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The Piston Helicopter Engine


The Piston Engine/ Piston engines are the most common type of engine to be found in modern,ght helicopters. Normally there will be 4 or 6 cylinders in a horizontally opposed configuration. Pistons move back and fourth inside the cylinders. Inside each cylinder, fuel is mixed with air and ignited. The energy produced by the combustion of the fuel air mixture causes the gases to expand and drive the piston down into the cylinder. The piston is connected by a connecting rod con-rod to a drive-shaft which is forced to turn due to the movement of the piston. Piston engines may operate on either a two stroke cycle or a four stroke engine cycle. The Four Stroke Engine Cycle/ A complete cycle of the four stroke engine comprises four strokes of the piston moving within the cylinder. This cycle is also known as the Otto cycle after its inventor in 1876. The four strokes are: ol liInduction liCompression liCombustion (or Expansion) (or Power) liExhaust /ol Induction/ During induction, the fuel and air mixture is sucked into the cylinder through an open intake valve (on the right) as the piston moves from the top of the cylinder to the bottom of the cylinder. The exhaust valve (on the left) is closed. Compression/ Early in the compression stroke, the inlet valve closes and the fuel/air mixture is trapped in the cylinder. The piston then moves back up to the top of the cylinder. This compresses the mixture and causes the temperature and pressure of the fuel/air mixture to rise. As the piston reaches the top of the cylinder and completes its compression stroke, the fuel/air mixture is ignited by a spark from the spark plug. This causes combustion which causes the gases in the cylinder to expand. Power/ As the piston has passed the top of its stroke, the expanding gases force it back down the cylinder. This is called the power stroke as the heat energy provided by the combustion process is now converted into mechanical energy. Just before the piston reaches the bottom of its stroke, the exhaust valve will open. Exhaust/ As the piston returns to the top of the cylinder again, the burned gases are forced out of the cylinder and into the atmosphere through the exhaust manifold. As the piston nears the top of its stroke and while the last of the burned gases are being expelled, the inlet valve opens in preparation for the next induction stroke. In one complete Otto cycle, only one of the four strokes provides power - but the crank-shaft has rotated two times. Engine manufacturers increase the power of the engine by adding more cylinders. This has the added bonus of making the engine run smoother. Each cylinder will have the power stroke occurring at different positions during the rotation of the crankshaft to try to even out the power impulses. Compression Ratio/ This is the ratio of the total cylinder volume when the piston is at the bottom of its stroke (bottom dead centre BDC/em) compared to the top volume of the cylinder when the piston is at the top of its stroke (emtop dead centre TDC). The compression ratio is designed to suit the type of fuel used. If the compression ratio is too high, the fuel may ignite early and excessive wear will occur. Valves/ Both the inlet valves and the outlet valves must open and close at the correct times in relation to the movement of the piston. The timing of the valve operation is controlled by a camshaft. The camshaft only rotates at half the speed of the crankshaft. The camshaft operates rocker arms and pushrods that push to relevant valve open. When the camshaft releases the pressure, a spring returns the valve to the closed position. A typical helicopter piston engine speed in flight is 2700 revolutions per minute (RPM). Each inlet valve and exhaust valve opens once during the four strokes of the Otto cycle. I.e. once in every two revolutions of the crankshaft. This means that at 2700 RPM, each valve will open and close 1350 times per minute = 22 times per second. This is a very short time to get the fuel/air mixture into the cylinder and exhaust the burnt gases again. To increase the efficiency of the fuel/air mixture induction, the inlet valve opens before the piston reaches top dead centre (TDC). This allows maximum time to induce the fuel/air mixture into the cylinder. It is referred to as valve lead. Similarly the exhaust valve opens before the piston reaches bottom dead centre (BDC) on the power stroke. It is worth noting that for a very short time at the start of the induction stroke, the exhaust gases are still exiting through the open exhaust valve while the fuel/air mixture is being forced into the cylinder through the open inlet valve. This period of overlap when both the inlet valve and the exhaust valve are open at the same time is known as valve overlap. For information about turbine engines, visit the Turboshaft Engines Turbine Engines on my website. br class=clearp

