Development of Variable Valve Actuation Mechanism for Multi-cylinder SI Engine
1Department of Mechanical Engineering, Sinhgad College of Engineering, Pune, India
Engine designers are prompted to consider Variable Valve Actuation system because of the inherent compromises with fixed valve events. The major goal of a VVA mechanism is to control the amount of air inducted into the engine. In this paper, a novel two-step VVA mechanism is presented to facilitate the variation in valve timing and lift of base engine. This mechanism helps the engine breathe effectively by changing the valve timing and lift after the set switch-over engine speed by dividing the operating range of engine in two zones viz, low speed and high speed zone. A simple, cost effective and feasible option to incorporate both low lift cam and high lift cam is presented in order to vary intake valve timing for improving engine breathing during low-speed low-load operation, and high-speed full-load operation. The engine is simulated in 1D simulation software GT suite to identify the potentials of variable valve actuation. The software is used to find suitable valve timings and lifts for low and high speed zone and the switch-over point engine speed. The simulation model has shown an average improvement in volumetric efficiency by 2.8%.
At a glance: Figures
Keywords: variable valve actuation, two step, volumetric efficiency, valve event, 1D simulation
American Journal of Mechanical Engineering, 2013 1 (5),
Received September 20, 2013; Revised October 10, 2013; Accepted October 20, 2013Copyright: © 2013 Science and Education Publishing. All Rights Reserved.
Cite this article:
- Mane, Prashant R., and Y.P. Reddy. "Development of Variable Valve Actuation Mechanism for Multi-cylinder SI Engine." American Journal of Mechanical Engineering 1.5 (2013): 131-137.
- Mane, P. R. , & Reddy, Y. (2013). Development of Variable Valve Actuation Mechanism for Multi-cylinder SI Engine. American Journal of Mechanical Engineering, 1(5), 131-137.
- Mane, Prashant R., and Y.P. Reddy. "Development of Variable Valve Actuation Mechanism for Multi-cylinder SI Engine." American Journal of Mechanical Engineering 1, no. 5 (2013): 131-137.
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The desire for higher fuel economy, improved performance and driveability expectations of customers from engines are gradually increasing along with stringent emission regulations set by the government. Many original engine manufacturing companies are prompted to consider the application of higher function variable valve actuation mechanisms in their vehicles as a solution.
The VVA is a generalized term used to describe any mechanism or method that can alter the shape or timing of a valve lift event within an internal combustion engine. It allows the lift, timing in various combinations of the intake and/or exhaust valves to be changed while the engine is in operation.
The variable valve timing and lift is designed to overcome the limitation of having a fixed opening period and valve lift throughout the engine’s speed range. The optimization of cylinder breathing is difficult in modern car engines since they operate over a wide speed range of 1000 to 6000 RPM. In the low-to-mid speed range, the engine performs better and the emission levels are lower if the intake valves opens late and closes relatively early. The low lift assists in better atomization of charge at lower velocities of flow. Conversely, at high engine speed, the cylinder breathing is improved by opening intake valve slightly earlier and closing late to make use of the ram momentum effect. The high lift helps engine to induct more air during suction. However, in fixed valve timing and lift, the engine performance is optimized in the mid-speed range but at the expense of below optimum performance at the two extremes, i.e. at low and high engine speeds .
Continuous [2, 3, 4, 5, 6] VVA systems vary the valve lift and timing continuously over the operating range. These systems can be electro-hydraulically actuated, electromagnetically actuated, or electro-mechanically actuated. However, these systems are relatively complex, costly and difficult to package for high-volume production applications. The control of valve seating velocity, high power consumption and speed limitations continue to present significant challenges for many continuous VVA systems. The variable valve lift mechanisms provide two lift profiles in its simplest form. These mechanisms have a set of cam lobe profiles for low-to medium and medium-to high speed range and arrangement for switching between the two cam lobe profiles. The one cam lobe profile is designed for low speed operation. The other cam lobe profile is independently designed for high speed operation. The discrete two-step VVA systems represent a practical alternative to continuously variable systems due to relative ease of application for a variety of valvetrain types. Overall, the optimization process yields a balanced system that satisfies powertrain requirements for fuel economy, emissions and performance [7, 8].
