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kap all phase & 6/11/03 11:35 am Page Student Workbook LV34 Engines (3) LV34/SWB Student Workbook for Technical Certificates in Light Vehicle Maintenance and Repair MODULE LV34 ENGINES (3) Contents Page Page Advanced Valve Systems: Effects of fuel on valves and valve seats Requirements of a valve Valve guides Valve springs Valve rotators Operation of valve rotators Camshafts Twin camshaft four valve per cylinder engine Camshaft layout of a V6 engine Fully assembled V6 engine Operation of the scissor gear 10 11 12 13 Hydraulic Valve Lifters: Operation valve closed Operation valve open Valve lash adjuster Progress check 16 17 17 18 19 Variable Valve Timing Systems: Conventional valve timing Variable valve timing During idling During normal driving During full acceleration During full power Varying the valve timing Large valve overlap Intake valve closes quickly 22 23 24 24 25 25 26 26 27 27 5 8 9 Operation of VVT - i Management of the VVT - i system Reason for the lock pin Retard Hold Advance Progress check 28 28 29 29 30 30 31 Timing Oil Control Valve: Valve timing varied Valve arrangement Operation Low and medium speed High speed Oil control valve Progress check 33 34 36 36 37 38 38 41 VTEC System: VTEC operation Basic principles of VTEC operation Main components of VTEC Effect of change-over on torque Hydraulically operated pin 43 44 44 46 47 47 Three Stages of VTEC Operation: Stage Stage Stage 49 49 50 51 (Cont.) -1Copyright © Automotive Skills Limited 2003 All Rights Reserved LV34: Engines (3) Issue Page Comparing the VTEC and VTEC–E Main advantages The VTEC-E Progress check 52 52 53 54 Continuous Variable Valve Timing Control: VANOS variable cam timing (BMW) Valve timing gear operation Operation of the VANOS Operation of the double VANOS Function 55 55 56 58 60 62 Page Components and Operation: Comparing VTEC with Valvetronic Variable valve timing (Ferrari) The future Progress check -2Copyright © Automotive Skills Limited 2003 All Rights Reserved LV34: Engines (3) Issue 64 66 66 67 69 Advanced Valve Systems Thomas Midgley, Jr To help to understand the need for improvement in materials that are used in valves and valve seats, it is helpful to investigate briefly the history of the development of fuel The story of leaded petrol goes back many years and involves Thomas Midgley, Jr (not usually ranked with the likes of Isaac Newton) who was born in 1889 and held a PhD in engineering In 1916 he joined forces with Charles Kettering the inventor of coil ignition Kettering was having trouble with a farm engine that ran on kerosene and knocked very badly Midgley added iodine to the fuel and knocking was reduced After six years he found that tetra-ethyl lead worked beautifully In the 1920s tetra-ethyl lead was a wonderful invention When the spark plug ignites the fuel mixture a flame front travels through the combustion chamber Tetra-ethyl lead made the flame front travel more slowly and less turbulently Lead virtually eliminated knock, and overnight compression ratios jumped from 4:1 to 7:1, therefore the modern high output engine was born In today’s modern engines, cylinder pressures reach forty times atmospheric pressure -3Copyright © Automotive Skills Limited 2003 All Rights Reserved LV34: Engines (3) Issue Originally, his ideas were thought to be brilliant, only later did they turn out to be disasters Lead emitted from vehicle exhaust was affecting children all over the world Today, of course, we use unleaded petrol Thomas Midgley was the man who put lead in our petrol; at the time it was hailed as a great advance but eighty years on we are still trying to deal with the consequences of it Midgley also invented chlorofluorocarbons and we know the damage that has occurred by using these wonderful chemicals They are responsible for punching a very large hole in the ozone layer which in 1992 was the widest and deepest ever recorded CFCs and the reduction of the ozone layer have increased ultra-violet radiation levels, which are responsible for the increase in skin melanomas Paradoxically, in 1939 Thomas Midgley predicted the control of the ozone layer in order to control the Earth’s climate In 1940, Thomas Midgley, Jr was paralysed by polio He built an assembly of pulleys and ropes so he could move himself between his bed and his wheel chair In 1944, he became entangled in the ropes and strangled to death in his own invention Effects of fuel on valves and valve seats As leaded petrol burns, the tetra-ethyl lead turns into a tan-coloured layer of lead oxide, which covers valves and the combustion chamber The valves hit their seats hard several thousand times a minute, the lead oxide acts as a cushion therefore protecting the valves and seats Lead oxide has lubrication properties, which reduces wear on the valve guides If an old leaded engine is run on unleaded fuel, damage occurs, but only if the engine is fitted with ‘soft’ metallurgy, and only in high temperature areas such as the exhaust valves, guides and seats On a ‘soft’ cast iron valve seat at high temperatures, iron oxides form and these oxides flake