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All diesel engines will fall into one of two basic designs (I am NOT getting into semi-diesels for narrow boat fans). Each design may appear in one of three forms. The advantages etc. are discussed below.



This design usually has on odd shaped hole in the centre of its piston into which the fuel is injected.

Do not worry about the technical aspects of this picture (you are sharing with mechanics 2).

The air coming in and then the piston moving up causes the air to rush round as shown.

The piston comes to within less than 0.5mm of actually hitting the valves and head. This squeezes the air so hard that it gets hot enough to ignite the fuel when it is injected (squirted in).

The air rushing about helps the fuel to ignite quickly which lessens diesel knock (the sound that makes all diesels sound as if bits are going to fly out when compared with a petrol engine.

The cetane rating of the fuel also influences diesel knock.


Until the advent of computer modelling, it has been difficult to design the injection system to do an efficient job, at low revs, on small capacity cylinders. It has been said that about 500cc is about the smallest reasonable possible, thus the number of 1, 2, or 3 cylinder direct injection diesel engines.

The low surface area of the combustion space gives low heat loss and thus easy starting, but tickover combustion can be "patchy" leading to smoke - especially at low speed, because this results in low speed swirl in the cylinder.

The larger piston sizes increase inertia which leads to slow(is) acceleration and low top speed.



These engines have a flat-topped piston with a special "hole" in the cylinder head called a pre-combustion chamber. The chamber shown is the common type known as a "Ricardo Comet Chamber", Mercedes have their own design.

The air is forced through the throat by the piston moving up, the friction thus created heats the air that then swirls about in the swirl chamber.

When the fuel is injected the burning mixture goes back down the throat (heating it) and mixes with the air compressed in the cylinder. The heat in the throat is transferred to the next charge of air.

The swirl is very great, which gives a lower degree of diesel knock.

The heat lost through the swirl chamber walls, especially when cold, requires a higher compression ration AND heater (glow) plugs.

This design gives good combustion at slow speed which tends to give a smoother/quieter tick over.

(The friction in the throat can give a worse fuel consumption than a petrol engine with prolonged very high speed use).

The high degree of swirl made it easier to design an injection system for smaller cylinder capacities, thus its predominance in small, four cylinder designs.



The engines available to most boaters are sourced from three (and a half) sources:

Full Marine Engines - There are few of this type. Bukh, Saab and some Penta are typical. They are likely to be direct injected, heavy, comparatively slow revving, and the parts will be under low stress.

They will be designed to operate at a temperature of 70C or less and the cooling system should have protection anodes fitted internally (these should be inspected regularly).

They may or may not be of Monoblock construction. (a car with all 4 cylinders made in one block is a monoblock, an air cooled Lister with individual cylinders is not). The more joints you have (non-monoblock construction) the more chances of a leak developing.

They will be expensive, but very long lived.


Automotive - Typified by BMC 1.5s & 1.8s, Perkins, Ford etc.

They will be monoblocks of four or more cylinders, be high revving, (for their age of design) and comparatively highly stressed. In normal inland marine sizes they were, until very recently, likely to be indirect injected.

Their cooling systems should be expected to operated at about 90C unless de-rated by the mariniser for direct cooling (see later section). If not direct cooled their cooling system should be pressurised unless long experience proves otherwise (BMC 1.5)

The parts should be relatively cheap, and with the right choice of engine, readily available.


Industrial - depending on age is likely to be somewhere between the above types in case of design, stress etc. Modern Japanese based units are likely to be more like automotive units, whilst older Listers have more in common with marine units.


Classic engine - these tend to be old engine, or old designs of engine from the industrial stable. They will be direct injected. In relation to output they will be big, heavy, slow revving, and with large torque. They are likely to be stressed at a low level, giving long life.


Two views of modern industrial based unit

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Choice of engine

There is probably no right or wrong choice of engine. A displacement boat which can swing a large propeller would probably do best with a fully blown marine engine or an older industrial engine. A high speed craft which can only swing a small propeller might find a modern automotive unit more suitable.

Once a propeller supplier has Ok'd your match of Hull, Engine, Prop, & Transmission your main considerations should be:-

How highly stressed are parts?

How likely are leaks to develop?

How expensive are the service parts?

Does it use a camshaft timing chain or belt?

The last one is very important. Whatever the manufacturer or their agent says, cam BELTS do not like oily water, and that is just what they will get covered in if the bilges fill up. A belt breaking is not uncommon, a chain breaking is very nearly unheard of. If a cam shaft stops turning and the engine keeps going the result will easily run into four figures on a diesel engine (unless you can do it yourself). You choose the safest option!

Your lecturer would also fight shy of a modern automotive unit with a complex heater plug control unit, this just gives added complication and opportunity for unreliability.


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