Repair 7

Carrying Away Engine Exhaust Gases

Headers are typically installed on cars by a vehicle owner seeking to increase the horsepower of his cars engines. Headers are simple exhaust tubes or exhaust pipes that allow for the faster exiting of burned engine exhaust gases. Traditional exhaust manifolds are somewhat restrictive to exiting engine exhaust gases and create a bit of engine backpressure, which reduces engine horsepower and restricts gas mileage. Headers, on the other hand, typically have larger exhaust tubing and special tubing designs that enable exhaust gases to exit a cars engine faster and in greater quantities. This leads to a reduction in engine backpressure and an increase in both engine horsepower and gas mileage.

Bolt Headers to Engine Exhaust Ports

Automotive headers, which take the place of factory-installed exhaust manifolds, simply bolt directly to an engines exhaust ports, which are the openings in a cars engine through which exhaust gases flow out. Headers have flat areas (header flanges) on their exhaust tubing ends that align directly against the side of a cars engine exhaust ports. Header bolts attach directly through the header flanges and create an airtight seal, through which exiting engine exhaust gases travel directly into engine headers.

Funneling Exhaust to Remaining Exhaust Components

As exiting engine exhaust gases flow through a set of headers, these exhaust gases are transported to the remaining components of a cars exhaust system. Headers, besides allowing exhaust gases to exit a cars engine faster and in greater quantities, serve to bridge the gap between the engine and its main exhaust system components. Normally, exhaust gases, after flowing through and exiting a set of headers, travel to a cars catalytic converter, where these exhaust gases are reignited and burned. After traveling through the catalytic converter, exhaust gases travel through a cars muffler and then out of the tail pipe.

A cars exhaust system is responsible for transporting the burned exhaust, or combustion gases, from its engine and out the tail pipe. The exhaust system is basically just a long tube attached to the engine and extending to the rear of the vehicle. However, there are certain components that enable the exhaust system to function properly.

Exhaust Manifold

A vehicles exhaust manifold is the upper end of the exhaust system. The exhaust manifold is attached directly to the side of the engine and is the first part of the exhaust system to receive the burned exhaust gases from the cars engine. The exhaust manifold, which is extremely hot, further burns any fuel that was inadequately burned by the engine and funnels it down into the main exhaust system.

Exhaust Pipes

The exhaust pipes make up the brunt of a cars exhaust system. The exhaust pipes, which can be metal or aluminum, are responsible for transporting the burned exhaust gases from the engine and the exhaust manifold and funneling them towards the cars tailpipe.

Muffler/Catalytic Converter

As a cars exhaust gases are transported along the exhaust pipes, they must pass through the muffler, which is an enclosed metal container responsible for "muffling" the sounds of the traveling exhaust gases, and the catalytic converter, which is an "after burner" responsible for igniting and burning any leftover exhaust gases not burned by the engine and exhaust manifold.

The Tail Pipe

The final destination for a cars exhaust gases is the tailpipe. The tailpipe is the end of the entire exhaust system, and it funnels the burned car exhaust out into the outside air, or atmosphere.

Catalytic Converter as Afterburner

The sole job of a catalytic converter is to reheat already-burned exhaust gases from a cars engine. As burned engine fuel, or exhaust, leaves a cars engine, it travels along a cars exhaust system. A catalytic converter is located in the middle portion of the exhaust system--between the engine and the tailpipe. Exhaust gas flows directly into the catalytic converter and is reignited and reburned to reduce emissions.

Catalytic Converter Makeup

The outside of a catalytic converter looks just like a car muffler. It is sort of oblong with an aluminum outer cover. A cars exhaust pipe flows directly into a catalytic converter, which houses a catalyst that is superheated by the flowing exhaust gases and ignites the incoming exhaust gases. This catalyst is usually made up of small pieces of palladin, a heat-reactive element that turns glowing hot when heated.

Reduction of Exhaust Emissions

As exhaust gas exits a catalytic converter and flows toward the cars tailpipe, the vehicle emissions are much lower than they were before entering the catalytic converter. Due to the heating and re-igniting effects of the catalyst elements on the exhaust gases, much of the unburned or improperly burned fuel is burned in the catalytic converter and prevented from exiting the cars tailpipe into the outside air. This greatly reduces vehicle emissions.

