A positive perspective on electric vehicles

Because we recently ran a story that was highly critical of electric cars, here’s a positive story for balance. – Anthony

Guest essay by Jan Kjetil Andersen, Jkandersen.no Csens.org

I want to share some thoughts and experiences about using electric vehicles (EV) and how they compare to traditional internal combustion engines (ICE).

My personal experience is based on being a user of Nissan Leaf in my daily commute for the last four years. We do also have two ordinary cars in the family, but what we see is that the EV is the car everyone chooses first. The reason is obvious; it is simply a much better car to drive. The noiseless engine let you hear the wind blowing and the birds singing, or you can turn on music and hear it without any disturbing engine in the background. The gearless drivetrain gives a unique smoothness, and the acceleration is just superb.

My experience so far have made me to be an enthusiastic EV supporter, not because I think I save the planet, but because I find the EV much more enjoyable to drive.

With that as a prologue, let us take a look on the more theoretical and technical constraints of electric versus fossil fueled cars.

Efficiency of combustion engines – theoretical limits

Efficiency is the relationship between the total energy contained in the fuel, and the amount of energy used to perform useful work.

Let us first analyze the theoretical limits given by the physical laws for an “ideal” frictionless engine.

The most fundamental limit of efficiency for a combustion engine is given by Carnot’s Theorem which states that:

The Maximum Efficiency = (T2-T1)/T2

Where

T2 = The maximum temperature in the process in Kelvin

T1 = The minimum temperature in the process in Kelvin

If for instance the minimum temperature is 300K (27 Celsius), and the maximum is 1200 K (927 C), the maximum theoretical efficiency is (1200 – 300)/1200 = 900/1200 = 75%

The main point to take from this is that the maximum theoretical efficiency is substantially below 100%, even if the machine is without any friction.

But even though all gasoline and diesel engines are covered by the general Carnot process, they are far from a Carnot processes. We come one step closer to reality by looking at the theoretical upper limit for Otto cycles and diesel cycles for gasoline and diesel engines respectively. The theoretical efficiencies of an Otto cycle, diesel cycle or any other thermal cycle can never beat the Carnot cycle, but they set an upper limit for those engines.

The maximum efficiency of an Otto engine is given by the compression ratio, the higher compression the higher efficiency. However, the compression ratio of Otto cycle engines is limited by the need to prevent the uncontrolled combustion known as knocking. Modern engines have compression ratios in the range 8 to 11, resulting in theoretical ideal cycle efficiencies of 56% to 61%.

The Diesel cycle is less efficient than the Otto cycle when using the same compression ratio, but this is more than compensated by the higher compression ratio. Diesel engines therefore have slightly higher efficiency than gasoline engines.

Efficiency of combustion engines – in practice

Real engines are obviously not ideal. The actual cycle of a four-stroke gasoline engine is very different from the idealized Otto cycle. In addition, there are of course frictions in all moving parts which results in truly existing engine efficiency in the range of 25% – 30% in ordinary gasoline automobiles.

In addition to that, there are losses in the drivetrain between engine and wheels, resulting in actual power to the wheels efficiency of only 18% – 25%.

So how does this compare to the efficiency in an EV?

Well first of all, there is no theoretical upper limit for efficiency like the Carnot theorem for EV. A frictionless electric engine has a theoretical efficiency of 100%.

In practice we see that there are losses in charging batteries, using batteries and friction in the electric drivetrain, but the actual power to the wheels is here about 82 percent, i.e. several times better than an ICE.

The figure above show development in the efficiency for steam, gasoline and electric engines. James Watt revolutionized the steam engine by improving the efficiency from Newcomen’s puny 0.5% to 3%. Later triple expansion engines reached about 10% efficiency. Nicolas Otto’s petroleum motor had 12% efficiency, and the Spague electric motor had about 70% efficiency.

The superior efficiency of electric motors is also illustrated by the fact that it makes sense for diesel electric railway locomotives to use an electric generator combined with an electric motor as a replacement for a mechanical transmission.

The Battery vs the gasoline tank

The electric automobile engine is in my opinion superior to the combustion engine. In low and moderate speeds, you get the noiselessness and smoothness of a luxury car, the acceleration of a sports car and the energy use of a moped. That combination is unbeatable by any single fossil fueled car.

However, when the features of energy storage in a battery is compared to a gasoline tank there is no doubt that the battery is far inferior.

The battery in my Nissan Leaf has a capacity of 24 KWh, which is equivalent to 2.6 liters (0.7 US Gallons) of gasoline.

Imaging having a car with 0.7 gallons gasoline tank, which it takes 8 hours to fill at home, or 25 minutes on a supercharger, would you, buy it?

Well I have, and I must say that in spite of the low range, I am overall very satisfied with it.

Due to the good energy economy, it has a driving range from 140 km with modest speed in the summer to about 80 km in the coldest winter months. Those ranges may seem puny, but in my experience, it covers the vast majority of most people’s driving needs.

Battery development

The prices of Li-ion batteries have dropped considerably recent years and the drop is projected to continue. How fast the prices drop can be debated, but approximately 14% annually, as is described in this article, is a conservative bet.

Fourteen percent drop each year translates to halving the prices in five years. This development can be seen on the new generation EV now brought to the market. The prices have not halved, but the battery size and range have approximately doubled compared to the ones we saw five years ago.

