A method for controlling a start-stop operation of a refrigeration unit to reach a desired setpoint temperature, the refrigeration unit being powered by an engine and having a compressor, the method comprising the steps of: operating the engine, by a controller, at a reduced first speed during a delay period; extending the delay period, by the controller, based on the reduced first speed, so as to at least partially compensate for the reduced low engine speed, wherein the extended delay period is determined from a fuel map of the engine being preprogrammed into the controller, the fuel map providing information pertaining to the resulting fuel consumption and efficiency of the engine operating at different engine speeds and for different runtimes or delay periods; wherein, in the fuel map, engine speeds are correlated with the resulting fuel consumption and efficiency of the engine, and with runtimes or delay periods of the engine, such that a reduction in engine speed results in an improved immediate efficiency and in a longer runtime or delay period; increasing a displacement capacity of the compressor, by the controller, based on the extended delay period, so as to further compensate for the reduced first speed and to minimize the time required to reach the desired setpoint temperature; and operating the engine, by the controller, at a reduced second speed, the reduced first speed being less than the reduced second speed, until the desired setpoint temperature is reached, and stopping the engine by the controller to conserve fuel. operating the engine, by a controller, at a reduced first speed during a delay period; extending the delay period, by the controller, based on the reduced first speed, so as to at least partially compensate for the reduced low engine speed, wherein the extended delay period is determined from a fuel map of the engine being preprogrammed into the controller, the fuel map providing information pertaining to the resulting fuel consumption and efficiency of the engine operating at different engine speeds and for different runtimes or delay periods; wherein, in the fuel map, engine speeds are correlated with the resulting fuel consumption and efficiency of the engine, and with runtimes or delay periods of the engine, such that a reduction in engine speed results in an improved immediate efficiency and in a longer runtime or delay period; wherein the extended delay period is determined from a fuel map of the engine being preprogrammed into the controller, the fuel map providing information pertaining to the resulting fuel consumption and efficiency of the engine operating at different engine speeds and for different runtimes or delay periods; wherein, in the fuel map, engine speeds are correlated with the resulting fuel consumption and efficiency of the engine, and with runtimes or delay periods of the engine, such that a reduction in engine speed results in an improved immediate efficiency and in a longer runtime or delay period; increasing a displacement capacity of the compressor, by the controller, based on the extended delay period, so as to further compensate for the reduced first speed and to minimize the time required to reach the desired setpoint temperature; and operating the engine, by the controller, at a reduced second speed, the reduced first speed being less than the reduced second speed, until the desired setpoint temperature is reached, and stopping the engine by the controller to conserve fuel.