Marine Diesel Engines E-Book

Peter Caplen

• Herausgeber: Crowood
• Kategorie: Fachliteratur
• Sprache: Englisch

Veröffentlichungsjahr: 2011

Beschreibung

The diesel engine is by far the most popular powerplant for boats of all sizes, both power and sail. With the right care and maintenance it is twice as reliable as the petrol engine as it has no electrical ignition system, which in the marine environment can suffer from the effects of damp surroundings. Self-sufficiency at sea and the ability to solve minor engine problems without having to alert the lifeboat is an essential part of good seamanship. Marine Diesel Engines, explains through diagrams and stage-by-stage photographs everything a boat owner needs to know to keep their boat's engine in good order; how to rectify simple faults and how to save a great deal of money on annual service charges. Unlike a workshop manual that explains no more than how to perform certain tasks, this book offers a detailed, step-by-step guide to essential maintenance procedures whilst explaining exactly why each job is required.

Leseprobe

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Contents

Introduction

1 A Brief History of Diesel Engine Development

2 How the Diesel Engine Works

3 Fuel Systems

4 Lubrication

5 Cooling Systems

6 Electrical Systems

7 Turbochargers, Superchargers and After-Coolers

8 Marinization

9 Troubleshooting

10 Engine Installation

11 On-Board Tools and Spares

Glossary

Useful Contacts

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Introduction

Despite recent changes in fuel taxation for privately owned boats, the diesel engine is still by far the most popular form of propulsive power for boats of all sizes, both power and sail. Although the loss of some taxation benefits has caused a rise in the price of diesel so it approaches that of petrol, there can be no doubt that diesel is still the best choice for boats. Gas oil, or ‘red diesel’, is the standard fuel for marine diesel engines; it is far less flammable than petrol, which is why it is unheard of for a diesel-powered craft to suffer from a fuel tank explosion – something which cannot be said of petrol-engined craft, as the number of incidents, although fairly low, is depressingly regular.

Diesel engines are also twice as reliable as the petrol engine – in theory at least – because they have no electrical ignition system, which in the marine environment is always prone to suffer from the effects of the damp surroundings. However, in the last ten years or so electronically controlled diesels have to some extent lost this advantage. Then again, the sealed computer-based electronic systems used for controlling diesel engines are much less exposed than the petrol engine ignition system. The high levels of reliability of modern electronics mean that a complete breakdown is very unlikely, and this, coupled with the high efficiency and fuel economy of modern common-rail diesels, still gives the diesel engine a big economy edge over the petrol version.

These virtues make the diesel engine the favoured choice of the serious boat owner, who regards safety and reliability of paramount importance. Also to be considered is the fact that the economy of the diesel engine allows a vessel with a given fuel tank capacity to travel that much further when powered by diesel rather than petrol.

The easy availability of gas oil from boat yards and marinas makes refuelling more convenient; many yards are unable to stock petrol due to the stringent regulations covering the storage of petrol, which requires the additional heavy expense of installing underground tanks. The only area of the UK where petrol is readily available at nearly all marinas is the south coast, and in this area there are many more petrol-powered boats than in other areas. Moreover the fact that gas oil is still somewhat cheaper than petrol despite the loss in 2008 of the derogation allowing non-road diesel fuel to be taxed at a lower rate means that motor boating can still be enjoyed on a reasonable budget without too many worries about how wide the throttles are opened!

Many boat owners are still unfamiliar with the workings of the diesel engine, although with the prevalence of high efficiency diesels now commonly being used in cars, their mysteries are less puzzling. Anyone with a basic working knowledge of petrol engines will have a good basis for understanding the diesel, as there is little real difference between the two apart from the fuel system and the lack of an electrical ignition system. The mechanics of the diesel engine are basically identical to that of the petrol engine, and once the differences in fuelling are understood, they should not present any problems to the enthusiastic DIY owner.

