CAMLESS ENGINE
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
Since the timing of the engine is
dependent on the shape of the cam lobes and the rotational velocity of the
camshaft, engineers must make decisions
early in the automobile development process that affect the engine’s
performance. The resulting design represents a compromise between fuel
efficiency and engine power. Since maximum efficiency and maximum power require
unique timing characteristics, the cam design must compromise between the two
extremes. This compromise is a prime consideration when consumers purchase
automobiles. Some individuals value power and lean toward the purchase of a
high performance sports car or towing capable trucks, while others value fuel
economy and vehicles that will provide more miles per gallon. Recognizing this
compromise, automobile manufacturers have been attempting to provide vehicles
capable of cylinder deactivation, variable valve timing (VVT), or variable
camshaft timing (VCT). These new designs are mostly mechanical in nature.
Although they do provide an increased level of sophistication, most are still
limited to discrete valve timing changes over a limited range.
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. The microprocessor issues signals to the
solid-state circuitry based on this algorithm, which in turn controls the
actuator, to function according to the requirements.
Camless valve train
In
the past, electro hydraulic camless systems were created primarily as research
tools permitting quick simulation of a wide variety of cam profiles. For
example, systems with precise modulation of a hydraulic actuator position in
order to obtain a desired engine valve lift versus time characteristic, thus
simulating the output of different camshafts. In such systems the issue of
energy consumption is often unimportant. The system described here has been
conceived for use in production engines. It was, therefore, very important to
minimize the hydraulic energy consumption.
UNEQUAL LIFT MODIFIER:
In a
four-valve engine an actuator set consisting of two solenoid valves and two
check valves controls the operation of a pair of intake or a pair of exhaust
valves. Solenoids and check valves are connected to a common control chamber
serving both valves (Figure 10). In a four-cylinder engine there is a total of
eight control chambers connected to eight pairs of valves. For each pair, the
volumes below the hydraulic pistons are connected to the
high pressure reservoir via a device called the lift modifier. In a neutral
position the modifier does not affect the motion of the valves, and activation
of the solenoid valves moves both engine valves in unison
. 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 modifier is shown schematically in Figure 11 as a Rotating
rod with its axis of rotation perpendicular to the plane of the drawing. The
rod is installed in the cylinder head between the two intake valves. A cutout
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. In the case A the modifier is in the
neutral position, and both valves operate in unison. In the case B the modifier
rod is shown turned 90 degrees clockwise. The exit of oil from the volume below
the hydraulic piston in the valve No. 1 is blocked and the valve cannot move in
the direction of opening. However, the entry of oil into the volume below the
hydraulic piston is permitted by a one-way valve installed in the modifier rod.
This guarantees that, whenever deactivation takes place, the valve No. 1 will
close and remain closed, while the valve No.2 continues its normal operation.
If the modifier rod is turned 90 degrees counter-clockwise (from the position
shown in the case A), the valve No.2 is deactivated, while the valve No. 1
would continue normal operation. In the case C the lift of one of the valves is
reduced relative to the second one. The rod is turned a smaller angle so that
the exit of oil from the valve No. 1 into the communication chamber is not
completely blocked, but the flow is significantly throttled. As a result, the
motion of the valve No. 1 is slowed down and its lift is less than that of the
valve No.2. Varying the angular position of the modifier rod 26 varies the degree of oil throttling, thus varying the lift
of the valve No. 1.
ADVANTAGES OF
CAMLESS ENGINE: `
Electro
hydraulic camless valve train offers a continuously variable and independent
control of all aspects of valve motion. This is a significant advancement over
the conventional mechanical valve train. It brings about a system that allows
independent scheduling of valve lift, valve open duration, and placement of the
event in the engine cycle, thus creating an engine with a totally uncompromised
operation. Additionally, the ECV system is capable of controlling the valve
velocity, perform selective valve deactivation, and vary the activation
frequency. It also offers advantages in packaging. Freedom to optimize all
parameters of valve motion for each engine operating condition without
compromise is expected to result in better fuel economy, higher torque and
power, improved idle stability, lower exhaust emissions and a number of other
benefits and possibilities. Camless engines have a number of advantages over
conventional engines. In a conventional engine, the camshaft controls intake
and exhaust valves. Valve timing, valve lift, and event duration are all fixed
values specific to the camshaft design. The cams always open and close the
valves at the same precise moment in each cylinder’s constantly repeated cycle
of fuel-air intake, compression, combustion, and exhaust. They do so regardless
of whether the engine is idling or spinning at maximum rpm. As a result, engine
designers can achieve optimum performance at only one speed. Thus, the camshaft
limits engine performance in that timing, lift, and duration cannot be varied.
But in a cam less engine, any engine valve can be opened at any
time to any lift position and held for any duration, optimizing engine
performance. The valve timing and
lift is controlled 100 percent by a microprocessor, which means lift and
duration can be changed almost infinitely to suit changing loads and driving
0conditions. The promise is less pollution, better fuel economy and
performance. Another potential benefit is the cam less engine’s fuel savings.
Compared to conventional ones, the cam less design can provide a fuel economy
of almost 7- 10% by proper and efficient controlling of the valve lifting and
valve timing. The implementation of camless design will result in considerable
reduction in the engine size and weight. This is achieved by the elimination of
conventional camshafts, cams and other mechanical linkages. The elimination of
the conventional camshafts, cams and other mechanical linkages in the camless
design will result in increased power output. The better breathing that a
camless valve train promotes at low engine speeds can yield 10% to 15% more
torque. Camless engines can slash nitrogen oxide, , pollution by about 30% by
trapping some of the exhaust gases in the cylinders before they can escape.
Substantially reduced exhaust gas HC emissions during cold start and warm-up
operation.
CONCLUSIONS
1. An electro hydraulic camless valve
train was developed for a camless engine. Initial development confirmed its
functional ability to control the valve timing, lift, velocity, and event
duration, as well as to perform selectively variable deactivation in a
four-valve multicylinder engine.
2. The system employs the hydraulic
pendulum principle, which contributes to low hydraulic energy consumption.
3. The electro hydraulic valve train is
integral with the cylinder head, which lowers the head height and improves the
engine packaging.
4. Review of the benefits expected from a
camless engine points to substantial improvements in performance, fuel economy,
and emissions over and above what is achievable in engines with camshaft- based
valve trains
.
5. The development of a camless engine with an
electro hydraulic valve train described in this report is only a first step
towards a complete engine optimization. Further research and development are
needed to take full advantage of this system exceptional flexibility.
