The ICE vs EVs

 

How Internal Combustion Engine (ICE) cars work is embedded in the DNA of our culture so deeply, we don’t even think about it. Driving electric is a different technology that works differently than ICEs. In most ways this technology is superior, but it has to be managed a bit differently than ICEs.

 

100 years ago there were three competing technologies for cars, ICE, battery electrics, and steam power. Each had their advantages and disadvantages.

 

Steam power took the longest to get going and steam powered cars were very heavy with the largest engines of any type of car. Once they got up a head of steam they were very fast though. Steam engines using the Stirling Cycle can also be more efficient than an ICE ever could be. High temperature steam from a leaking boiler is often invisible to the human eye and can be deadly if someone gets too close. The disadvantages killed off steam power for cars and that’s probably a good thing.

 

Electric cars had a niche, but the only batteries available were lead acid batteries, the same tech as used for the 12V battery in ICE cars today. Lead acid batteries don’t have great energy density compared to other rechargeable batteries available today, but they do well in ICE vehicles for starting the car. Early electric vehicles had very poor range and needed even longer charge times than today. 100 years ago electricity wasn’t nearly as common at it is today, so there was even more risk of getting stranded than today.

 

ICE cars were not very popular early on. They required a crank to start and broken arms from engines starting and then spinning the crank were common. They cost just as much as other types of cars and were dirty and high maintenance compared to electric cars.

 

Two things solidified ICE cars as the standard, first Henry Ford started making inexpensive ICE cars that more people could afford and soon after the electric starter came along. This sent the world down the ICE car route for most of the next century. Until the 1970s, cheap oil prices kept anyone from considering anything but petroleum fueled cars.

 

When oil prices went up in the 1970s, interest in electric cars started a new renaissance, but battery technology had advanced very little in the previous 60 years. The only other rechargeable batteries available were nickel cadmium (NiCd), which are not suitable for electric cars because they have a memory. Unless they are fully discharged before recharging, they will tend to establish the level of charge when charging started as the new “empty”.

 

The nature of the propulsion source is different between ICEs and electric motors. Electric motors convert a lot more of the energy in into motion than ICEs can. All thermal engines, whether steam powered or an internal combustion engine have a theoretical maximum efficiency. A steam engine can get as high as 60% efficiency, a gasoline ICE can theoretically get up to 56-61% efficiency, and diesel engines can achieve a bit higher efficiency than an ICE. However, in the real world, there are many reasons those efficiencies are never seen. All moving parts have friction, combustion is rarely 100% complete, real world fluids and gases don’t behave like theoretical ideal liquids and gases, a narrow range of RPM where the engine is most efficient, and energy is also lost due to aerodynamic forces.

 

In cars, gasoline engines are rarely better than 20% efficient and diesels can get a high as 40% efficient. In 2014 Toyota claimed to be working on an engine to be used in hybrids that will be capable of 37% efficiency, the best ever seen in a gasoline engine.

 

A standard gasoline ICE engine runs what is known as the Otto Cycle (named after the inventor), and a different type of ICE is used in hybrids called the Atkinson Cycle. The Atkinson Cycle is a bit more efficient, but it produces a lower power density. This is mostly only used in hybrids because most of the time the engine is charging the battery and the electric motor and gasoline motor together can propel the car, so the lower power density is not as critical. A car powered only with an Atkinson Cycle engine would likely have very poor acceleration. An Atkinson Cycle hybrid can get up to 30% efficiency in the real world, which is better than a pure gasoline ICE alone. Part of this efficiency gain comes from running the ICE at the most efficient RPM and load settings most of the time (when charging the battery).

 

Where does the lost energy go? Mostly heat, though some goes into making noise too. Most of the heat generated is released through the exhaust gases, but most ICE car engines have a liquid cooling system to absorb the heat that doesn’t get released in the gases. Without liquid cooling of some kind, heat will build up in some areas of the engine and cause it to fail. Some older ICE car engines were air cooled, but they don’t meet modern air quality standards, and some cars have the ability to run without any coolant in the engine (though it’s not recommended), this is mostly done by using extra engine oil to replace the job done by the water based coolant.

