Exam Study Notes

Air Injection into Exhaust System contributes to reducing harmful emissions by introducing a controlled amount of air into the exhaust stream, helping to facilitate the oxidation of unburned hydrocarbons and carbon monoxide. This process not only improves overall engine efficiency but also enhances the performance of catalytic converters, ensuring a cleaner release of exhaust gases. Overall, this technology plays a crucial role in meeting stringent environmental regulations and contributes to a more sustainable automotive future.

• Air injection is a method for reducing exhaust emissions.

• It involves injecting air into the exhaust ports of an engine.

• This air mixes with the hot exhaust gases, oxidizing hydrocarbons (HC) and carbon monoxide (CO).

• Components of a basic air injection system:

  • Air supply pump with filter

  • Air manifolds and nozzles

  • Anti-backfire valve

  • Check valve

  • Connecting hoses

• Secondary air injection pumps air into the exhaust system when the engine is cold.

• The oxygen in this air combusts remaining fuel in the exhaust gases.

• This reduces emissions and warms up the catalytic converter.

Development of Air Injection Technology

• The method of injection and entry point of air into the exhaust system have evolved.

• Early systems injected air close to the engine (cylinder head exhaust ports or exhaust manifold).

• The injected oxygen oxidized unburned and partially burned fuel before it exited the tailpipe.

• Vehicles in the 1960s and early 1970s had significant unburned fuel, so secondary air injection reduced emissions.

• However, the extra heat from recombustion, especially with rich exhaust, could damage exhaust valves.

• In some cases, it could cause the exhaust manifold to glow (incandesce).

• As emission control became more sophisticated, the amount of unburned fuel decreased.

• With the introduction of catalytic converters, the function of secondary air injection shifted to supporting the converter.

• The original air injection point became the "upstream injection point."

• Air injected upstream burns with the rich exhaust to quickly heat up the catalytic converter when it's cold.

• Once warm, air is injected downstream (into the catalytic converter itself) to assist with catalysis of unburned hydrocarbons.

Methods of Implementation

Pumped Air Injection

• Uses a vane pump (air pump or "smog pump") driven by the engine via a belt or electric motor.

• The pump's air intake is filtered to exclude dirt particles.

• Air is delivered under light pressure to the injection point(s).

• A check valve prevents exhaust from flowing back through the air injection system.

• Carbureted engines' exhaust tends to have raw fuel spikes when the throttle is released.

• A diverter valve is used to prevent explosive combustion of this raw fuel.

• The valve senses the decrease in intake manifold vacuum and diverts the air pump's outlet to the atmosphere.

• The diverted air is often routed to the engine air cleaner or a silencer to reduce noise.

Aspirated Air Injection

• Utilizes negative pressure pulses in the exhaust system at engine idle.

• An aspirator valve (sensitive reed valve assembly) draws air from the clean side of the air filter.

• During engine idle, negative pressure pulses draw air through the aspirator valve and into the exhaust stream at the catalytic converter.

• This system (marketed as Pulse Air) was used by American Motors, Chrysler, and other manufacturers starting in the 1970s.

• The aspirator offered advantages in cost, weight, packaging, and simplicity.

• It also eliminated parasitic losses since there's no pump requiring engine power.

• However, it only functions at idle and admits less air over a narrower range of engine speeds compared to a pump.

• This system is still used on modern motorcycle engines (e.g., Yamaha AIS - Air Injection System).

Thermal Reactors

• A thermal reactor system has a reaction chamber in the cylinder head.

• The reaction chamber is located immediately behind the exhaust valve.

• It has a predetermined capacity to induce oxidation of harmful exhaust gas constituents.

• Gas-phase oxidation of carbon monoxide slows as combustion products cool, but doesn't stop entirely.

• Carbon monoxide and hydrocarbons continue to react in the exhaust manifold.

• Oxidizing hydrocarbons homogeneously requires a holding time of approximately $50 ms$ at temperatures above $900 K$.

• Homogeneous oxidation of carbon monoxide requires higher temperatures, exceeding $1000 K$.

• A thermal reactor is an enlarged exhaust manifold that bolts onto the cylinder head and enhances the oxidation rate.

• It increases the residence time of combustion products at high temperatures, allowing oxidation reactions to proceed.

• Secondary air may be added to allow for fuel-rich operation and rapid mixing with combustion products.

• A multiple-pass arrangement is commonly used to shield the hot core of the reactor from the surroundings.

• This is critical because the reactions require nearly adiabatic operation.

• Typically, only about a factor of 2 reduction in emission levels for CO and hydrocarbons can be achieved even with adiabatic operation.

• Higher temperatures and long residence times are typically required to achieve better conversions.

• The heat released in the oxidation reactions can result in a substantial temperature rise.

• Removal of $1.5$ CO results in a temperature rise of about $490 K$ (Heywood, 1976).

