Month: September 2013

Emission Standards

Emission standards are requirements that set specific limits to the amount of pollutants that can be released into the environment. Many emissions standards focus on regulating pollutants released by automobiles (motor cars) and other powered vehicles but they can also regulate emissions from industry, power plants, small equipment such as lawn mowers and diesel generators.

Vehicle emission performance standard: An emission performance standard is a limit that sets thresholds above which a different type of emission control technology might be needed.

  • In the United States, emissions standards are managed by the Environmental Protection Agency (EPA). The state of California has special dispensation to promulgate more stringent vehicle emissions standards, and other states may choose to follow either the national or California standards.
  • California’s emissions standards are set by the California Air Resources Board, known locally by its acronym “CARB”. Given that California’s automotive market is one of the largest in the world, CARB wields enormous influence over the emissions requirements that major automakers must meet if they wish to sell into that market. In addition, several other U.S. states also choose to follow the CARB standards, so their rulemaking has broader implications within the U.S.



Exhaust Gas Recirculation

NOx Emissions

In many countries around the world, the emissions of NOx from diesel and gasoline vehicles are restricted by legislation. NOx is formed in the combustion chamber of engines, when high temperatures cause oxygen and nitrogen (both found in the air supplied for combustion) to combine.

Exhaust Gas Recirculation

A widely adopted route to reduce NOx emissions is Exhaust Gas Recirculation (EGR). This involves recirculating a controllable proportion of the engine’s exhaust back into the intake air. A valve is usually used to control the flow of gas, and the valve may be closed completely if required.

The substitution of burnt gas (which takes no further part in combustion) for oxygen rich air reduces the proportion of the cylinder contents available for combustion. This causes a correspondingly lower heat release and peak cylinder temperature, and reduces the formation of NOx. The presence of an inert gas in the cylinder further limits the peak temperature (more than throttling alone in a spark ignition engine).

The gas to be recirculated may also be passed through an EGR cooler, which is usually of the air/water type. This reduces the temperature of the gas, which reduces the cylinder charge temperature when EGR is employed. This has two benefits- the reduction of charge temperature results in lower peak temperature, and the greater density of cooled EGR gas allows a higher proportion of EGR to be used. On a diesel engine the recirculated fraction may be as high as 50% under some operating conditions.



Attempts were made to control the emission of carbon monoxide, hydrocarbons and oxides of nitrogen by modifications of engine design. But it was found that using the engine modification approach only, the emission levels of these gases were not reduced to the desired level appreciably. Hence, the emission engineers were bound to switch over to other alternatives. Devices developed to achieve control of exhaust emissions include catalytic converters—oxidizing catalysts for HC and CO, reducing catalysts for NO and three-way catalysts for all three pollutants; thermal reactors for HC and CO, and traps or filters for particulates.

Catalytic Converters

For engines with sufficiently low level of emission of oxides of nitrogen, only a single converter to oxidize carbon monoxide and hydrocarbons may be used. In order to get an efficient oxidation of CO and HC, it is necessary that exhaust gases have sufficient oxygen with them, which is in general not found, particularly when an engine operates on a rich air-fuel mixture. Hence, secondary air is injected in the stream of exhaust gas before the catalytic converter. A representative diagram of an engine with the oxidation catalytic converter and secondary air is shown in Figure.




(i)                 Hydrocarbons (HC)

There are some unburned or partially burned hydrocarbons in the exhaust. The amount is in significant from an energy standpoint, but it is objectionable from the viewpoint of its odour, its photochemical smog and from the standpoint of its having a carcinogenic effect. The products of photochemical smog cause watering and burning of the eyes, and affect the respiratory system especially when the respiratory system is marginal for other reasons.

Hydrocarbon emissions from SI engines

The most widely accepted causes for hydrocarbon emissions in exhaust gases of spark ignition engines are:

  1. Flame quenching at the combustion chamber walls, leaving a layer of unburned fuel-air mixture adjacent to the walls.
  2. Crevices in the combustion chamber small volumes with narrow entrances, which are filled with (lie unburned mixture during compression. and remains unburned after flame passages, since the flame cannot propagate into the crevices. The main crevice regions are the spaces between the piston, the piston rings and the cylinder walls. The other crevice regions are the threads around the spark plug. The space around the plug center electrode, crevices on the intake and exhaust valve heads, and the head gasket crevice.
  3. The oil (11m and deposits on the cylinder walls absorb fuel during intake and compression and the fuel vapour is desorbed into the cylinder during expansion and exhaust.
  4. Incomplete combustion, either partial burning or complete misfire, occurring when the combustion quality is poor. e.g. during engine transients when air-fuel, exhaust gas recirculation, and spark timing may not be adequately controlled. (more…)

Geometrical Tolerance

Geometrical Tolerance

Geometrical tolerance is defined as the maximum permissible overall variation of form or position of a feature.
Geometrical tolerances are used,
(i) to specify the required accuracy in controlling the form of a feature,
(ii) to ensure correct functional positioning of the feature,
(iii) to ensure the interchangeability of components, and
(iv) to facilitate the assembly of mating components.

Tolerance Zone

It is an imaginary area or volume within which the controlled feature of the manufactured component must be completely contained (Figs. a and b).


It is a theoretically exact geometric reference (such as axes, planes, straight lines, etc.) to which the tolerance features are related (Fig.)

Datum Feature

A datum feature is a feature of a part, such as an edge, surface, or a hole, which forms the basisfor a datum or is used to establish its location (Fig.).


Welding Joints and Symbols

Basic terms of a welded joint are shown in Fig. and the five basic types of joints are shown in Fig.


Various categories of welded joints (welds) are characterized by symbols which, in general are similar to the shape of welds to be made. These symbols are categorized as:

(i) Elementary symbols
(ii) Supplementary symbols
(iii) Combination of elementary and supplementary symbols and
(iv) Combination of elementary symbols.

Types of Joint

 Position of Weld Symbols on Drawing
The complete method of representation of the welds on the drawing comprises, in addition to the symbol (3), the following Fig.

Welding Position

(i) An arrow line (1) per joint,
(ii) A dual reference line, consisting of two parallel lines; one continuous and one dashed (2a, 2b) and
(iii) A certain number of dimensions (4) and conventional signs (3).

NOTE The dashed line may be drawn either above or below the continuous line (Fig.). For symmetrical welds, the dashed line is omitted.