How a Battery Functions

Outline of Exhaust and Evaporative Emissions

In general, gasses released to the atmosphere by a vehicle mainly consist of exhaust gasses coming from the exhaust pipe, blow-by gasses coming from the crankcase, and fuel vapors originating in the fuel tank or other similar locations.

Exhaust Gas

The term exhaust gas refers to the mixture of gasses that is released into the atmosphere following the combustion of fuel in the engine’s combustion chambers. Nitrogen (N2), carbon dioxide (CO2), and water vapor (H2O) account for a large portion of the overall exhaust gas volume and, while these gasses pose no threat to public health, other exhaust gasses such as the carbon monoxide (CO) and hydrocarbon (HC), produced in the combustion chambers as a result of incomplete combustion, are toxic substances. In addition, the air drawn in by the engine in order to supply the oxygen (O2) required for combustion also contains about 80% nitrogen (N2); when combustion temperatures are high, these two gasses can react with each other to produce nitroxide (NOX).

Blow-by Gas

Blow-by gas is the gas which escapes from combustion chambers through the space between the pistons and the cylinder walls. It is mainly composed of unburnt fuel gas (a mixture of air and fuel) and combustion gasses. Blow-by gas also contains some toxic substances – the most notable of these being hydrocarbon.

Fuel Vapor

Fuel vapor is caused by the evaporation and release to the atmosphere of fuel from the fuel tank and other fuel system components. Like the other exhaust gasses, it contains hydrocarbon and smaller quantities of other toxic gasses.

Emission Control System Outline

In order to prevent damage being done to public health and the environment, most Honda vehicles are fitted with the following emission control system components for the purpose of reducing emission levels of the above mentioned toxic gasses.

• Three-way catalytic converter (TWC)
• Exhaust gas recirculation (EGR) system
• Positive crankcase ventilation (PCV) system
• Evaporative emission (EVAP) control system
• Feedback control system (in PGM-carburetor engines only)
• Throttle control system (in carbureted engines only)

Three-way Catalytic Converter (TWC)

The TWC is cylindrical in shape and contains a honeycomb-type structure. Its function is to remove the CO, HC, and NOX that were generated during the combustion process from the exhaust gas. As they are passed through the honeycomb, these gasses take part in a chemical reaction allowing a certain amount of each one of them to be removed from the exhaust gas before it is released into the atmosphere. Most vehicles have a TWC installed mid-way along the exhaust pipe, although some (such as low emission vehicles) have the TWC located immediately after the exhaust manifold.

Exhaust Gas Recirculation (EGR) System

1. EGR control solenoid valve
2. ECM
3. Various sensors
4. EGR valve
5. EGR valve lift sensor

The major components of the exhaust gas recirculation system are the EGR valve, the EGR valve lift sensor, and the EGR control solenoid valve. The EGR valve, either acted on by the intake manifold vacuum or operated electrically, is opened whenever certain engine conditions have been met and is used to redirect a portion of the gas discharged from the exhaust manifold back to the intake manifold. In this way, the combustion temperature is lowered and the level of NOX contained in the exhaust gas is reduced.

Positive Crankcase Ventilation (PCV) System

During the final stage of the engine’s combustion stroke, unburned fuel and other gasses generated during the combustion process can leak past the engine’s piston rings and into the crankcase. This leakage is referred to as blow-by and the gas leaked is called blow-by gas. The PCV system is designed to prevent this blow-by gas from escaping into the atmosphere. Its PCV valve contains a spring-loaded plunger and, when the engine starts, the plunger is raised by an amount proportional to the intake manifold vacuum level. This causes the blow-by gas to be sucked directly into the intake manifold. In order that a vacuum does not develop in the crankcase at this time, fresh air is supplied via the breather pipe and cylinder head.

On some models, an oil breather chamber is provided at the outlet of the crankcase. This chamber removes oil particles from the blow-by gas in order to prevent the intake system from being soiled or contaminated.

Evaporative Emission (EVAP) Control System

1. EVAP purge control solenoid valve
2. From the alternator SP sensor
3. Various sensors
4. EVAP purge control diaphragm valve
5. Fresh air
6. EVAP control canister
7. Fuel tank
8. EVAP two-way valve
9. Fuel tank EVAP valve
10. Fuel tank cap

The fuel evaporative emission control system is designed to minimize the amount of raw fuel vapor (containing HC) that escapes from the fuel tank and enters the atmosphere.

The basic system consists of an EVAP control canister, a vapor purge control system, and a fuel tank vapor control system.

EVAP Control Canister

The EVAP control canister is used for the temporary storage of fuel vapor. When a specific temperature condition is met, this fuel vapor is drawn into the engine from the EVAP control canister and burned.

Vapor Purge Control System

In order to purge the fuel vapor from the EVAP control canister, fresh air is drawn through it and into a port on the throttle body. The purging vacuum (the intake manifold vacuum) is controlled by the EVAP purge control diaphragm valve and/or the EVAP purge control solenoid valve.

Fuel Tank Vapor Control System

When the fuel vapor pressure in the fuel tank exceeds a predetermined value, the EVAP two-way valve will open and the fuel vapor will flow into the EVAP control canister.

Feedback Control System on PGM-carburetor Engines

1. 1 O2 sensor
2. 2 IAC valve
3. 3 MAP sensor
4. 4 Speed sensor
5. 5 Control unit
6. 6 Vacuum switch
7. 7 Clutch switch
8. 8 A/C switch
9. 9 TW sensor

The standard feedback control system consists of an O2 sensor, an idle air control IAC valve, and a control unit.

The control unit uses data from the O2 sensor to determine whether the air-fuel ratio is richer or leaner than the stoichiometric ratio. It also receives data from various other sensors enabling it to ascertain the current engine condition, and, when necessary, it sends a suitable control current to the IAC valve to open it by the required amount. In its open condition, the IAC valve allows a quantity of air from the air cleaner case to feedback to the intake manifold.

The feedback system has the following four functions:

Air-fuel Control

It controls the air-fuel ratio so that a stoichiometric ratio may be achieved.

Shot Air Control

It supplies air to the intake manifold in order to reduce emissions and also to prevent the afterburning that can occur during short deceleration when the air-fuel mixture is overrich.

Deceleration Control

It supplies air to the intake manifold when decelerating at a relatively high engine speed in order to reduce the emissions that typically occur at these conditions.

Hot Engine Start Control

When the engine coolant temperature is very high, it supplies air to the intake manifold in order to improve startability.

Throttle Control System on Carbureted Engines

The throttle controller functions as both a dashpot and a cranking opener.

The former prevents a throttle valve from closing rapidly – this will create a thick mixture – and reduces hydrocarbon emissions during deceleration, and also improves driveability by preventing sharp deceleration when the accelerator pedal is released.

The latter opens the throttle valve by a certain amount in order to improve startability.