4-crankcase ventilation and induction systems(function, operation, inspection, and service) - automobile engineering



  
in the previous sections we had discuss :

- Classification of Internal Combustion Engines and Engine Components (Course)

- Internal Combustion Engines Components 2 (Course)

 3- Internal Combustion Engines cooling and lubrication system (Course)






 In This  section we will discuss :
-Describe the crankcase ventilation systems, function, operation, inspection, and
service.
-Describe the induction systems, functions, operation, inspection, and service
-Describe engine-starting devices/systems for diesel and gas engines.

 


 SECTION 4



Objective 8

Describe the crankcase ventilation systems, function, operation, inspection, and
service.



CRANKCASE VENTILATION

Function of the Crankcase Ventilation System

The crankcase ventilation system keeps the engine crankcase purged of blowby and
water

vapour. BIowby is burned and unburned gases that escape past the rings into the
crankcase.

The water comes from condensation or water vapour from incoming air and as a
product of

combustion. These gases and water together form corrosive acids and sludge in
the crankcase.


NOTE
The volume of the crankcase changes constantly as the pistons
move up and down, so the crankcase ventilation system has to "make up" air or breathe.
 

Operation of the Crankcase Ventilation System
The most common methods of crankcase ventilation are:

• Passive ventilation systems
A passive system usually has a breather on the valve cover, with a mesh filter
to let air into the engine and a vent tube along the side of the engine to allow crankcase
air to escape to the atmosphere. This system usually has some positive pressure in thecrankcase.


• Positive crankcase ventilation

Most industrial engines have a positive crankcase ventilation system. Positive ventilation systems can have either a positive pressure in the crankcase  (typical of automotive engines) or a vacuum in the crankcase. Both systems have breathers
for air to enter the crankcase, but the air is pulled out of the engine by a tube
attached from the crankcase to the intake manifold. On turbocharged engines with pressure in
the intake manifold, the line is connected before the turbo or, on some engines,
connected to a venturi set up in the exhaust system. The negative pressure positive
crankcase systems create a vacuum of 0 to 0.5 inches of water column in the crankcase. Changing the orifice size in the breather line sets the amount of vacuum created
in the crankcase. A vacuum in the crankcase helps prevent lube oil leaks, crankcase explosions and, since these engines are equipped with a crankcase manometer, provides early indication of engine problems.

Inspection and Service of the Crankcase Ventilation System
The breather on the crankcase ventilation system should be checked to make sure
it is present and not plugged. If it is not present, dirty air enters the crankcase and
contaminates the engine oil. Systems that have positive suction on the crankcase can have a problem with  overscavenging. This occurs when too much air is pulled through the system along with oil vapours, resulting in oil consumption. A breather assembly filled with a wire
mesh eliminates this 'pull-over' of oil and also an orifice limits the amount of air that passes
through the system. The crankcase ventilation system can be checked with a manometer for the
correct value. The manometer also indicates if there is too much blowby, which is caused
by leaking rings or scuffed liners.


Objective 9

Describe the induction systems, functions, operation, inspection, and service.


INDUCTION AND EXHAUST SYSTEMS

Function of the Induction System

The induction system must be able to supply clean air at the proper temperature
and Quantity to the engine for combustion.

• The induction system cleans the air so that it is free of abrasive particles
that can affect the life of the engine.

• The induction system ensures that a sufficient quantity of air is supplied for
the size of the engine.

• The air should be at the correct temperature to aid in combustion and engine efficiency.

• The air also aids in cooling the valves and other internal parts. This is
particularly important on turbo-charged engines.

• The inlet system must also silence the incoming air.

Industrial engines use a lot of air ! An inadequate volume of air for a given amount of fuel causes excessive fuel consumption and can lead to a loss in power. A typical
industrial engine can use approximately 4320000 cubic feet of air in one day. That is a lot of air
to clean. Clean air is important. Dirt in the air is the engine's worst enemy because it
is abrasive and prematurely wears the rings and liners. An engine without an air filter can be
ruined in a very short time. On industrial sites, the air may appear to be clean, but there is a
lot of dirt suspended in the air. Air temperature is also important. For maximum efficiency the best temperature is from 15ºC (60º P) to 32ºC (90º P). If the air is too hot the power drops off
approximately 1 % for every 10º C (l8º P) rise in temperature over 32º C (90º P). If the air is too
cold in winter, the low air temperature leads to a drop in compression temperature, which results in poorer fuel ignition and thermal shock to the engine.


Operation of the Induction System

Paris of the Induction System

The induction system is contains a pre-cleaner, an air cleaner and an intake
manifold and may have a turbo, blower, or a piston-type air pump.


