One of the most common arguments against EVs goes something like this; “EVs are not environmentally friendly, all they do is move the pollution to a big dirty power station and make people feel good about doing nothing helpful at all”. This article will dispel that myth.
Firstly, we need to talk about efficiency…this is where an internal combustion engine fares very poorly and an electric motor does very well.
An internal combustion engine, whether it be petrol or diesel, is a very inefficient device. There are many moving parts, each of which has friction. The pistons fly up and down the cylinders at enormous speed, yet at the end of each stroke they must be brought to a complete stop then accelerated back in the other direction in a few milliseconds. Each piston has rings sealing the gap between the piston and cylinder, which create drag. The lubricating oil and coolant must be pumped around taking useful energy to do so. Much of the energy obtained by burning the fuel in the cylinders is lost as heat, either down the exhaust or piped away by the coolant to the radiator where it is dumped as waste heat. Not all of the fuel is completely burned, and the by-products are lost in the exhaust. The noise made by the engine is also lost energy that comes from the petrol you are burning. A modern petrol engine is around 25% efficient, meaning that three quarters of the energy in that petrol you pour into the tank is wasted…older cars and cars in bad tune can be down to 15% efficient, and even the most advanced direct injection economy cars being developed now are only around 35% efficient.
Electric motors are by contrast very efficient, with most offering better than 80% efficiency and many getting 90% or better. Two of the most popular motors used in home conversions, the HPEVS AC-50 and the Netgain Warp 9, both have efficiency ratings of 88%.
The next step in the drive train is the transmission, which is another area where EVs shine over their counterparts.
Because an internal combustion engine needs to be spinning pretty fast to make enough power to move a car away from a stop, and it makes its power in a fairly narrow speed range, it needs to have a multi speed transmission, usually a manual transmission with 3 to 6 speeds, or an automatic with 2 to 8 speeds depending on how new or fancy the car is. Older cars with big flexible engines often had 3 speed transmissions where smaller less flexible engines needed more gears.
An electric car with a DC motor still usually has a multi speed transmission, but because an electric motor delivers full torque from standstill until near its maximum speed, they don’t need a clutch or torque converter and simply move off from stopped, and they usually need only two speeds in the transmission. The reason they need a two speed transmission is that DC motors don’t spin at very high speeds so if they have a single gear ratio it needs to be high enough to give the car highway speed without overspeeding the motor. This is like taking off in third gear in your petrol car, so you can imagine for this to work the motor needs to be very powerful. Even so, a two speed transmission has less gears than a 5 speed and is more efficient.
An AC motor on the other hand can spin at speeds of more than 10,000 revolutions per minute (RPM) which means that highway speeds can be obtained with a single gear ratio that is low enough to allow a relatively small motor to take off from a standing start. This is the drive system used in all production EVs and increasingly in home conversions, and offers both the increased efficiency of a 3 phase AC motor as well as the more efficient single gear transmission.
So, just how much energy is lost in the transmission? In the bad old days of “slushbox” automatic transmissions up to 30% of the power that left the motor would never make it to the road. Modern drive trains can be a lot better, losing only 10%, but the single speed transmission on a modern EV loses more like 5%. Not as big a factor as the motor efficiency, but not to be ignored either!
We have another loss that applies only to EVs though. Not all of the power that leaves the wall socket makes it into the battery, and not all of the energy in the battery makes it to the motor. I have measured the actual energy consumption of my i MiEV and the wall to motor efficiency is approximately 80%, so 10% of the power is getting lost in the charging phase, and another 10% is lost as the power is extracted from the batteries and run through the controller. In this analysis we will be calculating wall to wheel efficiency, so the worst possible scenario for the EV, but the most honest one.
So…we will do a comparison between an average modern petrol car and a modern EV.
Assuming petrol engine efficiency to be 30%, transmission efficiency to be 90%, with electric motor efficiency at 90% and single reduction gear efficiency at 95% and charging efficiency at 80%.
This gives a total efficiency of 27% for the petrol car and 68% for the EV. The EV is more than two and a half times as efficient with its energy as the petrol car. An older petrol car with an old tech automatic and carburettor engine would be even worse, with perhaps 10% total efficiency…..
This assumes constant speed driving…in reality most driving is in the city where we are either idling in traffic jams, or stopping and starting constantly at traffic lights or intersections. This is where the EV is at its most efficient…when stopped at a traffic light the EV is using no energy at all! The petrol car is idling and burning petrol, although modern “soft hybrid” cars are helping this by switching the petrol engine off while stopped and restarting it instantly when you go to move off. Even then, the petrol engines are operating in the less efficient part of their operating range whereas EVs are in their high efficiency conditions.
