2.1) Valve Timing Diagram (VTD)

The exact moments at which the inlet and outlet valve open and close with reference to the position of piston and crank, when shown diagrammatically, it is known as Valve Timing Diagram. The timing is expressed in terms of degrees of crank rotation.

Suction Stroke:  Inlet valve is open. Piston moves from the Top Dead Centre (TDC) to Bottom Dead Centre (BDC).  Air-fuel mix is sucked in by negative pressure in cylinder.

Compression Stroke: Inlet and outlet valves closed. Piston moves upwards from BDC to TDC. Air-fuel mix is compressed.

Expansion/Power Stroke: Inlet and outlet remains closed here also. Piston moves down from TDC to BDC. This happens as a result of ignition of the mixture inside the cylinder. Ignition is started by spark plug or as a result of compression ignition.

Exhaust Stroke: Exhaust valve opens. Piston moves up from BDC to TDC. Exhaust gases are pushed out of the cylinder.

The Actual Valve Timing Diagram has slight variations with respect to the Theoretical Valve Timing Diagram. The variations are made in order to maximize the engine performance. Refer the figures [2.1 & 2.2] given above and compare it with the Theoretical VTD to the see the difference.

Opening and closing of Inlet Valve

The inlet valve is made to open 10degree to 30degree before the piston reaches the Top Dead Center (TDC) during Suction Stroke and is allowed to close only after 30degree to 40degree

Figure 2.1: Actual Valve timing diagram Figure 2.2: Ideal Valve timing diagram

Figure 2.3: VVT for Idling Case

after the piston reaches and leaves the BDC in the beginning of compression stroke. The reason for doing this is to facilitate silent operation of the engine under high speeds.

The inlet valves are made to operate slowly to avoid noise and hence sufficient time should be provided for the air-fuel mix to get into the cylinder. Thus valves are made to open before the actual BDC. Since the inlet valve is a small opening sufficient mixture doesn’t enter the cylinder in such short time, as the piston reaches BDC. Thus the inlet valve is kept open for some time period of time after BDC, to facilitate sufficient flow of charge into the cylinder.

Opening and closing of Exhaust Valve: The exhaust valve is made to open 30degree to 60degree before the BDC in the exhaust

stroke and allowed to close only after 80 to 100 degree, in the beginning of the suction stroke. The gases inside the cylinder have a very higher pressure even after the expansion stroke. This higher pressure enables it to move out of the cylinder through the exhaust valve reducing the work that needs to be done by the engine piston in pushing out these gases. Thus the exhaust valve is made to open before the piston reaches the BDC thus enabling the gases to escape outside on its own and the remaining gases are pushed out by the upward motion of the piston during the exhaust stroke.

When the piston reaches the TDC, if the exhaust valve is closed like in actual timing diagram, a certain amount of exhaust gases will get compressed and remain inside the cylinder and will be carried to the next cycle also. To prevent this, the exhaust valves are allowed to close only a certain time after the piston reaches the TDC.

2.2) Variable Valve Timing (VVT) Diagram

The main task of variable valve timing (VVT) diagram is setting the most advantageous valve timing for the particular engine for the operating modes like idle, maximum power and torque as well as exhaust gas recirculation.

First I discuss the various valve timing diagrams which are obtained for different operating conditions of engine with the help of variable valve timing technology.

a) Idling case

At idle, the camshafts are set so that the inlet camshaft opens late and, consequently, loses late as well. The exhaust camshaft is set so that it closes well before TDC. Due to the minimal gas residue from combustion, this leads to smooth idling. For this case valve timing diagram is shown in figure [2.3]

Figure 2.4: VVT for Maximum Power case

Figure 2.5: VVT for Torque caseFigure 2.6: VVT for Exhaust

Recirculation Case

b) Maimum Power

To achieve good power at high engine speeds, the exhaust valves are opened late. In this way, the expansion of the burned gases can act against the pistons longer. The inlet valves open after TDC and close well after BDC. In this way, the dynamic self-charging effect of the entering air is used to increase power. Valve timing diagram shown in figure [2.4]

c) Torque

To achieve maximum torque, a high degree of volumetric efficiency must be attained. This requires that the inlet valves be opened early. Because they open early, they close early as well, which avoids pressing out the fresh gases. The exhaust camshaft closes just before TDC. Valve timing diagram shown in figure [2.5]

d) Exhaust gas recirculation

Internal exhaust gas recirculation can be achieved by adjusting the inlet and exhaust camshafts. In this process, exhaust gas flows from the exhaust port into the inlet port while the valves overlap (inlet and exhaust valves are bothOpen). The amount of overlap determines the amount of re-circulated exhaust gas. The inlet camshaft is set so that it opens well before TDC and the exhaust camshaft does not close until just before TDC. As a result, both valves are open and exhaust gas is re-circulated.

