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PISTON ENGINES
Part 7
Piston Engine Operations(Cooling and Thrust)
IntroductionOf the total heat produced in a piston engine,
about 50% is used as energy, 40% passes into the atmosphere,
and the remaining 10% is absorbed by
the pistons, cylinder heads, valves etc.
and by the lubricating oil that splashes against them.
Unless these components are cooled by some means,
they become over-heated,
causing distortion, increased wear, and a reduction in strength.
Excessive temperatures can also result in engine seizure.
Two methods of cooling are in common use;
Air Cooling and Liquid Cooling.
Air Cooling
A prime example of Air Cooling
Simply expose the cylinders to the slipstream air !
The advantages of Air Cooling include :-
Simple and less costly to produce.
Lighter engines.
But there are disadvantages :-
Difficult to cool all cylinders evenly, especially inline engines.
Difficult to get engine to run at most efficient temperatureon ground and at altitude.
Except for small amount of ‘prop-wash’, early systems only effective when aircraft or vehicle
is moving forward.
Air cooled engines tend to be noisier than liquid cooled engines.
Not so easy to streamline.
Air CoolingAir Cooling Flows
Modern flat-four air cooled engine
Cylinders
Air cooling is light, relatively cheap and little or no maintenance is required
as there are few working parts.
In air-cooled engines the outer surfaces of the cylinder head
is extensively finned and a flow of air is directed over
the exposed surfaces.
The fins are kept thin and close together
to obtain a maximum surface in contact with the air.
Air cooling is light, relatively cheap and little or no maintenance is required
as there are few working parts.
The aircraft cross section shows a typical cooling airflow
through the forward facing intakes,one each side of the spinner
directly in line with the cylinders.
An outlet at the bottom is provided to allow the cooling flow to escape.
Airflow circulation in different aircraftwould follow similar principals.
Air CoolingAir Cooling Flows
Cross section of typical light aircraft
Air Outlet
Air Intake
Air CoolingForced Air Cooling
Typical single cylinder engine
Fan
Cooling Air DuctCylinder
Crankcase
Fan assisted and ducted airflow can overcome some of the air cooling disadvantages.
Engines can be fitted with a fan driven by the crankshaft, and a duct to direct the air flow over the cooling fins.
The downside is that the fan takes power from the engine.
The engine shown hereis fan assisted air cooled, and is an engine for static
(or near static) industrial use
where a cooling flow would bedifficult to achieve.
The picture shows a typical light aircraft flat four installation,viewed from above.
Cooling air flow ducts are fitted to channel the cooling airdirectly over the cylinder cooling fins, ensuring maximum cooling.
Most modern vehicles have an electric fan instead of a crankshaft driven type.
This puts less load on the engine when forced cooling is not required.
The fan usually cuts in when in slow moving traffic, where forward speed is too low to provide ram air cooling.
Air CoolingForced Air Cooling
Light aircraft installation with cooling air ducts
to improve airflow over cylinders
Cooling Air Ducts
Air Cooling
Cooling Air Ducts
Air Outlet
Air Intake
Air cooling, however, cannot keep the engine
at a constant temperature under all conditions.
Consequently the engine components tend to have
a shorter life than those of a liquid-cooled motor.
Liquid CoolingThe advantages to Liquid Cooling include :-
The engine temperature can be maintained more evenly.
The operating temperature of the engine can be kept within a narrower range.
The more even temperature makes the engine more efficient.
The cooling ‘jacket’ makes the engine quieter.
The engine can be more easily streamlined and thus produce less drag.
But there are also disadvantages :-
Tend to be heavier.
More complex, more weight, costlier to produce.
The system needs more maintenance compared to air cooling.
Coolant / Engine COLD:-Coolant / Engine HOT:-
Thermostatic Valve CLOSED
Thermostatic Valve OPEN
Liquid Cooling
Coolant circulates around engine block onlyCoolant circulates around engine block and radiator
Pressure Relief Valve
Radiator
Cooling Fan
Liquid Cooling
Radiator
Thermostatic Valve
We will now look at these three areas
The temperature in the engine coolanteasily attains boiling point,
and could boil away.
Therefore boiling has to be avoided.
Pressure Relief Valve
This is achieved by pressurizing the cooling system.
If the system pressure rises above a design value, the excess will bleed off through the relief valve.
Constant loss of fluid with no obvious leaks probably means the relief valve needs replacing.
CAUTION: - NEVER REMOVE THE PRESSURE OR FILLER CAP
OFF A HOT ENGINE COOLING SYSTEM.
Radiator
Liquid CoolingThe Radiator
Hot Coolant In
Cooler Coolant Out
Expansion Chamber Connection
Top Hose Connection
Bottom Hose Connection
A radiator is a means to transfer heat from one medium to another,
so they are more correctly entitled Heat Exchangers.
The coolant flows along horizontal tubes,above and below which are
corrugated thin metal cooling strips; these tubes and cooling strips are called
the ‘Matrix’.
Hot coolant heats the metal strips,which are cooled by the passing air;
exactly the same as the cooling fins on an air cooled engine!
In this example we have air and coolant as the two mediums,other systems have the following: -
Fuel and Oil – unwanted heat in engine oil is transferred to the fuel, to prevent fuel icing.
Fuel Cooled Oil Coolers (FCOC) or Fuel Oil Heat Exchanger (FOHE).
