Diesels Afloat E-Book

Callum Smedley

• Herausgeber: Fernhurst Books Limited
• Kategorie: Lebensstil
• Sprache: Englisch

Veröffentlichungsjahr: 2022

Beschreibung

Diesel engines are installed in just about every yacht and in most large motorboats and, while professional help is often at hand, sometimes it is not. Indeed, engine failure is one of the most frequent causes of RNLI launches. This book explains how to prevent problems, troubleshoot and make repairs using safe techniques. It could also help you save money on expensive bills for yard work you could do yourself. Diesels Afloat covers everything from how the diesel engine works to engine electrics, from fault finding to out of season layup. With this guide and your engine's manual you can get the best performance from your boat's engine and be confident in dealing with any problem. The book covers the syllabus of the RYA Diesel Engine and MCA Approved Engine (AEC-1) courses. This edition has been thoroughly modernised and updated by former course lecturer and currently chief engineer on merchant ships, Callum Smedley.

Leseprobe

THE AUTHORS

PAT MANLEY & CALLUM SMEDLEY

PAT MANLEY

Pat Manley was a long-time contributor to the magazine Practical Boat Owner and regularly featured in their Ask the Experts pages. He also wrote a number of successful books for Fernhurst Books: Simple Boat Maintenance, Essential Boat Electrics, Practical Navigation for the Modern Boat Owner, Electrics Companion, Diesel Companion and Radar Companion.

His books, articles and the numerous RYA courses he ran inspired thousands of sailors with practical advice that meant that they could understand and repair their boats.

Pat passed away in September 2016.

This left Fernhurst Books with the problem of finding someone to update Pat’s work when the time came.

We are delighted that Callum Smedley has updated this book.

CALLUM SMEDLEY

Based in the Shetland Islands, Callum Smedley has spent a lifetime at sea or teaching seafarers.

He started off as an engineering cadet at Clyde Marine Training before working at sea with Mobil, P&O Nedlloyd, Shetland Islands Council Ferries and the Northern Lighthouse Board. He has risen through the ranks to be chief engineer – responsible for their engines, electrics and machinery.

While working at sea and between such roles he has taught full or part time at the North Atlantic Fisheries College / NAFC Marine Centre (now called UHI Shetland) teaching engineering and MCA classes. He was promoted to be a section leader, responsible for the day-to-day running of the department and ran the HNC programme for officer cadets and all Certificate of Competency (CoC) classes, including the MCA’s Small Diesel Course known as the Approved Engine Course (ACE1).

He also runs a small marine surveying company for coded vessels, pleasure craft and fishing boats.

PREFACE

Machinery failure is the biggest cause of leisure craft requiring assistance. It therefore must be true that some knowledge of the operation of diesel engines and their systems, their maintenance and the ability to carry out simple repairs would reduce the need for the rescue services to attend such craft significantly.

The RYA*’s desire to improve this situation led to the introduction of the RYA* Diesel Engine Course in 1996, and I became one of its earliest instructors. Diesels Afloat incorporates much of the feedback from yachtsmen who have attended my RYA* Diesel Engine courses since their inception and covers the complete syllabus in depth. It answers the questions that I’m asked most frequently, and which aren’t answered in other books. However, there’s much additional information that I consider essential for a proper understanding of a diesel engine’s working and its operation by the skipper.

Troubleshooting is often covered by tables giving no definitive answer. I believe that a proper understanding of the diesel engine makes these tables superfluous and, instead, use the much more helpful descriptive text combined with a proper understanding of the various systems.

A boat’s engine cannot be considered alone. There’s a need to convert its power to propel the boat and this part of the system is often considered a black art. As I am often being asked about propellers, both by students and Practical Boat Owner magazine readers, I have devoted a chapter to this subject.