Air Filters

Pre-cleaners are installed before the air cleaner to increase the service
intervals of the air cleaner and are usually installed in an area relatively free from dust.
Pre-cleaners remove larger particles of dirt or other foreign matter. Some pre-cleaners have a
spiral vaned drum that spins the air and forces the dirt out via centrifugal force. Dry air filters are the most common and have an efficiency of approximately 99%. The" density of paper used in the element controls the cleaning. As the element becomes dirty, the cleaning ability increases, but the flow through the element decreases. Dry  paper elements are versatile and are not messy to service.

Oil bath air cleaners have an efficiency of 95% to 98%. There are several
types, but most direct the air so that it has to make a sharp comer, at which point the air
encounters the oil. The dirt particles are caught in the oil. A mesh that is covered with oil traps
the remaining particles. Proper operation of the cleaner depends on maintaining the oil at the
correct level. It is also critical to use the correct viscosity and type of oil in the air
cleaner and that the correct size cleaner for the airflow of that engine be used; otherwise, the.
cleaning ability can be affected. An air cleaner that is too small for the application draws oil into
the intake. An air cleaner that is too large does not clean the air properly;


Spiral rotor air cleaners do not use an element; instead, they use the speed
of the air passing through a tube with a spiral inside. The air is spun by the spiral, throwing the
dirt particles outward and into a collector that is emptied by a venturi mechanism in the
exhaust stack. These systems can be used alone or in conjunction with dry cleaners as a
pre-cleaner.


Methods of Induction
Natural aspiration relies on atmospheric pressure to push air into the cylinders
when the piston moves downward. This is due to the fact that a slight vacuum is created
in the cylinder. If the cylinders are not sealing very well, the amount of air entering the
cylinder is affected. The speed of the engine also affects the amount of air entering the cylinder
because there is less time for atmospheric pressure to push the air in. Naturally aspirated
engines cannot reach 100% volumetric efficiency. Only four-stroke engines can be naturally aspirate?

Artificially aspirated engines use some mechanical means to push the air into
the cylinder.This can be a blower, turbocharger, or piston-type scavenge pump. With
artificial aspiration, the charge of air in the cylinder can be above atmospheric pressure.. If the
charge of air in the cylinder is above atmospheric pressure, it is referred to as supercharging or
boost pressure. The more boost the engine has, the more power the engine is capable of
producing. There are limits to the amount of supercharging because engine life can be threatened by too much boost. Two-stroke engines must be artificially aspirated; four-stroke engines can be artificially aspirated. Methods for moving the air are discussed later in the
module. Piston-type scavenging pumps (reciprocating blowers) are used on some very large
two stroke industrial engines. This is not the most effective way to move air into the
cylinder and the pump is often supplemented by a turbocharger. Very small two-stroke engines use the bottom of the piston and a sealed crankcase to push air into the cylinder.

Blowers are air pumps that are mechanically driven by the engine. They can be
belt or gear drive. The speed at which they are driven determines the boost given to the
engine. Blowers have a parasitic effect on the engine because of the amount of power required to drive the blower. There are two types of blowers:


• Lobed blowers (roots blowers), which are positive displacement units that are
used on engines. These blowers have two rotary members with two or three lobes. The
lobes are timed with gears to avoid contact. The air delivered is proportional to
engine speed and run at relatively low rotational speeds (2000 to 6000 rpm). These are mostly used on two-stroke industrial engines.

• Centrifugal blowers, which are non-positive displacement units that look like turbochargers except that they are mechanically driven by the crankshaft. These blowers use high velocity air to increase air pressure and have high rotational speeds (10 000 to 50 000 rpm). Their most common use is on large, stationary, two-stroke engines.

Turbochargers (turbo) are the most popular and effective method used to increase
the power on modern engines. Turbochargers are radial flow centrifugal compressors
driven by turbine wheels. The engine's exhaust gases drive the turbine wheel. There is no
mechanical link between the turbocharger and the engine. This may make it appear as if there are no parasitic

losses when using a turbocharger. This is only partially true; the turbocharger
causes back pressure in the exhaust system, leading to some pumping losses Turbo speed
is determined by the amount of fuel burned (exhaust produced); not by engine speed. Turbochargers are more efficient than blowers except that they do not start
turning until the engine is making exhaust. This makes it difficult to use a turbo on a two.
stroke engine. Twostroke engines that have turbochargers have mechanically driven blowers and a turbo, or have only a turbo with provision for jets of compressed air to star the turbo turning
for start-up and low speed operation. (More on turbochargers in turbocharger section of this
module).


Two-Stroke Scavenging
Supercharging should not be confused with scavenging. The scavenging process in
a two cycle engine is used to force out the spent exhaust gases and to replace them
with fresh air at approximately atmospheric pressure. The engine mechanically drives the
scavenging pump. The air density after cylinder scavenging is no greater than it would have been had atmospheric air pressure filled the cylinders. Scavenging pumps may be roots or
centrifugal blowers, piston-type pumps, or turbochargers.

