4-stroke exhaust length

[richard cohen]

 Hi all,

my first ever venture into a forum, only been on the net for a week !

My latest project is an SE5a and a 52' 4 stroke sits nicely at the noisy end, but i am considering a scale exhaust running down the outside of the cowling terminating below the cockpit . Would this cause issues with engine running ?. it would be approx 12" or 30cm in length without a muffler. Aside from a noise increase( which isn't much compared to the jet jocks at the field! ) will the length cause any running faults due to back pressure ?

all comments gratefully received

 Rich

[David Ashby - Moderator]

Hi Rich and welcome, I'm sure there must be some i.c. flyers out there who can help..........

[Former Member]

[This posting has been removed]

[Chris Channon]

No, no problem at all, just use the same size pipe as is on the exhaust outlet, (forget the silencer),with the lenth of pipe you want to use it will be very quiet, more than with the supplied silencer, it will of course look soooooo much better!

Chris.

[richard cohen]

Eric,

thanks for the advice. I will probably make up an extension pipe to match the manifold and monitor engine performance/temp. Is the expansion chamber required or can the exhaust terminate with a slash cut to give a larger exit area for the gas\flow. I thought a straight pipe with no silencer would reduce back pressure and increase power ?

 richard

[Former Member]

[This posting has been removed]

[Brian Parker]

Richard,

If despite the above sound reasons for caution, you still wish to proceed with a 'straight exhaust system. Then it is possible to tune the exhaust length to give the required scale performance using an engine test bed/stand, a rev. counter and monitoring the temperature of the engine and exhaust, (after first having noted the results with the standard exhaust system). The fuel mixture will need resetting at each alteration in exhaust length and also the idle performance (or lack of) noted.

The following may also be helpful.

At the begining of  the exhaust stroke a 'slug' of high pressure exhaust gas, caused by its initial inertia, starts a high-speed pressure wave  that travels down the exhaust until it reaches the end of the pipe. At this point a reversal takes place and a reflected low pressure wave (negative pressure) travels back up the pipe. If this reaches the port at the correct time (at/near TDC) it augments extraction of exhaust gases and the incoming inlet charge.

As a straight exhaust system has a natural frequency of vibration.The time taken for a wave to travel down to the end of the pipe and back to the port is constant. Hence the limited beneficial speed range. Altering the natural frequency of the system, by altering the diameter or length of pipe will alter the usable maximum and minimum speed.  Short and/or small diameter pipes raise frequency. For more flexibility and lower peak power a longer pipe is better. Exhausts can also be double the optimum length to achieve maximum/required  power for a more practical length.

Cutting the end of the pipe at an angle will have the same effect as shortening the pipe. Useful for 'fine' tuning.

I should add that I have never tried this on model aero engines, but have extracted maximum power from motorcycles many years ago and can vouch for the power increase and for the 'music' of a well-tuned exhaust.

At your own risk.

[Simon Chaddock]

Just to add that 4 strokes are much more forgiving to a bit of exhaust back pressure than a 2 stroke. After all the piston has a full stoke to push out the exhaust. Do remember the pipe will get pretty hot so keep it clear of the airframe. The original long pipes on WW1 aircraft were for the benefit of the pilot, not for noise reduction (which it did, as you will find), but to reduce the amount of exhaust fumes he had to breath!

I once ran a Mills 1.3 diesel on a 2 foot long exhaust pipe. Lost a little bit of power but all you could hear was the prop noise! 

[Brian Parker]

Actually it's more complex than that.

The four-stroke exhaust gasses are travelling above the speed of sound. Also the speed of sound increases with temperature and pressure.

In a four-stroke, a reasonable length exhaust pipe of reasonable bore will not in itself create back pressure.  If the reflected exhaust pulse reaches the port at the 'wrong' time the effective exhaust stroke will be reduced, reducing performance (see my posting above).

A baffle in a four-stroke silencer will break up the pressure wave, releasing the exhaust in a constant stream and thus reduce the sound, producing back pressure and reducing power as it does so.

A two-stroke on the other hand is more complex and can have sound wave velocities many  times higher than a four-stroke and requires an appropriate proportioned expansion chamber  to function effectively, (evacuating the cylinder radidly and applying a 'ram back' pulse). To achieve this the exhaust port and the front pipe to the expansion chamber need to be larger bore than the outlet pipe, often a restrictor washer is also fitted. It  provides some silencing but its prime function is not a silencer!    A straight pipe will adversely affect tune and performance of a two-stroke.

An open throttle two-stroke engine relies more heavily on its resonate induction system for its performance. Aided by a tuned exhaust (if fitted).

Incidentally the inlet charge on a four-stroke also behaves as a tuning pulse with a reflected wave dependant on induction port length. This is only fully effective over a limited range.

Am I running on?

[Former Member]

[This posting has been removed]

[Simon Chaddock]

Yes to all above but are we not in danger of missing the original request from Richard?  Do we think that a long exhaust pipe will reduce the performance of his 4 stroke engine so much that it will not fly? Would any lack of a "tuned" exhaust effect running at part throttle? Would a small power loss (only at full power?) be that significant anyway? It is after all a SE5a not a speed racer. 

[Brian Parker]

I thought that a had attempted to answer that in my first posting.

[richard cohen]

thanks for all the responses guys,

I tried some bench tests with differing pipe lengths and there seemed to be plenty of power but the problem arose when looking to fit up the airframe. Heat was a problem with the covering and also ( and possibly more importantly) the exhaust position interfered with the cabane strut fitment.

The ease of rigging is a priority  for me as a model which takes ages to rig is not likely to get many outings. I have seen many models with superb detail but they rarely appear on the flight line. instead the owners seem to bring other models which can be airborne relatively quickly. So i have paid a visit to Just Engines and now have a hidden in cowl exhaust and am rigging dummy exhausts which should give the right appearance without the heat and rigging issues.

 just need some balmy sunday evenings to begin patrols over the wiltshire downs looking for german Jasta squadrons !  

 Rich