EMISSION CONTROL BY ENGINE VARIABLES AND EGR

FUEL INJECTION VARIABLES

Demands Made on Injection System

To achieve low soot formation, rates of fuel-air mixing are to be enhanced.  Fuel injection and air motion in the cylinder are key parameters to achieve rapid fuel-air mixing.  The following strategy is adopted to improve fuel air mixing and the diesel engine combustion, which leads to reduction both in the soot and NO_x formation:

• Use of high fuel injection pressures  and  smaller nozzle hole size to produce very fine fuel atomization for rapid fuel evaporation and mixing with air.
• Fuel spray not to impinge on walls but fuel to be distributed mainly within the air inside the combustion chamber.
•  Matching of injection spray configuration and development with in-cylinder air motion for rapid fuel-air mixing throughout the injection duration period
• Use of variable injection timing, multiple –injection and injection rate shaping technology

High Injection Pressures

The mass flow rate of fuel injected, m_f is given by:

, kg / cycle

(6.1)

where C_d  is coefficient of discharge, A_n is nozzle flow area in m²,  ρ_f  is fuel density in kg / m³, (P_inj-P_cyl ) is the pressure drop across nozzle orifice in Pascals, Δθ is the injection duration in degrees crank angle and N is the engine speed in RPM.

Generally, P_inj >> P_cyl.,
Thus, for a given injection rate and injection duration Δθ in crank angles, the injection pressure should vary with speed as,

(6.2)

The speed of  engines for road vehicles  from  lowest working speed to rated speed may vary by  a factor of; N_max/N_min = 4 to 5..To achieve similar injection duration and spray penetration from the lowest to rated engine speed, the injection pressure therefore, is required to vary by a factor of 16 to 25. This of course is not possible in practice but it has led to use of as high an injection pressure as possible. In the pre-1990 engines,