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Since the entire job of an internal combustion engine (ICE) is to turn heat into mechanical energy, why aren't electric heating methods more popular? I know there must be a reason. I'm not talking about electric cars or anything, but using electricity to heat up and do work.

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    A piston engine converts a chemical explosion to rotation, heat is largely an unwanted by-product. Converting electricity to heat to motion would be inefficient compared to an electric motor (a-la Tesla / Prius etc.) – John U Feb 7 '17 at 17:25
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    Is the premise of this question incorrect, or have the edits to the question caused the intent of the OP to be misunderstood? The job of an ICE is not to turn heat into mechanical energy. Perhaps the OP is asking why electrical heating is not used in more engines overall (incorrectly referring to them as ICE) and did not necessarily mean internal combustion specifically. This would cause the question's answer to be entirely different. – Keeta Feb 7 '17 at 18:55
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    See this is why i am very confused. On my hand, when i research i constantly hear internal combustion engines are heat engines that turn heat into motion. Now your saying its not. May this stuff is confusing. – DeusIIXII Feb 7 '17 at 19:08
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    @immibis ICEs are undoubtedly heat engines, but the overall job is to turn a chemical energy into mechanical energy (by the means of heat). – Chris H Feb 8 '17 at 9:16
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    Deus, I think you're really confused about how an ICE works. John U's comment is the only reasonable answer I've seen to your question.. Technically, for a gasoline engine, electric heating is VERY popular - the extreme heat generated around the arc of the sparking plug is used to light the explosive fuel mixture and every gasoline engine in the world uses electric heating.. But the hot, expanding, exploding gas that drives the piston down is a by product of a chemical reaction, not an electrical one. You couldn't easily achieve a gasoline-free version of the same using only a massive spark – Caius Jard Feb 8 '17 at 10:40
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Because it wouldn't be very efficient.

The main advantage of internal combustion engines is that the energy density of their fuels (gasoline, diesel) is very good. You can go a long way on a relatively small, light tank. The drawback of them is that they aren't very efficient. Most of the energy in the fuel is lost to friction and heat, and only a very small portion (<35%) actually turns into mechanical motion.

Electric vehicles are great because they are incredibly efficient. The fuel they use (batteries) is no where near as compact and light as gasoline or diesel for the amount of energy they store, but an electric motor can turn 90%+ of that energy into mechanical motion.

Using electricity to heat air and run an engine based on the pressure change would be combining the worst aspects of both these system.

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    This is the key. Yes, you could use electricity to heat the air in a piston to drive the car forward. However, it would be grossly less efficient than using the same electricity to directly power an electric motor. The advantage internal combustion engines have is that they can store power in the form of gasoline, which is not convenient for generating useful work (the best we can do is one of these engines at 30-35% efficiency), but is very convenient in terms of energy density. If you already have electricity, which is a convenient form of energy, you might as well just use it directly. – Cort Ammon Feb 8 '17 at 20:37
  • @CortAmmon And even if you used gasoline to drive an electric car, the most reasonable approach we have now by far is to have an ICE driven at ideal RPM (which can give you substantially higher efficiency than a typical engine/transmission system) that produces electricity that drives the electric engine. Even then, it's hard to beat the extra weight (and cost) required for the second engine and batteries. – Luaan Feb 9 '17 at 9:30
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Although other answers provide good answers related to energy losses, there's an additional thing to consider.

How on earth are you planning to heat the air in a tiny cylinder at many kilowatts? This is required, because a typical four-cylinder car engine produces 100 kilowatts of power, and the efficiency is probably around 33% in the best case (assuming a non-Atkinson-cycle-engine here). So, you need 300 kilowatts of electric heating, of which one cylinder's share is 75 kilowatts.

Worse, you need to heat the air at a specific moment of time (between compression stroke and expansion stroke). For best efficiency, the heating should be very momentary period between these two strokes, but let's now assume that the entire expansion stroke can be used to heat the air. One of the four strokes means that momentary power needs to be 4 (number of strokes) times 75 kilowatts or 300 kilowatts. Per cylinder!

Have you seen 300 kilowatt electric heating elements? If you have, you probably realize that there is no way to put such a heating element inside a 86 mm stroke x 86 mm bore cylinder in a 2 liter engine. It would need actually to fit in much smaller space, because if the compression ratio is 10, only about 8.6mm is available in the vertical direction.

