Question

a) What does higher cetane number signify?

b) How does an air injection system works?

c) What is the main advantage of fuel injection in S.I Engines?

Answer 1

Answer. a-

Cetane number

Cetane rating, also known as cetane number is a measurement of the quality or performance of diesel fuel. The higher the number, the better the fuel burns within the engine of a vehicle. The cetane rating of a specific diesel mixture is based off of how much cetane—a clear, colourless hydrocarbon that ignites under high pressures—is in some fuel. Pure cetane represents the highest purity of diesel fuel possible, and thus has a cetane rating of 100.

The cetane number measures the delay in the ignition time of the fuel. In other words, it is how minimized the delay is between when the fuel is injected into the chamber and when the combustion begins.

A higher cetane number simply means the time between when the fuel is injected into the combustion chamber and when the fuel ignites is minimized. This means the fuel has the ability to ignite more easily and readily due to compression. This shorter delay time results in more complete fuel combustion.

Answer. b-

AIR ENJECTION SYSTEM

An air injection system forces fresh air into the exhaust ports of the engine to reduce HC and CO emissions. The exhaust gases leaving an engine can contain unburned and partially burned fuel. Oxygen from the air injection system causes this fuel to continue to burn. The major parts of the system are the air pump, the diverter valve, the air distribution manifold, and the air check valve.

Components of AIR INJECTION SYSTEM and work

1.The AIR PUMP

It is belt-driven and forces air at low pressure into the system. A hose is connected to the output of the diverter valve.
2.The DIVERTER VALVE

It Keeps air from entering the exhaust system during deceleration. This prevents backfiring in the exhaust system. Also, the diverter valve limits maximum system air pressure when needed, releasing excessive pressure through a silencer or a muffler.
3.AIR DISTRIBUTION MANIFOLD

It directs stream of fresh air toward each engine exhaust valve. Fittings on the air distribution manifold screw into a threaded hole in the exhaust manifold or cylinder head.

4.AIR CHECK VALVE

It is usually located in the line between the diverter valve and the air distribution manifold. It keeps exhaust gases from entering the air injection system.

Basic operation of the air injection system is as follows:

When the engine is running, the spinning vanes of the air pump force air into the diverter valve. If not decelerating, the air is forced through the diverter valve, the check valve, the air injection manifold, and into the engine. The fresh air blows on the exhaust valves.
During periods of deceleration, the diverter valve blocks air flow into the engine exhaust manifold. This prevents a possible backfire that could damage the exhaust system of the vehicle. When needed, the diverter valve will release excess pressure in the system.

3. C-

1.The main advantage of fuel injection in SI engine is increased volumetric efficiency.

2.Lower exhaust emission is obtained by using fuel injection in an SI engine.

3.Better thermal efficiency is obtained, by using fuel injection in an SI engine.

4.More accurate air-fuel mixture preparation in all points of operation.
5. Possibility of stocheometric air-fuel ratio (Lambda=1) operation, which is required for 3-way catalyst exhaust gas after treatment.
6.Possibility to apply factors for transient operation conditions (e.g. accelleration and decelleration behavior).
7.Possibility to compensate temperature and air pressure effects (e.g. cold start conditions, high altitude operation).

8.No carburettor nozzle and hole coking, no carburettor icing.