Straight Engine Transmission

Diesel

Today's buses using diesel engines are 100 times cleaner than those built prior to 1985.  In the past 3 years, diesel engines have been imposed strict regulations regarding their nitrogen oxide (NOx) and particulate matter (PM) levels.  In 2007, PM emission requirements for new diesel engines were reduced to 0.01 g/HP-hr, an overall reduction of 99% from 1978.  Similar legislation was introduced in 2010, reducing NOx emission requirements by 98%, since 1978, to 0.2 g/HP-hr. 

New Diesel Engine Technology

The tremendous advances made in diesel fuel technology have been mirrored in engine design. Electronic control systems that allow buses to diagnose engine problems automatically mean better engine maintenance and enhanced fuel efficiency. Technologies like cooled exhaust gas recirculation (EGR) and 2 stage variable geometry turbochargers (VGT), common rail, diesel particulate filters (DPF), selective catalytic reduction (SCR), diesel oxidation catalyst (DOC) and engine exhaust after treatment have permitted the diesel engine to meet new stringent emission regulations.

Technology is continually being refined. Depending on speed and load, the EGR valve in the exhaust system is either open or closed. When closed, exhaust gases exit the turbocharger in the normal way. When the EGR valve is open, a percentage of the exhaust gases is diverted, first through a cooler, and then into the intake manifold, to mix with incoming air. The resulting mixture of fresh air and re-circulated exhaust gas burns at a lower temperature in the cylinder, thereby reducing NOx. VGT features small electronically controlled vanes inside the turbocharger housing. The position of the vanes can alter turbo-boost and pressure in the exhaust system. A sufficient differential of pressure, between the exhaust and intake systems, is necessary for proper flow through the EGR.