Naif al Hilali
FULL MEMBER
- Joined
- Nov 5, 2016
- Messages
- 324
- Reaction score
- 24
- Country
- Location
Blogs
Basics Of Vehicle Efficiency
Comparing internal combustion and electric powertrains
By Kevin Cameron November 28, 2016
Cycle World
Technical Editor Kevin Cameron shares his wealth of motorcycle knowledge, experiences, insights, history, and much more.
On the surface of things it looks like an open-and-shut case.
A gasoline engine is doing well to turn 25% of the energy supplied to it as hydrocarbon fuel into mechanical power. For a Diesel or other advanced piston internal combustion machine, the number is more like 35% or more (up to just over 50% for large, slow-turning marine two-stroke Diesels). The balance of the fuel’s energy is split between exhaust heat and heat rejected to coolant (which includes engine friction and pumping loss).
On the other hand, over-the-counter electric motors can be had at 94% efficiency. That is, of the electrical energy supplied to the motor, 94% appears as mechanical power at the output shaft.
Based on these numbers, we’d think electric was nearly four times as efficient as internal combustion. But we’d be wrong, because electricity does not just “come out of the wall”. Its generation involves a long series of efficiencies as well, and when we include them, electric and internal combustion come out nearly equal.
With the combustion engine, we begin with a fuel that is expensive to extract from the earth and may have been transported over long distance and then refined (crude oil into gasoline or Diesel) and transported again.
With the electric, we begin at the coal mine, hydro-fractured shale natural gas deposit, or the uranium mined and enriched for use in nuclear powerplants, and then (often) transported over long distance. Coal at present supplies 33% of our electricity, natural gas 33%, and nuclear 20% (the rest is 6% hydroelectric, 4.7% wind, plus smaller amounts for Diesel, biomass, grid and private solar, etc).
The basic cycle efficiency of a thermal electric plant (coal, gas, nuclear) is usually stated as 35%, but after subtraction of power required for pumps, blowers, coal pulverizers, ash systems, etc., the number becomes more like 30%.
The large alternator driven by the thermal plant’s steam turbine is about 98% efficient. Electricity is generated at the highest voltage that the alternator’s insulation can contain, but must be stepped-up in voltage for efficient transmission over power lines. Step-up and step-down for this gives 96-98% efficiency, and line loss (radiated power loss plus plain old resistance) give us 96-98% again. Losses are a bit higher at the distribution end, giving 94-96%.
Aboard our electric vehicle, the charge-discharge efficiency of the battery itself is 80-90%. When DC battery power is converted to AC to drive the traction motor(s), the IGBT power supply efficiency is about 95%. Finally we have the electric traction motor itself at maybe 95%.
To get overall efficiency we multiply all efficiencies together as decimals (98% becomes .98, etc.). Plowing through all the steps, we get 20% as the overall efficiency of an electric vehicle. Not that different from the gasoline engine.
This shows that conversion of vehicle power to electric is not free lunch. Electric vehicles do not emit the usual pollutants, but the power stations that drive them do. These take the usual forms – waste heat, carbon dioxide, ash, mercury, nitrogen oxides, sulfuric acid, unburned hydrocarbons, scheduled stack releases of Iodine-131 and other radionuclides, and so on (makes me remember “Let him who is without sin cast the first stone.”)
Some will argue over the fine details. Yes, a large tanker ship uses fuel equivalent to 1-2% of its cargo on a long haul. Yes, energy is required for fuel refining, transportation of coal by rail, etc. Just the act of digging out and storing coal releases a lot of methane into the atmosphere. Other arguments exist – the tailing ponds of hydro-fracking operations have drawn a lot of criticism, but less visible (because they are in faraway countries) are the very large brine pools necessary for the processing of lithium (for the lithium-ion batteries of electric vehicles and portable electronics) from dry lake beds. It becomes a pointless “science contest” like that waged back when nuclear powerplants were first coming into use; pro and con alike employed scientific experts with the same degrees from the same high-prestige universities to make exactly opposite statements. The pointlessness of such exercises only serves to undermine public trust in science itself!
Yet there’s hope. In Europe, combined-cycle gas-fired electricity-generating powerplants are just touching 60% efficiency. In such plants natural gas powers a large gas turbine, and the exhaust heat from the turbine is used to raise steam to drive a steam turbine.
For those who require complete energy purity, here are 2015 figures for percentages of US electricity generated just by renewables, from the US Energy Information Administration;
Hydroelectric - 6.0%
Wind - 4.7
Biomass - 1.6
Solar - 0.6
Geothermal - 0.4
Root for the energy source you favor.