Pat Manley – March 2007

I was asked if I would like to update Pat Manley’s book, Diesels Afloat in early 2020. This was because, as a college lecturer, I used the book for the MCA Approved Engine Course Part One, along with some of my own teaching material. I thought this would be a worthwhile project as I could ensure the book fully covered the syllabus for this course as well as the RYA* Diesel Engine Course. Over the years the MCA 30-hour diesel course has become the Approved Engine Course Part One (AEC 1) and the syllabus has recently been updated (February 2020). This course is used for the following sectors in the UK maritime industry:

■ Workboats, up to 24 metres, operating in category 2, 1 and 0 waters

■ Fishing Vessel 16.5m Skipper’s Ticket, to remove the 20-mile limitation

■ Commercial Sailing Yachts of various powers and gross tonnage up to 60 miles from a safe haven

■ Commercial Power-Driven Yachts of various powers and gross tonnage up to 150 miles from a safe haven

The AEC 1 is also used as a starting point for the Marine Engine Operator’s Licence and the Small Vessel Certificates of Competency.

Callum Smedley – March 2022

* RYA is a trademark of the Royal Yachting Association

CONTENTS

Introduction

1. The Diesel Engine & Hull Types

2. Cycle of Operation & Construction

3. The Fuel System

4. The Air System

5. The Engine Cooling System

6. Lubrication

7. Engine Electrics

8. Power Transmission

9. Propellers

10. Electronic Engine Management

11. Hull Fittings

12. Pollution & Safety

13. Operation, Care & Maintenance

14. Troubleshooting

15. Layup & Recommissioning

16. Tools & Spares

Acknowledgements

INTRODUCTION

During the 1930s, with the increasing popularity of sailing cruising yachts as a leisure pursuit, fitting an auxiliary engine became common. The unwillingness of these small petrol engines to start, and their general unreliability, meant that their use tended to be confined to occasions when there was no wind, and even then, many yachtsmen sailed as if they had no engine. They were, in all senses of the word, auxiliary engines and were hated and distrusted by many.

As auxiliary engines became more common, a rule of thumb developed such that their size for any given yacht was about 2 horsepower for every ton of displacement. This was adequate for getting home when the wind was too light to sail but was not too much use for battling into wind and sea. But, as it was a sailing boat after all, that did not matter.

When small, reliable marine diesel engines became available in the 1960s, coupled with the need to get home to go to work, engine powers were increased, such that 4 horsepower per ton of displacement became the norm. This allowed sailing boats to be motored into wind and wave and make progress, and in calm water they could achieve their maximum ‘hull speed’. That is, they could go as fast as was economically sensible under power. Displacement motorboats could use the same rule of thumb but, as they had no alternative means of propulsion, were often given a bit more power.

Into the 1990s, there came a tendency by some boat builders to ‘up’ the power to around 6 horsepower per ton displacement for sailing boats. In many ways this was because, although the sailing performance of most current sailing yachts was very good, they were often used as motor sailing boats. Many owners reach for the engine starter if the boatspeed drops below 5 knots or if they have to go to windward.

However, this relative overpowering brings a hidden cost. Diesel engines must be worked hard, as we shall demonstrate later, so the boat must be cruised at a speed higher than its ‘hull speed’, with its attendant large increase in fuel consumption.

Some motor cruisers have a similar problem to such sailing boats, in that they are overpowered for the conditions in which they will be used. Displacement motorboats would be adequately powered at 4 to 6hp per ton displacement, which is generally fine. Planing boats need much more and, generally speaking, are ‘cruised’ at high power and high speed. It is these boats that experience problems when run at low speed, as may be dictated by inland waterway speed restrictions. The need for turbocharged diesels to be run under load is even more demanding than for those normally aspirated.

Boats that are going to be used only on inland waterways should be powered accordingly. This is sometimes seen as a potential liability when it comes time to sell the boat, so is often ignored. Twin-engine boats can be run safely on one engine at a time, so that the engine can be run at higher power, swapping engines to equalise the hours run. Be aware though that, to save production costs, some twin-engine boats have a power-steering pump fitted to just one engine, so single engine running on the ‘wrong’ engine can produce interesting results. I was assured by a salesman that a twin-engine boat had a pump on each engine, despite it being obvious that only one was fitted. Running the engines one at a time showed the salesman that he was in error.

Rather than the sailing boat’s engine being an auxiliary, it has become the alternative means of propulsion. Obviously for a single-engine motorboat, it’s the only means of propulsion, whereas a twin-engine motorboat still has an engine should the other fail, provided that you can still steer the boat.