off and actually embed themselves in the soft face of the exhaust valve These tiny particles work like very small grinding wheels as the valve operates, grinding away the seat taking up the valve clearance preventing the valve from closing properly They cause edges of the valve to burn since heat cannot be conducted away through the seat (burnt valve) -4Copyright © Automotive Skills Limited 2003 All Rights Reserved LV34: Engines (3) Issue Requirements of a valve Valves • must ensure a gas tight seal when closed Valves Valve operating mechanism • offer as little opposition to the flow of gas possible • have a reliable operating mechanism • operate with minimum wear under extreme conditions, long life • be accurately timed to open and close to provide maximum engine efficiency (power) The use of unleaded fuel demands more robust valve components as follows: Valve guides Shoulder limits depth into cylinder head This distance is important Why? The valve guide in this case is a machined hole in the cylinder head Common plain type Integral type Shoulder type Silicon-aluminium bronze valve guides are used (good heat conducting properties), usually in engines with double overhead cam (DOHC) Silicon-aluminium bronze is fairly soft therefore it wears rapidly In DOHC engines the valve moves up and down with virtually no side force on the valve stem -5Copyright © Automotive Skills Limited 2003 All Rights Reserved LV34: Engines (3) Issue In engines fitted with tappets, the end of the tappet moves in an arc, therefore it forces the valve into the wall of the valve guide, in this case steel or cast iron valve guides are used Note: Steel and cast iron valve guides may be used because they are less expensive Poppet Valve Valves are faced with hard stellite Alloys that have been developed to satisfy conditions during the operation of valves (they are subject to approximately 650°C during the exhaust stroke) are varying amounts of manganese, silicon, nickel and chromium A new material used is molybdenum and titanium, which makes them highly resistant to heat and also reduces the valve weight by approximately 20% Heat is passed from the seat directly to the cylinder head and along the stem through the guide to the cylinder head In some extreme operating conditions the valve stem is sometimes made hollow and filled with sodium Sodium is a soft metal with a low melting point of 98°C In its molten state it splashes up and down the valve stem, therefore assisting the transfer of heat from the head of the valve Some valves are coated with aluminium to improve heat transfer from the valve to the engine block These valves cannot be re-surfaced (ground) in the normal way due to the surface coating being thin Seventy five percent of exhaust valve heat is dissipated through the valve seat area -6Copyright © Automotive Skills Limited 2003 All Rights Reserved LV34: Engines (3) Issue Valve seat Valve seat Shrink Fit Valve Seat Laser Clad Valve Seat Valve seats are stellite or hard chromium (60% alloy valve seat inserts) An alternative is to use a laser clad valve seat, which is a highly wear resistant alloy, and is welded onto the cylinder head and subsequently machine cut to form the seat angle With this system the seat can be made thinner, the result is that the valve seat diameter can be made larger and the cooling effect around the valve seat is improved The valve seat is shaped like a cone and is normally at an angle of 45 degrees although manufacturers use alternative angles as shown above The valve seat is in the shape of a cone to conform to the shape of the valve The valve seat contact width is generally 1.2 to 1.8 mm Excessive valve seat contact width is likely to cause carbon intrusion between the valve face and seat, although the cooling effect will be high If it is too narrow, gas tightness will improve but the cooling effect will decrease -7Copyright © Automotive Skills Limited 2003 All Rights Reserved LV34: Engines (3) Issue Valve springs Valve springs are used to close the valves quickly, most engines have just one spring per valve but some use two springs per valve To prevent valve surging (bounce) when the engine is running at high speed uneven pitch springs or double springs are used The wider pitch is always installed at the top of the valve Valve surging causes abnormal noise generation from the engine when the engine is operated at high speed, it can also cause interference between the piston and the valve, which may lead to damage of both parts Valve rotators Valve keepers Valve A Rotator body Coil spring Spring plate B C Retainer Valve spring Rotator body Valve closed Some engines have valve rotators fitted rather than conventional valve retainers The purpose of the rotator is to prevent improper seating of the valve cause by lead compounds when leaded fuel is used or carbon sticking to the valve surface Coil spring Plate spring Valve open Valve closed -8Copyright © Automotive Skills Limited 2003 All Rights Reserved LV34: Engines (3) Issue Operation of valve rotators When the valve is opened, the valve spring is compressed therefore the tension on it becomes greater