emissions and fuel principles
Emissions and Fuel Principles
Composition of petrol
Petrol is used as a fuel for motor vehicles, it consists of:
And several other alkalines; other additives including detergents are also added; oxygenators can sometimes be added, these may be:
· MTBE (methyl tert-butyl ether)
Composition of air
Air consists of 78.09% nitrogen, 20.95% oxygen. The remaining 1% is made up of four more gases, these are Argon 0.93%, carbon dioxide 0.03% and other gas traces 0.003%. Water vapour is also present in air in varying amounts depending on location and weather conditions/climate.
Effects of lean fuel mixture
Effect on combustion: A lean air-fuel mixture contains a high volume air and low volume of fuel. Considering 14.7:1 is an ideal mixture, 20:1 (20 parts air to 1 part fuel) would be a very lean mixture. A slightly lean mixture is ideal for low fuel consumption and low exhaust emissions. Extra air in the cylinder results in complete combustion meaning that all the fuel will be burned; if the mixture is too lean, problems such as lack of power, missing, and even engine damage may occur.
Effects of rich fuel mixture
Effect on combustion: A rich air-fuel mixture contains a high volume of fuel and a low volume of air; if a petrol engine has a mixture of 8:1(8 parts air to 1 part fuel) this would be a very rich mixture. A richer mixture tends to increase power; however having a rich mixture will increases fuel consumption and exhaust emissions considerably. If the mixture is overly rich this may result in: reduced engine power, fouled spark plugs, and cause incomplete combustion.
The phrase Stoichiometric ratio describes the correct air-fuel ratio necessary to achieve complete combustion of the fuel and air. It is represented by the Greek letter, lambda. For petrol, the value of the ratio is 14.7 parts of air, to 1 part of fuel. So if lambda equals 1, the air-fuel mixture is at the Stoichiometric ratio, of 14.7 to 1.
If an air-fuel mixture has a higher figure e.g. 1.03, the mixture contains more air than in the 14.7 to 1 ratio, making the mixture slightly lean. A mixture with a lower lambda value, e.g. 0.97, has less air than fuel, compared to the lambda ratio, 14.7:1, this makes the mixture slightly rich.
The exhaust gas oxygen sensor is called the lambda sensor, this is because it can be used to maintain the air-fuel ratio to lambda, with only slight variations either way. It can be installed in the exhaust manifold or down pipe, where it measures the percentage of oxygen compared to carbon monoxide in the exhaust gases.A high percentage of oxygen may mean too little fuel is entering the engine, the mixture is too lean. The sensor delivers this information to the ECU, which adjusts the mixture accordingly. Equally, a low percentage of oxygen may indicate too much fuel is entering the engine, making the mixture too rich.
Flame spread is the spread of flames throughout the cylinder once the plug has ignited; if the plug is in the side of the cylinder, the flames will take longer to fill the cylinder. Whereas if the plug is at the top of the cylinder (directly above the piston) the flames will take less time to spread as the the spark is distributed equally inside the cylinder.
If the flames spread too quickly this can cause a knocking noise, this is because the fuel is igniting before the piston reaches TDC (top dead centre)
The spread of flames causes a dramatic rise in cylinder pressure due to the rapid combustion of the fuel that was injected on the first stroke. The rate of pressure rise controls the extent of the combustion knock, (commonly referred to as diesel knock).
Exhaust emissions are made up of several different gases. These are:
· CO₂ (Carbon Dioxide) is not a pollutant, but is a greenhouse gas and so plays a role in global warming. The only way to reduce CO2 emission is to burn less fuel.
· NOx (Nitrogen Oxide) these are made when nitrogen in the air reacts with oxygen at the high temperature and pressure inside the engine.
· H₂O (Water)
· HC (Hydro-carbons) this is made up of unburned or partially burned fuel and this substance is a major contributor to urban smog, and is also toxic. They can cause liver damage and even cancer.
· CO (Carbon Monoxide)
· Particulates soot or smoke made up of particles. Particulate causes respiratory health effects in humans and animals.
· Carbon monoxide is a colourless, odourless and tasteless, yet a very toxic gas. Its molecules consist of one carbon atom and one oxygen atom. If too much carbon monoxide is inhaled it cuts off oxygen supply which will result in extreme illness and possible death.
Fuel supply method
The fuel can be supplied to the cylinder by either carburettor or injection.
The function of a carburettor is to mix fuel with air, before the fuel enters the cylinder. A major components within the carburettor is the needle, this controls the amount of fuel that moves up to the top of the carburettor, next to the butterfly. As the air passes the butterfly and travels down the tunnel, the needle allows fuel particles to rise past the needle to mix with the air, the mixture then goes down the inlet manifold and into the engine.
The injection system operates on the same principles as a carburettor. The fuel injector can inject fuel either into the inlet manifold just above the valve or higher up inside the inlet manifold. As the piston moves down inside the cylinder, the air and fuel in pulled into the cylinder, where it is mixed just before the mixture is burnt.
Fuel system components
· Tank: the fuel tank is usually positioned at the rear of the car, usually underneath the vehicle but is occasionally fitted in the boot of the vehicle. The tanks purpose is to store fuel until it is needed. The tank has a pipe that runs from the top of the tank and protrudes on the outside of the vehicle for easy refuelling.
· Fuel filter: the fuel filter is located in the fuel line between the fuel tank and the fuel pump, its purpose is to filter the fuel that passes through it, and the filter removes any foreign objects from the fuel, like rust or paint from inside the tank. If the foreign objects enter the carburettor or injection system, costly damage may be caused.
· Air filter: the air filter cleans the air before it is used in the engine, if debris such as small stones/grit enters the carburettor it can severely the internal components, if the needle becomes damaged the engine will not get correct fuel and may no longer start or run.
· Pump: the fuel pumps delivers a constant supply of fuel to the carburettor or injectors, without the pump the vehicle would have to rely on gravity feed, which would not be reliable unless the tank is constantly full, for this reason a pump is used. The pump must also run at a certain speed otherwise not enough fuel will reach the carburettor, this may result in a slightly lean mixture.
· Pressure regulator: the pressure regulator supplies the injectors with fuel, its function is to retain a set pressure so that only the correct amount of fuel. The amount must be precise to gain the correct air fuel ratio. If debris gets into the pressure regulator or the regulator is not functioning properly the fuel will not reach the cylinder, the vehicle will then be unable to run.
· Injectors: the injector pipes carry fuel from the pressure regulator to the inlet manifold, these pipes are under high pressure when the engine is running, a small hairline crack in one of the pipes can result in a cylinder not being used due to the lack of fuel as most of the pressure is lost through the small crack. Each end of the pipe is flared to give a tight seal when bolted to the pressure regulator.