For all their impressive power, tanks won’t work without fuel, which in the First World War meant petrol. This post takes a look at how this petrol was stored and used aboard British tanks.
On the Mark I the engine was fed with fuel by a simple gravity feed. This required the petrol tank to be located above it. The location decided on was in the track horns, right at the front of the tank. Each horn contained a 25 gallon tank for a total of 50. On good ground this could give a radius of action of around 24 miles, but in poor conditions rather less.
Although having the fuel supply here worked well for normal driving, it soon became clear that it presented a major drawback under two circumstances.
The first of these was when the tank was nose down, as it could easily end up when crossing trenches or shell holes on the battlefield. If the fuel tanks spent too long below the level of the engine the petrol supply would be cut off, immobilizing the vehicle.
The second was when the tank came under fire from the front, as it could expect to in combat. If the armour was penetrated the petrol in the unprotected tanks posed a serious fire risk, and being next to the driver and commander the chances that either could escape if the fuel caught fire were slim.
Mark IIs and Mark IIIs, intended only for training, kept the same arrangement. The improved Mark IV, however, resolved this problem. The petrol was now stored in a single tank low down at the back of the vehicle. It was also armoured and, at 70 gallons, larger. This improved the range to 35 miles, again, on good ground.
The new location also helped the crew in another way. Fuelling up was a labour intensive process. There were no petrol tankers or powered pumps, the fuel was supplied in 2 gallon cans and manually poured into the tank by the crew.
The Mark I’s tanks had to be filled from inside. The fillers were at shoulder height next to the commander and driver’s seats, making this quite an awkward job. The tank on the Mark IV (and later Mark V and Mark V*) on the other hand, could be filled from outside with the cans held at chest height.
Putting the petrol below the engine presented a problem. It now needed to flow uphill. This would require some form of pump.
At first an air pressure system was used. Once it was manually pumped up to the required pressure (2lb) by the driver a mechanical pump took over and forced the air through a series of pipes. The air bore down on the petrol, which was forced up the pipes to the engine.
Although this was fairly effective, any air leaks or a loss of pressure would lead to the entire system breaking down. Something more reliable was needed.
The solution was the Autovac, which used reduced, rather than increased, air pressure to pump fuel.
It comprised an upper and a lower chamber. The lower chamber contained a reservoir of petrol, and the upper chamber contained a float, the inlet pipe from the petrol tank and the pipe from the engine air pump.
The pump reduced the air pressure inside the upper chamber. This created a suction effect that drew fuel from the main tank into it. As more fuel entered the chamber the float was forced upwards. Eventually the float rose high enough to block the air pump and fuel inlet pipes and open a pipe at the bottom of the Autovac, draining the fuel into the engine. The float would fall, and the process repeated itself. It was a successful system and it was reused on all British tanks for the rest of the war.
Find out more about tank development and First World War tanks in the books below.