Give an explanation of the operation of the following starters:
A starter motor is a high torque electric motor that is tasked with cranking over the engine to start up the process of combustion and running of the engine. Prior to starter motors engines were hand cranked to start which was an arduous process. They are feed power from the battery and different types have emerged over the years.
Inertia:
An inertia starter motor refers to the way that the drive pinion is engaged. The drive pinion slides freely up and down on a worm gear on the spline, when the motor is engaged the rotation of the armature drives the pinion up the spline and engages the pinion into the teeth on the ring of the flywheel, which in turn cranks the engine. when the key is released the momentum of the flywheel drives the pinion the opposite direction and the pinion goes back down the spline into the neutral position. Advantages are the inertia drives simplicity and ease of function, the disadvantages are that the drive pinion can wear and possibly damage the ring gear on the flywheel as the pinion is driven into the flywheel at high speed, it is also noisy.
Pre engaged starter motor:
A pre engaged starter motor is an improvement on the inertia starer motor. A solenoid attached by a lever to the drive pinion acts as the engagement mechanism. When the key is turned the solenoid forces the drive pinion to mesh with the gear on the flywheel, when it is meshed the motors starts cranking the engine over. When the engine has started the solenoid retracts the drive pinion. This starter removes the problem of damage by pre engaging the pinion, with slighly more complexity added by the solenoid.
Reduction gear starter:
A reduction gear starter was developed to counter the increasing size and weight of starter motors, larger engines needed larger and heavier starters so to counter this a normal sized high torque starter with a pre engaged mechanism (or inertia gear on some types) was housed with a set of gears this meant that a small motor could still start a large engine by multiplying torque through a gearbox. This is more complex than other types but makes up for it in weight savings.
A type of reduction gear starter
This one has a planetary gear system with a inertia drive
Alternators
An alternator is the charging mechanism on a engine. Drive is transmitted by a belt to create power in the alternator. This is done in the opposite way as a starter motor the central rotor of magnets is driven inside a stationary iron core with magnets called a stator and power is created by cutting the magnetic field lines on the magnets. the alternator charges the battery to run all the cars electrical devices and it creates the power for the spark of the spark plugs. An automotive alternator uses rotor winding this means the voltage delivered can be controlled and voltage peaks are evened out to stop damage to electrical components.
Generator
A generator or dynamo is an an older type of charging system for engines. it operates on much the same principle as an alternator, except it has a stationary magnet setup with a rotating commutator. The commutator rotates and intersects the field lines of the magnets. this creates less power than a alternator. This system was used widely on early cars and outboard motors for much longer. In an outboard setup there are rotating magnets on the inside of the flywheel which rotate around a stator of windings.
A diagram of a magneto system on a Johnson outboard motor
A warranty is an agreement between buyer and seller that that specific details and conditions of their transaction are true and that the word of the party can be trusted. A guarantee is a formal agreement usually a document that ensure the quality of a product or service. In the case of a marine business specifically a marine repair business a warranty could be a guarantee to the service being provided to the boat, or a guarantee to the quality of parts being installed into a boat.
Quotations and estimates
A Quotation is an agreement between two party's on the price of a item or service. A quotation can be taken as a formal agreement and if one party does not honour the quote they could be liable to legal charges. An estimation is a rough estimate of the price required for a item or service. Unexpected difficulty's can mean the original estimate is too low and a new estimate will be needed. In a Marine repair business a quotation or estimate could be changed severely because of unforeseen problems or malfunctions.
Charge out rates
A charge out rate is the amount of money required when charging for a service for example an automotive or marine workshop. A business owner will calculate a charge out rate for their business before it starts. When a charge out rate is being calculated all costs of the business are taken into account including overheads like rent and machinery costs. Other costs involved are labour costs and profit margins. Charge out rates should not be too low as to make the business unprofitable, and not too high as to make it unattractive to customers.
Courtesy service
A courtesy service is a service provided by a business which enhances the experience for the customer and entices them to come to that business again, this can include small things like a cup of coffee on entry or getting something for free with a purchase to more expensive things like providing a courtesy car to a customer who's car is being worked at the mechanics. These all help to create a customer base and customer loyalty (customers that come back to your business)
Cultural issues
The world we are in is far more culturally diverse than it used to be with many different cultures and ethnicity's living and working together. Cultural issues have to catered for in the modern working place. Different cultures have different customs in business and religion and so on, for a business to be seen a diverse and therefore welcoming to all types of people then cultural protocols must be in place. If cultures are respected then employees and staff will integrate better into the company.
