1.1 INTRODUCTION
The vehicle manufacturing industries faces two most important in the modern world because of the increasing competitions, reducing energy sources and increasing pollution levels. These problems are to reduce the fuel consumptions in engine by improving the performance of engine and secondly reduction in the exhaust emissions of CO2, NOX etc in order to meet the standards of environment and to reduce the pollution. (Jankovic, 2002) The control over the emissions of green house gases and reduction in the consumption of fuel are thus primary requirement for meeting the current and the future needs. There have been different solutions found out to solve this problem like lean burn, fast combustion, gasoline direct injection, variable valve events and actuation etc. The recent investigations on variable valve timings have shown that it can greatly affect the engine performance and the emission of exhaust gases from engines. (Nagumo, 1995) The objective of the present research is to carry out further investigations on the same.
Valve timing refers to the opening time or closing time of the intake valve and the exhaust valve respectively with respect to the piston positions of Top dead centre and Bottom Dead Centre. The valve motion is a typical engine is controlled using camshafts rotating at a speed half that of the crankshaft. During the total 4 stroke cycle of the engine the crankshaft rotates twice resulting into two piston cycles and therefore the camshaft rotates once, which causes one cycle of each of intake and exhaust valve. Variability of valve timings is related to both the time of opening of the valve as well as the duration of opening. Usually in customary IC engines we can say that the intake and the exhaust timings of the valve are fixed. Traditionally, the timings of the valve were designed in order to optimize the operations of the engine at high speed and during wide open conditions of throttle operation. But now with the increasing investigations on variable valve timings and the achieved advantages from that the shift is from fixed valve timings towards the variable valve timings.
1.2 PROBLEM STATEMENT
Variable valve timing has been investigated as best method for solving the problems of increasing the engine performance and reducing the exhaust gas emissions, but there issues of reliability, acceptable cost and durability related to implementation of variable valve timing and as a result of it so far only a few engines have been equipped with this mechanism. But in modern days because of the development of new technologies of controlling the valve timings like hydraulic and electromagnetic valve control, there are probable chances of variable valve timing becoming feasible in engines. At this, what is required is to investigate the effects of the variable valve timing phenomena’s on the engine performance. Moreover the recent requirements of gas mileage and the permissible emissions, emphasizing the lower speed of engine and lower pollution emissions, there is need of finding out alternate strategies for valve timings. This would thus require optimization of the valve events for various operating conditions. Thus there is also need of postulating some alternate strategies for optimal valve timing in order to minimize the fuel consumption and maximizing the torque from the engine in terms of duration of the intake valve and the timings of opening and closing of exhaust valve.
1.3 AIM
Investigation into effects of valve events on the engine performance and gas emissions and postulating an optimal valve timing strategy
1.4 OBJECTIVES
Understanding the functioning of the valves, their importance for engines
Understanding the effects of various valve timing events on engine performance
Exploring the industrial technologies used to achieve variable valve timing mechanism
Developing a theoretical approach for predicting the engine performance vs valve timings
Experimental study to analyze the performance of an engine using variable valve events
Proposing a new valve timing and valve lift mechanism for engines
1.5 METHODOLOGY
The approach that would be adopted for achieving the desired objectives is described below through a flow chart
Project Overview
Reviewing Objective
Literature Reviewing on engine performance, gas emissions and valve timing parameters and techniques
Brainstorm Section
• Project Goal/Requirements
• Generating different Ideas
• Reviewing Literature
Theoretical investigation on various valve events in relation to their effects on engine performance and gas emissions
Developing a theoretical approach for predicting the engine performance vs valve timings
Procuring materials for experimentation
Developing an experimental platform
Proposing a new valve timing and valve lift mechanism
Conducting experiments and analysis of results
Drawing out conclusions from the study
CHAPTER 2
LITERATURE REVIEW
2.1 BASICS OF ENGINE AND VALVE EVENTS
Figure 1 shows a conventional four stroke engine with the positions of intake and exhaust valve during the four strokes of the cycle. From the figure we can see that during the ignition and compression stage of the cycle the intake valve and the exhaust valve remain closed. The valve motion takes place only during the induction and the exhaust stages and thus the valve motion is always in the periods either side of piston Top Dead Centre during the non-firing stroke. (Dresner, 1989) Valve timing refers to the opening time or closing time of the intake valve and the exhaust valve respectively with respect to the piston positions of Top dead centre and Bottom Dead Centre. The valve motion is a typical engine is controlled using camshafts rotating at a speed half that of the crankshaft. During the total 4 stroke cycle of the engine the crankshaft rotates twice resulting into two piston cycles and therefore the camshaft rotates once, which causes one cycle of each of intake and exhaust valve. Variability of valve timings is related to both the time of opening of the valve as well as the duration of opening. (Dresner, 1989)
Figure 1: 4-Stroke engine cycle of a engine (Dresner, 1989)
Valve duration refers to the time for which the valve is open, in terms of the engine parameters it is defined as the angle of rotation of the crankshaft between the opening and the closing of the valve.
