Small-size direct-injection (DI) diesel engines are being developed as potential replacements for indirect-injection (IDI) or prechamber engines in automobile applications. Figures 10-lb and 10-2 show...

Small-size direct-injection (DI) diesel engines are being developed as potential replacements for indirect-injection (IDI) or prechamber engines in automobile applications. Figures 10-lb and 10-2 show the essential features of these two types of diesel. The DI engine employs high air swirl, which is set up with a helical swirl-generating inlet port (Fig. 8-13). The injector is centrally located over the bowl-in-piston combustion chamber and the injector nozzle has four holes, one in each quadrant. The ID1 engine (a Ricardo Comet swirl chamber), in contrast, has no swirl in the main chamber, but generates high velocities and a rotating flow in the prechamber during compression. Figures 15-21 and 15-23 show performance maps for typical versions of these two types of engines. Bmep, brake mean effective pressure, is plotted against engine speed. Brake specific fuel consumption contours are shown with the numbers in grams per kilowatt-hour. The heat-release-rate profiles for these two types of engine at a typical midload mid-speed point are shown versus crank angle in the sketch. 0 has units of joules per second.

(a) Explain the reasons for the differences in shape and relative timing in the cycle of the heat-release-rate profiles.

 (b) Suggest reasons for the differences (magnitude and shape) in the maximum bmep versus mean-piston-speed line for the DI and ID1 engines.

(c) Evaluate the brake fuel conversion efficiency of each engine at its maximum efficiency point, and at 2000 rev/min and road load (road load is the power requirement to maintain a vehicle at constant speed; it is 2 bar bmep at 2000 rev/min). Explain the origin of the observed differences in efficiency at these two operating conditions.