2 and 4 Stroke engines

Question 1

Course (du piston)

Answer 1

Stroke

Question 2

Moteur alternatif (moteur a combustion interne)

Answer 2

Reciprocating engine (internal combustion engine)

Question 3

Arbre manivelle

Answer 3

Crankshaft

It is made from a one piece forging, which are then machined (usinés).

Question 4

The 4 strokes of the piston

Answer 4

• Intake/induction
• Compression
• Power
• Exhaust

Question 5

What is the induction stroke

Answer 5

Crankshaft rotates clockwise, piston moves down the cylinder, inlet valve is open and fresh air is drawn in or pushed by the turbocharger.

Question 6

What is the compression stroke

Answer 6

The inlet valve is closed, the piston moves up to TDC, pressure and temperature increases.

Question 7

What is the power stroke

Answer 7

Just before TDC, fuel is atomised into droplets, a combustion appears and the piston is pushed down.

Question 8

What is the exhaust stroke

Answer 8

Piston at BDC, exhaust valve opens, gas escapes as piston moves up, the inlet valve opens and exhaust valve closes before it reaches TDC.

Question 9

Trunk piston

Answer 9

Jupe du piston, it takes the thrust caused by the connecting-rod and transmits it to the cylinder liner

Question 10

Use for medium speed 4 stroke engines

Answer 10

• Electricity

- Propulsion engines in ships with small head room or diesel electric

Question 11

Head room

Answer 11

Espace libre au dessus de la tête

Question 12

Bâti moteur

Answer 12

Engine frame, made of a single casting or cast steel sections and steel plates welded together.

The crankshaft is underslung (suspendu) and the load (effort/charge) on the bearing caps (chapeau de palier) is transferred to the frame via tie bolts (tirants)

Question 13

Tourillon

Answer 13

Journals

They rotate in the main bearings of the crankshaft

Question 14

Maneton

Answer 14

Crankpin

It is linked to the connecting rod, because of the large diameter, to decrease the load the width of the bottom end of the rod is greater than the diameter of the cylinder liner using a liner or splitting the bottom end obliquely (diagonale), in which case serrations (rénures) are used to locate the two halves.

Question 15

Joue de manivelle

Answer 15

Webs

They connect the pins to the journals and are fitted with counterweights to reduce vibration.

Question 16

How is the piston cooled?

Answer 16

Journals and crankpins have holes fitted for oil to go up the connecting rod, the piston pin is lubricated as the oil goes through it to cool the piston to cool it.

Question 17

La bielle

Answer 17

Connecting rod

In 4 stroke engines, it is subject to inertia that gives it stress

The top end is fitted with a bushing (douille), to maintain a steady oil film. The bottom end is fitted with a pair of bearing shells (coussinet de palier) lined with antifriction metal to avoid steel to steel rubbing in case of oil film breakdown.

Remove the bearing cap to disconnect the crankpin and the connecting rod.

Question 18

Issue with bottom end bolts

Answer 18

Because of stress, they must be treated carefully when overhauling the engine. They should be inspected for damage to the surface from which a crack could start.

The damage can come from corrosion (water in LO) or from incorrect handling.

Question 19

La chemise

Answer 19

Cylinder liner

Cast separately from the main cylinder to:

• Be made in a superior material
• Be replaced before the cylinder due to additional wear
• Be able to expand from the heat differently than the cylinder
• Less risks of effect (more complex castings are more likely to have defects)

Question 20

Pont d’eau

Answer 20

Bore cooling

At the top of the liner, where there is more stress, the walls are thicker and the water is closer to the liner’s surface to prevent breakdown in cooling or excessive stress.

Question 21

Bague anti polissage

Answer 21

Fireband (antipolishing ring)

It is slightly smaller in diameter than the liner, it is meant to remove any carbon deposit on the piston above the top ring to prevent it from polishing the liner and removing its oil retention ability.

