The Efficiency Benefits of Electric Motors

There are many reasons to choose electric motors to drive Autonomous Vehicles. Here we’ll just look at the efficiency. Later, we’ll see how our innovations incorporate other features.

Internal combustion engines are inherently inefficient, and electric motors are inherently efficient.  The maximum theoretical thermodynamic efficiency of a car type internal combustion engine is 37%, so it can’t be improved beyond that, and actual engines are significantly less efficient. For comparison, the maximum theoretical thermodynamic efficiency of an electric motor is 100%. Electric motors typically operate at 90% efficiency or better, and larger motors are typically more efficient than smaller ones. 

The Table below compares efficiency for specific 2015 and 2016 cars. The Scion iA is the most efficient gasoline car, with 37 mpg Combined mileage. For comparison, the minicompact Aston Martin DB9 is representative of the cars with the worst gas mileage, 15 mpg. The Toyota Prius c is the most efficient hybrid with 50 mpg. The Volkswagen e-Golf is the most efficient electric car with 116 MPGe Combined mileage (and since it’s electric, Volkswagen can’t mess with the pollution controls). Note electric cars don’t use gasoline, so the EPA computes an “equivalent mileage,” or MPGe.

The Volkswagen e-Golf is over 3 times more efficient than the best conventional car! The Volkswagen is also twice as efficient as the best hybrid car!

Note these are all very small cars.

 (https://www.fueleconomy.gov/feg/best/bestworstNF.shtml accessed 11/13/15)

Car Model	Combined	City	Highway
Scion iA	37	33	42
Aston Martin DB9	15	13	19
Toyota Prius c hybrid	50	53	46
Volkswagen e-Golf (MPGe)	116	126	105
Tesla 85D (MPGe)	95	100	106

I included my model of the Tesla with 95 MPGe Combined, to show two things: first, the Tesla is not a small car! The Tesla is a 5-seat luxury car, so it has a disadvantage in weight and accessories. Yet it still gets almost 2.5 times the mileage of the best gasoline car, and almost twice that of the best hybrid.

Second, the luxury Tesla gets better Highway mileage, 106 MPGe, than the tiny electric Volkswagen e-Golf, 105 MPGe. There are at least two possible reasons for this: larger electric motors are more efficient than smaller ones; and the Tesla is more streamlined than the Volkswagen, which makes a big difference at higher speeds. We’ll exploit both effects in our innovations.

The Table below shows aggregate gasoline powered Car Energy Use.

Only 18-25% of the Energy is delivered to the wheels, so cars waste 75-82% of the energy they use. Later you’ll see that the Transportation Sector of the US Economy is only 21% efficient – the worst Sector in overall efficiency. And, all the oil we import goes out the tailpipe of our transportation system as energy wasted – not to mention all the pollutants.

The inefficiency of the internal combustion engine accounts for 68-72% of the energy losses. The parasitic losses include things needed to run the engine, like the water pump and alternator, so that 4-6% counts against the gasoline engine too.

 (https://www.fueleconomy.gov/feg/atv.shtml, accessed 11/7/15)

Energy Use	Combined	City	Highway
Engine Losses	68-72%	71-75%	64-69%
Parasitic Losses	4-6%	5-7%	3-4%
Power to Wheels	18-25%	14-20%	22-30%
Drivetrain Losses	5-6%	4-5%	4-7%

Another reason for improved efficiency of electric vehicles is that clever designs, like the Tesla, virtually eliminate the Drivetrain, saving the 5-6% in the table above.

The Table below is a further breakdown of the Power to the Wheels for gasoline vehicles. In city driving, Braking is up to 10% of losses. At highway speeds Wind resistance is up to 19%, and Rolling resistance up to 9% of losses. You’ll see how our innovations improve each of these.

Braking is a significant fraction of City losses: 7-10%. Another efficiency advantage of electric cars is regenerative braking, that is recapturing the energy of decelerating, rather than losing it as heat in the brakes. The electric motor can also operate as an alternator, turning motion into electrical energy, and that electrical energy is returned to the batteries. Note, hybrid cars can gain this advantage as well. You can’t capture 100% of the braking energy but you can recapture perhaps 90% of the energy, further adding to the efficiency of electric cars.

Note how Wind Resistance goes up so much from City, 3-5%, to Highway driving, 13-19%. Generally Wind Resistance goes up with the cube of the speed – you’ll see how this drives our innovations for high-speed travel.

Power to Wheels	Combined	City	Highway
Wind resistance	9-12%	3-5%	13-19%
Rolling resistance	5-7%	3-5%	6-9%
Braking	5-7%	7-10%	2-3%

The Table below shows an example of Energy Use for Electric Vehicles. The battery is 90% efficient, but we need an Inverter to convert the DC voltage of the battery into AC to run the motor, losing 4%. And, we need a charger to convert the AC from the power source into DC to charge the battery, losing 4%. If we can find a better way, we might be able to save another 8%.

(https://matter2energy.wordpress.com/2013/02/22/wells-to-wheels-electric-car-efficiency/ accessed 11/6/15):

Energy Use	Value
Motor & Drivetrain	9%
Inverter	4%
Battery	9%
Charger	4%
Power to Wheels	75%

In the previous post, you saw that reducing the weight of a car is critical to efficiency. One of the challenges with current electric cars is the need for large batteries. Batteries are heavy, expensive, and use rare resources.

What if we could greatly reduce the size of the battery?

Our innovations make the batteries much smaller, and achieve many other benefits.

But before we see those innovations, we need to get back to something we postponed: the challenges of car crashes. In the next post you’ll see how dangerous and expensive those car crashes are.