Chasing the Numbers
Monday, October 25, 2010 at 03:02PMEdison2 made all our primary decisions on how to approach building a truly efficient car right at the beginning of our X-Prize campaign. Time has proved to be on our side here and we’re pleased that our initial analysis has turned out to be so watertight.
What has happened as time has gone by is that we have learned where to look to find other people’s numbers and what we find there gives us a chance to further explore our ideas.
The engineering department at Edison2 avoids the fashionable idea of simulation but we do spend a bit of time writing models because in our models we find clarity. Consider the graph below:
We’ll talk about the three straight lines first. These are iso-efficiency lines that give the amount of power available for different speeds at 100 mpge. Since a gallon of gas has 115,400 BTU of energy, the faster you can go while getting 100 mpge, the more power you have; in other words, you’ve got so much energy to go 100 miles so, the faster you can go and still make it to 100 miles, the faster you can spend that energy.
This is cool because if your car is as efficient as the Very Light Car, you have more power to spend than if you drive some obsolete barge.
To get back to the graph, the three straight shades of light blue lines tell how fast you can go at different levels of energy efficiency and still get 100 mpge. The top straight line is 100% efficiency, the middle 60% and the bottom 20%.
The graph’s curved lines are derived from published ABC coastdown data for the 2010 Escalade, Camry and Very Light Car. Where the curved and straight lines cross is how fast that car will go at that efficiency and still get 100 mpge.
Some interesting stuff comes out of this. First, both the Escalade and Camry take a large energy multiple of the Very Light Car to move down the road. Second, the Escalade power used line crosses the 100% efficiency line at slightly less than 60 mph while the VLC crosses the 20% efficiency line at slightly over; 20% would be a really bad IC engine efficiency, and even with that, the VLC is better than an Escalade with an obviously impossibly efficient power plant.
Interpolating between the lines, a realistic electric drivetrain efficiency is 80% and a very good IC might deliver 30%. Assuming an electric Escalade and Camry and an IC VLC, Edison2’s car will go twice as fast as the Escalade and about 50% faster than the Camry at 100 mpge.
Were we to project the lines above 100 mph, we would see the VLC line cross 100 % efficiency at about 187 mph. We wish that 100% efficiency was achievable: it is not.
The first thing to come out of this is that GM never had the mystical 300 mpg carburetor because it is literally impossible to get more than about 58 mpg out of an Escalade at highway speeds. The second is that at 30% efficiency, the Very Light Car has demonstrated it can deliver 100 mpge at 80 mph. An electric VLC would be even better in terms of efficiency and, depending on the choices made in its execution, could cure electric car range anxiety.
The bottom line? It is the car that matters: an enormous, square behemoth drinks energy while a light, low drag VLC does not. What you power them with also makes a difference but electric drive does not fix any underlying inefficiency in the car
Reader Comments (1)
The graph is difficult to read because the Key is cut off. Can you please repost?