In both the case where the compression ratio was increased and the case where the charged air pressure was increased, the degradation of the fuel consumption to the reduction of NOx was smaller when compared with only the retarded fuel injection timing.

When both the compression ratio and the charged air pressure were increased, the compression pressure was higher than the maximum pressure, and the reduction ratio of NOx to the degradation of the fuel consumption was leveled off.

　

4.4　Test to reduce Overlap Period of Inlet/Exhaust Valve

The overlap period reduction test where the inlet/exhaust valve is open, was carried out, expecting the effect to suppress NOx generation due to the residual gas by reducing the scavenging efficiency to leave the combustion gas in the cylinder.

The test was carried out for the overlap period from the standard value to the 75｡?A reduction in terms of the crank angle by retarding the inlet valve opening timing, and advancing the exhaust valve closing timing.

The test results were shown in Fig. 4.

The exhaust gas temperature was remarkably increased due to the reduction of the blow-by air volume caused by the reduction of the overlap period, and even in the condition of the overlap period of 55｡?A reduction in which the exhaust gas temperature was higher by approximately 50｡?han the limit exhaust gas temperature in the standard condition, reduction of the scavenging efficiency was small because of the low-speed 4-stroke engine, and little combustion gas was left, and the NOx reduction effect was small.

　

Fig. 4 Reduction in Overlap Period

　

On the other hand, the cycle efficiency was improved, and the fuel consumption was improved by 3% by increasing the indicated mean effective pressure in the gas exchanging process, and decreasing the charged air pressure from the value above the standard by 27% to the value above the standard by 17%.

　

4.5　Retarded Inlet Valve Closing Timing

When the inlet valve closing timing is delayed and the actual compression ratio is reduced in a condition where the charged air temperature is kept constant, and the charged air pressure is increased, the compression temperature is dropped, and the combustion temperature is also dropped by the corresponding reduction thereof, and NOx can be reduced.

The fuel consumption is also improved by the quantity equal to reduction of the compression process work.

Expecting all these factors, the test was carried out in a condition where the increase of the charged air pressure (by 16%), the increase of the compression ratio (by 16%), and the overlap reduction (by 55｡?A) were combined.

When the inlet valve closing timing was retarded by 60｡?A compared with the standard value, the reduction of the actual compression ratio became excessive (by about 9 in terms of the actual compression ratio), and the compression temperature and the compression pressure were too low, and the engine could not be started during the winter season.

　

Fig. 5　Retardation in Inlet Valve Closing Timing (^o CA)

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