Why does reality have to be so complicated?
Basics Of Vehicle Efficiency
Comparing internal combustion and electric powertrains
By Kevin Cameron November 28, 2016
Cycle World
Technical Editor Kevin Cameron shares his wealth of motorcycle knowledge, experiences, insights, history, and much more.
On the surface of things it looks like an open-and-shut case.
A gasoline engine is doing well to turn 25% of the energy supplied to it as hydrocarbon fuel into mechanical power. For a Diesel or other advanced piston internal combustion machine, the number is more like 35% or more (up to just over 50% for large, slow-turning marine two-stroke Diesels). The balance of the fuel’s energy is split between exhaust heat and heat rejected to coolant (which includes engine friction and pumping loss).
On the other hand, over-the-counter electric motors can be had at 94% efficiency. That is, of the electrical energy supplied to the motor, 94% appears as mechanical power at the output shaft.
Based on these numbers, we’d think electric was nearly four times as efficient as internal combustion. But we’d be wrong, because electricity does not just “come out of the wall”. Its generation involves a long series of efficiencies as well, and when we include them, electric and internal combustion come out nearly equal.
With the combustion engine, we begin with a fuel that is expensive to extract from the earth and may have been transported over long distance and then refined (crude oil into gasoline or Diesel) and transported again.
With the electric, we begin at the coal mine, hydro-fractured shale natural gas deposit, or the uranium mined and enriched for use in nuclear powerplants, and then (often) transported over long distance. Coal at present supplies 33% of our electricity, natural gas 33%, and nuclear 20% (the rest is 6% hydroelectric, 4.7% wind, plus smaller amounts for Diesel, biomass, grid and private solar, etc).
The basic cycle efficiency of a thermal electric plant (coal, gas, nuclear) is usually stated as 35%, but after subtraction of power required for pumps, blowers, coal pulverizers, ash systems, etc., the number becomes more like 30%.
The large alternator driven by the thermal plant’s steam turbine is about 98% efficient. Electricity is generated at the highest voltage that the alternator’s insulation can contain, but must be stepped-up in voltage for efficient transmission over power lines. Step-up and step-down for this gives 96-98% efficiency, and line loss (radiated power loss plus plain old resistance) give us 96-98% again. Losses are a bit higher at the distribution end, giving 94-96%.
Aboard our electric vehicle, the charge-discharge efficiency of the battery itself is 80-90%. When DC battery power is converted to AC to drive the traction motor(s), the IGBT power supply efficiency is about 95%. Finally we have the electric traction motor itself at maybe 95%.
To get overall efficiency we multiply all efficiencies together as decimals (98% becomes .98, etc.). Plowing through all the steps, we get 20% as the overall efficiency of an electric vehicle. Not that different from the gasoline engine.
This shows that conversion of vehicle power to electric is not free lunch. Electric vehicles do not emit the usual pollutants, but the power stations that drive them do. These take the usual forms – waste heat, carbon dioxide, ash, mercury, nitrogen oxides, sulfuric acid, unburned hydrocarbons, scheduled stack releases of Iodine-131 and other radionuclides, and so on (makes me remember “Let him who is without sin cast the first stone.”)
Some will argue over the fine details. Yes, a large tanker ship uses fuel equivalent to 1-2% of its cargo on a long haul. Yes, energy is required for fuel refining, transportation of coal by rail, etc. Just the act of digging out and storing coal releases a lot of methane into the atmosphere. Other arguments exist – the tailing ponds of hydro-fracking operations have drawn a lot of criticism, but less visible (because they are in faraway countries) are the very large brine pools necessary for the processing of lithium (for the lithium-ion batteries of electric vehicles and portable electronics) from dry lake beds. It becomes a pointless “science contest” like that waged back when nuclear powerplants were first coming into use; pro and con alike employed scientific experts with the same degrees from the same high-prestige universities to make exactly opposite statements. The pointlessness of such exercises only serves to undermine public trust in science itself!
Yet there’s hope. In Europe, combined-cycle gas-fired electricity-generating powerplants are just touching 60% efficiency. In such plants natural gas powers a large gas turbine, and the exhaust heat from the turbine is used to raise steam to drive a steam turbine.
For those who require complete energy purity, here are 2015 figures for percentages of US electricity generated just by renewables, from the US Energy Information Administration;
Hydroelectric - 6.0%
Wind - 4.7
Biomass - 1.6
Solar - 0.6
Geothermal - 0.4
Root for the energy source you favor.
Why does reality have to be so complicated?
