Early vehicles vented the fuel tank through the filler cap into the atmosphere. Some of the fuel in the tank would vaporize. Some vapors escaped from the filler cap, some from the carburetter.
Non-vented filler caps are designed to stop the exit of vapors. A vacuum relief valve can relieve low pressure in the tank when the temperature drops. This will also stop the tank from collapsing if its internal pressure falls below atmospheric pressure.
The fuel cap may also incorporate a pressure relief valve. If the tank’s internal pressure exceeds the set value of the relief valve, it can stop the tank from rupturing. Some modern caps have no valves at all, and are completely sealed to stop the entry of air and water, as well as the emission of fuel vapor.
Modern tanks also contain an expansion volume either directly in the shape of the tank, or in a separate chamber connected to the fuel tank by tubing.
A liquid vapor-separator may be connected to the tank by a number of tubes. This separator allows liquid fuel to separate from the vapors, and return to the tank.
A vapor line is connected to the vapor space in the tank, or the liquid vapor separator. It carries fuel vapors from the tank to a storage volume. This vapor line can incorporate check valves. If the vehicle is tilted too far from the horizontal, they stop liquid fuel entering the storage volume.
A storage device is used to store the fuel vapors. The fuel tank breathes through this storage device. Some vehicles use the engine crankcase.
When the temperature of the fuel in the tank increases, fuel vapors are forced along the vent line, past a liquid check valve, and into the crankcase.
When the engine starts, the Positive Crankcase Ventilation system flushes vapors out of the crankcase and into the intake manifold where it joins with the inlet air-fuel mixture. Once in the inlet manifold, the vapors are drawn into the engine where combustion can convert them into carbon dioxide and water vapor.
Modern petroleum based fueled vehicles are fitted with three-way catalytic converters. 3-way converters convert hydrocarbons and carbon monoxide to water and carbon dioxide, as well as convert the oxides of nitrogen, nitric oxide and nitrogen dioxide, back into harmless nitrogen and oxygen molecules.
Older catalytic converters converted hydrocarbons and carbon monoxide into water and carbon dioxide, but were not able to convert the oxides of nitrogen.
The term 'three-way' is in relation to the three regulated emissions the converter is designed to reduce: carbon monoxide, hydrocarbons or volatile organic compounds, known as VOCs, produced from evaporated unburned fuel, and nitrogen oxides.
The converter uses two different types of catalysts to reduce the pollutants: a reduction catalyst and an oxidation catalyst.
The exhaust gases first pass over the reduction catalyst in the converter. The platinum and rhodium coating helps to reduce the oxides of nitrogen, together known as ‘NOX’ emissions.
When a nitric oxide or nitrogen dioxide molecule comes into contact with the coating, it strips the nitrogen atom out of the molecule and retains it. This frees up the one or two oxygen atoms in the molecule which combine in pairs to form molecules of oxygen.
The nitrogen atoms bond with other nitrogen atoms that are retained in the catalyst and form molecules of nitrogen. So two molecules of nitric oxide become one molecule each of nitrogen and oxygen, or two molecules of nitrogen dioxide become one molecule of nitrogen and two molecules of oxygen.
The exhaust gases then flow over the oxidation catalyst in the converter. This has the effect of reducing any unburned hydrocarbons and carbon monoxide by oxidizing them over the platinum and palladium coating. This aids the reaction of the carbon monoxide and hydrocarbons with any remaining oxygen in the exhaust gas.
Each carbon monoxide molecule combines with an oxygen molecule to make one less harmful carbon dioxide molecule. Because of strict emission requirements, vehicles with a 3-way catalytic converter have a feedback system, called looping.
The electronic control unit, or ECU, monitors the air-fuel ratio by using an exhaust gas oxygen, or EGO, sensor, also known as a lambda sensor. This sensor tells the engine computer how much oxygen is in the exhaust and uses this information via the ECU to control the fuel injection system.
The ECU can increase or decrease the amount of oxygen in the exhaust by adjusting the air-to-fuel ratio. The system ensures that the engine runs at close to the stoichiometric point in normal driving conditions. It also ensures that there is always sufficient oxygen in the exhaust system to allow the oxidization catalyst to deal with unburned hydrocarbons and carbon monoxide.
