With people concerned about rising gas prices and increasing carbon emissions, all kinds of different alternative fuel systems for vehicles have been considered. Ethanol, biodiesel, solar power, compressed natural gas, hydrogen fuel cells, and hybrid electric cars have all been tested. Out of all these options, hybrid electric cars have certainly gotten the most attention. But do they deserve it? Is hybrid electric really the best solution to our current fuel and emissions problems?
The answer is no, at least for large vehicles. Big, heavy vehicles like garbage trucks can benefit from a more efficient and cleaner technology: hybrid hydraulics. Like hybrid electric motors, hybrid hydraulic motors use energy from regenerative braking to power vehicles. But whereas electric cars use this energy to charge a battery which then powers the car, hydraulic vehicles send this energy directly to the drivetrain.
Hybrid hydraulic systems work by harvesting the kinetic energy generated by friction in the brake pads as the vehicle slows. This energy is used to run a hydraulic pump, which moves fluid from a reservoir to an accumulator in order to pressurize nitrogen gas. When the brakes are released, the pressure in the accumulator is also released, and this energy powers the car and allows it to accelerate.
There are two different types of hybrid hydraulic systems: parallel and series. In a parallel system, the accumulator is hooked into a conventional drivetrain. This provides acceleration assistance but doesn’t allow the engine to shut off at any point. In a series system, two hydraulic pumps are used to turn the wheels more directly, bypassing many traditional components of the drivetrain. This provides increased efficiency and also allows the engine to be shut off when the vehicle is stopped.
Hydraulic hybrids have some serious advantages over electric hybrids. For example, because they do not require batteries, many of the environmental concerns associated with battery manufacture and disposal are eliminated. Hydraulic hybrids are also more efficient. While an electric hybrid can recover about 30 percent of the energy created during braking, a series hydraulic hybrid can recover 70 percent. Hydraulic components are light, simple, and low-maintenance, which increases fuel efficiency and cost savings even further.
One big disadvantage of hydraulic hybrids is that they cannot power air conditioning or radios like a battery can. This means that the driver will either have to keep the gas engine running or else do without these amenities while at rest.
It may surprise you to know that this technology is already in use, and has been since 2005, when UPS converted some of its fleet to hybrid hydraulics. The EPA estimates that UPS reduced their emissions by 40 percent and saved 1,000 gallons of fuel by taking this step.
While hybrid hydraulic vehicles are not likely to catch on for highway driving or passenger vehicles, it will be interesting to see how the technology fares for large vehicles.