2. Two-step VVA System
In this section the development, principle, strategy, construction, working and design considerations of proposed two-step VVA system are explained.2.1. Development
The proposed two-step VVA system is designed for multi-cylinder Spark Ignition (SI) engine by considering packaging and geometrical constraints. The development process of two-step VVA system for inlet valves begin with understanding of the existing base engine valvetrain. The base engine consists of 4 in-line cylinder, 2 valves per cylinder, Type III (centre pivot rocker arm) valvetrain. The Figure 1 shows the base engine valvetrain. The packaging constraints are bound by obstacles determined from the cylinder head. The geometry layout designs are evaluated to meet kinematic requirements, for packaging, compactness and adequate clearance between the components. The engine is simulated in 1D simulation software Gamma Technologies, Inc (GT) suite V7.2 for defining the target valve events for both low speed zone and high speed zone. Several iterations are carried out to achieve the best suitable valve events.
The two-step VVA system consists of intake and exhaust valves operated by single camshaft. The intake valve is operated by two cam-lobes while the exhaust valve is operated by single cam lobe. The first cam lobe provides reduced opening period and lower valve lift during lower speed range in operation because vigorous mixing of air and fuel charge is required. The second cam lobe provides an extended opening period and higher valve lift during upper speed range in operation because of more quantity of charge is required in the cylinder. The actual switch-over point speed where the cam-lobes change from low to high lift profile is derived from engine’s RPM. This variable valve lift and timing mechanism is applicable on both the intake and exhaust valves.2.3. Strategy
The low valve lift increases the velocity of charge flow which helps to promote better fuel atomization, reduced HC emissions and better cold start properties at low speeds. During the low speed of engine, it is desired to delay the opening of intake valve to reduce the overlap with exhaust valve for stable combustion and avoid mixing residual gases with fresh gases. Also, at low speeds, the intake valve closing is earlier to retain the maximum compression ratio and reduce back flow. Thus, Late Inlet Valve Opening (LIVO) and Early Inlet Valve Closing (EIVC) with low lift are desirable.
The higher lift helps the engine to breathe properly at higher speeds and take high amount of air in shorter time. During high speed, engine desires to have early opening of intake valve, thereby increasing the valve overlap with exhaust valve to reduce pumping losses. Also, at high speeds, the intake valve closing is delayed to take full advantage of ram charging. Thus, Early Inlet Valve Opening (EIVO) and Late Inlet Valve Closing (LIVC) with high lift are desirable.
The Figure 2 shows the valve lift (mm) vs. crank angle (degree) curve for low speed and high speed zones.
The two-step VVA mechanism consists of 3 rocker arms per cylinder, two for intake valve and one for exhaust valve. The two intake rocker arms are low speed intake rocker arm (Intake rocker arm_Low) and high speed intake rocker arm (Intake rocker arm_High). A single camshaft centrally located operates both the intake and exhaust valve opening periods. There are two intake and one exhaust cam-lobes on the camshaft per cylinder. The rocker arms are constantly in contact with the respective camshaft lobes via rocker arm rollers. An arrangement is made on the intake rocker arms to accommodate a lock pin. The lock pin is provided to engage and disengage the Intake rocker arm_Low with Intake rocker arm_High. The position of lock pin is such that it is always inside the Intake rocker arm_Low and disengaged from the Intake rocker arm_High. The lock pin is having flanges that are free to slide around the fork when rocker arms are in rocking motion.
The motion to lock pin is provided by fork connected to plunger A. It is free to slide inside the intake rocker shaft. The fork when sliding inside the rocker shaft moves the lock pin inside the Intake rocker arm_High engaging the two rocker arms and disengages the two when sliding in reverse direction. A linear solenoid connected to plunger A is used to actuate the switch-over from low speed cam-lobe to high speed cam-lobe and vice versa. A speed sensor monitors the engine RPM and signals linear solenoid when the cam-lobe switch-over should take place. The Figure 3 shows the cut section of two-step VVA system.
