The operation of a boat with an outboard motor is often overshadowed by a characteristic piercing sound that not only tires the pilot, but also scares the fish, and also causes the discontent of vacationers on the shoreline. Two-stroke engines, despite their simplicity and high specific power, have always been famous for louder exhaust compared to four-stroke counterparts, due to the peculiarities of their work cycle and cylinder purging.
There is a common misconception that there is nothing to do with this hum but to tolerate or change the power unit to a more expensive four-stroke. However, engineering practice and the experience of experienced water engines prove the opposite: competent exhaust system modernization and proper technical adjustment can reduce the decibel level by one of the most expensive. 5-10 dBThis is subjectively perceived as a two-fold decrease in volume.
In this article, we will examine the physical causes of noise in a two-stroke internal combustion engine and consider specific, technically sound ways to suppress it. You will learn how to refine a regular silencer, why sealing the casing is important and how to do this. high-quality It affects acoustic comfort while swimming.
Noise physics: why two-stroke is so loud
To effectively combat the problem, you need to understand the nature of the problem. The main source of sound in a two-stroke engine is not so much mechanical friction of parts as a sharp emission of exhaust gases under high pressure. Unlike four-stroke engines, where the exhaust occurs through a separate valve, in two-strokes, the gas exchange process is combined with purges, which creates powerful acoustic waves.
The key factor is resonance The gases leave the cylinder at a tremendous speed, creating a shock wave. If the exhaust path does not have the correct geometry and resonance chambers, this wave comes out almost without attenuation. Noise insulation of the lower unit itself in budget models is often minimal, allowing sound to flow freely through the metal casing.
And you also have to consider the rotational frequency of the crankshaft, and at high speeds, when the boat goes into planing, the frequency of the exhaust pulses increases, merging into a continuous roar. The greatest impact is the efficiency of gas expansion in the silencerThe volume and structure of the inner cavity determine how successfully the energy of the sound wave will be absorbed.
An additional source of discomfort is vibration. The two-stroke engine vibrates more strongly because of the asymmetry of the working cycles, and this vibration is transmitted to the transom of the boat, causing the entire PVC or metal boat body to resonate, turning it into a kind of sound membrane.
Upgrade of the exhaust system: installation of the resonator
The most effective way to reduce noise is to redesign the exhaust tract. Mercury, Yamaha or Tohatsu) is often a simple pipe with a minimum number of partitions, the installation of an additional resonator or the complete replacement of the "bank" of the silencer with a larger one can significantly reduce the noise level.
The resonator works by interfering with sound waves, and when you enter an extended chamber, the wave bounces off the walls and extinguishes itself, and it is important that the resonator has at least 70-80% of the engine's displacement to achieve a tangible effect. counterpressureToo narrow or long path can "strangle" the engine, depriving it of power at high speeds.
Calculation of resonator volume
For a two-stroke engine with a volume of 9.9 hp (about 170 cc), the minimum effective volume of the resonator should be approximately 120-140 cc Increase volume to 300-400 cc will give an even better result, but will require more space in lower unit.
When making or selecting a muffler, use materials with high thermal insulation properties. Stainless steel is preferable to conventional steel, as it holds the temperature better and is less susceptible to corrosion from salt water. Internal partitions can be filled with basalt fiber or special metal shavings, which will turn the muffler into an active sound absorber.
The main goal of modernization is to increase the path of gases and create zones of sharp expansion of volume without a critical increase in flow resistance.
Sealing of the casing and reducing vibration
Often the noise is not in the exhaust itself, but in the poor soundproofing of the lower unit, the metal casing of the engine works like a speaker, amplifying the sounds of the internal mechanisms. The first step to silence should be a thorough review of all joints and connections. gasket between the crankcase and the silencer, and between the lower unit and the lower unit leg.
To reduce the vibration that is transmitted to the boat hull, it is recommended to use vibration insulating gaskets for an outboard motor. If the regular rubber dampers are worn out or dampered with time, their replacement will give an instant positive effect.
Notice the attachment of the motor to the transom. Excessive puffing or backlash can increase resonance. Using high-quality anti-vibration gaskets under the rods or flange mounts will help to break the path of transmission of sound waves to the boat's hull.
Before sticking vibration isolation on the plastic hood, be sure to degrease the surface with alcohol or a special cleaner, otherwise the material will unstuck from heating and humidity at the most inopportune moment.
Remember that any additional insulation must not block the air for cooling. Two-stroke engines are cooled with off-shore water, but the top (carburetor, electronics) requires air purging. Leave the vents open, insulating only hard surfaces.
Impact of Fuel and Oil Quality on Noise Levels
The quality of the fuel mix directly affects the nature of combustion and, as a result, the sound of the engine. Using cheap mineral oils leads to the formation of soot on the piston and in the exhaust channel. The growths of sodium change the geometry of the exhaust tract, create turbulence and can cause detonation that sounds like a metal clang.
