EcoModder Forum EPA roll-down coef vs HP at 100 mph

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 06-16-2013, 12:29 AM #1 (permalink) Engineering first     Join Date: Mar 2009 Location: Huntsville, AL Posts: 792 14 i3-REx - '14 BMW i3-REx Last 3: 45.67 mpg (US) 17 Prime Plus - '17 Toyota Prius Prime Plus 90 day: 58.25 mpg (US) Thanks: 79 Thanked 196 Times in 126 Posts EPA roll-down coef vs HP at 100 mph Hi Recently we were discussing the HP-to-road power in a thread over at PriusChat. So I posted a recent chart showing the power required for a Volt vs Prius Plug-In (PiP): One of the posters correctly pointed out that my chart projects ~27 hp at 100 mph. Obviously something is wrong as this is off by a factor of four. Perhaps you' all can spot my error? I'm using the following:Test Car List Data Files | Cars and Light Trucks | US EPA - source of roll down coefficients drag HP = (A + B*v + C*(v*v)) / 7.5 :: "v-velocity mph", A, B, C from tables Now in the past, I've used an OBD scanner to record ICE rpm and MG1 torque to calculate shaft HP. This has been in fine agreement with the specs for our NHW11 (2003 Prius.) So I need to repeat this analysis using the EPA roll down coefficients and compare. But I'm really scratching my head on this one. Any thoughts? The reason this is important is the title of the thread,"64% of Prius Power gets to wheels during max acceleration?" The original poster is using some raw acceleration numbers and the vehicle weight to calculate the HP-to-road metric and found only 64% is getting there. This suggests an exceptional transmission efficiency loss. Understand, I'm OK with this as it may provide more insights about Prius transmission efficiency . . . if we can find independent confirmation. Unfortunately, I did a bad thing . . . loaned out my 2010 Prius OBD scanner. This one can read out ICE rpm and MG1 torque providing the raw HP data at the input to the transmission. Thanks, Bob Wilson __________________ 2019 Std. Range Plus Model 3 - 134 MPG3 || 2014 BMW i3-REx - 117 MPGe, 39 MPG JuiceBox 40 Pro (240 VAC, 40 A), KHONS portable (120-240 VAC, 12-32 A) Retired engineer, Huntsville, AL (five times AutoPilot saved.)
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 06-16-2013, 02:55 AM #2 (permalink) Eco-ventor     Join Date: Oct 2010 Location: sweden Posts: 1,503 Princess - '92 Mazda MX-3 GS House of TudorTeam Mazda 90 day: 53.54 mpg (US) Shirubāarō (*´ω｀*) - '05 Toyota Prius Executive Team Toyota 90 day: 53.13 mpg (US) Thanks: 64 Thanked 610 Times in 385 Posts The formula for power should have a component of velocity cubed. __________________ 2016: 128.75L for 1875.00km => 6.87L/100km (34.3MPG US) 2017: 209.14L for 4244.00km => 4.93L/100km (47.7MPG US)
 06-16-2013, 10:29 AM #3 (permalink) Master EcoModder   Join Date: Nov 2010 Location: Atlanta Posts: 403 Sweetie - '11 Hyundai Sonata GLS 90 day: 39.35 mpg (US) My Miles - '03 Combo GLS/KLX/Ninja 90 day: 40.49 mpg (US) Sipper' - '04 Kawasaki Ninja 250 90 day: 74.98 mpg (US) Thanks: 1 Thanked 37 Times in 28 Posts I don't know if you are wrong on the 27HP for 100 mph, a Ninja 250 has 25-26hp and tops out right around 100 mph, and it's far less aero efficient than most cars. __________________ This ain't a war, anymore than a war between men and maggots. Or, dragons and wolves. Or, men riding dragons, throwing wolves at maggots!
 06-16-2013, 11:25 AM #4 (permalink) Master EcoModder     Join Date: Oct 2012 Location: USA Posts: 1,408 awesomer - '04 Toyota prius Thanks: 102 Thanked 249 Times in 201 Posts fyi torque for android will give you lots of nice numbers and they have special pids for the prius http://ecomodder.com/forum/showthrea...4-a-25216.html And it is insanely cheap if you have an android already, I spent \$17 on software and adapter. It has torque/rpm for mg1,mg2,ice and all sorts of things (check out the spreadsheet of custom prius pids), I even use it to tell me the brake friction pad horsepower so I get a feel for how much I can brake on regen alone. But yes, under acceleration/high current it certainly feels "lossy". And all that mechanical to high current electrical, to a buck/boost/inverter to mechanical, it would not suprise me. It is basically a glorified torque converter, and mg1 always needs or generates power to oppose engine torque to get any useful work out of the planetary gear carrier. I have not gone 100mph, let alone looked at the power demands, but will let you know if I do But do check out the custom prius pids on a link in that link, good stuff.
 06-16-2013, 11:36 AM #5 (permalink) EcoModding Apprentice     Join Date: Feb 2013 Location: Saratoga County, NY Posts: 170 TheEgg - '18 Volkswagen Tiguan SEL Premium 90 day: 27.14 mpg (US) Thanks: 12 Thanked 17 Times in 17 Posts So what numbers would you expect for the PiP. I am not willing to take my Jetta to 100 mph, but it takes 12 horsepower to hold at a steady 60 mph, and 16 at 80 mph...which is pretty close to the PiP's curve in your chart. __________________
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Quote:
 Originally Posted by bwilson4web drag HP = (A + B*v + C*(v*v)) / 7.5 Bob Wilson
Hi Bob Wilson,
Do you have a source for this formula? In context of EPA dyno testing, it looks like the estimated road power at 50 mph. The proper conversion coefficient, considering units of lb and mph, should be 375 and there should be another mph term:
drag HP = v * (A + B*v + C*(v*v)) / 375
using 26.050,-0.01200,0.018200 as A, B, C for the Volt I get
9 at 50 mph
55 at 100 mph