The two-step VVA system operates in two speed ranges viz, low-to medium speed range and medium-to high speed range. The switch over point separates these two speed ranges.
2.5.1. Low to Medium Engine Speed Range
The linear solenoid coil is de-energized at below set switch over point RPM of the crankshaft. The lock pin remains disengaged from Intake rocker arm_High and the intake valve follows the timing and lift from low lift cam-lobe. The Intake rocker arm_High keeps rocking but does not transfer motion to intake valve. An instance when the lock pin is disengaged from the Intake rocker arm_High is shown in Figure 3.
2.5.2. Medium to High Engine Speed Conditions
As the speed of engine rises above the set switch over point RPM, the speed sensor sends trigger pulse which energizes linear solenoid coil. The push stroke of solenoid pushes the plunger A and thereby the fork. At a given instant when the two intake rocker arms bores align, the fork pushes the lock pin ahead engaging the two rocker arms, thereby making them to oscillate in unison. When the two rocker arms are engaged, the high lift cam lifts the Intake rocker arm_High early and closes it late with higher maximum lift as compared to Intake rocker arm_Low. As a result the inlet valve follows the timing and lift from high lift cam-lobe. The Intake rocker arm_Low keeps rocking but does not transfer motion to intake valve. The Figure 4 shows an instance when the lock pin is disengaged from the Intake rocker arm_High.
When the engine speed reduces below set switch over point RPM the linear solenoid coil de-energies which pulls the lock pin in reverse operation. This causes the two rocker arms to get disengaged and the intake valve follows the low lift cam lobe timing and lift.2.6. Design
The two-step VVA mechanism consists of structural, functional components and linear solenoid. The structural components are fork, inlet rocker shaft and plunger A. The lock pin is a functional component and the linear solenoid actuates the lock pin.
2.6.1. Lock Pin
The lock pin is a critical functional component designed by considering the possibility of shear failure and probability of engagement in two-step VVA system.
The lock pin is subjected to shearing mainly due to the action of valve spring force. The Figure 5 shows the forces acting on lock pin where Fshear and Fsp are shear and valve spring force respectively.
The total deformation and equivalent Von-Mises stress of lock pin are within allowable limit under the action of forces.
2.6.2. Linear Solenoid
A linear solenoid is an electromagnetic device that converts electrical energy into a mechanical pushing or pulling force or motion. The Figure 8 shows the pin actuation force and stroke.
The linear solenoid is selected for following specifications.
Force, Fpin = 40 N
Stroke, S = 18 mm
Operating Voltage, 12 VDC
3. 1D Engine Simulation Model
The base engine is analyzed with advanced 1D engine and gas dynamics simulation software GT suite V7.2  to predict and improve the performance before a prototype is built. GT-POWER is a module of GT suite which calculates the engine performance based on input data. The Table 1 gives details of engine characteristics required for the 1D simulation in GT-POWER.
The GT engine model is run to evaluate the volumetric efficiency of the engine. The results obtained are compared with the available testing data base and found to be in agreement with 5% deviation. The developed model is then used to study the effect of varying valve timing and lift.
The intake valve timing is varied by keeping exhaust valve timing fixed in the developed GT engine model. Two separate cam-lobe profiles are used for intake valve to achieve the variation in intake valve timing and lift. The cam-lobe profiles switch based on engine speed. The low cam-lobe rocker and high cam-lobe rocker move independently for engine speed below 3000 RPM, with the valves being driven from the low cam-lobe profile. When the engine speed rises above 3200 RPM, the two rockers lock together using a lockpin thus making the intake valve to be driven from the high cam-lobe profile.3.1. Trials to Finalize the Valve Events for Improved Performance
To determine the valve timings and lifts that match with the required volumetric efficiency performance, different valve timings and lifts trials are experimented, and compared with the base volumetric efficiency performance. The Table 2 shows the base engine valve events.