• Thermal reactors with fuel-rich cylinder exhaust gas and secondary air addition give greater fractional reductions in CO and hydrocarbon levels than reactors with fuel-lean cylinder exhaust.

• Incomplete combustion in the cylinder results in reduced fuel economy.

• The attainable conversion is limited by incomplete mixing of gases and any secondary air added.

• Exhaust gas temperatures of automobile spark ignition engines can vary from $600$ to $700 K$ at idle to $1200 K$ during high-power operation.

• Most of the time, the exhaust temperature is between $700$ and $900 K$, which is too low for effective homogeneous oxidation.

• Spark retard increases the exhaust temperature, but this is accompanied by a significant loss in efficiency.

• Noncatalytic processes can yield significant improvements in carbon monoxide and hydrocarbon emissions.

• However, controlling $NO_x$ emissions is challenging with such systems.

• Control of $NO_x$ emissions through noncatalytic reduction by ammonia is feasible only in a narrow temperature window.

• Ensuring a proper flow of ammonia presents a logistical problem for vehicular emission control.

Pressure and Void Coefficients in Thermal Reactors

• Thermal reactors with liquid coolant experience reactivity changes when voids in the coolant undergo concentration changes.

• The quantity of coolant in the core decreases as liquid boils, reducing absorptions in the coolant (positive reactivity feedback).

• The slowing down of neutrons to thermal energy decreases because of a reduced moderator concentration (negative reactivity feedback).

• In under-moderated reactors, the net effect is a negative void coefficient.

• The present invention provides a thermal reactor system which may reduce the harmful constituents to the required amount without providing an exhaust gas purification system in the exhaust system.

• The reaction chamber is provided in the cylinder head behind the exhaust valve and oxidation of exhaust gases occurs in the reaction chamber at a high temperature.

• In the conventional exhaust thermal reactor system, the thermal reactor is positioned in the exhaust passage after the outlet of cylinder head.

• According to inventor's experiments, it has been found that sufficient oxidation cannot be expected in the termal reactor provided in the exhaust passage, because of the exhaust gas temperature drop in the exhaust passage.

• In order to elevate the exhaust gas temperature, the spark timing is retarded and in order to maintain the temperature at a high level sufficient to induce the oxidation, a large scale insulation must be provided on a great part of the exhaust system which will increase the cost of the system.

Catalytic Converters

• Catalytic converters clean car exhaust emissions using chemical reactions with precious metals.

• They change harmful substances in exhaust gases (carbon monoxide, nitric oxide, nitrogen dioxide, and hydrocarbons) into less harmful substances (carbon dioxide and water vapor).

• The interior has a honeycomb structure coated with a catalyst.

• Precious metals like palladium, rhodium, and platinum are commonly used as the catalyst.

• These metals have value, making catalytic converters a target for thieves.

• Catalytic converters need to work at high temperatures (up to $400$ degrees) to maximize efficiency.

• Early units were positioned close to the engine, but this caused issues, so they were gradually moved further down the exhaust system.

• In today’s cars the catalytic converter is found underneath the vehicle towards the exhaust outlet.

• There are various types of catalytic converters.

• A 'two-way' oxidation cat turns carbon monoxide (CO) to carbon dioxide ($CO_2$) and hydrocarbons to carbon dioxide and water.

• 'Three-way' catalytic converters also reduce emissions of nitric oxide (NO) and nitrogen dioxide ($NO_2$) which together are more commonly known as NOx, a major cause of localised air pollution.

• Catalytic converters have been around since the 19th century.

• Frenchman Eugene Houdry patented the technology and founded Oxy-Catalyst, which fitted catalytic converters to industrial chimneys.

Stratified Charging Engine

• Stratified charge engines improve engine efficiency and reduce fuel consumption while addressing ecological concerns.

• They rely on understanding phenomena inside the engine cylinder, optimizing fuel injection, and designing combustion chamber and piston head shapes.

• The increase in engine efficiency results from regulating the petrol-air mixture composition based on rotational speed and load.

• A stratified charge engine injects fuel into the cylinder just before ignition.

• This allows for higher compression ratios without knock and leaner air/fuel ratios than conventional engines.

• Traditionally, Otto cycle engines draw a mixture of air and fuel into the combustion chamber during the intake stroke, creating a homogeneous charge.

Homogeneous Charge vs. Stratified Charge

• In a homogeneous charge system, the air/fuel ratio is kept close to stoichiometric (the exact amount of air for complete combustion).

• This provides stable combustion but limits efficiency.

• Running a lean mixture with a homogeneous charge results in unstable combustion.

Periods in Fuel Injection

• Period $t_1$: From fuel injection to contact with the piston head, including air resistance.

• Period $t_2$: From entry into the piston head curvature to half the curvature length, including frictional resistance.

