A unique feature of John Deere engines is that the engines are designated differently based upon the type of the air intake systems used in its varied industrial, construction, agricultural, oil and gas or other applications. Understanding these different designations is crucial in deciphering the application of the engine and in identifying the correct maintenance parts.

The five (5) different designations that are used for John Deere engines are D, T, A, S and H. The D is the oldest and simplest air intake system for John Deere engines. This is a an engine whereby the reciprocating positions will pull air into the air intake system directly through an air filter, and directly into the combustion chamber where combustion occurs.

The T designated engines are all turbocharged engines. The turbocharged engine process was a latter addition to the "D" engines to reduce fuel combustion and boost engine power. The turbocharged unit is fitted between the air intake system and the ducts leading to the combustion chamber. Exhaust gases are channeled through radial placed vanes on a shaft, which produced motion in one side of the turbocharged unit. The central shaft then turns a compressor in the second section of the turbocharger, pulling filtered air in and compressing it to a high density. The high density air is then compressed into the combustion chamber. The higher air density ensures more efficient fuel combustion and improved fuel economy.

The third John Deere engine designation is A. The A abbreviation is an indication that the engine is fitted with a turbocharger and an Aftercooler, and the cooling medium in the engine is air to water/coolant. The aftercooler is a cooling device like a radiator with its cooling medium being coolant or water.

In the John Deere 4650 tractor, the intercooler fits into the intake manifold of the engine and cools the compressed air that’s being routed from the turbocharger before it enters the combustion chamber. In the turbocharger, the compressed air makes it denser; however, the temperature of the air has also increased. However, hot air will expand less when burned in the combustion chamber and may also cause diesel knock. The liquid coolant intercooler is used to reduce the temperature of the compressed air to prevent these combustion problems.

The H designation John Deere engine abbreviation is another option to the A engine in that the engine is fitted with an intercooler, however, the cooling medium in the H engine is air. Ambient air is forced into the tubes of the intercooler, absorbing the heat of the compressed turbocharged air, and then being discharged. The H engine aftercooling system is called "air to air".

The final designation of John Deere engines is the S designation. The S John Deere engines are the marine engines, and the S symbol indicates that the marine engine is turbocharged and aftercooled, however cooled by a combination of air and seawater. There are different configurations of this cooling system, which may include additional pumps and pipes to pull in seawater and then to expel the heated water back into the river or sea.