This causes the outside circumference of the plate spring to flex upward slightly, causing the coil spring to flatten even more, the rotator body then turns At this time point A slides, but points B and C not slide When the valve closes, the spring extends and the tension in it weakens The spring returns to its original condition, this causes slipping to occur at points B and C but no slipping occurs at point A Therefore the rotator body remains in the same position as when the valve is open Camshafts On single and double overhead valve engines the crankshaft drives the camshaft via a belt, gear or chain system Double overhead cams are used on engines with four valves per cylinder, inline engines have two camshafts and ‘V’ engines have four Having more valves per cylinder increases the flow of gas, therefore increasing the power output of the engine Camshafts are made from steel, either forged or cast, and then machined, case hardening is used on the cam lobes, while cast shafts are usually hardened by chilling during casting More compact and lighter camshafts are made from high carbon, high chromium alloy and are then tempered to withstand increased pressure between the cam and valve operating mechanism of high lift high-pressure cam lobes Camshafts are supported in plain bearings but sometimes roller bearings are used -9Copyright © Automotive Skills Limited 2003 All Rights Reserved LV34: Engines (3) Issue Continuous Variable Valve Timing Control Hydraulic control unit Mechanical timing mechanism Lexus variable valve timing VTC (variable valve timing control) is a generic implementation and is used by Lexus in their VVT- i Alfa Romeo use a system similar to the BMW VANOS and double VANOS and other manufacturers use similar arrangements VANOS variable cam timing (BMW) Shown camshaft and timing chain layout The VANOS (Variable Onckenwellen Steuerung), (Variable Camshaft Angle Control) consists of a hydraulic and mechanical camshaft control device and is managed by the DME (Digital Motor Electronic) -55Copyright © Automotive Skills Limited 2003 All Rights Reserved LV34: Engines (3) Issue The DME, is a microprocessor-based system that controls ignition and fuel injection, it also performs a number of ancillary duties The DME is better known as the electronic control unit (ECU) If an electrical fault occurs the DME reconfigures itself and by-passes the problem, it then carries out a self-diagnosis and records the fault For diesel engine/BMWs the term DDE is used which stands for Digital Diesel Electronics An OBD (on board diagnostics) is also incorporated in the DME so that faults can be downloaded for reference The processor, which is inside the DME, can work at speeds of twenty million instructions per second (20MIPS) figures used are for the current (2003) BMW M3 The estimated life span of the DME is 150,000 hours Valve timing gear operation Camshaft sprocket Camshaft Intermediate gear (cup gear) The timing gear arrangement is shown, which serves to illustrate the basic principles of operation of the VTC generic valve timing arrangement The timing chain drives the camshaft sprocket and the camshaft is driven by the intermediate gear (cup gear) The cup gear has helical gears machined on the outside and this gear links to the camshaft sprocket, which has matching helical gears on the inside The camshaft fits into the cup gear by means of splines Because the helical gears are interlinked, if the cup gear is pushed along its axis the camshaft is forced to rotate, therefore the valve timing is changed -56Copyright © Automotive Skills Limited 2003 All Rights Reserved LV34: Engines (3) Issue Return spring Camshaft sprocket Camshaft splines Camshaft rotates in relation to camshaft sprocket position Hydraulic pressure pushes cup along splines on the camshaft As the splined carrier (cup gear) is pushed in, it twists the camshaft therefore advancing the cams, as it moves back it retards The operation of variable valve timing is to change the relative timing between the camshaft and its sprocket In the VANOS type the relative timing between the inlet and exhaust camshafts is changed, therefore valve overlap is changed Note: In this system no other valve timing parameters change, such as valve lift or opening duration By adding the VTEC valve arrangement valve opening overlaps can be continuously varied which provides higher power output from the engine, both variable valve timing and lift complement each other The timing adjustment is carried out by modifying the position of the camshaft compared to the position of the crankshaft A double VANOS adds adjustment to the inlet and exhaust valve timing The VANOS operates on the inlet camshaft, depending upon engine speed and accelerator position When the engine is at low speed the inlet valves are opened later, this improves smoothness and steady idling of the engine At medium engine speeds the inlet valves open much earlier, this boosts engine torque and provides internal exhaust gas re-circulation Fuel consumption and exhaust emissions are reduced At high engine speeds the inlet valve opens later therefore full engine