Even my 1900W electric interior heater which I use during the cold Finnish winter is much larger than 86 mm x 8.6 mm. And that's only 1.9 kilowatts, much less than 300 kilowatts.

Note that having the heater externally, i.e. heating the air before it passes to the engine, is not possible. In this case, the pressure of the air would rise in the compression stroke too, countering the pressure increase of the expansion stroke. You really need the air to be cold during the compression stroke and hot during the expansion stroke. So the heating element really has to be internal to the engine.

  • While this is also a problem, it's much less important than the fundamental efficiency issue. If you adjust the numbers just a bit this actually gets feasible – consider that there are >50 kW continuous-flow water heaters that would well fit in a car's engine department. Now, heating water does work always better than heating air, but heating compressed air to very high temperatures allows for much higher power densities than heating atmospheric-pressure air to just safe warm home temperatures, so comparing to interior heaters doesn't make sense. – leftaroundabout Feb 8 '17 at 0:56
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    "How on earth are you planning to heat the air in a tiny cylinder at many kilowatts?" I dunno but I'd start by putting a petrol-powered generator in the back of the car! :-D – David Richerby Feb 8 '17 at 12:03
  • @DavidRicherby I'd rather start by injecting the petrol into the cylinder and igniting it with a spark. But actually, if we're talking only about electricity, you have a valid point: batteries cannot match the energy density of gasoline. – juhist Feb 8 '17 at 12:47
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    As you mention, (evenly) heating one out of four strokes is extremely generous; in practice the peak heating rate per cylinder is probably in the 1-10 megawatt range. – Nick T Feb 8 '17 at 20:31
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tl;dr
Internal combustion engines cannot turn all the heat into mechanical work. And if there is heat source, why not to use it for heating?


All thermal cycles, for example Carnot cycle (Ideal, the most effective), Otto cycle (idealised petrol engine), Diesel cycle (idealised diesel engine), Clausius-Rankine cycle (ideal steam turbine), dissipates some portion of heat by definition. The real engines turn even less energy into mechanical work and more energy to the heat.

Part of the mechanical energy is consumed by alternator to charge the battery, power the spark plugs, ECU and other system. The rest is used for the actual motion.

If we use elecrical heaters only, we will dump all the heat from the engine and use part of the mechanical work to heat the car up. It is double-waste. If we use air heating we had foud usage for part of the wasted energy from the engine.

As an example compare fuel comsumption and driving expirience of Skoda Fabia 1.2 HTP (honem to prodej [sell it ASAP]) with stereo, AC and seat heaters. When all systems are on, the consumption is significantly higher and acceleration is significantly worse - all because more power direced to the alternator rather to the wheels. Turning AC off is commonly tactics used when attempting to overtake with such cars.

Electrical independent heating and electrical seat heaters are used as optional extra for enhanced comfort. It takes less than a minute to heat them up regardless the ambient temperature, on the other hand warming up the engine takes from minutes to tens of minutes with respect to the ambient temperature.

Side note, if your engine is overheating it is recommended to turn heating fully on - it will redirect part of the heat from failing/overloaded radiator to the cabin.

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    Actually, instead of electric resistance heating you could in theory use a heat pump which has >100% efficiency. The >100% efficiency of a heat pump multiplied by the <100% efficiency of the internal combustion engine can mean the overall efficiency is arbitrarily close to 100%. However, in practice, heat pump is even more ridiculous than resistance heating. As I pointed out in my answer, heating elements need to be very big. Heat pump? Even bigger. – juhist Feb 7 '17 at 18:38
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    @juhist Heat pumps need mechanical power to pump heat from colder heater to warmer cooler. Thats's why fridge and AC have compressors. Why to use heatpump to transfer energy from >100°C engine to <30°C cabin? Peltier cell works simillar; compressor is replaced by alternator and gas medium by electric charge. – Crowley Feb 7 '17 at 18:47
  • @juhist You approach the question from different POW. You discuss electrical heating of the working gas - replacing petrol by electricity. :) – Crowley Feb 7 '17 at 18:48
  • Ah, yes, indeed. I didn't read your answer carefully enough. – juhist Feb 7 '17 at 18:49
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    @juhist Heat pumps do not have >100% efficiency. You're thinking of the coefficient of performance (COP), which can be a factor of 3 or 4. Put 100 watts in and you can move heat at a rate of 300 or 400 watts. This looks like overunity! But no. Reality is that, one, simply moving heat doesn't make any new energy. Two, the heat pump's output temperature will not be very different from ambient. This drastically reduces the amount of energy you could recover from the output (see "Carnot efficiency"). – Jamie Hanrahan Feb 9 '17 at 2:43
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The purpose of an ICE is to convert chemical energy into motion. It does this by igniting fuel not to generate heat per se but to cause rapid expansion of particles in the cylinder, which in turn creates pressure, and therefore applies force on the piston. Heat is one of a number of factors that come into play in that expansion. However a number of other factors come into play in the chemical reactions involved in combustion. These could not be simulated by simply heating air rapidly with electric heat. Converting chemical energy to heat is also, it turns out, much easier to do efficiently that converting electricity into heat.