The ignition system is one of the most important systems on the internal combustion engine. The purpose of the system is to ignite the fuel-air mixture in the combustion chamber. There are several components in the system which are used to create spark and regulate timing. Diesel systems do not have an electronic ignition system as pressure is used to ignite the fuel, however glow plugs are used to preheat the combustion chamber. Virtually all petrol engines have spark ignition.
What two substances are mixed together to form electrolyte a battery?
An automotive battery has lead plates and an electrolyte solution which is made up of Sulfuric acid and distilled water. This cause a chemical reaction and releases electrons and produce electricity.
What three elements are required to make a petrol engine run effectively?
For a petrol engine to run effectively and efficiently it requires: Spark, Air, and Fuel. This include the quality and delivery of these elements. The spark needs to be powerful to overcome the compression in the cylinder and ignite the fuel. it also has to be delivered at precisely the right moment. The flow of air into the engine has to be consistent and the amount of air has to sufficient otherwise the engine will not run. Fuel must also be delivered consistently and in the right amounts. to little and the engine will not run or will run poorly, too much fuel and the engine will flood.
Explain the operation of a 'kettering ignition system', including all components from the battery to the spark plug
The Kettering ignition system is a mechanical ignition timing system used on outboard motors and early cars. Electricity from the battery travels to the coil and charges it up there are capacitors on the line which stop voltage spikes , the release of spark is controlled by a cam on the crankshaft which opens a contact breaker and creates a spark. the spark is distributed to the spark plugs by a distributor or on multi coil engines through a C coil on cylinder head. disadvantages of the Kettering system is the potential for wear on the contact breakers and cam lobe, which in turn affects sparking.
Give an explanation of dwell angle in a distributor
Dwell angle is a measurement of the amount of time that a coil has to charge before it releases another spark. An average dwell angle on a 4 cylinder engine is 52 degrees. Which means that the coil has 52degrees to recharge and fire a spark on the distributor. if the dwell angle is too small the spark may not be fully charged and if it is too large the engine may misfire. Dwell angle can be adjusted on a distributor engine with a mechanical adjustment ( a screw, or movement). Dwell angle is adjusted on electronic ignition systems through a computer.
Explain why the 'Heat range' of a spark plug is so important.
A spark plug operates in a specific heat range to maximise efficiency some plugs have more thermal efficiency than others. If a spark plug is not hot enough when it is idling then the spark plug can be fouled up quite easily. If a spark plug is not 'cold' enough when the engine is at high RPM then the tip of the spark plug make cause detonation or knocking because of high temperature. The spark plug needs to operate in the heat range that is required by the engine.
What is a capacitor discharge system and how does it operate?
Capacitor discharge is a type of electronic ignition. It is used widely in outboard motors. It operates on similar principles to other electronic ignition systems. The voltage in the system is increased to about 400-600 V by a transformer in the capacitor module. This system has a large capacitor which stores a high amount of voltage and then fires this voltage to a coil when it is triggered. When the release is triggered a rectifier allows current to flow to the coil and voltage is increased further from the 400-600V of the capacitor discharge to about 40kV at the spark plug. This system is quite simple and reliable with no parts that can wear and affect the ignition. it also has a very fast recharge time for the capacitor reducing the dwell angle. however the limited spark duration can be too short to provide reliable ignition in some engines.
The capacitor module contains the transformer and capacitor.
Power boats and yachts need a transmission to control the power of the engine and the ability to select reverse. Marine transmissions are very similar to automotive transmissions, although a marine transmission is usually much simpler.
One of the biggest super yachts in the world Octopus
1. Explain the difference types of gear selection systems found.
There are many types of marine transmission for different types of motors. There is a variety of transmission types to suit different vessels. A large ship has to have incredible amounts of torque and low down power to drive a large propeller and accelerate a huge amount of weight.
A In outboard gear cases and stern drives
On outboard motors the most common type of gear selection system is the 'Dog clutch system ' this is a fairly simple mechanism that consists of a small dog clutch on a spline shaft which can be moved forwards and backwards from the central neutral position into engagement with the forward or reverse gears of the gear case. A small power boat does not need different ratios of forward gears as they operate at a fairly constant speed to reach a destination, so a simple dog clutch is all that is required.
While they are simple and reliable there are some disadvantages to the dog clutch. There are no synchromesh units on a dog clutch which can make gear changes quite violent as the dog crashes into the gear. This can also damage, wear or even break the teeth on the dog clutch especially when gears are changed at high engine revolutions. The selector mechanism is also susceptible to corrosion and gear selection can be compromised.
The dog clutch can be seen in the centre.
