Any internal combustion engine, whether it's an automobile or a boat, turns only a fraction of the fuel's energy into useful work, and dissipates the rest into enormous heat. If you don't take that energy away, the metal of the pistons and cylinders will quickly reach critical temperatures, leading to bullying and complete destruction of the unit. Which is why the cooling system (CO) is a vital node without which operation is essential. outboard It is impossible even for a few minutes.
Unlike cars that use a forced-blown radiator, water provides unique heat exchange capabilities, with a heat capacity 4 times higher than air, allowing for more compact and efficient heat removal systems, but aggressive water environments, sand, silt and salts, impose their own stringent requirements on the design and materials of the circuit parts.
Understanding fluid circulation will help the owner not only to properly maintain the equipment, but also to quickly diagnose problems by external signs. In this article, we will discuss in detail the physical essence of the process, design differences of systems and typical errors that water engines make during operation.
Physical bases of heat dissipation in aquatic environment
The basic principle behind any cooling system is that the coolant is forced to circulate in a closed or flowing circuit, and the water, through special channels in the cylinder block and the head of the block, heats up, taking excess heat energy from the hot walls, and then the heated liquid is thrown out or heat released through the heat exchanger.
The key performance parameter here is the flow rate and contact area of the liquid with the surface being heated. Engineers design the cooling shirt to avoid the formation of "steam plugs" - zones where water boils ahead of time, creating an insulating layer of steam. Thermostat plays the role of a regulator, providing rapid heating of the engine at idle speeds and intensive heat exchange under load.
⚠️ Warning: The operation of the motor without water "jet" (control ejection) even for a few seconds can lead to irreversible overheating and jamming of the piston group.
It is important to consider that the density of water varies depending on salinity and temperature. Sea water is denser than river water, which affects the performance of the pump and requires more careful selection of anticorrosion materials. Aluminum alloys, widely used in block production, are especially sensitive to galvanic corrosion, so the system is often installed in the system. sacrifice-anode.
Why does water boil at 100 degrees and the motor work at 120 degrees?
The cooling system creates an excess pressure that raises the boiling point of the liquid. However, in flow systems (open loop) the pressure is atmospheric, so the critical temperature is considered to be 100 ° C, although modern antifreeze in closed circuits allows you to work at higher values.
Open loop (Open Loop)
The most common scheme on two-stroke and low-power four-stroke motors is a flow system, or Open Loop. In this embodiment, the off-shore water is directly sucked in by the pump, passes through the engine cooling shirt, is heated and ejected back into the pond with the exhaust fumes or through a separate hole.
The main advantage of this scheme is the simplicity and cheapness of maintenance, there is nothing to break, except the pump and thermostat itself. However, there is a significant disadvantage: direct exposure of the aggressive environment to the internal channels of the engine. Rust, scale and biological fouling (shells, algae) can quickly disable the engine if you do not monitor the state of the system.
- 🌊 Water intake occurs through water intakes located at the bottom of the lower unit, often in the form of slots or holes.
- 🔄 Circulation is provided by an impeller pump, which creates the necessary pressure to lift water to the height of the cylinder block.
- 💨 Exhaust gases are often excreted through the screw hub along with hot water, which creates a characteristic gurgling sound and visible jet.
Owners of open-loop motors should remember that when driving at high speeds, water enters the water intakes under pressure, creating additional pressure, a phenomenon called "pump acceleration." When idling, when the boat is standing still, the pressure is created only by the impeller, so the risk of overheating at low speeds is higher.
Closed circuit and heat exchangers
Powerful four-stroke engines, as well as diesel units, are most often equipped with a closed-type system (Closed Loop), in which case a technical fluid (antifreeze or rosine) circulates inside the engine, which does not contact the off-shore water.
This design allows special additives in the coolant to prevent corrosion and cavitation within the thin channels of the unit. In addition, the thermostat in the closed system works more accurately, maintaining optimal thermal conditions regardless of the temperature of the off-shore water, which is especially important for the environment, since it eliminates the release of oil and fuel into the reservoir through the cooling system.
The heat exchanger is a complex design, often of the pipe-in-the-pipe type or plate radiator type. The off-shore water washes the outer surface of the tubes, which contain hot antifreeze, and the efficiency of such a unit depends on the purity of the external channels, which can be clogged with dirt and algae.
| Parameter | Open outline (Open) | Closed circuit (Closed) |
|---|---|---|
| Coolant | Off-shore water | Antifreeze/Tosol |
| The risk of block corrosion | Highly | Low. |
| Service | Wash, impeller replacement | Replacement of antifreeze, cleaning of radiator |
| Cost of ownership | Low. | High. |
Despite the complexity, the closed system ensures more stable engine operation in all conditions, but it requires regular checks of the antifreeze level and the condition of the pump glands to prevent water from entering the oil.
Design and operation of the water pump
The heart of any outboard motor cooling system is a water pump, and in the vast majority of cases, it's a volumetric pump with a flexible impeller, and it works by deforming the rubber plates that grab water and push it up the channel to the engine.