This book explains everything the beginner needs to know to keep their boat’s diesel engines in good order, how to rectify simple faults, and how to save a great deal of cash on annual service costs. It will also be of benefit to the more knowledgeable owner who wants to fill in the gaps in their knowledge. It covers all the basic maintenance procedures, and also explains the workings of power-boosting equipment such as turbochargers, superchargers and intercoolers, and the way different types of marine cooling systems operate.

While there is a huge selection of practical material there is also plenty of detail about the history and development of the diesel engine. Hopefully this will give the owner a much better insight into (and interest in) their power unit than a straightforward workshop manual could provide. Unlike a workshop manual that explains no more than how to perform certain tasks, this book attempts to provide a basic understanding of the workings of the diesel engine; furthermore, while offering step-by-step instruction on practical maintenance procedures, it also explains exactly why each job is required.

Several sections concentrate on areas that many owners may not have considered of great importance. Fuel tanks are a case in point. Give a diesel engine a supply of clean, air-free fuel and even if it is almost totally ‘clapped out’ it will probably still run; but introduce dirt or air into the fuel, and even a new engine will soon stop in protest. A clean fuel supply is the key to reliable running. Therefore, although these items may be unglamorous and unpleasant to deal with, they will ultimately reward the owner with a trouble-free season’s boating.

The largest chapters necessarily concentrate on the fuel, cooling and electrical systems, as these are where problems are most likely to occur. The cooling system will cause problems if it is not properly cleaned and maintained; in the worst case scenario the engine will overheat to the point of seizure, so it is most important to keep the cooling system in good order, yet this does not require any great degree of skill. Similarly some basic maintenance of the electrical system will ensure the engine starts when the key is turned, and continues running and charging the batteries until it is switched off.

Much of the practical content of this book is drawn from a long experience of keeping ailing diesel engines running on a minimal budget and with the minimum of facilities. This can be part of the pleasure of owning a vessel with inboard power, and for those who must work to a tight budget it is an almost essential skill!

It is a fact that a job completed by the owner, once the necessary knowledge has been acquired, will be of a higher standard than that performed by a service agent or boatyard as the owner knows only too well that if problems arise at sea it will be up to him alone to find the remedy, and that if a job is performed properly in the first place a problem will not arise later.

However, regardless of the financial considerations of the reader and whether or not they choose to have their annual maintenance performed professionally, self-sufficiency at sea and the ability to solve minor engine problems without having to alert the lifeboat is a part of good seamanship, and being able to rectify minor breakdowns should be a matter of pride. Your engine is your lifeline, and it is as well always to remember that the welfare state ends at the sea wall!

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CHAPTER 1

A Brief History of Diesel Engine Development

Although the early development of the diesel engine is naturally connected with the name Rudolf Diesel, like other complex machinery its progress was a combination of the work of many separate individuals. While Rudolf Diesel was a first class engineer in his own right, serving his basic engineering apprenticeship in various engineering fields, it was without doubt his final designs and prototypes that formed the first true diesel engines as we know them today.

Due to the lack of effective communications in the nineteenth century there was little chance of liaison between engineers in different countries. Thus engineers working alone were designing and constructing experimental engines on similar lines, without any knowledge of anyone else’s advances.

Internal Combustion: the Way Forwards

The diesel engine was conceived along with the petrol engine over a period of years as an answer to the horrendous inefficiency of the steam engine, which was at best only about 10 per cent fuel efficient, wasting 90 per cent of the fuel energy being used. Furthermore the development of the steam engine had reached its peak, and no more significant improvements in fuel efficiency would be possible. It was well known that internal combustion was the way forwards, rather than the external combustion of the steam engine. Internal combustion would immediately save the loss of heat from which the steam engine suffered when transferring heat energy from the boiler to the cylinder.

During the early days the most conveniently available fuel source for stationary engines was town gas, and it was with this fuel that early experiments with internal combustion engines were conducted. The principle of the four-stroke cycle for internal combustion had been furnished by Frenchman Alphonse Beau de Rochas in 1862, while at the same time in Germany Nikolaus August Otto, unaware of the work of de Rochas, actually built a four-stroke engine; for many reasons this could not be made to run smoothly, but although it did actually manage to run, the idea was shelved.

