The Theory Behind the Very Light Car
Edison2 took a close look at the interplay between weight, drag, regenerative braking and acceleration before deciding on a conventional power source for the Very Light Car. Our examination demonstrated the key importance of low weight and low aerodynamic drag in automotive efficiency, and led us away from a hybrid or electric drivetrain.
Here is a graph showing power requirements for a Prius on the EPA city cycle. The specially written Edison2 software behind it is part of what allowed us to explore in detail the benefits and consequences of different drivetrain philosophies. The key think shown in this graph is that the Prius - by conventional standards aerodynamically "slippery" - takes appreciable power to overcome its drag. This energy is not available to regenerate: it's gone.
Further, Edison2's analysis of the EPA city cycle for a spread of vehicle attributes demonstrated that the benefits of regenerative braking are, at best, mixed. A hybrid vehicle squanders much of the recovered energy accelerating the equipment required to do the recovery. In fact, only in a narrow set of circumstances can the help from a hybrid drive overcome the weight and cost disadvantages of batteries and electric motor. Pure electric vehicles fare worse because their batteries are necessarily even larger and heavier.
This is illustrated below in a graph showing the energy consumption of varying the weight of a Prius while holding aero drag constant. The graph is best read horizontally: a 3000 lb hybrid Prius with regeneration is not mare efficient than the same car weighing 2000 lbs without the hybrid drive. Other conclusions that can be drawn include 1) there is less energy available to regenerate in a light car because less has been spent accelerating it 2) As weight shrinks, the benefit of regenerative braking becomes vanishingly small and 3) A 5000 lb SUV is a good candidate for a hybrid drive but no matter how good the hybrid system, it will still drink fuel.
In the Very Light Car we have an unprecedented combination of low weight and low aerodynamic drag. Our theoretical work told us this approach is the most efficient, and so far our testing is validating the theory, expressed in the following chart.
This graph is based on published car weights and manufacturers claimed drag coefficients. Edison2's Very Light Car goes about 5 times as far on the same amount of energy as a Chevy Tahoe and more than twice as far as a Prius. Aptera comes close, but faces serious limitations on range per charge and has only 2 seats.
The X Prize events begin at the Michigan International Speedway on April 26 with the Shakedown stage, followed by the Knockout stage in June and the Finals in July. Some cars are electric, some are hybrid, and a few are internal combustion. Soon we will find out just how successful the various approaches to auto efficiency are.
Reader Comments (9)
You guys are the "real deal" and a class act ! In a world full of hype, I admire your logical, analytical and "back to basics" approach. You embrace technology while avoiding unnecessary complication.
My only suggestion is that it seems that a small turbo-diesel engine may have provided even more fuel efficiency and the acoustics and longevity would probably be better than a high-strung 250cc. Given that you have a diesel "expert" on the team - what is his opinion ? Regarding "fuel of the future" - either bio-ethanol or biodiesel will eventually be our choices. Here's a real data point of a similar approach that validates yours: www.treehugger.com/files/2008/07/vw-282-mpg-1-one-liter-car-production-2010.php .
I also think that a successful "real world" production design needs to bias horsepower upward to give higher speeds and higher accelerations than the EPA cycle, to cater to people's actual driving habits, and for hills and (of course) air conditioning. I live in Southern California, and NOONE drives slower than 70 MPH on the highway.
Good luck! I think you guys are going to clean-up (unless the rules favor electrics too strongly). Keep us posted on your progress !
Would love to see your reply to Kevin's question above... also, I know the Xprize requires a mass-marketing plan; has yours been made public? Estimated cost for this car? Estimated date of availability?
The real reason we did not use a small diesel is because it does not exist and we did not want to build our own engine diluting our effort. The only small diesel engines are generator sets or larger engines like the Smart diesel which are too large for this effort.
That being said the car in the competition is optimized for the X Prize. The "better" efficiency of a diesel comes from two places:
One: it is because 1 gallon of Diesel contains about 15% more BTUs. Because the competition is about MPGe which balances out the fuel to be 100 miles with the BTU content of one gallon of gasoline this part of the advantage is adjusted away.
Two: A diesel engine runs an open throttle, It is managed through amount of fuel as opposed to a gasoline engine which is throttled by restricting intake air. This restriction through a butterfly or throttle body in the gasoline engine creates a vacuum. This vacuum creates pumping losses and thus reduces efficiency in all but full open throttle conditions.
