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INTAKE AND EXHAUST SYSTEMS 101 (FOR DUMMIES)
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Indra ChitreIntake and Exhaust Systems 101By Indranil ChitreWhat does this essay cover?Air FilterThrottle BodyIntake ManifoldExhaust Gas Recirculation (EGR) ValveEngine / Combustion ChamberINTAKE SYSTEMThe intake system deals with allowing 3 elements in to the combustion chamber of the engine:a) Airb) Fuelc) Partial Exhaust Gases- Note: even though EGR valve deals with EXHAUST gases, I’ve put it on the intake side since these exhaust gases are “going in” the combustion chamberEngine / Combustion ChamberEXHAUST SYSTEMExhaust ManifoldCatalytic ConverterMufflerTailpipeThe exhaust system deals with allowing 2 elements from the combustion chamber of the engine to the atmosphere:a) Exhaust Gasesb) Sound WavesAdditionally, there are a few sensors that we shall cover in the essay. The sensors covered are in no way comprehensive to those required for the optimal functioning of a vehicle. There are also some ancillary systems like secondary air injection systems that are not covered in this essay.AIR FILTERFunction – A single air filter is used primarily to ensure that no dust, debris, biomaterials like insects and faeces, stray materials, etc enter the vehicle components and systems. Such particles can accelerate the wear and tear of the components once they enter them. Air filters also serve secondary functions like reducing intake noise and acting as a flame arrester in case of a backfire.Make – Air filters are made of paper, and the housing is made in 2 parts of plastic (most of) or steel. Some high performance air filters are made of foam or cotton. The microfibers contained within air filters stop particles as small as 10 – 25 microns in size (diameter). To give a perspective of size, human hair is 50 microns in diameter, and our naked eye cannot see particles less than 40 microns in diameter. As noted in the picture above, air filters can be box-type (most of); some are also conical or circular in shape.Maintenance – Overtime air filters get clogged up i.e. the ducts and spacing within the air filters get filled with unwanted particles. This affects the flow of air through the air filters and consequently within the intake system. Since the rate of air flow and fuel injection is highly critical to the optimal performance of an engine, a clogged air filter should be taken care of immediately. Air filters are relatively inexpensive, less than $50.00, and easy to replace (no need for an expert mechanic) hence they should be periodically inspected and replaced. Manufacturers of air filters provide guidelines for their replacement either in terms of months in service or miles driven by the vehicle. These guidelines assume “normal” operating conditions. Should the vehicle be used for off-road or other applications e.g. in stormy or dusty weather, the air filter should be replaced more frequently than the stated guidelines.Sensors – The 2 main sensors used in conjunction with air filters are:a) Mass Air Flow (MAF) – this sensor is used to measure the amount of air passing through the intake system after the air filter.b) Air Filter Restriction – this sensor measures the pressure drop in air after passing through the air filter; used to estimate whether the air filter needs cleaning or replacement. The “replace air filter” message lights up on the vehicle dashboard if the drop in pressure warrants it. Depending on the vehicle make and model, and the fuel type, general guidelines are 15 – 30 in H2O drop in pressure will trigger the message.Other Key Items – Air filters should not be placed close to the throttle body or the turbocharger.THROTTLE BODYFunction – Throttle Body controls the flow of air within the intake system. When someone presses the accelerator, the throttle body valve opens and allows more air in. This in turn causes the fuel injectors to inject the necessary fuel to maintain an optimal Air:Fuel ratio in the combustion chamber.Note – The above air speeds can exceed 300 feet p/sec. High air speed helps maintain fuel in suspension. When air speeds drop below 50 feet p/sec, as is the case in some idle conditions, fuel droplets begin to separate.INTAKE MANIFOLD – THROTTLE BODY FUEL INJECTEDFunction – The intake air-fuel mixture needs to be distributed equally, in terms of amount and quality, to all engine cylinders. This is accomplished through the use of the intake manifold. Note in the picture above; air and fuel mixture enters the top portion of the manifold, and is distributed to the individual cylinders through the 4 runners.Make – The diameter of the manifold is designed with some compromise. Small diameter manifolds