Air and Oil – unwanted heat in the engine oil is transferred to the air, which like the air cooled engine, is then dumped overboard. Air Cooled Oil Cooler (ACOC).
Liquid CoolingThe Radiator
Cool Air Flow In
Warm Air Flow Out
Cooled Coolant
Heated Coolant
The Thermostatic Valve is usually housedin an easy to access bell housing
on the engine block, generally where the top hose
is connected to the engine.
Consisting of a temperature sensitive wax pellet,
within a rubber diaphragm,it is no more than a heat switch.
Liquid CoolingThermostatic Valve
Typical Thermostatic
Valve
Thermostatic Valve
Thermostatic Valve
Wax Pellet
Connection
Rubber Diaphragm
With a cold engine, the valve is closed;isolating the radiator flow,
and forcing coolant around the engine block only.
As the coolant heats up, it heats the wax pellet, which expands against the diaphragm,
pushing the valve open.
When the valve opens, coolant is directed to flow
through the radiator.
A ‘Jiggle-pin’ in the valve plateallows air to escape,
and a small coolant flow to the radiator so it starts to heat up
before the hot coolant is allowed full flow.
Liquid CoolingThermostatic Valve
CLOSEDNo FlowOPEN
with Flow
To Radiator
Rubber Diaphragm
‘Jiggle’ pin
Wax Pellet
Liquid CoolingOther Features
Expansion Tank
Heating Unit
Heater matrix
Air intake duct Fan
Feet
Screen Demist
Facia
Control valves
Pressure Relief Valve
The expansion tank provides for the heated coolant to expand.Most systems have the pressure relief valve on this tank,
and a simple gravity fill priming point on the radiator.
Hot coolant goes through a heater matrix, to heat the vehicle.The air passes through the matrix via the control valves,
and a multi speed fan can be switched on to increase the air flow into the vehicle.
Thrust Augmentation
What is it?A method of extracting more power
from Internal Combustion Engines.
It’s a form of ‘Power Boosting’
There are two types of booster: -
Supercharging and Turbochargingboth working on air compressors
Between the Throttle Valve and the Engine is a partial vacuum.
This vacuum works against the piston movement, and the cylinder may not be fully charged with Air/Fuel.
If Air was forced into the engine, there would be a better Air/Fuel mix in the cylinder.
Because more air means more fuel can be added.
More air and fuel means more power is developed. more THRUST !
Thrust Augmentation
AIR COMPRESSOR
CARBURETTORAIR
FILTER
ENGINE
INLET MANIFOLD
EXHAUSTMANIFOLD
The Air Compressor
Let’s take a closer look here.
LET’S TAKE THE LID OFF
The Air Compressor
TAKE A CROSS SECTION
CROSS SECTION
ROTOR
ROTOR
ROTOR VANES
STATOR VANES
OUTLET DUCT
The compressor works with a rotor, using centrifugal force.
The rotor spins the air between the rotor vanes,forcing it from the centre out to the rim,
then pushing the air through the static fins or stator vanes.
This compressed air is then piped to the fuel controller, and the engine, via the outlet ducts.
Thrust Augmentation
The Air CompressorThe air enters the ‘eye’ of the rotor,
and is forces outwards.
It is then pushed into the stator vanes,
and forced through the outlet ducts.
Thrust Augmentation
The Air Compressor
The Super ChargerSupercharging is an air compressor
driven by the engine via a gear train or belt drive.
They can take some 20% of the engine power to drive,but can increase overall engine power up to 50% or more,
giving a net gain of 30%+ power.
GEAR OR BELT DRIVE COMPRESSED AIR TO ENGINE
The Turbo ChargerTurbocharging is a similar air compressor,but driven by the engine exhaust gases,
passing through a turbine.
Turbo’s take very little power from the engine to drive them,therefore the net gain in power is relatively greater,
although they can suffer from what is called ‘turbo-lag’.
TURBINE BLADES
THROUGH EXHAUST PIPE
COMPRESSED AIR
TO ENGINE
Check of Understanding
What is the main benefit of using liquid, as opposed to air, to cool an engine?
The engine will be less costly to maintain
The engine will be lighter
The engine will be simpler
A steady temperature is easier to maintain
Of the total heat produced in a piston engine,
approximately how much is used as energy?
60%
20%
50%
40%
Check of Understanding
What is the purpose of the fins arranged about the cylinder and cylinder head
of an air-cooled engine?
To prevent the engine coolant from boiling
To reduce the engine weight
To allow heat to dissipate rapidly
To allow air to move freely through the engine compartment
Check of Understanding
Fan assisted airflow can overcome some of the air cooling disadvantages.
What is the major downside to this?
A partial vacuum forms between the throttle valve and the engine
The fan takes power from the engine
The fan makes the engine noisier
The temperature of the coolanteasily attains boiling point
Check of Understanding
In a liquid cooled engine, how is coolant boiling avoided?
By pressurizing the cooling system
By making the cylinder fins thinner and closer together
By forcing the coolant through a heater matrix
By the use of heat exchangers
Check of Understanding
What is the purpose of a thermostatic valve?
To bleed off excessive coolant
It isolates the radiator flow
It maintains pressurein the cooling system
It provides for the heated coolantto expand
Check of Understanding
Up to what increase in overall engine power
can supercharging produce in a piston engine?
60%
20%
50%
30%
Check of Understanding
PISTON ENGINES
End of Presentation