Stern drive units have a taper drive system. Drive is engaged by a brass clutch unit with a tapered face that mates to another tapered face on the forward or reverse gear. The neutral position is in the centre and the gear is moved to engage the required gear. The advantages of a taper drive is that gear changes are very smooth unlike the dog clutch, this reduces wear on the components, however the taper drive can slip if it is worn.
B. Inboard marine gearboxes
Inboard marine transmissions are divided up into: Hydraulic marine transmissions and Manual marine transmissions. This refers to the way power is transmitted. In a Hydraulic marine transmission drive is transmitted by the circulation of hydraulic fluid. Different gear ratios are achieved by the use of clutch packs. These clutch packs are engaged to change the engine output speed or the rotation of the output (for reverse).
Very high oil pressure is used to engage the clutch packs. Only 2 or three ratios are required from this gearbox including reverse.
Cutaway of a Hydraulic gearbox
One of the most common type of manual marine transmission is the planetary gear system. This system achieves different gear ratios with a planetary gear case.
Planetary Gears - Planetary gears are the most common and provide both strength and smooth operation with good resistance to torque loads. The planetary gear systems have efficiency 65% and have a tendency to free spool when loaded, therefore a braking mechanism is needed.
Different parts of the planet gear system are held while others are allowed to rotate, this creates different gear ratios, many different ratios can be created with the use of two planetary gear cases.
A stern drive is a drive unit that is essentially the bottom leg of an outboard attached to an inboard engine. It offers the space benefits of an outboard. and the practicality of an inboard..
2 What type of gears are used in outboards and stern legs
Outboards and stern legs typically use helical gears and pinions.
A Give an explanation of why manufactures choose this type of gear.
Hellical gears are an efficient way of transferring drive, they are strong and do not wear as aggressively as spur gears, they are also quieter. The pinion transfers drive from the power head on a 90 degree angle to the propeller it has to be Strong to deal with this load.
A forward gear and pinion off a Johnson outboard
3 Explain what a duo propeller system is and how it works/operates
A duo propeller is a prop system developed by Volvo penta that utilises two contra rotating propellers to drive instead of one. A shaft and pinion from the power head drives the two propellers in opposite directions, no other gearing is necessary. The main benefit of having two propellers is the increased thrust produced, this can increase acceleration, top speed and increase fuel efficiency as well as getting a boat on the plane faster. A duo prop eliminates the lateral movement that a single propeller can give, the two propellers even each other out so the boat travels straight. Cavitation that is caused on single propellers is also reduced as the second propeller reduces slipping which creates bubbles and cavitation. Overall vibration is decreased and handling is increased.
Volvo penta duo prop
An exploded duo prop view
4Why would a v drive be used instead of a standard shaft drive system.
A V drive unit is a part of some marine transmissions that can transfer drive through tight angles. The purpose of a v drive is simply for space. Because the drive can be transferred at tight angles less space is needed for the prop shaft, this system is common for yachts where space is at a premium.
The v drive unit transfers drive underneath the engine.
A simple yet effective v drive diagram
5 Compete a jet unit with a surface drive unit and give reasons why you would choose one system over the other.
Jet units were invented by New Zealander Bill Hamilton for navigating shallow south island rivers.
A jet unit operates on the function of drawing in water and accelerating the water through impellers out of a nozzle which can be controlled to turn the boat. The advantages of a a jet unit are: It can operate in very shallow water, Very high acceleration, good handling, and savings in space. A jet boat can operate in shallow water as there is no propeller to be caught on the ground some jet boats can operate in just 7.5cm of water. Acceleration and handling are very responsive as the water jet can be directed at will, and space is saved by having the jet unit on the transom of the boat. A jet unit is disadvantaged by its fuel efficiency which can be greatly affected if the jet intake is clogged by something, a jet unit is also affected by engine mismatch greater that a propeller boat.
Diagram of a jet unit
Jet unit can be seen on the back of the boat
Surface drive units are a conventional propeller which is positioned on the surface of the water. Half of the propeller is out of the water and half is submerged. Having the propeller at the surface level reduces drag and reduces the vessels draft, making the surface drive suitable for many applications where the owner is looking to reduce fuel costs, increase speed and create a more efficient vessel. A surface drive can deliver very high performance and is relatively simple, however it is disadvantaged by more drag and less acceleration and handling than a jet unit. The surface drive can also be unsafe as the propeller is out of the water and could make the boat unstable at high speeds due to the propeller slapping the water which increases lateral forces.
For these reasons i would choose a jet drive unit over a surface drive unit. It is reliable and provides very good performance, whilst being developed and improved by company's worldwide. Also I'm a New Zealander so I'm being patriotic.