A closed loop is usually part of what is called a feedback system. A feedback system collects information on how a system is operating, then feeds that information back to affect how the system is working. They can be very simple, and very sophisticated.
This vehicle has a cruise control unit to help it maintain a set speed. When it falls below it, a computer sends a signal that moves the throttle linkage and increases the fuel reaching the engine, and speed. It has the opposite effect when the vehicle exceeds the set speed. A cruise control unit that continually monitors the system is called a closed loop system.
A closed loop system in an engine exhaust system can monitor the amount of oxygen in exhaust gases, to maintain a constant air-fuel ratio.
Air pollutants are classified as either primary or secondary contaminants.
- A primary air contaminant, such as carbon monoxide gas, or particles of unburned fuel, is added to the atmosphere as a by-product of burning gasoline in an internal combustion engine.
- Secondary emissions are emitted as gases and can combine with other airborn substances to form particles once in the atmosphere.
Air contaminants can be divided into gases and particulates.
- The primary pollutant gases from vehicles include: carbon monoxide, nitrogen oxides, hydrocarbons, and sulfur dioxide. These pollutants can have a damaging effect on the atmosphere and the natural environment.
- Particulates, often referred to as Particulate Matter, or PM, are tiny particles of solid or liquid suspended in the air. They are graded in a size range from 10 nanometers to 100 micrometers in diameter. Particulates of less than 10 micrometers are dangerous to humans because they can be breathed and reach the lungs. Smaller particles also tend to stay airborne longer than larger particles, which settle more quickly.
Different countries have recognized the global effects that the continual dispersion of contaminates in the earths atmosphere can have. As a result they have either introduced emission control standards and/or are signatories to international protocols to limit and control global pollution through emissions.
Vehicle manufacturers are required to comply with these standards and are required to ensure that the emissions from the vehicles they produce are strictly limited and have systems incorporated into them that monitor and control such emissions.
Many manufacturers are producing “hybrid” vehicles that use alternative power sources to limit emissions, however, even with alternative lower-pollutant fuels, the use of the internal combustion engine is set to remain a key power source for many years.
As a result, ever stricter emissions control is at the forefront of vehicle design and construction.
Typically the regulated emissions are:
- carbon monoxide,
- hydrocarbon,
- nitrogen oxides, and
- particulate matter.
Limit values are normally given in grams per brake horse power-hour in USA and grams per kilowatt-hour in Europe. These regulations are likely to become progressively more stringent.
While the engine is running, some gases from combustion leak between the piston rings and the cylinder walls, down into the crankcase.
This leakage is called blow-by. Unburned fuel, and water from condensation, also find their way into the crankcase, and sump. When the engine reaches its full operating temperature, the water and fuel evaporate. To prevent pressure build-up, the crankcase must be ventilated.
In older vehicles, crankcase vapors were vented directly to the atmosphere through a breather tube, or road-draught tube. It was shaped to help draw the vapors from the crankcase, as the vehicle was being driven.
Modern vehicles are required to direct crankcase breather gases and vapors back into the inlet system to be burned.
A common method of doing this is called positive crankcase ventilation, or PCV.
A valve called a PCV valve, regulates gas flow between the crankcase and the inlet manifold. It is controlled by the pressure in the manifold. With the engine off, the valve is closed, and air cannot enter the inlet manifold. This allows the engine to start.
At idle, low pressure in the manifold draws the valve to the other end of the body. This lets a small, measured amount of vapor pass the valve.
At wider throttle openings, the valve plunger position allows maximum flow through the body, which gives maximum crankcase ventilation.
The system is designed to remove more air than just blow-by, so there’s a fresh air intake, usually at the air cleaner, to direct filtered air to the crankcase. This intake is usually as far as possible from the PCV valve.
Wide throttle openings produce maximum blow-by. Gases that can’t be handled through the vacuum system, are directed back through the inlet connection to the air cleaner, where they join the carburetter intake air, and are drawn into the cylinders for burning.
