100 MPH and 100 MPGe
Oct 31, 2016
How a Cd = 0.12 body form for a Chevy Bolt world result in a 186 MPGe Hwy vehicle that would still be capable of getting over 100 MPGe at 100 MPH with S rated tires and a electric overdrive.
By John Gilkison and Phillip Knox
As many of you know we published an article with the title 100 MPH and 32 MPG at EV World on September 8th, 2013 the link is ( http://evworld.com/focus.cfm?cid=188 ). In that article we were trying to outline how an ICE driven passenger automobile with a lower drag body form of Cd = 0.12 would be able to achieve this metric routinely on the interstate highway system. Our baseline car was a 1996 Honda Accord with a CdA of 6.73 and weight of 3,000 lbs. The vehicle described would get dramatically better fuel mileage at lower speeds with values as high as 111 mpg at 55 mph.
Over the decades I have seen articles trying to explain why the automobile manufacturers cannot produce 100 MPG cars, or if possible they would very expensive, needing exotic technology, and would not sell. Readers at EV World know that 100 MPG cars are already here in the form of the Battery Electric Vehicle or the BEV. The Nissan Leaf is rated at 101 MPGe on the highway. The soon to be released Chevy Bolt is rated at 110 MPGe on the highway. The Leaf and the Bolt have Coefficient of Drag’s of Cd = 0.28 and Cd = 0.31 respectively.
This set me to thinking could a BEV get 100 MPGe at 100 MPH with a more aerodynamic body form? The answer turns out to be “YES” surprisingly enough. The reason lies in a little known metric strangely titled Brake Specific Fuel Consumption in lbs of fuel per hour (BSFC). Before we get into the math behind this little known gem of car science lets lay out some basic metrics we will be using.
1: Gallon of Gasoline 87 octane 10% ethanol blends 111,836 BTU’s (British Thermal Units).
2: One Kilowatt hour = 3,412 BTU’s.
3: Gallon of Gas Electrical energy equivalent 111,836/ 3412 = 32.777 KWh’s. With charging losses being calculated to be 12% or using a divisor of .88 x 32.777 KWh = 37.25 KWh’s. We do not include well wheel losses for drilling, transporting, or processing fossil fuels so I am not doing it here for electricity except to use charging losses for cost analysis.
4: Gallon of Gas weighs 6.138 lbs.
5: Coefficient of Drag or Cd is typically a number less the 1.0 that represents the reduction in drag force over the idealized frontal area due to the form that allows the air assume laminar flow and come back together behind the vehicle with reduced disturbance. Modern cars have typical Cd’s around .32.
6: Cd A or Cd Area = Cd X the frontal area for the Honda example a Cd .32 X Frontal Area of 25 sq ft = Cd A = 8
7: Weight is given in pound avoirdupois.
8: Kilowatt conversion to Horsepower multiplier is 1.3409.
9: Rolling Resistance is related to weight and is given in Horsepower needed to overcome it. While rolling resistance tends to increase linearly at speed, aerodynamics is a cubic function.
Some of you may remember the GM Ultralite 100 MPG concept car GM produced in 1992. The car was capable of 100 MPG but only at 50 MPH. The car was powered by a 2 stroke 3 cylinder engine which is not normally used in passenger cars today. The project was apparently just considered to be a prototype to explore the technology, but few if any of the concepts made it in to general production.
Both Phil Knox (of iT Works) and I became curious just what would a modern BEV already capable of 100 MPGe would be able to do if the body form drag on these cars were to be reduced to Cd = 0.12. Could they achieve 100 MPGe at 100 MPH? What would their performance be at lower speeds like 62 MPH or 100 KPH? We calculated that the BSFC of both the Leaf and the Bolt stood at an astonishingly low figure of 0.1397 lbs of electrical fuel equivalent per hour in BTU content.