A manual transmission is a type of trasmission that requires the driver to change gears. It is a relatively simple system and is widespread in the automotive industry. A manual gearbox is engaged by a driver operated clutch and gears are selected with a gear stick.
A manual gearbox
1. What type of gears are used for reverse in a manual gearbox?
Reverse gears in a gearbox are straight cut gears, meaning the teeth are straight. reverse gears are usually small ratios as a car does not need to travel in reverse at high speed.
A set of reverse straght cut gears (idler, driver and driven.)
a. Give an explanaiton for your choice
Straight cut teeth are used on reverse gears because spur gears are far easier to mesh together than helical gears. In a lot of cases all of the foward gears in a modern gearbox are helical (angle cut teeth), Because reverse gears have to engage with an idler, how the gears mesh is critical. The disadvantage's to spur gears are: Spur gears wear more as only one tooth is in contact while the gear is spining, and they make more noise than a helical gear.
2. What type of gears are used for all foward gears in a manual gearbox?
Helical gears are used in the foward gears of a gearbox. These gears have angled teeth which means that more surface area and more teeth are in contact at one time while the gear is spinning. Herring bone type gears are used in large applications like trucks or earthmoving equipment, these gears can take the very high amounts of load required in a large application however they are more complex and more expensive to manufacture.
A herringbone gear
A. Give an explanation
Helical gears are stronger and quieter than spur gears. Because of the amount of teeth in contact with the meshing gear a helical gear does not wear as agressively as a spur gear, giving them a longer lifespan. As a gearbox is usually in a foward gear lifespan is critical.
3. What is the purpose of a synchromesh unit.
A synchromesh unit is a part of the geartrain, its job is to make the changing of gears smoother, this is done by sychronising the teeth of the two gears that are about to be engaged. This synchronation of the gear teeth ensures the gears are travelling at the right speed and the gear teeth are in the right position. A synchromesh unit can save the gear teeth from damage and extend the life of the gearbox because it is saved from violent gear changes. If the synchromesh unit is damaged then gear changes may be clunky or may not change at all.
A simple Synchromesh diagram
A. Explain the operation of a synchromesh unit?
A synchromesh unit has to smooth the changing of gears by lining up the gear teeth and to ensure the gears are running at the same speed. This is achieved by a unit called a baulk ring. This unit is positioned on the geartrain between gears, when there is a gear change the baulk ring engages the next gear with a tapered face that grips to another tapered face on the gear and makes them travel at the same speed. There is a small clearance on the baulk ring which allows it to rotate and let the teeth of the rings line up. Gears can now be changed with ease.
Position of the baulk ring on the geartrain.
Synchro or baulk ring
C. How do we check a baulk ring for serviceability?
Because a Baulk ring is under a high amount of stress it can be damaged. The baulk ring has to deal with stress of engaging a gear with a tapered face especially at high speeds. and over time the stress could warp and chip the teeth of the ring. A damaged baulk ring may not engage properly or could damage other gearbox components.
Wear to the tapered face of the baulk ring
4. What type of bearings do we find in gearboxes?
A gearbox needs many types of bearings to run. Caged roller bearings are used on the main and counter shaft. These bearings allow smooth rotation movement of the geartrain. Thrust bearings are used to keep the shafts from 'floating'. lateral movement of these shafts can damage the gears. Needle bearings are used on the inside of shafts, and flat caged needle bearings are used in the final drive assembaly.
Gearbox thrust bearing and tapered roller bearings.
5. Give an explanation of a gear ratio.
A gear ratio is the difference in speed of two connected shafts or gears. The gears travel at different speeds because of the amount of teeth on the driver and driven gear and the size of the gear. A gear ratio is measured in the amount of revolutions one shaft does in one revolution of its connected shaft. A ratio of 2:1 means that the shaft does two revolutions in one revolution of its connected shaft. Small gear ratios allow high speeds, while high gear ratios multiply the torque produced. Gear ratios are important in controlling an engine and keeping the engine in its optimum operating speed.
The cylinder head has to deal with very high stress and temperatures. Because of this the parts that make up the cylinder head have to be checked for damage, and measured to make sure they are within the manufacturers specifications.
Inspection of the cylinder head
The face of the cylinder head is checked visually for damage like: chips, scratches, corrosion, broken edges, and broken threads. The face of the cylinder head is also checked for warpage by placing a straight edge across lengths of the face, a feeler gauge checks the warpage of the face. This kind of damage could effect how the cylinder head mates with the cylinder block or possibly make the engine lose compression, which can shorten the life of the head gasket.
