Transcript Chapter 5 PowerPoint

Lubrication and Cooling
Chapter 5
Aim
To understand principals of operation of
aircraft lubrication and cooling systems
Objectives
1.Describe the function of oil in combustion
engines
2.Describe the properties of oil
3.State the types of oil and how they are classified
4.Explain the design of typical oil systems
5.State operational aspects associated with oil
system operation
6.Describe the design of typical engine cooling
systems
7.State operational aspects associated with cooling
system operation
1. Function of oil
Oil must perform a number of functions in the engine
including
• Lubrication
• Cooling
• Cleaning
• Protection
• Sealing and shock absorption
1. Function of oil
Lubrication
The primary job of the oil is to interpose a film of oil between the moving
parts of the engine to reduce friction
Without this power losses due to heat would be great and the engine would
seize up resulting in complete engine failure
1. Function of oil
Cooling
Oil is pumped in and around the moving parts of the engine where high
temperatures exist
As it is circulated through the sump and the engine cooler the heat is
dissipated before entering the hot areas of the engine again
1. Function of oil
Cleaning
Over time dirt, grit, combustion bi-products and metal flakes will end up in
and around the moving parts of the engine
In order to prevent friction losses and engine damage that these
contaminants could cause oil is used to remove them
An oil filter is used to prevent these contaminants from continually
circulating through the engine
When the filter is replaced the cartridge is inspected for metal contaminants
as this may be an indication of high engine wear
1. Function of oil
Protection
When exposed to moisture, salt or chemical products rust or pitting can form
on internal engine parts
Cylinder walls and parts of the crank shaft that have been hardened are
particularly susceptible to corrosion
1. Function of oil
Sealing and Shock Absorption
Oil provides a seal between the piston and the cylinder wall preventing the
charge or exhaust gases from escaping into the crank case
Oil around the crankshaft, bearings, connecting rods, etc helps cushion the
shocks produced by reciprocating forces
2. Properties of oil
Viscosity
Viscosity is a resistance to flow of any liquid
A liquid with high viscosity will be thick and not flow easily,
As a liquid is heated it becomes less viscous
Practically when an engine is cold, such as just after start up, the oil will not
be performing its functions to the best of its ability
We must allow the oil to reach working temperatures (inside the green
range) before applying high power settings. On cold days this may mean
keeping the power in the low range before running it up
Conversely if the oil temperature is too
high oil will have too low viscosity to be
able to perform its functions
The type of oil used is determined by the
outside air temperatures in which the
aircraft will be operating
2. Properties of oil
Ignition Point
Oil must have a sufficiently high flashpoint to ensure it does not
vaporise or catch fire within the normal operating temperatures of the
engine
Stability
Oil must be chemically stable and not change its characteristics over the
operating range of the engine
3. Classification of oil
Straight Mineral Oil
In modern aircraft straight mineral oil is used in a new engine to
encourage the moving parts to rub against each other and bed-in
It is typically only used up to the first oil change (10-25 hours
depending on the engine), a placard will be placed in the engine bay
and the maintenance release will be endorsed
Mineral oil has a tendency to oxidise when
exposed to high temperatures. It also has a
higher tendency to pick up accumulated carbon
deposits associated with the normal combustion
process, clogging the filter
3. Classification of oil
Ashless Dispersant Oil
AD oils are the most common in general aviation
Does not have the carbon forming properties of straight mineral oil,
the dispersant additives prevent the collected carbon deposits from
forming into larger masses
The oil will begin to darken soon after an oil change due to the
suspended carbon particles
3. Classification of oil
Synthetic Oil
Synthetic oils are used on aircraft operating over a wide temperature range
Typically only found in turbine engines
Unless specified in the flight manual do not use synthetic oil in piston
engines as they may destroy seals
3. Classification of oil
Oil Grade
The correct grade of oil must be matched to the engine in line with its
operational requirements, manufacturers recommendations and
maintenance cycle
Oils are graded in line with the society of automotive engineers rating
system and are assigned an SAE number. Aviation oil is given a
commercial aviation number which is double the SAE rating
The oil grades used in UniSA aircraft are:
