Quote Of The Day II: 230 MPG "May Be Overly Optimistic?" Edition

by Edward Niedermeyer

Published: September 30th, 2009

quote of the day ii 230 mpg may be overly optimistic edition

Official fuel economy testing for all vehicles is conducted on chassis dynamometers, which are basically treadmills for cars and trucks. One subtlety of chassis dynamometer testing is that vehicle fuel economy measurements using decades-old standard speed profiles may be overly optimistic compared to today’s average on-road fuel use. Official methods exist to adjust the test cycle fuel economy of conventional vehicles to better estimate expected real-world fuel use, but a similar adjustment method has yet to be finalized for PHEVs.From a National Renewable Energy Lab paper on plug-in hybrid efficiency testing [via Green Car Congress].The paper by scientists from the NREL and the Idaho National Lab ( full PDF here) continues by explaining how even their newly proposed methodology has shortcomings:The Blended Method…assumes that the increase in gasoline use during CD [charge depleting] mode is the same as the increase calculated for CS [charge sustaining] mode. This works well for blended PHEVs that have lower electric power capabilities for CD mode, and would thus require additional engine power (blended with the electrical power output) for more aggressive driving. The downside of this method is that PHEVs with high electric power capabilities may not need help from the engine and therefore would not use more gasoline in CD mode but would simply deplete their battery energy over a shorter distance. It is also possible that a blended PHEV would actually increase its depletion distance in the event that the vehicle controller commanded the added engine output in CD mode to be high power (to achieve high engine efficiency) and thus prolonged its battery depletion.Even so, such tradeoffs between CD fuel consumption and depletion distance should somewhat balance out through UF [utility factor] application. For instance, though this Blended Method for applying adjustments may penalize the high electric power PHEV with some excess CD fuel use, the method assumes a longer CD distance than the vehicle actually achieves. This gives it an inflated UF weighting for CD fuel displacement (relative to its CS fuel use). These two factors may roughly balance each other out when calculating the total combined consumption. A similar balance (in the reverse direction) could work out for the longer depletion distance blended PHEV.It’s worth a read as long as you’re comfortable with this kind of prose. Measuring plug-in efficiency is no picnic…

Edward Niedermeyer

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Carve on Oct 02, 2009

That's the whole reason mpg is listed on the sticker- so you can determine operating cost. 18 mpg 10 years ago means something totally different than today, and means somethind different in Japan than in the US, but it is still 18 mpg no matter when and where they are. This is because the context changed. Would you rather the number be fixed at 1979 dollars per $.85/gal 1979 gas? The same 18 mpg car in 1979 would rate at 21.2 miles per dollar, and today it would be 7.2 miles per dollar with NO CHANGES othe than the cost of gas and the value of the dollar. Furthermore, that car will get 18 mpg whether the gas is expensive gas in Japan or cheap gas in Saudi Arabia, but the miles per dollar would be dramatically different. The dollar changes too much, and the price of electricity and fuel are too variable for this to be a useful, published measure. Remember when diesel was more than gas recently? Could you imagine having to go check the miles per dollar of your car every few days? Just tell us the efficiency, and the energy providers will tell you the price of the fuel. Of course, the volt is even using miles per dollar since that is entirely dependent on the length of your trip. They're making it even more confusing. On top of all that, to truly be miles per dollar, you'd have to consider the insurance, operations, and maintenance cost of your car. What you really need to know to make comparissons is a car's energy efficiency and the cost of that energy FOR YOU. A blanket label for this will be correct for almost NOBODY.

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KarenRei on Oct 02, 2009

It's really, really simple. Three rules: ------------- 1. If You Have More Than One Distinct Drive Mode, You Get More Than One Sticker. 2. Everything That's On One Sticker Goes On The Other(s). 3. All Units Of Fuel Consumption Should Be Presented In Terms Of The Fuel(s) Actually Used, With "Consumption" Relative To The Moment The User First Acquires The Fuel(s). ------------- So, if you normally would have a sticker that says "30mpg city/36 mpg hwy/500 miles per tank", and you make that into a series-style plug-in hybrid, you now have two stickers: one that says "30mpg city/36 mpg hwy/500 miles per tank", and one that says "200 Wh/mi city/260 Wh/mi hwy/40 miles per charge" (with the Wh/mi being wall to wheels, not pack to wheels) I don't see why anyone would possibly want any other kind of measurement. Why wouldn't you want all the info from one sticker for each distinct drive modes your vehicle has? Why wouldn't you want your units in terms of when you acquire the fuel (I.e., wall-to-wheel energy efficiency instead of pack-to-wheel energy efficiency)? How could you possibly make an informed decision with everything lumped together into a contrived "mpg" rating that has no reflection on how you'll actually drive the car?

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