The impeller is made of special rubber that is water and temperature resistant, but highly sensitive to dry operation. When the engine is started, the rubber blades press against the walls of the body, creating inlet pressure and outlet pressure, which is why it is critical to start the engine only submerged in water or connected to the forced feed system (“boot”).
Always lubricate the lips of a new impeller with soap solution or silicone lubricant before installation – this will prevent the rubber from rolling in at the first start and prolong the life of the part.
The pump body is usually made of stainless steel or brass. Between the metal body and the rubber impeller there is a minimum gap, the violation of which (for example, due to production or improper assembly) leads to a drop in performance. Also, the design often provides a reverse valve to prevent water from draining from the system when the motor is stopped, which allows you to keep the "water lock".
The pump's performance is directly dependent on the rotation of the crankshaft. At idle speeds, the flow is minimal, but sufficient to remove heat. As the speed increases, so does the volume of water pumped. However, if the water intakes get clogged with grass or sand, even a proper pump will not be able to provide the desired flow, which will lead to cavitation and overheating.
Thermostat: Thermal Regulator
The thermostat is an automatic valve that controls the flow of coolant according to its temperature, and its main task is to ensure that the cold engine is quickly warmed to operating temperature and maintain it in a narrow range when the load changes.
When the engine is cold, the thermostat is closed and the water is circulating in a small circle (or simply does not enter the unit fully, depending on the design), which allows the cylinder block to heat up quickly. Once the temperature reaches a predetermined value (usually about 50-60 ° C for opening and 70-80 ° C for full opening), the valve begins to open, launching a full stream of off-board water.
- 🌡️ A faulty thermostat can jam in the closed position, causing instant overheating and steam ejection.
- 💧 If the thermostat is jammed in the open position, the engine will warm up for a long time, working on the re-enriched mixture, which leads to increased fuel consumption.
- 🔧 Replacing a thermostat is a simple procedure that is recommended every 2-3 seasons or at the first sign of temperature instability.
Many aqueous motorists mistakenly remove the thermostat as an extra element, which is absolutely impossible. Without the thermostat, the engine operates in a sub-optimal thermal mode, heat gaps increase, efficiency drops and the wear of the cylinder-piston group parts increases dramatically.
The thermostat is not just a valve, it's a guarantor of the engine's life, and its health ensures that the thermal gap between the piston and the cylinder is correct.
Diagnosis of malfunctions and control jet
The main visual indicator of cooling system serviceability is the tell-tale, a thin stream of water that must flow from a special hole in the body of the motor or lower unit when the engine is running, and the absence of a jet or its weakening is the first alarm.
However, there are nuances to consider: In some modern four-stroke engines, the jet can be very weak or intermittent at idle speeds due to the design of the thermostat, in which case you should focus on the body temperature (caution!) or use an infrared thermometer, and the whistle of the pump or extraneous sounds when working can also be a sign of problems.
⚠️ Warning: If the water jet is gone on the move, immediately release the gas and turn off the engine. Further movement even in idling mode can be dangerous for the piston group.
A common cause of the lack of a jet is not a pump failure, but a banal clogging of the control hole with salts or sand, which can be cleaned with a thin wire or a fishing line, and if the pump is worn out, water can circulate inside, but not create enough pressure to be ejected outwards, which also can cause local overheating.
☑️ Diagnostics of the cooling system
Maintenance and conservation for the winter
Regular maintenance of the cooling system is the key to a long life of the engine. At least once a season (and when active in salt water - more often), you need to carry out preventive washing, to this end, the motor is run in a container with fresh water or connected to the water supply through a special adapter, adding means to remove scale.
Special attention should be paid to conservation for winter: residual water in the channels, freezing, expands and can break the cylinder block or pump body. There are two main ways of wintering: storage in water (only if the water does not freeze) or storage on land with a full purge system.
To dry the engine completely, you remove the boat, disconnect the fuel hose and let it work for a few seconds to expel the water. A more professional method involves using compressed air to blow the channels through the control hole or water intakes. It is also recommended to remove the thermostat and leave it to dry separately if the motor is stored in an unheated room.
How to wash the engine with fresh water?
Use ears to flush, put them on the water intakes, turn on the water, and then start the engine. Let's work 5-10 minutes on the idle. Never start the engine without water, even for a few seconds, because the impeller can get damaged.
Can I use antifreeze in an open loop?
Absolutely not. Antifreeze is toxic to aquatic life and is prohibited from being discharged into water bodies. Open systems use only pure water. Anti-corrosion additives are added as special spray preservatives for winter storage, but not for permanent operation.
Why is the engine warmed, even though there is a jet?
This may indicate partial clogging of the internal channels of the cylinder block with salts or scale. Water circulates, but does not wash critical heating zones. Chemical washing of the system with special solutions (deckyler) is required to remove sediments.
How often do I change the pump impeller?
The average life of the impeller is 300 hours or 3 years of operation, whichever comes first, but if you are driving in shallow water or in muddy water, you should replace it annually.