Basic Working Principles of a Vehicle Air Conditioning System
What we will attempt is to describe, in layman’s terms is how the air condition system on a vehicle works and what happens when we press the “A/C” button on the vehicles dashboard.
The basic working principle of all cooling systems, be it the domestic freezer, the Air Conditioning system in our office or the one fitted to our vehicle is the same. The fundamental processes at work are four basic principles in physics, which we all have probably experienced in everyday life Compression, Expansion, Evaporation and Condensation.
When you pump up a bicycle tyre, the body of the pump where the air is compressed above the pressure inside the tyre, causing the transfer of air into the tyre gets hot. As the tyre inflates and more effort is needed to compress the air to an ever higher pressure the pump gets even hotter. The tyre also becomes heated by the now hot gases entering through the valve.
When you discharge an aerosol can the body of the can gets cooler because of the reduced pressure inside the can as the liquid contents are expelled turning to gas.
The old sailors trick to find the wind direction by wetting a finger and feeling which side is chilled by the passing flow of air.
Whenever warm wet air touches a cold surface such as the outside surface of a glass of ice cold beer heat is from the air and transferred to the cold surface reducing its temperature turning the water vapour back to liquid droplets of water.
The air conditioning system in our vehicle has specific components that employ the above physical processes in order to reduce the temperature of the air circulating in the vehicle cabin. In addition to cooling the air the Air Conditioning system have the added benefit of removing excess moisture from the air entering the cabin reducing the amount of condensation that forms on the inside of the cars windows, improving visibility for the driver.
Working Components of a Vehicle Air Conditioning System
Let us now go to the workings of an automobile air conditioner and how the above principles are applied. We will take it component by component.
Compressor: This is the heart of the air conditioning system. The compressor similar in size to the vehicles electrical alternator can usually be identified as the component sited low down in the engine bay driven by the engine belts via a pulley and connected to the rest of the air conditioning system by two reinforced hoses. When you turn on the air conditioner in your car an electrical circuit operates a clutch in the compressors pulley causing the compressor to start pumping refrigerant gas into the rest of the system under extremely high pressure. By increasing the pressure the refrigerant gas leaving the compressor becomes hot.
Condenser: The condenser can be identified as a second radiator that shares the air flow with the main engine coolant radiator. Usually the condenser will have its own electric cooling fan/s that become/s active when the air conditioning system is switched on. The condenser takes the heated high pressure refrigerant gas from the compressor and cools it. Condensing the refrigerant gas into a liquid releases heat in the process. This heat is expelled into the atmosphere by the air flowing through the condenser.
Receiver or Dryer: This can be identified as a small reservoir or canister sitting in in-line with the outlet hose from the condenser. Here any moisture that has contaminated the refrigerant is captured. If moisture or other contaminants are allowed to circulate it can damage the air conditioning system and ice crystals being formed can cause blockages.
Expansion Valve: The refrigerant next flows into the expansion valve where the pressure is reduced causing the liquid to revert back to a gas which causes rapid cooling of the refrigerant vapour. Often on humid days ice can be seen forming on the pipe work immediately after the Expansion valve.
Evaporator: This component is rarely seen, other than by service engineers as it is buried deep under the dashboard of the vehicle and shares the space occupied by the cabin heating system. Here the highly cooled refrigerant vapour absorbs the heat from the air inside the car by pushing the air from either the outside or re-circulated air from inside the cabin across the outside of the now super chilled evaporator circulating cold air inside the vehicle’s cabin..
A/C Tubes: These components are connecting all the components with different temperature and pressure conditions. Rubber material need to be tailor made based on used refrigerants and related oils. Several solutions are available for improved NVH performance based on whole systems vibration performances.
REFRIGERANTS R12, R134A, R1234YF
There are still numerous vehicles in the market with air conditioning systems originally designed for refrigerant R12. 2001 was the official final end for R12 in vehicle air conditioningssystems. Starting from that date, R12 system had to beconverted during maintenance or repair work. R134a was and is used as replacement refrigerant besides several”drop-in” refrigerants (refrigerant mixtures).