Synthetic or semi-synthetic oils for two-stroke engines (standard) JASO FD or TC-W3) They burn cleaner and make the engine work smoother, they also have better lubricating properties, which reduces the mechanical noise of rubbing vapors. The correct proportion of the mixture (usually 1:50 for modern motors) is critical: over-enrichment with oil leads to coking, and the lack of dry friction and ringing.
The octane number of gasoline also plays a role. If the engine is designed for an AI-92, and you pour the AI-95 without adjusting the angle of advance of ignition (which is impossible in simple engines), there can be a stiffer combustion. Conversely, the use of low-octane fuel in forced engines causes detonation that is not only noisy, but also destructive to the piston group.
Set up a carburetor for quiet work
Incorrectly adjusting the carburetor is a common cause of increased noise. If the mixture is too poor (lots of air, not enough fuel), the engine runs at high idling speeds and emits a piercing screech. In addition, the poor mixture burns at higher temperatures, which increases heat load and exhaust noise.
Adjustment of the quality and quantity of the mixture must be strictly manual for a particular model. Usually the procedure involves heating the engine to operating temperature and then twisting the screws to achieve stable minimum rpm. carburetor It provides a smooth, low-frequency hum instead of a ragged, ragged sound.
☑️ Checking the carburetor settings
It is worth noting that after winter storage or prolonged downtime, jellyplants can get clogged with resins from old gasoline. Cleaning the carburetor with ultrasound or special aerosols often returns the engine to factory silence specifications without the need to replace parts.
Comparison of noise reduction methods
When choosing a way to control noise, it is important to evaluate labor and efficiency. Some methods require intervention in engine design, others require only preventive actions. Below is a comparative table of the main approaches.
| Method | Efficiency | Complexity | Impact on power |
|---|---|---|---|
| Replacing the oil with synthetics | Low (subjectively) | Minimum | Missing. |
| Sealing of the casing | Medium | Low. | Missing. |
| Customization of the carburettor | Medium | Medium | Possible correction |
| Installation of the resonator | High. | High. | Possible fall in BB |
| Replacement of silencer | Maximum | High. | Depends on the design. |
As the table shows, the most radical measures yield the best results but require technical skills. A comprehensive approach combining maintenance and moderate upgrades is the optimal solution for most boat owners.
When welding or soldering exhaust system elements, be sure to use argon or quality fluxes so that the seams do not flow from condensation and vibration through the season.
Frequent mistakes when trying to make the engine quieter
In the quest for silence, inexperienced water-engines often make mistakes that can lead to engine failure, one of the most common being trying to completely close the exhaust hole or install an exhaust pipe that is too long without regard to drag.
The shutdown of the exhaust gases causes the piston group to overheat rapidly. The two-stroke engine relies on the flow of gases for partial cooling and purging. If the gases cannot escape, the fresh mixture will not enter the cylinder, the engine will suffocate, warm up and eventually jam.
⚠️ Warning: It is strictly forbidden to jam the exhaust outlet more than 30-40% without accurate calculations and tests on the stand, this is guaranteed to lead to overheating and piston bullying.
Another mistake is using the wrong materials to stuff the muffler, and the regular glass wool will burn out in a couple of hours, and the metal shavings without corrosion treatment will quickly rust and turn into porridge that will fill the exit holes. Use only heat-resistant materials designed for exhaust systems.
Also, don't ignore the condition of the lower unit leg, if water gets into it (through the glands), it can turn into a resonator, check the presence of water in the lower unit crankcase and replace the paddle shaft glands if necessary.
⚠️ Warning: Any modification of the exhaust system may result in the loss of warranty for the new engine.
Remember, the absolute silence of a two-stroke engine is impossible to achieve because of the physical principles of its operation, but reducing noise to a comfortable level is a very real task, requiring a competent engineering approach and regular maintenance.
FAQ: Frequently asked questions
Will the installation of a silencer from a four-stroke engine on a two-stroke?
No, it's impossible and dangerous. Four-stroke mufflers are different in design, often activate catalysts, and are designed for different temperature and pressure, and if you install a muffler, you'll create critical backpressure, which will overheat and break the two-stroke engine.
How often should the muffler be cleaned of the stain?
It is recommended to perform a visual inspection and cleaning of the exhaust tract every 50-100 hours or once a season, especially if you use mineral oils or often work on part-time throttle.
Does the length of the lower unit (S, L, X) affect the noise level?
A longer lower unit leg (L or X) lowers the motor deeper, and water absorbs exhaust sound better, especially at low revs. In addition, the long exhaust tract inside the leg can act as an additional resonator.
Can I use car silencers for a outboard motor?
In theory, some models of direct-flow mufflers can be adapted, but they must be made of stainless steel and have sufficient volume. However, car mufflers often have a complex internal structure that creates high back pressure, which is detrimental to a two-stroke.