Hybrid Gasoline-Electric Cars

The purpose of this section is to explore the pros and cons of hybrid cars. (At the moment we have no software products related to this subject.)

Issue 1 - Complexity

A hybrid car is generally more complex than an equivalent conventional gasoline or diesel model, but the situation is not as bad as you might expect:

Component Conventional Gasoline or Diesel Car Hybrid Car (Toyota Prius)
accessory drive belts one or more none
starter motor one replaced by AC motor
alternator one replaced by AC motor
rectifier AC to DC, simple power converter, much more complicated
differential one, assuming 2-wheel-drive one
gearbox multi-speed manual or automatic, complicated epicyclic, simple
clutch or torque converter one none
12V accessory battery one one
high-voltage traction battery none one
Table 1: Complexity Comparison

Although the hybrid has added an expensive traction battery, it has eliminated some mechanical moving parts, such as the expensive multi-speed gearbox. The hybrid uses a computer-controlled electric motor to vary the torque on one shaft of the epicyclic "power splitter" gearbox to achieve the same goal as a conventional automatic multi-speed transmission, but without the stepped gear changes. (This is the reason the Prius transmission is called an electronic-Continuously-Variable-Transmission, e-CVT, to distinguish it from the mechanical CVTs that use drive belts between a pair of tapered cones.)

Issue 2 - Efficiency

Fuel consumption for a hybrid car can vary by large amounts depending on driving style, length of trip, temperatures, and speeds. When comparing to other cars, the numbers can be deceptive. For example, though a 4 miles per gallon (mpg) variation sounds large in absolute terms, it might amount to only 10%. Also, driving at high speeds (where the aerodynamic drag is a major factor) and using a heavy foot on the brake pedal (where the hydraulic brakes are used more than the regenerative electric braking) can both decrease the hybrid's efficiency advantage.

The chart below shows the miles per US gallon for a new Prius V wagon over about 6 months and 18,000 miles. There are two points for each tank of gasoline, one from the car's mpg display and the other calculated from the car's trip odometer and gasoline station's pump display. The driver was a conservative one but not a hypermiler, usually traveling around the speed limit. More than half of the distance was on highways. The EPA efficiency estimates for this car are 44 mpg city and 40 mpg highway (not as good as the standard Prius due to increased size and weight), so the predictions matched the real world in this case.

Note that the mpg low points (in the 30's) included long periods parked with the electric air conditioning running, when the outside temperatures were close to 100 deg F. Under those conditions the car automatically started the combustion engine periodically to maintain battery charge, but lost only about 3 miles of range per hour. That amounts to 3 miles/45 mpg or about 0.067 US gallons per hour. If the car is parked with the cabin heater running and outside temperatures around freezing point, it will give similar results.


Chart 1: US Miles Per Gallon

Notes: To convert to British gallons, multiply by 1.2. In the UK this vehicle has seven seats and is called the Prius+.

Some have argued that diesel cars are better for highway driving, but unfortunately they lack the regenerative braking feature. Real world freeways are not always on flat terrain and heavy traffic often requires braking. Hybrids take advantage of these conditions by converting excess speed to battery energy. Often the combustion engine will be off for minutes at a time and the traction battery will be fully charged at the end of a downhill section. In highway traffic jams, whether stop and go or merely 10 mph below the speed limit, the car's trip-average mpg display will start increasing, since the combustion engine shuts down more often.

In summary, a hybrid car can meet or even exceed EPA mpg estimates. The abilities to run air conditioning and other accessories on battery power, to shut down the combustion engine when it is not required, and to recover excess kinetic energy through regenerative braking give the hybrid car an advantage over conventional gasoline and diesel vehicles.