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70. Carburetion

22/08/2013

We’ve spent a good deal of time wondering about the engine controls on the Bristol Scout.

All petrol engines run on a mixture of petrol and air, as you know, and getting exactly the right mixture under all circumstances is one of the most difficult bits of engine design.

Most modern car engines use injection, where a little piston injects a precisely controlled amount of ptrol, either into the intake, or directly into the combustion chamber. The piston is controlled by a computer which uses all sorts of information (engine speed and load, throttle pedal position, air density, humidity, etc.) to decide how much petrol to inject.

Before the advent of computers, the most common device was a carburettor, which used a variety of methods to meter the petrol, but all of them were based on the idea that the air being sucked into the engine would draw fuel in as it passed through a narrow tube which created a partial vacuum. Obviously the more air was going in, the greater the vacuum, and so the more petrol.

Carburettor design is extremely skilful, and took years and years of experience to get right, and in 1915 it wasn’t particularly well developed, and for aircraft engines there was a further complication owing to the changing requirements at higher altitudes.

This is bad enough on a conventional engine, where the carburettor is fairly close to the combustion chamber, so that changes in the carburetion affect the engine almost instantly.

On the rotary engine, the petrol is injected at the back of the crankshaft. The mixture then has to pass up about 300mm (1ft) of crankshaft into the crankcase, and from there up the transfer ports to the cylinder head, so that what should start be a fine mist of petrol droplets evenly distributed through the incoming flow of air, ends up coating all the surfaces of the engine and arriving in the combustion chamber in the form of rather large drops, some considerable time after it was injected into the engine. Oh, and in the case of the early engine installations (including ours), the petrol tank wasn’t pressurised, so that the petrol tended to dribble in through the carburettor, instead of spraying in, and it wasn’t always possible to get full power even on the ground.

Engine controls on a Sopwith pup. The large lever is the air throttle and the small one is the petrol valve, or mixture control.

Engine controls on a Sopwith pup. The large lever is the air throttle and the small one is the petrol valve, or mixture control.

With all these issues, it wasn’t possible to get a fully automatic carburettor to work on a rotary engine, and it was necessary for the pilot to control not only the throttle opening, but also the mixture. On most rotary-powered aircraft, you’ll see two quadrant levers placed next to each other. One controls the flap valve that regulates the amount of air going into the engine, just like on a car, and it controls the amount of power delivered in the same way. The other lever controls the petrol valve, and hence the mixture. Generally you’d only ever want the engine to run flat out or slowly, so before takeoff you’d push both levers forward cautiously, adjusting the petrol lever position to give optimum running with the air lever fully open and partly closed.

 

Even this only gives partial control of the engine – to get the engine to run slow enough to land, there’s a ‘blip’ switch on the top of the stick that cuts out the ignition. The engine will continue to turn over even with the ignition cut out, and you used the blip switch intermittently in order to make the engine tick over.

But when we looked at the drawing of the Scout cockpit we could only see one control – a lever that operated a cable just like on a motor mower. Indeed, we’d spoken to Gene Demarco, of The Vintage Aviator, who’d said that in some installations there was only one control for the mixture, the air throttle being left fully open and speed control being achieved entirely with the blip switch.

But in an idle moment, I decided to have another look at the relevant drawing, coupled with the invaluable parts list, and managed to establish that the Scout does indeed have two controls – just placed unconventionally. Most throttle controls are placed on the left side of the cockpit, so that the right hand holds the main control, the joystick. For some reason the engine controls on the Scout are in the right side; maybe Frank Barnwell the designer was left handed!

 

The 1915 drawing of the engine controls. the throttle control is shown just aft of the instrument panel mounted on the top longeron, but it too kme a while to work out that the mixture control is an aluminium wheel with a long tube running down diagonally to the petrol valve just behind the engine.

The control that looks like a motor mower is indeed the throttle valve that controls the air and therefore the power output. The lever sticks up and you push it forward for more power, back for less. All quite conventional. But underneath it, tucked out of the way under the instrument panel, is an aluminium knob which you turn clockwise and anticlockwise, and that’s the mixture control.

 

The photo from the parts list of the cockpit area showing the engine throttle lever (not very clearly, but I think it's actually mounted under the fuselage longeron on a vee-shaped bracket) and the mixture control which is the diagonal tube running from top right to bottom left and terminating in a round knob under the instrument panel.

The photo from the parts list of the cockpit area showing the engine throttle lever (not very clearly, but I think it’s actually mounted under the fuselage longeron on a vee-shaped bracket) and the mixture control which is the diagonal tube running from top right to bottom left and terminating in a round knob under the instrument panel.

It’s certainly unconventional, and we’ll have to wait a while before we get to see how practical it is!

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