*3.6. Onboard Power Consumers*

The main onboard power consumers of an HD vehicle are the air brake compressor, power steering, air conditioner compressor, engine oil and coolant pumps, engine fan, and electrical accessories. In a conventional powertrain, most of these are powered mechanically by drives connected to the engine shaft. Those of them not having disconnecting mechanisms, consume some power even in the idle state. In the literature [13,33], one can find typical ranges of the consumed power for each of these components as a function of the engine shaft speed as well as their operating cycles based on the recommendations of the SAE J1343 standard [34]. An operating cycle is defined by two numbers, namely Δ*t* being the averaged period of the operating cycle and α being the fraction (i.e., percentage) of the operating cycle when the component is engaged. Table 3, based on the information from References [13,33], summarizes the power and duty cycle data for the auxiliaries included in the model of the conventional powertrain. In this table, "A/C" stands for the air conditioning and "Electrical acc." denotes the low-voltage electrical accessories. The columns "Min. power" and "Max. power" contain the ranges of power consumption corresponding to the engine rpm range, which was from 500 rpm (the lower power value) to 2000 rpm (the higher power value).



The power feeding the electrical accessories is drawn from the engine shaft through the alternator, whose efficiency, obtained from the known literature [35,36], was taken into account in the elaborated model.

The engine coolant and oil pumps are not listed in Table 3 since in this work, they were considered as parts of the engine system. Their power demand is taken into account by the fuel consumption characteristics of the engines since the latter underwent the bench tests with the said auxiliaries installed. The coolant and oil pumps were also considered identical for both the conventional and the hybrid powertrains.

In a hybrid powertrain, the electrified auxiliary components are regulated independently from the engine operating regime, therefore consuming the exact amount of power required for their operation [13]. Table 4, based on the information from [13], shows the average power and operating cycle data for the auxiliaries assumed to be electrified in the hybrid powertrain. Besides the conventional auxiliaries, the cooling system of the high voltage components was taken into account (denoted as HV cooling).


**Table 4.** Power and duty cycle parameters of the auxiliary loads in the hybrid HD powertrain.

A DC–DC converter was considered as the interface between the low voltage auxiliary system and the high voltage traction system.
