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The main disadvantages of HCCI engines are the knocking tendency at high engine loads, the challenge of the start of the combustion, control of the combustion phase, and the narrow operating range. In this study, we aimed to control the combustion processes in HCCI engines and to expand their working range by improving the fuel properties of fusel oil by the addition of diethyl ether. Thus, the variations in the in-cylinder pressure, rate of heat release, indicated mean effective pressure, start of combustion, combustion duration, CA50, indicated thermal efficiency, mean pressure rise rate, hydrocarbon and carbon monoxide emissions were investigated. It was observed that the in-cylinder pressure and rate of heat release were taken into advance and the test engine could be operated for a wider range by increasing the diethyl ether ratio in the blend. The indicated mean effective pressure increased by 67.5% with DEE40 fuel compared to the DEE80. Under the same operating conditions, HC and CO emissions decreased by 41.6% and 56.2%, in use of DEE40. Furthermore, the highest indicated thermal efficiency was obtained as 42.5% with DEE60 fuel. Maximum hydrocarbon and carbon monoxide emissions were observed with DEE80 fuel as 0.532% and 549 ppm, respectively.

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There are many criticisms for the association between the Six Sigma concept and the two statistical metrics associated to 6σ processes: 1.5σ shift for maximum deviation and 3.4 PPM non-conformities for the long-term performance. As a result, the paper aims to carry out an analysis of this problem, and the first result obtained is that a stable process can reach a maximum drift, but its value depends on the volume of the sample. It is also highlighted that, using only the criterion “values outside the control limits” for monitoring stability through the Xbar chart, a minimum value can be calculated for the long-term performance of a process depending on the sample size. The main conclusion resulting from the calculations is that, in the case of a 6σ process, the long-term performance is much better than the established value of 3400 PPB: For small volume samples of two pieces it is below 700 PPB, for three pieces it is below 200 PPB, and for samples with a volume greater than or equal to four pieces the performance already reaches values below 100 PPB! So, the long-term performance of 6σ processes is certainly even better than the known value of 3.4 PPM.