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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