Piston Engine Operations PISTON ENGINES Part 7 Piston Engine Operations (Cooling and Thrust)

Introduction Air Cooling and Liquid Cooling. Of 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 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 temperature on 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. A prime example of Air Cooling Simply expose the cylinders to the slipstream air !

Air Cooling Air Cooling Flows Air cooling is light, relatively cheap and little or no maintenance is required as there are few working parts. Cylinders 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. Modern flat-four air cooled engine

Air Cooling Air Cooling Flows 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 aircraft would follow similar principals. Air Intake Air Outlet Cross section of typical light aircraft

Air Cooling Forced Air Cooling 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 here is fan assisted air cooled, and is an engine for static (or near static) industrial use where a cooling flow would be difficult to achieve. Cylinder Cooling Air Duct Fan Crankcase Typical single cylinder engine

Air Cooling Forced Air Cooling The picture shows a typical light aircraft flat four installation, viewed from above. Cooling air flow ducts are fitted to channel the cooling air directly 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. Light aircraft installation with cooling air ducts to improve airflow over cylinders Cooling Air Ducts

Air Cooling 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. Air Outlet Air Intake Cooling Air Ducts

Liquid Cooling The 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.

Liquid Cooling Coolant / Engine COLD:- Coolant / Engine HOT:- Thermostatic Valve CLOSED Thermostatic Valve OPEN Pressure Relief Valve Cooling Fan Radiator Coolant circulates around engine block and radiator Coolant circulates around engine block only

Liquid Cooling We will now look at these three areas Thermostatic Valve Pressure Relief Valve The temperature in the engine coolant easily attains boiling point, and could boil away. Therefore boiling has to be avoided. Radiator 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.

Liquid Cooling The Radiator A radiator is a means to transfer heat Expansion Chamber Connection Hot Coolant In Top 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’. Radiator Cooler Coolant Out Bottom Hose Connection

Liquid Cooling The Radiator 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). Heated Coolant Warm Air Flow Out Cool Air Flow In Cooled Coolant

Liquid Cooling Thermostatic Valve The Thermostatic Valve is usually housed in 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. Thermostatic Valve Thermostatic Valve Connection Typical Thermostatic Valve Rubber Diaphragm Wax Pellet

Liquid Cooling Thermostatic Valve 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 plate allows 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. OPEN with Flow CLOSED No Flow To Radiator ‘Jiggle’ pin Rubber Diaphragm Wax Pellet

Liquid Cooling Other Features 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. Pressure Relief Valve Air intake duct Fan Feet Screen Demist Facia Expansion Tank Heater matrix Control valves Heating Unit

Thrust Augmentation What is it? Supercharging and Turbocharging 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 Turbocharging both working on air compressors

Thrust Augmentation The Air Compressor Let’s take a closer look here. 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 ! CARBURETTOR AIR FILTER ENGINE INLET MANIFOLD EXHAUST MANIFOLD AIR COMPRESSOR Let’s take a closer look here.

Thrust Augmentation The Air Compressor 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. STATOR VANES OUTLET DUCT ROTOR VANES ROTOR CROSS SECTION LET’S TAKE THE LID OFF TAKE A CROSS SECTION

Thrust Augmentation The Air Compressor The 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.

The Super Charger The Air Compressor Supercharging 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. COMPRESSED AIR TO ENGINE GEAR OR BELT DRIVE

The Turbo Charger Turbocharging 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’. COMPRESSED AIR TO ENGINE TURBINE BLADES THROUGH EXHAUST PIPE

Check of Understanding What is the main benefit of using liquid, as opposed to air, to cool an engine? The engine will be lighter A steady temperature is easier to maintain The engine will be simpler The engine will be less costly to maintain

Check of Understanding Of the total heat produced in a piston engine, approximately how much is used as energy? 20% 40% 50% 60%

Check of Understanding What is the purpose of the fins arranged about the cylinder and cylinder head of an air-cooled engine? To reduce the engine weight To allow air to move freely through the engine compartment To allow heat to dissipate rapidly To prevent the engine coolant from boiling

Check of Understanding Fan assisted airflow can overcome some of the air cooling disadvantages. What is the major downside to this? The fan takes power from the engine The temperature of the coolant easily attains boiling point The fan makes the engine noisier A partial vacuum forms between the throttle valve and the engine

Check of Understanding In a liquid cooled engine, how is coolant boiling avoided? By making the cylinder fins thinner and closer together By the use of heat exchangers By forcing the coolant through a heater matrix By pressurizing the cooling system

Check of Understanding What is the purpose of a thermostatic valve? It isolates the radiator flow It provides for the heated coolant to expand It maintains pressure in the cooling system To bleed off excessive coolant

Check of Understanding Up to what increase in overall engine power can supercharging produce in a piston engine? 20% 30% 50% 60%

PISTON ENGINES End of Presentation