14-02-2013, 12:29 PM
CAMLESS ENGINE
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INTRODUCTION
The cam has been an integral part of the IC engine from its invention. The cam controls the “breathing channels” of the IC engines, that is, the valves through which the fuel air mixture (in SI engines) or air (in CI engines) is supplied and exhaust driven out. Besieged by demands for better fuel economy, more power, and less pollution, motor engineers around the world are pursuing a radical “camless” design that promises to deliver the internal – combustion engine’s biggest efficiency improvement in years. The aim of all this effort is liberation from a constraint that has handcuffed performance since the birth of the internal-combustion engine more than a century ago. Camless engine technology is soon to be a reality for commercial vehicles. In the camless valve train, the valve motion is controlled directly by a valve actuator – there’s no camshaft or connecting mechanisms .Precise electrohydraulic camless valve train controls the valve operations, opening, closing etc. The seminar looks at the working of the electrohydraulic camless engine, its general features and benefits over conventional engines. The engines powering today’s vehicles, whether they burn gasoline or diesel fuel, rely on a system of valves to admit fuel and air to the cylinders and let exhaust gases escape after combustion. Rotating steel camshafts with precision-machined egg-shaped lobes, or cams, are the hard-tooled “brains” of the system. They push open the valves at the proper time and guide their closure, typically through an arrangement of pushrods, rocker arms, and other hardware. Stiff springs return the valves to their closed position. In an overhead-camshaft engine, a chain or belt driven by the crankshaft turns one or two camshafts located atop the cylinder head.
A single overhead camshaft (SOHC) design uses one camshaft to move rockers that open both inlet and exhaust valves. The double overhead camshaft (DOHC), or twin-cam, setup does away with the rockers and devotes one camshaft to the inlet valves and the other to the exhaust valves.
WORKING OF PUSH ROD ENGINE
Pushrod engines have been installed in cars since the dawn of the horseless carriage. A pushrod is exactly what its name implies. It is a rod that goes from the camshaft to the top of the cylinder head which push open the valves for the passage of fuel air mixture and exhaust gases. Each cylinder of a pushrod engine has one arm (rocker arm) that operates the valves to bring the fuel air mixture and another arm to control the valve that lets exhaust gas escape after the engine fires. There are several valve train arrangements for a pushrod.
Crankshaft
Crankshaft is the engine component from which the power is taken. It receives the power from the connecting rods in the designated sequence for onward transmission to the clutch and subsequently to the wheels. The crankshaft assembly includes the crankshaft and bearings, the flywheel, vibration damper, sprocket or gear to drive camshaft and oil seals at the front and rear.
Camshaft
The camshaft provides a means of actuating the opening and controlling the period before closing, both for the inlet as well as the exhaust valves, it also provides a drive for the ignition distributor and the mechanical fuel pump.
The camshaft consists of a number of cams at suitable angular positions for operating the valves at approximate timings relative to the piston movement and in the sequence according to the selected firing order. There are two lobes on the camshaft for each cylinder of the engine; one to operate the intake valve and the other to operate the exhaust valve.
Working
When the crank shat turn the cam shaft the cam lobs come up under the valve lifter and cause the lifter to move upwards. The upward push is carried by the pushrods through the rocker arm. The rocker arm is pushed by the pushrod, the other end moves down. This pushes down on the valve stem and cause it to move down thus opening the port. When the cam lobe moves out from under the valve lifter, the valve spring pulls the valve back upon its seat. At the same time stem pushes up on the rocker arm, forcing it to rock back. This pushes the push rods and the valve lifter down, thus closing the valve. The figure-1,2 shows cam-valve arrangement in conventional engines
Valve opening and closing
A more detailed step-by-step illustration of the valve opening and closing process is given in Figure 9. It is a six-step diagram, and in each step an analogy to a mechanical pendulum is shown. In Step 1 the opening (high-pressure) solenoid valve is opened, and the high-pressure fluid enters the volume above the valve piston. The pressure above and below the piston become equal, but, because of the difference in the pressure areas, the constant net hydraulic force is directed downward. It opens the valve and accelerates it in the direction of opening. The other solenoid valve and the two check valves remain closed. In Step 2 the opening solenoid valve closes and the pressure above the piston drops, but the engine valve continues its downward movement due to its momentum. The low-pressure check valve opens and the volume above the piston is filled with the low-pressure fluid. The downward motion of the piston pumps the high-pressure fluid from the volume below the piston back into the high-pressure rail. This recovers some of the energy that was previously spent to accelerate the valve. The ratio of the high and low-pressures is selected so, that the net pressure force is directed upward and the valve decelerates until it exhausts its kinetic energy and its motion stops. At this point, the opening check valve closes, and the fluid above the piston is trapped. This prevents the return motion of the piston, and the engine valve remains fixed in its open position trapped by hydraulic pressures on both sides of the piston. This situation is illustrated in Step 3, which is the open dwell position. The engine valve remains in the open dwell position as long as necessary. Step 4 illustrates the beginning of the valve closing. The closing (low-pressure) solenoid valve opens and connects the volume above the piston with the low-pressure rail. The net pressure force is directed upward and the engine valve accelerates in the direction of closing, pumping the fluid from the upper volume back into the low-pressure reservoir. The other solenoid valve and both check valves remain closed during acceleration. In Step 5 the closing solenoid valve closes and the upper volume is disconnected from the low-pressure rail, but the engine valve continues its upward motion due to its momentum. Rising pressure in the upper volume opens the high-pressure check valve that connects this volume with the high-pressure reservoir. The upward motion of the valve piston pumps the fluid from the volume above the piston into the high-pressure reservoir, while the increasing volume below the piston is filled with fluid from the same reservoir. Since the change of volume below the piston is only a fraction of that above the piston, the net flow of the fluid is into the high-pressure reservoir. Again, as it was the case during the valve opening, energy recovery takes place. Thus, in this system the energy recovery takes place twice each valve event. When the valve exhausts its kinetic energy, its motion stops, and the check valve closes. Ideally, this should always coincide with the valve seating on its seat. This, however, is difficult to accomplish. A more practical solution is to bring the valve to a complete stop a fraction of a millimeter before it reaches the valve seat and then, briefly open the closing solenoid valve again. This again connects the upper volume with the however, is difficult to accomplish. A more practical solution is to bring the valve to a complete stop a fraction of a millimeter before it reaches the valve seat and then, briefly open the closing solenoid valve again. This again connects the upper volume with the low-pressure reservoir, and the high pressure in the lower volume brings the valve to its fully closed position. Step 6 illustrates the valve seating. After that, the closing solenoid valve is deactivated again. For the rest of the cycle both solenoid valves and both check valves are closed, the pressure above the valve piston is equal to the pressure in the low-pressure reservoir, and the high pressure below the piston keeps the engine valve firmly closed.