Preheat system for cold weather operations below 70ºF/21ºC

by Had Robinson
updated February 20, 2021

As the temperatures drops, increasing fuel viscosity and air density become acute causing a lean condition severe enough to cause fading or stumbling at throttle positions greater than 50% or so, depending on the engine size and other factors.  Installing a system to preheat the air entering the carburetor is recommended where temperatures are 50F/10C to 70F/21C, what we may call "cold weather operations".  When temperatures get colder, the preheat system is mandatory, along with allowing the engine to heat up to normal operating temperature as would be experienced when flying in warm conditions (>70F/21C).

In order to run efficiently and smoothly at all throttle settings, the carburetor must also be at the normal operating temperature as when the engine is run when it is >70F/21C.  The preheat system helps with this, including techniques to decrease the cooling of the engine from cold/freezing ambient air.  When this is done, adjustments to the carburetor are unnecessary.

Pilots would be surprised at the great difference in the speed of fuel moving through filters on a warm day and when it is 30 degrees cooler.  Nonetheless, fuel in the tank will be close to the ambient temperature and must travel through various filters BEFORE it enters the carburetor.  This is why that part of the fuel system must be in perfect order, especially when fuel is sucked into the engine (almost all paramotors) rather than pumped in (as with most aircraft).  We are working on a fuel system modification (FSM) that will greatly assist moving fuel from almost 20" below the carburetor up through all the various fittings, filters, and lines.  It is being thoroughly tested at this time.

Note: We do not have the luxury of fuel injection with paramotors.  So, why not install it on these motors?  It is a bad idea and is discussed on this page.

The preheat system must be reliable, simple, durable, and vibration proof.  Installing a preheat system on engines with a cooling fan and a shroud is easier than on motors with passive cooling systems and/or without shrouds around the cylinder.

Tests have shown that operating in temperatures colder than 50F/10C also require steps to prevent excessive cooling of the cylinder and head.  This can be done safely with duct tape as illustrated in the photos below.  For example, the Polini Thro 130 has (2) openings in the back of the cooling shroud.  The larger opening is ducted to the preheat system.  When the ambient air gets below 50F/10C, the 2nd cooling opening can be covered with tape, as needed.

Parts needed:

• CHT  The preheat system cannot be calibrated unless the engine has a CHT
• 6cm/2.5" x 76cm/30" automotive preheater reinforced flexible pipe (typically used in older engines and available in auto parts stores).  Some types have a hardened steel spiral core and some do not.  Either type can be used but the former is much more durable.
• perforated steel tape (available at hardware stores in the plumping section)
• ordinary duct tape – We tried aluminum duct tape but it tended to split and was harder to change out or modify when adjusting engine temperature.  Duct tape does 100% of what we need it to do: it's easy to remove, change, holds securely, and does not care about vibration.  It looks so-so but who cares if your engine runs and performs much better?
• nylon ties, medium duty x 25cm/10"
• miscellaneous hardware
• TTO water temperature gauge (not shown in the photos) is a practical way to monitor the air temperature entering the engine.  It should be around 22C/70F but can be much greater.

The more expensive flexible pipe is made with a spiral of hardened steel which should only be cut with a wheel (unless you want to ruin a hand tool).  The most important aspect of the preheat system is that it *not* come off during engine operation.  The photos below show a Polini Thor engine and what has to be done to make it absolutely secure.  Purists can spray paint the steel tape and the duct tape a flat black to improve esthetics.

The duct tape covering the pipe openings (red arrow) may be cut away, as needed, to obtain a good running temperature of the engine.  We discovered that the duct tape on the shroud (white arrow) is not needed.  The 2nd cooling shroud opening (not visible) is directly below and left of the opening covered by the preheat system pipe.

In tests, having all shroud openings sealed may be necessary when the air temperature is less than 50F/10C.  Pilots must test the system on the ground and remove material accordingly.

Rather than have some sort of movable valve, duct tape does the same thing without the weight or complexity.  It is easy to change.  Some ingenuity is required to be certain that nothing comes loose and gets into the propeller.  Duct tape is amazing stuff and, in tests so far, sticks to everything and does not loosen or tear.

Here is a later photo.  Note the zip tie to ensure that the duct tape does not peel away from the tubing.

The steel tape with holes is secured at the bottom to the bolts which hold the redrive/clutch assembly together.

The holes in the wall of the preheat pipe were made with a Dremel cut-off wheel that cut the steel reinforcing wire and a razor knife was used to cut the flexible pipe material.  After hours of use, I used duct tape to line and protect all of the cut edges of the flexible tubing.

After a thorough test in cold weather.  Nothing moved.  It is important the air box end of the flexible tubing be tightly connected to the air box entrance.  The engine must be able to suck the heated air from the engine.  The tubing could be connected to a baffle around the exhaust pipe but this would add more weight to the system.

The nylon zip tie (red arrow) is threaded through a hole in the pipe (around the steel reinforcing wire) and then around the frame in order to secure the end of the pipe in case the duct tape did not hold for some reason.  No part of it must touch the propeller. 

In this modification, a fuel line loop was added to the system to help warm up the gasoline & oil mix.

These nylon ties are critical to ensuring that the pipe stays put.  The tie to the left is secured to the upper engine mount but is unnecessary (it was removed later). 

Note the white nylon zip tie that makes sure the flexible tubing does not get loose if the duct tape fails.  So far, the duct tape has held well.

Once the preheat system is in place, test it thoroughly on the ground when ambient temperature is about the same as you plan to fly in.  The engine temperature must be the same or greater temperature as when ambient conditions are warm (>70F/21C).  It is better to have the engine on the warm side than too cool.

I continue to test these modifications and the results will be posted here.