Performance and Emissions at exhaust lambda 1.0

Figure 5.3c shows the combustion duration over the CAI operating range, which in this paper is defined as the deg CA from 10% mass fraction burnt to 90% mass fraction burnt.

It can be seen that the combustion speed was the highest at low speed mid-load operation in the pure CAI operation zone, where the combustion duration was less than 8 CA.

When the engine speed increased, the combustion speed increased but the combustion duration in deg CA increased by a few degrees. At low load operation, the combustion duration became longer because the combustion process was slowed down due to dilution by the large amount of the residual gases, of which the quantity was mainly determined by the intake pressure and air flow rate. In the spark assisted CAI operation above the pure CAI operation range, the combustion duration was longer and the heat release process was slower due to retarded combustion phasing of CA50 as shown in Figure 5.3d, the crank angle at which 50% mass fraction is burnt. It is noted that at the higher load region of low engine speeds, although the combustion phase was retarded to 16 or 18 degree ATDC, the combustion speed was still high enough to cause excessive pressure rise due to insufficient dilution effect of less residual gas at lower engine speed.

Therefore, it is envisaged that the upper load region can be extended further by

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Figure 5.4a shows the Indicated Specific NOx (ISNOx) emission values over the CAI operating range. It can be seen that the NOx emissions under pure CAI operation were below 2g/KWh due to the dilution of the large amount the residual gas in the cylinder.

When the load increased and the operating mode transferred from pure CAI to spark assisted CAI, more NOx emissions were produced because of the reduction in the residual gas fraction and the increase in the in-cylinder temperature. At the knock limit boundary, the NOx emissions increased to 6 g/kWh.

Figure 5.4b shows the distribution of Indicated Specific CO (ISCO) emission over the CAI operating range. It can be seen that the CO emission was very high and its value tended to be higher at higher load and higher speed where the intake pressure was higher as shown in Figure 5.4a. Such results are likely to be caused by the air short-circuiting prevalent in the two-stroke operation. During the gas exchange process in the two-stroke engine operation, there is a valve overlap when both of the intake and exhaust valves are open to achieve the scavenging. During this period, a portion of the intake fresh air can flow across the combustion chamber and into the exhaust directly. The portion of the fresh air dilutes the exhaust gas and leads to the measurement of the air to fuel ratio in the exhaust pipe to be leaner than the actual air to fuel ratio of the in-cylinder mixture.

Therefore, the mixture in the cylinder during the combustion process is richer than that measured from the exhaust pipe. Figure 5.5 shows the effect of the short-circuiting on the relative air to fuel ratio (Lambda). The in-cylinder lambda (Lambdac ) was derived from the measurement of instantaneous CO2 concentration in the exhaust port with a fast response Cambustion CO2 analyser as to be described in Chapter 4, whereas the lambda in the tailpipe(Lambdatp) was measured by a lambda sensor. It can be seen that the short-circuiting rate increased with the intake pressure for the fixed valve timings. As a result, the in-cylinder lambda (Lambdac) became increasingly richer while the lambda in the tailpipe (Lambdatp) was ketp at 1. Thus the CO emissions would be expected to increase with the intake pressure when the exhaust lambda signal is used as a feedback for the air fuel ratio control on the two-stroke engine.

a) b)

c) d)

Figure 5.4 ISNOx, ISCO, ISHC, and Exhaust Temperature over the Operating Range

Figure 5.4c shows the distribution of Indicated Specific uHC (ISHC) emissions over the CAI operating range. The value varied from 10 g/kWh to 60 g/kWh depending on the engine load. The high values of uHC emission were caused by richer mixture whilst the exhaust lambda was maintained at 1.0 because of the short-circuiting process discussed above. Since the oxidation rate of the uHC during the combustion process in the cylinder is mainly determined by the in-cylinder temperature. When in-cylinder temperature increases, more uHC is converted to the CO2 and H2O. Therefore, the distribution of the uHC emission is much related to that of the exhaust temperature, as shown in Figure 5.4d.

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Figure 5.5 Effect of Short-circuiting on the Measurement of Lambda at 1500rpm

Figure 5.6 shows the distribution of Indicated Specific Fuel Consumption (ISFC) over the operating range. Within the pure CAI operating range, the ISFC varied from 270 g/kWh to 320 g/kWh depending on the engine load. Above the pure CAI operation, the fuel consumption increased with engine load because of the delayed combustion and too rich mixture caused by the short-circuiting stated above.

Figure 5.6 ISFC over the Operating Range 27

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5.3 Short-circuiting Effects on Two-stroke CAI Combustion and