Thursday, July 7, 2011

Trailers

Trailer types:

Simple single axle boat trailer

 Single axle:

This is the smallest type of boat trailer, Usually made for carrying small pleasure craft under 15 ft. This trailer makes boat ownership easier for more people. 

Double Axle:
 
A double axle trailer is for Larger boats. Four wheels means that the larger weight of the boat is spread onto a larger surface area. Double axle trailers usually contain a braking system as there is more weight extra breaking power is necessary.


Triple axle:

This type of trailer is only for the largest of boats. Boats larger than this usually cannot be taken in and out of the water. This type of trailer is uncommon in New Zealand.



Trailer design:
A frame design:

An A frame boat trailer is the simplest type of boat trailer, however it is a strong and easy to fabricate design. 

A simple trailer for a sailboat

Draw bar:

A draw bar type trailer, utilises a simple frame design and a draw bar. This is a moving attachment to a trailer which allows a better turning ability. 



Trailers are often personalised to individual boats, this is because of different hull shapes and different sized boats.

Bunk Type trailer:

A bunk type trailer is a trailer with a solid skid for the boat to rest on.

Roller type trailer:

A roller type trailer has rollers that allow a boat to easily be retrieved from the water, this also reduces the damage to the bottom of the hull. the rollers are prone to corrosion as water can become stagnant.
A tandem roller type trailer

Trailer materials:
 
Trailers are made of highly corrosion resistant materials, as they have to be submerged into salt water constantly. Usually stainless steel or mild steel with galvanising layer to protect it. Galvanisation is the process of protecting metal from corrosion by bonding a layer of metal over the original metal. 
Aluminium is also used as trailer material as it is light and is resistant to corrosion.
OVERALL DIMENSIONS & RULES

  • If you own a boat you can tow a trailer that can be 2.5m wide, up to 4.25m high when the boat is sitting on the trailer, and 11.5m long, assuming that the car plus the boat doesn't exceed 20m which is the limit.
  • When you load your trailer the I deal position is to have the weight is so about 10% of the weight is taken on the tow ball and the rest supported and distributed over the single/tandem axles of the trailer. If there is to much weight on the tow ball the trailer coupling will not be parallel to the ground therefore the car will be hard to steer (and good luck pulling your boat up a steep launch ramp if the car is front wheel drive). If the load is to far to the back of the trailer  it will have the tendency to fish tail around at certain speeds.
  • Another thing to consider is what gear you have in the boat, “is it heavy” if so where is the weight adding to the load.
  • The tongue of the trailer is also very important – the A-frame design has the coupling of the trailer where the apex of the frame meets.
  • The draw bar style has a strut running through the apex of the A - frame which extends out further and is attached to the tow bar by a coupling.
Trailer Weight Estimates
This chart relates to single axle trailers only
Trailer
Capacity
No.
Axles
Trailer
Length
Trailer
Width
Weight
1000 lbs.115'4"62"295 lbs.
1500 lbs.115'4"62"305 lbs.
2000 lbs.117'2"88"492 lbs.
2450 lbs.118'2"96"522 lbs.
2999 lbs.118'2"96"625 lbs.
3500 lbs.119'8"96"677 lbs.
5000 lbs.120'8"96"933 lbs.
7000 lbs.127'9"96"1512 lbs.

Suspension types:

Boat trailers can utilise a number of suspension types. Leaf type springs on boat trailers are quite common as they can deal with the weight of heavy boats. Leaf type suspension is susceptible to corrosion though as water can get caught in the crevices between suspension leaves.



Another type of suspension is the strut type, this is a sealed unit so it does not suffer the same problems with corrosion however the strut type cannot support as much weight as the leaf type, so is  used on small single axle trailers.


Wheels and tyres:

The wheels on a boat trailer suffer from corrosion as they are submerged when the boat is being loaded. salt can build up and crystallise in the crevices on the wheel and tyre. Salt can also degrade the rubber in the tyre.

