mph vs HP using EPA data, spreadsheet
 

Hi,

Already shared at PriusChat, I decided to take three sets of EPA roll-down data and plot the horsepower needed to overcome vehicle drag at 25-85 mph for:

• NHW11 Prius (2001-03)
• NHW20 Prius (2004-09)
• ZVW30 Prius (2010-2013)

http://hiwaay.net/~bzwilson/prius/CMAX_020.jpg

• 02-Prius (BYJ) - The 2002 Prius, column BYJ, is known to have higher rolling and aerodynamic drag. So it consistently takes more power across the board.
• 08-Prius and 13-Prius - These have lower rolling drag from improvements in transmission and aerodynamics. The smaller NHW20 Prius has lower aerodynamic drag above 50 mph while the ZVW30 Prius with a redesigned transmission has lower rolling drag.
• 52-55 MPG zone at 15 hp - there is a curious cluster in all three models of middle 50s MPG but there is a welcome, 5 mph spread.
• 39 MPG zone at 19 hp - there is another curious cluster at 39 MPG with a greater 7 mph spread.

Reducing rolling drag can be accomplished by:

• tire inflation - at some loss of ride softness, up to maximum sidewall pressure is OK. Given the loss of ~1-2 psi per month, have and use an air pump every 2-3d fill-ups.
• four-wheel alignment - Firestone and possibly Goodyear offer life-time alignment, ~$150, but just for toe of the front wheels. In reality, toe and camber (the tilt), of both the front and rear wheels can be adjusted using camber bolt(s) on the front and shims on the rear. This lets the tires wear down uniformly so the $250-500 tire sets run for their rated miles and don't have to be replaced early. It also improves handling and reduces rolling drag.
• transaxle oil replacement - unfiltered, the oil accumulates wear particles and drag increases. Our best studies indicate an early change, 5k service miles, is needed to flush left-over manufacturing debris and early sealant bead loss. Due to 15-20% carry forward from each change and the reduced particles from the gears as they self-polish, the subsequent changes should be 15k, 45k and thereafter 90k service miles.
• larger diameter tires - a very advanced technique, it in effect adds a permanent, increase in gear ratio. This will throw off the odometer and speedometer calibration and lower the true 0-60 mph time. The indicated MPG will appear to be lower but if you measure and apply the calibration correction factor and drive 'true speed', there is a small reduction in rolling drag. It also reduces the rotational rate of the transaxle gears and the higher gyroscopic forces will tend to improve steering tracking.
• save weight - part of rolling drag comes from the vehicle weight but given vehicle weights of ~3,000 lbs, it would take 300 lbs to have a 1% effect.
• user expectations - rolling drag effects are only dominant at slow speeds, 50 mph or less. But these techniques help mitigate the gradual increases in rolling drag as bearings and parts wear.

Reducing aerodynamic drag:

• cooling drag - fully enclosing the engine compartment air inlets reduces the drag by minimizing how much air flows through and robs energy. Typically done in the winter where the cold air can easily carry away the engine heat, it remains an area of study at Ecomodder.com. Active air inlet vanes is an area of advanced study.
• wheel wheel drag - there are small air-dams in front the tires and they help. However, a lot of air is entrapped and spills out the wheel wells. Wheel well covers can reduce this significantly.
• wheel spoke drag - moon caps
• bottom panels - already built into the ZVW30 and partially in the NHW20, the NHW11 has none. An advanced technique (speculation) would be a 'shark jaw' air inlet, temperature controlled that lowers down to take under-car air just behind the front bumper and routes it up to the radiator(s). This allows complete coverage of the ordinary bumper and front air inlets.

Follow-up

The curious falloff of MPG, 50s->39 MPG, between 15 and 19 hp suggests something is going on in the engine and/or transmission. In theory, we can use the BSFC data along the operating line to build a model for MPG vs mph. This is an interesting area for Prius performance investigation.

I have uploaded a zip file with the excel workbook used for this chart. Feel free to download and play with it. It includes all vehicle data from 2002, 2008, and 2013 so other makes and models can be easily compared. It includes a set of instructions. Also, thanks to Ecomodder.com which hosts those who know what this spreadsheet can and can not do.

MY APOLOGIES!

When I realized neither my 1.4 MB '.xlsx' or a zip file could be uploaded (size limits,) I saved it as a '.xls' file forgetting it would truncate the columns in the EPA data. You can get a copy of the '.xlsx' file from either PriusChat or the YahooGroup 'Prius_Technical_Stuff.' Sorry, I forgot but there are at least two places to pickup the full size file.

The other alternative is to read the instructions spreadsheet and after converting to a more capable spreadsheet, one with a larger number of columns, it should be easy to make it work.

Bob Wilson