The control of combustion by self-ignition shows advantages with respect to thermal efficiency, NOx reduction, and process stability in both SI and CI engines. As the main stage of energy conversion in an engine, the combustion process and its control play a determining role in the chain of process sequences. New self ignition concepts are replacing the conventional forms of initiation and propagation of combustion in SI and CI engines.
Various self-ignition methods such as retention of burned gas, recirculation of exhaust gas, fuel additives, and heated intake pipes have been successfully applied at partial load operation of different engines. To qualify the effectiveness and controllability of various self-ignition methods, thermodynamic analysis followed by experimental validation is necessary.
Initiation of combustion in SI engines by external energy is succeeded by chain-like branching and propagation of the reaction within the fuel– air mixture, which is more or less homogeneous and isotropic. Each propagation from a single initiation center into a medium with homogeneous properties occurs as a diffusing front, and in combustion as a flame front.
The corresponding increase in internal energy, representing the kinetic energy of molecules, occasionally leads to the disruption of molecules resulting from combustion; thus, the main products can form dissociation radicals, which are the seeds for new molecules such as NO or NO2.
Increasing the distance between exothermic centers leads to a decrease in heat transfer intensity from one center to the next, because of the low temperature of the medium between centers. Therefore, an ideal support for the combustion process is obtained when the temperature of the medium between the centers is high enough to transport heat from one center to the next, but low enough to avoid dissociation. Obviously, the lower limit is given when using a fuel with additives. In other cases, media that can transfer the heat for reaction are dispersed homogeneously within the combustion chamber.
Combustion by self-ignition without diffusion and without propagation of the flame front is based on the same physical and chemical processes but at very different levels of pressure and temperature. Homogeneous charge compression ignition (HCCI) is a very popular method, but not very exact if applied to the CI process in diesel engines. The method consists of the distribution of exothermic centers within a mass of burned gas, and not in the generation of a homogeneous charge.
Increase in Compression Ratio
The advantages of increasing the compression ratio, especially for SI engines, were discussed from the point of view of the thermodynamic cycle, and their realization, favored by direct fuel injection, was argued. It was also mentioned, that the tendency to increase engine speed leads to an increase in the bore/stroke ratio, a situation that limits the increase in compression ratio. In SI engines, an increase in compression ratio at partial load leads to a decrease in combustion chamber volume, which is a means of maintaining a fresh mixture density more or less at the level corresponding to full load, when the air is throttled.
Management of Engine Cooling
It is generally considered that only a third of the heat input into a piston engine is transformed into work, another third is ejected as burned gas, and the last third is lost as heat transfer through the cooling circuit. The improvement measures discussed in this section are related to the first third, the transformation of heat into work, and to the second third, utilization of exhaust gas in turbines. The loss of heat by cooling has been more or less neglected until now. Management between engine and cooling circuit has appeared recently in the opposite direction: water in the cooling system is occasionally heated by an external combustion system to warm the engine at start.
Last word
Furthermore, a considerable problem is actuation of the water pump by the engine itself, as a function of engine speed. Therefore, the volume flow of the coolant is strongly speed dependent; a load-dependent adjustment is very limited. A better alternative is actuation of the pump electrically, with speed control as a function of the momentary load–speed combination, for optimum operation between cooling effect and engine efficiency. Such applications are increasingly being introduced in series production of advanced four- and six-cylinder engines.
