Combustion Chamber Design

a. Dead Volumes

Air in the combustion chamber is contained in several different volumes like piston bowl, top land crevice, piston – cylinder head clearance, valve recess and head gasket clearance. Typical distribution of clearance volume at tdc among different components for a DI diesel engine is shown in Table 3.2. The piston bowl in DI diesel engines contains slightly more than 50% of total clearance volume at tdc. The air contained in top land crevice, head gasket clearance and valve recess is nearly 15 % and is poorly utilized during combustion. Even the air contained in the volume between piston crown and cylinder head at tdc is poorly utilized. Piston-cylinder crevice volumes store morethan proportionate air due to lower temperature in the crevice region compared to the temperature of air in the cylinder. A reduction in crevice volume therefore, increases air utilization. Similarly, a lower clearance between piston and cylinder head increases air utilization and reduces the possibility of fuel entering the crevices. Reduction in ‘poor air utilization' volumes results in lower particulate emission and fuel consumption..

b. Multi-Valves and Air Motion

Use of multiple valves (3 or 4) per cylinder increases flow area and hence, the volumetric efficiency of the engine. Four valves per cylinder are now common in gasoline engines. In the direct injection diesel engines use of four valves enables a centralized location of injector and combustion bowl in the piston. The injector can be placed more centrally and vertically. Typical PM-NO_x characteristics for 4- and 2- valve per cylinder passenger car diesel engines are compared on Fig. 3.5.With two valves designs, the injector is always offset and inclined. Injector inclination of 20 and 10 degrees from vertical has been observed to give an increase of about 25 and 5 %, respectively in PM emissions due to poor fuel distribution in the cylinder compared to a vertically located injector. Reduction of up to 4 to 7 % in specific fuel consumption is also obtained at the same NO_x emission level in a multi-valve engine with centrally placed combustion bowl and injector.

A centrally placed combustion bowl has lower swirl requirements and results in more equal fuel distribution and availability of equal air to each spray for mixing. In four-valve engines, symmetrical air motion in the piston bowl and equal fuel distribution between different sprays lead to optimum mixture formation and combustion with very low smoke levels.

Use of lower air swirl in 4-valve engines compared to 2-valve engines for the same PM and NOx emission levels, results in reduction of fuel -air ‘overmixing' during premixed phase of combustion. As ‘overmixing' of fuel is an important source of HC emissions, lower HC emissions in 4-valve engines are obtained.