Question 22

Mesurer

Answer 22

To gauge

Parts must be gauged to establish the wear rate and check that it’s within manufacturer’s tolerances.

Question 23

The piston on 4 stroke engines

Answer 23

The crown (tête) and skirt are made of different materials.

The crown is a heat resisting steel forging which may be alloyed (allié/alliage) with chromium, molybdenum and nickel to maintain strength at high temperatures and resist corrosion. It is dished (creusé) to form a combustion chamber with cut-outs to allow for the valves’ opening. The top land (top ring and above) may be tapered (lègerement conique) to allow for thermal expansion.

The skirt can either be a nodular cast iron or forged/cast silicon aluminium alloy. Aluminium is light thus reducing bearing load.

Question 24

Les sègments

Answer 24

Piston rings (fire, compression, scraper)

Chrome plated or plasma coated to resist wear, oil scraper ring is fitted to the piston because the liner is splash lubricated.

Question 25

How is the piston cooled

Answer 25

A jet of oil directed upwards from a hole in the top of the con rod onto the underside of the crown. Or using an oil catcher, this directs oil into the cooling spaces on the underside of the crown.

Question 26

La culasse

Answer 26

Cylinder head They house the inlet and exhaust valves, the fuel injector, the air start valve, the relief valve and indicator cock. They are normally manufactured from cast iron, it maintains a reasonable strength under load. Adequate cooling is essential to prevent thermal fatigue due to uneven expansion throughout the casting and bore cooling has been introduced along with cooling spaces. Poor cooling from scale build up inside the cooling spaces (from inadequate water treatment) or from incorrect tightening down can cause the cylinder head to crack.

Question 27

Les soupapes

Answer 27

On 4 stroke engines, inlet and exhaust valves are doubled to reduce the individual size (thus reduce their inertia and the necessary cooling) to keep the cylinder head at a correct thickness and reduce power lost to actuating the valves. Valves have 2 springs of different vibration characteristics to avoid the valve falling down if one spring breaks.

Question 28

Culbuterie

Answer 28

Push rods (tiges de culbuteur) and rocker arms (culbuteurs) are used to open and shut valves, operated by cams through rollers (gallets) and roller guides (douilles). The rocker arms actuate bridges (étriers) that push both valves at the same time.

Question 29

Le jeu normal

Answer 29

Clearance For the rocker arm it is the space between the top of the valve spindle (tige de soupape) and the rocker arm. It ensures that the valve closes properly when expanded from the heat. Clearances are according to the manufacturer when the engine is cold. If the clearance is too small, the valve won't close fully and it'll burn out from the combustion gasses pushed at high pressure through, weakening the material. The first indication of a valve burning out will be the rise in exhaust temperature.

Question 30

Arbre à cames

Answer 30

Camshaft On small engines they are from single forging, in bigger ones they can be built up (assemblés) cylinder by cylinder, each parts connected by a flange (bride). The flange must have as many holes as there are cylinders to adapt the camshaft to the firing order (ordre d'allumage). On bigger engines cams are shrunk onto the steel shaft using heat or hydraulic means. Keys (clavettes) aren't used as they are stress raisers. Due to oil imperfection, the cams' surface is hardened via the nitriding process. The camshaft is either gear or chain driven by the crankshaft, it turns at half the speed on 4 stroke engines.

Question 31

Pompe à injection

Answer 31

On medium speed engines, it's a jerk type fuel pump, one for each cylinder with a helix plunger (piston plongeur) operated by a cam reciprocating in a barrel, it has a vertical groove. The pump has spill ports connected to the suction side, the plunger is keyed to a sleeve (douille) which has a pinion machine into it, which meshes with a rack (crémaillère) which can rotate the plunger. The rack is connected to the engine governor.

Question 32

Injection within the pump

Answer 32

The plunger moves up, blocking the spill port, pressure builds up. Depending on the helix position, the spill ports opening timing changes and the fuel quantity changes, pressure then falls down, if the vertical groove is in front of the spill ports, no pressure builds up, no injection, the engine stops.