Tesla is leading the range contest with 500 km (310 mile) range and a supercharging rate of 270 km (170 miles) in 30 minutes. With those figures, the range and filling time properties starts to close in on fossil fueled cars.

In practice no more time on filling station than for a gasoline car.

Personally, I do not use more time on supercharger stations than I used to use on gasoline stations. The reason is that I charge at home, and do not use supercharges station more than approximately 10 times per year. I may stay there 20 minutes each time, which amounts to 200 minutes annually. A petrol car with the same driving distance would have to be filled about 50 times per year, which would have taken about the same time in total when the stop, opening tank, payment et cetera is included.

Toque and rotational speed

Torque is a measure of the turning force on an object such as a bolt or a crankshaft. It is important to understand this unit to get a grip of a fundamental benefit of the EV, so let us examine it a bit.

Torque is measured internationally in Newton*meter. As an example to illustrate the amplitude of the unit; you should use about 100 Nm on each bolt if you want to fasten your wheels on your car.

The conversion factor between torque and power delivered is that power in watt equals torque multiplied by rotations per second multiplied by two Pi:

P = T * R*2*Pi

The reason it has to be like this, is that Watt is just Nm per second and the perimeter of the circle with on meter radius is 2 Pi as seen on the figure below.

If the crankshaft for example has a rotation speed of 10 rotations per second and 100Nm torque is applied, the power delivered is 6.26 Kilowatt (KW). The same torque applied at 100 rotations per second thus gives 62.8 KWFigure: If you push a handle of 1 meter one rotation in one second you deliver a power in Watt of 2 Pi times the torque.

The rotation speed given by tachometers in automobiles usually show rotations per minute (RPM), not per second, so I will continue with the most common form here.

Figure, the tachometer in an ordinary petrol vehicle. Here showing 2000 RPM on a scale going to 7000RPM.

The reason we are interested in torque is that it gives valuable information about the engine behavior with different rotational speeds. A typical plot for petrol and electric automobile engines is shown in the figure below.

Figure. Typical torque/RPM diagrams for traditional gasoline engine, modern electronically regulated gasoline engine and electric vehicles.

Gasoline engines have a useful rotation range approximately between and 1500 to 6000 RPM. However, in ordinary smooth driving you want to stay between 2000 and 3000 RPM.

The electronics in modern cars modern cars usually cap the torque to an upper fixed value, which is seen as a flat torque curve. There are two advantages with this. The first is that the drive chain must be scaled to handle the maximum toque, and it is uneconomical to have those dimensions just for a narrow peak range.

The second is that a flat toque curve feels smoother because, as long as the air resistance is negligible, constant toque gives constant acceleration. The G-force you feel against the seat is therefore constant, and that feels better than a varying acceleration.

The torque delivered by an EV is high and even from zero to about 4000 RPM, and thereafter slowly decreases. An EV operate over a very broad rotation spectrum. This eliminates the need for a gearbox.

You can do without shifting gears on a gasoline car too, just put it in second gear, start with some careful clutching and you may accelerate up to motorway velocity and stay there without using any other gears. The tachometer will then show around 6000 RPM. It is of course not recommendable to drive like that since it may damage the engine. You will also use extra petrol and it gives a lot of vibrations and noise.

Nevertheless, this demonstrates one aspect of the difference between ICE and EV; an EV has no engine noise even at 12 000 RPM.

The torque curve and wide rotational spectrum show that an EV has some features that is just better than what you find on a similar ICE.

Comparison

The table below gives a side by side overview of EV vs ICE features

There is a large uncertainty concerning the total economy because of the yet unknown lifetime of the battery.

The warranty for most EVs batteries today is that there shall be at least 70% capacity left after 8 years or 160 000 km (100 000 miles). This guarantee may not seem very assuring since a modern car of good quality should at least last twice as long as that. That means that the owner run a substantial risk of having to replace the battery at least one time in the car’s lifetime.

The battery pack is the most expensive item in an electric vehicle. The current cost is approximately 300 USD/KWh which gives a price of USD 22 500 for a car with 75 KWh battery. If the prices continue to drop by 14 % annually, the price will be USD 6732 eight years from now, still a considerate amount, but at least it is more acceptable than the current price.

My experience there is that after four years and 91 000 km, I see no performance drop at all. I use my daily commute as a benchmark, and on days with mild temperatures, I have always used exactly 20% battery capacity on 29 km.

Conclusion

The EV driving experience is superb, but the range and recharging time is still inferior compared to traditional cars.

However, the technology is now evolving quicker for EV than for traditional cars and the battery prices are cut in half every fifth year.

Many different sources all forecast that the market share of EV will grow from the current 0.2 percent. BP forecast a slow growth up to six percent market share in 2035, while Bloomberg new energy forecast that EV will outsell ICE in 2038.

Personally, I think the evolution will go even quicker. The much better energy efficiency and torque curves are revolutionary improvements which are impossible to match for any ICE. The EV will soon have both better total economy and better driving performance than any ICE, and most people will then buy the best and most economical vehicle. My bet is that EV will outsell ICE before the year 2030.

I do recommend them now in 2018, may be not yet for the economy, but definitely for the driving experience.

References:

1. Fuel economy: https://www.fueleconomy.gov/feg/atv.shtml

2. Nature: http://www.nature.com/nclimate/journal/v5/n4/full/nclimate2564.html?foxtrotcallback=true

3. Bloomberg: https://www.bloomberg.com/news/articles/2017-07-06/the-electric-car-revolution-is-accelerating