R134a has a high GWP (global warming potential) of 1430. Withthe current EC Directive 2006/40/EC it was decided to only userefrigerants with a GWP of less than 150 in the future. Thus, air conditioning systems of vehicles of class M1(passenger cars, vehicles for passenger transport with a maximum of 8 seats) and of class N1 (commercial vehicles witha gross weight limit of up to 3.5 tons), for which type approval isissued within the EU starting from 01.01.2011, may not be filled anymore with R134a. Starting from 01.01.2017, vehicles filled with R134a cannot be initially type-approved anymore. However,the use of R134a shall be further permitted for service and maintenance work on already existing R134a systems. R1234yfwith a GWP of 4 shall be used as new refrigerant. However, the use of other refrigerants is possible, as long as the GWP valuesis below 150. It remains to be seen to what extent all vehicle manufacturers will convert to the same or different refrigerants.
The power steering system in your vehicle enables you to steer your vehicle in the direction you would like it to proceed. Power steering is really “power assisted” steering. “Power assisted” steering will allow you to steer your vehicle manually when the engine is not running or if you have a failure in the power steering system which disables it.
Power steering utilizes a hydraulic pump running off a belt driven by the engine, this pump enables a small amount of fluid to be under pressure. This pressure in turn assists the steering mechanism in directing the tires as you turn the steering wheel. The power steering system typically includes a pump, power steering fluid, a pressure hose assembly, a control valve and a return line.
There are two basic types of power steering systems used on vehicles. The rack and pinion steering system and the conventional/integral steering gear system, which is also known as a recirculating ball steering system. The rack and pinion steering system is the most commonly used power steering system on todays’ vehicles. The steering shaft turns a gear that moves the rack side to side, utilizing a power unit built directly onto the rack assembly. The steering gear system is generally used most often on trucks, it has a series of steel balls that act as rolling threads between the steering shaft and the rack piston. The steering wheel shaft connects to a gear assembly and a series of links and/or arms that turn the wheels to the left or right
Power steering helps the driver of a vehicle to steer by directing some of the its power to assist in swiveling the steered roadwheels about their steering axes. As vehicles have become heavier and switched to front wheel drive, particularly using negative offset geometry, along with increases in tire width and diameter, the effort needed to turn the wheels about their steering axis has increased, often to the point where major physical exertion would be needed were it not for power assistance. To alleviate this auto makers have developed power steering systems: or more correctly power-assisted steering—on road going vehicles there has to be a mechanical linkage as a fail safe. There are two types of power steering systems; hydraulic and electric/electronic. A hydraulic-electric hybrid system is also possible.
A hydraulic power steering (HPS) uses hydraulic pressure supplied by an engine-driven pump to assist the motion of turning the steering wheel. Most recent PC & LDV use now Electric power steering (EPS) when HPS is still wide applied in MD & HD market
The function of the vehicle fuel system is to store and supply fuel to the engine. The engine intake system is where the fuel is mixed with air, atomized, and vaporized. Then it can be compressed in the engine cylinder and ignited to produce energy or power. Although fuel systems vary from engine to engine, all systems are the same in that they must supply fuel to the combustion chamber and control the amount of fuel supplied in relation to the amount of air.
The fuel is stored in the fuel tank and the fuel pump draws fuel from the tank. It then travels through the fuel lines and is delivered it through a fuel filter to the fuel injectors (carburetors and throttle body injection were used on older vehicles). As the fuel is delivered, the final conditions for providing complete combustion are atomization and the spray pattern of the fuel. Atomization is accomplished as a result of the injection pressure, due in part to the diameter of the holes in the injector. The spacing, angle and number of holes in the injector tip determine the spray pattern.
Depending on whether your vehicles fuel system is a return type or returnless type system, the fuel pressure is regulated differently. A return type system has a fuel pressure regulator that varies the fuel pressure based on the amount of vacuum from the intake system. This is so the amount of fuel pressure and flow of fuel as it reaches the injectors remains consistently the same. Whereas a returnless type system uses the powertrain control module (PCM) to regulate fuel delivery. There is a fuel pressure sensor mounted to the supply rail of the fuel injectors to allow the PCM to monitor fuel pressure. When the fuel pressure and flow starts to drop due to increase of engine speed or load the PCM compensates by increasing injector duration and/or operating speed of the fuel pump.
