The Detroit Diesel Series 50 is a straight-4 diesel engine, that was introduced in 1993 by Detroit Diesel. The Series 50 was built off of the existing block of its sister engine the Series 60 which itself was initially designed by Detroit Diesel The heads were cast by John Deere at one time. Deere is known for quality engines used in highway vehicle applications. The Detroit Diesel Series 50 runs on diesel and the Series 50G on compressed natural gas (CNG). A few thousand units engines are now in the buses on the streets of major cities such as New York City and Philadelphia.