Quieter Engines

Jan 1, 2012   //   by admin   //   Tech Info  //  No Comments
  • Quieter Engines (Exhaust & Induction Noise)

Modern “super-quiet” types of silencer have been designed to achieve greater reductions in noise level without too much reduction in power output. In general, the larger the internal volume of the silencer, the greater is the reduction in noise output without loss of power.

With reference to the diagram:
Baffle position (A) can give you the lowest noise output.
Baffle position (B) can give you a better performance, (200-300 rpm+) but at a higher noise level.

Some silencers have aluminium baffles at (A) and (B) to create a silencer with three chambers. These are normally alloy washers that can be drilled out or sleeved to suit.
Steel ‘penny’ washers can be often fitted into single chamber silencers by reducing their diameter to fit using a lathe. This simple modification will reduce the loud, hollow sound of older types of silencer.

On some, you can open up the baffle holes by using a pointed pair of pliers, for more performance, or close them using a hammer and a piece of dowel on a flat surface, to reduce noise. But be careful, closing the baffle holes too much can cause an engine to overheat and create running problems. If this happens, just open up the baffles until satisfactory running is achieved.

Note C:  The noise output can be further reduced by cross drilling and tapping the outlet pipe to fit an M3, or M4 grub-screw. This will reduce the outlet area and reduce the noise output. Some care and attention is needed to do this without damaging the silencer.

Firstly, only drill thro’ and tap at 90 degrees to the die line. Never drill through the die line or it can split. Secondly, this will increase ‘back pressure’ to the engine if the screw is too big and reduces the outlet area by too much, the engine will lose power and overheat. (The needle adjustment may also become sensitive). Once you are happy with the noise reduction and performance of the engine, remove the grub-screw and re-fit with permanent thread lock (e.g. Loctite) then fit a silicone extension pipe.

An alternative is to tap the outlet bore to a suitable thread size then screw in a threaded insert with a smaller hole through the middle. And again fit a silicone extension pipe to direct exhaust residue away from the airframe.
These super quiet silencers normally assemble with a long through bolt that often screws into the rear outlet piece.

Please note: The nut at the outlet end is normally the lock nut for the assembly. If you try and tighten the silencer by only using this nut you will strip the threads!! To dis-assemble, or re-position the outlet to suit the model, undo the lock nut anti-clockwise, then loosen the assembly bolt from the front of the silencer.

Re-assembly is the opposite. Use the correct screwdriver to tighten the bolt into the outlet piece, then fit the lock nut for added security, using a small spanner.
Should any part of the silencer leak, simply take it apart, (as above) clean up the components with cellulose thinners, and re-assemble with high temperature silicone sealant around the joints. Then clean off any excess silicone.

Note: The actual silencer joint to the engine exhaust outlet is best sealed with a very small amount of epoxy adhesive. This ensures a secure, gas tight seal. How many times have you seen loose silencers with black oily residue coming from the joint ?  Please ensure the mounting bolt threads are lightly oiled to ensure they are not epoxied in solid !

The silencer outlet stub is always best extended with a short length of straight, or cranked, silicone tube. Fitting one of these these will reduce the sound output by about 1 dB(A), as well as directing exhaust residues away from the plane.

JP products make small add-on silicone expansion chamber silencers for .30 to .50 size motors. They can be fitted to both two stroke and four stroke engines. The outlet is moulded internally to accept an outlet reducer sleeve. But you will have to make your own sleeve from alloy or silicone tube. These are easy to fit and cost about £4.00 each.

Induction Silencers & Aircleaners  Many carburettors are noisy due to “induction roar”.  On engines fitted with effective silencer systems, this “induction roar” can be quite loud at full throttle. On most carburettors, there is a “bell mouth” to direct air to the inlet venturi. It is this bell mouth, acting like a megaphone, that amplifies the sucking noise of each induction stroke.

This can be reduced by fitting induction silencers or air filters to the carburettor inlet.

JP products make a range air filters to suit model engines. They come in various sizes to suit the inlet spigot diameter. A metal ring secures a disc of fine metal gauze at the top and this in turn is connected to the carburettor spigot by a short length of silicone tube and held in place with a small cable tie. I find it is best to shorten the silicone tube by about 5 mm to create more clearance from the propeller.

These cost about £2.00 each, and as they won’t break the bank, they are worth fitting, as they will also prevent dirt from getting into the engine. My own OS 20, Thunder Tiger 40, RMX 40, MDS 48 and JEN 56 engines, all run well with air filters fitted, although the mixture needle may need adjusting slightly to prevent richness at full throttle.

On larger engines fitted with Tillotson or Walbro carburettors, the induction roar can be extremely loud when the exhaust noise has been subdued by an effective silencer. I will be fitting a large, lightweight air filter to my 35cc Webra Bully glow motor that is fitted this type of carburettor.

The BMFA / Department of the Environment recommended maximum noise level, is 82 dB(A) at 7 metres, measured at every 90 degrees from the model. Even if your model does pass the noise test, it is no reason NOT to attempt to make it quieter, if possible.  These days, it is extremely rare for a model to be underpowered. Model noise is a technical challenge where we can all help each other to achieve satisfactory results.

Richard and I, hope these notes will help members to make their engines quieter. It is only by everyone ensuring that their models are as silenced as “effectively” as possible, that any noise complaints can be managed in a proper manner.

This is an article based on notes and ideas submitted by Richard Winter. (Club Safety Officer).

Nigel Rollason.

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