I’ve been doing a lot of comparisons lately, so I’m going to continue the trend and compare hydraulic and electric hybrid vehicles.
Electric hybrid vehicles, first made popular by Toyota’s Prius, are well known to many of you, but hydraulic hybrids are still rare. I’ll start by defining a hybrid: an auxiliary vehicle propulsion system using the recovery of kinetic energy to be re-introduced during acceleration. Essentially, a hybrid uses energy normally wasted during braking to charge batteries or accumulators, which will use the stored energy to power motors to aid acceleration, reducing fuel consumption. They are called hybrid, because they utilize two forms of drive, rather than just the usual single gas or single diesel engine.
The electric hybrid comes in a couple forms: series hybrid and parallel hybrid. A series hybrid has no direct connection between the engine and the wheels. Electric motors drive the wheels, and the sole purpose on the onboard engine is to turn a generator to charge the electric batteries. A parallel hybrid uses both electric motors and mechanical transmissions to put power to the wheels, and both can operate simultaneously to increase forward motivation.
The hydraulic hybrids us a combination pump/motor for converting between recovery and propulsion. During vehicle braking, the pump/motor is in pumping or recovery phase. The momentum of the vehicle keeps the wheels turning, which through driveline enhancements, turn the pump/motor to pump fluid under high pressure into the accumulator bank. During propulsion phase, our pump turns into a motor, using the energy stored in the hydraulic accumulators to add torque to the drive train, reducing the energy needing to be created by the engine.
Electric batteries are limited in their ability to give and take electrons quickly, but excel at gradual transfer in and out over an extended period of time. This allows an electric hybrid to operate in electric-only mode for a reasonable period of time, and continue to supplement the engine for acceleration runs even longer before their usefulness is depleted or they are recharged. The exception is the use of capacitors, but I won’t discuss those at this point.
Hydraulic accumulators can absorb and expel massive amounts of energy for a short period of time. As far as I know, hydraulic hybrids are being used only on commercial vehicles, like garbage trucks and delivery vehicles, so it makes sense that they can output large amounts of power over a short time. When a truck stops, it has a massive amount of inertia ready to be absorbed, even if it was travelling at a slow speed to start. Conversely, a truck requires a lot of power to get moving from a stand still.
Both systems work well in either application, because is each is tailored appropriately. Although relatively heavy, batteries packs in electric hybrids can form to any shape, so they’re easy to install into a car. Accumulators for hydraulic hybrids look like torpedoes, are even heavier than batteries and require a lot of space to install, which is easy to find on a commercial truck. Vehicles like garbage trucks and delivery vehicles have rapid stop/start cycles, give plenty of opportunity to recover kinetic energy. Hydraulic hybrids won’t see a lot of action on long-haul rigs because they don’t stop often enough to recharge the accumulators, and any hybrid vehicle loses its advantage in efficiency over a long enough distance.