Brakes:

Boat trailers that haul a large boat must have a braking system. The weight of a large boat is too much for the towing car to safely control. they are controlled in conjunction with the towing vehicles brakes. These also suffer from corrosion as salt water degrades them.


These brakes are also activated by a cable much like a hand brake when the boat trailer is stationary.


Hubs and stubs:

Wheel hubs are usually galvanised to protect them from corrosion, wheel stubs can seize if crevice corrosion takes hold. so are protected with galvanisation.



Lights

The lights on a boat trailer are connected to the towing vehicle by a wire, brake lights and indicators are controlled in conjunction with the towing vehicles lights, as a long trailer could obscure the vehicles lights.
the electrical coupling is waterproof to prevent failures. 
 

Coupling


simple coupling and electrical connection.

Winches:

A winch on a boat trailer is used to haul the boat in from the water. on small craft a hand winch can be operated, but for larger craft a high torque electric winch is required.

A hand operated winch

an electric winch


An extreme case of moving a boat by trailer.








 







Wednesday, July 6, 2011

Two stroke engines

Two stroke motorbikes dominated the sport for decades

Two stroke engines are a type of internal combustion engine, it was invented by Scottish engineer Dugald Clerk in 1881. The principle of a two stroke engine is that the process of intake, compression, power and exhaust is completed in two strokes of the piston, compared to an engine with four strokes. The result is a very high power to displacement ratio, potentially even double the power from a similar sized four stroke.. Two stoke engines come in all sizes and also come in diesel variations. When two strokes were introduced they quickly became popular on motorbikes for their power to weight ratio, alternatively diesel two strokes became popular in Trains, large boats, and power generation applications.

Ridiculous double two stroke engine drag bike.

Explain how a two stroke engine works?

As mentioned above a two stroke works completes the internal combustion process in two strokes. Also different to a four stroke engine is the intake and exhaust system. A two stroke engine utilises ports or openings in the cylinder to deliver fuel and remove exhaust.

The two stroke cycle

The piston in a two stroke engine also acts as a valve, the up and down movement of the piston uncovers the intake, exhaust and transfer ports at precise moments. Some two stroke engines has different systems but most operate on the principle. Fuel and air mixture coming from the carburettor enters the cylinder through the intake port below the piston, this mixture enters the crankcase and is pressurised by the cylinder which is now on a power stroke. The downward motion of the piston now uncovers the transfer port, this port transfers fuel-air mix from below the piston to above the piston. This intake also helps expel the the exhaust from the last power stroke. The piston now on its upward stroke covers all of the ports and compresses the fuel air mixture, when the piston reaches T.D.C the spark plug ignites the fuel and the process begins again.


Although a two stroke engine has power and simplicity, there are some disadvantages, the largest of these problems is scavenging. When the piston in a two stroke engine is on it's power stroke there is a point where both the intake and exhaust ports are open, the exhaust gas is 'pushed' out of the cylinder by pressure and the incoming intake of petrol, some of this petrol mixture will also leave through the exhaust port, this is known as scavenging and can make a two stroke engine emission heavy as rich fuel is being wasted and sent into the atmosphere. efforts to limit scavenging include shaping the head of the piston to direct the incoming air-fuel flow upwards into the combustion chamber, this does not stop the problem though. In the 1980's and 1990's many countries implemented strict emission controls on vehicles, this effectively ended petrol two stroke engines being made widespread. Two stroke petrol engines are still common in marine applications and some motorbikes.


While two stroke petrol engines have lost popularity the two stroke diesel engines have maintained their popularity. Some of the biggest and most power full engines in the world are two stroke diesel engines, they maintain the simplicity and power of a two stroke petrol but a diesel two stroke does not suffer form the problem of scavenging. This is because a diesel only allows air to enter the cylinder when the intake port is open and the exhaust is not a port but multiple valves, because fuel is injected into the cylinder with a high pressure injector and ignited by the hot compressed air there is no chance of fuel escaping the cylinder unburnt. A diesel follows the same cycle as a petrol but air is forced in with a supercharger (compulsory on a two stroke diesel) and exhaust is pushed out of the exhaust valves by the rush of incoming air.

