The Science Behind the Spitfire – Part 3: Excellent Engines
In celebration of British Science Week we continue our exploration of the Science Behind the Spitfire. The aeroplane is renowned for its performance and the important role it played during the Second World War. We will peel back the panels and discover some of the science behind the Spitfire’s success.
Spitfires were powered by Rolls-Royce Merlin engines, and later Rolls-Royce Griffon engines. Just like the aeroplanes in which they were fitted, engines were constantly tuned and developed throughout the war.
So how do piston-engines actually work?
The Merlin engine contains 12 cylinders. Inside each is a piston that is pushed up and down. The pistons are connected to a crankshaft that turns the ‘up and down’ motion of the piston into a rotating motion. In the Spitfire this motion is used to drive the propeller. So how do we get those pistons moving?
There are two valves at in the top of the cylinder, think of them as little gates. One opens to let things in (inlet), one opens to let things out (exhaust). By opening and closing in various combinations they produce the four-stroke cycle:
- The exhaust valve closes and the inlet valve opens. The piston moves down and draws air and fuel into the cylinder.
- The inlet valve closes (both valves are now closed). The piston moves back up and compresses the air and fuel.
- The spark plug in the top of cylinder ignites the fuel-air mixture which burns and expands, pushing the piston back down. This is called the power stroke because its generates the power to keep the piston moving during all the other stages.
- With the inlet valve still closed, the exhaust valve opens. The piston moves back up and pushes the exhaust gases out of the exhaust valve.
The Carburettor Problem
One engine issue that had to solved was the Merlin’s carburettor. This is an important part of the engine where fuel and air are mixed before they enter the cylinders described above. A float was used to control how much fuel entered the carburettor. This worked well when flying normally, but if the Spitfire attempted to perform a negative g-force manoeuvre, the float wouldn’t operate properly. The result was loss of power or even the engine cutting out!
Messerschmitt engines were fuel injected, and didn’t suffer the same problem. Luftwaffe pilots knew that a steep dive could be one effective way to lose a tailing Spitfire.
The problem was first solved by Beatrice Shilling, an engineer who came up with a simple flow restrictor that allowed just enough fuel through to keep maximum engine power. Later, pressurised carburettors were developed to do away with the issues of negative G-force.
Another key feature for the Merlin engine was the improvement of superchargers.
A supercharger uses a fan-like mechanism known as an impeller to compress the air that enters the engine. The result is that more oxygen is present inside the cylinders, burning more fuel, and producing more power.
By the end of the war the Spitfire was equipped with engines that boasted two-stage, two-speed superchargers.
‘Two stage’ means there were two impellers, a big one and a small one, to compress the air twice. More compression meant more oxygen which meant more power. Two speed means that the supercharger could run at different intensities. From the cockpit, the pilot could move a lever to switch the supercharger between low or high settings. More compression was needed at higher altitudes because the air is thinner. Being able to switch the speed of the supercharger gave the pilot the ability to tune the engine power to suit different altitudes.
And that’s a wrap on another bit of Spitfire Science, stay tuned for more Spitfire blogs in the near future.