Another part of the equation for EVs is the power loss between the power station and the end user. This is often cited as being a major factor in making EVs no better than petrol cars, but in fact power losses in transmission are pretty low, as long distance transmission is done at high voltages where losses are around 1%, while lower voltage lines in suburban areas have higher losses. The total loss is usually around 6-8%, with the USA average being 6%.
So…..we have established that the EV is far more efficient with energy than an internal combustion car, even taking into account transmission losses. We now need to consider where that power is coming from.
Of course, if you are in an area where the power is all generated by renewable sources, such as Tasmania where I live where all of our power is from Hydro, wind, or solar, then you are truly running a zero emission vehicle.
Most of us are not that lucky, and live in an area where the power is generated by a mixture of renewable and fossil fuels. Almost nowhere is the power completely generated by fossil fuels, but for the sake of it, let’s look at the worst of them all, brown coal.
An ordinary coal fired power plant is around 35% efficient. Some high tech coal plants are over 50% efficient, but we will look at the worst ones we are likely to see in Australia. Taking the coal plant as being 35% efficient and the grid as 94% efficient and using our previously calculated 68% efficiency for the EV, we get a total system efficiency in the worst case scenario of 22%, which is an efficiency slightly worse than the petrol car….but this is the absolute worst case scenario. In fact Australia’s overall average is 86% fossil fuel and 14% renewables, and if we take this as the average mixture being used for your EV then the total efficiency of the system is now around 27%, or about the same as for the petrol car.
So…this merely proves the claim of pollution relocation right?
Not quite. We haven’t finished with petrol cars yet for the true system efficiency.
Oil as it comes out of the ground isn’t ready for your petrol tank. First of all it has to be shipped halfway around the world, then it needs to be refined. This involves heating it to boiling point and subjecting it to a process called fractional distillation. The lighter elements in the oil rise and the heavy ones sink, and each one is tapped off and subjected to further refining, such as catalytic cracking and blending etc until the final petrol is produced. This uses enormous amounts of energy.
The actual amount of energy used to refine petrol is the subject of debate and controversy, and is variable dependant on a number of factors including the quality of crude oil being used (really good crude needs less energy to refine, tar sands need HUGE amounts of energy, and a range of grades in the middle…) as well as the efficiency of the refinery and the energy mix being used at the refinery. Making accurate analysis harder is the reluctance of oil companies to provide the raw data, however several studies have been done which give some idea. Refinery process is around 85-90% efficient, and the total process of extracting the oil and producing petrol from it seems to be approximately 80% efficient.
This would give a total system efficiency of the petrol car as approximately 21%, which is slightly worse than the EV being run on pure brown coal electricity.
Several studies have concluded that it takes between 4 and 6 kWh of electricity to refine a gallon of petrol (around four and a half litres) and that amount of electricity could power an EV for the same distance as that gallon of petrol could power an average modern petrol car. As I said above, the actual numbers are hard to verify, and on the face of it the 6 kWh figure seems plausible, but looking deeper at it I believe that figure is somewhat too high. Whatever the actual figure is, it is clear that a lot of energy is used in refining and shipping petrol and this needs to be remembered when calculating the efficiency of petrol cars vs electric.
So, in conclusion, we have no clear cut exact difference, but rather a range of numbers dependant on variable factors.
An EV running on purely renewable power generated close to its charging location, such as home solar panels, will be around 68% efficient.
One running on renewable energy drawn from the grid will be around 64% efficient.
If running off the current (2015) Australian average mix of renewable and fossil fuelled grid, it will be around 27% efficient and if running on pure coal fired power the figure is 22%.
A petrol car of a modern type in good tune will be around 21% efficient if you take the lower numbers for power used in refining petrol. By the higher estimates it will be more like 10%.
Of course if the petrol car we are comparing is an older carburettor engine with an automatic transmission then we are already down at 10% or less before we even factor in the energy used refining fuel…
To summarise, in the worst case scenario, where the EV is being run on pure fossil fuel electricity, the “moving the pollution” argument is not far from the truth. If you are running an EV on “green power” or fully renewable generated electricity, the EV is between two and three times as efficient as the petrol car. Mixtures of power source will skew that number in either direction.
And there is one more part of the argument that is often ignored….what is wrong with moving the pollution anyway? If two modes of transport are going to generate the same amount of pollution, would you rather that pollution be generated in the midst of where people live, work, play…or is it better to generate that pollution away from population centres? I know which one I would pick…