There different parameters of valve timing involved which are useful in analysis. The parameters of valve timing are always discussed with respect to the position of the piston which in turn is derived by the crankshaft rotation measured between the TDC position and the BDC position of the piston. (Gray, 1998)
The main parameters of valve timing that would form the basis of this study are: (Gray, 1998)
1. IVO: Intake valve opening time
2. IVC: Intake valve closing time
3. Peak Valve Lift
4. EVO: Exhaust valve opening time
5. EVO: Exhaust valve closing time
Figure 2 shows the events of the intake and the exhaust valves as they appear at the end of the exhaust stroke
Figure 2: Intake and exhaust valve events (Dresner, 1989)
It can be easily seen in the figure that the valve events never coincide with the BDC and TDC. The overlap region shown in the figure is the difference between the EVC and IVO.
2.2 PREVIOUS RESEARCHES DONE
In the traditional engines the valve timing and the events are designed in order to optimize the operation conditions at wide open throttle and high speed of engine (Dresden, 1989). Controlling the events and the timings of the valve can lead to improvements in the torque curve, the curve of the indicator power and the brake power curve of a given design of engine. The fuel consumption and the exhaust emissions can also be controlled with the help of variable valve timings (Nagumo, 1995). To achieve the above objectives the maximum temperature of the in cylinder is controlled and the residual amount remaining after the completion of the compression stroke is controlled, also called as EGR. Kohany et al, 1999 presented a varying valve timing strategy for a throttled spark ignition engine which aimed at the maximization of engine torque. It was also found that the benefits of varying valve timing are more significant in case of an un-throttled engine. In case of gasoline engines as found by Stone et al , 1989, with the help of variable intake closing load control can be done without using throttle, thus helping in reducing the throttle loss which normally occur at low loads so as to regulate the quantity of the induced mixture. Many different methods of regulating the load with the help of variable valve timing have been suggested. Lenz, 1988, suggested that by performing the early closure of of intake valve. The charge in the spark ignition engine can be controlled without the use of throttle. During the low speed of the engine, the proper closing of the intake valve at proper time will help in increasing the volumetric efficiency of the process of gas exchange. The early closure of the intake valve before the bottom centre will result in the expansion of the fresh mixture before bottom dead centre and hence resulting into lower temperature at the starting of the compression process. This will reduce the amount of NOx in the exhaust and will increase the amount of hydrocarbons. Ma TH, 1988, suggested a method of the load control along with improvements in the fuel economy. He concluded in his research that closing the intake valve late, for engines having 2 intake valves, is practically applicable concept for the using which the the two intake valves can be made in phase relative to one another in order to extend the total opening period of the intake. For the case of diesel engines the most promising and effective application is the valve overlap control in a turbocharged engines (Stone, 1989) It has also been stated and argued that the starting performance is much better with diesel engines having controlled valve overlap mechanism. During initial years there were problems in providing a mechanism for variable valve timing with acceptable durability, cost and reliability. As a result not many of the automotive engines were equipped with such kind of mechanisms (Dresden et al, 1989) (Gray, 1988). But with the recent developments in the hydraulic and electromagnetic valve control mechanisms and the recent progress in the field of microprocessor utilization for variable valve timings it has become feasible to go for such automotive engines (Schechter, 1996) (Moriya, 1996). But with the time the federal demands for emission requirements and the gas mileage has also increased at it is demanding to look for some other mechanisms of valve timing which can result in to low pollution emissions and lower engine speeds. Some of the researches done, like of Duckworth et al , 1996, have tried to find out the effect of the valve overlap on the performance of engine with giving an emphasize on internal exhaust gas regulation techniques.
Leone et al, 1996, studied the effect of the different strategies of the variable camshaft timing with respect to reduction in NOx, HC and CO emissions from an engine which is part loaded. Shiga et al , 1996, tried to study the effect of closing the intake valve earlier on the performance of the engine and the cylinder pressure. At the same time Ueda et al, 1996 and Badami et al, 1996 tried to study the effect of the late closing of intake valve on the performance of engine both at the idle and at the part load. Moro et al, 2001 presented a theoretical strategy of controlling the engine load using variable valve timings for both intake and exhaust valves. They found that when the Exhaust Valve Closing and the Intake Valve Closing are in linear correlation with the engine load, like control of the EGR amount and the intake pressure, then the efficiency of the cycle at partial loads improved from 31% to 37%. Bozza et al, 2001 developed a 1-Dimensional mathematical model for the simulation of the Spark Ignition Engine operation equipped with variable valve timing mechanism. In their study they tried to examine the effect of intake and exhaust valve timings on the performance of engine and tried to demonstrate that there is potential of employing variable exhaust and inlet valve timings. However they did not presented any method of the valve timing optimization using which the engine performance can be maximized. Nakayasu et al, 2001, developed a variable exhaust and intake valve control system for a 4 cylinder in-line motorcycle engine. They provided two modes for the intake control and 3 modes for the exhaust control. They concluded that the engine performance was increased by around 10% with new design with respect to earlier design. But they also did not perform any further studies for optimizing the valve timings. The present research is thus aimed at that. In this first the performance of an un-throttled spark ignition engine installed with variable valve timing mechanism would be analyzed and then an alternate strategy for optimal valve timing in order to minimize the fuel consumption and maximizing the torque from the engine in terms of duration of the intake valve and the timings of opening and closing of exhaust valve would be postulated.
