What are the pros and cons of electric motors, compared to internal combustion engines, from a sustainability perspective?
Are electric motors better for the economy, more efficient, or more versatile/powerful than internal combustion engines?
Electric motors generally:
Consequently, heavy-duty applications such as trains and ships, if diesel powered, typically have hybrid diesel-electric drivetrains, with the diesel engine generating electricity that powers the electric motor.
Furthermore, a pure electric system has the advantage that there is no local pollution: one of many problems with combustion engines is the amount and range of pollutants generated: particulates (PM10, PM2.5), NOx, SOx, CO, CO2.
Electricity can be generated from a wide range of sources, so using an all-electric transmission makes it much easier to switch to a cleaner supply.
And as a large demand from electric engines improves the economics of electric storage, by providing income for electric-storage providers, economies of scale, and demand-pull for innovation, that makes it even easier to incorporate into the electricity grid exogenously-variable renewables such as run-of-river hydro, PV, CSP, and wind.
I agree with everything in EnergyNumbers' answer, but would like to add some more:
Along the lines of "easier to maintain", an electric motor is likely to last much longer, for example, in an automotive application, than an internal combustion engine (ICE). An EV traction motor may very well last 1,000,000 miles, versus perhaps one quarter of that for most ICEs. While it's true that current EVs tend to use batteries to power the motors, and batteries may need to be replaced at a comparable interval to an ICE timing belt, an EV with a new battery is going to run much more like a new vehicle than an ICE with a new timing belt. The regenerative braking associated with the electric motor will also prolong brake life.
Electric motors are more quiet than ICEs. This is a controversial topic. I believe noise pollution to be a problem, and thus, a quieter car to be better (especially with respect to sustainability). Some are concerned about pedestrian safety with quieter electric motors. While this can be, and is being, addressed by adding noise makers to electric cars, I believe this solution is actually misguided. The basis for this aversion to quiet cars partly draws from a multi-year study in the US concerning pedestrian accidents with hybrid vehicles. The hybrid vehicles had a 40% higher rate of pedestrian accidents than non-hybrids. However, the study did not correct for the fact that in the US, hybrids tend to be owned by city-dwellers, and the rate of pedestrian accidents is twice as high in cities vs. rural areas, for all cars. I believe this factor alone can explain the 40% result. Also, during this study, which involved thousands of hybrid cars, not a single blind person was killed by a quiet hybrid.
Size matters. In an application-agnostic answer, it's important to note that for some applications, electric motors are more versatile because of the variety of sizes they come in. Hopefully, it's obvious that an ICE to make your phone vibrate is not an option, but a motor works quite well.
Without specifying your application, I'll try to discuss multiple transportation applications. For cars or trains, the excess weight of a battery pack that drives an electric motor is not a huge penalty. The road provides extra normal force to balance the vehicle weight. However, in aerospace applications, power-to-weight ratio is more critical. So, for airplanes, using electric motors for propulsion is still not practical. Liquid fuels have a higher energy density compared to batteries, which makes petro-fuels or biofuels more attractive for airborne propulsion.
This is less of a problem for cars, which do not yet have to levitate. Also, it should be noted that comparisons of energy density for liquid fuels vs. batteries distort the true picture, because total weight is what's important, and an ICE system requires a heavy ICE, and a heavy transmission. When the whole system is considered, battery EVs suffer a noticeable, but acceptable, weight penalty for their lower energy-density fuel.
Battery range (capacity) is also a problem for EVs. However, I believe this issue to be exaggerated, as the average US driver (who drives a lot!) still only drives 40 miles on an average day. In the future, more charging stations will also minimize the range problem for electric cars. However, for now, range versatility has to be considered an advantage of ICE vehicles. An interesting compromise is made by Chevy's Volt EV, which is fundamentally an electric car, but which carries an on-board generator for recharging the battery by burning liquid petro-fuel.
Temperature constraints can be a disadvantage for electric motors, due to the batteries themselves. High temperatures can shorten battery life, and very low temperatures can (temporarily) reduce the useful capacity of the battery. Note that normal temperatures here on earth are well-suited to battery EVs, so this issue is more a problem if your area has temperature extremes.
This is difficult to answer without a more specific question. Electric motors will draw their fuel from different sources than internal combustion engines. Each country will have different economics regarding their electric grid, and oil consumption. If your country has more abundant resources to produce electricity (coal, natural gas, solar, wind, geothermal, nuclear), you may consider electric motors an economic benefit. If you're in an oil country, maybe not.
When you add biofuels to the mix, you add agriculture as a factor. Most internal combustion engines can use varying levels of biofuels. An Otto cycle engine needs modest engine modifications to run rich ethanol blends. Many modern diesel engines need no modifications to run biodiesel (although winter cold can limit the blend).
So, ICEs can provide a domestic economic boost if your country can grow its own biofuel feedstock.
I believe that pollution (which includes greenhouse gas) and energy efficiency are the most important factors here, and those favor electric motors for many applications. EnergyNumbers' answer covers those well. I simply wanted to add a few more pros and cons to round out the topic.
Disclaimer: biased owner of an electric vehicle, but I've certainly owned ICEs, too :)
Note: more sources coming later ...
Specific to electric vehicles in the USA, the Union of Concerned Scientists created a good comparison in 2009. Here's the latest update, from 2018-03-08:
Based on data on power plant emissions released in February 2018, driving on electricity is cleaner than gasoline for most drivers in the US. Seventy-five percent of people now live in places where driving on electricity is cleaner than a 50 MPG gasoline car. And based on where people have already bought EVs, electric vehicles now have greenhouse gas emissions equal to an 80 MPG car, much lower than any gasoline-only car available.
