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