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April 13, 2007

Cars, electric motors, and batteries – an in-depth look

Category: General,Plug-in Hybrids – Dan 8:58 am

Ever since I wrote the Mythbuster’s post “Hurdles remain for Plug-in Hybrids?“, I’ve been thinking more about the engineering logic of electric and plug-in hybrid vehicles. Here’s a quick rundown.

Batteries – Bad news *but*…

First, in doing the research for the earlier article on Hydrogen for Cars, I looked up information on energy density and found a complete table[1] showing both “volumetric density” (energy content for a given volume) and “gravimetric density” (energy content for a given mass) for a long list of common and not so common energy storage choices. Here’s a few lines of data based on this table:

Material Volumetric (kwH / liter) Gravimetric (kWh/kg)
Diesel 10.9 13.8
Gasoline 9.7 12.2
Ethanol 6.1 7.9
Liquid H2 2.6 39
Lithium-Ion Batteries 0.3 0.1

In terms of conventional fuels, Diesel and Gasoline deserve their popularity rating — they have high energy density, both in terms of volume (taking up a smaller part of your car) and weight (less mass to accelerate). And frankly, battery energy density stinks. How is it possible to even conceive of an electric car? Fortunately, battery energy density is the only bad news in this article.

Electric motors – A big win

When you do the comparison of electric motors to gasoline or diesel engines, you get an entirely different picture, in numerous dimensions.

  • Power efficiency – Gas and diesel engines get hot. They require complex cooling systems to eliminate waste heat. The result is that a gas engine typically only uses about 15% of the potential energy in the gasoline to drive the car, whereas electric motors are 75% efficient or higher. Tesla claims their engine is 85% to 95% efficient.
  • Full torque at zero speed – Gas and diesel engines don’t produce power unless they are spinning, and thus require a clutch to get things moving. Electric motors can generate full torque at a stand still. Indeed this is the reason that modern powerful railroad trains are pulled by “Diesel-electric” engines — in effect a hybrid engine with electric motors powering the wheels and no clutch.
  • Regenerative braking – Once you’ve burned the gas, the only thing you can do with excess energy is to use conventional brakes to convert the car’s kinetic energy to heat. With an electric motor, you can slow down and recover the kinetic energy, and put it back into the battery. Typically you can recover over 40% of a car’s kinetic energy in this fashion. This is why a Toyota Prius gets such superior city gas mileage — nearly half of the energy “lost” when the car stops at the light can be reused to start it again.
  • Fewer moving parts – The typical DC electric motor, as would be used in these cars has only a dozen or so mechanical parts, and only the main wheel bearing has friction and wear from mechanical contact. Contrast this with typical gas or diesel engines, that have literally thousands of mechanical parts and hundreds of bearings. Can you say “reliability”?
  • No extra rotating stuff – Typical gas powered cars have hundreds of pounds of rotating steel. When you step on the gas to accelerate, you not only have to get the car up to speed, you have to get all this rotating steel in motion. This huge rotating inertia means you need a bigger engine. A car powered by an electric motor can four small electric motors, one for each wheel, eliminating all this rotating metal.
  • No clutch, no transmission, no differential – We already pointed out that you can get rid of the clutch because electric motors produce torque at zero speed. They also produce relatively flat torque throughout up to several thousand RPM. Unless you want an ultra high performance electric car (like the Tesla for example), you don’t need a transmission. And if you put one motor on each wheel, you don’t need a differential or a transfer case. Four wheel drive comes along for free!
  • Less weight – A Tesla has one electric motor, which weighs about 70 pounds and is designed for a high performance sports car. Compare this with a typical gasoline engine, which can easily weigh 300 to over 500 pounds[3]. Not to mention the weight of the clutch, transmision, and differential, which can easily add up to another 300 pounds or more.
  • No oil changes – Without the many moving parts, and without the high heat, explosions, and pressures of a gas engine, and you eliminate oil and oil changes. Yes, there is lubrication in the bearings of an electric motor — it’s generally good for the life of the motor.

Bottom line here is that electric motors offer huge advantages over gas and diesel engines.

So there’s good news and bad news

So batteries have terrible energy density, while electric engines are great. So what?

Let’s do some math.

Working from the gas tank forward with the gasoline powered car…

  • A typical car has a 15 gallon gas tank.
  • Gas weighs between 5.8 and 6.5 lbs/gallon, so our tank will hold about 90 pounds of gasoline.
  • Gas has the energy equivalent of 12.2 kWh/kg, which means our tank of gas has about 450 kWh of energy.
  • The gas engine is only about 15% efficient, so the useful energy we have on board is about 67 kWh.

Now let’s work backwards to see how much battery we really need on board for the electric.

  • First, the electric motor is about 90% efficient, so whereas the gas car stored about 450 kWh of energy in gasoline, we’d only need to store about 75 kWh (67 / 90%).
  • Then we can factor in regenerative breaking, which in city driving provides us with another 40+ percent. So we can reduce the needed 75 kWh by 40% to 45 kWh.
  • We’ve run out of efficiency magic, and it’s time to buy batteries. 45kWh of batteries, at 0.1 kWh/kg, yields a battery pack weight of 450 kg, or about 990 lbs.
  • While 990 lbs sounds like a huge weight, remember to subtract the weight of the gas (90 lbs) and the weight of the gas engine, transmission, differential, and clutch. So we’re really only off by about 300 pounds.

The point here; yes, batteries don’t have anywhere near the energy density of gasoline, and that is and issue. But electric engines are so efficient, and they allow you to eliminate so much weight, that they compensate for a good portion the battery’s weakness.

How do you go the last mile (pun intended) to make batteries adequate? Plug in hybrids! Reduce battery size by half or more (say 300 pounds max), put in an ultra efficient gas engine to recharge the batteries (it can run at a constant speed, it needs no clutch, it needs no transmission), and most of the time your car can run happily on battery power, and when you need the extended range, you have your on board charging unit. Smart, green energy.

Note there were a couple articles that were especially useful to write this post:

Tesla Motors white paper “The 21st Century Electric Car”
Aurica Labs paper “Building the World’s Fastest Electric Car”