Types of Scavenging
Loop flow scavenging occurs when the flow of the air into the cylinder and the
exhaust flow out of the cylinder come from the same side of the cylinder. This forms a loop
in the gas flow.

Uniflow scavenging is used on engines. The inlet and exhaust ports are at
opposite ends of the cylinder; thus, the airflow is from one end of the cylinder to the opposite.
This is a very affective scavenging arrangement.The crossflow scavenging arrangement has the exhaust ports on opposite sides of the cylinders. The flow of the gas is from one side (intake side) of the cylinder to the top and

back down to the opposite side (exhaust side) to the exhaust ports.




Inspection and Service of Induction Systems

Air Cleaner Service
For all filter types it is important that the ducting after filtration does not
leak. Check to ensure that the filter elements have sealed properly (some use rubber strips).
Leakage through the piping or past the air filter allows dirt into the engine and reduces engine
life considerably.


Air cleaner condition can be checked for restriction with several methods:
• Air cleaner service indicators are commonly used on intake manifolds. They
flag when restriction in the manifold reaches a predetermined amount.

• The most accurate method of measuring air cleaner restriction is using a water manometer.


Dry Air Cleaners
Dry or paper element filters can be checked visually by placing a light behind
the element and looking for the amount of light that comes through. The element can easily
be replaced. Dry elements can be cleaned by carefully using compressed air (blow from the
inside outward). Some paper elements can be washed and dried when they become too
dirty. The element should be inspected for holes or tears if it is to be reused. Never use
an air cleaner that has holes in the element.


Oil Bath Air Cleaners
Service oil-type filters by removing the oil cup, then cleaning and replacing
the oil to the correct level. Allow no more than ½ inch of sludge to accumulate before
servicing the filter.

(USE THE CORRECT VISCOSITY OIL. This is very critical in cold weather).


NOTE
Oil bath air cleaners should be the correct size for the engine. Failure to
use the correct size air cleaner results in several problems. If the cleaner is too small, oil
"pull over" into the engine can result. If the filter is too big for the engine, the engine
does not filter well because of the reduced air velocity.


Pre-Cleaner Service
Some pre-cleaners are serviced daily, depending on the dust conditions. Others
have a oneway rubber ejector valve in the canister to eject dirt and water. Most larger air
cleaners eject the dirt automatically with the aid of a venturi in the exhaust system. Check
pre-cleaners for damage; otherwise, they will be rendered useless.


Manometers
Manometers are valuable tools for checking air cleaner restriction, turbo boost,
and manifold pressure or vacuum. The V-tube manometer is a primary measuring device that indicates pressures or vacuums by the difference in the height of two columns of fluid. It is usually used for low-pressure applications. There are two types: water-filled and mercury-filled.


Water Manometers
Connect the manometer to the source of pressure or vacuum. When a pressure is
imposed, add the number of inches by which the water in one column of fluid travels
upwards to the amount the other column travels downwards to obtain the pressure (of vacuum)
reading.


Mercury Manometers
The height of a column of mercury is read differently than that of a column of
water. Mercury does not wet the inside surface of the tube; therefore, the top of the column
has a convex meniscus (shape). Water wets the surface and therefore has a concave meniscus. A mercury column is read by sighting horizontally between the top of the convex mercury
surface and the scale. A water manometer is read by sighting horizontally between the bottom
of the concave water surface and the scale. Should one column of fluid travel farther
than the fluid in the other column, due to minor variations in the inside diameter of the tube
or to the pressure imposed, the accuracy of the reading obtained is 1101 impaired. Refer
to the Conversion Chart to convert the manometer reading into other units of
measurement.

Intake Manifold Boost Pressure (Turbo Boost)
Use a mercury manometer to check pressure in the intake manifold of a
supercharged engine. The engine must be run at full load in order to check a turbocharged engine. The pressure ranges from 10 to 50 in. Hg.


Function of the Exhaust System
The exhaust system has an exhaust manifold, piping and a silencer. The exhaust
system must remove the exhaust with a minimum of restriction, good flow characteristics, and
adequate silencing.


Operation of the Exhaust System
Exhaust systems are tailored for the engine. Piping that is too large or too
small can affect the efficiency of the exhaust flow. The length of the piping can also affect the
exhaust flow. Connecting the exhaust systems of several engines causes condensation in the
cylinders of the engine that is not running. Silencers or mufflers should have a minimum
backpressure, but must reduce the noise from pulsations in the exhaust system. Some newer
engine installations are equipped with catalytic converters to comply with government
exhaust emission standards. Many natural gas engines have water-cooled exhaust manifolds. This is because of  the high exhaust temperatures experienced by the engines. Water-cooled exhaust manifolds help prevent warping and cracking of the manifold, but also reduce the radiant heat  to other parts of the engine and surroundings. .


