The advantage in fuel economy that comes from driving a hybrid-electric car instead of a non-hybrid has not one major contributing factor, but three. Even if you don’t drive a plug-in hybrid, these innovations drastically improve the vehicle’s efficiency using clever applications of physics and optimization. 

Regenerative Braking

Likely the most familiar method of reducing loss in hybrid vehicles is regenerative braking. It uses the same principles that drive the car forward, except energy flows from the wheels to the battery instead of from the battery from the wheels. 

When the spindle of a motor is spun, its interior magnet and coils rotating relative to each other generates a voltage, or electromotive force (EMF). When the voltage from the battery is applied to the motor, spinning it, the EMF generated by the coil works in the opposite direction of the applied voltage. Known as back EMF, this is what causes the perceived ‘resistance’ of the motor. If the spindle is held in place, there will be no back EMF, and thus no resistance (AKA a short circuit). This is why a motor overheats if the mechanical load on the spindle is too high. Conversely, if the spindle is forced to rotate faster than the applied voltage would cause it to, the back EMF will be higher than the applied voltage. Electrical current will flow in the opposite direction, charging the battery. Instead of using a friction-based hydraulic brake to slow the vehicle, regenerative uses the motion of the wheels to spin the motor. The motor is able to extract energy from the wheels, slowing them down, and use it to charge the battery. Otherwise, the energy from the moving wheels would be dissipated as friction. Regenerative braking also has the side benefit of not wearing down the brake pads.

Optimal Engine Speed

In a non-hybrid vehicle, the most efficient driving speed is typically around 50 miles per hour. Generally speaking, the engine itself has a power level where it is most efficient, which depends on the specifics of its design and construction. However, it is air resistance that limits the efficiency of vehicles at high speeds. As air resistance applies a force that is proportional to the square of the velocity, fuel consumption increases drastically at high speeds. Since the internal combustion engine is the non-hybrid’s only source of power, the engine has no choice but to operate at the driver’s desired power level. With a hybrid, the engine can always run at its most efficient because the battery is there to provide additional power to the wheels if needed. If the energy produced by the engine at its most efficient level is more than needed at the moment, the excess can be used to charge the battery for future use. If the battery is sufficiently charged, the engine can turn off and the car can run on only battery power.

Atkinson Cycle

In many hybrids, the traditional Otto cycle (intake – compression – spark – power – exhaust) in the internal combustion is not used. Instead, the intake valve is held open longer than normal, which essentially shortens the compression stroke. Lengthening the power stroke relative to the compression stroke allows for more heat and pressure rejection from the cylinder during the power stroke. Essentially, this enables every bit of energy from the compression and spark to be utilized. It can also be thought of as decreasing the pressure in the cylinder during the compression stroke, which reduces the input force required (force = pressure * area), boosting efficiency. Unfortunately, this process greatly reduces available power, so it is not used unless there is a hybrid battery to provide additional power when needed. 

In non-hybrid vehicles, the energy saved by the preceding three methods would be wasted as heat. While it is true that energy cannot be destroyed, it can be dissipated, or converted into a non-useful form. In thermodynamics, useful energy is known as exergy, which CAN be destroyed. In any non-reversible (or non-ideal) process, the useful energy is decreased, and exergy is destroyed. Conversely, entropy, which is a measure of heat-based disorder and a non-recoverable form of energy, is increased. Reducing the amount of exergy destroyed means less fuel use, less greenhouse gas emitted from your tail pipe, and less frequent trips to the gas station.

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