Most workboats and fishing vessels are solely powered by diesel engines, with fishing vessels tending to be single engine and newer workboats tending to be twin engine. In addition to using diesel engines to move the vessel, a lot of fishing vessels and workboats will have an auxiliary engine for electrical power and sometimes hydraulic power. These vessels also tend to have a more complex gearbox, because of power take offs (PTOs) for hydraulic pumps, that can be used for deck winches, cranes and bow thrusters.

Because of these gearboxes and the size of workboats and fishing vessels, the diesel engines fitted can be quite powerful, but they work in just the same way as a small engine in a sail yacht.

Because car engines have become so reliable, many boat owners take the same attitude towards their boat’s engine as they do their car’s. But there’s a significant difference – the boat is at sea, where there can be significant corrosion problems, and you can’t just pull to the side of the road and stop as you can on the road.

This book will look at all the aspects of the various parts of the engine and its systems, maintenance, troubleshooting and use of your marine diesel engine so that you can get the best out of it with the least chance of it letting you down, while following the syllabus for the RYA* Diesel Engine and MCA AEC1 courses.

The engine’s handbook is there to be read. It’s quite amazing the interesting and useful things you will find in it. There’s a whole generation of Volvo Penta 2000 series engine owners who just do not know how the engine should be started from cold, despite clear instructions being included in the handbook. As we tell all our students – READ THE HANDBOOK!

CHAPTER 1

THE DIESEL ENGINE & HULL TYPES

HOW IT ALL STARTED

Rudolf Diesel was granted the first patent for a diesel engine in 1892, when petrol engines were in their infancy. Whereas petrol engines could be built small enough to be put in a motor car, the diesel engine was on a different scale completely. Early examples were 3 metres tall!

An early diesel engine

Although diesels were used in German flying boats and even Zeppelins in the 1930s, they were really too big and heavy and were not considered a success.

It was not until the late 1950s that any real success was achieved in building small, relatively lightweight diesels for use in small leisure craft. These were one-, two- and three-cylinder engines revving at around 2,300rpm and developing from 7 to 35hp. They were quite heavy and bulky, but the smallest could be fitted into a 20-foot boat.

In 1970 Petter produced a 6hp single-cylinder engine built mainly from aluminium and derived from one of their small industrial units. This was very compact, light in weight and revved at 1,500rpm. This was quickly followed by a two-cylinder 12hp version.

Larger boats needed more power and, by this time, there were a number of smaller diesel-powered cars on the market, some of which were marinised by independent companies to give engines producing 35 to 45hp. Larger motor and workboats used marinised truck engines.

Planing motor cruisers need lots of power and relatively light weight, and it’s here that the modern turbo-charged truck engine plays its part.

All modern marine diesels destined for the leisure market, and some of the commercial market, are marinised versions of automotive or industrial engines. This marinisation may be carried out by the original engine builder or by an independent marinising company.

THE MODERN DIESEL ENGINE

Compared with its forbears, the modern diesel is light in weight and relatively high revving. It is in all ways comparable to the modern petrol engine but more economical to run and a little more expensive to buy.

Modern diesels range from around 10 horsepower right up to 1,000 or so in the leisure and commercial engine ranges, for under 24m vessels. Some of the newer larger engines do have electronic control and / or management systems, but the vast majority of small diesels are purely mechanical, apart from the starter motor and sometimes a fuel solenoid.

The type of engine we use should be dictated by the use to which it will be put.

Above: Cutaway drawing of a small modern diesel engine and its gearbox

Modern Yanmar diesel engine

Displacement Hulls

Where the boat’s speed is limited by its waterline length, relatively low power is required. The old rule of thumb was 2hp per ton displacement. Much more realistic in these days of needing to get home to go to work would be 4hp per ton. Many builders seem to be offering as much as 6hp per ton or even more, but this brings with it problems of high fuel consumption and engines that are run at far too low a power for normal cruising. Diesels need to be worked hard, so it’s no good saying that I won’t use all that extra power that I’ve installed ‘just in case’. If you don’t work them hard you are storing up problems for later, and sometimes sooner, in their life.