In 1876, after further research, Otto developed an improved version of the four-stroke engine. One of the problems with early designs was the harsh clattering noise that the piston made as it reached the top of its stroke. To try to overcome this, the piston was allowed to travel further up the bore and thus increase the air pressure within to act as a damper for the piston before ignition. This development did indeed dramatically cut the mechanical noise of the engine, but more importantly it was found that the efficiency of the engine was hugely improved. At this point Otto patented the four-stroke cycle in Germany, and in fact it is still occasionally known as the ‘Otto’ cycle, especially by older engineers with more traditional apprentice training.

The next challenge was to free the engine from the gas pipe supplying it with fuel! Powdered coal was tried unsuccessfully, which led to the idea of using liquefied fuel. The first engines were petrol-fuelled and utilized crude carburettors and glowing platinum igniters. After much experimentation a low voltage electric spark was used for the ignition system, and the forerunner of today’s spark plug was born – an idea which incidentally had been tried in Italy some twenty years previously!

Rudolf Diesel

As a young engineer Rudolf Diesel began his career with Sulzer of Switzerland building ice-making machines. His first attempts at engine development were in the form of an improved type of steam engine but using ammonia rather than water vapour as the power source; this was not a success, however. He had followed with fascination the developments being made in the field of internal combustion, and having the benefit of a first class education and a sound understanding of thermo­dynamics, he turned his attention to his own theories of internal combustion.

At this stage in the development process fairly crude internal combustion engines were being produced, for use in factories to power machinery, and also modified for use as marine engines, particularly in fishing boats. Although all these machines tended to be low revving and produced relatively low power, they were vastly more fuel efficient than the steam engine.

Yorkshireman Herbert Akroyd Stuart designed a ‘hot bulb’ engine which was also known as a ‘semi-diesel’. It operated using the four-stroke cycle, with fuel sprayed into the combustion chamber at the top of the compression stroke. This fuel was ignited by a glowing hot metal bulb (as the name implies), which had to be preheated with a blow lamp for eight minutes before the engine could be started. Once it was running the heat of combustion maintained the temperature of the bulb so that the engine ran continuously. Although this engine was low powered and very unsophisticated it was built in the thousands for industrial and marine use, where in both cases it could be relied upon to give a long and reliable life.

Diesel’s designs were largely aimed at attaining better fuel efficiency, with the ultimate goal of 100 per cent – which as a realist he knew could not be achieved. He turned his attention back to the many lessons he had learned in his early years, and decided that for maximum efficiency an engine would need to run at extremely high pressure. He was impressed with the theoretical efficiency of the four-stroke cycle, and set about designing a four-stroke engine with compression ignition, working with a cylinder pressure of 300kg/cm2. At that time these pressures were only found in volcanoes and bombs, so to give his design greater credibility he halved the maximum pressure to 150kg/cm2 – but even these figures were considered unattainable and were declined by manufacturers.

After reworking his figures he submitted a revised design with a working pressure of 44kg/cm2, and this was accepted by Maschinenfabrik Augsburg (later to become the famous diesel engine manufacturers MAN), who agreed to build an experimental engine to see if the principle would work. With engineering standards at that time being fairly low due to the lack of high precision machinery, it took a great deal of time and experimentation just to achieve an effective seal within the cylinder; even­tually, however, the test equipment was reading pressures on the compression stroke approaching the designed levels.

On the first ignition test in 1894 the glass pressure indicator on the cylinder head exploded due to the unexpectedly high pressure that rapidly developed within the cylinder when the fuel was sprayed in. The rest of the engine was undamaged, and the compression ignition principle was dramatically proven.

The engine was redesigned and could be coaxed into life for short periods of time; although accompanied by violent detonations and clouds of smoke, it proved that a compression ignition engine could work. From then on it was a case of trial and error to improve on the original design.

