diesel engine

Are you talking about the engine or the transporting vehicle?

If it's the engine then you have to consider its power capacity--(KW or HP)

A high load would be a load that is near capacity.
A low load would be a fraction of capacity.
A full load is the same as a high load.
A part Load is the same as a low load.

Diesels come in all types and sizes. There are small air cooled diesel engines that are used on small equipment, there are diesel engines that are the size of a large house (AKA Cathedral) that are used to propel ships at sea, there are scavanged diesels that are almost two stroke diesels. The term diesel really means a thermodynamic cycle that is used in an engine as opposed to a gas turbine, a reciprocating steam cycle, a Stirling cycle, an Otto cycle and so on.

See Thermodynamic Cycle

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