Detroit Diesel Series 50

The Detroit Diesel Series 50 is an inline four-cylinder diesel engine, that was introduced in 1993 by Detroit Diesel. The Series 50 was developed from the existing block of its sister engine, the Series 60, which itself was initially designed by Detroit Diesel. The cylinder heads were cast by John Deere at one time.
History.
The Series 50 engine is used as a major bus engine in North America, especially from buses built in the mid to late 1990s and early 2000s. The power plant was offered for several applications: trucks, buses, motor homes, construction and industrial equipment, and military vehicles. It is unusual to find an inline four-cylinder engine propelling heavy duty buses, which traditionally use inline six, V-6 or V-8 diesel engines.
In 2000, in order to better respond to more stringent EPA emissions standards, Detroit Diesel announced revisions of the Series 50 for diesel applications. The changes included the addition of an exhaust gas recirculation (EGR) system, and a variable geometry turbocharger system to improve torque. 
In September 2004, Detroit Diesel ceased production of the Series 50 heavy duty diesel engine for on highway applications due to emissions standards. It is still available for off-road applications. A replacement was scheduled to take place in 2007, then was pushed back until 2009. According to Detroit Diesel, Series 50 engines will no longer be produced for non-Daimler AG trucks or buses beginning in 2010.
Alternative fuels.
In addition to being powered by diesel fuel, the engine can also use alternative fuels such as compressed natural gas (CNG) or liquified natural gas (LNG) under the Series 50G label. Several thousand Series 50 propelled buses are now on the streets of major cities such as New York City and Los Angeles.