Working of camless engine with electromechanical valve actuator?

Answer 1

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. 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 optimized, 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 valve train 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 exhaust valve.

PROBLEMS RELATED TO CONVENTIONAL VALVE TRAIN

The 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 valve train. 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 single lobed cam is designed to operate the valves at only specific periods of the Otto cycle, thus preventing the engine from achieving maximum torque at higher rpms. The opening and closing of the valves is constrained by the geometry of the cam profile.

ELECTROMECHANICAL CAMLESS VALVE ACTUATOR

In recent years camless engine has caught much attention in the automotive industry. Camless valve train offers programmable valve motion control capability. An EMV system consists of two opposing electromagnets, an armature, two springs and an engine valve. The armature moves between the two magnets. When neither magnet is energized, the armature is held at the mid-point of the two magnets by the two springs located on either side of the armature. This system is used to control the motion of the engine valve. The engine valve is then in turn used to control the flow of air into and out of a combustion engine cylinder. The camless engine, where lift and valve timing can be adjusted freely from valve to valve and from cycle to cycle. It also allows multiple lift events per cycle and, indeed, no events per cycle-switching off the cylinder entirely.

Computer controlled- opening and closing of valves make it possible to optimize the various phases of engine running. During idling phases, specific intake valve opening strategies make it possible to admit just the necessary quantity of air without having recourse to throttling the intake with a butterfly valve, something that generates consumption of fuel not used by the engine. Timing of valve opening or the latitude to only open a single intake valve make it possible to stabilize the engine on idling points which consume little fuel while ensuring a good level of drive ability or the driver.

During urban driving and on the open road, both adequate opening and timing of the valves make it possible to admit a quantity of air limited to the requirement so the engine mixed with a massofburned gases purposely retained in the engine. This strategy ensures reduction of fuel consumption, polluting exhaust emissions, in particular, nitrogen oxides, produced by the engine. In terms of performance, the modularity of the system makes it possible to maximize the massof fresh air trapped in the cylinder at all engine speeds, ensuring both good torque and high power.

Apart from these advantages, deactivation of the cylinder also delivers additional savings in terms offuel consumption and exhaust emissions when the engine is only using a small amount of its power as, or example, in urban use. In this mode, only halfof the cylinders are used to provide energy to the wheels, significantly limiting losses due to poor engine efficiency. The camless system is there or system which, on an air aspirated supercharged engine provides the customer with a significant improvement in engine features. In addition, it is a system that has a strong potential or evolution and its functions will be consumption required in order to implement combustion through auto-ignition, such as the HCCI system, which is under consideration as the next stage in the battle to reduce fuel consumption.

Electromechanical Valve Train is considered the next evolution of VVT. With the potential to dial in any conceivable valve timing point of the combustion cycle for each individual cylinder, valves can be opened with more lift and/or duration, as the computer deems necessary. Just imagine that you have your latest 2-litre 16-valve EMVT powered engine on the dyno after installing an exhaust. Simply changing a couple of numbers on the computer will have a set of completely revised valve timing maps to suit your exhaust - or cold air intake for that mater. There will be no need for expensive cam changes that may not even give the results you are after. Electronically altering valve events will have a far more major impact on engine performance than any current electronically-controlled item.

CONTROL DESIGN

When the valve-closing event starts, the lower solenoid coil is deactivated, and the valve moves up towards its seating position by the mechanical spring force. An electro mechanical valve actuator works according to the spring-mass pendulum principle, which means that the system follows its own natural oscillation frequency, and external electromagnetic force is only needed for overcoming the friction loss. The electromagnetic actuator is only effective in a relatively short range closing to the seating position, and so it is not efficient in the sense of energy consumption to apply closed-loop control when the valve is still far away from the

seating position. The system goes unstable as the engine valve moves to the region within one-third of the total lift.

This type of system uses an armature attached to the valve stem. The outside casing contains a magnetic coil of some sort that can be used to either attract or repel the armature, hence opening or closing the valve.

Most early systems employed solenoid and magnetic attraction/repulsion actuating principals using an iron or ferromagnetic armature. These types of armatures limited the performance of the actuator because they resulted in a variable air gap. As the air gap becomes larger (ie when the distance between the moving and stationary magnets or electromagnets increases), there is a reduction in the force. To maintain high forces on the armature as the size of the air gap increases, a higher current is employed in the coils of such devices. This increased current leads to higher energy losses in the system, not to mention non-linear behaviour that makes it difficult to obtain adequate performance. The result of this is that most such designs have high seating velocities (ie the valves slam open and shut hard!) and the system cannot vary the amount of valve lift.

The electromechanical valve actuators of the latest poppet valve design eliminate the iron or ferromagnetic armature. Instead it is replaced with a current-carrying armature coil. A magnetic field is generated by a magnetic field generator and is directed across the fixed air gap. An armature having a current-carrying armature coil is exposed to the magnetic field in the air gap. When a current is passed through the armature coil and that current is perpendicular to the magnetic field, a force is exerted on the armature.When a current runs through the armature coil in either direction and perpendicular to the magnetic field, an electromagnetic vector force, known as a Lorentz force, is exerted on the armature coil. The force generated on the armature coil drives the armature coil linearly in the air gap in a direction parallel with the valve stem. Depending on the direction of the current supplied to the armature coil, the valve will be driven toward an open or closed position. These latest electromechanical valve actuators develop higher and better-controlled forces than those designs mentioned previously. These forces are constant along the distance of travel of the armature because the size of the air gap does not change.

The key component of the Siemens-developed infinitely variable electromechanical valve train is an armature-position sensor. This sensor ensures the exact position of the armature is known to the ECU at all times and allows the magnetic coil current to be adjusted to obtain the desired valve motion.

The ability of the electromechanical valve actuator to generate force in either direction and to vary the amount of force applied to the armature in either direction is an important advantage of this design. For instance, varying the value of the current through the armature coil and/or changing the intensity of the magnetic field can control the speed of opening and closing of the valve. This method can also be used to slow the valve closure member to reduce the seating velocity, thereby lessening wear as well as reducing the resulting noise.

A special software algorithm is used to control the actuator coil currents such that the valves are decelerated to a speed near zero as they land - in conjunction with a switching time of barely three milliseconds. For the valves this means minimal wear and minimum noise generation. The 16-valve four cylinder engine that is currently undergoing tests in Germany, by Siemens, is equipped with 16 valve actuators and the corresponding armature-position sensors. A ECU is used and two cable rails connect the actuators to it. A 42-volt starter-generator provides the power.

WORKING

Camless engines generally employ one of two types of camless actuators: electro-hydraulic or electro-mechanical valve actuators. The actuators receive input from the ECU via a dedicated CAN bus to open and close the poppet valves at a prescribed crankshaft angle timing, transition time and lift, matching the valve timing request sent by the ECU. Feedback is then sent by the actuators through the CAN bus to verify the actual occurrence of the operation.

Electromechanical actuators are generally made with two solenoids and two springs. As can be seen in Figure 1 the ECM receives input from the crankshaft position sensor to close the valve, which activates Solenoid 1 by taking current from the battery. The current is passed through a pulse width modulator which tunes the amplitude of the current to control the speed of valve seating. The magnetic field created by Solenoid 1 attracts the armature in the upper position. Spring 1 is compressed and thus closes the valve. Solenoid 2 pulls the armature down to open the valve as shown in Figure 2.