power is produced The camshafts are connected to the crankshaft by means of a belt or chain, sometimes gears are used -57Copyright © Automotive Skills Limited 2003 All Rights Reserved LV34: Engines (3) Issue Operation of the VANOS The crankshaft drives the sprocket on the exhaust cam by a chain A second set of teeth moves a second chain that transmits its drive to the inlet camshaft The larger inlet cam sprocket is not bolted directly to the camshaft, it has a hole in the middle of it Inside the hole is a set of helical teeth, on the end of the camshaft is a gear that is also helical on the outside, but is too small to connect with the teeth on the inside of the larger sprocket A small metal cup has helical teeth to match the camshaft on the inside and also match the sprocket on the outside The ‘cup’ gear is pushed by hydraulic pressure and is controlled by the DME Hydraulic pressure is applied to the left of the piston and the camshaft advances valve timing Hydraulic pressure is applied to the right of the piston and the camshaft retards the valve timing -58Copyright © Automotive Skills Limited 2003 All Rights Reserved LV34: Engines (3) Issue What happens is that when the helical cup is moved along its holder in the cam sprocket, because it is interlinked with the helical gears, the helical cup will rotate along its axis when oil pressure is applied Since the camshaft is attached to the cup, it will rotate on its axis also causing the relative alignment between the camshaft and the driving cam sprocket therefore the valve timing has changed The DME determines the position of the oil control valve and hydraulic pressure is applied to the left hand side of the piston, this pressure causes the piston to move to the right, thus advancing the valve timing due to the action of the twist of the helical splines When the oil control valve is in the retard position the cup moves to the left and rotates to the retard position, also the oil control valve shuts off the oil passages to maintain hydraulic pressure on both sides of the cup, therefore the timing phase is maintained in the required position At idle the valve timing is retarded, when the engine speed just starts to increase the DME energises a solenoid which allows oil pressure to push the piston (cup) to advance the camshaft by 12.5 degrees at mid engine speed Increasing valve timing advance improves cylinder fill at mid engine speeds, which produces improved performance (increased torque) Occasionally a noise can be heard from the mechanism, this is caused by tolerances that allow the sprocket to move slightly as the cup gear moves in and out -59Copyright © Automotive Skills Limited 2003 All Rights Reserved LV34: Engines (3) Issue Operation of the double VANOS Double VANOS provides variable control for both the inlet and exhaust camshafts, this improved system provides a much improved power output On single VANOS the valve timing is only changed at two distinct engine speed points, whereas on the double VANOS the inlet and exhaust valve timing is continually variable, almost throughout all engine speeds On double VANOS the intake valve opening is extended by 12 degrees High oil pressure is demanded in order to adjust the camshafts with the speed demanded by the engine, it must also maintain accuracy Unburnt fuel mixture is reduced and engine idle smoothness is improved The DME manages the warm-up phase ensuring that the engine and catalytic converter reach operating temperature sooner (An air pump can be fitted to blow oxygen into the exhaust system during the warm-up phase, by generating an oxygen surplus the catalytic response time is reduced by about 50%) Double VANOS improves low engine speed power and widens the power band by flattening the torque curve Double VANOS has a lower torque peak about 450 rev/min lower than the single VANOS and a 200 rev/min higher power peak The torque curve is improved between 1500 and 3800 rev/min and the torque does not fall off as fast when the power peak is past The comparison is a general overview of both systems The figures given will obviously vary depending upon the type of engine and designed output power and is only to aid understanding of the two systems explained -60Copyright © Automotive Skills Limited 2003 All Rights Reserved LV34: Engines (3) Issue A main advantage is that the double VANOS controls the exhaust gas into the inlet manifold individually during all operating conditions providing accurate internal exhaust gas re-circulation EGR Under part load EGR is increased allowing the engine to run on a wider opening of the throttle, which provides greater fuel economy When the engine is under full load the system has a low EGR therefore providing the cylinders with maximum oxygen The purpose of variable valve timing is to change the relative timing between the camshaft and its sprocket In the VANOS type the relative timing between the inlet and exhaust camshafts is changed, therefore valve overlap is changed Note: In this system no other valve timing parameters change, such as valve lift or opening duration By adding the VTEC valve arrangement, valve