http://www.autoweek.com/apps/pbcs.dl...024/LATESTNEWS

The drag is a function of the velocity squared so the wonder is not that the Prius gets worse mileage at high speeds but why do other cars get worse mileage at low speeds! The rest of the article is pure speculation not based on physics taught even in high school.

Bob Wilson

 

The Prius is optimized for city/suburban driving not sustained highway speeds. Even the Prius, if driven at a low enough average speed, will suffer a lower MPG.

Other cars can suffer at highway speeds and generally have a "sweet spot" less than say 75 mph. Anecdotal observations indicate that my HAH gets the best mileage at about 55mph, say 42-45 mpg. Things are worse at 75mph with say 36-38mpg being typical.

 

Near as I can tell, the Prius maximum range speed is ~18 mph and it really doesn't start to go down until under 14 mph. In short, it takes speeds close to parked to see a low-speed MPG hit. <grins>

Bob Wilson

 

The worst part is that the average reader will nod his or her head in a agreement as they read it. Just one more off the target article for the average Joe to add to their hybrid myth toolbox.

 

Actually, I tend to agree with most of the article with this change to one paragraph:

But at steady highway speeds, the Prius is just another REALLY EFFICIENT gasoline-powered car. Worse, it lugs around parts that the gasoline engine doesn't need (thats true when only the ICE is in use). That's one reason that its EPA fuel economy rating is less on the highway - 51 mpg - than it is in the city - 60 mpg.

The WIND drag is a function of velocity squared. In addition, I think rolling and ICE drag are proportional to velocity. The Prius, with its low CDa and low friction everything else, makes it "just another REALLY EFFICIENT gasoline-powered car".

On the other hand, my old Jeep Cherokee was much more consistant than many hybrids - always got about 17-18mpg at most speeds and driving conditions

 

So in the "WIND" paragraph, drag is a function of velocity squared but in the next paragraph the Jeep always gets "17-18mpg" . . . something is wrong with the 'theory' and the 'practice.' The simpler explanation is the Jeep has a real problem at low speeds.

Bob Wilson

 

Even at 60 mph, rolling resistance, ie the drag of the tyres on the road, accounts for ~50% of the power requirement.

Only at speeds beyond this does aerodynamic drag become much more relevant.

As rolling resistance is directly proportional to vehicle weight, any reduction in weight will translate to a considerable reduction in fuel consumption, even at steady highway speeds. This is the main reason for the great highway mileage figures of the 850 kg Insight (the aerodynamic Cd of 0.25 is only a minor contributor).

 

Using Wayne Brown's Prius MPG simulator, rolling resistance accounts for 20.3 % of power requirements at 60 MPH for the NHW20 Prius. This assumes the following:

• Goodyear Integritys inflated to 44/42
• Smooth dry pavement
• 75F ambient temp
• No winds
• No AC use
• 250 pound load, occupants + cargo

The energy breakdown:

• Aero drag: 6.37 kW (48.4%)
• Mechanical drag: 3.64 kW (27.7%)
• Tire rolling resistance: 2.25 kW (17.1%)
• Road rolling resistance: 0.43 kW (3.3%)
• Miscellaneous: 0.46 kW (3.3%)
• Total energy use: 13.15 kW

According to the simulator, increasing speed to 70 MPH in the scenario above increases the aero drag component to 56.6%, and decreases the rolling resistance component to 17.4%. It also drops fuel mileage from 58.11 MPG to 49.83 MPG.

As an aside, this simulator is available for download and purchase from Wayne's site, $22 for either the Windows version or Palm version, $27 for both. It's a handy tool for Prius owners.

 

Just for giggles, for the scenario in my previous post I adjusted the Prius MPG simulator to a load of -750 pounds, or 1000 pounds less. At 60 MPH, energy requirements drop to 12.26 kW, or a 6.8% drop. Fuel mileage increases to 61.9 MPG.

A disclaimer: Though the simulator allows entry of a load down to -1000 pounds, I don't know if its mathematical models are designed to account for a negative number.

EDIT: In an e-mail dialog with Wayne, he confirms: "Yes they can handle negative numbers in the 'Load' area; I went to a an extreme to modularize the programs just in case they become popular in order to crank out versions for several different vehicles."

 

These numbers seem about right:


Use 746 watts/hp to get the equivalent kW:

60 mph ~= 17.5 hp -> 13,055 watts (~52 MPG, measured)
70 mph ~= 25 hp -> 18,650 watts (~49 MPG)

The challenge is adding the ICE efficiency as a function of load and this remains a non-trivial problem.

Bob Wilson