Displacement motorboats are normally cruised at a constant speed and the engine is in use all the time. The engine gets warmed up properly and, as long as it doesn’t have too many hp per ton, it gets a reasonably easy life.

A sailing boat’s engine has a much harder time, as often it doesn’t reach normal running temperature before it’s stopped. It’s also often used at relatively low power when ‘motor sailing’. These conditions are not good for a diesel, and even less good if it’s turbo-charged. Unless there’s just no suitable non-turbo engine of the power required, we’d suggest avoiding a turbocharged engine in a sailing yacht.

A modern 36-foot yacht weighing 6 tons needs 24hp by the 4hp per ton rule. This would give a cruising speed of 6.6 knots with a fuel consumption of 11.5mpg. Install a 40hp engine, as many builders do, and you get a 7.3-knot cruising speed and 7.3mpg. Cruise that 40hp engine at 6.6 knots and you are using only 12.5hp, under a third of its rated power rather than the minimum recommended 50% (that’s power, not rpm). The argument about having extra power for heavy weather has a serious hole in it. If you bear away about 20 to 30 degrees from the direction of the waves, you’ll go faster, use less fuel and have a much more comfortable ride!

While all of the above is true for a pleasure yacht, workboats and fishing vessels do tend to have more power. For example, this table shows information from three different workboats, for the hp per ton:

Power

hp to tons

Workboat 1

Unloaded displacement 22 ton

200hp

9.1

Loaded displacement 30 ton

200hp

6.7

Workboat 2

Unloaded displacement 36 ton

500hp

13.9

Loaded displacement 50 ton

500hp

10

Workboat 3

Unloaded displacement 70 ton

760hp

10.9

Loaded displacement 130 ton

760 hp

5.8

It is interesting to see how much this ratio changes when fully loaded. Workboat 1 and workboat 2 boat have a PTO (power take off) for deck hydraulics such as capstans and a crane. While workboat 3 has an auxiliary for this, so all power is used to drive the vessel. Some workboats and a lot of fishing vessels have to tow, so this means a lot of power. The propeller for a workboat that tows is very different from a fast motor cruiser. A bit like a race car and a tractor, both can have the same power, but use it completely differently.

Semi-Displacement & Planing Hulls

These hulls need much more power than a displacement hull. Because of the demands that the engine should be as light and compact as possible, these engines are normally turbocharged and can have electronic engine management. To save carrying redundant weight, these engines are normally cruised at about 300rpm below maximum continuous rpm. Heavy weather will require a reduction of speed, so you don’t need any extra power.

Hull design and desired cruising speed affects the power requirement and it’s not easy to use any rule of thumb, as it is for a displacement hull. Once the hull gets beyond ‘displacement speed’ you’ll almost certainly be using enough power to avoid problems caused by running a diesel at too low a power. If you are forced to slow to displacement speed and you’ve got two engines, shut one down if safe to do so.

These hulls are used for pleasure craft and fast workboats. These workboats don’t tend to carry much cargo, but often up to a maximum of 15 people.

GETTING THE POWER TO THE PROP

This can be done in a number of ways; some of the methods shown below would only be found on yachts, such as sail drive, whereas most commercial vessels will use a form of shaft drive. Some vessels will not use a propeller! In which case a water jet is used, this is basically a large pump, and the discharge can be vectored to give ahead or astern motion, as well as helm control.

Shaft Drive

Traditionally, the propeller, or screw, is mounted directly on a shaft extending aft from the engine’s gearbox and exiting through a waterproof gland towards the rear of the hull. Traditional hulls were relatively deep, and the shaft could exit more or less horizontally. Modern hulls are relatively shallow so, if the downwards angle of the shaft is not to be too great, the engine needs to be mounted fairly well forward in the hull, but this may then intrude on the accommodation space.

