Ever wonder how the exhaust system on your helicopter effects the engine and its ability to make power?

Enter the world of 2-Stroke engines and learn a little!

First, the basics...Most of the engines we use in R/C helicopters are 2-stroke (or 2-cycle) engines and use what is called Schneurle porting. This type of porting uses a hollow crank shaft that has a cutout in it under the carburetor that is "timed" with the piston to open and allow the most efficient amount of fuel/air mixture into the crankcase. As the piston moves up in the cylinder it causes a low pressure area in the crankcase of the engine.

This low pressure draws the fuel/air mixture from the carburetor into the crankcase through the crank shaft cut out and into the area under the piston. As the piston begins the down stroke (or power stroke), the cutout in the crank shaft is closed and the charged mixture in the crankcase is forced through transfer ports that run up the outside of the cylinder wall (you can see these on the outside of you engine, they are the cast bulges on the bottom half of the cylinder and then through the ports in the cylinder).

If you have ever taken the head off of your engine and looked into the cylinder, the little "windows" are the ports. This fresh charge of fuel and air being pushed through the ports forces the exhaust gasses out of the engine through the exhaust port and fills the cylinder.

When the piston has reached the bottom of the stroke and begins upwards (compression stroke) the fuel/air mixture in the cylinder is compressed and as the piston reaches the top of the stroke the mixture is ignited by the glow plug and causes combustion.

This combustion drives the piston down and, at the same time, the crankcase begins drawing a new fuel/air mixture charge for the next stroke and the cycle starts all over again. Thus, the name "2-Cycle". What makes a 2-stroke engine work is the timing of the transfer of the charged fuel/air mixture to the position of the piston in the cylinder. The exhaust port will be opened before the transfer ports are and it will shut before transfer ports are opened completely. This timing is what makes the power and torque properties of the engine. This is a simple description of what happens. I know, most of you know this already...right?

The Exhaust System:

The manufacturers have the timing issues taken care of for the most part. There are people that have their engines ported (changing the timing of the port by cutting the top or bottom and widening of the port) for more power or a different type of power, but for most of us, the way to get our engines 'happy' is by changing the exhaust system to suit our particular style. The reason that a change in exhaust systems can make such a difference in the way the engine works all boils down to one thing...changing the timing of the engine.

Let me explain, 2-strokes produce power from back pressure in the exhaust port. If there is no pressure on the port, the escaping gas is allowed to "free flow" out. What the back pressure does is control the way the exhaust leaves the engine. The exhaust leaves the engine in pulses. By controlling these pulses, the transfer of charged fuel/air into the cylinder is controlled, thus the engine can be tuned by back pressure. The length, diameter, shape and size of the muffler and header are the major factors that determine the power of the engine.

In general, the longer the system (to a point), the power is produced in the lower RPM range, and the shorter the system (to a point) the power is produced in the high RPM range.

The shape of the pipe also determines the way the engine makes power. Remember, the idea of a tuned pipe is to time the back pressure pulse to control the mixture in the cylinder.

Mufflers:

Many of us are running the Hatori 666 style of muffler or some variation of it. These are nice units that are designed so they work by volume to control the back pressure. Engines are nothing more than an air pump. By controlling the amount of air pumped through the engine, (you can only put so much air in a container without compressing it) the muffler effectively controls the back pressure (timing). Mufflers as a whole, work effectively across the RPM range and give nice power across the board. They usually have some sort of internal baffling to control noise and are smaller in length than most tuned pipes. Mufflers also usually bolt right to the engine and have no header pipe.

Tuned Pipes/Headers:

For those of you that want an engine to work in a particular style, the tuned pipe/header is the way to go. By shaping the pipe, tapering up from the header to the body of the pipe and then tapering down to the outlet, the exhaust pulses and back pressure are controlled (timing) to be effective at a particular RPM range. This is where length really plays a part (short-High RPM, long-Low RPM). You can hear the difference when the engine "comes on the pipe." There will be a change in tone and the engine seems to jump to life. The header pipe (the part that connects the engine to the tuned pipe) also plays a large roll here.

The header must be matched to the pipe. For example, the Novarossi in my Futura SE came with a header that was about 10 inches long. Running a Chapman tuned pipe, I was only able to get power up to about 2/3 throttle and then the engine went flat and wold bog. By cutting the header length down to 6 1/2" the power went up all the way to full stick. If you have a problem with an engine feeling sluggish and your buddy has the same type that just screams, check out the exhaust system he's running. Chances are his exhaust system is different from yours. There are many types of exhaust systems available, so figure out what your goals for your engine are.

Get the information you need to make a smart decision on you selection of a muffler or tuned pipe/header. What engine do you have? What type of flying you do? Do you want a quiet system or can you put up with a bit of noise? Do your homework and you should be happy with your choice! Author: Richard W. Kosar Date: March 19, 1996

Author: Richard W. KosarDate: March 19, 1996