GRIP CEE 2017 / Main Report

5.4.3 EMISSIONS OF NO X

AND PARTICULATEMATTER When compared to diesel engines, the average combustion temperature of NG is almost 260 °C lower at around 1,890 °C, which results in considerably smaller amounts of NO x being emitted into the environment. The principle mechanism of NO x formation is thermal dissociation and the subsequent reaction of nitrogen with oxygen molecules in the combustion air. Three major factors play a crucial role: oxygen concentration, combustion temperature, and time of exposure at the combustion temperature. As these factors increase, NO x emission levels increase accordingly. Since natural gas has much lower combustion temperatures and does not require as much excess air, the NO x emissions of CNG-powered vehicles are significantly lower than those from diesel. The same is true for the emission of particulate matter (PM). NGVs are known to emit very little or no particulate matter. PM emission is closely associated with the residual combustion of fuel, which is a typical process for high molecular hydro­ carbons, such as those in diesel fuel. Thus, the pollutant emissions of CNG vehicles have always easily met EURO 6 standards, even in former times when only EURO 1 and EURO 2 were in force. Since the EURO 5 and EURO 6 emission standards came into force, the manufac- turers of diesel vehicles must compensate for these handicaps by using costly after- treatment of the exhaust gas in order to reach the high standards shown in Figure 5.10. Among these treatments are the Selective Catalytic Reduction of NO x (SCR) and the Diesel Particulate Filter (DPF).

50 PM (mg/km)

EURO 3 (2000)

25

EURO 4 (2005)

5

EURO 6 (2014) EURO 5 (2009)

500

80

180

250

NO

(mg/km)

x

CNG vehicles emissions from EURO 3 to EURO 6

Figure 5.10: Development of EURO emissions limits of NO x

and PM pollutants for diesel

passenger cars in the course of time from EURO 3 to EURO 6

Central Eastern Europe GRIP 2017 |

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