With Alternatives to the Petrol Engine Being Announced Ever So Often You Could Be Forgiven
CYLINDER DEACTIVATION
1. INTRODUCTION
With alternatives to the petrol engine being announced ever so often you could be forgiven for thinking that the old favorite the petrol engine is on its last legs but nothing could be further from the truth and possibilities for developing the petrol engines are endless. One of the most crucial jobs on the agenda is to find ways of reducing fuel consumption, cutting emissions of the green house gas CO2 and also the toxic emissions which threaten air quality. One such fast emerging technology is cylinder deactivation where a number of cylinders are shut down when less is needed to save fuel.
The simple fact is that when you only need small amounts of power such as crawling around town what you really need is a smaller engine. To put it another way an engine performs most efficiently when its working harder so ask it to do the work of an engine half its size and efficiency suffers. Pumping or throttling losses are mostly to blame. Cylinder deactivation is one of the technologies that improve fuel economy, the objective of which is to reduce engine pumping losses under certain vehicle operating conditions.
When a petrol engine is working with the throttle wide open pumping losses are minimal. But at part throttle the engine wastes energy trying to breathe through a restricted airway and the bigger engine, the bigger the problem. Deactivating half the cylinders at part load is much like temporarily fitting a smaller engine.
During World War II, enterprising car owners disconnected a spark plug wire or two in hopes of stretching their precious gasoline ration. Unfortunately, it didn’t improve gas mileage. Nevertheless, Cadillac resurrected the concept out of desperation during the second energy crisis. The “modulated displacement 6.0L V-8- 6-4” introduced in 1981 disabled two, then four cylinders during part-throttle operation to improve the gas mileage of every model in Cadillac’s lineup. A digital dash display reported not only range, average mpg, and instantaneous mpg, but also how many cylinders were operating. Customers enjoyed the mileage boost but not the
side effects. Many of them ordered dealers to cure their Cadillacs of the shakes and stumbles even if that meant disconnecting the modulated-displacement system
Like wide ties, short skirts and $2-per-gallon gas, snoozing cylinders are back. General Motors, the first to show renewed interest in the idea, calls it Displacement on Demand (DoD). DaimlerChrysler, the first manufacturer to hit the U.S. market with a modern cylinder shut-down system calls its approach Multi- Displacement System (MDS). And Honda, who beat everyone to the punch by equipping Japanese-market Inspire models with cylinder deactivation last year, calls the approach Variable Cylinder Management (VCM)
The motivation is the same as before — improved gas mileage. Disabling cylinders finally makes sense because of the strides achieved in electronic power train controls. According to GM, computing power has been increased 50-fold in the past two decades and the memory available for control algorithms is 100 times greater. This time around, manufacturers expect to disable unnecessary cylinders so seamlessly that the driver never knows what’s happening under the hood.
2. CYLINDER DEACTIVATION-MECHANISM
Cylinder Deactivation is the method of deactivating Cylinders as per the Power requirement of the Engine in order to achieve better Fuel efficiency as well as Emission Control. Cylinder deactivation works because only a small fraction of an engine’s peak horsepower is needed to maintain cruising speed. Passenger cars require 25 or so horsepower to cruise at 60 mph while 35 horsepower is enough to drive a large SUV at that speed. Using a reduced number of hard working cylinders to produce the necessary power is more efficient than employing the full complement of lightly loaded cylinders. The energy savings come from reduced pumping losses. In all gasoline engines, energy is consumed drawing the air needed for combustion past a partially closed throttle plate.
Shutting down half the cylinders demands a wider throttle setting to ingest the same amount of air drawn in when all the cylinders are working. The wider throttle position yields lower pumping losses and better mileage. Operating half the valves also diminishes the energy spent turning the camshaft. Minimizing the load carried by half the pistons and connecting rods trims friction and reduces energy losses to the cooling system. The mechanical components needed to disable cylinders are surprisingly simple. All three systems scheduled for 2004 implementation use a computer-controlled squirt of oil pressure to slide a pin inside either selected valve lifters (DaimlerChrysler and GM systems) or half of the rocker arms (Honda). The normal transfer of motion from a cam lobe to a valve stem is thereby interrupted. In DCX and GM pushrod applications, the outer portion of each disabled lifter telescopes over the inner portion to maintain contact with the cam lobe without opening the valve. The Honda approach is a variation of the VTEC (variable valve timing electronic control) cam-lobe-switching scheme used for more than a decade. Instead of skipping between high- and low-lift cam lobes, VCM selects a rocker-arm alignment that delivers no valve lift.