Over the years there were various improvements, each offering greater fuel efficiency, but the overriding problem was the fuel injection system, which in early engines used an air blast system requiring a compressor to supply the pressurized air. It was not until the end of World War I that a satisfactory airless injection system was perfected and put into production.

A simplified view of direct diesel injection.

Indirect Injection

During the period between the first successful running of Rudolph Diesel’s first engine and the end of World War I there was also progress made with pre-­combustion chamber design, now known as indirect injection. This was developed by another engineer, Prosper l’Orange, along with an airless injection system which improved efficiency by removing the need for the auxiliary air compressor, which in itself cut down on power loss.

The pre-combustion chamber was a separate chamber in the cylinder head above the cylinder, into which air was forced by the rising of the piston during the compression stroke. This improved combustion and reduced detonation when the fuel was injected.

One of the benefits of the pre-combustion chamber design was found to be quieter combustion, although at a slight loss of thermal efficiency. It was an Englishman, Sir Harry Ricardo, who later perfected the system of indirect injection with his designs of swirl chamber; although this differed from the early pre-combustion chamber designs in that it was very much more sophisticated – air is induced to swirl around the specially shaped chamber, thereby ensuring a good air/fuel mix and an even burn – the concept is basically very similar. Despite being designed in the 1930s, the Ricardo Comet Mark V combustion chamber was still one of the most popular designs of swirl chamber among engine manufacturers until more recent developments superseded them.

A simplified view of indirect diesel injection.

In 1925 another personality well known in the field of modern diesel fuel systems was making his name with the successful development of an improved fuel injection system following the experiences of other engineers. This was Robert Bosch, who had already enjoyed much success with his development of magneto spark ignition systems on petrol engines. Bosch fuel injection systems are today used on a wide range of modern engines, while the basic design is incorporated into the products of many other manufacturers.

The Two-Stroke Engine

In the quest for greater efficiency most early diesel engineers agreed that if a four-stroke engine was reasonably efficient, then surely a two-stroke with twice as many power strokes for the same revolutions would be doubly efficient. This was not the case, however, as the two-stroke required an external means of getting sufficient air into the cylinder for adequate combustion. This was provided in the form of a mechanical blower (or supercharger), which supplied air under pressure to the cylinder.

The two-stroke diesel became the accepted type for use in large power units, especially in ships, where the slow revving nature of the engine allowed plenty of time for the intake of air and the outflow of exhaust. Not so many small two-stroke diesels were produced, however, although such notable names as Foden and Detroit Allison were (and still are) very successful with their smaller two-strokes.

In fact the Detroit two-stroke diesel is still seen on many large American cruisers, although British owners tend to opt for four-stroke options simply because the two-stroke diesel is even more alien to them than the four-stroke. From the late 1920s diesel technology progressed at a steady pace, with further refinements being added and greater power being extracted from ever smaller and lighter engines.

A Bedford 466 from the 1960s, producing a modest 140hp from a 7.6ltr block: a sought-after engine of its day.

Diesel Engines for Pleasure Craft

Although diesel engines advanced rapidly in the commercial and industrial field and became the standard power unit for ships and commercial craft of all types, it is only in the last forty years or so that it has come to be considered the standard powerplant for pleasure craft. After World War II the majority of smaller private vessels were petrol powered, and even when mass production of GRP craft began in the early sixties, the petrol engine was the favoured unit.

No doubt this was mainly due to the fact that petrol engines have always been cheaper to buy than diesels, and at the time petrol was a lot cheaper in real terms than it is now. The fact that small diesels were also significantly heavier and more bulky than their petrol equivalents probably also affected this decision. The significant number of cruising craft around today still fitted with their original petrol engines is evidence of this fact, although as they wear and spares run out, more and more are being converted to diesel.

In contrast the large number of US-built power boats now being sold in the UK has seen an increase in petrol power again, but this is simply because petrol is still cheap in the USA and there is little point in installing diesels at extra cost. Since the early 1970s, however, diesel progress has accelerated at an astonishing rate, and with the realization that oil supplies are not infinite, the quest for greater fuel economy naturally still leads to the diesel engine.