Useful Facts on US Passport Renewal you should know


In most cases you don't need to apply in person for US passport renewal. If your current US passport is undamaged and under your current name, was issued to you when you were 16 years or older, and issued within the past 15 years, then you do not have to submit the application in person. For citizens within the US, the undamaged passport can be mailed with the relevant form DS-82. Passport agencies will help you download/fill in the form and expedite the US passport renewal process. It has been strongly advised by the US Department of State, Bureau of Consular Affairs, that current passports sent for renewal and any other valid documents must be sent via couriers that have traceable delivery method so applicants are aware of the delivery status of their documents. The envelopes containing the passport and other documents must be large enough for your US renewal application and documents to comfortably fit in without folding. After renewal, the old passport will be returned to you. Some countries require that your passport should be valid for at least 6 months ahead of the dates of your trip. Some airlines also follow the same policy. It is advisable to apply for renewal approximately 9 months prior to its expiry. Professional agencies effectively handle the whole procedure of US passport renewal for applicants who seek out their services. They charge fees for getting your renewed passport in the least time possible and with minimum fuss. Contact your local agency if you need your renewal done in a short time. In holiday season, it is difficult for authorities to promise to get your passport renewed within a time-frame. You can expedite matters if you use an agency for facilitating the renewal. For US passport renewal outside the US, those US citizens residing abroad, except in Canada, must do the needful at the US Embassy or Consulate. The 9/11 recommendations have made it mandatory for all US citizens to carry their US passport for travelling anywhere including Mexico, Canada and the Caribbean. For your unfettered movement anywhere outside the US make sure to keep track of when your US passport renewal is due. Expedited services at most agencies make sure that you get your renewed document early on. If your documents are in order, some agencies would offer same day service and delivery. Don't miss out on your holiday to Cancun or a trip to Rio for the Carnival because you forgot to renew your passport!In most cases you don't need to apply in person for US passport renewal. If your current US passport is undamaged and under your current name, was issued to you when you were 16 years or older, and issued within the past 15 years, then you do not have to submit the application in person. For citizens within the US, the undamaged passport can be mailed with the relevant form DS-82. Passport agencies will help you download/fill in the form and expedite the US passport renewal process. It has been strongly advised by the US Department of State, Bureau of Consular Affairs, that current passports sent for renewal and any other valid documents must be sent via couriers that have traceable delivery method so applicants are aware of the delivery status of their documents. The envelopes containing the passport and other documents must be large enough for your US renewal application and documents to comfortably fit in without folding. After renewal, the old passport will be returned to you. Some countries require that your passport should be valid for at least 6 months ahead of the dates of your trip. Some airlines also follow the same policy. It is advisable to apply for renewal approximately 9 months prior to its expiry. Professional agencies effectively handle the whole procedure of US passport renewal for applicants who seek out their services. They charge fees for getting your renewed passport in the least time possible and with minimum fuss. Contact your local agency if you need your renewal done in a short time. In holiday season, it is difficult for authorities to promise to get your passport renewed within a time-frame. You can expedite matters if you use an agency for facilitating the renewal. For US passport renewal outside the US, those US citizens residing abroad, except in Canada, must do the needful at the US Embassy or Consulate. The 9/11 recommendations have made it mandatory for all US citizens to carry their US passport for travelling anywhere including Mexico, Canada and the Caribbean. For your unfettered movement anywhere outside the US make sure to keep track of when your US passport renewal is due. Expedited services at most agencies make sure that you get your renewed document early on. If your documents are in order, some agencies would offer same day service and delivery. Don't miss out on your holiday to Cancun or a trip to Rio for the Carnival because you forgot to renew your passport! Looking for a company who can help you with US passport renewal? You can get in touch with Express Passport for their cost effective service.