are essential to maintain high charge velocity, which in turn ensures thorough mixing of air and fuel, hence improves the fuel economy. Large diameter runners pass the most air and are helpful for high-speed, race driving applications.INTAKE MANIFOLD – PORT FUEL INJECTEDMake – The PFI intake manifold has to be optimized only for air flow (as opposed to air and fuel in the above). Fuel is injected right before the intake valve. The manifold is usually manufactured in 2 separate parts; upper manifold, also known as the Plenum, and lower manifold made of runners. The length of the runners represent a design compromise. Long runners are suitable for low-RPM torque; short runners for high-RPM torque. Some intake manifolds called Variable Intakes have both long and short runners. Depending on the particular application of high or low RPM, either the short or long runner is respectively activated through computer control.Diagnosis – In the event the upper intake gasket is blown, a vacuum leak may form. In the event the lower intake gasket is blown, a vacuum leak, coolant leak or oil leak may occur.EXHAUST GAS RECIRCULATION (EGR) VALVEFunction – A portion (5 – 10%) of the exhaust gases are recirculated back in the combustion chamber through the EGR valve. Exhaust gases are inert gases i.e. they do not burn in the combustion chamber. Since the exhaust gases take up a portion of the combustion chamber, less space remains for the fresh air and fuel. This reduces the peak combustion temperatures which results in lower NOx emissions.Make – In V-type engines, gas passages are cast within the inlet manifold. In In-line engines, an external tube is used to carry the exhaust gases to the EGR valve.EXHAUST MANIFOLDFunction – Exhaust manifolds are quite the opposite to intake manifolds. Exhaust manifolds collect the exhaust gases from each of the cylinders and pass them to the rest of the exhaust system through a single port. Exhaust manifolds are also designed to minimize backpressure and keep exhaust noise at a minimum.Make – Exhaust manifolds carry gases at high temperatures often exceeding 1,500F, hence are made of sturdier construction compared to intake manifolds. Exhaust manifolds are made of either cast iron or steel.Maintenance – Exhaust manifolds tend to expand during operation. This creates stresses within the manifold material. When an exhaust manifold is removed from the cylinder head, the manifold tends to get warped. This can be straightened out using a suitable jack. While replacing the exhaust manifold, exhaust manifold gaskets should be used even if they were not in the original factory configuration. These gaskets provide room for the manifold to slide on top of the cylinder head.CATALYTIC CONVERTERFunction – To reduce harmful emissions contained within the exhaust gases. The Cat reduces NOx, HCs and CO through a 2-step process, by converting them in Nitrogen, CO2 and Water, which are relatively harmless.Make – The Cat is made of 2 ceramic blocks; the first one containing Platinum and Rhodium, the second one containing Platinum and Palladium. Step 1 reduces NOx levels in the exhaust gases; Step 2 reduces the HCs and CO.Sensors – There are 2 Oxygen (O2) sensors used in conjunction with the Cat. 1st O2 sensor measures the level of Oxygen before the exhaust gases enter the Cat. This sensor sends rich/lean signals to the vehicles’ ECU, which then directs fuel injection accordingly. 2nd O2 sensor measures the effectiveness of the Cat in reducing the harmful emissions.MUFFLERFunction – The combustion chamber of the cylinders produce pressure pulses while the exhaust gases are removed in the exhaust stroke. These pressure pulses create air vibration, which causes loud exhaust noise. The muffler reduces this exhaust noise before emitting the exhaust gases to the tailpipe and eventually to the atmosphere. Sometimes resonator tubes are used in addition to the mufflers to further dampen exhaust noise.Make – The muffler is divided in 4 different chambers. Each chamber uses specific engineering principles to dampen the exhaust noise. For example, chamber 1 uses perforated holes to disperse and dampen the exhaust noise; chamber 4 works on Helmholtz principle of noise cancellation through opposing sound waves.Note – Exhaust gases expand and cool at the inlet to the muffler; this causes the gases to become denser. Hence the outlet to the mufflers can be of smaller diameter compared to the inlet, without any loss of flow efficiency.References:a) Level 1 Inspector Training, STI, Chapter 10 – Intake and Exhaust Systemsb) Wikipedia
Owners and operators of various smog check stations across southern California.
www.starsmogcenter.com.
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