Diesel two stroke for a Large ship

What is happening below and above the piston?

Above the piston is the inlet of fuel through the transfer port, the compression of that fuel, and the ignition of that fuel resulting in a power stroke. Below the piston is the inlet port where fuel-air mix enters through a reed valve, then the mixture is pre compressed by the downward motion of the piston, the mixture then travel through the transfer port when the port is uncovered.

Why must roller and ball race bearings be used on 2 stroke SI engine crankshafts?

Because a two stroke engine is a sealed unit there is no way for oil to be circulated to lubricate the parts. Lubrication in a two stroke is done by the fuel entering the system. two stroke fuel contains a preset amount of oil which lubricates the parts and bearings of and engine before it is ignited, this oil also cause some of the emission problems that two strokes have. The reason for ball race bearing is so this oil can penetrate between the ball bearing and lubricate them, if this was a closed bearing the oil would not perform it's job and the engine could seize.

Two stroke ball race bearing

Why are rollers caged in crankshaft bearings?

Crankshaft roller bearings are caged to prevent the individual rollers from moving around the big bearing. The constant movement of the bearing from the piston would make the rollers bunch together on the bearing leaving some surfaces with no roller protection.

Roller bearings of different sizes

Why must ‘split’ type bearings be used on one piece crankshafts?
A one piece crank shaft cannot be disassembled, the bearings on the centre of the crankshaft need to be replaced after an amount of time as they suffer engine wear. This is the reason for the split bearing, it can be split and removed from the crankshaft and replaced with a new bearing.

Split type caged roller bearing

Why do we have pins between the piston ring gaps on a 2 stroke?

A pin on the piston stops the piston ring from moving around the piston. the repeated motion of the piston can move the piston rings and in a two stroke if the piston ring gap is in the wrong place then this could cause pressure loss.

A piston with piston rings, gudgeon pin and piston ring pins.

How does a reed valve work on a two stroke?
A reed valve is a type of one way valve. It is essentially just a flap that opens one way. They are used on a two stroke engine to allow the entry of fuel i the crankcase. The movement of the piston creates a vacuum this vacuum 'pulls' air and fuel through the reed valve when the vacuum stops the valve closes.

A two stroke reed valve

How do you check a reed valve?
The constant opening of this flap can damage the reed valve where it becomes inefficient and does not close properly. Dirt can also make the valve not work properly. The reed valve can be checked by holding it up to light and inspecting it, if there is a gap the reed valve needs repairing or replacing.

Explain the term ovality and taper and how are they formed?
Taper and Ovality are terms for wear in the cylinder bore of an engine. Due to the constant stress that an engine operates with the cylinders and bore are subjected to wear. Taper is the wear which makes the cylinder bore taper or wear outwards, It is caused by the constant sideways movement of the piston translating linear motion into rotational motion. Ovality is the wear on a cylinder bore which turns the circular bore into a more oval shape, this is again because of the repeated movement of the piston.

an example of cylinder bore wear

How does the piston rings seal in the bore?
The piston rings purpose is to ensure there is a seal in the cylinder around the piston, and to transfer heat from the piston through convection to the cylinder block. Without a proper seal much power would be lost and proper compression would not be achieved. They achieve this seal due to the ring pushing the bore and also due to the rings design. When combustion happens in a cylinder some of the force of combustion goes in the piston ring gap and behind the piston ring and force the ring into the cylinder bore creating a strong seal. the piston ring also prevents excess oil from being burnt in combustion.

Another example of cylinder wear

What is meant by the terms groove depth, side clearance and end gap on the pistons rings?
Groove depth:

This is the depth of the groove that the piston ring sits in. This groove is prone to carbon build up and effect the piston rig operation.




Side clearance:

This is the amount of space that is between the piston ring and the piston groove wall, this is measured with a feeler gauge.


