Fuel efficiency calculations

Some maths to try and understand the comparative fuel efficiency of vehicles using petrol, diesel and electricity as their fuel. For example, is a 3 mile per kWh electric car more or less fuel efficient than a 80 mile per gallon diesel car? (Spoiler: more)

First we need to understand the energy content of the fuels in question. According to the UK Government there is 9.6kWh of energy in a litre of petrol and 10.9kWh in a litre of diesel. We can then work out the miles per kWh by dividing the miles per gallon figure by 43.5 for petrol cars, and by 49.5 for diesel cars.

So a 50 mpg diesel car is doing roughly 1 mile per kWh. And a 22 mpg petrol car is achieving 0.5 miles per kWh. Almost all electric cars can manage at least 2.5 miles per kWh, so a diesel car would have to do 123.75 mpg to be using its fuel as efficiently as that, and for a petrol car it would be 108.75 mpg. Electric motorbikes can usually achieve twice the fuel efficiency of an electric car, so those figures would be double.

Why is this? Optimistically only about 35% of the actual energy in diesel and petrol is converted into motion, the rest is mainly wasted as heat from combustion of the fuel. Meanwhile electric motors generally turn over 90% of their fuel into motion, with the batteries wasting energy as heat at the extremes of their performance envelope.

Raw fuel efficiency is one aspect, but fuel cost is equally important, i.e. £ per mile. Right now in our part of the UK the cost of petrol is about £1.20 per litre, and diesel about £1.24 per litre. Electricity can cost up to £0.30 per kWh from a commercial fast charger, or as little as £0.045 per kWh on an EV-friendly domestic energy tariff.

To work out the £ per mile we simply divide £5.45 by the fuel efficiency of a petrol car, or divide £5.63 by the fuel efficiency of a diesel car. So our 50 mpg diesel car costs £0.113 per mile, and the 22 mpg petrol car costs £0.248. The electric car calculation is the same, divide the cost of a kWh, say £0.30, by the fuel efficiency, say 2.5 miles per kWh, and in this case get £0.12 per mile.

Purely on fuel costs, a 50 mpg car filled with £1.24 per litre diesel is cheaper to run than a 2.5 mile per kWh electric car charged with £0.30 electricity.

However, charge the electric car overnight at home at £0.045 per kWh and the £ per mile drops to 1.8 pence per mile. Almost a tenth of the £ per mile of the diesel car. And the electric motorbike mentioned above is going to easily cost less than a penny per mile to run.

Finally, if the electricity you charge your car or motorbike with has come from the solar PV panels on your rooftop, then the pure fuel cost is zero...

Key fob battery change

Recently the car started to complain that the battery in the keyfob was "low", so it was time to find out how the car and keyfob work together.

Our 2015 Outlander has "keyless" operation, meaning you don't have to insert the key into a lock to operate the car. However if the battery in the keyfob does go completely flat then the car can still be used, via the physical key stored in the keyfob.

Flip the keyfob over onto the side with the Mitsubishi logo.

Slide the releasing mechanism, just above the logo, and slide the key out of the keyfob.

You can use the physical key to unlock the driver's door, then put the key back into the keyfob, and place the keyfob into the slot in the central console, just to the right of the cigarette lighter socket. You can then operate the car as per normal. I'm going to infer that the slot has an RFID reader built into it, and the keyfob has a passive RFID tag, with the code required for the car's immobiliser to allow the car to be used.

Keyless operation is far more convenient, and changing the battery in the keyfob is very quick. At the end of the keyfob where the physical key is located there is a notch in the plastic. Using a screwdriver you can pop the two halves of the keyfob case apart.

The top case of the keyfob, with the buttons, has a CR2032 "coin cell" battery in a holder. The C of CR2032 indicates that the battery is 3V lithium, the R indicates that it is round, 20 is the diameter in millimetres, and 32 indicates a thickness of 3.2mm.

If you can't get hold of a CR2032 you can substitute a BR2032 battery instead. BR2032 are also 3V lithium batteries, but use a different material for the positive electrode. CR2032 are better at delivering the short bursts of current needed for keyfob operation than the BR2032, whereas BR2032 batteries are better for supplying a constant current, as you might find in the real-time clock of a computer. However in a pinch a BR2032 will work in a keyfob.

The old battery can be lifted out.

And a new battery fitted with the positive side,  indicated by a "+", upwards. Try to not touch both the positive and negative sides of the new battery while you're fitting it, as you'll discharge the new battery if you do.

Simply snap the two halves of the case back together, slide the key back into the keyfob, and you're good to go for another few years.

Autonomous cars & speed limits

The progression of electric vehicles is being mirrored by the progression of autonomous vehicles, with most mainstream manufacturers now offering varying levels of driver assistance and autonomy. SAE Level 2 autonomy enables "hands off" driving, where the vehicle maintains speed, braking and steering, whilst the driver remains "in control" and ready to take over should the vehicle not choose the correct course of action. Essentially any vehicle with adaptive cruise control and lane departure warning technology is knocking on the door of Level 2 autonomy.

This year the UK government gave the go-ahead for the first trials of platooning vehicles, which is expected to happen later in 2018. The significance of this trial is not in the technology; this has been proven in the lab and on closed tracks and there is no way that the public highway would or should be used to find out if technology works. The significance is to characterise the known unknowns, especially how other drivers and vehicles on the road interact with the platooning vehicles, and the emergent behaviours this will create.