Improved fuel economy also brings with it increased overall efficiency, which leads to higher power-to-weight ratios. Thus in the last thirty years we have seen the change from petrol to diesel engines in leisure craft designed for serious cruising, and more recently even the high-speed express cruiser has gone over exclusively to diesel, with almost no loss of performance but a great improvement in fuel economy and lower fuel costs.

The number of diesel-powered cars on our roads has increased dramatically in the last two decades, and the staggering performance of many of them as they rush silently down the outside lane of the motorway is evidence enough of the progress made. This progress is now also being felt in the marine industry, with even small planing craft often being equipped with diesel powerplants.

One of the latest high-tech common-rail diesels from Volvo-Penta: the D4-300 producing 300hp from a 3.7ltr block.

As we have already seen, early diesel engines were very heavy and slow-revving, and compared to such old stalwarts of the small cruiser as the BMC 1.5 and 2.2, the later BL 1.8 and 2.5, the Ford 4D and the Bedford 220, all of which were popular with the DIY mariner in the not-too-distant past, they too in their turn are heavy, unsophisticated and slow-revving compared with the latest electronically controlled common-rail diesels now used in cars and trucks.

This new technology has found its way into the marine field, where it is not only high efficiency and low fuel consumption that is gained, but the clouds of smoke when starting on cold mornings are thankfully gone. Their compact size and high power-to-weight ratios indicate the advances that have been made in a relatively short time, and make the comparatively low cost automotive diesel engine available for marinizing and installing in smaller and smaller vessels.

In the same manner the larger engines from manufacturers such as Volvo, Cummins and Caterpillar are all producing sophisticated power units with power-to-weight ratios that could only be dreamed of a few years ago. Furthermore, with the added power output comes greater fuel economy per horse-power, increased reliability and clean exhausts.

As the worldwide demand for diesel engines increases there are sure to be many more improvements to performance, fuel efficiency and exhaust emissions before engineers come to the same conclusion that they did with steam engines: that no further progress is possible!

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CHAPTER 2

How the Diesel Engine Works

Before looking at the diesel engine in detail it is first worth considering the marine engine. The vast majority of modern marine engines on offer today begin their lives as basic vehicle or industrial units, and are then modified and adapted to suit the marine environment by the manufacturer.

With the high performance demanded from modern craft, the traditional marine engine with its low revs and heavy build designed to last for years cannot offer the power-to-weight ratio to satisfy this need. This is one reason why the lighter vehicle engine with high power-to-weight ratio has taken over the market; the other reason is cost. As we have seen, diesel technology is advancing all the time, with more power being extracted from smaller capacity units, which when carried to extremes inevitably leads to a shorter working life.

However, the modern high-performance diesel when in standard trim offers exceptional reliability and long life, and when fitted in a cruiser will almost certainly last the lifetime of the boat if maintained correctly at the appropriate intervals. What we know and accept as the marine diesel of today is far removed from the low-­revving, heavy monster with its origins in the earliest of diesel engine designs. The real marine engine is still in evidence in the vastly larger sizes used in ships, but the design of small high-performance engines, although greatly refined, would not be at all alien to Rudolf Diesel.

The Four-Stroke Diesel Engine

The four-stroke is by far the most popular of today’s small marine diesel engines, although two-strokes are still available from at least one manufacturer. As we have already seen, the four-stroke cycle (at least in theoretical terms) has been around for well over a hundred years, and nothing has yet been developed to better it.

The cycle is identical for both petrol and diesel engines, and the name refers to the four strokes (two up and two down) that are needed to complete one power cycle, and which encompass two revolutions of the crankshaft. The basic four-stroke diesel engine comprises a heavily built block to withstand the high internal pressures that develop during the combustion process. This contains the cylinders that may be a combination of any number from one to twenty or more, although in the smaller sizes which we are dealing with, six would be a normal maximum and occasionally eight, probably in V formation.

The internal workings of a simple diesel engine viewed from the front.