End gap:


This is the size of the gap that is on the piston ring, this gap can change due to the changing ductility of the ring. This is measured with a feeler gauge while the ring is in the cylinder to determine it's natural position.


Explain the difference between a 2 stroke and a 4 stroke engine?
While they are both internal combustion engines there are several differences between a four stroke engine and a two stroke engine, the main differences are in the fuel delivery systems. A two stroke engine completes the engine process of intake,compression, power and exhaust in two strokes of the piston that is one movement down and one movement up. As mentioned above a two stroke utilises the movement of the piston and intake and exhaust ports on the cylinder bore to deliver the fuel to the system, as opposed to the valve system on a four stroke. The fuel in a two stroke engine, unlike 4 stroke, also contains oil as the engine is a sealed unit and needs lubrication. Two stroke engines are predominantly air cooled because of their simplicity, apart from marine two strokes.


What is meant by each of the following terms:
Mechanical Efficiency:

Mechanical efficiency is a term used to describe how efficient a machine is (in this case an engine) 100% efficiency would mean that all of the energy produced by the engine would be used, this is quite impossible though as engines lose energy through friction heat and noise. 

A maglev train has very high mechanical efficiency as the train does not come into contact with the rails and is sitting on a 'cousion' of  magnetic field. The electromagnets on the train are propelled by the opposite
 magnetic force from the magnets on the track.

Thermal Efficiency:
Thermal efficiency is a measure of the amount of heat produced by a machine, and how much of that heat is wasted. This figure is put into a percentage of thermal efficiency.

Volumetric Efficiency:
Volumetric efficiency is a measure of the movement of a fluid ( air, fuel) into a space. In internal combustion engines this is how easy it is for a charge to enter or exit the cylinder. This efficiency can be increased by removing bends in intake and exhaust pipes. A turbo charger decreases volumetric efficiency by slowing down the exhaust gas leaving the engine to drive the turbine, however makes up for this energy sap with the power it produces.

List 2 methods of lubricating the internal parts of 2 stroke S.I. engines?


As mentioned before may two stroke engines have oil in the fuel which lubricates the engine parts. Another method of lubrication is having a wet sump, this system utilises oil that is poured into the crankcase and is a high enough level that the crankshaft and big end bearing are submerged at BDC, the movement of the piston splashes oil up on to the walls of the cylinder and the piston. In four stroke engines the oil is pumped to the top of the cylinder.

This is a four stoke system.

What happens if the piston gap is too big?

If the Piston ring gap is too big then the cylinder could lose compression due to the large gap on a piston ring. 

What happens if the piston gap is too small?
If the gap is too small then the piston ring will not be able to expand as much. If the piston ring cannot expand then it may not seal the bore properly, leading to poor compression.

Although Two stroke engines have lost some popularity due to emission constraints they are still a popular choice for small engines like chainsaws as they are very simple and easy to produce and they can operate sideways or upside down due to the ports. Nearly all of the biggest ships in the world use low speed two stroke diesel engines, so in a way two stroke engines have revolutionised the world by allowing goods to be
transported vast distances efficiently.

You got to have balls to ride death machine like a two stroke racing bike


References:

http://en.wikipedia.org/wiki/Two-stroke_engine

http://science.howstuffworks.com/transport/engines-equipment/two-stroke.htm

http://auto.howstuffworks.com/diesel-two-stroke.htm

http://en.wikipedia.org/wiki/Volumetric_efficiency

http://en.wikipedia.org/wiki/Thermal_efficiency

http://en.wikipedia.org/wiki/Mechanical_efficiency

http://en.wikipedia.org/wiki/Piston_ring

Tuesday, July 5, 2011

Cooling: Marine engineering

Cooling in marine engineering is vital, an engine or part that creates friction and heat needs to cooled down, if it is not the engine can create excessive wear run poorly and eventually stop or seize due to the immense heat created. cooling systems have developed over the years from simple water pump systems to complex heat exchanger systems.



Explain the terms:

Conduction:

Conduction is the transfer of heat between objects of matter (solids). Heat moves from an area of high temperature to areas of lower temperature this is the basis of heat transfer in engines.

