# Impact of Exhaust Gas Recirculation on Gaseous Emissions of Turbocharged Spark-Ignition Engines
## 2 Materials and Methods
However, these emissions will be affected not only by the EGR rate, but also by the spark timing (due to the impacts on combustion velocity and the temperature evolution inside the cylinder) and the VVT settings (since it controls the amount of internal residuals trapped inside the cylinder, especially at low loads). Therefore, in order to have a meaningful assessment of the exhaust emissions impact, it is necessary to have a proper combination of these calibration parameters as a function of the EGR rate. For this purpose, a design of experiments (DoE) was performed for six operating points, in a range from 1500 to 3000 revolutions per minute (rpm) and 6 to 15 bar of brake mean effective pressure (BMEP). This design of experiments was used to extract optimal calibrations at different levels of EGR, from zero to the maximum possible at each keypoint, aimed at optimizing the brake thermal efficiency. To successfully conduct the DoE, lower and upper boundaries had to be determined for each of the variables for every DoE. Intake and exhaust VVT boundaries were set according to safety limits to avoid contact between valves and the piston head. On the other hand, spark advance and EGR rate boundaries were set based on preliminary tests: at high loads, combinations of low EGR and advanced spark were excluded to avoid knocking combustion; at lower loads, combinations of high EGR and retarded spark actuation were also taken out to avoid excessive combustion instability and misfires. Finally, the throttle actuation or the VGT were used to establish the engine torque once the rest of the parameters were set. Table 2 shows the ranges covered for each keypoint, while Figure 2 shows an example of the configuration of the DoE for the engine point of 1500 rpm and 6 bar BMEP. Once the DoE was completed and data were processed, every test must overcome two barriers: combustion stability and knock. If one of these limits was surpassed, the test was discarded before generating the surface response model. The combustion stability was defined in terms of a COV (coefficient of variance) limit for the indicated mean effective pressure (IMEP), set to 3.33% for the 6 bar BMEP points and 2.5% for the higher BMEPs. As widely accepted [22], COV of the IMEP was computed using information from a 200-cycle acquisition:  
\[COV_{IMEP}=\frac{std(IMEP)}{mean(IMEP)}\times 100 \tag{3}\]  
Knock intensity was assessed based on the MAPO (Maximum Amplitude of Pressure Oscillations) parameter. The method consists of in-cylinder pressure analysis using a high-pass filter to analyze oscillation in the time domain, calculating the maximum absolute value of the filtered pressure signal as stated by [32]. A maximum of 3% of a 200-cycle acquisition over a 0.5 bar limit was considered acceptable. This limit was established based on previous characterization of knocking cycles at the same engine speed. A 1-D engine model based on GT-Power software, previously developed and validated by [33], was used in order to estimate the amount of residual gas trapped in the 6 bar BMEP operating points due to the high interaction between these residuals and the external EGR for analysis of the engine-out emissions. For a proper estimation of these residuals, it was necessary to have fine control of the intake and exhaust manifold conditions (pressure and temperature). For that purpose, the variable geometry turbine was controlled to match the experimental pressure in the exhaust manifold, while the temperature was achieved thanks to a heat transfer multiplier in the exhaust pipes. In the intake side, the compressor speed was set to obtain the experimental compressor outlet pressure. Then, depending on the operating condition, the engine's main throttle position was changed to achieve the experimental intake manifold pressure. Finally, the intake manifold temperature was controlled through the operation of the charge-air cooler.