Caterpillar D4

The Caterpillar D4 track-type tractor is a small bulldozer manufactured by Caterpillar Inc.Caterpillar first introduced the D4 in 1938 as the diesel follow on to the successful CAT 30 gas model. [http://www.tractordata.com/td/002/td2982.html] In 1935 Caterpillar started the naming convention of 'D' for diesel or 'R' for regular gas followed by a number to indicate the relative size. [http://www.aghistory.org/cgi-bin/default.asp?AID=168] At the time of introduction the D4 engine produced about 40HP so the 4 indicated the relative engine power. The D4 series engines quickly increased in power so the numbers just became a figure of merit rather than indicating actual engine power.