Study of Gasoline Particulate Filter to Reduce Particulate Emissions from Spark Ignited Direct Injection Vehicle

Study of Gasoline Particulate Filter to Reduce Particulate Emissions from Spark Ignited Direct Injection Vehicle

ABSTRACT

Increased fuel efficiency and high power output are the main assets of Spark Ignited Direct Injection Engine (SIDI). Likewise, Diesel Particulate Filter (DPF) in diesel engines is an efficient way for reducing the particulate emissions, Gasoline Particulate Filter (GPF) could also play the same role for SIDI engine. Investigation and benefits of introducing the GPF into the SIDI engine with two methods is proposed in this paper. The particle diameters for SIDI engine used to be in the range of 50-70 nm over both US06 and FTP-75 drive cycles. The filtration efficiency over the FTP-75 and US06 cycles was 82 and 76 % for SIDI engine. The first method analyzes the effect of filtration efficiency, pressure drop and particulate matter regeneration on the new design concept of GPF. Second method focuses on computation of gaseous compounds, particle number and their size distribution and comparison is carried out between Port fuel Injection (PFI) and SIDI vehicles for gasoline (E0) and 10% ethanol gasoline (E-10) fuel.

INTRODUCTION

In the today’s era, protection of environment and its resources has become a very serious issue. According to CAIT 2.0, global CO2 emissions were 150 times higher in 2011 than they were in 1850. From the NOAA media release data, for the past ten years (2005-2014), the average rate of CO2 increase is 2.11 ppm which is more than double the increase in 1960s. The world automakers are now shifting towards the SIDI vehicles instead of PFI engines. The couple of reasons are the potential savings from fuel efficiency and the potential performance boost, especially if used in concert with a turbocharger.  

Particulate Matter (PM) is a type of pollutant which is under special concern, is a complex physicochemical mixture of solid and volatile particles ranging in size from a few nanometers to up to around one micrometer in diameter. Gasoline fuels were not subjected to the regulation for PM mass due to its low levels as compared to Diesel engines. But with the introduction of Diesel Particulate Filter (DPF) was introduced to reduce Particle emissions, the growing consensus of better health, a particulate filter introduction in gasoline engines was also taken into consideration. Later it was found that in spite of the fuel and lower CO2 emission advantages, SIDI engines spew unsightly and unhealthy particulate matter or soot and particle number emissions. SIDI combustion systems can emit 2 to 8 times more PM mass than indirect SI engines (PFI) which are more harmful and toxic for health. [Ref. 8]. Lean and stratified operation of SIDI engine can be one of the reason for increasing the PM emissions. PM emission in stratified combustion is identified as mainly originating from the inhomogeneous gas phase distribution of the stratified mixture .In September 2009, EURO 5a introduced the PM mass emission limit (5 mg/km) for the first time which was updated to 4.5 gm/km for SIDI engine in September 2014 by EURO 6 [Ref. 3].

The design concept of GPF is homogenous to DPF. The fundamental principle of the both filters are same. The particulate matter in the exhaust accumulates in a heavy quantity in DPF. As, more PMs are collected, this leads to a higher filtration efficiency. But in case of GPF, the filtration efficiency is not so high because of the low gathering of PMs. Moreover, the regeneration operation is also not possible in GPF as compared to DPF. Due to no PM layer formed in GPF, pressure drop is also low. Higher exhaust temperature in case of SIDI engine with low concentration of O2 at higher speed leads to increase in volumetric efficiency.                                                                                

METHODOLOGY

Two methods are employed for the study of particulate filter. In first method, a new concept of design of GPF for SIDI vehicle was evaluated in terms of certain parameters like filtration efficiency, pressure drop and regeneration performance. In the second method, gaseous compounds, particle number and size distribution measurements on SIDI engine equipped with GPF were conducted over the two cycles US06 Supplemental Federal Test Procedure (US06) and U.S. Federal Test Procedure 75 (FTP-75) on gasoline fuel.

Table no. 1 [Ref. 1] shows the comparison between SIDI and Diesel engines with respect to corresponding particulate filters.

Table 1. Impact on GPF and DPF

[pic 1]                       

According to this new concept design of GPF in 2011, PM filtration was the main aim of GPF as up to that time there was no official standard regulation was set for PN emissions, it was only proposed. But for the first time, European emission standard set up the regulation for PN emissions in September 2014 to a limit of 6×1011/Km [Ref. 4] for SIDI engine vehicles. So, now PN filtration is also added to the main aims of GPF.

PARTICULATE FILTER WORKING MECHANISM  

First of all, filter is equipped to the exhaust side of the SIDI engine. When particulate matter mixed with exhaust gases is passed through filter, the particles are entrapped to the porous walls and the filtration efficiency starts to increase. As, when more and more particles are filtered, a PM layer is formed which when increases also increase the filtration efficiency.

But in case of GPF, PM accumulation is very less which results in a very thin or no PM layer which decreases the filtration efficiency which is just opposite with respect to Diesel Particulate Filter. This can be seen in the fig. 1 [Ref. 1].

[pic 2] 

[pic 3]

[pic 4]

Fig.1 PM Filtration mechanism

EVALUATION PROCESS & CONDITIONS

For reducing PN emissions, 1.4L and 1.8L SIDI engine was used which was tested over New European Driving Cycle (NEDC) cycle. PN emissions were measured according to an evaluation process whose model [Ref. 1] is shown in fig. 2.

Fig.2 Vehicle emission evaluation system

         [pic 5] 

First of all, exhaust gases from the engine diluted with the dilution air. Thereafter, the particles were heated so that particle size greater than 2.5µ were eliminated. For more oxidation of the particles, they were followed to secondary heating zone followed by dilution and less than 20 nm size particulate matter were found.

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