 

ICE engines do have one added benefit, once the engine is warm, channeling some of that heat into the car’s cabin can heat the car as a “freebie”. Running the heater actually helps the engine cooling a little bit.

 

ICE engines produce no torque at zero RPM (revolutions per minute), and will rip themselves apart if the RPM gets too high. If an ICE gets to too low an RPM, it will stall. There is also a sweet spot RPM where the engine is producing power most efficiently. Because of these limitations, any ICE capable of more than a few miles per hour top speed requires a transmission.

 

If an ICE car didn’t have a transmission, it would either stall when you tried to start moving, or it would be ripping itself apart by the time you got to about 10 mph. A transmission is a necessary evil with ICE cars. They do reduce efficiency some from extra sources of friction, as well as add weight and complexity to the car design.

 

In recent years car makers have tried to squeeze as much efficiency as they can out of cars by making transmissions with more gears, or CVT transmission which have an infinite number of gears. All this is to try and keep the engine in the efficiency sweet spot as much as possible.

 

Electric motors are a completely different technology with completely different characteristics. Real world efficiency for an electric vehicle from the wall socket to the wheels is around 75%. That is 75% of the power that came out of the wall socket is used propelling the car. The ideal efficiency of the motor alone is closer to 95%.

 

Electric motors produce some heat, but far, far less than an ICE. The drive train can be much simpler which reduces places where friction eats up energy. Tesla’s design places the motor between the wheels where it can drive the wheels directly.

 

Electric cars don’t need transmissions. A transmission may help at extremes, like over 80 mph, but it’s generally not needed. Electric motors produce lots of torque even at 0 RPM, so when the car is sitting at a traffic light or stop sign, the motors can be turned completely off. Electric motors can also work just fine over a wide range of RPM and once the motor has overcome the static friction when stationary or moving at low speed, it’s extremely efficient.

 

There are two types of friction, static and dynamic. If you’ve ever pushed a heavy object across a floor without wheels, you probably found it was more difficult to get the object moving than it was to keep it moving than it was to get it moving in the first place. That’s because the coefficient of static friction is always higher than the coefficient of dynamic friction, so once something is moving, it’s easier to keep it moving.

 

Cars have the same issues with friction. When starting or just moving at low speed, static friction is coming into play. Once everything is moving, dynamic friction is in play and the losses from friction alone go down. Aerodynamic drag is a factor that comes into play as cars get going faster. The faster the car goes, the worse the aerodynamic drag gets and it increases exponentially with speed not linearly.

 

Friction and aerodynamic drag affect all vehicles. ICE cars usually have a bit more friction at low speed because there are more moving parts, but in general, both of these factors affect all cars equally.

 

Most ICEs are geared to get the best motor efficiency around 45-60 mph. Aerodynamic drag is kicking in at those speeds, but because the engine is geared for best efficiency there, the two cancel out to some degree allowing the car to be efficient at that speed. Because of all the other inefficiencies of ICEs, aerodynamic drag is not noticed as much as with an EV.

 

Think of it like tracking your grade in a class in school. If you’re getting an average of 30% in the class and you get a 20% on a midterm, it’s not going to lower your grade all that much. But if you’re averaging a 90% and get a 50% on a midterm, it’s going to drop you down a grade or two. With an EV, there are so few sources of loss that the few sources of loss are really felt whereas with an ICE, another source of loss is just another one among many and the contribution from that loss source isn’t noticed as much. It’s there, it just gets lost in the noise to some degree.