Here below possible type of alternative engine fuels to standard Petrol (with different % of Alchool) and Diesel:
Oil cooling is the use of engine oil as a coolant, typically to remove surplus heat from an internal combustion engine. The hot engine transfers heat to the oil which then usually passes through a heat-exchanger, typically a type of radiator known as an oil cooler. The cooled oil flows back into the hot object to cool it continuously.
If air-cooling proves sufficient for much of the running time (such as for an aero-engine in flight, or a motorcycle in motion), then oil cooling is an ideal way to cope with those times when extra cooling is needed
Transmission coolers are a simple solution to help prolong transmission life. Transmission heat is the prime reason for tranny failure. High performance applications like towing and high torque engines can build heat in the transmission and break down fluids. Transmission fluid works best at lower temperatures.
Because your automatic transmission works harder when you tow, it can get hotter, and heat is one of the major enemies of your transmission. An aftermarket transmission cooler can keep your transmission from getting too hot, helping you get the best performance and long life out of it.
Fluid heated by the transmission, engine, or power steering pump flows to the cooler. The air moving over the fins of the cooler cools the fluid, which is then routed back to the transmission, engine or power steering pump in a continuous loop through the return line
Selective Catalytic Reduction (SCR) is an advanced active emissions control technology system that injects a liquid-reductant agent through a special catalyst into the exhaust stream of a diesel engine. The reductant source is usually automotive-grade urea, otherwise known as Diesel Exhaust Fluid (DEF). The DEF sets off a chemical reaction that converts nitrogen oxides into nitrogen, water and tiny amounts of carbon dioxide (CO2), natural components of the air we breathe, which is then expelled through the vehicle tailpipe.
SCR technology is designed to permit nitrogen oxide (NOx) reduction reactions to take place in an oxidizing atmosphere. It is called “selective” because it reduces levels of NOx using ammonia as a reductant within a catalyst system. The chemical reaction is known as “reduction” where the DEF is the reducing agent that reacts with NOx to convert the pollutants into nitrogen, water and tiny amounts of CO2. The DEF can be rapidly broken down to produce the oxidizing ammonia in the exhaust stream. SCR technology alone can achieve NOx reductions up to 90 percent on Diesel Engine.
A SCR system consist in these components:
Water cooling is a method of heat removal from components and industrial equipment. As opposed to air cooling, water is used as the heat conductor. Water cooling is commonly used for cooling automobile internal combustion engines
Heat engines generate mechanical power by extracting energy from heat flows, much as a water wheel extracts mechanical power from a flow of mass falling through a distance. Engines are inefficient, so more heat energy enters the engine than comes out as mechanical power; the difference is waste heat which must be removed. Internal combustion engines remove waste heat through cool intake air, hot exhaust gases, and explicit engine cooling.
Engines with higher efficiency have more energy leave as mechanical motion and less as waste heat. Some waste heat is essential: it guides heat through the engine, much as a water wheel works only if there is some exit velocity (energy) in the waste water to carry it away and make room for more water. Thus, all heat engines need cooling to operate.
Cooling is also needed because high temperatures damage engine materials and lubricants. Cooling becomes more important in when the climate becomes very hot. Internal-combustion engines burn fuel hotter than the melting temperature of engine materials, and hot enough to set fire to lubricants. Engine cooling removes energy fast enough to keep temperatures low so the engine can survive
Some high-efficiency engines run without explicit cooling and with only incidental heat loss, a design called adiabatic. Such engines can achieve high efficiency but compromise power output, duty cycle, engine weight, durability, and emissions
Most internal combustion engines are fluid cooled using either air (a gaseous fluid) or a liquid coolant run through a heat exchanger (radiator) cooled by air. Thus, engine coolant may be run through a heat exchanger that is cooled by the body of water.
