unburnt hydrocarbons in the exhaust are the principal cause of the unpleasant smell of a diesel engine, though the lubricating oil also makes a small contribution. There are three main reasons for this. First, at low temperatures and light loads, the mixture may be too lean for efficient
burning so the precombustion processes during the ignition delay period are partially inhibited. This is why some of the mixture subsequently fails to burn.
Secondly, because of the low volatility of diesel fuel relative to petrol, and the short period of time available for it to evaporate before combustion begins, HCs are generated during starting and warming up from cold. In
these circumstances, fuel droplets, together with water vapour produced by the burning of the hydrogen content of the remainder of the fuel, issue from the cold exhaust pipe in the form of what is generally termed white smoke, but which is in fact largely a mixture of fuel and water vapours. At about 10% load and rated speed, both HC and CO output are especially sensitive to fuel quality and, in particular, cetane number.
Thirdly, after cold starting and during warm-up, a higher than normal proportion of the injected fuel, failing to evaporate, is deposited on the combustion chamber walls. This further reduces the rate of evaporation of the fuel, so that it fails to be ignited before the contents of the chamber have been cooled, by expansion of the gases, to a level such that ignition can no longer occur. Similarly, the cooling effect of the expansion stroke when the engine is operating at or near full load can quench combustion in fuel-rich zones of the mixture.This is the fourth potential cause of HC emissions.
Unburnt HCs tend to become a problem also at maximum power output, owing to the difficulty under these conditions of providing enough oxygen to burn all the fuel. As fuel delivery is increased, a critical limit is reached above which first the CO and then the HC output rise steeply. Injection systems are normally set so that fuelling does not rise up to this limit, though the CO can be removed subsequently by a catalytic converter in the exhaust system.
Another potential cause of HCs is the fuel contained in the volume between the pintle needle seat and the spray hole or holes (the sac volume). After the injector needle has seated and combustion has ceased, some of the trapped fuel may evaporate into the cylinder. Finally, the crevice areas, for example between the piston and cylinder walls above the top ring, also contain unburnt or quenched fractions of semi-burnt mixture, Expanding under the influence of the high temperatures due to combustion and falling pressures during the expansion stroke, and forced out by the motions of the piston and rings, these vapours and gases find their way into the exhaust
In general, therefore, the engine designer can reduce unburnt hydrocarbons emissions in three ways. One is by increasing the compression ratio; secondly, the specific loading can be increased by installing a smaller, more highly rated, engine for a given type of operation; and, thirdly, by increasing the rate of swirl both to evaporate the fuel more rapidly and to bring more oxygen into intimate contact with it.