  • The chemical energy is used to release the heat. And particles expand by changing the temperature, that's if i understand the gas law correctly. – DeusIIXII Feb 7 '17 at 20:29
  • @DeusIIXII I see your point. On slightly further research it looks like the gaseous expansion involved is more complex than I thought, and several factors come into play, the largest one being (I think) increased temperature causing increased pressure. – Michael Lay Feb 7 '17 at 20:52
  • I am with you. Its more intricate then i thought as well. I knew it would be a vast topic but man. Lol. – DeusIIXII Feb 7 '17 at 21:19
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    @Michael - Please edit your answer accordingly. As you have it, your answer is not correct. – Pᴀᴜʟsᴛᴇʀ2 Feb 7 '17 at 21:19
  • @DeusIIXII The terminology gets tricky quickly too. In many contexts, when you talk about heat, you really talk about waste heat - the part that cannot be used to do work. But that's not really appropriate when dealing with heat engines, whose sole purpose is to extract as much work as possible from a heat differential :) But do note that it's exactly this "squeezing" that gives you the rather low efficiency, not the gasoline burning. Why would you use an electric heat engine (~30% efficiency) instead of an electric motor (~90% efficiency)? And electric motors are light and cheap :) – Luaan Feb 9 '17 at 9:36
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But we do use electrical systems to create heat in internal combustion engines. They're called Block heaters.

As most of the comments and posts point out, it's not heat that's converted to mechanical energy, it's the force from the ignited fuel (the combustion) via the pistons. The heat is mostly wasted energy, which is absorbed and transferred out by the coolant.

However at extreme temperatures oil becomes more viscous, requiring more energy to move. This can lead to engine damage when trying to run cold, or the inability to run the system at all. Block heaters were designed to keep certain parts of the engine closer to their ideal running temperature in order to make it easier to start.

So while we don't need electricity to create extra heat in an already running engine, we do use it to keep things warm when it's off.

  • I'm not sure how your reference to a block heater answers the OP's question? – Pᴀᴜʟsᴛᴇʀ2 Feb 7 '17 at 22:29
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    Block heaters aren't used to put power to the wheels, they are a housekeeping system. – whatsisname Feb 7 '17 at 22:29
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I don't know what you mean by heat up and do work but the pre-86 Lancia Delta featured electric cabin heating is that's what you are referring to?

If you mean to heat up air inducted into the engine, this is a bad idea because cold air has the highest density and therefore the most oxygen available for combustion.

  • Regarding the second paragraph: That is why turbocharged engines use intercooler between turbo and intake. – Crowley Feb 7 '17 at 17:43
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First question - where do you get the electricity from? The electricity we use comes from other sources hydro, geothermal etc Combined with losses of conversion mean that using other fuels that are directly available coal etc to provide heat has been effective. Now, with global warming then we are moving away towards renewables...

  • Um, no. Even with the losses of conversion, electric power is more efficient than an ICE. The issue that hindered adoption of electric cars has been a lack of battery capacity/range. – Hobbes Feb 7 '17 at 17:28
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Heat is a form of energy, but compared to electricity it's lazier. It's hard to get work out of it. So if you had a source of electricity you'd be much better off using it in an electric motor, with 99% efficiency, than using it to heat something and then extract work from the heat with maybe 30% efficiency on a really good day.

And I didn't even mention Boltzmann's constant!

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