PN emission measurements. Thus, higher integrated luminescence, indicating higher soot formation due to pool fires on the piston top, was associated with higher PN levels. When coolant and oil temperatures were both varied, PN emissions were found to decline dramatically with increasing temperatures. At lower temperatures, diffusion combustion occurred on the piston due to persistence of a non-vaporized fuel film. At 15oC coolant and oil temperature, this phenomenon was strong, but it gradually declined as the temperature increased. When the temperature reached 60oC, diffusion flames started to disappear, resulting in a dramatic decrease in PN

Visualization of soot formation in load transients during GDI engine warm-up

Reducing the emissions of pollutants, and particularly soot particles, from internal combustion engines is one of the greatest challenges faced by car manufacturers. Although modern gasoline direct injection (GDI) engines produce relatively low particulate emissions during steady state operation under near-stoichiometric conditions, they can produce much higher particulate emissions during transients that cause abrupt changes in load, fuel consumption, and the air-to-fuel ratio. Emissions during transients are particularly high when the engine coolant temperature is low, as occurs during engine start-up. Consequently, there is a need to find ways of reducing particulate emissions during load transients. This paper therefore investigates particulate formation during load transients in a single-cylinder GDI engine equipped with an endoscope in the cylinder head. A transient sequence was designed in which the engine load was increased from 4 bar NMEP to a maximum of 12 bar NMEP in 2 s at an engine speed of 2000 rpm. During the transients, the engine’s particulate emissions were measured in terms of particulate number (PN) and images of the combustion process inside the cylinder were captured via the endoscope using a high-speed camera to identify locations where soot formation occurred. Experiments were conducted at a range of coolant temperatures and using different injection strategies to determine how these parameters affect PN emissions. The coolant temperature was found to be the dominant factor governing PN emissions during transients. Luminescence data obtained by analyzing the flame images agreed well with the measured PN emissions during transients. Under all varied parameters in the transients except delayed injection, soot was mainly formed from wall films. For transients with delayed fuel injection, much of the piston film could be avoided but soot formation instead became mixing-dominated. Variation of the air-fuel ratio had little effect on PN emissions during transients. At all coolant temperatures, PN emissions were lowest when using a split injection strategy but delaying the injection timing increased PN emissions even though the endoscope images suggested a lower frequency of diffusion flame formation. No conditions were found under which the PN emissions during transients with low coolant temperatures could be reduced to levels comparable to those seen with warm coolant

History Effect on Particulate Emissions in a Gasoline Direct Injection Engine

Soot formation in internal combustion engines is a combination of complex phenomena. Understanding the formation mechanism that influences particulate emissions can help to make gasoline direct injection (GDI) engines comply with increasingly stringent emission standards. It is generally accepted that the deposition of liquid fuel wall films in the combustion chamber is a significant source of particulate formation in GDI engines. The injection timing, which can help avoid interaction between the pistons and fuel spray, has been identified as the parameter with the greatest influence. Traditionally, the start of injection (SOI) sweeps one can find in the literature are carried out by changing the timing one value at a time. To quantify the influence of SOI, variations in our study were carried out in a novel way using cycle-to-cycle parameter control. Instead of motoring or turning off the engine between different SOI variations, the motor was run continuously with combustion and SOI sweeps carried out online in a series of preprogrammed perfectly deterministic SOI sequences to provide evidence of so-called history effects on particulate number (PN). The variation in SOI produces a change in engine combustion and liquid fuel impingement, leading to a state that acts as a precursor for the next state. The different preprogrammed sequences provided excellent data repeatability between engine runs but very different results, depending on the order in which the SOI timings were set. In-cylinder combustion was visualized with an endoscope connected to a high-speed camera. Two SOI timings were chosen (based on piston deposit level data from stationary measurements) to investigate the history effect of preceding conditions on PN. The results show that the preceding engine states influence PN formation and emission that is established as history effect in the study. The history effect is pronounced and was most noticeable under impinging conditions such as early injection timings like -340 crank angle degrees (CAD). History effect was also found to depend on the duration and SOI of the preceding state. More importantly, the history effect depends on how SOI is varied, which in turn influences PN emissions. In the cycle-to-cycle variation of SOI, PN levels at relatively later injection timing of -250 CAD resulted in similarly high levels at an early injection timing of -340 CAD

Effect of Renewable Fuel Blends on PN and SPN Emissions in a GDI Engine

To characterize the effects of renewable fuels on particulate emissions from GDI engines, engine experiments were conducted using EN228-compliant gasoline fuel blends containing no oxygenates, 10% ethanol (EtOH), or 22% ethyl tert-butyl ether (ETBE). The experiments were conducted in a single cylinder GDI engine using a 6-hole fuel injector operated at 200 bar injection pressure. Both PN in raw exhaust and solid PN (SPN) were measured at two load points and various start of injection (SOI) timings. Raw PN and SPN results were classified into various size ranges, corresponding to current and future legislations. At early SOI timings, where particulate formation is dominated by diffusion flames on the piston due to liquid film, the oxygenated blends yielded dramatically higher PN and SPN emissions than reference gasoline because of fuel effects. For particulates >23 nm and with optimized SOI timing, the use of oxygenated blends significantly increases SPN and conversely decreases raw PN emissions at low load (4.5 bar IMEP). At high load (9 bar IMEP), overall SPN emissions were significantly higher and there were no clear differences between the blends. Additionally, SPN measurements showed that soot formation and emissions of volatile organic compounds (VOC) depended strongly on blend composition. Finally, adding oxygenates (up to 22%) to gasoline did not reduce emissions of SPN in the size ranges addressed by current regulations