There appear to be two main objections to platooning vehicles on UK motorways; firstly that they'll obstruct slip roads, and secondly that they'll obscure signage. On a personal level I disagree with both of these points. Slips roads are signposted a mile in advance, giving vehicles plenty of time to filter into the left hand lane, and bigger vehicles can already obscure signage from smaller vehicles, so this is not an issue caused by platooning vehicles.

However the huge significance of this trial for me is that permission has been given for platooning vehicles to break rule 260 of the Highway Code, i.e. "...keep a safe distance from the vehicle in front". In this trial the platooning vehicles will not be a safe distance from each other, if they were being driven manually. And this is an important point, as platooning vehicles are operating at Level 3 autonomy, where the driver is not able to take control should the vehicle not choose the correct course of action. At Level 2 autonomy the vehicle is always being operated safely, as the driver is ready to take over if the autonomous systems are not operating the vehicle safely.

So this trial is setting a precedent whereby a Level 3 autonomous vehicle need not meet all the rules of the Highway Code. Rules 124 and 125 immediately spring to mind, as they deal with speed limits. The key to this debate is the unbalanced nature of these rules, especially as captured in rule 125: "The speed limit is the absolute maximum and does not mean it is safe to drive at that speed irrespective of conditions." I don't think many people would argue that in the depths of a snowy winter driving exactly at the 30mph limit down a street crowded with festive shoppers may not be not safe. But I also don't think many people would argue that exceeding the speed limit by a few mph on a dry, deserted motorway is any less safe than at the speed limit.

So the debate that is looming is this: if a Level 3 autonomous vehicle can determine when it is safe to platoon, surely a Level 3 autonomous vehicle can also determine when it is safe to exceed the current speed limits, and by how much. And this will be a debate based on opinions and feelings rather than cold hard facts. There is hard data to prove that autonomous systems can safely drive a car well in excess of the speed limit, but many drivers are still wary of much simpler driver aids. For example I know many experienced drivers who shun manual cruise control. These are the people that need to be won over.

Hopefully they will be won over, because higher levels of vehicle autonomy are key to free-ing up the UK's motorway network. Instead of smart motorways with variable speed limits and roadworks to add lanes to already congested roads, we could have more cars, travelling faster, closer together, and all safer than we are at the moment. What's not to like about that?

Electric Vehicles

There's been a lot of press recently, created by the UK government's policy to prohibit the sale of new petrol and diesel cars from 2040, about the inability of the country's power infrastructure to support a nation's worth of electric cars. Putting aside for one moment that it will likely be sometime between 2050 and 2060 before the last of the new petrol or diesel cars sold in 2040 are no longer in regular use, we still have at least 23 years to come up with a reliable power infrastructure to support widespread EV adoption. Personally I don't think this is a major issue, especially when there's a deadline to galvanise efforts. Also, given that we already have the solutions, in my opinion it is just a case of implementing them at scale.

The first thing to consider is the generation of sufficient electricity for all these additional EVs. Some analysts have calculated that there is a need for handful of new nuclear power stations to cope with the extra demand for electricity. This is based on an assumption of a high peak load when everyone tries to charge their cars simultaneously. Whereas, if every car had a sibling storage battery from which the car was charged, the challenge becomes charging the storage batteries rather than the cars. This is significantly easier, as the storage batteries are always connected, and a simple load balancing algorithm would spread the load on the grid. Ensure that the storage battery has twice the capacity of the EV battery, and you can charge the storage battery at any time of the day or night. This technology already exists, under the banner of Demand Side Management, so if every household across the country had a storage battery tomorrow, the grid would continue to work just fine.

For sure millions of EVs will require more electrical power than is currently generated. And localised micro power generation is a potential solution for this additional power. Even in the northern latitudes of the UK during winter there is solar energy to be obtained from roof top PV panels. When the likes of Ikea can sell you PV panels and a storage battery for a few thousand pounds, you can be sure this is a mature, proven technology. There is still a major need for a country-wide power grid though, as solar and other renewable energies are not evenly distributed. But considering the vast storage battery capacity now available from the paragraph above, the contribution of renewable energy can now be significantly higher than fossil or nuclear. In theory, with sufficient storage capacity, there is no reason why all the country's energy cannot come from renewable sources.

But where are all these batteries going to come from? Are there sufficient rare-earth metals to produce them. This is another bit of scare mongering that has made the mainstream media, perpetuated by journalists that can't even fact check on Wikipedia. For starters rare-earth metals are not rare, as in scarce, but are rare, as in not found conveniently in seams of ore. This does make them more expensive to extract, but also makes them easier to find, because they are pretty much everywhere. Secondly, rare-earth metals aren't widely used in batteries, so there's no need to extract large quantities of these plentiful metals to build batteries. Rare-earth metals are used in electric motors though. But the good thing about that is that electric motors are about the simplest mechanical system you can build, and are extremely reliable. Essentially the electric motor in today's EV is maintenance free for life.

So far my thoughts have been based on the state of current technology. Imagine the world 23 years ago, and now think forward 23 years, noting that the rate of technological development is accelerating. Electric motors and EVs will be more efficient, requiring less energy. Batteries will have higher energy density and charge faster. Micro generation of solar energy will be widespread, as will macro generation from wind, wave and tidal sources. If anything, having stored energy distributed widely around the country will given us a more resilient and robust power infrastructure, not the frail brown-out prone grid the mainstream media portrays.