Combustion Improvement and Control for a Natural Gas HCCI Engine by the Internal EGR by Means of Intake-valve Pilot-opening

A novel EGR (exhaust gas recirculation) method by means of the intake-valve pilot-opening has been demonstrated using a single-cylinder test engine, in order to control the combustion and to reduce the energy loss due to intake-gas pre-heating in a natural gas HCCI (homogeneous charge compression ignition) engine. The intake valve, together with the exhaust valve, is slightly opened at the beginning of the exhaust stroke. Then, part of the burnt gas, which has a high temperature, is introduced into the suction pipe backward, resulting in an increase in the intake-gas temperature. The EGR rate can be varied successfully up to about 40% by using the specially designed camshaft and the valve control device, which can delay the closing timing. The effect of the EGR rate on engine performance and emissions has been investigated under the condition that the temperature of the fresh mixture and the fuel consumption rate are kept constant. The results show that the combustion phase can be advanced by the increase in the rate of EGR. As a result, a higher combustion efficiency and a higher indicated thermal efficiency can be obtained in comparison with when there is no EGR.