Waste Gates and Dump Valves
If too much boost is produced in the intake the engine can generate more power
than the engine parts can withstand; therefore, the maximum pressure boost is regulated.
Waste gates are used on some turbochargers to regulate the turbo air pressure by dumping
the. exhaust gas to a bypass lireaches the turbine; thus, the turbine speed decreases and is not
able to compress as much air. Waste gates may have various controls, from very simple to sophisticate with multiple controls.'


Intercoolers and Aftercoolers
Intercoolers and aftercoolers cool the air between the turbocharger and the
cylinder. These units are heat exchangers placed between the supercharger (turbocharger) and the  engine. Some are located in the engine intake manifold (intercooler) or elsewhere (afrercooler), and they can use engine coolant or ambient air to absorb the heat from' incoming
compressed air from the supercharger. By cooling the air, the air's density is increased; thus,
more fuel can be added and a further power increase can be obtained from the same displacement  engine. Air at 32°C (90°F) compressed by a turbocharger to double atmospheric pressure reaches approximately 150°C (300°F). Aftercooling results in cooler and more air
entering the cylinder. This helps in cooling engine parts (valves and pistons), lowering
cylinder peak pressures, and lowering the exhaust temperature. The most common aftercoolers
are air-towater or air-to-air. Many larger engines have a separate cooling system and water pump for the aftercooler.


Objective 10

Describe engine-starting devices/systems for diesel and gas engines.



STARTING METHODS
In order to start an internal combustion engine it is necessary to rotate the
engine by some means. This rotation will allow a charge of air (diesel engine) or air-fuel
mixture (gasoline engine) to be drawn into the cylinders and compressed prior to firing. The
rotation will also cause certain auxiliaries to operate such as the fuel injection system in the
diesel or the carburetor and ignition system in the gasoline engine. Several different methods of rotating the engine may be employed. In the case of a very small engine, a hand turned crank may be used. With larger engines, the method usually used is an electric motor driven by a storage battery. Large diesel engines most commonly use compressed air, which is admitted into one or more cylinders to drive the engine pistons.


Electric Starters
An electric starter motor powered by a storage battery may start small engines
and even engines up to 1000 kW. The motor cranks the engine until ignition takes place.
The engine speed increases and disengages the starter motor gearing. The operator shuts off
power to the starting motor, when the engines fires up. An electric starter motor is shown in
Fig.41. The pinion drive gear connects it mechanically to the engine flywheel. The electric  motor is a dc type, enclosed to make it weatherproof.


Air Starting Systems
Some medium sized and most large diesel engines are started with compressed air.
Starting by compressed air takes one of two forms: the air might be directed to an air
motor to carry out cranking, or it might be led through a distributing device to the main power
cylinders of the diesel engine. This system requires a two or three-stage compressor to fill
the starting air receiver with air at a pressure of 2000 - 2500 kPa. The air receiver capacity
should be sufficient for twelve starts without recharging from the compressor. When air is to be fed to the main power cylinders, the air-distributing device  is operated by the camshaft and serves to direct the high-pressure air to each cylinder on its  power stroke. The air, usually at about 2100 kPa, is fed through air-starting valves fitted in the cylinder heads. The air admitted drives the engine in a manner similar to that of steam driving a steam engine.

The expansion of the starting air cools the cylinders and makes ignition
difficult. Therefore, only a few of the cylinders are used for starting. The engine is turned to a
specified starting position indicated on the flywheel. The engine must have timing valves to supply starting air through starting valves (see Fig.42) to the cylinders on their power strokes.


The engine should fire after the first full compression stroke and no more
starting air is supplied. A starting system using an air motor is shown in Fig.43. The air starting
system has its own tank, compressor and drying system. A push button valve activates flow of air to the starting motor via a relay valve. The air, which turns the starting, motor and is vented
to atmosphere. As the air is dry and has no lubrication qualities, lubricating oil is added to
the air motor. The air motor will turn over the diesel engine until it starts or until the air
supply runs out. The air motor system will turn the engine faster than an electric starter. This is an
advantage in building up the temperature for the diesel to start.


Hydraulic Starting Systems
The hydraulic starting system uses a hydraulic motor to turn over the diesel for
starting. This type of system is shown in Fig.44. The accumulator holds a quantity of oil under
high pressure, 10 000 to 20 000 kPa. To start the motor the oil flows from the
accumulator to the hydraulic motor, causing rotation. The low-pressure oil flows back to a
reservoir. The starter motor has a pinion, which turns the diesel engine flywheel. When
the diesel starts, the flow of oil is shut off and the pinion drive disengages. The engine driven
hydraulic pump restores the pressure in the accumulator. A hand pump is also supplied as a back up should the engine driven hydraulic pump fail.

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