Checking a cylinder head for warpage
Valve Guides
Valve guides provide support and lubrication. for the valve as it travels up and down. Valve guides are visually inspected for damage like bending or chips. The guides are also measured in diameter at 6 locations with a telescopic gauge and micrometer to determine the amount of wear, taper and ovality. Because of the up and down movement of the valve in the valve guide, the guide can taper at the top and become oval as the valve has a thrust side where more wear happens. if a valve guide is bent then the valve can suffer excess wear.
Valves
The condition of the valves is very important as their operation is vital to engine operation valves operate under very high temperatures and ave to been tough to cope with forces.. The valves are visually inspected and measured for wear, cuts, carbon build up and operation. Valve stems are measured with a micrometer at 3 positions, the top middle and bottom. The top and bottom have the largest amount of wear as it is the largest thrust area. the stem measurement is subtracted from the valve guide measurement to find the valve oil clearance. If this clearance is too small the valve could seize.
Valve Collets and keepers
Valve collets and keepers hold the valve onto the valve spring. if they are damaged or bent then the valve could come off the spring and it would no return to the valve seat.
Valve seats
The valve seats are inspected for damage, wear, seat angle and carbon build up. The valve seat seals the valve with the cylinder head so that no compression is lost. If the seat is damaged or worn the valve may not sit properly. The angle of the seat also determines how well the valve sits. Small carbon deposits that build up from combustion can collect on the valve seat and make the valve not sit properly.
Carbon and damage on a valve seat
Valve Spring
The job of the valve spring is to return the valve to the valve seat after it is opened, and hold it there. The forces on the spring can bend the spring which affects its operation. This is measured with a set square and vernier calliper. if the valve spring is bent by more than 2.0 mm then it should be replaced as the spring may not be returning the valve to its seat properly. The valve springs are also checked for: free length, installed height and spring tension. All of these checks ensure that the spring is working properly. Spring tension is measured on a valve spring pressure tester, the valve spring can lose it's tension and elasticity over its lifetime and its ability to return the valve to its seat.
Camshaft
The camshaft is responsible for the opening and closing of the valves, any amount of damage or wear to the camshaft can affect engine timing and engine operation. The camshaft is visually inspected for cuts, metal imperfections, or scuffing on the cam lobe. The camshaft lobes are measured for lift and wear. If they are worn enough then the timing of opening the valves would be affected greatly.
Lift is measured by subtracting the base Circle from the base circle and lobe lift.
Camshaft journals are visually inspected for wear and damage. Damage to the journals could make the camshaft wobble or have increased wear. Camshaft journals are measured for wear as well. If a camshaft bearing is tapered or has become oval from wear then it's operation will be affected, the journal may not spin true which may affect timing. The Journals are measured at both ends to calculate the taper from one end to the other, A high amount of taper could mean that the journal is running unevenly. Ovality is measured with a micrometer on the y and x axis, ovality is the wearing of the circular journal into an oval shape, this can make the journal and camshaft run poorly. Cylinder head bearings on modern engines are usually integral and cannot be replaced, leading to the entire head being replaced.
Camshaft Run out
Camshaft run out occurs over time as the stresses on the camshaft journals wear and twist until the journal has points of wear. this also contributes to taper and ovality of the journals. Run out is measured with a DTI gauge that is positioned on the journal to record the slight changes in wear.
Camshaft bearing oil clearance
The bearing oil clearance is the space that oil can travel through to lubricate the bearing. If the oil clearance is too large then the camshaft will not rotate smoothly, if the clearance is too small not enough oil can enter for lubrication and the camshaft may seize due to the immense heat the cylinder head works at. Oil clearance is measured with a plasti
Bucket pad thickness
On some DOHC engines the camshaft lobes follow a bucket pad type system which is on top of the valves. These pads which are case hardened to lengthen their life span and protect from the constant wearing of the camshaft lobe acting upon it. The valve clearance and timing can be adjusted by changing the height of these pads. If the pads are severely worn then they may not be opening the valve properly, affecting the timing and performance of the engine. Bucket pads are measured with a micrometer and along with the valve clearance measurement are used to figure out the thickness of the replacement pad.
Simple bucket valve system
More complicated variable valve timing mechanism
Camshaft End float
Camshaft end float is the lateral movement of the camshaft while it is capped down. Every camshaft has a small amount of end float, this amount can be increased as the engines gets older by wearing the thrust bearings that stop lateral movement. These bearings can easily be replaced. Too much end float can make the camshaft wear more aggressively as it may be out of place laterally. End float is measured with a DTI gauge positioned on the end of the camshaft as the camshaft is moved back and forth.
