Diesel Engine Proper 4

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(5) Piston Material

Piston must be made of a material that meets the following requirements :

• Low Thermal expansion. The coefficient of thermal expansion must be low. It is best to use the same material for both pistons and cylinders.
• High heat conductivity.
• Low spesific gravity (to decrease inertia during high speed operation).
• Sufficient strength and large abrasion resistance even at high temperatures.
• Easy to cast

Alumunium alloys is currently used because they satisfy all of the above requirements. Special cast iron is used as well. A piston made of special cast iron has the same coefficient of thermal expansion as the cylinder, but tends to be heavy.

Alumunium alloys has a larger coefficient of thermal expansion than iron,but has high heat conductivity, therefore the temperature of the piston head can be lowered.

However, alumunium alloy has a weak point (poor lubricating oil retention). For this reason,pistons are usually plated with lead to eliminate this shortcoming. Seizure can be prevented by lead plating.

Some pistons have a special cast iron ring carrier that is cast into the top ring groove to prevent abrasion. A piston usually tin plated to improve initial breaking in performance and to prevent rusting.

(6) Thermal Problem of Pistons

The strength and hardness of the alumunium alloy used for manufacturing pistons will suddenly decrease when temperature exceeds 400^oC. As a result, abrasion and cracking will begin to occur. When Lo-Ex alloy is used, the piston head cavity temperature is designed to be 300 - 330^oC and the bottom of thetopof ring groove is designed to below 230 - 250^oC.

The overheating of piston can be prevented by various methods.  For example the cooling efficiency can be raised to lower the temperature of the cylinder liner. The thermal flow type shape (dome shape that promotes the flow of heat from the top of the piston to the ring) can be adopted for the back of pistons so that the piston temperature will be even. Pistons can also be oil cooled.

(7) Handling of Piston

Clearence between piston and cylinder

When the piston is installed in the cylinder, there must be a specified clearence between them. Insufficient clearence will cause seizure due to thermal expansion, while excessive clearence will lead to compression leakage, inefficient heat radiation by the piston, over-consumption of lubricating oil, and piston slap.

Measurement of piston dimensions

A piston is designed to maintain an even clearence with the cylinder during operation when thermal expansion is taken into consideration. Therefore the dimensions of the piston in the cold stage are supposed to be smaller than in the operating state by the amount of  thermal expansion that takes place. The upper part of the piston is heated more than the lower part. Therefore its diameter isthe smallest and the top and increases toward the bottom. In other words, a piston has conical shape. Since heat is transmitted through the ribs that connect the bosses of the piston head and the piston pin, the ribs and bosses are heated more than the other parts. This mean that the expansion in the axial direction of the piston is larger. Therefore the diameter in the pin direction is smaller than the diameter in the perpendicular direction. (this called Ovality) (A cast iron piston is exactly round). Since a piston has the complicated shape explained above, its maximum diameter is measured when finding its dimension measurements.

Diesel Engine Proper 3

My apologies to the loyal readers because it has been a long time I did not continue writing about diesel engines. I hope I can still continue writing until the end. And now I will continue from Diesel Engine Proper 2.

[4] Pistons
(1) Functions of the Pistons
A piston is one part of combustion chamber. Its function is to transmit the pressure generated by combustion to the crankshaft through a connecting rod. In addition, it has intake, compression and exhaust functions.
The Piston and the cylinder must form a completely enclosed space in order to ensure efficient intake compression, and exhaust.To maintain complete enclosure under every type of operating condition, the material and the structure of the piston must have high heat conductivity and its thermal expansion must be small even when exposed to high temperature (up to about 1,000degree Celcius, even though momentarily). A piston must have enough strength to withstand the combustion pressure (about 80 kg/cm2) so as to transmit the combustion pressure correctly to the crankshaft.

(2) Piston Structure

Pistons have various types of structures. The basic structure is shown below :
Piston head....Since the piston head directly recieves the combustion pressure, its must be very durable so that it is able to withstand the pressure. Some pistons have reinforcing ribs toincrease strength.
Ring Land.....Piston rings are inserted into rings lands. Ordinarily,a diesel engine has 3 compression rings and 1 or 2 oil rings. Some recent models have 3 rings :  2 compression rings and 1 oil ring.
Oil drain hole.....The oil rings groove has an oil drain hole. Oil that is scrapped off by the ring passes through this hole and drops into the oil pan.
Piston pin mounting hole.....This holds the piston pin that connects the piston to the connecting rod.
Skirt.....The part below the piston pin center.

Oval Piston

Conical Piston

(3)Piston Pin
The piston pin connects the piston and the small end of the connecting rod, and has a hollow cylindrical shape. Piston pin ca be classified into the full floating type and the half floating type, depending on the mounting method. The full folating type is ordinarily used on diesel engine.