CHAPTER 3
VALVE EVENTS AND EFFECTS ON ENGINE PERFORMANCE
3.1 Changes IN Exhaust Valve Opening Timing and its effects
The pressure built up inside the cylinder because of the combustion is made to escape to the exhaust by opening the exhaust valve. In order to make the process more efficient i.e in order to get more amount of work during the expansion process the timing of valve opening should be at least after the bottom dead centre and not before that. At the same time it also desired that the cylinder pressure drops to the minimum possible value before the piston rises again. This results in minimizing the total work done by the piston during the expelling of the combustion and products and before the intake of new and fresh charge. Thus this are two contradicting desirables pone requires Exhaust valve opening to be after bottom dead centre while the other requires it to be before bottom dead centre. Thus the proper choice of the exhaust valve opening timing is a trade-off between lost works due to escape of combustion products before the full expansion and the work done in raising the piston against the exhaust back pressure. In Conventional mechanisms the valve lift slowly starts providing flow restrictions for quite a time after it starts lifting and as result the valve lift tends to begin before the bottom dead centre. Usually it is 50-60 degrees prior to the bottom dead centre. The ideal timing of EVO to optimize these effects changes with engine speed and load as does the pressure of the gasses inside the cylinder. During the par load operation conditions, it is desirable for the exhaust valve opening to be as close to bottom dead centre since the pressure of the cylinder is much nearer to exhaust back pressure and as result will take lesser time in escaping from the valve. While the full load operation conditions require the exhaust valve opening to be as early as possible because of the time required by the pressure in the cylinder to drop to exhaust back-pressure. Figure 1 shows the effects of Exhaust Valve Opening and the position of opening.
Figure 1: Effects of Exhaust Valve Opening Time
3.2 Changes IN Exhaust Valve closing Timing and its effects
The amount of the exhaust gas present in the cylinder before the start of the intake stroke is very important in governing the performance of engine and this is governed by the timing of the exhaust valve closing. Exhaust valve closing also a parameter which governs the valve overlap which also has significant affects on cylinder contents during the start of the intake stroke. The requirement of the exhaust valve closing is discussed with respect different load conditions:
For the case of full load operation conditions, it is required that the amount of the residual gas in the cylinder is at its minimum level because it will result in allowing maximum amount of fresh charge to enter the cylinder before the combustion stroke and hence it requires the Exhaust valve closing to be as nearer to the top dead centre in order to allow maximum amount of residual gases to go out. In case of engines in which the exhaust systems are fairly active i.e. there is pressure wave generation by the flow of exhaust gases from different cylinders, the timing of the exhaust valve closing greatly affects the on whether these pressure waves will result into pushing of the gas back in the cylinder or drawing of gas out of the cylinder. The pressure wave timings depend on the speed of the engine thus the exhaust valve timings optimized for one speed can be not effective at other speeds of engine operation.
For the case of part load operation conditions it is desired that some of the residual gas is left inside the cylinder, as this will help in taking less amount of fresh charge. With the retaining of the exhaust gases the restriction for the intake by the throttle plate gets reduced and hence the pumping losses get lowered. Thus for this case, closing of the exhaust before the Top dead centre helps in retaining the exhaust gases.
But there is limit to which the amount of the exhaust gases can be allowed in the cylinder because it reduces the combustion process and the combustion becomes unstable. This limit becomes lower with the reduction in engine load and the density of the charge. The combustion rate becomes increasingly slow with the increase in the EGR level till a point after which the combustion becomes unstable. The ratio of oxygen to fuel may remain constant but exhaust gas recirculation leads to reduction in the total cylinder contents of the two. It is because of the reduction in this ratio which results in instability of combustion.
Usually in most of the systems, Exhaust Valve Closing Timings are 5 to 15 degrees before the top dead centre.
Figure 2: Effects of Exhaust Valve Closing Time
CONCLUSION
Till now the various researches done in the field of valve events and the mechanisms for improving the engine performance and reducing the exhaust gases have been reviewed and presented. The effects of the valve events like exhaust valve opening time and exhaust valve closing time have also been presented. In the further part of the research, the effects of some other valve events like intake valve closing timing, intake valve opening timing, valve overlap etc would be found and presented. The current industrial technologies used to achieve variable valve timing mechanism would also be explored and discussed. Then a theoretical approach for predicting the engine performance vs valve timings would be presented. Experimental study would then be carried out to analyze the performance of an engine using variable valve events. And then finally a new valve timing and valve lift mechanism for engines would be proposed
