Diesel Fuel Combustion
Since diesel combustion is compression induced and depends to a major extent on successful vaporization and mixing over an extremely short time, it is much more complex and difficult than that of a spark ignition engine. In a spark ignition engine, a pre-mixed charge of air and fuel is supplied to the cylinders and the source of combustion is a single point. In the diesel engine, on the other hand, air alone is initially supplied to the cylinders. It is then compressed so that its temperature exceeds than that necessary for spontaneous combustion of the fuel. The fuel is then injected into the cylinder as very fine spray. Consequently, combustion is initiated at a number of points throughout the charge. The auto-ignition point of diesel fuel can be around 493 K, which is much low as compared with the ultimate compression temperature of about 873 K attained almost throughout the charge. Due to heat losses, temperatures of charge adjacent to the cylinder walls are significantly lower than the latter figure.
The droplets of fuel in the spray have to mix thoroughly with the air, evaporate, and then burn completely in a very short time remaining in the engine cycle. To meet this requirement the fuel has to be consistently of a high quality, with properties suitable specifically for diesel engines. This is more relevant for starting in very cold conditions, when the temperature on completion of compression can be as low as 673 K and the auto-ignition temperature as high as about 773 K. This is why some diesel engines incorporate glow plugs to facilitate starting. Once auto-ignition starts, a small flame, or flames, may be alternately initiated and quenched. The temperature at which combustion spreads is generally approximately between 773 to 873 K.
In a gasoline engine, control over power output is achieved by throttling the supply of mixture to the cylinders. Whereas in a diesel engine it is carried out by regulating the quantity of fuel delivered through the injector nozzles. If more fuel is supplied than can combine with the oxygen available, the hydrogen content burns preferentially, causing unpleasant black smoke in the exhaust. Thus maximum fuel supply for maximum power output has to be limited just below at which black smoke is emitted. As combustion is initiated simultaneously at a number
of centres distributed throughout the air-fuel mixture, detonation associated with gasoline engines, is not a problem in this case. However, because of the very high air-fuel ratio under idling and light load conditions, an explosive combustion termed diesel knock may be heard.
The fuel is generally injected into the diesel engines cylinders about 15 degrees before TDC, at pressures about 30 MPa for indirect injection and about 100 MPa for direct injection engines. Currently, however, direct injection is more preferred at pressures up to 130 MPa, though as high as 150 MPa is contemplated for reducing the output of NOx in the exhaust. Compression ratios in these engines range from about 14 : 1 to 24 : 1, typical values being 18 : 1 for direct injection engines and 22 :1 for indirect injection engines.
Ignition delay is the time interval between the evaporation and mixing of the fuel in the air and the initiation of combustion. It is generally of the order of 1000th of a second, however varies according to the properties of the fuel, size of droplets, their rate of mixing with the air, and the temperature. In general the delay can be reduced by increasing turbulence, but there is practical limitation of turbulence that is effective. During the delay period, mixing continues and pre-flame reactions occur where free radicals and aldehydes are formed. Until the visible flame appears, the curve of pressure against crank angle (Fig. 8.10) follows the line that it would have taken if the engine had been motored without fuel injection. Subsequently heat of combustion causes the pressure to rise rapidly and then falls again as the piston begins to descend on the power stroke.
Fig. 8.10. Release of heat following ignition delay.
The delay period in terms of time remains constant for any given engine, fuel quality and compression temperature. Therefore, it becomes larger in terms of crank angle as the speed of the engine increases. Consequently, it is desirable to advance the injection timing with increasing speed. The longer the delay period, the steeper is the subsequent pressure rise and the noisier the engine. This noise, diesel knock, is loud when the engine is running under very light load or idling, specifically after starting from cold. It also becomes louder as the volatility of the fuel is increased and the cetane number reduced. The main reason for the increased noise under cold starting is that, temperatures in the combustion chamber are low so the ignition delay is long, due to which a higher proportion of the total fuel charge is injected before combustion starts. Also, only minute quantities of fuel are being injected, which may not be well atomised. The quantity of air available for combustion is large with respect to fuel. This causes a sudden late release, at or near TDC, of a fairly large proportion of the total energy supplied giving rise to the explosive combustion, and, therefore, the diesel knock.
The ignition delay period as indicated in Fig. 8.11, is divided into two parts :
(£) The mixing period, which is the time required for atomisation and evaporation of the fuel, and physical mixing with air,
(ii) The interaction period, in which the molecular interaction prepares the mix for, and initiates, the ‘actual burning’ phase of combustion. This is the larger of the two periods.
Ignition delay of the fuel is of extreme importance in CI engines primarily because of its effect on both the combustion rate and on detonation. It may also exert an influence on engine starting ability, and on the presence of smoke in the exhaust.
There are a number of engine variables that may also affect the ignition delay, and thereby affect engine performance as follows.
(a) Compression Ratio. For a given fuel, an increase in compression ratio decreases ignition lag. This is apparently due to the increased density of the compressed air, resulting in closer contact of the molecules, which thereby hastens the time of reaction when fuel is injected. As the difference between cylinder air temperature and the minimum auto-ignition temperature increases, the ignition delay has been found to decrease. With the increase in the compression ratio, the minimum auto-ignition temperature decreases and maximum air temperature increases, and hence the difference between these two temperature also increases. This results in a decrease in ignition delay.
(6) Inlet Air Temperature. An increase in the intake air temperature decreases the ignition delay, because it raises cylinder air temperature, thereby increases the difference between minimum auto-ignition temperature and maximum air temperature.
(c) Coolant Temperature. Ignition delay increases with increase in coolant temperature. Increase in coolant temperature tends to decrease the heat transfer from the combustion chamber, thus producing higher cylinder air temperature, as a consequence the difference in temperature increases.
id) Engine Speed. The degree of turbulence within the combustion chamber has a definite effect on the ignition delay. Turbulence increases with engine speed. However, increase in turbulence normally increases the heat loss to the combustion chamber walls, hence, lower combustion temperature results. On the other hand, it is also possible to increase the engine turbulence as well as cylinder temperature and hence its pressure, thus a decrease in ignition delay may be realised.
Fig. 8.11. Pressure-time diagram illustrating ignition delay (not to scale).
When the engine is running normally, combustion proceeds in three phases. The source of energy for igniting the mixture is the high temperature of the air that has been compressed in the cylinder.
During the first phase, immediately following injection, the droplets of fuel tend to break up more finely and some vaporization occurs.
The second phase begins when flames appear. Since these, as previously indicated, are initiated simultaneously at a number of centres distributed throughout the combustion chamber, detonation is not a problem. This phase is characterized by rapid rise is pressure, which ultimately halted and reversed as the piston begins to descend.
In the third phase, the period during which the piston is descending, the remainder of the fuel is injected, mixed with and evaporated into the air, and finally burnt.