In every case, the engine control computer triggers cylinder deactivation after studying coolant temperature, vehicle speed and engine load parameters. Each cylinder is disabled by interrupting not only the operation of the intake and exhausts valves but also spark and fuel delivery. Deactivation requires only 40 or so milliseconds and is timed to occur immediately after the power stroke so the disabled cylinder remains filled with exhaust gas. That creates what amounts to a gas spring.
As the piston rises and falls, nearly all of the energy required to compress the gas spring on the up stroke is returned to the crankshaft on the down stroke. From this common ground, the three makers implementing cylinder deactivation go their separate ways. In the valley between cylinder banks, a Lifter Oil Manifold Assembly (LOMA) routes oil pressure to four solenoid control valves. When commanded by the power train control computer, the solenoids direct oil to valve lifters equipped with the switching mechanism. (One solenoid operates both of the lifters for each disabled cylinder.) To maintain even firing intervals, the end cylinders on the left bank and the center cylinders on the right bank are disabled.
3. METHODS USED FOR CYLINDER DEACTIVATION
One of the most employed techniques for cylinder deactivation process now is the Displacement on demand method using Lifter Pin Control Mechanism developed by Delphi Automotive Systems which is an Italy based automobile component manufacturer. In this method we use a system of four components which sense the power requirement and the displaces or de activate the cylinder. This method is mainly used by General Motors in its automobiles. This method will be explained in the following pages
Another method used mainly for cylinder deactivation is the Deactivation using Variable Profile Cam shaft developed by Lotus automotive systems. This method is similar to the V tech engines used by the famous automobile manufacturer Honda.
Another method which can be employed is Active Valve Train technology (AVT) which does away with camshafts. But this method’s feasibility and technology is still in experimental stage.
4. DISPLACEMENT ON DEMAND USING LIFTER PIN CONTROL MECHANISM.
Principle of Operation.
There are four subsystems in this deactivation hardware system
• Electronic control module (ECM)
• solenoid valves
• hydraulic subsystem
• lifter locking pin mechanism
When the solenoid valve in the system is energized, the engine oil pressure increases in the control port. At the same time, the lifters are in continuous contact with the camshaft. When a lifter is on the cam base circle, the locking pins inside that lifter are free to move. If the control port is pressurized while in this state, then the pressure force acts on the pins, de-coupling the camshaft from the valves. The cylinder deactivation hardware is based on control algorithm which has been developed to characterize the dynamic response of the deactivation system
5. DISPLACEMENT ON DEMAND USING LIFTER PIN CONTROL MECHANISM.
BLOCK DIAGRAM
5.1 Electronic Control Module.
The Electronic Control Module optimizes engine performance by measuring multiple instantaneous events to enable real-time control of the Solenoid valve of Lifter Pin Mechanism. For ECM control purposes, it is crucial to know the dynamic responses of the deactivation hardware system in order to coordinate the deactivation hardware control with other engine control functions. Specifically, the total actuation time is critical for proper design and control of the ECM deactivation system. The total actuation time consists of three elements: the solenoid plunger response time, the hydraulic subsystem response time and the lifter locking pin mechanical response time. Characterization of the total actuation time is not easily achieved given the fact that laboratory testing cannot encompass every possible engine operating condition.
Therefore, one objective of this study is to develop a high fidelity, yet simple enough analytical model in order to characterize the dynamic behavior of the deactivation
hardware system. This physical model can provide guidance in calibrating the deactivation timing and improving robustness of the system.
