TIPS for Hybrid-Electric Driving


       The first tip for getting good gas mileage applies to all vehicles: slow down gradually in anticipation of stopping to minimize the use of the brakes—any heat produced in the brakes is wasted energy.  The friction of engine braking, as in down-shifting for steep downhill runs, is also wasted energy, but the brakes are spared wear and overheating.  Here is why you see signs coming into small towns on main highways, “Engine braking permitted only in emergencies.”  Don’t take it personally—it applies only to certain trucks.   Some heavy trucks have even more efficient engine braking by rearranging the valve action temporarily so the engine acts as an air compressor.  The pop of compressed air into the exhaust system at the end of the compression stroke is too noisy for small towns.  The energy required for compression would be largely restored to the next down stroke of the piston even with no fuel burned.  These special engine brakes for trucks are called “Jake” brakes.

       In a hybrid electric vehicle, when you slow down gradually without applying the brakes, all the energy that would be wasted in the brakes or the spinning engine appears as electric energy and is restored to the 200 volt battery (twenty-eight 7.2 volt nickel nickel-hydride cells in series for a Prius—an ordinary 12  volt battery is six 2 volt cells in series).  The mechanism for regenerative braking in a hybrid vehicle is simple in principle.

       A computer controls the gasoline engine and only starts it when electric power in insufficient either because the accelerator pedal is calling for more power than can be produced by the electric motor or the battery is low.  Also simplifying the hybrid system is the fact that an electric motor and an electric generator can be one and the same machine without loss of efficiency for either function.  When it is acting as a generator, it resists being spun.  The energy required to spin it in spite of the resistance is the source of the energy that appears in the battery.  Obviously, when producing power, it consumes from the battery an appropriate amount of electricity measured in watts or kilowatts.

      So the electric motor can be continuously in the drive train spinning at a rate proportional to road speed.  The computer and the accelerator pedal determine whether it is acting as a motor or a generator at any moment.  The gasoline motor is connected to the drive train by a freewheel which connects it to the drive train only when it is trying to spin faster than the rest of the drive train (like the pedals of a bicycle when you pedal as fast as the bicycle is already moving).  The position of the transmission selector lever marked “B” creates a solid connection in the freewheel permitting engine braking as in a conventional automobile.

       Now to understand the most subtle point in achieving the very best fuel economy in hybrid vehicles: imagine traveling through rolling hills on cruise control driving one of these cars.  The cruise control will maintain speed uphill with gasoline power; relatively little electric power will be used because maximum gasoline power is greater than maximum electric power.  The result will be a nearly fully charged 200 volt battery at hilltops preventing recovery of a maximum amount of electrical energy on the downhill portions.  This problem can be overcome by using more electrical energy on the uphill by permitting loss of speed as the top of the hill is approached (turn the cruise control off).  One’s overall progress can be nearly normal by gradually speeding up on the downhill portions (if the speed limit permits). Remember, if electrical motor power is sufficient for any assigned task for the hybrid system, the gasoline engine will not be invoked.  Rather exact monitoring of this procedure is possible by keeping the instantaneous gas mileage readout at or only slightly above the expected consumption (experience will soon be close enough).

       Similarly in stop and go driving, if you accelerate mostly with electric power, the battery will be ready to more fully capture the benefit of  regenerative braking when you slow down.  This is why slower acceleration is more important for fuel economy in driving a hybrid than it is for a conventional automobile.

       As you learn more by experience, I will appreciate hearing from you so I can add your new discoveries to these tips (after I test them out for myself).

John Frantz, NASW,  April 26, 2007  <   >   

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