Page 366 - Advanced thermodynamics for engineers

16.4 DIESEL (COMPRESSION IGNITION) ENGINES 355

The majority of small diesel engines originally designed for passenger cars used the indirect
injection (idi) process. In these engines, see Fig. 16.5(b), the fuel was injected not directly into the
main combustion chamber but into a small swirl chamber (e.g. Ricardo Comet V) connected to the
main chamber by a throat. This approach was adopted because it was possible to achieve good mixing
of the fuel and air in the pre-chamber due to the high swirl velocities that could be generated there as
air was forced into the pre-chamber by the piston travelling towards tdc. This approach had a further
advantage that relatively simple fuel injection equipment could be used, with a single-hole nozzle and
low-pressure injection pump. Combustion of the rich mixture commenced in the pre-chamber, and the
burning gases entered the main chamber, which contained pure air, and generated high turbulence
which ensured good mixing of the burning plume and the ‘secondary’ air. Such combustion systems
are very tolerant of fuel quality, need relatively simple fuel injection equipment, and can be run at
relatively low air–fuel ratios before producing black smoke. Their disadvantage is that their fuel
economy is about 10%, or more, worse than their di counterparts.

Fuel injector
(a) (b)
Fuel injector
Swirl chamber

(c) (d)

FIGURE 16.5
Some combustion chamber arrangements for diesel engines (a) basic schematic of direct injection (di) diesel
engine (b) basic schematic of indirect injection diesel engine (c) combustion chamber arrangement of low- or
medium-speed diesel engine (d) combustion chamber arrangement of high-speed di diesel engine.