Particulates from a CNG DI SI Engine during Warm-Up

To assist efforts reducing harmful emissions from internal combustion engines, particulate formation was investigated in a compressed natural gas (CNG) Direct Injection single-cylinder SI engine in warm-up conditions. This involved tests at low engine speed and load, with selected engine coolant temperatures ranging from 15 to 90 \ub0C, and use of a gasoline direct injection (GDI) system as a standard reference system. Total particulate number (PN), their size distribution, standard emissions, fuel consumption and rate of heat release were analyzed, and an endoscope with high-speed video imaging was used to observe combustion luminescence and soot formation-related phenomena.The results show that PN was strongly influenced by changes in coolant water temperature in both the CNG DI and GDI systems. However, the CNG DI engine generated 1 to 2 orders of magnitude lower PN than the GDI system at all tested temperatures. The PN decreased in both systems when the coolant temperature increased. The results also show that PN was sensitive to a broader engine coolant temperature range in the GDI system. However, PN was around two orders of magnitude higher at the lowest coolant temperature (15 \ub0C) than at the highest temperature (90 \ub0C) in the CNG DI system. In homogeneous CNG combustion (unlike gasoline combustion) high-speed video images revealed no diffusion or yellow flame anywhere in the cylinder, even at the lowest coolant temperature. Thus, no soot formation location could be determined from the images in CNG cases. Overall, engine measurements showed that the CNG DI engine emitted lower standard emissions (CO2, CO, HC, NOx) and PN than the GDI system across the experimental range of engine coolant temperatures

Particulate Emissions in a GDI with an Upstream Fuel Source

Public health risk and resulting stringent emission regulations for internal combustion engines pose a need for solutions to reduce particle emissions (PN). Current PN control approaches include increasing fuel injection pressure, optimizing spray targeting, multiple injection strategies, and the use of tumble flaps together with gasoline particulate filters (GPF).Experiments were performed using a single-cylinder spark-ignited GDI engine equipped with a custom inlet manifold and a port fuel injector located 500 mm upstream. Particulate emissions were measured during stationary medium/high load operation to evaluate the effect of varying the mass split between the direct and upstream injectors. Mixing quality is improved substantially by upstream injection and can thus be controlled by altering the mass split between the injectors. Additional particulate measurements were performed using a thermodenuder and a catalyst to remove major part of the volatile organic compounds (VOCs) from raw emissions. This made it possible to determine particle numbers (PN) both raw emissions and solid particulates, and the size distribution of the solid particulate emissions.Upstream fuel source was found to reduce PN emissions by almost a factor of 10 under optimal conditions, and significant reductions were achieved even when only 10% of the fuel mass was injected upstream. At a fixed load, as mass percentage from PFI increases, PN decreases. However, the PN reduction due to PFI is load-dependent and can be sensitive to engine speed. Solid PN decreased almost linearly with the PFI mass percentage, independently of engine speed. This implies that upstream injection improved mixing and thus reduced rich zone formation and/or wall-wetting compared to exclusive direct injection

Particulates in a GDI Engine and Their Relation to Wall-Film and Mixing Quality

This paper investigates how particulates number PN is influenced by fuel wall-film, liner wetting, and the mixing quality for different start of injection timings (SOI). Both experimental data with PN measurements, endoscope images from a high-speed camera from a single-cylinder engine, and CFD simulations were used for the analysis. Engine geometry was a spray-guided system with 300 bar fuel pressure and with single injections. Data was captured for 2000 rpm / 9 bar IMEPn. The results show that fuel film on the piston was only found to significantly increase PN for over-advanced SOI (in our engine geometry, earlier than -310 CAD). This results in luminescence from diffusion burn on the piston surface, which strongly contributes to PN. For an SOI timing of -310 CAD, fuel film on piston reaches a maximum of 3#x00025; of the injected fuel, vaporizes, and no remaining fuel film is found at the time of ignition. Approximately 0.5-1#x00025; of the fuel ends up on the liner. Because of the slower evaporation, the liner film is exposed to scraping by the piston rings late in the compression stroke. For tested SOI timings of -310 CAD and later, all piston film evaporates before combustion, and the mixing quality starts dominating the PN formation. The mixing time has the strongest effect, leading to the reduction in PN with earlier SOI up until -310 CAD. Spray-tumble flow interactions are also shown to have appreciable effects on the mixing quality, and the usefulness of these interactions varies depending on the SOI