The advantage of internal exhaust gas recirculation over external exhaust gas recirculation is the fast reaction of the system and very even distribution of the re-circulated exhaust gases. Valve timing diagram shown in figure [2.6]

Figure 2.7: Variable Valve Timing System

2.3) Design of Variable Valve Timing system

The variable valve timing system consists of the following components: also shown in figure[2.7]

a) Two fluted variators

The fluted variator for adjusting the inlet camshaft is fitted directly on the inlet camshaft. It adjusts the inlet camshaft according to signals from the engine control unit. The fluted variator for adjusting the exhaust camshaft is fitted directly on the exhaust camshaft. It adjusts the exhaust camshaft according to signals from the engine control unit. Both fluted variators are hydraulically operated and are connected to the engine oil system via the control housing.

b) The control housingThe control housing is attached to the cylinder head. Oil galleries to both fluted variators

are located in the control housing.

c) Two solenoid valvesThere are two solenoid valves located in the control housing. They direct oil pressure to

both fluted variators according to the signal from the engine control unit. Inlet camshaft timing adjustment valve -1- (N205) is responsible for the inlet camshaft, and exhaust camshaft timing adjustment valve -1- (N318) is responsible for the exhaust camshaft.

2.4) Honda VTECH

The basic mechanism used by the VTEC technology is a simple hydraulically actuated pin. This pin is hydraulically pushed horizontally to link up adjacent rocker arms. A spring mechanism is used to return the pin back to its original position as shown in figure [2.8].

To start on the basic principle, examine the simple diagram below. It comprises a camshaft with two cam-lobes side-by-side. These lobes drive two side-by-side valve rocker arms.The two cam/rocker pairs operate independently of each other. One of the two cam-lobes is intentionally drawn to be different. The one on the left has a "wilder" profile, it will open its valve earlier, open it more, and close it later, compared to the one on the right. Under normal operation, each pair of cam-lobe/rocker-arm assembly will work independently of each other.

VTEC uses the pin actuation mechanism to link the mild-cam rocker arm to the wild-cam rocker arm. This effectively makes the two rocker arms operate as one. This "composite" rocker arms, now clearly follows the wild-cam profile of the left rocker arm. This in essence is the basic working principle of all of Honda's VTEC engines.

2.5) Variable Valve Timing system with Intelligence (VVT-I)

VVT-i, or Variable Valve Timing with intelligence, is an automobile variable valve timing technology developed by Toyota, similar in performance to the BMW's VANOS. The Toyota VVT-i system replaces the Toyota VVT offered starting in 1991 on the 5-valve per cylinder engine.

 VVT-i, introduced in 1996, varies the timing of the intake valves by adjusting the relationship between the camshaft drive (belt, scissor-gear or chain) and intake camshaft. Engine oil pressure is applied to an actuator to adjust the camshaft position.

The VVT-i system is designed to control the intake camshaft within a range of 50° (of Crankshaft Angle) to provide valve timing that is optimally suited to the engine operating conditions. This improves torque in all engine speed ranges as well as increasing fuel economy, and reducing exhaust emissions. By using the engine speed, intake air volume, throttle position and engine coolant temperature, the ECM (Engine Control Module) calculates optimal valve timing for each driving condition and controls the camshaft timing oil control valve . In addition, the ECM uses signals from the camshaft position sensor and the crankshaft position sensor to detect the actual valve timing, thus providing feedback control to achieve the target valve timing.

Figure 2.8: VTECH System

Figure 2.10: Normal valve opening System

Engine control module (ECM) is a type of electronic control unit that determines the amount of fuel, ignition timing and other parameters in an internal combustion engine needs to keep it running efficiently at different conditions.

2.6) BMW Valve Tronic

Valve-tronic engines use a combination of hardware and software to eliminate the need for a conventional throttle mechanism.

Valve-tronic varies the timing and the lift of the intake valves. This system has a conventional intake cam, but it also uses a secondary eccentric shaft with a series of levers and roller followers, activated by a stepper motor. Based on signals formerly taken mechanically from the accelerator pedal, the stepper motor changes the phase of the eccentric cam, modifying the action of the intake valves.

a) Working & Need of Valve-Tronic Technology

Fuel injection systems monitor the volume of air passing through the throttle butterfly and determine the corresponding amount of fuel required by the engine. The larger the throttle butterfly opening, the more air enters the combustion chamber. 

At light throttle, the throttle butterfly partially or even nearly closes. The pistons are still running,

Figure 2.9: VVT-I System

Figure 2.11: Valve-Tronic opening System

taking air from the partially closed intake manifold. The intake manifold between the throttle and the combustion chamber has a partial vacuum, resisting the sucking and pumping action of the pistons, wasting energy. Automotive engineers refer to this phenomenon as "pumping loss". The slower the engine runs, the more the throttle butterfly closes, and the more energy is lost. Valve-Tronic minimizes pumping loss by reducing valve lift and the amount of air entering the combustion chambers.