opening overlaps can be continuously varied, which provides higher power output from the engine, both variable valve timing and lift complement each other Some designs such as the BMW double VANOS has cam-phasing at both inlet and exhaust camshafts which provides greater overlap and higher efficiency Overlap On some engines the inlet camshaft timing is within a maximum range of 40 degrees while the exhaust camshaft is 25 degrees There will be less top end power than in the case of the cam-changing VVT without the use of variable lift and variable valve opening duration -61Copyright © Automotive Skills Limited 2003 All Rights Reserved LV34: Engines (3) Issue Function The Valvetronic is incorporated within double VANOS technology providing a fully variable valve drive with additional infinitely adjustable intake valve lift, which keeps the valve opening periods even shorter V8 Valvetronic layout The engine DME (ECU) controls a lever between the camshaft and each pair of inlet valves Depending on its position the lever transmits cam lift either as a higher and longer opening period or a lower and shorter opening period Engine breathing is controlled by the intake valves therefore eliminating the conventional throttle butterfly valve and with it the pumping losses it creates when it is not fully open When the vehicle is running downhill with the throttle partially open or fully closed, the pistons are still trying to draw air into the cylinders This causes a partial vacuum which resists the pumping action of the pistons and wastes energy, this is known as “pumping loss” The slower the engine runs the greater is the energy loss caused by the throttle butterfly To compare this with our own individual breathing requirements, when we make a high degree of physical effort we breathe, more deeply and longer Whenever we require less air we not throttle the air supply by holding our nose or mouth, we simply breathe in a shorter and flatter manner -62Copyright © Automotive Skills Limited 2003 All Rights Reserved LV34: Engines (3) Issue The throttle butterfly is comparable to holding the nose or putting a hand over the mouth to throttle breathing Valvetronic sometimes has deep long ventilation and sometimes shorter and flat ventilation which allows the engine to breathe like we do, taking in air according to current requirements without any throttling effects and therefore with maximum breathing efficiency The reduction in fuel consumption provided by the throttle free load control amounts to approximately 10% The additional saving in fuel consumption, will improve as the driver is more reserved in the use of the accelerator pedal, which could lead to a fuel saving of around 18% To compare a standard litre engine with VANOS/Valvtronic fitted engine, about 21% more power can be gained 11% more torque and up to 20% improvement in fuel economy and improved acceleration times Each valve is given its own individual drive function operating according to specific requirements, the technology using an electromechanical valve drive ensures enhanced variability of valve lift Valvetronic does not require sulphur free fuel as in the case of direct injection engines, which provides full economic benefits even when driving in countries without a nationwide supply of sulphur free fuel Electric motor Eccentric Shaft Double VANOS The illustration shows the main components of the Double VANOS and Valvetronic cylinder head assembly -63Copyright © Automotive Skills Limited 2003 All Rights Reserved LV34: Engines (3) Issue Components and Operation Stepper motor Eccentric fitted to secondary shaft Intermediate lever Exhaust Camshaft Inlet camshaft Roller Valve rockers Valve lash adjuster Inlet valve Exhaust valve The camshafts are conventional but act through intermediate levers carried on and positioned by eccentrics on a secondary shaft The intermediate levers are springloaded and bear against a roller cam follower that opens the valve through a hydraulic valve lash adjuster Stepper motor Eccentric Intermediate lever Rollers -64Copyright © Automotive Skills Limited 2003 All Rights Reserved LV34: Engines (3) Issue Eccentric shaft The secondary shaft is rotated by an actuator motor which provides a rotary motion of the eccentric to raise or lower the lever, this alters the height of the contact area wiped by the inlet cam, thus the valve lift is varied on each induction stroke Mixture intake low (low lift) Mixture intake high (high lift) Motor operation is controlled by the engine management system Sensor inputs are used, a sensor on the accelerator pedal in the form of a potentiometer measures the rate of change and position of the driver’s foot on the pedal, thus registering power demanded This input device provides the engine management system with information so that the valve is opened the correct amount, therefore providing the engine with the correct amount of air/fuel mixture Valvetronic permits a fully variable valve stroke ranging from 0.0 to 9.7 millimetres Not only does the valve mechanism operate with precision it is also very swift to respond, the actual adjustment from minimum