Doing the Math

 

Electric race cars require a substantially different design than conventional internal combustion engine (ICE) powered race cars. So, sticking an electric motor and batteries in a conventional race car is the wrong path for electric racing
 

Part of the problem is that a number of teams (including ProEV) have created impressive electric powered race cars by converting ICE designs. These electric race cars are able to go out and turn a lap, at, or better than, their gas cousins. Building powerful electric race cars is not a problem.

What is a problem is carrying enough energy to allow the EV race car to do those lap time for more than a few laps.

Gasoline is light. Batteries are heavy. If we add more batteries, the car can no longer turn fast enough lap times.

Here is some basic math to help explain how important the difference is:

Gasoline contains 33.41 kW-hrs per gallon (8.83 kW-hrs per liter)[i].
Gasoline weighs in at about 6.073 lbs. per US gallon (.728 kg per liter).
So each pound of gasoline contains 5.50 kW-hrs (12.129 kW-hrs per kilogram).

Since, when racing, the gasoline race car starts with a full tank and ends with an empty tank, the average weight of the gasoline carried through the race is 1/2 the starting weight. This effectively doubles the energy per pound in gasoline compared to batteries in racing. So this puts gasoline at 11.00 kW-hrs per pound (24.258 kW-hrs per kilogram) in comparison to batteries.

However, the internal combustion engine is very inefficient. Even Formula 1 engines only manage 30% efficiency [ii]. This gives us 3.30 kW-h per pound (7.28 kW-hrs per kilogram) usable energy in gasoline.

There are a number of types of lithium batteries. They have different advantages and disadvantages. Wikipedia puts the specific energy at a range of 0.045-0.120 kW-hrs per pound (0.100-0.265 kW-hrs per kg) [iii].

Electric motors range around 80%-99% efficient [iv]. My experience with racing EVs indicates that they tend towards the lower figures but let's use an optimistic 95%. This gives us 0.043-0.114 kW-hrs per pound (0.095-0.252 kW-hr per kilogram) usable energy in the batteries.

So, whatever the starting weight of fuel in the gasoline race car, the electric race car needs to carry 29 to 77 times that weight.

To illustrate how poorly suited current race vehicle designs are for electric power, let us convert a Formula 1 (ICE) car to electric. We assume we are able to swap gasoline engine, exhaust pipes, and large radiators for the same weight and performance in electric motors, controllers and smaller radiators.

The Formula 1 car starts the race with a full tank of around 55 gallons (200 liters) of gasoline. To match the weight and thus equal the performance of the Formula 1 car during the first few laps of a Grand Prix, our battery pack cannot weight more than the 330 lbs. (150 kg) of fuel.

Knowing that it takes around 10 pounds (4.5 kg) of gasoline to complete a lap at Circuit de Catalunya [v], how may laps can our electric Formula 1 car with it's 330 lbs. (150 kg) battery pack complete?

Best case, ten pounds multiplied twenty-nine times give us a required battery weight of two hundred and ninety pounds (130 kg). We can do a single lap.

Gasoline race cars are well evolved designs that make use of power intensive techniques to produce grip, especially aeronautical down force. EV race cars can produce the same power, but the weight penalty of carrying the energy, make it a terrible design choice.

We can, and will (eventually), build electric race cars that will be able to run races at F1 speeds. We will not get there by doubling or quadrupling battery specific energy. We will get there by designing electric race cars to work with the different design challenges of batteries. For information regarding ProEV's ideas about what this vehicle will look like, read "Rewriting the Rules".

 
References:

i. Wikipedia "Gasoline gallon equivalent", n.d.,
http://en.wikipedia.org/wiki/Gasoline_gallon_equivalent (accessed: July 6, 2013)

ii. Technical F1 Dictionary "Fuel thermal efficiency", n.d.,
http://www.formula1-dictionary.net/thermal_efficiency.html (accessed: July 6, 2013)

iii. Wikipedia "Lithium-ion battery", n.d.,
http://en.wikipedia.org/wiki/Lithium-ion_battery (accessed: July 6, 2013)

iv. Wikipedia "Gasoline gallon equivalent", n.d.,
http://en.wikipedia.org/wiki/Gasoline_gallon_equivalent (accessed: July 6, 2013)

v. Sam, "Formula 1 2013 Round 5: Spanish Grand Prix", May 10, 2013,
http://www.racecar-engineering.com/articles/f1/formula-1-2013-round-5-spanish-grand-prix/ (accessed: July 6, 2013)

 

 

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