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Trucks and buses have a Compression Ignition type engine running on Diesel. In a petrol engine a mixture of petrol and air is ignited by an electric spark from the spark plug but a diesel engine has no spark plugs. Diesel fuel, less easily ignited (low auto ignition temperature) than petrol, is not drawn in with air as a mixture, but is sprayed under high pressure from an injector into the combustion chamber, where it ignites on contact with the hot, compressed air. Each injector is supplied with fuel in metered quantities and at a high pressure by an engine driven fuel injection pump. The accelerator controls the amount of fuel delivered by the pump, and hence the power delivered by the engine.

The diesel's advantages lie in its greater efficiency (resulting in lower fuel costs), longer life and lower maintenance costs.

Its disadvantages include a high initial cost, greater weight, a somewhat rougher idling, some smell, a higher noise level and slower acceleration.

In a medium sized vehicle, petrol is compressed to about one-ninth of its original volume, giving a compression ratio of 9:1; in a diesel engine it may be as high as 22:1, to increase the temperature of air to the auto ignition temperature of the diesel fuel.

A diesel engine has a much smaller combustion chamber than a petrol engine and its higher compression ratio results in greater efficiency, because more potential heat energy is converted into power and less heat is wasted.

To ensure that the correct amount of fuel is injected at the right moment, each cylinder on a diesel engine is fitted with an injector. A pump, driven at half crankshaft speed, forces fuel into the combustion cylinders in their firing order.

In the diesel 4 stroke cycle, pure air is drawn into the cylinder on the suction stroke; fuel is injected and starts to burn towards the end of the rising compression stroke; pressure from expanding gases forces the piston down on its power stroke; and burnt gases escape as the piston rises on its exhaust stroke.

In some diesel engines, a heater plug is fitted to help starting in low temperatures. This does not produce a spark but glows continuously until air temperature in the engine cylinders is high enough to ensure self-ignition of the fuel sprayed into the combustion chamber.

Why the engine needs oil :-

The oil in an engine does more than simply cut down friction and wear by lubricating the pistons, bearings and other moving parts. It also helps to seal hot high-pressure gases; take heat away from hot areas and disperses it to the air in the sump; reduces corrosion; and absorbs some of the harmful waste products of combustion. As the loads in Diesel Engine will be much higher, they should also have adequate antiwear and EP properties. But one of the most important properties which is not critical for pertol engines oils is its alkalinity reserve to combat ill-effects of sulphur in the diesel fuel. Sulphur present in the diesel fuel burns to form oxides of sulphur, which in the presence of water, form sulphur acids, which are highly corrosive to engine parts. Hence the need for high alkalinity reserve in diesel engine oils.

Oil is carried in the sump, at the bottom of the engine, and forced by a pump through a filter to the main bearings. The pump will normally deliver several litres of oil a minute, at a pressure controlled by a relief valve.

From the main bearings, the oil passes through feed holes or grooves into drilled passages in the crankshaft and on to the big end bearings of the connecting rods. In some engines the oil is taken to the gudgeon pins through passages drilled in the connecting rods.

The cylinder walls and gudgeon pin bearings are lubricated by oil fling oil - oil escaping from the end of the bearings and dispersed by the rotating crankshaft. The excess is taken off the cylinders by scraper rings on the pistons and then drops back to the sump.

A bleed or tributary from the main supply passage feeds each camshaft bearing; and in many overhead valve engines there is another bleed leading to the rocker shaft bearings. The oil then drains back from the cylinder head to the sump, where excess heat is dispersed to the surrounding air. Another bleed supplies the timing chain or gears on the camshaft drive.

Bharat Petroleum manufactures a range of lubricants for trucks and buses. Given below are the different types of lubricants available for you to choose from.

CNG AS FUEL IN ENGINES

The CNG (Compressed Natural Gas ) Engines are coming in a big way in Buses and heavy vehicles plying in the Metro cities to reduce the Environmental Pollution caused by vehicles.

Unlike in a passenger Cars where retrofitting the Engine will switchover to CNG use, for Buses and Trucks, it is not possible to retrofit gas conversion kit due to the CI type of Engines used by these class of vehicles. A specially designed CNG Engine has to fitted to use CNG. Also the vehicle can only run on CNG unlike Petrol or CNG in a passenger cars.

CNG / LPG is a clean fuel with high octane number and fully burns in the engine without leaving any unburned carbon deposits in the engine and this will help to run with less excess air (near stoichiometric ratio). This leads to higher operating temperature of the engine and causes severe nitration on the oil, Use of Gas in the Engine will also lead to non lubrication of the inlet valve otherwise lubricated by the liquid fuel particles and exhaust valve which is lubricated by the carbon deposit from the unburned petrol.

The Oils for Gas Engines are specially developed to meet the operating requirement of the conventional Diesel engines and also to take care of the additional demands of higher operating temperature & nitration of the Oil fortified with carefully selected additives to give ash deposits to lubricate the vale seats.