Tag Archives: transport

Mercury and aeroplanes

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One of the many items you are not permitted to bring aboard aeroplanes is a mercury thermometer (or more accurately “mercurial thermometers and barometers”). Why?

mercury

Aeroplanes are made largely of aluminium, as it has one of the best available strength-to-weight ratios.* When aluminium is exposed to air it forms a tough coating of aluminium oxide that doesn’t flake away like iron oxide (rust) does and which prevents chemicals from reacting with the aluminium.

But if the raw elemental aluminium is exposed (e.g. by a scratch) and comes into contact with mercury it forms an amalgam, tearing away at the aluminium and causing it to lose its structural integrity. As the aluminium is eaten away it combines with the air to form aluminium oxide and falls away (as seen in this video). This allows the mercury to reach fresh aluminium and the process then repeats, so a small amount of mercury can do a large amount of damage. If a mercury thermometer were to leak aboard an aeroplane the aeroplane would need to be taken out of service and disassembled to assess the damage the mercury might cause. There have been at least two incidents in which aircraft exposed to mercury have been written off by their insurers.

The forty second timelapse video above shows the effect of a small amount of mercury on an aluminium I-beam over the course of two hours.

* Titanium has a better strength-to-weight ratio (288 kNm/kg compared with aluminium’s 214 kNm/kg) but it costs about five-and-a-half times more. Titanium is used in jet engines, where strength-to-weight ratio is key.

Why 80mph is not a good idea

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The UK government has announced that it is beginning a consultation on raising the speed limit on UK motorways from 70 mph to 80 mph. I don’t think this is a very good idea, and my reasons are listed below.

Time Saving

You simply don’t save that much time by increasing your speed to 80 mph. If you drove the 400 mile distance from London to Edinburgh, you would only save three-quarters of an hour, and that assumes that you maintain a constant speed of 80 mph the entire time, neither accelerating or decellerating.

Assuming no acceleration or deceleration is entirely unrealistic and as it takes longer to accelerate to 80 mph than it does to get to 70 mph, the graphs above represent a best case scenario. The real time savings are likely to be much lower.

Kinetic Energy and Stopping Distance

Kinetic energy at different speeds for my car, a Peugeot 207 with a mass just over 1000 kg.

The kinetic energy of a vehicle is what makes it dangerous; transferring that kinetic energy to an object – be it a pedestrian or another car – is what causes damage. Kinetic energy depends on the square of the speed; if you double the speed of an object you quadruple its kinetic energy. An increase in speed from 70 mph to 80 mph is a 14% increase in speed but this results in a 31% increase in kinetic energy, making the car – in a sense – 31% more dangerous to other road users.

Increasing kinetic energy also increases stopping distance. Increasing the speed limit to 80 mph would increase the average stopping distance on motorways to 120 metres (30 car lengths!), a 25% increase on the 96 metre stopping distance found at 70 mph.

Fuel Economy

The relationship between speed and fuel economy is not linear; fuel economy is poor at both low and high speeds and peaks somewhere between 40 and 60 mph. Driving at 80 mph will decrease fuel economy and therefore increase fuel consumption; fuel costs will be higher and CO2 (and other harmful gas) output will be higher. Our limited supply of petrol will be depleted faster if we all drive at 80 mph than if we drive at 70 mph.

SkySails

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On July 7th the German company SkySails GmbH was awarded the Sustainable Shipping Environmental Technology of the Year Award (for the second time) for its SkySail technology.

The SkySails system uses a computer controlled kite with an area of more than 160 square metres to harness the power of wind as an auxilliary power system for large marine vessels. The SkySails company claims it can reduce fuel consumption over long journeys by between ten and fifteen percent.

The SkySails system is a form of high altitude wind power (HAWP). HAWP systems are viable because the power available to wind power systems increases with the cube of the wind’s speed (e.g. if you double the speed the energy produced increases by a factor of eight) and wind speed increases rapidly with height. Companies like KiteGen are even working on using HAWP systems for electricity generation.