Marine Diesels From Air Filters to Exhaust Systems

Marine Diesels From Air Filters to Exhaust Systems

Fuel by itself is of no use whatsoever: it needs oxygen from the air outside in order to burn. At the most basic level, this happens its own accord: as the piston falls during the induction stroke, air rushes in past the open inlet valve to fill the expanding space. Then, when the compression and power strokes are complete, the exhaust valve opens and the rising piston pushes the exhaust gas out ready for a fresh charge of clean air.

In practice, though, the engine needs an air filter to stop dirt, moisture and bits of rubbish being sucked into its cylinders, and needs an exhaust system to dispose of the hot exhaust gases safely and quietly. To save having a separate filter and exhaust pipe for each cylinder of a multi-cylinder engine the incoming air is fed to the cylinders through tubular structures called inlet manifolds, and the exhaust gases are carried away through similar structures called the exhaust manifolds.

Air filters

Unlike their cousins that power tractors and earth-moving machinery, marine diesels usually operate in a relatively clean environment: there's little danger of them having to contend with straw, dust or roadside litter. This means that their air filters can be relatively simple, so some engines operate perfectly well for years with little more than a metal box with a few baffles in it.

Most, however, have something a little more sophisticated, involving either wire gauze or porous paper.

Paper tends to restrict the air flow, so to make up for this its area has to be increased by being folded into a concertina shape. It's also difficult to clean, so once a paper filter becomes clogged it has to be replaced with a new one.

Wire gauze doesn't restrict the air flow as much, but it is less effective because the gaps between the strands of wire are bigger than those between the fibres of paper. To counter this problem - and to minimise corrosion - wire gauze filters need to be dipped in oil from time to time, so that dust sticks to them instead of passing straight through.

Exhaust systems

When it comes to exhaust systems, the boot is on the other foot: road vehicles and agricultural machinery have an easy time of it. Their engines are in compartments that are open to the atmosphere but sealed away from their drivers and passengers, so all that's required is a pipe connected to the exhaust manifold, with a few baffles to reduce the noise. A few marine installations adopt a similar 'dry' exhaust system, usually in the form of an exhaust pipe sticking straight up from the engine compartment, with a weighted flap to stop rain or spray running down inside and heat resistant lagging to minimise the risk of fire or burns.

For pleasure craft, though, wet exhaust systems are pretty well standard, with water from the engine's cooling system used to cool the exhaust gas. The water is mixed with the exhaust gas in the injection bend, where it almost immediately turns into steam but in doing so reduces the temperature of the exhaust gases from almost 500° C to about 70° C - cool enough to allow flexible tubing and GRP to be used for the rest of the exhaust system.

At that reduced temperature, the steam condenses back into water. That is why the mixing takes place in a bend: it protects the engine against the possibility of the cooling water running back through the system and into the cylinders.

If the engine is below the waterline, or very close to it, however, the injection bend alone is not enough: there's a danger that water already in the exhaust might set up a siphon effect that would allow sea water from outside to make its way back through the exhaust system and into the engine. To stop this, many boats have an extra loop in the exhaust system, known as a swan-neck. To guard against the possibility of waves pushing water up the exhaust pipe, some boats have a one-way flap covering the end of the pipe where it emerges from the hull; on some sailing yachts you may even find a hand-operated gate valve that seals the exhaust pipe completely when the engine is not being used.

The vital thing about any exhaust system that it must not restrict the flow of exhaust gases beyond a certain limit, because if exhaust can't get out of the cylinders, there will be no room for fresh air to get in. The effect is exactly the same as if the air filter were clogged: starved of oxygen, the engine will not be able to burn its fuel, so it will lose power and produce black smoke.

More power

Any engine is simply a device for converting the energy released from burning fuel into mechanical power. None of them are very good at it: well over 60 per cent of energy released from the fuel is expended as heat and vibration, rather than as use mechanical work. Engine designers are continually working to improve efficiency but the fact remains that the power an engine can produce will always be limited by the rate at which it can burn fuel.

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Marine Diesel Engine Maintenance

Marine Diesel Engine Maintenance

In a conventional marine diesel engine the power is produced by hot compressed air igniting fuel sprayed under very high pressure into the cylinder head. A marine diesel engine does not use a carburetor to mix fuel and air or spark plugs to ignite the mixture. Instead it employs the pistons to compress the air to 3000 kPa which causes it to become extremely hot and the fuel is ignited as soon as it is injected into the cylinder.

Some marine diesel engines are fitted with a heater plug in the inlet manifold or a glow plug in the pre-combustion chamber of each cylinder to provide additional heat to the combustion air during starting.

Diesel engines are heavier and slower revving than petrol engines but they are also more reliable because they do not rely on external carburetion or an electrical spark for ignition.

Newer engines use an electronic fuel injection system whereby fuel and air are mixed more thoroughly in the pre-combustion chamber before entering the cylinder. This system maximizes power and fuel economy and is also less polluting.

Every boater should have an understanding of how their engine works so let's start by explaining the mechanical cycles.