EGR valves
If valve overlap is maintained, or even increased, oxides of nitrogen can be reduced by an exhaust gas recirculation, or EGR, valve, connected between the exhaust port, or manifold, and the intake system.
If engine conditions are likely to produce oxides of nitrogen, the EGR valve opens, letting some gases pass from the exhaust, into the intake system. During combustion, these exhaust gases absorb heat from the burning air and fuel. This lowers peak combustion temperatures, which reduces the formation of the oxides of nitrogen.
The EGR valve usually opens when the engine is at normal operating temperature and likely to be using a lean mixture.
When the engine first starts and until it warms up, a temperature-sensitive valve prevents manifold pressure reaching the EGR valve, and stops it from operating. Manifold pressure reaches the valve when the engine is warm, and when the EGR port at the carburetter is influenced by the position of the throttle. This is generally at light throttle openings, when a lean mixture could cause increased oxides of nitrogen.
It does not operate at idle, or at wide-open throttle.
Oxides of nitrogen can also be reduced by retarding ignition timing. This lowers the maximum temperature reached during combustion. The maximum ignition-advance setting is then said to be “emission-limited”. However, it also lowers engine output, and increases fuel consumption.
Spark control systems use ignition timing to optimize engine output, and fuel consumption, with minimal emissions. They control vacuum levels to the vacuum advance unit on the distributor.
In some designs, vacuum signals are delayed, with vacuum valves in the signal line.
In other designs, the vacuum signals are sustained.
The signals can also be applied to dual diaphragm advance units on some distributors. The degree of control needed depends on many other factors, and each application should be considered individually.
Controlling air-fuel ratios
If the exhaust gas is hot enough, combustion of remaining hydrocarbons and carbon monoxide can be completed by adding extra air to them at the exhaust port or manifold. This afterburning produces water and carbon dioxide, which are then exhausted to the atmosphere.
The pulse air method uses the pulsations of the exhaust gas to open and close a reed valve. It admits air into the exhaust manifold in short bursts. Air drawn from the air filter enters the exhaust manifold. This method suits engines with 4 cylinders, or less, because their exhaust pulsations are further apart.
Larger capacity engines may use an air pump to supply a larger volume of air. The pump is normally driven by a V-belt, from the engine crankshaft, and forces air into the exhaust ports. Some of the engine’s output is used in driving the pump.
Electronic fuel injection and engine management systems deal with emissions more effectively than carburetted engines by more closely controlling the air-fuel ratio entering each cylinder, and by ensuring the ignition timing matches operating conditions.
Sensors around the engine send the ECU information about air-flow, coolant temperature, throttle position and engine speed. The ECU uses this to set fuel and ignition settings, from its programmed memory.
Most systems also use a sensor in the exhaust manifold to gauge the amount of oxygen in the exhaust gases leaving the engine. This indicates how well combustion is being completed, which in turn indicates the air-fuel mixture entering the engine.
A voltage signal is then relayed back to the control unit, to indicate whether the mixture is rich or lean. More oxygen indicates a lean mixture, less indicates a rich mixture. The control unit then adjusts the amount of fuel being injected into the engine, to keep the air-fuel ratio very close to the chemically correct composition for complete combustion.
By mass, this is 14.7 parts of air to 1 of fuel, known as the stoichiometric ratio.
Charcoal storage devices
Another kind of storage device is a canister of activated charcoal. “Activated” means the charcoal is porous, with a large surface area. It can store large quantities of fuel vapour. It has connections for the fuel tank vent line and the purge line, which carries the vapours to the intake manifold. In some designs on carburetted engines, it is also has a connection from the carburetter bowl.
When the engine is running, the action of the piston during the intake strokes, creates a low-pressure area in the inlet manifold. This can be used to open a purge valve, which draws fresh air into the bottom of the canister. The air collects the vapour and directs it to the inlet manifold where it is drawn into the engine and burned. The purge valve is designed to operate only at speeds well above idle.
If it operated at low speeds, the extra fuel vapours could upset the air-fuel mixture, which could cause poor idling and rough running.
The evaporation of automotive fuel is a major source of hydrocarbon emissions.
The rates of evaporation are higher with gasoline than diesel, because gasoline is more volatile.
