Automotive SI Engines - Exhaust System.pptx

Automotive SI Engines - Exhaust System

1. Exhaust System

2. COURSE OUTLINE Exhaust System : Need for exhaust system, exhaust manifold design, exhaust pipe calculation including application of choke flow, and exhaust testing and analysis.

3. FUNCTION OF EXHAUST SYSTEM The exhaust system provides the means to allow exhaust gases escaping through ports formed in the cylinder head to be collected together in the exhaust manifold . The exhaust manifold is in the form of branch pipes bolted to the cylinder head . These pipes merge at a common junction so that the exhaust gases can pass through the exhaust – gas silencer system before expelled into the atmosphere.

4. REQUIREMENTS OF EXHAUST SYSTEM The exhaust system must be quiet in operation, absorb the least possible engine power, minimize heat transference to the car interior and isolate the car structure from vibrations set up in the system. To meet international regulations on reduction of harmful exhaust emissions, a catalytic converter may be included in the exhaust system.

5. Main Components The exhaust system consists of three main parts. These are: a. the exhaust manifold, b. the exhaust pipe, c. and the silencer.

6. Exhaust Manifold The exhaust manifold collects the exhaust gases from a number of cylinders and leads them to a single pipe. Exhaust manifolds are made of iron castings or fabricated from steel pipes and plate by welding to withstand high temperatures of about 800 C or more . The exhaust manifold must offer minimum resistance to gas flow. Hence , the exhaust manifolds for an in – line cylinder are designed with two outlets . These are connected to separate downtake pipes, from where the exhaust gases enter a large single pipe via a breeches piece.

7. Exhaust Manifold For an in – line four – cylinder engine, the outer pair of exhaust ports communicates with one manifold outlet and the inner pair with the other outlet . In the case of six – cylinder in – line engines, a pair of three – branch manifolds may be used to serve the exhaust ports of the front and the rear halves of the engine.

8. Exhaust Manifold For V and horizontally opposed engines two – separate exhaust manifolds are used. On modern cross – flow cylinder heads, where the exhaust and intake manifolds are sited on opposite sides a separate exhaust manifold is employed. The exhaust manifold may have either a central or and end outlet, which then connects directly with the downtake pipe of the exhaust system proper.

9. Flange The exhaust system components may be either joined together by welding to form a one – piece assembly, or coupled together by means of slip joints, flanged joints or ball – and – socket joints . The latter type of joint facilitates correct alignment of the system , so that the supporting mountings are relieved of unnecessary strain. On some transverse – engine cars, a spring – loaded version of the ball – and – socket joint is used at the lower end of the downtake pipe.

10. Exhaust Pipe Exhaust pipes are made of steel tubing which may be coated with aluminium by a metal – spraying process to protect it against corrosion and oxidation . Engines are usually mounted on flexible mounting so provision is made for the relative movement of the exhaust pipe to the frame. In one form, there is a length of flexible exhaust piping between the pipes or manifolds bolted to the ports and the rear part of the pipe which is attached to the frame by rigid brackets. In another form, the whole exhaust system is made up of rigid pipes and connections which is supported from the frame by flexible attachments.

11. Exhaust Silencer/Muffler The exhaust silencer suppresses the exhaust noise caused by the release of hot gas into the atmosphere in a series of high – pressure pulsations , to a legally acceptable level. This is done by breaking up the intermittent stream of high – velocity exhaust gases into numerous small streams. This consequently reduces both the velocity and temperature of the exhaust gases so that they leave the tail pie in a constant flow at low velocity.

12. Types of Silencer   Expansion With the expansion type of silencer, the incoming gases from the down- take pipe are initially allowed to expand into the main chamber of the silencer to reduce both their pressure and their temperature. Then the direction of flow of gases is changed by passing them through side – by – side perforated tubes. In the process more energy of vibration of the exhaust gases is converted into heat energy by frictional loss before they pass into the tail pipe. This method is suitable for the suppressing of low and medium – frequency exhaust noise.

13. Expansion Silencers

14. Absorption Type The absorption types are mainly effective in damping out high – frequency noise . The sound is damped through the medium of an acoustically dead material of a soft and porous nature. A heat – resistant material such as glass fibre mat or stainless steel wool is packed into the annular space between a perforated or a louvred central tube and the expansion chamber through which it expands. Sound waves leaving the central tube and striking the absorbent material have their energy of vibration partially converted to heat energy.

15. Absorption-Type Silencer

16. ENGINE EXHAUST TESTING The exhaust gas analyzer measures the amounts of various air pollutants in the exhaust gas. Analyzing these measurements tells if the exhaust gas contains excessive pollutants . It also indicates if the engine is mechanically sound and operating properly. These tailpipe measurements can help determine the condition and performance of the engine, ignition, fuel system,, and emission controls.   Exhaust-gas analyzers measure from two to five gases. A two gas analyzer measures only HC and CO. The four-gas analyzer measures HC, CO, O2, and CO 2 . A five gas analyzer also measures NO x .

17. Relationship Among Four Gases Measured by the Gas Analyzer Exhaust-gas analyzers measure from two to five gases. A two gas analyzer measures only HC and CO. The four-gas analyzer measures HC, CO, O2, and CO 2 . A five gas analyzer also measures NO x .

18. ENGINE EXHAUST TESTING HC and NOx are measured in parts per million (ppm). An HC or NO x reading of 200 means that every million parts of exhaust gas contains 200 parts of HC or NO x . The gases CO, CO 2 , and O 2 are measured as a percentage of volume. For example, a CO reading of 1% means that one percent of the exhaust gas is CO . The relationship among the exhaust gases at the stoichiometric ratio of 14.7:1 and as the air-fuel ratio various richer and leaner.

19. ENGINE EXHAUST TESTING Tailpipe readings are made downstream or after the catalytic converter . These readings taken with a two gas analyzer on a vehicle with a catalytic converter may have little value. Normal action of the catalyst reduces the HC and CO to nearly zero. However , the two gas analyzer can be used by sampling the exhaust gas upstream or ahead of the catalytic converter. exhaust gas.

20. ENGINE EXHAUST TESTING Four-and five gas analyzers provide the most complete analysis of the tailpipe Carbon dioxide (CO2) and oxygen (O2) in the exhaust gas pass unchanged through the catalytic converter. Therefore , measuring CO2 and O2 at the tailpipe gives more complete reading of the air fuel mixture entering the cylinders and their combustion process . Sample the exhaust gas ahead of the catalytic converter if engine HC and CO measurements are needed. A good catalytic converter can lower HC to between 5 and 100 ppm (parts per million). CO can be between zero and 0.5 percent.