Typical caterpillar tractors, applied to wood gathering in forests, are powered by internal combustion (IC) engines, which are sized to meet the full drivetrain load. However, the maximum power is required only in a part of the total operation time during work cycles of logging tractors. For this reason the hybrid electric propulsion system, installed instead of IC engine-hydrostatic transmission system, may considerably improve vehicle performance indices such as fuel consumption, pollutant emission and noise level. In this paper we present a series hybrid electric powertrain configuration for a small (2100 kg of mass) crawler tractor used for logging in forests. The drivetrain is composed of a thermal engine, an electric generator, a group of lead-acid batteries and electric drive motors with controllers. To calculate energy balance of the designed vehicle a special "forest cycle" was elaborated and discussed in the paper. The cycle was created on the basis of experimental data, measured in the actual tractor work. Since average power required during the "forest cycle" is much lower than the maximum one, the hybrid electric system needs a smaller IC engine than one in the conventional vehicle. This engine, coupled with electric generator, works in operation conditions yielding better fuel economy. The electric drive motors are fed both by the generator and the batteries in the "full load" cycle phase. The batteries are recharged during vehicle down-hill moving and standing phases.
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