 

The charts below show the efficiency curves for some ICE vehicles and another chart for the Model S and the Tesla Roadster. The curves for the ICEs vary a lot more, but they tend to plateau around 45 mph, then aerodynamic drag becomes a bigger and bigger factor over about 55-60 mph and the curves roll off. The Tesla curves peak around 25 mph and roll off from there.

mpg-vs-speed-all

Range_Constant_Speed_Tesla

Despite the dropping efficiency at speed, electric vehicles are significantly more efficient than ICE cars. In the graph below you can see the different EPA energy expended per mile for ICE cars, hybrids, and EVs. ICE get the biggest increase in efficiency going from city driving to highway. Hybrids see a little increase, and EVs see only a slight improvement For an EV the slight improvement comes from driving at a constant speed instead of the stop and go driving in city driving.

 

EconomyICEvEV

Additionally you can see on the graph that EVs overall get much better energy economy. ICE engines aren’t normally measured in Wh/Mi, they are normally measured in Miles per Gallon (mpg) or Kilometer per Gallon depending on where in the world you are. Gasoline has around 33 KWh/gallon, so you can convert back and forth between the two. The EPA uses 33.7 KWh/Gal for their conversion from Miles/Wh to MPGe. If you look at EVs at the EPA’s website: http://www.fueleconomy.gov/ you will see all EVs have pretty high MPGe values. All Teslas are near or over 100 MPGe.

 

So why are EVs so short ranged? It’s because the energy density of batteries is much lower than gasoline. The volume of the battery pack on the Tesla Model S and X is about 96 gallons, but the energy density of the highest capacity Li-ion batteries is only about 1 KWh/gallon of volume, 1/33 the energy density of gasoline.

 

Gasoline is an amazingly dense energy source, which does mean it will be around for some years to come in some applications. However for personal cars, EVs are superior to ICE passenger cars in many ways:

 

  • EVs have much better acceleration. This will become even more apparent as the CAFÉ standards get stricter and stricter. ICE cars will have to get wimpier to meet new MPG standards.
  • Interior volume – By putting the batteries under the floor, Tesla was able to give much more interior space than other cars of similar size and shape.
  • Ability to refuel at home – This is still a sticking point for people who don’t have a way to charge an electric car at home such as people who don’t have a garage or live in an apartment, but with infrastructure improvements to charge on the street or in the lot of apartment complexes, this can be solved. It’s a big advantage to be able to always leave home with a “full tank” of gas.
  • Cheaper to fuel – Electricity in the US costs on average 12 cents a KWh. Coupled with the better efficiency of EVs, the cost per mile to drive an EV is significantly cheaper than an ICE.
  • Quieter – EVs only produce much less noise than ICEs.
  • Maintenance – EVs have fewer moving parts and much simpler drive trains than ICEs which can lead to low maintenance costs over the long run and much longer lasting cars.

 

The only downsides have to do with the speed of charging, driving very fast, and the current cost. Batteries take longer to charge than filling a tank with gas. And though the price of batteries is always coming down, battery powered cars still cost more than gas powered cars. As the cost of batteries comes down, this difference will be erased.

 

The time it takes to charge batteries is not normally a factor. As long as a car has enough battery capacity to allow someone to do whatever driving they need to do in a day, the down time to charge the car overnight is not usually an issue. The only time it becomes a factor is when you need to charge during a road trip. Tesla currently is the only car company with an effective long distance travel network. The superchargers go a long ways towards mitigating the hassle of charging on the road.

 

The last issue with driving fast can be mitigated by driving slower, though it is embarrassing by many to drive along in the slow lane with everyone screaming past. As EV battery packs get larger capacity this will become less of a factor over time.

 

It takes rethinking the way you drive a bit when transitioning to an EV, but in the long run it’s worth it. I’ve only driven an electric for three months at this point, but I definitely think it’s vastly superior to driving my old ICE. I’ve had a hard time parting with my old ICE for sentimental reasons, but I don’t want to drive it anymore.

 

Reference links:

http://oppositelock.kinja.com/the-effect-of-speed-on-vehicle-energy-consumption-1699304974

https://blog.automatic.com/the-cost-of-speeding-save-a-little-time-spend-a-lot-of-money-5e8129899fec#.t41ei05dt