How To Understand Exhausts
Exhausts, mufflers, silencers, tuned-pipes, expansion chambers, exhaust diverters – there are lots of names and types but what do they do and how do they work? Nigel Fraser Ker tells all…
In this article I’m going to take a look at two-stroke exhaust pipes. Looking back over the years you’ll find that an exhaust pipe of some kind has always been a necessity for helicopter fliers. Unlike their fixed-wing counterparts, the engine is usually buried inside the airframe and without some means of carrying away the exhaust gasses the model would become hopelessly messy – covered with the oil that passes through the engine. But keeping the model clean is only one relatively minor benefit of an exhaust pipe. Far more important are two other functions:
- 1. Reduction of noise
- 2. Dissipation of heat energy
Let’s see how these are achieved and the problems associated with them…
I imagine that most readers will be too young to remember when two-stroke model aircraft were flown unsilenced – the exhaust gas came out of the engine and straight into the atmosphere. Needless to say, the noise was unbearable, certainly as far as the rest of the population were concerned, and now nearly all national governing bodies such as the AMA and the BMFA insist that some form of exhaust muffler is used. In some places the issue of noise is taken extremely seriously and aircraft must be electronically tested to ensure that the noise they create falls below a certain level. Mufflers are a necessity and a good one as far as I’m concerned.
It would be possible to make mufflers much more effective (in terms of noise reduction) than the ones we can buy in the shops. For example, the inside of mufflers could be coated with some absorbent material and they could be longer and contain a more complicated series of baffles. However, apart from the added expense, size and weight, the addition of such features would tend to resist the movement of the gas as it passed through the silencer and create what is called ‘back pressure’.
The big problem with back pressure is that it works against the second requirement of an exhaust pipe which is to dissipate heat. Anything which prevents the waste heat from the engine getting away will make the engine run hotter – not what we want at all! So you can see that exhaust designers have to balance on the one hand the need for noise reduction and on the other the detrimental effect it will have on the engine and what you end up with is necessarily a compromise.
Fuel tank pressurisation
One last function of the exhaust is that of pressurising the fuel tank. Some form of pressure take-off nipple is either screwed into or welded onto the exhaust and a length of silicon tube connects it to the fuel tank. The slightly higher pressure in the exhaust pressurises the tank and pushes fuel into the carburettor. This arrangement is not ideal because the pressure will vary with engine speed but it’s okay for most purposes and if you really want perfection then there are various gadgets that you can buy to regulate the fuel pressure.
Types of exhaust
As far as a helicopter pilot is concerned, exhaust pipes can be divided up into three types. These are mufflers/silencers; tuned pipes; and hybrids.
Mufflers - A muffler consists of three elements – a manifold, an exhaust chamber and an outlet pipe. The manifold attaches the exhaust to the engine and should therefore match the type of engine you have although it is possible to buy adapters. On almost all designs of muffler the manifold attaches to the side of the chamber (rather than to the end) which creates a compact arrangement. Occasionally they are provided as separate parts with different designs to suit various engine/model types. For example, sometimes they are available as either straight or 90û versions which can be very handy when space is tight inside a scale model.
The chamber is either a simple void or may have some form of baffle arrangement inside to increase the internal surface area and absorb some of the sound energy. The outlet pipe is just that – not exactly space-age technology – just a tube of metal. The important characteristic that separates mufflers from tuned pipes is that they do little or nothing to improve the performance of the engine – they simply make it quieter.
Tuned pipes - A tuned pipe consists of a tubular manifold or header pipe, an expansion chamber and an outlet pipe. The header pipe which is nearly always a separate part is made in such a way that the exhaust gasses encounter no sharp bends on their way to the expansion chamber. Compared to a muffler, expansion chambers are generally longer and thinner, often making them rather awkward to fit to a helicopter. Now for the clever bit…
The expansion chamber can be considered as having three sections – the diffuser, the dwell section (or belly) and the cone (or convergent section). As the exhaust port in the engine begins to crack open, the gasses inside the combustion chamber are forced out into the exhaust pipe. This creates a wave of pressure which travels down the header pipe and into the diffuser where it is allowed to expand creating a wave of low pressure to be reflected back down to the exhaust port. This effect, aided by the force of new fuel/air mixture entering from the engine’s inlet port, helps to scavenge the combustion chamber of waste gas, effectively sucking it into the exhaust pipe.
But that’s only half the story. That wave of high pressure that was travelling along the expansion chamber eventually hits the convergent section of the pipe and is reflected back down towards the engine. If the timing is right, the wave hits the exhaust port just as it is closing prior to the next compression. Normally a little fuel/air mixture is drawn with the exhaust all the way through the combustion chamber and starts to enter the header pipe just as the high pressure wave hits it. This gets pushed back again by the pressure wave, rather like a supercharger.
As you may have guessed, the problem with the principle of the expansion chamber is that the timing has to be right otherwise it doesn’t work. The pressure wave inside the exhaust pipe travels at a certain speed and if its journey along the pipe and back again is too short then the engine won’t scavenge properly – too long and the supercharging effect will be lost. Clearly, expansion chambers have to be exactly the right length to work properly for a given RPM. In other words they have to be tuned, which is where the name ‘tuned pipe’ comes from and it’s exactly the same principle as tuning an organ pipe. So in simple terms we can see that for any given engine speed there will be an ideal length of pipe – longer for low revving engines and shorter for higher revving engines. This is quite convenient because it means that the larger models, which tend to run at lower revs, tend to need the longer exhausts! Ideally, a pilot will adjust the length of the header pipe to best suit the engine he is using. This can be achieved either by cutting it or by adjusting the length of the connector – usually a length of high temperature silicone tubing.
Although tuned pipes offer the most overall power, the fact that they work best at a specific engine speed RPM can be a disadvantage, especially for helicopters. Imagine a situation where the pilot suddenly adds a lot of collective pitch to make the helicopter rise rapidly into the air. The load on the engine will increase which will cause a drop in revs and if this moves it away from the ideal engine speed for the tuned pipe then it will cause a drop in power making the engine slow down even more. This becomes a chain reaction and the engine power can drop off alarmingly. A way to partially counteract this effect is to run the engine speed slightly higher than the optimum speed so that as the engine loads up during aggressive manoeuvres its speed drops into a range where there is more power rather than less.
Hybrids - The last type of pipe to mention is the hybrid which attempts to make the best of both worlds, incorporating some of the tuned characteristics of an expansion chamber with the compact arrangement and forgiving nature of the muffler. This has been achieved most successfully in recent years by Curtis Youngblood’s ‘Muscle Pipes’. These are ingeniously designed to be similar to a tuned pipe internally except that the length of the expansion chamber is wrapped up inside itself. This reduces has the advantage of retaining a little of the tuned nature whilst being compact and easy to mount.
Exhaust diverters are the little rubber tubes that pilots fit to the end of their exhausts to keep the oil from the engine off their model. Although in theory they increase the back-pressure of the system (and therefore could be associated with overheating problems) I have never found them to cause too much of a problem although for all-out performance they should not be fitted. Another factor to consider is that 3D-pilots fly their helicopters in all directions so an exhaust diverter is unlikely to achieve much. Unless you have a specific reason for fitting one (such as keeping the exhaust from burning some part of the canopy) I’d advise you not to.
Rotorworld would like to thank J Perkins Distribution for their help with this article. Many of the exhaust pipes featured in this article are available from J Perkins. Visit www.jperkinsdistribution.co.uk for more details.