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Motoculture: exhaust

The evacuation of the burnt gases resulting from the combustion of the fuel mixture in the cylinder can not take place directly in the open air at the outlet of the combustion chamber. (Non-contractual photo, does not necessarily reflect the drawing)

Indeed, it is essential that these gases pass through an exhaust device whose function is to cool them, but also to relax, to facilitate the flow and reduce noise.
Hence the interposition of a pipe device and a silencer.
Directive 2000/14 / EC, which aims to combat the noise emitted by equipment used outside buildings, is tomandatory application since 3 January 2002 in all countries of the European Community. For a large number of devices, it brings profound changes in terms of technical requirements, user information and the certification methods used by manufacturers.
This new directive also considerably extends the scope of the directives it replaces.
Indeed it concerns more than 50 types of machines used mainly in gardening, maintenance of green spaces and construction.
While imposingguaranteed sound power level labeling for all types of equipmentit distinguishes two categories of materials. For one of them, the guaranteed sound power levels must not exceed the limit values ​​which it is planned to reduce as from 3 January 2006.
The maximum acceptable sound level is of the order of 100 dB, knowing that many machines currently on the market are already well below this threshold.

Exhaust from a mower

Exhaust from a mower


All silencers work the same way: they brake the exhaust. They thus reduce the power of the engine and also induce a back pressure wave whose intensity is reduced by limiting as much as possible the length of the exhaust pipe, which is important on a single cylinder. That's why small monocylinders have a silencer (called exhaust) directly attached to the cylinder head or the engine block.
This silencer, crossed by hot gases containing corrosive residues, deteriorates rapidly. This corrosion can be delayed by spraying the interior of the silencer, all ten uses, a product of aerosol corrosion inhibitor. When pierced, the silencer must be changed. Some high-power two-stroke engines are also equipped with an intake silencer, because by its operating principle, the two-stroke engine also emits a significant noise on admission.
This type of silencer usually includes baffles incorporated in the housing containing the air filter.
Unlike the exhaust muffler, it does not corrode much and it is enough to oil it slightly when wintering the equipment to keep it in good condition.

Spark arrestor

Because of its reduced dimensions, the silencer of a small single cylinder may leak sparks; these come either from the combustion of calamine particles detached from the walls of the combustion chamber and from the piston head, or from the post-combustion of unburnt heavy elements contained in the fuel (oil droplets on a two time, in particular). This phenomenon is quite common on Diesel.
Moreover, a mixture that is too rich is never completely burned, the exhaust gases sometimes contain combustible elements that can ignite at the exit of the exhaustespecially when the engine has been running for a long time at full throttle. Spark sparking or auto-ignition at the exhaust outlet can ignite grass or objects in the vicinity of the machine.
That's why most manufacturers of gardening and DIY tools offer an optional spark arrester.
This is usually a sheet metal cowling that acts as a chicane and attaches directly to the output of the silencer. The mounting of this accessory is almost mandatory on chainsaws as well as on motor pumps running inside a room (unless their exhaust is connected to the outside).
The presence of carbonaceous residues fouls it quite quickly. They are easily removed by soaking the spark arrester for 12 hours in a trichlorethylene bath. Let it dry before reassembling it. If it shows traces of early corrosion, treat them with a corrosion inhibitor product.
When reassembling, observe the exit direction of the spark arrester. It must always open in such a way that it ejects the hot gases as far as possible from the fuel tank, the air filter and the hands of the operator. Always be sure to let the exhaust cool down before attempting any repairs, including power.

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