Unleaded Gas Only

All cars equipped with catalytic converters require the use of unleaded gas only. The catalyst elements inside a catalytic converter are highly sensitive and can be damaged by gas that contains lead. Catalytic converters were first installed on American cars in 1975, the first year of unleaded gasoline.

In a car with a manual transmission, the function of the clutch is to interrupt the transfer of power between the engine and the drivetrain. This interruption, along with the gradual reapplication of power as the clutch is engaged, allows the car to be started smoothly from a stop and prevents the grinding of gears during shifting. The clutch mechanism consists of several parts that work together to perform this function.

Flywheel

The flywheel is a large metal disc attached the crankshaft of the engine. It is turned by the action of the engine, and it is the point of contact through which power is transferred from the running engine to the transmission and drivetrain. The flywheel has teeth around its circumference, which are engaged by the starter motor in order to turn the engine over; it also has a smooth outer surface that is gripped by the clutch disc when the clutch is engaged.

Clutch Disc and Pressure Plate

The clutch disc is a metal circle covered with a high-friction coating; the disc is pressed against the flywheel when the clutch is engaged. The pressure plate is a mechanism that uses a system of springs and levers to clamp the clutch disc against the flywheel when the clutch is engaged and to release the clutch disc when the clutch pedal is depressed. The hub of the clutch disc is attached to the input shaft of the transmission, and when the flywheel, clutch disc and pressure plate are squeezed together, power is transferred from the engine, causing the transmission shaft to rotate.

Clutch Linkage

The clutch pedal is connected to the clutch mechanism via a linkage system that can be either mechanical or hydraulic. A mechanical linkage consists of rods or cables connecting the clutch pedal to the clutch, while a hydraulic linkage uses lines filled with hydraulic fluid. The function of the linkage is to move the throw-out bearing -- a sliding bearing that moves the pressure plate away from the clutch disc -- when the pedal is depressed.

Automatic Transmissions

Automatic transmissions use a much more complicated mechanism in which sensors and electronics control when the clutch is engaged and disengaged. Hydraulic pressure activates either a number of steel plates and friction discs called a clutch pack -- much like the clutch in a manual transmission -- or a series of transmission bands that tighten and release around the clutch housing. When either of these systems is engaged, power is transferred from the engine to the transmission, and the car moves.

Automotive oxygen sensors, or 02 sensors, are little electronic devices that measure the amount of oxygen in a cars exhaust, helping to reduce pollution and increase fuel economy. Oxygen sensors were first implemented in 1976 when automaker Volvo engineered them into the exhaust system of a car called the 240.

Basic Function

O2 sensors are designed to help regulate emissions from the exhaust. They are often located in the exhaust manifold; their basic function is to monitor the amount of unburned oxygen that passes through a cars exhaust system. O2 sensors send messages to the automobiles computer telling it if the fuel mixture is burning too much or not enough oxygen.

Significance

In the early 1980s California required that all new cars sold in the state must have oxygen sensors. Several other states followed suit, and by the late 1990s O2 sensors are required equipment on all automobiles sold in the United States.

Design

O2 sensors are equipped with a zirconium ceramic bulb located within a vented cover that screws into the exhaust manifold. With a coated layer of porous platinum, the bulb also has two strips of platinum inside to act as electrodes.

How They Work

As hot gases pass through the cars exhaust system, they pass through the vented end of the oxygen sensor. The ceramic bulb on the O2 sensor gets exposed to the hot gas and experiences an increase in voltage output that represents the difference in oxygen levels between the exhaust and the outside air. A higher release of voltage indicates a great difference in these two factors. The automobiles computer reads this information and compensates for the lopsided ratio accordingly by leaning the fuel mixture with either the carburetor or throttle-body, depending on type of vehicle it is.

Warnings

O2 sensors, like all automobile components, wear out with age. Dirt, grease and grime are the most common factors that result in damaged or faulty oxygen sensors on automobiles. Driving with a faulty O2 sensor will cause increased fuel consumption as well as higher outputs of exhaust pollution than with a working O2 sensor.