Question 33

L'injecteur

Answer 33

Fuel injector, it atomises the fuel into tiny droplets that penetrate far into the combustion chamber to mix with oxygen. The hot air ignites the mix. This is done with a spring-loaded needled valve, the fuel pushes up, once it reaches the right pressure, the valve is lifted and the fuel goes through tiny nozzles atomising it. Springs are ajustable according to manufacturers' demands. They are cooled by the fuel circulating inside (also keeping the right viscosity when the engine is stoped) and from the bore cooling of the cylinder head.

Question 34

Common rail injection

Answer 34

Low speed running creates drop of injection pressure and makes poor atomising and thus poor combustion and loss of power/pollution. Electronicaly controlled injectors fed by accumulator (the rail) connected to a high-pressure pump, electronic controls allows accurate fuel dosage and injection timing.

Question 35

Two stroke engines summary

Answer 35

- Combustion space and scavenge space are separated from the crankcase by diaphragm plate. - Piston rod is bolted to piston and passes through stuffing box mounted in the diaphragm plate (seals the two spaces, stopping oil from going up to scavenge space and air going down to the crankcase) - The foot of the piston is bolted to the crosshead pin, the top of the connecting rod swings around it. Guide shoes (patins) are attached to either side of the crosshead pin to guide the movement of the piston, allowing for greater strokes. - Using oil in the combustion chamber means lower grade fuel can be used as the oil alkalinity counters acid attacks from the fuel's sulfur.

Question 36

The bedplate

Answer 36

Foundation on which 2 stroke engine is built, it carries the weight of the engine and holds the crankshaft sitting in bearing housings in the transverse girders, modern bedplates are constructed from fabricated longitudinal girders with cast steel.

Question 37

A-frames

Answer 37

Carry the crosshead guides and support the engine entablature, it is mounted on the bedplate

Question 38

Entablature

Answer 38

Cylinder block, incorporates scavenge air space and cooling water spaces. It houses the cylinder liner and is made of cast iron.

Question 39

Tie bolts

Answer 39

Tie bolts withstand the force of the combustion which would separate the A-frames and the bedplate. They are tightened hydraulically.

Question 40

La crosse

Answer 40

The crosshead pin connects piston rod and con rod. On either side there are slippers which run up and down in the crosshead guides as the piston reciprocates. It keeps the connecting rod short and prevents it from moving too much sideways while having a long stroke. To accommodate high downwards thrust and good lubrification, the crosshead pin has a large diameter, the bottom halves of the bearing shells have oil gutters cut in to assist distribution of oil, which is supplied to the crosshead using a swinging arm/telescopic pipe. The slippers are mounted on journals (tourillons), the slippers float on the journals to allow for any misalignment of the guides. The surfaces are white metal lined, oil is supplied to the slipper from the crosshead oil supply. Guides are either built into the A-frames or machined and mounted into them.

Question 41

La bielle (2 temps)

Answer 41

Single forging incorporating the top half of the crankpin bearing housing and the bottom half of the crosshead pin bearing housing. Oil is supplied down to the crankpin bearing via drillings in the con rod. When inspecting crankpin bearing and journal, check ovality, excessive will lead to a failure in lubrication.

Question 42

Le gland

Answer 42

Stuffing box/gland contains a series of rings which are made up of 3-4 segments, on the inside of each set is a garter spring which provides tension to hold the ring segments against the piston rod to remove any residual oil liable to pollute crankcase oil.

Question 43

La chemise (2 temps)

Answer 43

The bottom has scavenge ports, they are also fitted with bore cooling with increased top wall thickness. The lower part should not be cooled to avoid condensation and thus corrosion. Oil is provided by a pressurised system to the combustion chamber via drillings and then grooves to be spread circumferentially around the liner, the piston rings assist the spreading. Scuffing: Breakdown in lubrication which creates localised welding between points of the rings and liner surface with subsequent tearing of microscopic particles (severe form of wear).