3.1.1. Low Speed Zone Valve Events
The Late Inlet Valve Opening (LIVO) and Early Inlet Valve Closing (EIVC) with low lift are desirable at low speeds. Several combinations can be formed and optimized valve events can be selected. In this case all trials have less duration and lift as compared to base engine valve event. The Table 3 reflects some of the trials carried out for selection of suitable valve events for low speed zone by keeping maximum lift of 6.5 mm.
3.1.2. High Speed Zone Valve Events
The Early Inlet Valve Opening (EIVO) and Late Inlet Valve Closing (LIVC) with high lift are desirable at high speeds. In this case all trials have more duration and valve lift as compared to base engine valve event. The Table 4 reflects some of the trials carried out for selection of suitable valve events for high speed zone by keeping maximum lift of 9.5 mm.
The Figure 9 and Figure 10 shows valve lift (mm) vs. crank angle (degree) curves of trial intake valve events for low speed zone and high speed zone respectively. The trials show different valve overlap between intake and exhaust valve.
The Figure 11 and Figure 12 shows volumetric efficiency (Air) lift vs. engine speed curves of trial intake valve events for low speed zone and high speed zone respectively. The volumetric efficiency is in the range of 70%-88% upto 3000 RPM and then falls progressively as the speed rises. This is due insufficient inlet valve late closing ram effect and valve lift to compensate for the shortening cylinder filling time.
The volumetric efficiency in the high speed zone is upto 70%-88% in the range of 4000-6000 RPM. The momentum of incoming charge is insufficient to oppose the upward moving piston just before the IVC below 4000 RPM. As a result, a portion of the newly arrived charge will be pushed back and returned to the induction manifold. Hence, the shortening of the effective compression stroke by the LIVC reduces the nominal compression ratio and lowers the volumetric efficiency. It can be observed from Figure 11 and Figure 12, the intake valve events are suitable for low speed and high speed zone respectively based on which the best performance curves are selected.
The Figure 13 shows the valve lift (mm) vs. crank angle (degree) curves for intake valve of the base engine and selected valve events for high speed zone and low speed zone along with the base engine valve events.
The Figure 14 shows volumetric efficiency (Air) lift vs. engine speed curves of the base engine and selected valve events for high speed zone and low speed zone. The Table 5 and Table 6 shows low speed and high speed zone valve events respectively for intake and exhaust valves. The improvement in volumetric efficiency is achieved in the speed range of 1000-3500 RPM using low speed zone valve events and in 3500-6000 RPM using high speed zone valve events.
The valve events are selected such that, when considered together their volumetric efficiency performance is above base engine throughout the speed range.
Figure 15 shows the combined effect of two cam lobes to improve volumetric efficiency of engine. It can be observed that the volumetric efficiency using two valve timings is improved over the base engine throughout the speed range.
The Table 7 shows percentage improvement in volumetric efficiency at various engine speeds obtained by the comparing base engine and VVA engine intake valve events. It is found that there is an average improvement of 2.8 % in engine volumetric efficiency with maximum improvement of 7.2 % at 6000 RPM.
A novel two-step VVA system is designed and presented to improve the volumetric performance of 4- cylinder, Single Overhead Cam (SOHC), SI engine. The VVA system is developed to facilitate variation in intake valve timing and lift. The two-step VVA system can be applied to both inlet and exhaust valves. The performance of base engine in terms of volumetric efficiency is analyzed by using GT software. The combination of LIVO and EIVC with low lift in low speed zone and EIVO and LIVC with high lift in high speed zone shows the engine volumetric efficiency improvement of approximately 2.8 %. The necessary information required to initiate a prototyping work of the VVA system is presented. The development of this system continues to explore further options to improve the engine performance and driveability.
The authors would like to thank all the authorities in department of Power Train Engineering (PTE), The Automotive Research Association of India (ARAI), Pune for providing the facilities and expertise during the course of this work.
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