Prius Facts and Hype

Toyota PRIUS Hybrid Vehicle: It is not a car but a phenomenon

The Myths, Facts, and Hype ...

by M. Kostic * 2010-Prius

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Car  Fuel Economy & Racing Car Performance * Rail-Road Transport

We could increase ANY car efficiency (everything else being the same, including Prius too) , i.e., mileage per gallon (mpg), if we: (1) drive with lower speed(note that air resistance increases with speed squared), (2) minimize breaking and sudden acceleration (regenerative breaking recovers good part but not all of energy, thus it still wastes energy; unnecessary and sudden acceleration as transient process is less efficient than an optimized steady-state driving, it also increases unnecessary breaking and wear of tires and other car components, not to mention safety risks), (3) minimize use of "energy hungry" accessories like Air-Conditioning (A/C), defrosters, etc., and (4) minimize frequency of cold engine warm-ups (combine short trips). Everything else you read online or in print is more product of imagination than reality.

Myth 1: Prius is very efficient since it uses two sources of energy, from gasoline and electrical energy from battery.

Fact 1: In fact Prius uses only one (primary) source of energy from gasoline. Prius is never plugged-in and does not use external electrical energy at all. The electrical energy generated and stored in its capacitors and battery is produced by gasoline engine only (thus not free), directly from gasoline engine during driving and indirectly during "regenerative breaking" from the car kinetic energy, which in turn was again generated from gasoline engine work. However, having an energy storage battery provides for much better (more efficient) energy management and regenerative breaking (partial recovery and storage of otherwise wasted energy), in addition to taking exceptional care of  details and optimization of many energy-(re)generation and energy-utilization processes (HSD), making Prius a fine, innovative car. Some even say, PRIUS is not a car but a phenomenon!

Myth 2: Prius has better city mileage (60 mpg spec) than on expressway (50 mpg spec) since it uses "free" electrical energy from battery, charged during "regenerative breaking." Note that real mileage is smaller than specs (low/mid 40s mpg) due to cold engine warm-ups in short city trips, use of A/C, and excessive speeds on expressways (over 55 mpg; note that air resistance increases with speed squared).  

Fact 2: The better city mileage is due to much lower air resistance and thus much lower power needed at lower speeds. Everything else being the same, the less stop-and-go and less "regenerative braking," the better city mileage will be, see below. If you would drive on expressway with steady city speed (35 mph, would be illegal, or in city without any stopping/breaking), the mileage will be much better than 50 mpg or 60 mpg, say 85+ mpg, without charging battery at all. During usual city stop-and-go driving, a large part of Prius kinetic energy while breaking (say 50%) is regenerated (called "regenerative breaking"), that is, it is stored in the battery as opposed to all of it being lost in heat as in classical cars, so that  Prius mileage is reduced less than in regular cars, say from 85 mpg without any breaking to 60 mpg with usual city breaking (if "regenerative"), instead to say 35 mpg with regular breaking using efficient economy cars. Thus, breaking always waste energy (the less breaking the better mileage, always, and for Prius too!) -  however, Prius is not as bad (recovers half of it) as classical cars are (they waste all of it).

Myth 3: Prius does not use gasoline for Air-Conditioning since its A/C compressor is driven by electrical motor that uses "free" electrical energy from battery.

Fact 3: Since Prius is never plugged-in to charge its battery from electrical outlet (still would not be free), the battery is charged by gasoline engine directly and indirectly during regenerative breaking using the car kinetic energy which had been generated in the first place from gasoline engine work during car acceleration. Thus, everything else being the same, the mileage is always better if A/C is not used, since it is driven by electrical energy ultimately produced from gasoline.

My two questions (to be answered by Toyota: my 01/23/07 email has been documented at Toyota National Headquarters under file #200701241154):

Question 1:

Is the Prius engine Compression ratio 13:1 and how it could use regular gasoline if it is true?

I believe its compression ratio is smaller since the intake valve does not close when piston is at the BDC (they called it Atkinson cycle; the compression ratio is smaller than the expansion ratio), thus no self-ignition problem with regular-octane gasoline. Prius engine expansion ratio is 13:1 to enhance fuel efficiency, but compression ratio should be smaller to allow use of regular gasoline. Please verify with the technical department and if I am correct, please respond why misleading specification information.

Question 2:

If the engine rated power is 76 HP and the traction battery is 28 HP which adds up to 104 HP, how come the Hybrid System Net Power is 110 HP (I assume continuous for some time, not burst power), as if the energy conservation law is violated. What are the power sources (continuous for some time, when both gasoline engine and the battery are used) to add up to 110 HP? Note that for a very long up-hill driving the battery will discharge and the continuous System Net Power will  be 76 HP gasoline engine only.

Thank you in advance for your response.


Prius * Hybrid Synergy Drive * Graham's Prius * Prius Power Train 

John's Stuff - Toyota Prius Education - Energy Flow * HC

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