Most liquid-cooled engines use a mixture of water and chemicals such as antifreeze and rust inhibitors. The industry term for the antifreeze mixture is engine coolant. Some antifreezes use no water at all, instead using a liquid with different properties, such as propylene glycol or a combination of propylene glycol and ethylene glycol. Most “air-cooled” engines use some liquid oil cooling, to maintain acceptable temperatures for both critical engine parts and the oil itself. Most “liquid-cooled” engines use some air cooling, with the intake stroke of air cooling the combustion chamber.
There are many demands on a cooling system. One key requirement is to adequately serve the entire engine, as the whole engine fails if just one part overheats. Therefore, it is vital that the cooling system keep all parts at suitably low temperatures. Liquid-cooled engines are able to vary the size of their passageways through the engine block so that coolant flow may be tailored to the needs of each area. Locations with either high peak temperatures (narrow islands around the combustion chamber) or high heat flow (around exhaust ports) may require generous cooling. This reduces the occurrence of hot spots, which are more difficult to avoid with air cooling. Air-cooled engines may also vary their cooling capacity by using more closely spaced cooling fins in that area, but this can make their manufacture difficult and expensive.
Only the fixed parts of the engine, such as the block and head, are cooled directly by the main coolant system. Moving parts such as the pistons, and to a lesser extent the crank and rods, must rely on the lubrication oil as a coolant, or to a very limited amount of conduction into the block and thence the main coolant.
Liquid-cooled engines usually have a circulation pump. The first engines relied on thermo-syphon cooling alone, where hot coolant left the top of the engine block and passed to the radiator, where it was cooled before returning to the bottom of the engine. Circulation was powered by convection alone.
Conductive heat transfer is proportional to the temperature difference between materials. If engine metal is at 250 °C and the air is at 20°C, then there is a 230°C temperature difference for cooling. An air-cooled engine uses all of this difference. In contrast, a liquid-cooled engine might dump heat from the engine to a liquid, heating the liquid to 135°C (Water’s standard boiling point of 100°C can be exceeded as the cooling system is both pressurised, and uses a mixture with antifreeze) which is then cooled with 20°C air. In each step, the liquid-cooled engine has half the temperature difference and so at first appears to need twice the cooling area.
However, properties of the coolant (water, oil, or air) also affect cooling. As example, comparing water and oil as coolants, one gram of oil can absorb about 55% of the heat for the same rise in temperature (called the specific heat capacity). Oil has about 90% the density of water, so a given volume of oil can absorb only about 50% of the energy of the same volume of water. The thermal conductivity of water is about 4 times that of oil, which can aid heat transfer. The viscosity of oil can be ten times greater than water, increasing the energy required to pump oil for cooling, and reducing the net power output of the engine.
Comparing air and water, air has vastly lower heat capacity per gram and per volume (4000) and less than a tenth the conductivity, but also much lower viscosity (about 200 times lower: 17.4 × 10−6 Pa·s for air vs 8.94 × 10−4 Pa·s for water). Continuing the calculation from two paragraphs above, air cooling needs ten times of the surface area, therefore the fins, and air needs 2000 times the flow velocity and thus a recirculating air fan needs ten times the power of a recirculating water pump. Moving heat from the cylinder to a large surface area for air cooling can present problems such as difficulties manufacturing the shapes needed for good heat transfer and the space needed for free flow of a large volume of air. Water boils at about the same temperature desired for engine cooling. This has the advantage that it absorbs a great deal of energy with very little rise in temperature (called heat of vaporization), which is good for keeping things cool, especially for passing one stream of coolant over several hot objects and achieving uniform temperature. In contrast, passing air over several hot objects in series warms the air at each step, so the first may be over-cooled and the last under-cooled. However, once water boils, it is an insulator, leading to a sudden loss of cooling where steam bubbles form (for more, see heat transfer). Steam may return to water as it mixes with other coolant, so an engine temperature gauge can indicate an acceptable temperature even though local temperatures are high enough that damage is being done.