Exhaust Gases

The polluting emissions by the diesel engines exhaust usually consist of a mixture of gas, vapour, liquid aerosols and solid substances in the particulate state. Analysing in detail the composition of the exhaust gases of engines using HFO and MDO, it is made up of 90% nitrogen and oxygen (non-polluting substances), and the remaining 10% of: carbon dioxide (CO2), water vapour and, in minor quantity, carbon monoxide (CO), sulphur oxides (SOx), nitrogen oxides (NOx), unburnt hydrocarbons (HC) and particulate matter (PM), originating from the combustion reactions.

The amount with which these substances are presented in the gases depends on several factors:

  • Quality of used diesel fuel
  • Engine type
  • Engine tuning
  • Regulation of the fuel feed pump
  • Motor load
  • Motor temperature
  • Engine maintenance status

Table 2.1 shows the values relating to the typical composition of the exhaust gases of a medium-fast 4-stroke diesel engine.

Table 2.1. Typical composition of the exhaust gases of medium-fast 4-stroke diesel engines


Main components of the exhaust gases		Concentration of the component [% in vol]Emission index[g/kWh]

Nitrogen

N2

74.0-76.0

5020-5160

Oxygen

O2

11.6-12.6

900-980

Carbon dioxide

CO2

5.2-5.8

560-620

Vapour

H2O

5.9-8.6

260-370

Nitrogen oxides

NOx

0.08-0.15 (Relatively high, if not controlled)

9.6-16

Sulphur oxides

SOx

0.08 (Linked to the type of fuel used)

12

Carbon monoxide

CO

0.006-0.011 (Relatively low, thanks to the good quality of combustion)

0.4-0.8

Hydrocarbons and volatile organic compounds

HC, VOC

0.04-0.13

0.4-1.2

Particulate Matter

MGO*

[mg/Nm3]

HFO**

[mg/Nm3]

MGO4

[g/kWh]

HFO5

[g/kWh]]

Soot (elemental carbon)

50

50

0.3

0.3

Fuel ash***

4

40

0.03

0.25

Lubricant ash

3

8

0.02

0.04

*Marine Gas Oil DM-A grade with 0.01% fuel ash content and 1.5% lubricant ash content
**Heavy Fuel Oil RM-B grade with 0.1% fuel ash content and 4% lubricant ash content
***Due to low load, start-up phase and rapid load increase

Table 2.2 shows the values relating to the typical composition of the exhaust gases of a 2-stroke diesel engine.

Table 2.2. Typical composition of the exhaust gases of 2-stroke diesel engines

Main components of the exhaust gases

Concentration of the component

 [% in vol]

Emission index

[g/kWh]

Nitrogen

N2

74.0-76.0

5020-5160

Oxygen

O2

13.0

1070

Carbon dioxide

CO2

5.2

590

Vapour

H2O

5.4

250

Nitrogen oxides

NOx

0.15

18.63

Sulphur oxides

SOx

0.05

8.52

Carbon monoxide

CO

0.006

0.41

Hydrocarbons and volatile organic compounds

HC, VOC

0.02

1.17

Particulate Matter

120 mg/Nm3

0.66