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02-05-2012, 01:23 PM
Post: #1
ELECTROMECHANICAL CAM ACTUATOR

ELECTROMECHANICAL CAM ACTUATOR


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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.

Cams, lifters, pushrods... all these things have up until now been associated with the internal combustion engine. But the end is near or these lovely shiny metal objects that comprise the valve train hardware in your pride and joy. Camless engine technology is soon to be a reality for mass-produced vehicles. In the camless valvetrain, the valve motion is controlled directly by a valve actuator - there's no camshaft or connecting mechanisms. Various studies have shown that a camless valve train can eliminate many otherwise necessary engine design trade-offs. Automotive engines equipped with camless valve trains of the electro-hydraulic and electro-mechanical type have been studied for over twenty years, but production vehicles with such engines are still not available. Conventional valve train
The issues that have had to be addressed in the actuator design include:
• reliable valve performance cost
• packaging
• power consumption
• noise and vibration



Noise has been identified as the main problem with the electromechanical actuator technology, arising from high contact velocities of the actuator's moving parts. For this noise to be reduced, a so-called soft-landing of the valves has to be achieved.

The valvetrain in a typical internal combustion engine comprises several moving components. Some are rotating and some are moving in a linear manner. Included are poppet valves that are operated by rocker arms or tappets, with valve springs used to return the valves to their seats. In such a system the parasitic power losses are major - power is wasted in accelerating and decelerating the components of the valvetrain. Friction of the camshaft, springs, cam belts, etc also robs us of precious power and worsens fuel economy, not to mention contributing to wear and tear. The power draw on the crankshaft to operate the conventional valve train is 5 to 10 percent of total power output.
Another factor working against the conventional valve train is that of the cam profile. Usually , it is fixed to deliver only one specific cam timing. The cam lobes have to be shaped such that when the valve travels up and down at the engines maximum speed it should still be able to slow down and gently contact the valve seat. The valves crashing down on their valve seats results in an engine that is real noisy and has a short life expectancy.


Having different cam profiles will result in different engine

characteristics. While high-rpm power and low rpm-torque can be each optimised, a compromise is required to obtain the best of both in the same engine. With Variable Valve Timing (VVT) technologies the compromise is getting better and better - reasonable low down torque and high-speed power are being produced by many sub 2-litre engines.
But the problem remains that the cam grind is still a fixed quantity - or two fixed quantities in the case of Honda V-TEC engines. That's why the Electromechanical Valve Train is considered the next evolution of VVT. With the potential to dial in any conceivable valve timing at any point of the combustion cycle for each individual cylinder, valves can be opened with more lift and/or duration, as the computer deems necessary.

Conventional valvetrain mechanism

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 exhaustvalve.
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