Thermal Mosquito Fogger

Overview

A thermal fogger kills flying and resting insects by filling an outdoor space with a dense cloud of insecticide droplets fine enough to drift, settle on surfaces, and reach the shaded harbourage where mosquitoes rest by day. It produces that fog thermally: insecticide solution is pumped onto a surface heated to several hundred degrees, flash-vaporised, and then condenses back into a visible white aerosol the instant it meets cooler outside air. The heat comes from a Pulse-Jet Engine, the same valveless-resonance engine principle as the WWII V-1 flying bomb, scaled down to a handheld barrel.

The operator carries the unit by a Shoulder Strap, pumps up the Fuel System, starts the jet, and then squeezes the Dosing Valve trigger to release Formulation Tank solution into the hot tube. A heat-shielding Housing & Guard keeps the glowing resonator away from the operator.

How the pulse-jet works

The engine has almost no moving parts. To start, the Hand Priming Pump pressurises the Fuel Tank to about 0.3 bar, which pushes petrol through the Fuel Metering Jet into the Carburettor, where it mixes with air drawn past the Reed Valve. The Ignition System system fires the Spark Plug and the charge ignites in the Combustion Chamber. Combustion raises pressure, slamming the reed valve shut, and the burnt gas blasts out down the Resonator Tube. The exiting slug leaves a momentary low pressure behind it that re-opens the reed valve, draws in a fresh charge, and the residual heat re-ignites it. The cycle then repeats on its own 60–110 times a second, once running, the spark can be switched off entirely. The acoustic resonance of the tailpipe length tunes this self-sustaining beat, which is why these machines have a characteristic loud staccato bark.

Within a minute the Resonator Tube glows dull red at 600–700 °C. That hot tube is the working surface of the whole machine.

Generating the fog

With the engine settled, the operator squeezes the Trigger Valve. Solution is drawn from the Solution Tank up the Pickup Tube, metered through the Needle & Seat and a selectable Dosing Orifice, and delivered into the Solution Heating Coil wrapped around the hot resonator. There the liquid flashes instantly to vapour. As that superheated vapour leaves the Outlet Nozzle and hits ambient air, it cools and recondenses into a fog of droplets with a volume median diameter around 10–20 µm, small enough to hang in the air for minutes and penetrate foliage, large enough not to be a pure inhalation hazard. Releasing the trigger stops the flow at once via the Valve Return Spring, so no solution bakes onto the coil when idle.

The choice of carrier matters. Oil-based formulations (diesel or light kerosene) produce the classic dense white visible fog and resist evaporation; water-based formulations fog less densely and are used where the oil smoke is unacceptable. Either way the active ingredient is usually a pyrethroid such as permethrin or deltamethrin at low percentage in the carrier.

Safety and the heat hazard

Everything about the operation centres on a glowing-hot tube being carried by hand. The Barrel Shroud is a perforated guard that keeps hands off the 600 °C surface while venting its heat, and the Hand Guard holds the grip well back. Foggers must never be pointed at people, animals, or open flame, the fog from an oil carrier is flammable. Operators wear respirators and run the machine walking forward into clear air so the cloud drifts behind them. After use the unit is parked on its Rest Stand to cool, never set down on grass while the tube is hot.

Use and maintenance

Effective fogging is done in still air at dawn or dusk, when mosquitoes are active and a temperature inversion holds the fog low rather than letting it rise away. The operator walks at a steady pace, swinging the nozzle to lay an even blanket; a single 5–6 L fill treats up to half a hectare. Common faults are a clogged Fuel Filter or fuel jet (the engine will not sustain firing) and a gummed dosing needle from dried solution. After each session the solution circuit is flushed with clean carrier so residue does not harden in the orifice, and the fuel tank is depressurised for storage.