The BSFC of these cars is at least three (3X) times better than comparable ICE vehicles. Replace the Chevy Bolt Cd = 0.31 body with a Cd = 0.12 body and we found it would be capable of getting 186.7 MPGe at 62 MPH or 100 KPH. This means that the 1.83 gallon gas tank equivalent battery pack of the Chevy Bolt could drive the car up to 340 miles on the highway instead of the rated 217 miles.
Chevy Bolt 1: Frontal Area = 25.147 sq ft.
2: Cd 0.12 3: CdA = 3.012.
4: Weight 3,650 lbs but it is EPA rated at 3,950 lbs.
5: Transmission Efficiency 98% so.
Road Load @ 62 MPH (100 KPH) 3,950/3,000 x 7.101 HP = 9.357 HP.
Aero @ 62 MPH (100 KPH) 3.107/6.73 x 11.023 HP = 4.941 HP.
Road Load = 9.357 HP + 4.941 HP = 14.289 HP.
Brake HP = 14.289 HP/0.98 = 14.590 BHP.
Fuel 14.590 BHP x .1397 BFSC = 2.038 lbs/6.138 lbs per gallon = .322 gallons.
MPGe = 62 MPH/.332 gallons = 187.6 MPGe
Then I ask Phil Knox the $64,000 question. Could the Chevy Bolt with a Cd of 0.12 body form assuming the same weight achieve 100 MPGe at 100 MPH. We did the math and came up with 107.67 MPGe at 100 MPH assuming a level road, ideal conditions, and no wind. Since the Bolt is currently limited to 90 MPH we think the car would require S rated tires and possibly an overdrive unit to lower motor RPM’s. With the addition of S rated tires the rolling resistance would increase and the MPGe would drop to at least 103 MPGe. Given those two parameters we think it is safe to say that the Chevy Bolt platform is certainly capable of 100 MPGe at 100 MPH or 161 KPH. Here are our calculations.
Road Load @ 100 MPH (161 KPH) 8.25 KW x 1.3409 = 11.6204 HP x 3,950/3,000 = 14.565 HP.
Aero @ 100 MPH (106 KPH) 42 KW x 1.3409 = 56.31 HP x 3.017/6.73 = 25.243 HP.
Road Load = 14.565 HP + 25.343 HP = 39.909 HP.
Brake HP = 39.909/.98 = 40.806 BHP.
Fuel 40.806 HP x .1397 BSFC = 5.7005 lbs/6.138 lbs per gallon = .9287 gallons.
MPGe = 100 MPH/.9287 gallons = 107.67 MPGe.
Note (A switch to S rated tires might involve a 4% loss in MPG or a .96 multiplier x 107.67 MPGe = 103.36 MPGe.) We think we can safely call a Chevy Bolt with a low drag body aero platform Cd or 0.12 routinely capable of 100 MPGe at 100 MPH.
This is a pretty exciting claim really. Conceptually it involves a complete paradigm shift in ground transportation. This paper and math exercise really does show what applied aerodynamics means for cars and trucks. The boat tail part of the body that makes these low drag forms possible would of course have to be automatically deployed at highway speed as we outlined in our article 100 MPH and 32 MPG. They are not suitable, safe, or needed in routine city driving.
Please go view my You Tube video
where I will show the graph of a standard car these calculations are based upon and where I explain the math in three other separate pages. I recommend you use a large monitor and a full screen to be able to see the display screen I am shooting. I made a few verbal miscues but they are not serious enough for me to re-shoot this 13 minute video. The most important part of the video is the graph which I want you to see. The engine of any car (any engine) only knows about Rolling Resistance, and Aerodynamic Loading. The BSFC is a function of these two variables and the throttle body is being adjusted to meet these needs at a specific speed regime.
We are all used to the metric City/Hwy/Comb metric used to show the MPG’s of ICE vehicles where city driving is the lowest MPG figure and highway driving is the higher MPG return with combined driving being the middle figure. BEV’s have traditionally reversed this metric because of the electric drive train lack of idling loses and regenerative braking capabilities. Aerodynamic drag normally lowers the MPGe return of the BEV on the highway.
Imagine a new Chevy Bolt with a Cd of 0.12 with a window sticker reading.