The engine in the VLC is pushing the Exhaust Gas Recirculation to an unprecedented level. Here we substitute the throttle restriction with diluting the atmospheric charge by mixing it with exhaust (thereby displacing and effectively reducing the oxygen content without creating the efficiency robbing vacuum). In theory with this we should be able to mitigate all the diesel advantages. We have achieved a specific efficiency over .4 lbs (less than) of fuel per hp / hr. This puts us square into highly efficient territory and we have this with a wide range of rpms. This last driveability part is really important and not widely understood. Essentially we have the eficiency of a stationary constant rpm engine.
The downside and opportunity is that this engine has a very sophisticated and costly intake and exhaust system and engine management system. However do not forget that a single cylinder (twin in production) is inherently cheaper to manufacture and as such there is a budget to splurge on extra sophistication where it counts. Our engine further has a Turbo which we use to keep the pressures at optimum.
While we do not disagree with the development of electric cars we see the future of the automobile developments such as these... because the energy density and low cost of gasoline is hard to replace and the infrastructure is there.
Our engine in development has had over 50 hp and in X prize trim will top out at about 37 hp which is enough to make the car much faster than a new VW Jetta TDI...
As far as production is concerned "we know what we know how to do". We are one of the best suspension and chassis companies and we know how to navigate uncharted waters effectively and efficiently. We do not know how to produce anything in large quantity. It is our intention to license our technology to people who know how to manufacture things... or to find the appropriate partner. We are finding interest and in time we are certain that a VLC will be available to you at a very attractive price because it is so simple and non material intensive. We actually believe that we are creating a whole new automobile segment and that in some ways much of what is on the drawing boards today, that is termed low cost or green, is obsolete. We know this because we own the proof.
We do not believe in setting expectations and then disappointing, thus I will refrain from giving you a price or a date. Our method has serious hurdles to overcome. We are in totally uncharted territory and there are no parts scaled to our applications available from anywhere. When the virtues of our methods are recognized and people start to produce to this scale then the world will have a whole new range of cars, cars which have between 70mpg and 120 mpg with no range restrictions, cars which emit less pollution and cars which cost less.
Five of our lug nuts weigh 1 ounce. The first itinerant cost $65.-- each. Our current version "3" which we make 200 units at a time cost $12.-- each. When produced in real quantity the nuts will cost <25 cents each. The average American car costs $6.-- per pound. An aluminum chassis luxury Audi costs $20.-- per pound. A VLC weighs under 1000 lbs... You do the math. Cars are nothing but a package with good engineering and materials processed and marketed...
I hope this answered your questions. If you like what we do then help spread the word. The sooner many people understand what we are up to the sooner there will be an economic environment in which our ideas become commonly accepted.
I consider this an unprecedented opportunity for our nation as the full re-scaling of this type of automobile opens the possibility of a new very large and very beneficial industry. One supported by physics.... One which can have very large implications for pollution and energy independence...For our children.
Thank you
Oliver
Oliver,
Thanks for the extremely transparent and informative response ! Once again, I'm so impressed with how well thought-out the VLC choices are. Truly a "dream team" effort.
I follow your engine comments, and was initially going to say that there's also typically a 3rd efficiency advantage to diesels - higher compression ratios. However, I realized that your choice of ethanol fuel allows you to increase C.R. enough to make this pretty-much a moot point . "Smart Mega-EGR" seems like a brilliant idea to reduce part-load pumping losses, while controlling emissions too ! Bravo.
Both our engine types run a Cr above 14 and under 17 to one. (with a turbo). Any more than that, besides the pre-detonation, starts to give up efficiency to ring drag.... there is an optimum and we are there.
I always appreciate intelligent people asking good questions. We want to sell some of our hard learned lessons but we are essentially quite open. You can always ask me directly on my personal facebook page.
We believe that the rescaling we propose opens a whole new industry with new unanswered questions. There are remarkably few people working on this even though once you understand the positive feedback loops this may be one of the most overlooked and obvious design opportunities ever...
Regards and thank you
Oliver Kuttner
It really looks like you are working toward the best overall 'output' that can be achieved with available technology. I wish it were with electric drive, but maybe that time just has not come yet. The only suggestion I would make at this time is, please, consider lifting the passenger compartment of the vehicle up off the ground. Not many people want to sit in traffic looking into the bumpers of all the huge conventional vehicles surrounding them. I know may people who have chosen SUVs simply because you sit nice and high and can see traffic so well. Would it really sacrifice that much in drag to let some air run under the streamlined cockpit? I would think there is some real, and certainly some perceived safety benefit, in lessening the chances of ending up -under- whatever vehicle you might collide with.
My two cents worth, I respect the work you are doing.
R. G. Sheehan
how much is it?
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How can I plot theoretically recoverable energy on various cycles (NEDC, FTP, EPA,...) regardless of motor power?