• 100 (SAE 50) – A straight mineral oil used to wear in new engines,
typically for the first 50 hours
• W 100 (SAE 50) – Ashless dispersant
mineral oil used during normal
operations for C172SP and BE55
aircraft
• W 100 Plus (SAE 50) - Ashless
dispersant mineral oil with anti wear
and corrosion additives. Used in the
older aircraft in the fleet C172RG,
C172N and PA28
4. Design of lubrication system
Wet sump
Most light training aircraft use a wet sump type of oil system
In this system a mechanically driven pump pumps oil out of the sump and
though the engine via the oil filter and cooler
The filter and cooler have bypass valves incorporated into them in order to
bypass the system should either component fail
After the oil has done
its work it sinks back
down to the sump via
gravity
The disadvantage of
this system is that
inverted flight is
limited (10-30
seconds)
4. Design of lubrication system
Oil System
The dry sump oil system is mostly used on aerobatic aircraft where extended
periods of inverted flight and unusual attitudes mean if the wet sump type
were used, oil starvation could potentially occur
The major difference is that a scavenger pump is utilized in the engine to
take the used oil to an external tank
As with the wet sump
system, oil
temperature and
pressure is read just
before the oil enters
the engine
5. Operation of lubrication system
Pre-flight
During pre-flight it is important to check for any leaking fluid or blocked air
intakes (in front of the oil cooler)
Oil quantity must be within manufacturers limitations
Only use the correct grade of oil if refilling and do not mix oil types
If the oil is over-filled, excess will escape though the breather valve once the
engine has started
5. Operation of lubrication system
Malfunctions – Low oil pressure
Can be caused by a number of factors including:
• Low oil quantity due to poor pre-flight, leaking tank or broken supply
line
• Loss of oil pressure due to pump failure, failure of pressure line or
bearing failure, etc
• High oil temperature causes the oil to become less viscous, reducing
pressure
• Faulty gauge, if a corresponding rise in oil temperature is not observed
gauge failure is the likely cause
5. Operation of lubrication system
Malfunctions – High oil pressure
Most likely caused by a faulty pressure relief valve
If the pressure is excessively high damage to the system such as seal failures
may occur resulting in complete engine failure
Malfunctions – Fluctuating oil pressure
A fluctuating gauge is an indication that the oil quantity is dangerously low and
the pump is drawing in air from either the sump or storage tank
In the case of a dry sump system it may indicate a failure of the scavenge pump
5. Operation of lubrication system
Malfunctions – High oil temperature
High oil temperature may be the result of general overheating of the engine
Low oil content results in less cooling, giving a higher oil temperature
It could also indicate an issue in the oil cooler (bypass valve stuck open)
If a corresponding decrease in oil pressure is noted it may indicate immanent
failure of the system
6. Cooling System Design
Cooling Systems
During the combustion process, high
temperatures are generated within the engine.
The majority of this heat is dissipated through
the exhaust system however up to 33%
remains in the engine and must be dissipated
by the cooling system
Some older, large piston engines use a liquid
cooling system (radiator). These can be heavy
and expensive to maintain
Most modern piston engines use air cooling
systems where air is taken in through ducting
and directed around the engine with baffles
before spilling overboard
Some aircraft are fitted with adjustable cowl
flaps to enable the pilot more control of engine
cooling
6. Cooling System Design
Cooling Fins
In order to help dissipate the heat in the engine, cylinders are cast with cooling
fins around the cylinder heads and barrel
These work by increasing the surface area of the cylinder
6. Cooling System Design
Propellor
Slipstream from the propeller will aid in cooling
Most aircraft are fitted with spinners on the propeller hub to help direct airflow
into the engine
7. Operating the Cooling System
Operational aspects
Monitor the cylinder head temperature where possible
In general terms, to avoid overheating you should avoid:
• Running the engine on the ground for extended periods with power at idle,
where the slipstream from the prop can not aid in cooling. Park into wind if
possible to aid cooling
• Climbing the aircraft with high power settings and low airspeed for
extended periods
• Descending with the throttle closed for extended periods (shock cooling)
• Sudden, large decreases in throttle/manifold pressure
Proper use of cowl flaps and carburettor heat should ensure the engine is
maintained within the correct operating temprature range
Refer where possible to the aircraft flight manual
Questions?