Question 44

Le piston (2 temps)

Answer 44

The crown carries 4-5 piston ring grooves. In engines with top exhaust valve, the skirt is short. The rod is hollow and has a tube running up its centre to cool the piston from within. The rings are stronger than the liner to give them maximum life. Piston rings seal the gas pushing them against the liner, and spread lubricating oil, they also transfer heat to the liner walls. When overhauling, measure and record the butt and axial clearances of ring grooves.

Question 45

La soupape d'échappement (2 temps)

Answer 45

- Hydraulicaly operated air sprung exhaust valve. The cam operates the hydraulic pump. - 7bar air is mixed with LO and has a small leak to avoid air lock. - Damping arrangement prevents hammering of the valve seating when closing. - Winged valve rotator, rotating the valve keeps it at even temperature and reduces carbon deposit.

Question 46

Casque de protection

Answer 46

- Safety helmet | - Hardhat

Question 47

Veste haute visibilité

Answer 47

- High visibility vest | - Yellow jacket

Question 48

Bottes de sécurité

Answer 48

- Safety shoes | - Foot protection

Question 49

Masque à poussière

Answer 49

Dust mask

Question 50

Gants

Answer 50

- Gloves | - Hand protection

Question 51

Masque de soudure

Answer 51

Welding mask

Question 52

Protection auditive

Answer 52

- Hearing protection | - Ear muffs/plugs

Question 53

EPI

Answer 53

Personal Protective Equipment

Question 54

ARI

Answer 54

Breathing apparatus

Question 55

Visière

Answer 55

Face shield

Question 56

Harnais

Answer 56

Safety harness

Question 57

Masque à gaz

Answer 57

Respirator

Question 58

Lunettes

Answer 58

- Safety glasses | - Goggles

Question 59

Bleu de travail

Answer 59

- Coveralls - Overalls - Boiler suit

Question 60

The 2 strokes of a piston

Answer 60

- Scavenge, exhaust, compression | - Power

Question 61

Combustion of dual fuel engines

Answer 61

Gas is fed into the cylinder in the air inlet channel by electronically actuated valves, micro-pilot injection with less than 1% fuel injection at nominal load, to ignite the gas mixture. Engine starts in diesel mode, past 300rpm it switches to gas.

Question 62

Natural gas feeding system

Answer 62

Fed through a valve station to be filtered, then pressurised according to engine load. The system has shut-off valves, gas is supplied through large common rail pipes running along the engine. The system is of double-wall design with alarm of leakage.

Question 63

Diesel fuel supply (2 temps)

Answer 63

Divided into two, one for pilot fuel and one for the back-up fuel. Pilot fuel is pressurised by a pump unit incorporating duplex filters, pressure regulator and engine driven radial piston-type pump, the pilot fuel is fed to a common rail pipe to the injection valves (injected at 900 bar). Back-up fuel is fed to normal camshaft-driven injection pump, pumped at high pressure to spring-loaded injection valve. Larger needle of the twin-needle injection valve is used in Diesel engine mode, smaller for pilot fuel. Pilot injection is electronically controlled, main diesel is hydro-mechanically controlled. Solenoids time pilot fuel injection for minimum NOx production.

Question 64

The pilot fuel pump

Answer 64

Stand-alone unit, radial piston pump and necessary filters, valves and control system. Receives signal and required pressure level from engine control unit (ECU) and maintains the pressure at that level. Fuel is fed to the rail which acts as accumulator and damper against the pressure variations.

Question 65

Air-fuel ratio

Answer 65

2.2:1, it is essential for optimum performance and emissions, helped by exhaust gas waste gate valve, which helps EG bypass turbocharger adjusting the air-fuel ratio to the correct value according to varying conditions. It is actuated electro-pneumatically.