Diesel exhaust emissions

7.13.4 Diesel exhaust emissions

and pressure are higher than in a conventional diesel injection engine at the start of injection.

Exhaust emissions from diesel engines have been The higher temperature and pressure reduces

reduced considerably by changes in the design of ignition lag to a minimum, so that the controlled

combustion chambers and injection techniques.

130 Advanced automotive fault diagnosis

Figure 7.32 Sectioned details of the CR injector (Source: Bosch Press)

More accurate control of start of injection and are always run with excess air to ensure better spill timing has allowed further improvements

and more efficient burning of the fuel. A normal to be made. Electronic control has also made a

catalyst therefore will not strip the oxygen off the significant contribution. A number of further tech-

NO x to oxidise the hydrocarbons because the niques can be employed to control emissions.

excess oxygen will be used. Special NO x convert- Overall the emissions from diesel combustion

ers are becoming available. are far lower than emission from petrol combus- tion. The CO, HC and NO x emissions are lower

7.13.6 Filters

mainly due to the higher compression ratio and excess air factor. The higher compression ratio

To reduce the emission of particulate matter (soot), improves the thermal efficiency and thus lowers

filters can be used. These can vary from a fine the fuel consumption. The excess air factor ensures

grid design made from a ceramic material to cen- more complete burning of the fuel.

trifugal filters and water trap techniques. The prob- The main problem area is that of particulate

lem to overcome is that the filters can get blocked, emissions. These particle chains of carbon mol-

which adversely affects the overall performance. ecules can also contain hydrocarbons, aldehydes

Several techniques are employed including cen- mostly. The dirt effect of this emission is a pollu-

trifugal filters.

tion problem but the possible carcinogenic effect of this soot gives cause for concern. The diameter of these particles is only a few ten thousandths of

7.14 Diagnostics – diesel

a millimetre. This means they float in the air and

injection systems

can be inhaled. In much the same way as with petrol engines EGR is employed primarily to reduce NO x emis-

7.14.1 Test equipment

sions by reducing the reaction temperature in the Note: You should always refer to the manufac- combustion chamber. However if the percentage of

turer’s instructions appropriate to the equipment EGR is too high, increased hydrocarbons and soot

you are using.

are produced. This is appropriate to turbocharged engines such that if the air is passed through an

Smoke meter (Figure 7.33)

intercooler, as well as improvements in volumetric efficiency, lower temperature will again reduce the

This is an essential device in the UK and other production of NO . The intercooler is fitted in the

countries where the level of smoke in the exhaust

same way as the cooling system radiator. forms part of the annual test. Most devices use infrared light to ‘count’ the number of soot parti- cles in the exhaust sample. This particulate matter

7.13.5 Catalytic converter –

(PM) is highly suspected of being carcinogenic.

diesel

On a diesel engine a catalyst can be used to reduce

Injector tester (Figure 7.34)

the emission of hydrocarbons but will have less The pressure required to ‘crack’ (lift the nozzle) effect on nitrogen oxides. This is because diesels

on an injector can be tested.

Engine systems 131