Above: Conventional shaft drive: engine further aft, but more down angle of thrust making it less efficient

Conventional shaft drive: engine further forward, giving more horizontal thrust line but with the engine impinging on the accommodation

Advantages:

■ Simple design

■ Relatively cheap to make

■ Easy maintenance

■ Thrust bearings can be used so that no thrust load goes into the gearbox or through the engine mounts

Disadvantages:

■ Engine and shaft need proper alignment if wear and vibration are to be reduced

■ The thrust line may be angled downwards

An alternative solution is to use several shafts and angled gearboxes, either in the form of a Z drive or a V drive, to keep the engine further aft. This solution may help the weight distribution on some planing boats. Z and V drives are heavier and more costly than simple shaft drives.

Z drive (above)

V drive

Stern Drive

Many planing motor cruisers have stern drives. The engine is mounted right at the rear of the boat and drives the propeller mounted to the rear of the boat’s transom through a stern drive leg and gearbox. The leg tilts to adjust the planing trim, and swivels to achieve steering. The boat has no rudder. If you like, it’s a bit like an outboard engine but with the engine unit inside the boat. Driving a boat with a stern drive needs a different technique than that for a shaft drive.

Stern drive

Advantages:

■ Engine weight can be kept far aft, an advantage in planing boats

■ Installation costs are reduced with no engine alignment costs

■ With aft cockpit boats, engine accessibility is good

■ With aft cockpit boats, engines do not intrude into the accommodation

■ Thrust angle can be ‘trimmed’ from a basic horizontal thrust line

■ Speed is potentially greater than with a shaft drive

Disadvantages:

■ More expensive to build

■ Externally mounted leg and drive unit needs frequent and expensive maintenance

■ Electrolytic corrosion of ‘out-drive’ unit in salt water

■ Boat has to be out of the water to service the gearbox / leg unit

■ With a deep ‘V’ hull configuration and twin engines, the engines have to be mounted so close together that servicing can be almost impossible

Sail Drives

A sail drive engine has its gearbox, leg and propeller all mounted as one unit, with the leg exiting through a hole in the bottom of the boat. This allows the engine to be mounted where it won’t interfere with the accommodation but keeps the propeller’s driving axis horizontal.

Sail drive: normal arrangement (above)

Sail drive: engine reversed on leg giving more room for accommodation but also more weight aft

Advantages:

■ Installation costs are minimal

■ No engine alignment required

■ More choice of engine position, so its intrusion on the accommodation can be minimised

■ Often less vibration (no shaft vibration as there would be in a poorly aligned shaft drive)

Disadvantages:

■ Large rubber diaphragm sealing hole in hull requires expensive replacement (every seven years for Volvo Penta, but not for Yanmar which has a double diaphragm and a moisture detector)

■ Some types of engine require the boat to be out of the water for oil changes

■ Possible corrosion of aluminium leg components in seawater

■ Electrolytic corrosion of larger propellers as the leg anode is relatively small and often electrically isolated from the propeller

■ Propeller mounted much further from the rudder, requiring more anticipation in close quarter manoeuvring

■ External water temperature may result in a non-optimal gearbox / leg lubricant

■ External water temperature may require non-standard battery charging until leg oil temperature has risen sufficiently to reduce friction drag

■ Cannot use a thrust bearing, so all thrust is taken by the engine mounts and gearbox

View of Yanmar sail drive sealing arrangements, showing the double diaphragm type seal

Volvo IPS

Introduced in 2004, Volvo’s revolutionary IPS combines most of the advantages of the shaft and stern drives in one unit. It’s a bit like a forward-facing sail drive with a steerable leg protruding from under the hull. The engine and drive are supplied complete and installed in pairs in fast motor cruisers.

Volvo Penta IPS

Advantages:

■ Horizontal thrust line for higher speed potential

■ Propeller in front of ‘leg’ in clear water

■ Exhaust about 80cm below the waterline to give very quiet running

■ Steerable legs, giving good manoeuvrability

■ Cast bronze underwater unit, giving good corrosion resistance

■ Lower installation cost

Disadvantages:

■ High unit cost

■ Available only with a couple of 4-500hp Volvo Penta engines

COMPRESSION IGNITION

A diesel engine has no ignition system or sparking plugs. Diesel fuel ignites at a temperature of around 320° Celsius. (Some writers give the ignition point as 900°C. This arises from one document which translated °C to °F but then labelled the result in °C – many other writers followed suit!) So what ignites the fuel and allows the engine to run?