(4)Offset Piston
In an offset piston, the piston pin hole is slightly offset from the piston center.
Adequate clearence must be maintened  betwen the piston and the cylinder bore. When the piston changes from the compression stroke to the combustion stroke, as the connecting rod inclination changes the piston moves from right to left. The distance of this movement is equal to the clearence. This is type of shock, which produce of hammering sound. This shock can be decreased by decreasing the clearence of the bore. However, the lubricating oil will be unable to pass through if the clearence is too small. If the rings can not be lubricated, air-tightness cannot be maintened. Or the clearence may become too small due to expansion of the piston. Therefore, the specified minimum clearence must be maintened.

BMW Diesel engines.

The small letter ‘d’ in the BMW model names stands for a difference with big results: every BMW diesel engine – whether four, six or eight-cylinder – delivers refined power and superb fuel efficiency.

The diesel engine transforms the chemical energy contained in diesel into mechanical energy. While petrol engines compress a mixture of air and fuel and ignite it with a sparkplug (externally-supplied ignition), diesel engines first compress pure air, which causes it to heat up. The fuel is later injected directly into the combustion chamber and ignites itself (self ignition). The direct injection with common rail technology enables the precise control of the ignition process, an even increase in pressure and the targeted burning of remaining soot particles by post injections. This minimises burning noises, makes running smoother and reduces emissions. 

The high compression ratio and pressure deliver generous torque even at low speeds and ensure efficient fuel combustion and thus lower consumption. Modern turbo technologies like the two-stage charging in the Variable Twin Turbo also increase the spontaneous power delivery and, in combination with modern automatic transmissions, minimise the response delay at low speeds. Thus BMW diesel engines combine the typical BMW dynamics and sportiness with smooth and refined running, low consumption and excellent acoustics.

The four-cylinder diesel in the BMW 3 Series Sedan 320d EfficientDynamics Edition, with 120 kW (163 hp) offers a powerful engine and a temptingly low average consumption of 4.1 litres of diesel per 100 km. The common rail injection technology ensures maximum running smoothness; and the BMW BluePerformance technology means BMW diesel engines already fulfil the EU6 emission norm that only come into effect in 2014. 

The BMW 740d Series six-cylinder diesel engine in the BMW 7 Series Sedan boasts an impressive 225 kW (306 hp) and a torque of 600 Nm. Thanks to innovative TwinPower Turbo technology the vehicle can be started at just above idling speed and has an electronically-controlled top speed of 250 km/h. The combination of both a large and a small turbocharger enable a powerful start and thrust even at high speed. 

Fuel consumption / CO2 emissions
Combined fuel consumption: 4.1 to 7.5 l/100 km
Combined CO2 emissions: 109 to 183 g/km

Brief Review on Truck Repair Services

Your truck must be considered as a machine instead of something that will allow you to haul cargo over long distances. As with all other machines, this machine must also be given appropriate care. If you fail to do so, you will be stranded someday in the highway. Repairing the truck at a truck repair center can turn out to be a highly expensive procedure. Why repair the truck when you can take precautions to avoid repairing it? Yes, that is what we will be looking into in the following passages. Given below are certain tips and strategies that must be kept in mind while driving the truck.

The next time you are driving, try to assimilate the sound generated by the truck. It might be noise to others, but you should not brush it aside so easily. This constant noise or hum is the only means by which your truck communicates about its condition. Here is an insider tip – always heed the noise levels generated by the truck. If it increases, then you must be doing something wrong. One of the primary symptoms that are being displayed prominently by these monsters is the alteration in the engine noise levels. The next time you think that the noise levels have increased, please take it to the nearest authorized service truck repair center. Let the experts look into it. Get all your doubts cleared by them. Replacing or repairing any parts of your truck is a costly affair. So, the economic way to own and utilize a truck is to study it inside out and avoid all possibilities that might lead to an untoward breakdown. The performance levels of your vehicle would certainly diminish over a certain number of years, but if it is served with regular maintenance and periodic checks, then certainly it would cover more miles before dying on you.

 The truck will be spending a major part of its life chugging along the road. If you wish to learn about the dirt levels present in our roads, please look at the underneath of the truck. If possible, try to wash it away with the help of pressurized water. The next time you cross a truck repair center, please take time to stop by and get the vehicle cleaned. Keeping the undersides clean will prevent rusting of the metallic components. Rust is the primary enemy of a machine. If viable, also invest on anti-rust coating for the truck’s body.

Overloading the vehicle with additional cargo might fatten your pockets. On the other hand, you are literally putting yourself and the truck at risk by doing so. Never exceed the threshold stated by the manufacturer. It is better to weigh the overall cargo before venturing out into the wilderness.

Every aging machine needs certain minimal degree of maintenance and repair jobs done on it. However, the quality of work done on the components of your truck determines the quality of its afterlife. So, before leaving your truck with a truck repair company make sure that they have the best mechanics available in the market.

Source : Trucksgo.com