Fig.2 Performance of the CCD system at various engine speeds and loads.
This may be attributed to the fact that the turbulence is generated late in the combustion process, and as the initial combustion, where a large part of the NOx forms, is kept similar to the standard combustion. Decreasing the CCD fuel amount and injecting it at the main injection tuning, the deterioration in BSFC and hydrocarbons at partial loads were effectively prevented. Extensive experiments were conducted for a variety of operating conditions and engine configurations, and all results showed a significant effect of the turbulence in reducing smoke.
Next, particulate was sampled from the combustion chamber during the combustion process to investigate the formation and oxidation process of particulate. Here, three cases are compared: with CCD injection, without CCD injection, and the case when the amount of fuel corresponding to the main fuel was injected in the main chamber without CCD injection. The excess air ratio is 1.5 except for the third case, here the excess air ratio is 1.8 because the total amount of injected fuel is reduced by the amount of the CCD injection fuel. The particulate concentrations peak early in the combustion period, and then decrease as the oxidation proceeds. The peak was lower with CCD than without CCD due to smaller injected fuel in the main chamber; the rest was injected in the CCD, which had much less smoke emission characteristics. When the CCD injection starts, the particulate concentration reduces rapidly, even though the amount of total fuel has increased due to the addition of the CCD fuel. These results of the gas sampling experiment show that the smoke reduction with the CCD system is due to two mechanisms: 1) promotion of particulate oxidation by strong turbulence generated by the CCD, and 2) reduction in particulate formation due to the decreased fuel quantity injected into the main chamber.
4. NOx REDUCTION POTENTIAL OF THE TWO-STAGE COMBUSTION
This section discusses the experimental results of several trials with two-stage combustion for NOx reduction. To realize the initial fuel-rich combustion in the two stage combustion, a variety of pistons were examined as shown in Fig.3. All of the pistons are equipped with a small cavity of 72% of the basic piston cavity. The rest is the air cavity, where the extra air is for the secondary combustion. The total volume of the air and the main cavities is equal to the basic cavity volume. The type 1 piston has a circular air cavity around the top clearance area, separated from the main cavity. The type 2 piston has the air volume at the top of the cavity, while in type 3 it is in the bottom of the cavity. Both of the last two pistons do not have clear separation between the air and the main cavities.
Figure 4 compares the results with these pistons with that of the basic piston. A 10 hole nozzle was equipped for the small cavity pistons, and this gave better emission performance than the original 4 hole nozzle. With the basic piston, the original 4 hole nozzle was used as the best matched injector. All of the three pistons from types 1 to 3 for the two-stage combustion, where CCD injection is applied, show significantly lower NOx emissions than the basic engine. However, without CCD injection significantly more smoke was emitted. With CCD injection, smoke levels apparently decreased, and became lower than the basic level, when injection timing was optimized as shown in the figure. The type 1 piston decreases smoke at relatively early injection timings, while the type 2 piston decreases smoke at late injection timings. Type 3 shows higher smoke emissions of the three.
In order to decrease NOx emission more significantly, two additional pistons were examined as shown in Fig.5. The OSKA type piston has flat-top projection at the center of the cavity, and a single fuel spray was to hit the projection and spread in 360 degrees to form uniform mixture in the cavity. The comma-shaped air cavity (CSAC) has air cavity at the side of the main cavity, and the CCD jet was intended to carry the air in the air-cavity into the main cavity in the secondary combustion process.
Figure 6 shows the result for the OSKA type piston. Two directions of CCD jets, to swirl direction and to center of the cavity as shown in Fig.5, were examined. With this piston NOx decreased one third of the original piston engine, and the smoke and fuel consumption were maintained at base level except for the heavy load range.