When air is compressed, the effect on the air is to increase its internal energy and thus its temperature. Provided that the air is compressed rapidly enough so that the heat has little time to escape to its surroundings, the air in a diesel engine cylinder can be made to rise to above the ignition temperature of the fuel by compression alone. If diesel fuel is then injected into the hot air, the mixture will ignite, releasing energy. This is known as compression ignition, unlike a petrol engine which uses spark ignition to ignite the fuel / air mixture (see later).

Let’s imagine an elephant jumping from a height onto a bag of cool air! And let’s imagine that, at the same time, an archer shoots an arrow full of diesel fuel aimed to arrive at the bag of air at exactly the same time as the elephant. As the bag of air is very rapidly compressed by the arrival of the elephant, the arrow with exactly the correct amount of fuel arrives and penetrates the bag of now very hot air. There’s only one inevitable outcome: the elephant gets a free ride!

Very simplistic, I know, but the basic diesel engine is as simple as that. If the air is heated to above the combustion temperature of the fuel very rapidly AND if the correct amount of fuel is injected into this hot air at the correct time, the engine will run. No electricity is required, except to turn the engine over fast enough to start the engine, and this can be done by hand on a small engine.

The principle of compression ignition

Compression of the air takes place in the engine cylinder by reducing the volume of the air by around 20 times. In other words, there is a compression ratio of 20:1 (see page 28 for more detail on this). The compression ratio by itself is of no use unless the air cannot leak out of the cylinder as the volume is reduced. Air is prevented from leaking past the piston as it moves in the cylinder by means of one or more piston rings, which press outwards against the cylinder wall to form a seal.

SPARK IGNITION

The spark ignition, or better known as the petrol engine, is used on various vessels, normally for outboard engines and deck mounted hydraulic power packs for a hauler on a small fishing vessel used for potting.

The main parts of the engine are similar to a diesel engine but, instead of using the hot air from compression to ignite the fuel-air mix, a spark is used. The air in a petrol engine is still compressed, but not as much as in a diesel engine. Then, rather than waiting for the fuel to start burning because of the heat in the combustion chamber, a spark is introduced at the right time, and the fuel burns rapidly.

Because of the lower pressures inside a petrol engine, they tend to be lighter. In fact, the compression ratio can be in the region of 10:1 to 14:1.

The voltage used to make the spark can be quite high, but the power used comes from the battery at 12v and is then transformed up to between 12,000v to 25,000v in the coil. This is why the leads to the tops of the spark plugs are well insulated. If the coil, plugs or leads get damp this can affect the voltage, and hence engine power and starting.

Petrol engine showing HT leads

The principle of spark ignition

CHAPTER 2

CYCLE OF OPERATION & CONSTRUCTION

THE FOUR-STROKE CYCLE

Most diesel and petrol engines use the four-stroke cycle; however, we are only going to look at the diesel four-stroke.

The elements in the cycle are shown below:

1 (above):Induction: Air is ‘sucked’ into the cylinder as the piston moves down with the inlet valve open – induction stroke

2 (above):Compression: Air is compressed and heated as the piston rises in the cylinder – compression stroke

3 (above):Injection: Fuel is injected into the hot air as the piston nears the top of its compression stroke – ignition

4 (above):Power: Work is done on the piston by the rapid spontaneous combustion of the air and fuel mixture pushing the piston down – power stroke

5 (above):Exhaust: The piston rises, pushing the burned air / fuel mixture out of the cylinder with the exhaust valve open – exhaust stroke

Note that the piston moves up and down twice for each ‘bang’ or power stroke. The valves operate only once for each two revolutions of the engine crankshaft. You get only one bang for each two revolutions in a single-cylinder, four-stroke engine.