-mort

 06-16-2013, 04:26 PM #7 (permalink) ...beats walking...   Join Date: Jul 2009 Location: . Posts: 6,190 Thanks: 179 Thanked 1,522 Times in 1,123 Posts That is the EPA standardized formula and it's what each manufacturer uses to establish the dynamometer Test Load HP coefficient they derive FROM their "road load" coastdown tests. However, that EPA equation is (to my knowledge) ONLY valid for 50 MPH since it is actually published as (EPA test speed is 50 mph): TLHP = (a + 50·b + 2500·c)/7.5 Last edited by gone-ot; 06-17-2013 at 06:59 PM..
 06-17-2013, 03:01 PM #8 (permalink) EcoModding Apprentice     Join Date: Apr 2011 Location: Southern France Posts: 159 AX - '95 Citröen AX 307 - '04 Peugeot 307 SW Thanks: 12 Thanked 67 Times in 37 Posts Fr = Cr*m*g Fa = 0,5*rho*CdA*v² Fg = m*9,81*sin(atan(slope/100)) (slope in %) P = (Fr+Fa+Fg)*v __________________ Citroën AX 1.5 D 430.000 km Peugeot 307 SW 2.0 HDI 136 195.000 km
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Quote:
 Originally Posted by PressEnter[] So what numbers would you expect for the PiP. I am not willing to take my Jetta to 100 mph, but it takes 12 horsepower to hold at a steady 60 mph, and 16 at 80 mph...which is pretty close to the PiP's curve in your chart.
Power required to overcome aerodynamic drag is proportional to velocity cubed. If you know how much power is needed at a specific speed to overcome aero drag, you can figure it out at other speeds.

If it takes 12hp to keep your car going at 60mph on level ground (assume 10hp for aero, 2hp for rolling resistance) you'll need 24hp at 80mph and 46hp at 100 just to overcome aero drag.

Rolling resistance is directly proportional to velocity, so it would be about 26hp and 50hp total, respectively.
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Quote:
 Originally Posted by HypermilerAX Fr = Cr*m*g Fa = 0,5*rho*CdA*v² Fg = m*9,81*sin(atan(slope/100)) (slope in %) P = (Fr+Fa+Fg)*v
Of course, figuring out Cr, Cd, and A are not simple feats...

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