The comparisons are comprehensive:
For a gasoline car, that means looking at emissions from extracting crude oil from the ground, getting the oil to a refinery and making gasoline, and transporting gasoline to filling stations, in addition to combustion emissions from the tailpipe.
For electric vehicles, the calculation includes both power plant emissions and emissions from the production of coal, natural gas and other fuels power plants use.
There's also an calculator to estimate emissions for a specific vehicle and location:
I think you had pretty much of advice pro electrical. I do not preach combustion is better than electrical, but would like to make some side notes in favor of combustion.
Energy efficiency: The energy for your electrical engine must come from somewhere. These sources are probably combustion engines, so basically you are adding an extra conversion which will lose energy.
Batteries: Your batteries will be heavy. Assuming that you use the engine for locomotion, you will lose more energy because you accelerate more mass. Batteries will also be environmentally unfriendly and produced under bad condition labour. These batteries will also need transportation and replacement and old batteries might become a source of heavy metal waste. An empty battery is as heavy as a full one. Your fuel tank on the other hand will become lighter as you go. Technology however promises better batteries and you might also google 'fuel cell'.
Pollution: Combustion engine pollution depends on what you are burning and how. If you would use hydrogen your pollution might be zero.
Engine complexity and durability: Your electrical engine will likely last longer than your internal combustion engine. It will also be easier to use, maintain, design, build etc. From durability perspective you might want to have a look at external combustion engines (for example steam engines). This guy claims they have some of them running for over a hundred years: http://www.mikebrownsolutions.com/steamart.html
From engine simplicity perspective it might be possible to use a more simple engine like a rocket engine. Of course they are not energy efficient but the bottom line I try to make is that the force you need for your vehicle is directional and not rotary. Combustion (and also electrical) engines complicate the issue by first converting to a rotating force and than to a directional one. One might simply propel a rocket engine against a rail to create a directional force at once. This would of course apply more to trains and not so much to cars.
Don't get too much into the electrical car hype yet. I believe they are still more expensive even though heavily sponsored by governments. That latter being a good indication for how environmentally friendly they are.
But don't understand this wrong: Electrical cars will become more advanced if batteries and/or fuel cells improve.
My opinion is that the electrical motor is not last long as piston engine.Because:
1.The brush connection will be rust and ruined out.
2.The brushless motors require complex electronic circuit which is hard to maintain.
3.Futuristic battery and capacitor are high technologies which isn't suitable for fixing,regenerating and it also costs a lot of money.
4.Motors are actually more complex of design and material compare to piston engine,it can't handle high temperature that lead into electrical component damages.
5.In cold weather,gas engines create much heat that maintains the good operating temperature for mechanical parts,the e-motors will not do this and the rotor can be frozen as well.
6.Reverse the current when braking can cause an over heat,inductive energy that can damage the teflon coat or destroy the soldering(that's why trains still use regular brake combine with electric brake)
Conclusion: e-bike is a result of civilization high technology while it has a lot of advantage,however it does have a lot of disadvantage of maintenance and recreation if apply for a specific civilization low technology reality.
Although hybrid cars are efficient on paper - the reality is far from that. Until battery technology advances well enough to have a cost-effective product in the long run, my choice will always be on the side of diesel, or even petrol engines - there are still other options still, take a look at this article: https://www.marlincash.com/fuel-efficient-cars/
Both advocates for the different vehicles, EVs vz ICEs, have very good arguments in their defense with some obvious bias because of feelings of connection to their vehicle. The internal combustion engine,ICE, has served humanity well for hundreds of years thanks to its Belgian inventor Nikolaus Otto Etienne Leonor in 1858. However, innovation has to advance and the most practical solutions have to emerge. In this case it seems like we are at an inflection point and the tilt is favoring the EVs. Yes, the demand on electricity would be overwhelming and the grid would not be able to handle it if suddenly all vehicles were EV, but that would not actually happen because the increase in EVs production would go hand-in-hand with new power generation from renewable sources. All things considered the EVs win the future.
Lots of good answers here, but they miss one important characteristic:
Charging time of electric motor powered battery car versus refuel time of an internal combustion engine car.
To get any useful amount of charge, even the best charging solutions require at least half an hour, and these fast charging solutions don't charge to full level, they stop at around 70%-80%. Not only that, but this speed requires a fast charger. Many charging stations are slow ones.
In contrast, it's only few minutes to refill a gasoline or diesel car.
Hydrogen may be a solution to the charging time problems. It can be generated using electricity on site on the refueling station, and hydrogen refueling doesn't take that much longer compared to gasoline / diesel refueling. Hydrogen cars ideally use fuel cells and electric motors. The obvious drawback is lower energy efficiency compared to direct electricity use in batteries. Also, hydrogen fuel cells are today quite expensive.
So far as replacing most Internal Combustion engines with electrical cars we need to remember that if everyone starts using them then we will need to produce more electricity. There is no free lunch, because electric power is produced by burning non-renewable (and still polluting) forms of energy -- if not coal, then natural gas or nuclear power). Hydro electric generation has its own set of environmental concerns. Oh yes, there is solar and wind power but just imagine the areas that will be required to accommodate these very large solar panel farms and win turbine generators to satisfy all the increased demand. There will be NIMBY concerns if located near urban areas and even in rural areas, and given that these solar and wind farm locations have to be in areas where there are either significant number of cloud free days or where there is a reliable steady flow of wind at the surface there are not too may alternate locations