Calculating Optimum Model Engine Intake Choke Areas

By Maris Dislers

The most fundamental requirement for good model engine performance is the reliable and consistent supply of fuel/air mixture. This is easy if you use pressure feed via a rubber bladder, crankcase pressure or the weaker muffler pressure method. But many of our engines rely on atmospheric pressure to do the job by filling the partial vacuum which occurs in the engine’s crankcase during the piston’s up-stroke.

Actually, this “suction” method works quite well providing the carburettor is carefully designed to make best use of the Venturi effect. This effect is named after Italian physicist Giovanni Battista Venturi, who observed that a fluid (air in this instance) forced through a restriction in its flow path will speed up in that section. Without going into scientific detail, suffice it to say that the laws of physics require a trade-off in the form of reduced pressure in the constricted area or “choke point” as the air speeds up. This is the key element involved in model engine carburettor design.

Put simply, the smaller this “choke point”, the faster the air flow and the greater the pressure drop, giving steadier fuel/air mixture supply to the engine through manoeuvres. Go too far and the induced friction in this flow path restricts the volume of air entering the engine. That can obviously limit potential power output. Clearly, there’s an ideal effective choke area for every model engine running on suction feed, depending upon the user’s intended application.

Now you’d expect the manufacturer to get that right, wouldn’t you? Well, they usually do their best, but they’re working against the fact that the optimum choke area will vary for a given engine depending upon the use to which it will be put. This is why some manufacturers supply a range of differently-sized venturi inserts for their products.

That said, there are plenty of instances where it seems that attention to this point has been lax or where compromises have been made for the sake of production expediency. For example,  the Davies Charlton 0.75 cc Merlin, 1 cc Spitfire and 1.5 cc Sabre all have the same effective choke area of 6.6 square millimetres. Moreover, the Mills 0.75 and 1.3 also share a common choke area. It’s surely obvious that they can’t all be the “optimum size”!  

As a general observation, engines with smaller swept volumes tend towards relatively large choke areas in relation to their displacements, presumably with the intention of enhancing power output. It’s therefore no wonder that they seem to be crankier than their larger counterparts. On the other hand, it has been quite common for larger glow-plug engines to feature a venturi insert (good for aerobatics) that could be removed for extra power in less acrobatic applications.

Determining Suitable Choke Area

The fundamental question requiring an answer is - what should the effective choke area of a particular model engine be when running on suction? As noted earlier, this will of course depend very much upon the engine’s intended application. Let’s assume that the user is after acceptable consistency for a particular purpose, not simply maximum “bench test” horsepower potential.

Most people simply try to work with what they have and blame themselves if the engine won’t quite run as they want. If they want to go further into the question, they might consult the few published tables of recommended choke sizes for various engine sizes, or perhaps seek an answer from one of the internet forums, often with conflicting replies. If they get really fussy, they might apply a simple time-honoured formula like;

Choke area (sq. in.) = 0.045 x swept volume (cubic in.)  

That’s getting better, but built into the above formula is the underlying assumption that all engines will be running within some specific and reasonably narrow RPM range. It might imply that the user wants a certain level of consistency in flight, but stops short of incorporating adjustments to reflect the intended use to which the engine will be put. Accordingly, this formula probably won’t suit all possible applications, although it seems to be generally quite good for engines used in C/L aerobatic applications.  

One of the trickiest parts is working out the effective choke area when a spraybar intersects a round carburettor throat. Luckily, Ahmed Moustafa’s spreadsheet table giving these values can be downloaded from the Barton Model Flying Club’s website. It includes a table of recommended choke sizes – probably by Claus Maikis.

A More Refined Approach

The “eureka” moment for me came when I finally understood that the velocity of air going through the carburettor has to be right for the application and that no carburettor having a fixed choke area can deliver this optimum velocity across the entire range of possible running speeds or for all possible user applications. It became clear that we should be tailoring this to suit our particular needs. So, how could I progress down the path of calculating the “right” choke area for a given engine that will give the desired consistency in flight?

I quickly got out of my depth when pursuing the theoretical fluid dynamics avenue. However, a path towards salvation became apparent when I realized that we already had a wealth of empirical data based upon extensive experience in the field. Basically,  we can derive effective working values for choke areas from engines that are known to behave well for the intended purpose. Using these as a guide, it should be possible to optimise the choke area for other engines operating at the desired RPM and used in the same application to make them perform equally consistently.

I worked through quite a few known “good” performers, finding that the required velocity for a particular application seems to be about the same irrespective of engine displacement, induction type, carburettor design or the type of fuel used (glow, diesel or petrol). I also noted that there are significant differences depending on the desired characteristics for particular applications. These findings led me to condense things into a simple “rule of thumb” equation which gives the practical minimum RPM for a given engine setup when used in a given application;

Practical minimum operating RPM = C × Effective choke area (in sq. mm.) ÷ Engine swept volume (in cc)

“C” is a constant that varies in value depending on the desired operating characteristics. Here are my recommendations;

Putting Theory into Practice

If you’ve stayed with me so far, the above equation allows you to do some homework to establish if things are right with your engine. To take the pain out of the relevant calculations, I’ve prepared an Excel Calculator Worksheet which does all the math for you. Let’s look at a few examples.

If you have an engine with a carburettor already fitted, measure the choke size at its most constricted point. A vernier calliper or a range of drill bits will do the job – just be as accurate as you can. If there’s a spraybar across the choke point, also measure the spraybar’s diameter. Its effective area within the choke must be deducted from the total.

Enter your measurements in the white cells within the blue field corresponding with your intended use type. If there’s no intersecting spraybar, such as with a Cox TD carburettor, leave the spraybar cell blank. The calculator will give you the effective choke area and the desirable minimum operating RPM.

If the latter figure is higher than the speed at which you want to run the engine (e.g. where it delivers peak BHP, or maximum torque) the choke size may be too generous. A reduction would likely improve running consistency. A spraybar of increased diameter might do the trick, or you may be able to fit a restrictor insert. Alternatively, you could fit a new carburettor of the correct size. 

This formula does not indicate maximum reasonable operating RPM before power drops off too much. You’d have to experiment with that, perhaps by filing flats on the spraybar sides to slightly increase the choke area. However, bear in mind that this step will raise the minimum operating RPM somewhat. You’ll have to recalculate that as you go.  

If you have an engine needing a carburettor, and know at what RPM you want it to operate, plug the values (including the appropriate value for “C”) into the orange field and it will return the recommended choke area. Then, if you intend to use a spraybar through the venturi, plug the spraybar's diameter into any of the other three blue fields to obtain an estimate of the required choke diameter to achieve the calculated effective choke area. Alternatively, you could consult Ahmed’s spreadsheet table.

Conclusion

Suction fuel feed systems are an elegantly simple arrangement. This spreadsheet calculator allows you to get the most important factor - air velocity through the carburettor -  about right to ensure that your engine runs consistently and with positive response to needle valve adjustments. Further experimentation is of course necessary to optimise an individual set-up. If nothing else, it might be enlightening to measure up your current pride and joy and do the calculations to see whether I’ve got the value of “C” anywhere near right.

Of course, none of this will cure other factors that can scuttle a reliable fuel supply, such as excessively outboard or rearward fuel tank location, vibration-induced air bubbles in the fuel line or air leaks past crankcase gaskets. That’s down to you as the builder of the model!

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 Article © Maris Dislers

First published December 2016