A hybrid excavator uses both a diesel engine and an energy storage system to do construction work more efficiently. One of the best places to save energy is the swing motion, where the upper house rotates to move soil from a digging area to a dump area. In a normal excavator, much of the energy used to speed up the swing is lost as heat when the operator slows the machine.
A hybrid system can recover part of that energy and use it again, reducing fuel use and emissions.
Key Facts
- Kinetic energy of the rotating house is E = 1/2 Iω^2, where I is rotational inertia and ω is angular speed.
- Power is P = E/t, so recovering the same energy in less time requires higher power.
- Hybrid swing systems can use an electric motor generator or hydraulic accumulator to capture braking energy.
- Fuel savings come from reusing recovered energy instead of making all swing power from the diesel engine.
- Energy efficiency can be estimated by η = useful energy output / energy input.
- If 30 kJ is recovered each swing and the machine makes 100 swings, recovered energy is 3000 kJ.
Vocabulary
- Hybrid excavator
- A construction excavator that combines a diesel engine with an energy recovery or energy storage system to improve efficiency.
- Swing energy
- The rotational kinetic energy stored in the excavator upper house while it is turning.
- Regenerative braking
- A method of slowing a moving machine while converting some of its kinetic energy into stored energy.
- Accumulator
- A device that stores energy in pressurized hydraulic fluid for later use.
- Motor generator
- An electric machine that can act as a motor to drive motion or as a generator to recover energy.
Common Mistakes to Avoid
- Thinking the hybrid system creates free energy, which is wrong because it only recovers energy that would otherwise be wasted as heat.
- Ignoring the slowing part of the swing cycle, which is wrong because regeneration happens mainly when the rotating house decelerates.
- Assuming all recovered energy can be reused, which is wrong because storage, conversion, and motor losses reduce the usable amount.
- Confusing the tracks with the swing system, which is wrong because swing energy comes from the rotating upper house, not from the machine driving forward.
Practice Questions
- 1 An excavator recovers 25 kJ of energy each time the upper house slows after a swing. If it makes 80 swings in one hour, how many kilojoules of energy are recovered?
- 2 A rotating upper house has rotational inertia I = 6000 kg m^2 and angular speed ω = 0.8 rad/s. Use E = 1/2 Iω^2 to find its rotational kinetic energy before braking.
- 3 Explain why a hybrid excavator saves the most energy on jobs with many repeated swing and stop motions, such as loading trucks from a trench.