Conduction in a simple experiment.

Convection:

Convection is the transfer of heat in fluids (Air,Liquid), heat is transferred by molecules and follow a current of heat transfer called a convection current. heat can be removed from one area and be localised in another area, like in a heat transfer unit in a engine.
Simple diagram of a convection current

Radiation:

Radiation is the transfer of heat through empty space, heat emitted from an object or liquid travels out into cooler empty space or air. This principle is the basis of the radiator in a car.

a simple thermal radiation diagram

A nuclear explosion gives off an incredible amount of thermal radiation.

What is Direct cooling

Direct cooling in a marine application, is the cooling of an engine with a raw water pump system. This system works by drawing raw water (either salt water or fresh water) in through a port on the bottom of the boat called the sea cock. This raw water is then drawn through filters by a powerful pump and circulated through the water jackets of the engine, the heat from the engine is transferred to water. this water is then expelled from the engine. This type of cooling is very effective, however drawing raw water (especially salt water) through an engine can severely corrode the engines water jackets and damage the engine unless corrosion protection measures are in place. Another type of Direct engine cooling in automotive use is air cooling. This is where an engine is cooled by air flow over the hot cylinder block. Large fins are moulded on to the block to increase the surface area and therefore increase the radiation heat transfer quality's.

Simple Raw water intake system diagram

Just because i can, the most powerful air cooled radial engine. The pratt and whitney R-4360 major, Four radial engines connected to a common crank shaft with 28 pistons!

What is indirect cooling?

Indirect cooling in a marine application is a cooling system that draws heat away from the engine with water and cools the water using a heat exchanger. In some respects it is similar to a car radiator system. Water that is circulating around the engine block draws heat away from the engine parts. This hot water is pumped to the heat exchanger, this device works by drawing raw water through a similar system to direct marine cooling but the raw water is only pumped through the heat exchanger the hot water circulating the engine transfers it's heat to the colder raw water which is then expelled from the system. This solves the problem of direct cooling corroding an engine block with raw water, this system leaves only the interior of the heat exchanger corroded which car be quite easily replaced. The now cooled fresh circulating water is sent back to the cylinder block water jackets. Most modern marine engine use this cooling system. In the automotive trade a water pump circulates water around the engine and the water is cooled in a radiator which is exposed to air flow.

A heat exchanger system.

What is the purpose of a thermostat?
A thermostat is a device in engines that is designed to keep temperature at a preset constant. Engines have a optimum operating temperature which the thermostat trys to control. It works like a switch and opens to release a flow of cold water through the cooling system when a preset temperature on the thermostat is reached the thermostat will open. A thermostat that does not operate correctly can make an engine overheat.


a typical thermostat

Thermostat diagram

What is the purpose of a pressure cap?

The pressure cap on a radiator is for maintaining pressure in the cooling system. When water is under pressure it's boiling point increases so this makes the cooling system more effective. The pressure cap also has a bypass valve to an overflow reservoir which is activated when the engine is very hot and boiling (expanding) water needs somewhere to go to relieve pressure.

Pressure Cap diagram

Why are engines fitted with core plugs? What is the purpose of core plugs?

The core plug is a small plug on the outside of the water jacket on the cylinder block. The main purpose of the core plug is to plug up the small holes in the water jacket which are created when the cylinder block is cast. They serve other purposes too, one of which is to prevent leakage from the water jacket and also to provide a pressure outlet if the water inside the jacket freezes, ice expands and without the core plugs as an outlet the cylinder block could be cracked.


What is the core of a heat exchanger made of?

The core of a heat exchanger has to deal with high temperatures and the corosiveness of fresh and salt water. So therefore it is made of a strong metal with a high corrosion resistance. Heat exchangers come in different types like Plate type and shell and tube type heat exchanger. A plate type is now the most common and efficient type, it utilises plates with flowing hot water being cooled by cold raw water flowing through an adjacent plate.

