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