to maximum stroke can take as little as 300 milliseconds To provide for this ultra high performance of Valvetronic, a separate computer is used it consists of a 32-bit, 512 kilobytes of memory for the program data and the internal RAM computer is linked to the engine management unit -65Copyright © Automotive Skills Limited 2003 All Rights Reserved LV34: Engines (3) Issue Valvetronic provides for smooth running and excellent cold starting When the engine is under part load with very little valve stroke (0.5 to 2.0 millimetres) the air/fuel mixture only has a narrow gap through which to enter the combustion chamber, this causes the mixture to enter at high speed and the resultant effect is a more atomised mixture even with a cold engine At low operating loads the valves move marginally providing very smooth engine operation Comparing VTEC with Valvetronic Valvetronic is less efficient at high engine speeds than conventional engines, let alone VTEC Quite a lot of friction is generated by means of its mechanical components therefore the efficiency of Valvetronic engines drops off at speeds above 6000 rev/min Variable valve timing (Ferrari) 3-dimensional profile cam Camshaft slides laterally as engine speed and load varies Low valve lift Increased valve lift Ferrari varies the valve timing and lift by using a camshaft that has lobes with a three-dimensional profile The profile varies along the length of the cam lobe One end of the cam lobe has a less aggressive cam profile than at the other end The shape of the cam smoothly blends these two profiles together A device is fitted that causes the camshaft to move laterally causing the valve to be operated by the various profiles As the camshaft rotates it is caused to slide gradually in a lateral movement as the speed and load of the engine varies therefore optimising the valve timing and lift -66Copyright © Automotive Skills Limited 2003 All Rights Reserved LV34: Engines (3) Issue The future So far every system described operates by a camshaft, the cam profile is the only limiting factor with regards to improvement in engine performance Research has led to a focus in the use of electro-magnetically operated valves A coil surrounds the valve stem and a powerful magnetic force is generated in the coil when electric current is passed through it, this force pushes the valve down against a spring opening the valve THE FUTURE This system would provide infinite variable valve timing The valves are operated by computer and would be capable of providing maximum fuel economy and power throughout all engine speed range load BMW prototype engine Experiments have been carried out to eliminate the valve spring altogether by reversing the magnetic field to close the valve Timing could be as precise as ignition or fuel injection timing, but at the moment magnetically operated valves limit the engine to 6,000 rev/min It also requires a large amount of electrical power, although pulsed, the electrical load would vary depending on engine speed -67Copyright © Automotive Skills Limited 2003 All Rights Reserved LV34: Engines (3) Issue A side effect of electro-magnetic valve operation is that the rapid movement of the valve would provide a high full load potential at low engine speeds and would boost the torque by about 5% Another variation of this system is to use electrical power to control a hydraulically operated valve, less power would be required and there would be greater control of valve motion, but at the moment it would only be able to operate within a maximum engine speed of about 2,500 rev/min This system is unlikely to be used on light vehicles for a considerable time, if ever -68Copyright © Automotive Skills Limited 2003 All Rights Reserved LV34: Engines (3) Issue Progress check Answer the following questions: Discuss in groups of three The operation of the VANOS and double VANOS List the advantages of the double VANOS over the single VANOS Discuss in groups of three The operation of the Valvetronic system, also the future developments of valve operating mechanisms Describe the meaning of ‘pumping loss’ Explain why it is possible to eliminate the throttle butterfly in a Valvetronic system (Note: A throttle butterfly may be used for emission control by generating a small vacuum in the inlet manifold, which is not normally present in the Valvetronic system Its purpose is to prevent hydrocarbons leaking into the atmosphere Discuss this comment -69Copyright © Automotive Skills Limited 2003 All Rights Reserved LV34: Engines (3) Issue ... Reserved LV34: Engines (3) Issue The diagrams show the degree of valve lift and change in valve timing for the VVT-i -40Copyright © Automotive Skills Limited 2003 All Rights Reserved LV34: Engines (3). .. on the valve stem -5Copyright © Automotive Skills Limited 2003 All Rights Reserved LV34: Engines (3) Issue In engines fitted with tappets, the end of the tappet moves in an arc, therefore it forces... Automotive Skills Limited 2003 All Rights Reserved LV34: Engines (3) Issue Valve springs Valve springs are used to close the valves quickly, most engines have just one spring per valve but some