Detail of the closing of a valve (either inlet or exhaust).

Detail of the opening of a valve (either inlet or exhaust).

A typical injector mounting arrangement.

The tops of the cylinders are sealed with a cylinder head; this is usually a single unit where the engine is an automotive derivative, but it can also be in the form of separate heads for each cylinder on larger units and in heavy-duty designs such as used by Gardner engines. The advantage of separate cylinder heads is that work can be performed on one cylinder without the need to disturb all the rest, as happens with engines with a single universal head.

The reason for using a single universal head on the majority of engines is, of course, cost. Within the cylinder head are passages to allow combustion air and exhaust gases to enter and leave the cylinder. The movement of these gases is controlled by inlet and exhaust valves, which open and close at predetermined times during the cycle, and which seal the cylinder during the compression and firing strokes.

Timing gears must be correctly aligned to ensure that the valves open and close and that fuel is injected at the correct moment.

The cylinder head also contains one injector for each cylinder, its purpose being to inject fuel at a high enough pressure to overcome the internal pressure within the cylinder produced on the compression stroke.

Detail of a typical crankshaft.

The opening and closing of the inlet and exhaust valves is controlled by the camshaft, which is in turn driven via either a chain, gears or belt from the crankshaft. Within each cylinder is a piston which travels up and down. The gap between the cylinder walls and the piston is sealed with rings of cast iron, which are located in machined grooves in the piston body.

The rings are split at one point to allow them to expand outwards and maintain a constant pressure on the cylinder walls. There are usually three or four rings near the top of the piston and one near the bottom. The bottom ring and the lowest of the three or four top rings are usually oil-control (or scraper) rings, which instead of being solid iron are formed in a lattice arrangement. The grooves in which the oil-control rings are fitted have drillings through the piston body so that oil which has splashed or sprayed onto the cylinder walls for lubrication can be scraped off and returned to the sump via the drillings in the ring grooves.

When a piston and rings are assembled and fitted into a cylinder the rings are compressed by the cylinder walls and the split openings (known as gaps) are reduced to around 0.5 per cent of the cylinder diameter, which in a small engine would be in the region of 0.35mm. Nevertheless, it is very important to ensure that the gaps are staggered around the piston during assembly to ensure minimal cylinder pressure loss when running.

The lower part of the block below the cylinders supports the crankshaft in bearing carriers that are lined with replaceable white metal bearings. The number of carriers and bearings varies with engine make and design, but in general terms for maximum crankshaft support a 4-cylinder engine will have five bearings and a 6-cylinder engine will have seven. These would be known as a five-bearing and seven-bearing crank respectively. There are notable exceptions to this flexible rule, one of which is the old but very well known Perkins 4107 and its successor the 4108, which although being 4-cylinder engines use three-bearing cranks.

Four-stroke cycle induction.

Four-stroke cycle compression.

It is the job of the crankshaft to convert the up-and-down motion of the pistons into the rotary motion required to drive the propeller shaft via the gearbox. It is connected to the pistons via connecting rods, which also use replaceable white metal bearings at their lower end where they are clamped to the crankshaft, and bronze bearings at the top where they connect to the piston via the gudgeon pin. A sump at the bottom of the engine covers the crankshaft and bearings. It is used as the reservoir for lubricating oil, which is circulated around the engine after being picked up from the sump by the oil pump, and delivered via the oil filter to all parts of the engine.

The Four-Stroke Cycle

The inlet or induction stroke begins with the piston at the top of the cylinder; the inlet valve is already open, as it opened just before the piston reached the top of its stroke. As the piston begins its descent down the cylinder it draws fresh air in through the inlet valve, which closes as the piston reaches the bottom of the first stroke.

The compression stroke begins with both the inlet and exhaust valves closed to seal the cylinder. As the piston rises it compresses the air that was drawn into the cylinder on the inlet stroke. While the air pressure rises, the air becomes hotter until maximum compression is reached at top dead centre with the air temperature reaching more than 525°C.

Four-stroke cycle combustion.

