What are Catalytic converters
Current emissions legislations in many countries mean two-stroke engines must be fitted with a catalytic converter. Chainsaw and other garden power tools are now fitted with catalytic converters, most major two-stroke engine manufactures like Stihl, Husqvarna, Tanaka etc use catalytic converters in their equipment.
Two stroke engines help in reducing harmful emissions by up to 70-80%. An example of the environment laws in America state that “all new hand-held power equipment sold in the US, must meet emission specifications set by the Environmental Protection Agency (EPA)”.
What does the EPA regulation do to chainsaws?
The EPA regulation is complex, but basically it requires manufacturers to design chainsaws and other power tools that produce less harmful emissions – less carbon monoxide (CO), less hydrocarbons (HC), and less nitrogen oxide (NOx). The amount each saw model is allowed to emit varies and is based on its power output. Large and powerful pro saws are allowed to produce more emissions than smaller less powerful saws. You will find that the chainsaws and other power equipment manuals now show the letters EPA in their wording.
The catalytic converter reduces the pollutant components with a chemical reaction called afterburning. Afterburning is triggered by a special precious metal coating. Cold air is mixed in to cool the exhaust gases.
Tanaka Pure-Fire 260PF Catalytic Converter Breakdown
The CARB Tier 2 Tanaka Pure-Fire 260PF trimmer/brushcutter employs a
catalytic muffler and a unique induction system. The engine has a 33-mm
bore and a short 28-mm stroke for a displacement of 24 cc. With a dry
weight of only 12.4 lb and a power output of 1.3 hp at 11,000 rpm, the
260PF has one of the best power-to-weight ratios in the industry.
At first glance, the PureFire could pass for any other piston-ported
engine. Air and fuel are drawn into the crankcase during the upstroke and
forced out through transfer ports on the downstroke. Fuel and air move out
of the crankcase and into a channel that leads to the transfer ports. A small diameter
orifice connects the crankcase cavity with the transfer-port feed.
This orifice gives velocity to the charge, which absorbs heat in its journey
to the transfer ports. Velocity and heat aid in fuel atomization, a precondition
for efficient combustion.
HC and CO emissions that survive are oxidized in the catalytic muffler.
The catalyst consists of a thin veneer of platinum, palladium, or rhodium
applied over a ceramic or metal matrix. A two-way converter, that is., one
that oxidizes HC and CO, is all that’s needed. NOx emissions do not present
much of a problem for carbureted two-strokes.
In the presence of heat, the converter oxidizes hydrocarbons into carbon
dioxide and water. The basic reaction is
HC + O2 → CO2 + H2O.
This exothermic reaction boosts exhaust temperatures to 1000°C and higher.
Carbon monoxide combines with oxygen to form carbon dioxide:
CO + 1/2O2 → CO2.
Other reactions involving CO release water and hydrogen.
The first version of PureFire technology—the 250PF—relied upon the
converter to scrub 40% of exhaust emissions. The recently introduced 260PF
cuts the percentage to 30% through a redesign of the combustion chamber.
Consequently, the converter runs cooler and should last longer.
Catalytic Converter Maintenance
A catalytic converter intensifies maintenance requirements. Rich mixtures
resulting from improper carburetor adjustments or dirty air filters must be
avoided. Converter burnout occurs at 1400°C or about 300°C more than normal
for high-output engines. Excessive oil or the wrong type of oil fouls the
catalyst. Finally, silicone adhesives or carburetor cleaners containing
lead, phosphorous, or silicone must not be used. But, even with these precautions,
converters are sacrificial items that need periodic replacement.