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 and all this General working of engine is discussed in Best polytechnic college. 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 engines 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 is no camshaft or connecting mechanisms. Precise electro hydraulic camless valve train controls the valve operations, opening, closing etc.
AN OVERVIEW OF CAMLESS ENGINE
To eliminate the cam, camshaft and other connected mechanisms, the Camless engine makes use of three vital components the sensors, the electronic control unit and the actuator.
Mainly five sensors are used in connection with the valve operation. One for sensing the speed of the engine, one for sensing the load on the engine, exhaust gas sensor, valve position sensor and current sensor. The sensors will send signals to the electronic control unit. The electronic control unit consists of a microprocessor, which is provided with a software algorithm. This system is similar to EFI system which is studied in top polytechnic college in pune.
TYPES OF CAMLESS VALVE TRAIN
- Electromechanical Poppet Valves
- Electrohydraulic Poppet Valve
- Hydraulic Pendulum
WORKING OF HYDRAULIC PENDULUM
The Electro hydraulic Camless valve train, (EC) provides continuously variable control of engine valve timing, lift, and velocity. It uses neither cams nor springs. It exploits the elastic properties of a compressed hydraulic fluid, which, acting as a liquid spring, accelerates and decelerates each engine valve during its opening and closing motions. This is the principle of the hydraulic pendulum. Like a mechanical pendulum, the hydraulic pendulum involves conversion of potential energy into kinetic energy and, then, back into potential energy with minimal energy loss. During acceleration, potential energy of the fluid is converted into kinetic energy of the valve. During deceleration, the energy of the valve motion is returned to the fluid. This takes place both during valve opening and closing. Recuperation of kinetic energy is the key to the low energy consumption of this system. Figure illustrates the hydraulic pendulum concept. The system incorporates high and low - pressure reservoirs. A small double-acting piston is fixed to the top of the engine valve that rides in a sleeve. The volume above the piston can be connected either to a high- or a low-pressure source. The volume below the piston is constantly connected to the high-pressure source. The pressure area above the piston is significantly larger than the pressure area below the piston. The engine valve opening is controlled by a high pressure solenoid valve that is open during the engine valve acceleration and closed during deceleration. Opening and closing of a low -pressure solenoid valve controls the valve closing. The system also includes high and low-pressure check valves.
Detailed view of hydraulic pendulum
During the valve opening, the high-pressure solenoid valve is open, and the net pressure force pushing on the double-acting piston accelerates the engine valve downward. When the solenoid valve closes, pressure above the piston drops, and the piston decelerates pushing the fluid from the lower volume back into the high-pressure reservoir. Low-pressure fluid flowing through the low-pressure check valve fills the volume above the piston during deceleration. When the downward motion of the valve stops, the check valve closes, and the engine valve remains locked in open position. The process of the valve closing is similar, in principle, to that of the valve opening. The low-pressure solenoid valve opens, the pressure above the piston drops to the level in the low pressure reservoir, and the net pressure force acting on the piston accelerates the engine valve upward. Then the solenoid valve closes, pressure above the piston rises, and the piston decelerates pushing the fluid from the volume above it through the high-pressure check valve back into the high-pressure reservoir. The hydraulic pendulum is a spring less system. Figure shows idealized graphs of acceleration, velocity and valve lift versus time for the hydraulic pendulum system. Thanks to the absence of springs, the valve moves with constant acceleration and deceleration. This permits to perform the required valve motion with much smaller net driving force, than in systems which use springs. The advantage is further amplified by the fact that in the spring less system the engine valve is the only moving mechanical mass. To minimize the constant driving force in the hydraulic pendulum the opening and closing accelerations and decelerations must be equal (symmetric pendulum).
DESIGN APPROACH FOR CAMLESS ENGINE
The camless engine was created on the basis of an existing four cylinder, four-valve engine. The original cylinder head with all the valves, springs, camshafts, etc. was replaced by a new cylinder head assembly fully integrated with the camless valve train. The camshaft drive was eliminated, and a belt-driven hydraulic pump was added. There was no need for lubrication, and the access for engine oil from the engine block to the cylinder head was closed off. No other changes to the engine have been made.
COMPONENTS OF CAMLESS ENGINE
Main components of a camless engine are-Engine valve, solenoid valve, high pressure pump, low pressure pump, cool down accumulator, etc.
The solenoid has conically shaped magnetic poles. This reduces the air gap at a given stroke. The normally-closed valve is hydraulically balanced during its movement. Only a slight unbalance exists in the fully open and the fully-closed positions. A strong spring is needed to obtain quick closing time and low leakage between activations. The hydraulic energy loss is the greatest during the closing of either the high- or the low-pressure solenoid, because it occurs during the highest piston velocity. Thus, the faster the solenoid closure, the better the energy recovery. The valve lift and the seat diameter are selected to minimize the hydraulic loss with a large volume of fluid delivered during each opening. Both high-pressure and low-pressure solenoid valves are of the same design
Unequal Lift Modifier
To enhance the ability to vary the intake air motion in the engine cylinder, it is often desirable to have unequal lift of the two intake valves, or even to keep one of the two valves closed while the other opens. In some cases it may also be used for paired exhaust valves. The lift modifier is then used to restrict the opening of one the paired valves. The rod is installed in the cylinder head between the two intake valves. A cut out in the rod forms a communication chamber connected to the volumes below the hydraulic pistons of both intake valves. The communication chamber is always connected to the high pressure reservoir.
- Enables the development of higher torque throughout the entire rev range which in turn improves fuel economy.
- Cylinder Deactivation can be achieved during the idling phase.
- Exhaust gas recirculation is improved.
- Reduces friction losses.
- Reduces the inertia of moving parts
- Even though some disadvantages are present, we can expect electro hydraulic & electromechanical valves to replace the conventional camshaft technology.
Prof. Shanteshwar Dhanure
Mechanical Department (Second Shift)
Pimpri Chinchwad Polytechnic