Model Engine Maker

Supporting => Engine Ancillaries => Topic started by: ackers on February 17, 2022, 05:20:13 PM

Title: A throttle and mixture control unit
Post by: ackers on February 17, 2022, 05:20:13 PM
A throttle and mixture quadrant for a model aero engine.

Although it is many years since I built my model radial aero engine, the running time in total is probably only a few tens of hours at most, and I never did get to the bottom of some of the large questions which emerged concerning the behaviour of the several carburrettors which I had tried at various times. Having recently picked up the threads of the model engine work again I have begun some more methodical testing, and quite quickly realised that one of the difficulties lay in adjusting and keeping track of the carb settings. It is all too easy to forget whether the needle valve is already one and a half turns open and so on, and depending upon the detail of the carb itself and its position on the engine the adjustments may not even be easily accessible.

In full size installations in the golden age of the piston aero engine there would often be an engine control unit consisting of a pair of levers controlling throttle opening and mixture strength. I decided to build a miniature control unit along those lines which would be positioned remote from the engine and provide convenient control and, importantly, measurement of the throttle and mixture settings. Of course some slight modifications in the region of the carburettor are needed to allow remote control, but I judged that the advantages would more than outweigh that complication. I used light-duty Bowden cables to link the control levers and carburettor; the kind of thing used for bicycle gear controls. The carburettor I tried this out on already had a throttle lever intended for servo operation and it was simple to make up a small lever to clamp on to the mixture screw.

The basic layout is straightforward. A central vertical plate carries the pivot pins for the levers, and the top surface is machined to a radius about the pivot centres. A light gauge alloy top plate bent to shape fits on the vertical plate. Scribed lines on the top plate indicate the lever positions. A design detail, often seen on the originals, is that shallow cut-outs or steps in the side of the top plate define certain positions of the levers. For instance the mixture lever in the “normal” position might sit within such a cut-out, and the throttle lever might encounter a step in the top plate at the “cruising” position. Pushing the levers slightly sideways disengages them from these features and allows full travel. I wonder if in the full size units the flexibility of the levers themselves might allow the necessary small sideways movement. In the model unit the levers are quite short and relatively stiff so the sideways movement has to be accomodated by flexible mounting.

A second complication to the lever assemblies is that I wanted to have provision for some degree of mechanical mixing between the throttle and mixture settings. To achieve this, a cam rotating with the throttle lever will be used to modulate the mixture control output. My intention is to use this to achieve better mixture compensation over the full range of throttle opening. Initial tests must be done of course to establish the necessary optimum or normal mixture settings at different throttle openings, which can then be implemented in the cam profile. This is the theory anyway; if I pursue that idea I will report on how well or ill that goes, but for the time being I am using the controls simply as independent throttle and mixture levers.

On each pivot pin the lever assembly consists of the following components:  next to the vertical plate is a thin neoprene rubber friction washer, then the cam disc (throttle) or plain disc (mixture), then the lever itself and finally a drum to which the Bowden cable inner is attached. The three outer components are joined by a pair of screws on a diameter at right angles to the axis of the lever. These bolts pass through clearance holes in the lever and the lever is located between the inner cam/disc and the outer drum by thick soft rubber washers. This means the lever is rigidly located within the assembly in the fore-and-aft sense but has the desired sideways compliance. The rubber washers were actually the side cheeks of small electrical grommets cut away from the centre portion.

The whole lever assembly is held in place by a washer, spring washer and self-locking nut on the threaded end of the pivot pin. Slight adjustments of the nut controls the pressure on the neoprene friction washer and thus the stiffness of the lever action. I was pleasantly surprised to find that this allowed quite delicate control over the “feel” of the lever even though the securing nut was quite a coarse thread; a standard M8 of 1.25 mm pitch. I was expecting to need a little trial and error and perhaps finer pitches and softer spring washers but I was lucky first time. The Bowden cable outers fit into adjustable sockets mounted on a small plate at the front of the central plate.

As with most of my model work, this unit is not a scale model of a prototype but “in the spirit of ” ; a suitable accessory for a model engine inspired by the radial aero engines of the 1920s and 30s.
In use, the device has already proved to be thoroughly practical and useful, making it easy to set repeatable conditions and to observe the settings. This has already provided me with new insights into carburation issues and I wish I had made the thing years ago!

Title: Re: A throttle and mixture control unit
Post by: Admiral_dk on February 17, 2022, 06:32:30 PM
A very neat setup - simple and very useful  :ThumbsUp:

Any chance of a picture of the complete setup ?
Or even better a short video of it running  ;) ?

Best wishes