City 128 MPGe/ Hwy 186 MPGe/ Combined 157 MPGe
Instead of the current sticker that reads.
City 128 MPGe/ Hwy 110 MPGe/ Combined 119 MPGe.
The equivalent price of a gallon of gasoline needs to be calculated for comparison purposes. The table below shows these prices for a energy equivalent gallon and for charging loses gallon of gasoline to get a standard gallon into the battery pack.
THE PRICE OF A GALLON OF GASOLINE EQUVILENT AT KWH RATES 6 CENTS THROUGH 12 CENTS
6 cents 8 cents 10 cents 12 cents
$1.966 $2.622 3.277 $3.933
GALLON OF GAS EQUIVILENT WITH 12% CHARGING LOSES FACTORED IN (32.777 KWh/.88 )
6 cents 8 cents 10 cents 12 cents
$2.235 $2.980 3.743 $4.470
As you can see here, electricity normally is not cheaper than gasoline in per gallon equivalent prices, especially when you factor in charging loses. The phenomenal savings for Battery Electric Vehicles comes from their lower (by a factor of 3x lower) Brake Specific Fuel Consumption in lbs of equivalent fuel per hour, for example my Ford Truck BSFC .414 v the Chevy Bolt BSFC .1397.
With all this information we have been discovering about BEV performance my wife and I have decided we need to get ourselves a BEV as soon as we can. We are currently waiting on our 2017 tax return in February so we will have the money for a down to do a lease. We bought our 2014 Ford F-150 XLT 4x4 in Sept of 2014 primarily for towing a fifth wheel trailer and other trucks duties. It just is not an ideal daily driver. Averaging 17.8 mpg over a whole year (at best) over 16K miles results in 898 gallons of fuel. Let’s just say 900 gallons of fuel at $2.00 a gallons which means a $1,800 fuel bill per year or $150 a month. These are conservative numbers. Towing the fifth wheel can really skew these averages dramatically.
A Chevy Bolt (if I can get my hands on one) rated at a 119 MPGe combined driven 12K miles a year would require only 100 gallons of gasoline energy equivalent or 32.77 X 100 = 3,277 KWH/.88 = 3,724 KWH X 8 cents a KWH = $298 a year for unsold solar PV power to the electric company. This amounts to $24.83 a month in fuel cost. Leaving 4K miles for the truck driving per year leaves only $449 per year or $37.45 per month fuel cost. Adding the two together $24.83 + $37.45 = $62.28 a month on average, so, let’s just round this figure off to $62 a month total. Take my present cost of $150 a month and subtract $60 and this means I could save $88 a month on fuel cost alone with this new driving scenario. I may end up paying more in the summer for electricity because air conditioning reduces our power exports but the most I need to pay is 12 cents a KWH. This could reduce our average monthly saving to $83 a month over the whole year because of higher KWh charges for summer months.
My wife and I have made this new car lease a priority for the New Year. We are hopeful we can get it done by the end of February. We are hoping we can get into a 39 month lease for $299 a month or less. Since we have 5.1 KW of grid tie solar which produces at least $400 of excess electricity per year that we sell to the local utility we could certainly power our basic transportation needs carbon free for at least eight months out of the year. I estimate we could trim our carbon footprint by 6.6 tons of emissions per year. 12,000/17.8 = 674 gallons of gas times 19.6 lbs of CO2 per gallon = 13,213 lbs/2,000 = 6.6 tons of Carbon emission reduction.
We are just hoping the rest of the people on the planet don’t discover the inherently 1/3rd lower BSFC of electric drive technology before we can get our own. Just kidding, we hope everyone does discover it ASAP! This is why we have written this blog, that is, to try to outline in detail both the inherent efficiency of electric drive technology and the radical potential of low drag body form to increase the resulting MPGe by an even greater amount. Happy Battery Electric Vehicle (BEV) driving everyone, and don’t keep your Brake Specific Fuel Consumption (BSFC) a secret, display if proudly, if you know it.
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