Many people consider Powered Paragliding to be one of the simplest forms of aviation. These wonderful machines may seem simple but if we consider just for a moment, that they are made to fly and carry a passenger (sometimes even two) we soon come to realize that safety and reliability should be very seriously considered.
These machines mainly use two stroke engines because of the lighter weight and more power delivered per pound of engine compared to the four stroke engines.
In the same time, two stroke engines are less reliable than their four stroke brothers.
In this article, we will briefly discuss the importance of the engine operating temperatures, one of the major contributing issues for a safe engine operation.
Engine overheating is directly caused by different factors:
air/fuel mixture ratio, tuned pipes, type of muffler( more restricted for a quieter sound or less restricted for a better cooling) design and size of cylinder and cylinder head and RPM, to name a few.
In order to prevent overheating, all of the above factors should be in tune. This is not very easy to achieve and most engine manufacturers will try to compromise. The cylinder and cylinder shape, the tuned pipe and the muffler are parts that the pilot can not really tune.
The only variables that the pilot is able to change are the air/fuel mixture and the RPM.
Once the carburetor is correctly tuned, the pilot can fly safer.
However, it is a good thing to know what is really happening with the engine in flight. It is especially true with the newer and very small engines used by different manufacturers. These engines are run at very high RPM in order to achieve the needed power.
The higher the RPM of an engine the higher the generated heat will be. Most engines will run perfectly at lower RPM settings but will have a tendency to overheat at very high power settings.
In-flight temperature monitoring becomes obvious.
There are two systems used: Exhaust Gas Temperature measurements and Cylinder head Temperature indications.
Which system is better?
This is a very complicated question and has been debated for a long time.
In my opinion, the Exhaust Gas Temperature gauge will be very useful while attempting to tune the carburetor.
The Cylinder Head Temperature Gauge is useful for getting a general idea of the engine's temperature and if used in flight, it can show the pilot when it is time to reduce the power to avoid overheating or if monitored for a longer time, it will show if the engine has a tendency to run cold, right or too hot.
However, we must stress that the CHT will not totally eliminate the possibility of an engine seizure!
This device will always have a delay in showing the real temperature.
Again, it will show the pilot only what the general temperature tendency is.
A very hot engine may seize in a matter of seconds and by the time the instrument will show the real temperature, the engine will have seized long before.
I have used a CHT for many years and it really helped me to control the engine's temperature.
Here is a good example: if the pilot flies at full power and notices the temperature to "creep"up slowly, he will know when to reduce the power before any problems may develop.
I usually tune my carburetors on the slightly rich side and if I notice a temperature of 390 degrees F, I know that it is still safe. If I see the temperature going past 390 degrees, I just throttle back and let the engine cool. On my Solo and Cors-Air powered machines, I usually achieve a temperature of 350 degrees F in mid-range and about 370-380 degrees at full power for extended periods of time.
On the other hand, the Exhaust Temperature Gauge is much more sensitive and may show variations on every throttle setting. This accuracy is very good when tuning the carburetor but in flight, it may be disturbing because it will divert the pilot's attention all the time.
In this article, I will present the CHT gauge I have used for years on all my paramotors and with very good results. In all these years of flying, I have never had an engine seizure.
The CHT gauge and probe I use is manufactured by WESTAC Instruments (www.westach.com
The principle of this instrument is very simple: the probe (sensor) is mounted under the spark plug and consists of a thermocouple.
A thermocouple is a "cable" made of two different metals.
When heated, it will generate an electric current which is measured by a gauge.
The hotter the thermocouple becomes, the more electricity is generated.
The Westach CHT has one little problem: because of the vibrations, it usually brakes by the spark plug after a few hours.
I reinforced the ring (see pictures below) and anchored the cable between two cylinder head fins. This significantly helps to increase the life of the cable.
After some failures (the cable kept breaking) I decided to further dissipate the stress generated by the engine vibrations.
I turned a portion of the cable into a spiral which acts like a spring and considerably dampens the shocks.
Installing the CHT is very simple:
Since the probe ring is very thin, the spark plug ring does not need to be removed. There are however other brands of CHT sensors that are thicker and more shock resistant but using them, the spark plug ring may have to be removed in some cases.
Once the plug with the probe is inserted, we anchor it between two cylinder head ribs to prevent it from being turned by the spark plug (which in some cases will bend the probe's ring) and torque the plug.
Next, we make the "spiral" and begin routing the cable to a convenient, handy and easy to see spot on the harness. Cable ties can be used to fasten the cable while routing.
The gauge can be fastened to the harness using Velcro straps.
Before connecting the two cables ( one cable is attached to the gauge and the second is supplied with the ring) we can additionally reinforce the gauge cable with heat shrink tubing.
Once the connections are made, our CHT system is ready to work.
Here are some general cylinder head temperatures:
For the Solo 210 and Cors-Air M21Y, I found the idling temperature to be about 200 degrees F.
This is a very important reading since the pilot before his take off, should bring the engine at least to this temperature.
Having a CHT gauge will eliminate any guessing.
Note: on other engines, the idling temperature may vary and each pilot should experiment and find it. Running the engine at idle for a longer time will allow to find this temperature.
Another important issue is: the idling temperature will be affected by the carburetor setting. For example, with a very lean mixture, if the idle temperature reaches say, 300 degrees F, it is a first warning that this engine, at higher power settings will definitely have overheating tendencies or even suffer a seizure.
For the same engines discussed above, a safe mid range temperature will be about 320-350 degrees F.
Again, if the mid range temperature is higher, the pilot should readjust the carburetor settings.
High RPM Temperatures
An engine , depending on its construction, will seize at different temperatures. For example I have run the Cors-Air at 450 degrees F for short periods of time when I tested this engine and no overheating marks inside the cylinder, piston or rings could be found.
I have also run a Solo 210 at 400 degrees F with no damage.
As a general rule, I usually throttle back at 390 degrees. It does not happen often because I always tune the carburetor before each flight of the day.
It is said that most two stroke engines will seize around 450 F.
If the engine shows a tendency to overheat at high power settings, the air/fuel mixture should be changed (enriched).
If the tuning does not help, there may be other causes that need to be eliminated. On old engines one of the major causes is a restricted muffler or exhaust manifold (carbon deposits)
Also, any restrictions in the muffler made by some manufacturers to reduce the engine noise, will increase the normal operating temperature of an engine.
Tuned pipes will also cause the same problem and to eliminate this, while upgrading an engine with a tuned pipe, it may be necessary to use on oversized cylinder head.
A CHT system is very useful and I would recommend it to any serious pilot. However as mentioned above, if not used properly, it will NOT prevent an engine seizure.
An EGT system on the other hand is very sensitive and is more useful to have while tuning a carburetor. Its installation is more complicated if one wants to achieve correct readings.
A very critical issue is how far from the exhaust port the probe is placed. If it is too close, the readings will be too high and if it is too far situated, the readings will be too low. In the last case, this instrument will not show the critical temperature before a seizure and therefore its purpose will not be served at all.
The EGT sensors will be addressed in another article.
Recently the Cors-Air M25Y/Black Devil and the Cors-Air M19/Black Magic engines have an option of adding a cooling shroud.
This shroud will lower the CHT by 100F!!! (contact us fro more info)