Development of an NOx Reduction System Using Exhaust Valve Controls in Small Engines for Motorcycles

Exhaust gas recirculation (EGR) systems are used as a technology to reduce NOx emissions. However, the charging of the fresh air-fuel mixture decreases due to the inflow of the burnt gas from the EGR system, and the drivability deteriorates. Therefore, an internal EGR system that uses the thermal energy of the burnt gas is generally designed. Such a system employs a variable valve timing mechanism using the hydraulic or the electromagnetic to reduce the exhaust emissions and the fuel consumption and to obtain favorable drivability, generally. However, because such a system is expensive, there are few small motorcycles with the internal EGR system. In the present study, the developed internal EGR system that slightly opens the exhaust valve at the beginning of the compression stroke or the end of the intake stroke has been designed. With this opening and closing timing, the charging rate of fresh air-fuel mixture decreases very little because the intake stroke nearly completes. Enough burnt gas to decrease NOx emissions flows into the cylinder because the in-cylinder pressure is negative, and the air-fuel mixture in the cylinder is activated by increasing the temperature in the cylinder at the beginning of compression due to the thermal energy of the burnt gas. Therefore, this system does not need a variable valve timing mechanism; the EGR cam is arranged on the same shaft as the exhaust camshaft, and is operated by a solenoid. As a result, NOx emissions have been decreased by 50%, while providing favorable drivability. Moreover, a reduction in fuel consumption has been enabled by increasing of the air-fuel ratio since NOx emissions have been reduced significantly.