Altitude Compensating Carburettors, Pt.1: Allied

[Bletchley]

This post is partly to inform, partly to seek clarification, and partly to confirm my own understanding of the development of WWI aero engine carburettors. It is difficult now to find much information on altitude compensation in WWI engines from modern books, although some of this information is still available from old textbooks published after the war and from original manuals and documents. The later development of engine supercharging and turbo-supercharging in the 1930s eclipsed these early attempts, and they are now, as a result, largely forgotten. The subject of early altitude compensation is, however, a very large one, including not just carburettor design but also development of new over compressed, high compression and over dimensioned engines, spark timing, cooling, and changes in fuel technology and propeller design (to name just a few), and is too large a subject to look at here. So I will merely outline the information that I have on changes in carburettor design, in the hope that others can fill in the missing details and correct any errors. Feedback would be appreciated, as this information is being used in the modeling of altitude compensation in Gabi's new ReLoad mod for the RB3D flight simulator.

The need for altitude compensation was not apparent at the start of the war, as the aircraft rarely flew above 5000 feet and the early rotary and stationary engines of that period therefore had no need for any additional mechanism to compensate for the effects of the reduction in air density experienced at higher altitudes. The early Gnome rotaries, for example, had a blocktube carburettor but no throttle or mixture control levers in the cockpit for the pilot to adjust either engine speed or fuel/air mixture - this was done on the ground, before the aircraft took off - and the early stationary engines appear to have had somewhat primitive float-chamber carburettors that were almost identical to those used on motor vehicles (but cast in aluminium, rather than bronze, to save on weight). It was nevertheless well understood that, as an aircraft ascended, there would be a corresponding reduction in engine power roughly proportional to the reduction in air density (itself dependent on changes in temperature and pressure, and therefore variable according not just to height but also to season, geographic location and weather). But it was at that time less well understood that there would also be an increase in fuel consumption and eventually a drop in engine power and rpm associated with a growing imbalance in the fuel/air mixture - a gradual over enrichment of the mixture as air density decreased. This was not fully understood at the time as the full effects of this were not immediately apparent at altitudes much below 10,000 feet.

The first response to the problem was to build bigger engines. As it was known that the 50 hp or 80 hp engines then in use would only deliver up to half this engine power at higher altitudes, because of the lower air density, a 100 hp or 160 hp engine would therefore be required to deliver the same power up to these altitudes. An aircraft that was required to fly and operate at these higher altitudes would need a correspondingly larger engine (I know that there are other reasons for using a bigger engine, and that this is a gross oversimplification of the effects). But it was only when they reached these operational heights above 10,000 feet that they appear to have realised that, even with these bigger engines, they were still not getting the power expected, and the engines were consuming more fuel than expected. The Allies appear to have been the first to have realised why this was so, or at least the first, in mid 1916, to design and implement a mechanism for correcting this imbalance in the fuel/air mixture. They designed and developed a new type of float chamber carburettor employing a "vacuum" or "leak hole" mixture control for their stationary engines (rotary engines did not require one as, with the exception of the early Gnome monosoupapes, the use of a simple blocktube carburettor with separate throttle and mixture controls gave the pilot full manual control over the air/fuel mixture for both load and altitude changes). The first of these carburettors was probably a Zenith, and probably the 48 D.C. Zenith carburettor fitted to the 150/180 hp Hispano-Suiza engines used to power the Spad VII in mid to late 1916, followed by the Zenith 55 / 58 D.C. and the Claudel C7 carburettors used in the 200 hp Hispano-Suizas and the Wolesley Viper aero engines; RAF1A carburettor fitted to the RAF4a engine of the RE8 and BE12 (not to be confused with the RAF1a engine, that had an early non-compensating Claudel carburettor); the Claudel Hobson HC7 and HC8, and various other 'custom' adaptations of these Zenith and Claudel carburettors added to the Siddeley, Salmson, Rolls Royce and Liberty engines; the Zenith D.C. 65 used on the 300 hp Hispano-Suiza; and the 48A carburettor used on the ABC "Wasp". They were all different in design detail, but they all worked in much the same way:

"The altitude control is similar in principle to other 'vacuum' systems...The petrol present in a carburettor may be regarded as a U-shaped column, the surface of the petrol in the float chamber forming one tip of the U, whilst the surface of the petrol in the jet forms the other tip. When the engine is running at ground level, one tip of the U, namely the petrol in the float chamber, is subject to the ordinary atmospheric pressure...but the petrol in the jet...is subject to a reduced pressure because of the engine suction. Thus there is greater pressure on one end of the U than the other; consequently, the flow of petrol from the float chamber to the jet is due to differences of pressure. As an aeroplane climbs, the air drawn into the carburettor becomes less and less dense with increasing altitude. Unless the petrol supply is proportionately reduced, the mixture will become too rich; the engine will begin to slow down, and will consume an abnormal amount of fuel. Vacuum control systems reduce the flow of petrol at high altitude by diluting or weakening the pressure on the float chamber end of the U-column of petrol. The lid of the float chamber is made with an airtight joint, and a passage known as a 'leakhole' communicates with the atmosphere. At ground level and at low altitude this passage governs the pressure inside the float chamber, which therefore corresponds to that of the atmosphere, so long as only the 'leakhole' is in action...The pressure is reduced, in proportion to the altitude reached, by opening a 'suction hole', controlled by a cock...the leakhole remaining open at all times. This suction hole communicates with the mixing chamber at a point when the depression is high. Consequently, whenever the suction hole is open, the reduced pressure or suction existing in the mixing chamber acts upon the float chamber, and reduces the pressure therein. The flow of petrol from the float chamber to the jet is correspondingly reduced" (from the "Wasp" maintenance manual).

The difference this 'vacuum' control made is illustrated by a test flight made with this type of carburettor, where "the aeroplane had a ceiling of no more than 12,000 ft so long as the vacuum control was not used", but with the control in action "the aeroplane climbed steadily to 17,000 ft", reaching the limit of this altitude control's ability to keep the fuel/air ratio constant. (ref. as above).

This is from the instructions for use of the Zenith 65 D.C. carburettor:

"Below 6000 ft use of the altitude control lever will not perceptibly effect the power or rpm; but if the control is left shut after climbing to 3000 ft the petrol consumption will be unnecessarily high. To obtain the maximum air endurance...the altitude control lever should always be partially opened at heights of 3000 ft and over. The precise amount of opening should be determined by the tachometer reading, the control being opened to the point at which any further opening will cause loss of rpm. The throttle and altitude control levers open and shut in the same direction. The pilot pulls them both towards him when climbing, and pushes them both away from him when diving". (from the 300 hp Hispano-Suiza maintenance manual).