The essential requirements for a diesel engine to start are:

■ Adequate compression, supplied by the cylinder bore, piston rings and valve seats all being in excellent condition so that the temperature of the air is raised to the ignition temperature of the fuel

■ Rotating the engine quickly enough to obtain rapid compression to minimise the escape of heat from the cylinder – this requires a well-charged battery of sufficient power if electric starting is used

■ The correct quantity and grade of fuel injected at the correct time

It can be seen that a diesel engine in good mechanical condition will start if it is turned over rapidly enough to raise the air temperature to ignition point and the correct quantity of fuel is

injected at the right time.

It should be noted that other than powering the starter motor, the basic diesel engine requires no electricity for its operation.

THE TWO-STROKE CYCLE

This is a very different beast from the four-stroke engine and also bears little resemblance to a two-stroke petrol engine. Again we are only going to look at the diesel cycle and there is only one player in the field – Detroit Diesels.

Detroit two-stroke diesel engine

Because the four parts of the induction – compression – power – exhaust cycle are compressed into only one up and one down stroke in the two-stroke engine, it cannot compress the air enough to raise its temperature to the ignition point of diesel fuel unless there is some additional compression. The additional compression is provided by a gear-driven supercharger that compresses the air before it passes into the cylinder. Thus, the cylinder is supplied with pre-heated compressed air.

The smallest engine in the Detroit marine engine range is 400hp.

The cycle is as shown below:

1 (above):Inlet: At the lower portion of the piston’s stroke, inlet ports in the side of the cylinder wall are uncovered, allowing compressed (and therefore heated) air from the super-charger to enter the cylinder – inlet phase

2 (above):Compression: Once the inlet ports are covered by the upward moving piston, the air is compressed in the cylinder. Because the air has been compressed in the super-charger, by the time the piston reaches the top of its stroke, the air is above the ignition point of the fuel – compression phase

3 (above):Ignition: With the piston approaching the top of its stroke, fuel is injected into the very hot air, where it ignites and burns very rapidly – ignition phase

4 (above):Power: The force created by the rapidly burning mixture pushes the piston downwards – power phase

5 (above):Exhaust: As the piston approaches the bottom of its stroke, the two exhaust valves open, allowing the exhaust gases to escape. At the same time, the inlet ports are uncovered, allowing compressed air into the cylinder. This incoming air helps scavenge the exhaust gases from the cylinder – exhaust phase

The two-stroke diesel has a power ‘stroke’ once every revolution and so has twice as many ‘bangs’ as a four-stroke engine at the same rpm, so should give twice as much power as a similar sized four-stroke engine. However, its ‘breathing’ is not as efficient as a four-stroke, so, in reality, their power outputs are similar. Detroit Diesels claim very good fuel efficiency, but they now use them only for ‘off-road’ engines.

MULTI-CYLINDER ENGINES

Two- and four-stroke multi-cylinder engines have the crankshaft and valve timing arranged so that the ‘bangs’ don’t all occur at the same time. The ‘firing order’ for the cylinders is designed to give the smoothest running. Some engines use gear-driven counterbalance weights in the crankcase to give even smoother running.

ENGINE COMPONENTS

In mechanical terms, the internal construction of a diesel engine is similar to its petrol counterpart. Components such as the block, pistons, connecting rods and a crankshaft are present in both, but they do need to be stronger in the diesel due to the higher internal pressures and temperatures found. A lot of the parts, such as the turbo-charger and injectors are covered in detail in their own chapters.

The cylinder block is the foundation on which the engine is built. Most small diesel engines have the cylinder block and crankcase cast as one piece. It must be rigid enough to support the weight of rest of the engine and the crankshaft, which sits in the main bearing housings of the crankcase.

In line 4-cylinder engine block

The cylinder liners sit in the block and are held in place by the cylinder head. The pistons move up and down inside the liners.

Diesel engine cylinder liners (note this is a two-stroke liner as inlet ports can be seen)

The crankshaft is the part of an engine that translates reciprocating linear piston motion into rotating motion. It also drives the camshaft and is where the power output for the engine is taken from.