Most reciprocating piston internal combustion engines work on one of two mechanical cycles-either the four-stroke cycle or the two-stroke cycle. These cycles designate, in correct sequence, the mechanical actions by which the fuel and air gain access to the engine cylinder, the gas pressure - due to combustion - is converted to power and, finally, the burnt gas is expelled from the engine cylinder.

The Basic Four-Stroke Diesel Engine

From its name, it is obvious there are four strokes in one complete engine cycle. A stroke is the movement of the piston through the full length of the cylinder and - since one such movement causes the crankshaft to rotate half a turn - it follows that there are two crankshaft revolutions in one complete engine cycle.

The four strokes in the order they occur are:

1. Inlet stroke. With the inlet valve open and the exhaust valve closed, the piston moves from top dead center (TDC) to bottom dead center (BDC), creating a low-pressure area in the cylinder. Clean, filtered air rushes through the open inlet valve to relieve this low-pressure area, and the cylinder fills with air.

2. Compression stroke. With both valves closed, the piston moves from BDC to TDC, compressing the air. During this stroke the air becomes heated to a temperature sufficiently high to ignite the fuel.

3. Power stroke. At approximately TDC, the fuel is injected, or sprayed, into the hot, compressed air, where it ignites, burns and expands. Both valves remain closed, and the pressure acts on the piston crown, forcing it down the cylinder from TDC to BDC.

4. Exhaust stroke. At approximately BDC the exhaust valve opens and the piston starts to move from BDC to TDC, driving the burnt gas out of the cylinder through the open exhaust valve.

The Two-Stroke Diesel Engine

The two-stroke engine uses two piston strokes to complete one power stroke and, therefore, fire twice as often as a four-stroke engine. A two-stroke engine is smaller and simpler with fewer moving parts. A two-stroke engine has the potential to produce twice as much power as a four-stroke engine of the same size, however, because of the extra fitting required in a two-stroke diesel engine, for example blowers and governors, they become more expensive to produce. There has been a shift towards four stroke diesel engines which have become more efficient and smaller.

Protect Your Marine Diesel Engine

Protect your engine by avoiding long periods (more than 10 minutes) of idling in a "no-load" situation. This is often done to charge batteries or cool refrigeration but if done repeatedly it will glaze the bores of the engine and cause premature engine failure. If the vessel is in a berth the engine can be put in gear to create load at idle.

All boat owners should have an understanding of basic marine diesel engine maintenance to keep themselves and their families safe on the water.

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Understanding A Marine Exhaust System

Marine exhaust systems are an important and often neglected engine of a ship. Most ships have a system of "wet" exhaust. The outlet for the exhaust gases from the exhaust manifold - the salt water is injected into the column. This mixture of gas and water is then passed through a series of curves to it from the boat, preferably in the back.

Exhaust fumes mixed with salt water to create a highly corrosive compound. This is why exhaust systems are often non-corrosiveComponents such as nitrile rubber reinforced hose (brown - not green - stripe), galvanized steel, fiberglass or plastic. The purpose of these components, which vary in size and shape depending on the size of the engine and the layout of the engine room is to prevent the engine hydraulicing. This occurs when an engine full of salt water that entered through the exhaust and can cause considerable damage if left for more than two hours. The water enters the exhaust gas to swell in the tail and armDrain design. In some sea conditions, such as by sea, the water can again be forced to exhaust when the engine is not running. Exhausts poorly designed, that water will flow back and fill the waterlock / muffler box then the pipe into the exhaust manifold, exhaust valves and combustion chamber. With the engine full of water can not compress the engine crank over, it is not like water.

Exhaust System

You will know that your engine is if you do not turn around hydrauliced ​​-usually after a long period of sailing - and removing the starter and make sure it works. To remove remove water from inside the combustion chambers, first of all injectors, crank engine blowing over the water, replace vent injectors, injector lines and restart. Then leave the motor running until exhaust fixed.

Check that the mast is not coke up or corroded - a common problem. To check the exhaust riser remove the drain hose from the riser (often a difficult procedure)and look at the line, to see if it is the accumulation of exhaust / salt restricted. When building the riser is too large to be removed in order to check the engine side of the line. Coke can be scraped away to be a short term fix although often the riser must be replaced. There are aftermarket systems which vary in quality. Make sure you fit the right model for your application.

Make sure that the water-lock is low enough and large enough to hold all the water in the exhaust system. There is aA gooseneck or central vertical loop in the vent pipe to rely on the mirror? It is a siphon break and function - without valves leak?

Exhaust is toxic and can cause motion sickness and headaches. Replace defective components immediately. Use double hose clamps on each joint or, preferably, super, the style of concrete drain bolt, if necessary. Hot sections should be insulated with fiberglass tape to prevent burns.

Asbestos Warning delayed. Many larger vessels andMarine engine installations had exhaust systems that were lagged with asbestos tape and rope. Asbestos was common in older boats noise.

Understanding A Marine Exhaust System

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