5.2 Solenoid Valve Operation.
A 3-way normally closed direct current ON/OFF solenoid valve controls cylinder deactivation. The common port of the control valve is connected via an oil gallery to a pair of spring-biased locking pins inside the valve lifter (a simplified sketch is shown). The common port is then switched to engine oil pressure for deactivation (valve energized) or to engine sump for activation (valve de-energized). Lost motion between the camshaft and the engine valve occurs when engine oil pressure is applied to the spring-biased locking pins inside the lifter, de-coupling the camshaft from the
engine valve. A typical solenoid consists of a plunger, a spring, a coil, and a pole piece. The plunger dynamics can be represented by Equation (1).
5.3 Locking Pin Mechanism.
Lost motion between the camshaft and the engine valve occurs when engine oil pressure is applied to the spring-biased locking pins inside the lifter, de-coupling the camshaft from the engine valve. The locking pins inside the deactivation lifter are designed to change states only on the base circle of the camshaft upon pressurization.
The common port of the control valve is connected via an oil gallery to a pair of
spring-biased locking pins inside the valve lifter (a simplified sketch is shown in Figure 4 later). The common port is then switched to engine oil pressure for deactivation (valve energized) or to engine sump for activation (valve de-energized).
inside the lifter, de-coupling the camshaft from the engine valve. It is desirable to control the switching sequence of the valves cylinder-by-cylinder and to complete the transition between V8 and V4 modes within one engine cycle. These requirements define the switching window of the base circle, the size of which is dependent on the number of cylinders switched at the same time, the cam profile and the firing order of the engine. As the exhaust valve must deactivate/reactivate first, the switching must occur after the intake event begins but before the exhaust event commences. The time it takes for the pins to move to the full travel point (determined by the design criteria) from the time when the control pressure rises to the critical pressure level, is called the locking pin response time p Dt , which can be characterized by:
Locking Pin Mechanism
Locking Pin Model Diagram
6. METHODS USED IN DISPLACEMENT ON DEMAND USING LIFTER PIN CONTROL MECHANISM.
Currently two methods are used for this type of Control.
1. Cylinder Bank Control.
2. Individual Cylinder Control.
The Control type is decided by the Control algorithm used in the Electronic control hardware. Hardware for the cylinder deactivation system exists for both individual cylinder control mode and complete bank control mode. An OHV (Over Head Valve) V8 cam-in-block (pushrod), 2-valve-per-cylinder valve train engine is used as a test engine for the deactivation mechanism. Cylinder selection for deactivation is determined by engine firing order and the desire to keep an even firing order in the deactivated mode. This results in two cylinders from each bank being selected for activation/deactivation .In bank control mode both cylinders of each bank are switched at the same time. In cylinder control mode, each cylinder is switched independently. Cylinder control mode requires four control valves compared to two control valves for bank control mode. The operating window for cylinder control
mode (intake valve opening to exhaust valve opening of a cylinder) is 180 crank degrees greater than the operating window for bank control mode (intake valve
opening of one cylinder to exhaust valve opening of the second cylinder). This virtual doubling of the switching window for individual cylinder control permits mode switching at higher RPM and increases robustness to variation.
Cylinder Bank Control is commonly used in Engines which is having Cylinders in more than one bank. e.g. V engines, Boxer engines.
Individual Cylinder Control is mainly used in inline cylinder engines.
7. DEACTIVATION USING VARIABLE PROFILE CAM SHAFT
This technique was developed by lotus automotive systems that allow the engine to effectively have multiple camshafts. As the engine moves into different rpm ranges, the engine's computer can activate alternate lobes on the camshaft and change the
Cams timing. This technique uses a Cam Profile Switching tappet (CPS) to switch between two different Cam profiles. Cylinder deactivation can be attained by switching between a normal Cam lobe and a plane circular lobe, which does not produce any lift at all. In this way, the engine gets the best features of low-speed and high-speed camshafts in the same engine.
This is similar to the patented V-Tech technology used in Honda engines in which whenever the engine speed increases the combustion time reduces in order to keep the combustion time nearly constant. Here the valve should be opened a little earlier than normal condition. This is achieved by the varying the position of the cam shaft which is placed in pressure actuators which is controlled by a micro processor and electronic circuits. Whenever the engine speed increases these pressure actuators presses or pushes the cam shaft towards the valve lifter. By doing this the valve can be kept opened for a longer time and the valve will be opened more than the usual or normal condition.