Diesel engine crankshaft

The flywheel is located on one end of the crankshaft and serves three purposes:

■ It stores energy from the power stroke in its mass and reduces vibration by releasing that energy which continues to turn the engine through the 3 non-power strokes until the next power stroke

■ It is the mounting surface used to bolt the engine up to its load (e.g. gearbox)

■ The gear teeth around its perimeter allow the starting motor to engage and crank the diesel engine

Diesel engine flywheel

The piston forms the lower part of the combustion chamber and is forced down by the rapid rise in pressure.

Pistons: top 2 cut-aways to show inside; bottom shows external view

When the piston is forced down, the connecting rod, which links the piston to the crankshaft, oscillates, turning the crankshaft (see crankshaft photo opposite).

The cylinder head is bolted to the block and is used to hold the cylinder liner in place. The cylinder head forms the top of the combustion chamber. It also houses the valves, that allow air into and exhaust gas out of the combustion chamber. The air inlet and exhaust manifolds are bolted onto the cylinder heads. When the valves are shut, the combustion chamber is effectively gas tight. The fuel injector is also mounted in the cylinder head.

Cylinder head

Cylinder head

The camshaft is driven by the crankshaft, via the timing gear, chain or belt. The purpose of the camshaft is to control the valves in the cylinder head and, in some engines, drive the high-pressure fuel pumps. In other engines, the timing gear drives the high-pressure fuel pumps.

In a four-stroke engine the crankshaft has to complete two full revolutions per complete cycle; the camshaft controls the whole cycle, so it only has to turn once. In other words, the camshaft is turning at half the speed of the crankshaft.

In a two-stroke engine the crankshaft has only to turn one revolution to complete the cycle, just like the camshaft. Therefore, the crankshaft and camshaft are both turning at the same speed.

Camshaft

Below: The cam rotates giving the push rod linear motion, up and down; this is used to operate valves in the cylinder head

Cylinder Head Valves & Their Drive From The Camshaft

There are a number of methods to open and close the valves.

Push-Rod-Operated Valves

The camshaft is mounted low in the engine and is gear driven. Long push rods operate the overhead valve gear. This is a cheap and efficient option, but the inertia of the push rods makes it unsuited to high-speed engines. Where the engine is based on an industrial engine or is a dedicated marine engine, this will be the norm. Adjustment of valve clearance is relatively easy (see page 187).

Push-rod-operated valves; the timing gears are also shown

Decompressors

Older non-automotive engines, with push-rodoperated inlet valves can have decompressors. These allow the engine to be turned over with no compression as an aid to starting or for maintenance by partially opening the inlet valve. It’s essential that this is adjusted so that there’s no contact between the inlet valve stem and the decompressor arm when in the ‘parked’ position to avoid damage.

You can’t hand start a diesel against compression, but if the decompressor(s) is / are used, the engine can be rotated by hand up to a speed at which it will start. Closing the decompressors will then allow the engine to start. The flywheel is very important here. Some multi-cylinder engines allow cylinders to be decompressed individually. When up to speed, close only one decompressor until the engine starts on one cylinder, then close the rest.

With a low battery, starting with the decompressors open will allow the depleted battery to turn the engine fast enough to start; the decompressors can then be closed.

Decompression levers on an engine

Overhead Camshaft-Operated Valves

Engines designed for high rpm will be fitted with one or more overhead camshafts. These engines will be derived from car diesel engines. The overhead camshaft will be driven by a gear train, chain and sprocket or by a rubber cambelt.

Gear-Driven Overhead Camshaft

This is reliable, heavy and expensive.

Timing gears

Chain-Driven Camshaft

This is lighter and cheaper than using gears. It uses engine oil pressure to operate the automatic chain tensioner and, provided that the engine oil is changed at least as regularly as specified, it is reliable.

Chain-driven camshaft

Toothed Cambelt Drive

The cheapest and least reliable option is the use of a rubber cambelt. This has long been popular in the car industry but requires the cambelt to be changed at regular intervals if hugely expensive engine repair bills are to be avoided. Independent car experts agree that the cambelt should be changed at 40,000-mile intervals, whatever the manufacturer says, and that the tensioner, if plastic, should be changed at the same time. The Ford Escort engine (which forms the basis of some older marinised engines) requires a cambelt change at 30,000-mile intervals. (40,000 road miles equates to around 800 engine hours at sea.)