• Period $t_3$: From half the curvature length to when the fuel exits the head, including frictional and air resistances for evaporating fuel.

• Period $t_4$: From exit of the piston head curvature to when the fuel reaches the spark plug points.

Advantages of Direct Fueling

• Direct fueling offers advantages over port-fueling (injectors in intake ports, giving homogeneous charges).

• Powerful electronic management systems mean there is no significant cost penalty.

• Direct fueling allows for higher mechanical compression ratios for better thermodynamic efficiency.

• Since fuel is not present until combustion, there is no risk of pre-ignition or engine knock.

• The engine may run on a leaner overall air/fuel ratio using stratified charge.

Disadvantages

• Increased injector cost and complexity

• Higher fuel pressure requirements

• Risk of carbon build-up on the intake valve

• Increased $NO_x$ formation due to local rich zones

• Combustion management can be problematic if a lean mixture is present at the spark plug. Fuelling a petrol engine directly allows more fuel to be directed towards the spark-plug than elsewhere in the combustion-chamber.

• Charge stratification can be achieved where there is no 'in cylinder' stratification.

Comparison with Diesel Engine

• Petrol can burn faster than diesel fuel, allowing higher maximum engine speeds.

• Diesel fuel has a higher energy density and can deliver strong torque and high thermodynamic efficiency.

• Diesel motor operation on the other hand inhales and compresses air only by the motion of the piston moving to top dead centre.

• At this point maximum cylinder pressure has been reached. The fuel is now injected into the cylinder and the fuel ' burn' or expansion is started at this point by the high temperature of the, now compressed, air

• As the fuel burns it expands exerting pressure on the piston, which in turn develops torque at the crankshaft.

CVCC (Compound Vortex Controlled Combustion)

• Honda's trademark for an engine with reduced emissions; the first engine to be installed with the CVCC approach for testing was a single-cylinder, 300 cc version of Honda's EA engine installed in a modified Honda N600 hatchback in January 1970.

• A type of stratified charge engine, it first appeared on the 1975 ED1 engine.

• As emission laws advanced, Honda abandoned CVCC and introduced PGM-FI (Programmed Fuel Injection).

• Honda CVCC engines have normal inlet and exhaust valves, plus a small auxiliary inlet valve providing a rich air–fuel mixture near the spark plug.

• The remaining air–fuel charge is leaner than normal.

• Upon ignition, flame fronts emerge from perforations and ignite the remainder of the air–fuel charge.

• The engine cycle is as per a standard four-stroke engine.

• This combination allowed stable running and more complete combustion, reducing CO and hydrocarbon emissions.

• This method allowed the engine to burn less fuel more efficiently without the use of an exhaust gas recirculation valve or a catalytic converter, although those methods were installed later to further improve emission reduction.

Advantages Over Previous Stratified Charge Engines

• Honda's big advancement with CVCC was that they were able to use carburetors and they did not rely on intake swirl.

• It can be seen from this method of charge stratification that the lean charge is 'burnt' and the engine utilising this form of stratification is no longer subject to ' knock' or uncontrolled combustion.

Early Design Flaw

• Early CVCC engines had problems with auxiliary valve retaining collars vibrating loose.

• Motor oil would leak into the pre-combustion chamber, causing power loss and smoke.

• Honda corrected this with metal retaining-rings.

• The 1983 Honda Prelude used a CVCC design and a catalytic converter to reduce emissions, called CVCC-II, along with two separate sidedraft carburettors.

Exhaust Emissions

• The development of Internal Combustion Engines is one of the most revolutionary moves in the history of mechanical engineering.

• Major emissions from exhaust pipe contain Particulate Matter (PM), Nitrogen Oxides (NOx), Carbon Monoxide (CO) and greenhouse gases which exploit the environment due to which the air quality index is degrading.

• Harmful emissions from exhaust are a major cause of degrading air quality.

• Exhaust gases can trigger acute diseases in humans.

• It is essential for design engineers to have exhaust emissions in compliance with norms set by government globally.

Exhaust Gas Recirculation (EGR)

• EGR is a nitrogen oxide ($NO_x$) emissions reduction technique.

• It recirculates a portion of an engine's exhaust gas back to the engine cylinders.

• This dilutes the $O_2$ in the incoming air stream and provides gases inert to combustion, reducing peak in-cylinder temperatures.

• $NO_x$ is produced in high temperature mixtures of atmospheric nitrogen and oxygen that occur in the combustion cylinder, and this usually occurs at cylinder peak pressure.

• Another primary benefit of external EGR valves on a spark ignition engine is an increase in efficiency, as charge dilution allows a larger throttle position and reduces associated pumping losses.

• Gases re- introduced from EGR systems will also contain near equilibrium concentrations of NOx and CO; the small fraction initially within the combustion chamber inhibits the total net production of these and other pollutants when sampled on a time average.