The combustion or firing stroke begins just before top dead centre is reached, as the piston is still rising at the end of the compression stroke. At the appropriate moment just before top dead centre a precisely metered amount of fuel is injected at a pressure high enough to overcome the already considerable pressure within the cylinder via the injector (or atomizer) in the form of a fine spray, which is immediately ignited by the heat of the compressed air within the cylinder. As the fuel and air mixture burns, it expands rapidly, forcing the piston down the cylinder until just before bottom dead centre when the exhaust valve opens.

Four-stroke cycle exhaust.

The exhaust stroke begins with the opening of the exhaust valve just prior to the piston beginning its ascent of the cylinder on the final stroke of the cycle. With the exhaust valve open, the piston forces the burnt gases out of the cylinder ready to accept a fresh charge of air on the next inlet stroke. The inlet valve opens just before top dead centre and while the exhaust valve is still open. This allows the fresh inlet air to be partially drawn in by the last of the exhaust gases, and also helps to ensure that the exhaust gases are completely cleared from the cylinder. The exhaust valve then closes as the fresh charge of inlet air is drawn in on the new inlet stroke.

The Two-Stroke Diesel Engine

We will not be dealing in depth with the two-stroke diesel as it is not particularly popular in the UK with owners and operators of small craft, although there are exceptions. This section is included to indicate the differences in operation between the two-stroke and four-stroke engine should the reader ever come across a two-stroke or be contemplating buying a boat powered by one. As we have already seen, the two-stroke diesel is normally found in large, low-revving ship engines where it really excels itself, and the principle of operation is the same whether the engine is producing 10,000hp or 200hp.

As already mentioned, when the two-stroke engine was first conceived the theory existed that as it produced a firing stroke on every second stroke instead of on every fourth stroke of the four-stroke engine, it must be twice as powerful as a four-stroke of similar bore and stroke. Unfortunately, additional practical problems were found that negated a lot of the theoretical additional efficiency, and the modern two-stroke diesel is now considered to be about one and a half times more powerful than a comparable four-stroke. The biggest problem was found to be how to remove the burnt exhaust gas from the cylinder and replace it with a fresh charge of combustion air in the space of one stroke of the piston. The eventual answer was to force the fresh air into the cylinder under pressure from an air pump or supercharger.

Being mechanically driven, the air pump required a proportion of the engine’s power to drive it, which further lowered the theoretical power output. However, having an excess amount of air forced into the cylinder by a supercharger ensured that exhaust gases were rapidly expelled and a plentiful charge of combustion air was available for the power stroke. This explains why the two-stroke is popular in large low-revving engines, in that the low revs allow plenty of time for the exhaust gases to be expelled from the cylinder and replaced with a charge of fresh combustion air in the space of one stroke of the piston. As engine speeds increase, the time available for the transference of exhaust gases and air becomes progressively shorter.

Unlike the four-stroke diesel engine, which is mechanically almost identical to the petrol four-stroke, the two-stroke diesel is very different to the two-stroke petrol engine used in motorcycles and outboard motors. In fact it bears more resemblance to the four-stroke diesel in that it has a heavily built block containing the cylinders, the tops of the cylinders are sealed with a cylinder head, the cylinders contain pistons connected to the crankshaft via connecting rods, and the crankshaft is located in bearing carriers at the bottom of the block, which is covered with an oil reservoir sump.

The main differences occur in the cylinder head and the cylinder itself. Instead of having inlet and exhaust passages, the two-stroke cylinder head has only an exhaust passage plus an exhaust valve controlled by a camshaft. The inlet passage is located towards the bottom of the cylinder, and enters the cylinder at the inlet port in the cylinder wall. The port opens and closes due to the movement of the piston, and has no mechanical valve gear like the exhaust valve. When the piston moves up the cylinder the piston body covers and seals the inlet port, and when it moves towards the bottom of the cylinder it uncovers the port. In practice the inlet port consists of several openings around the cylinder to allow the admission of as much air as possible in the short time that the ports are uncovered.