RCM&E Peter Chinn's Radio Motor Commentary: April 1977. Reproduced with the kind permission of the Editor, Nexus Special Interests.
Above: the fundamental difference between the Robart and the British TK unit seen here is that the Robart is a pump where as the TK is a pressure regulator .
Above right: the Robart "Super Pumper". This is the smallest and lightest fuel delivery device to date.
This article is significant in the history of the Reshift 60 Internal Fuel Metering system. (Ashley)
Another Fuel Pump
During the past five years, renewed efforts have been made to devise fuel systems that will provide better control over fuel delivery pressure in order to maintain fuel mixture strength within an acceptable combustible range at different throttle openings and through wide variations in fuel level , model attitude or tank location. The first of these was the Japanese Yamada 'YS' system first marketed as an accessory in 1972 and subsequently incorporated into the YS-60 engine. This utilises a rotary valve timed pressurised fuel supply and a complex pressure regulator which depends on
crankcase pressure to operate a valve and meter fuel to the carb. A less complicated system is the British TK pressure regulator (used, incidentally, by the top four competitors in last year's Nationals R/C aerobatics event) which also operates from a pressurised fuel system but is much simpler and less expensive.
The alternative to using a sealed and pressurised fuel tank to force feed fuel to the engine is, of course, a pump which will draw fuel from the tank and this is the basis of the more recent American 'Perry Pump' system which combines a diaphragm pump and a regulator to provide an accurately metered supply of fuel to the carburettor at all times.
The Perry Pump works very well but it is fairly expensive (about £24 including the special carburettor for use with it) and, at the present time at least, is available only for certain .40-.60 cu. in. shaft-valve engines having detachable back plates since the pump/regulator unit replaces the engine's existing back plate. An interesting development, therefore, was the introduction in the United States, towards the end of last year , of the Robart 'Super Pumper', which can be used with any engine, large or small.
Just as the TK was a simpler method than the YS of regulating a pressurised fuel system, so the Robart is an infinitely simpler form of pump. The device itself is very small and extremely light: it weighs a mere 5 grams or less than 0.18 oz. It does not need to be attached solidly to the engine or model but can be inserted in the fuel line in the same way as a fuel filter and can be transferred from one engine to another .
The Robart Super Pumper consists of a hexagonal section aluminium body approximately 1.10 in. long and 0.375 in. across the flats. It contains a tubular diaphragm (through which the fuel flows) and non-return valves at each end, one functioning as an inlet valve and the other an outlet valve. Thus, any contraction of
the tubular diaphragm will tend to force fuel through the outlet valve while expansion of the tube towards the sides of the body will draw fuel into the pump. To activate the diaphragm it is only necessary to tap crankcase pressure via a back plate nipple, connecting this with fuel tubing to the pressure pulse inlet fitting on the side of the
pump. The negative pressure developed in the engine crankcase as the piston rises, then causes the tubular diaphragm to be expanded and fuel is drawn into it through the inlet valve. On the return downward stroke, positive crankcase pressure compresses the diaphragm and the fuel is pumped through the outlet valve to the carburettor. The pressure pulse inlet fitting is on a screw thread and can be rotated a
few degrees each way to adjust diaphragm displacement to reduce or increase fuel flow. A few points need to be borne in mind when fitting and using the Robart Super-Pumper. First, the best position for it is alongside the crankcase above the mounting lug, between the carburettor fuel-inlet and the back plate so as to keep the connecting tubes to the back plate nipple and carburettor as short as possible. The inside volume of the pressure line obviously adds to the dead volume of the crankcase and, while the risk of a measurable power loss is small, it is worth bearing this in mind when the pump is fitted to very small engines. (The unit has, in fact, been used successfully on a wide variety of engines from the very largest down to the half-A (.049 cu. in. size.)
Unlike the TK regulator, the Robart pump must not be connected to a rotary-valve controlled crankcase pressure nipple, irrespective of whether this is crankshaft controlled or rear rotary-valve controlled, since such an outlet provides positive pressure only and will not operate the pump. A threaded brass nipple is supplied with the engine and can be fitted into a suitably drilled and tapped hole in the crank-
case back plate or transfer passage. However, it will call for a 6-32 ANC (American National Course) tap. The alternative, where available, is the standard pressure fitting made by some manufacturers for use with their engines and which usually fits in place of one of the upper back plate screws. These will need to be
reamed out to approximately the same i.d. as the Robart fitting.
The Robart pump is assembled as a sealed unit and should not be dismantled. Provided that a fuel filter is installed between the tank and pump, cleaning should not be necessary. As with an engine, however, fuel left in the pump after use will , in time, evaporate, leaving the oil content to coagulate and, possibly, gum the valves. This can be prevented by removing the pump (if one is not intending to use it for a
few weeks) and blowing out the fuel remaining in it. Don't use a petroleum solvent and don't poke anything through any of the three openings in the unit: damage to the valve or diaphragm will result. One other thing to watch is fuel composition: home-brewers who persist in adding petrol to their fuel will find that this (and
certain other additives) will ruin the pump diaphragm.
Lacking a pressure regulator, the Robart pump will not tolerate such abnormally wide variations in fuel head as the Perry pump but inter-adjustment of the carburettor's full throttle and idle mixture controls and the pump's pressure pulse inlet fitting will enable the most effective use to be made of it. As already noted, it can be used with practically any engine and in numerous special applications, such as twin motor installations in which a pump on each engine draws its fuel from a
single fuel tank in the fuselage.
Richard Fisher closed the whole business of fuel management discussion by
producing the Internal Fuel Metering (IFM) system fitted to Redshifts. (See later report from Peter Chinn).