PRIUS fule economy AGAIN
Hello -
Trick question - how many pints of Gasoline are in a Standard EPA Gallon, used for testing emissions and estimating fuel economy?
CORRECT ANSWER - NONE! There are NO measurements of gasoline flow or consumption made during the EPA emissions or fuel economy test, believe it or not!
Gasoline consumption is an ESTIMATE by the EPA fabricated from exhaust gas by-products, which are sampled during the famous EPA emissions tests, and hence the term ESTIMATED City (or Highway) mileage.
However, the Engine does not burn all the fuel it receives; some fuel is converted into non-gas by-products called particulates. Hence, sampling only the gas by-products gives unrealistically high economy ESTIMATES (but we all have learned that)! The most notorious of these is particulate carbon.
The EPA must filter out the particulates (eg, carbon) from the exhaust gas stream - to prevent damage of their expensive gas sensors - and knowing they are 'missing' a by-product, they use a gasoline-like fuel that possesses a uniform length hydrocarbon chain to provide minimal (or approximately known) particulate carbon. Hence, the ESTIMATED fuel economy is based on exhaust gas-sensor readings and estimates of the particulate carbon present.
Unfortunately, you can not buy the uniform length hydrocarbon gasoline the EPA uses in their emissions test - your corner gas station sells a blend of liquid hydrocarbons that range in boiling point temperature from approximately 65F to 400F degrees...and the higher the boiling point, the more particulate carbon is produced. Ever wonder why OIL fires have such dense black smoke - that is due to presence of particulate carbon in the exhaust smoke. Diesel engines, of course, use FUEL OIL - and what is their smoke color?
Thus far we know: Carbon provides the majority of gasoline's combustion heat, and particulate carbon is un-oxidized (unburnt) gasoline that produces NO energy as it passes through your engine, and is unmeasured (only minimally estimated) during EPA testing. So, now you know about the EPA Fuel Economy testing? NOPE, more to come!
Lets talk about wind resistance or drag - the EPA emissions tests are performed on a STATIONARY dynometer, where the vehicle does not move and the body experiences no air drag. Hence, the EPA ESTIMATES can not take into account the shape of - say a Prius - or comparatively of a Pickup truck or van.
Hence, the EPA Higway Fuel Economy ESTIMATE is for an average speed of 49 MPH, where vehicle air drag has minimal effect on the (emissions or economy) result. Ever wonder why there was no TOURING speed (65+ MPH) economy estimate? They can't estimate that until EPA places their dynometers within a programmed wind tunnel to create air flow that matches the EPA emission test speeds (CITY AND HIGHWAY).
Now, why does the Prius have a higher City MPG ESTIMATE than Highway? because Prius shuts off the IC engine at stoplights and severe traffic, and traction batteries do not produce particulate carbon or other exhaust byproducts when they move the vehicle. The EPA must use the same corrections for the battery that they apply to idling IC engines, and hence their ESTIMATE overstates the result.
Because EPA do not account for air drag, their Highway Prius estimates might reasonably be expected to understate mileage. My own 2005 Prius delivered 53+ MPG (recent 270 mile trip from Glen Burnie Maryland to Pittsburgh PA) from pump gasoline, which is more than the EPA Highway estimate of 51 MPG. Expect more improvement as the engine accumulates more than the current 2,700 miles and 'breaks in' the planetary gears and the piston rings. The actual city mileage is about 46 MPG; when it was purchased it delivered between 43 and 45 MPG until 1,000 miles were reached whereupon things kept getting better.
When was the last time you owned a vehicle that BEAT the EPA numbers? Well, don't expect accurate EPA fuel ESTIMATES until EPA begin to accept particulate carbon as; a pollutant, and have to actually measure it.
WilliamD