Diagram of heat flow in a plate heat exchanger

How does the centrifugal circulating water pump work?
In a liquid cooled engine, water or another liquid transfers heat from the engine to the the water, if the water is not circulated then the water will become very hot and will lose all cooling heat transfer. A centrifugal water pump has a impeller powered by an electric motor or belt from the crank shaft, this impeller draws in water and forces it out circulating water through the water jacket. Because the water always flows one way through an engine, metal edges inside the engine can be rounded and obstructions can occur.

Centrifugal pump cutaway

How does a raw water pump work?

A raw water pump works in the same way as a centrifugal water pump, the only difference is that the raw water pump is used to 'suck' water through the sea cock from the ocean or lake for purposes of cooling the engine.

Raw water pump housing and drive gear

Why is it important to lubricate the impeller when fitting a new one?
The impeller serves the same purpose as the raw water pump. When a new impeller has been installed it is important to lubricate the impeller and its housing because for the first few turns of the impeller there will be no water flow to lubricate it. As a rubber impeller touches its housing this could rip the impeller apart.

Rubber Impeller removed from housing

When fitting a impeller what should you check before refitting the housing and why?
When refitting an impeller, checks should be made to ensure that the impeller is housed the right way but more importantly that the lock key is secured. This small lock key ensures that impeller spins with the shaft in the center by locking the impeller to it. If this is not secured the impeller will spin freely on the shaft and not draw in raw water which could overheat and damage an engine, it could also damage the impeller.

What is the purpose of an oil cooler and how does it work?

Oil just as much as coolant water can heat up to the point where it is inefficient at its job (lubrication), oil percolates at high temperatures and loses its qualities. An oil cooler transfers heat from the oil in a similar fashion to a heat exchanger. Water diverted from the raw water intake is sent to cool the oil via heat transfer. Oil has a higher boiling point than water so requires less cooling.

Automotive oil cooler

What is the process of a inter cooler/after cooler how does it work?

An inter cooler is essentially a radiator fitted to engines with forced induction. The inter cooler cools down air after it has left the turbocharger but before it has entered the engine. Cold air is more dense than hot air so more cold air can fit into the combustion chamber, this is how an inter cooler increases engine performance. An inter cooler also reduces the chance of knocking in the engine as hot air from the turbocharger could make early detonation possible.
Automotive Inter cooler

Why do we have Inhibitors & antifreeze?
Antifreeze performs a variety of tasks in an engine including sealing and raising the boiling temperature of the water it is mixed with as well as lowering its freezing point. Overheating can damage parts of the engine and if the temperature is cold enough for the water to freeze the expanding ice could crack and destroy a cylinder block. Different strength mixtures of antifreeze are available. Strong mixture ratios are required in very cold or very hot areas where boiling or freezing is more common. Inhibitors are a liquid that prevents corrosion on the inside of an engine. A thin film layer of inhibitor coats the inside of the water jacket and pipes, this layer helps protect the metal from cavitation, oxidation, water flow and general wear.

An antifreeze tester measures the strength ratio of antifreeze to water

How do you test Antifreeze?
Antifreeze strength is tested with an antifreeze tester, this measures the strength of the antifreeze and tells you a percentage, from this percentage you can see whether you need to add antifreeze.

What is the purpose of a water muffle?
The purpose of a water muffler on a marine engine is to reduce the noise of the engine. Water is drawn into the engine by the raw water pump for cooling, when this water is expelled from the engine it leaves with the exhaust fumes from the engine. The water helps muffle the sound of the exhaust which is then expelled underwater at the propeller.



And just because i can again, The Strand Craft 22, a beautiful invention, with custom built super car that fits in the garage!

References:

http://www.boatsafe.com/nauticalknowhow/cooling.htm

http://www.dsmtalk.com/forums/showthread.php?t=204767

http://en.wikipedia.org/wiki/Shell_and_tube_heat_exchanger

http://en.wikipedia.org/wiki/Heat_exchanger

http://en.wikipedia.org/wiki/Plate_heat_exchanger