  Compared with conventional twin-cam engines with finger followers, Valve-Tronic employs an additional eccentric shaft, an electric motor and several intermediate rocker arms, which in turn activates the opening and closing of valves. If the rocker arms push deeper, the intake valves will have a higher lift, and vice-versa. Thus, Valve-Tronic has the ability to get deep, long ventilation (large valve lift) and flat, short ventilation (short valve lift), depending on the demands placed on the engine.

b) Advantages

Valve lift is variable between 0 and 9.7 mm.   In Valve-Tronic engines coolant flows across the head, resulting in a temperature

reduction of 60%. The water pump size is cut in half, reducing power consumption by 60%.  The power steering fluid is warmed quickly, reducing the power used by the hydraulic

pump.  Mounting the water and power pump on the same shaft and a heat exchanger between

coolant and engine oil reduces oil temperature by 30%. 

2.7) CHOKE OPERATION

A choke valve is a valve that lifts up and down a solid cylinder (called a "plug" or "stem") which is placed around or inside another cylinder which has holes or slots.

The design of a choke valve means fluids flowing through the cage are coming from all sides and that the streams of flow (through the holes or slots) collide with each other at the center of the cage cylinder, thereby dissipating the energy of the fluid through "flow

Figure 2.12: Choke Operation

impingement". The main advantage of choke valves is that they can be designed to be totally linear in their flow rate.

A choke valve is sometimes installed in the carburetor of internal combustion engines. Its purpose is to restrict the flow of air, thereby enriching the fuel-air mixture while starting the engine. Depending on engine design and application, the valve can be activated manually by the operator of the engine (via a lever or pull handle) or automatically by a temperature-sensitive mechanism called an auto-choke.

Choke valves are important for carbureted gasoline engines because small droplets of gasoline do not evaporate well within a cold engine. By restricting the flow of air into the throat of the carburetor, the choke valve raises the level of vacuum inside the throat, which causes a proportionally greater amount of fuel to be sucked out of the main jet and into the combustion chamber during cold-running operation. Once the engine is warm (from combustion), opening the choke valve restores the carburetor to normal operation, supplying fuel and air in the correct stoichiometric ratio for clean, efficient combustion.

Chokes were nearly universal in automobiles until fuel injection replaced carburetion in the late 1980s. Choke valves are still extremely common in other internal-combustion applications, including most small portable engines, motorcycles, small prop-powered airplanes, and carbureted marine engines.

2.8) Tuning of Intake & Exhaust pipe of Four Stroke engine

a) Exhaust System An exhaust system is usually tubing used to guide reaction exhaust gases away from a

controlled combustion inside an engine. The entire system conveys burnt gases from the engine and includes one or more exhaust pipes. Depending on the overall system design, the exhaust gas may flow through one or more of:

Cylinder head and exhaust manifold A turbocharger to increase engine power A catalytic converter to reduce air pollution A muffler (North America) / silencer (Europe), to reduce noise

Exhaust System Tuning:Many automotive companies offer aftermarket exhaust system upgrades as a subcategory

of engine tuning. This is often fairly expensive as it usually includes replacing the entire exhaust manifold or other large components. These upgrades however can significantly improve engine performance and do this through means of two main principles:

By reducing the exhaust back pressure, engine power is increased in four-stroke engines

By reducing the amount of heat from the exhaust being lost into the under bonnet area.

This reduces the under bonnet temperature and consequently lowers the intake manifold temperature, increasing power. This also has positive side effect of preventing heat-sensitive

components from being damaged. Furthermore, keeping the heat in the exhaust gases speeds these up, therefore reducing back pressure as well. Back pressure is most commonly reduced by replacing exhaust manifolds with headers, which have smoother bends and normally wider pipe diameters.

Exhaust Heat Management is the term that describes reducing the amount of exhaust heat loss. One dominant solution to aftermarket up-graders is the use of a ceramic coating applied via thermal spraying. This not only reduces heat loss and lessens back pressure, but provides an effective way to protect the exhaust system from wear and tear, thermal degradation and corrosion.

b) Intake system

A modern automobile air intake system has three main parts;

Air filter Mass flow sensor Throttle body

Some modern intake systems can be highly complex, and often include specially-designed intake manifolds to optimally distribute air and air/fuel mixture to each cylinder. Many cars today now include a silencer to minimize the noise entering the cabin. Silencers impede air flow and create turbulence which reduces total power; so many performance enthusiasts often remove them.

All the above is usually accomplished by flow testing on a flow bench in the port design stage. Cars with turbochargers or superchargers which provide pressurized air to the engine usually have highly-refined intake systems to improve performance dramatically.

Production cars have specific-length air intakes to cause the air to vibrate at a specific frequency to assist air flow into the combustion chamber. Aftermarket companies for cars have introduced larger throttle bodies and air filters to decrease restriction of flow at the cost of changing the harmonics of the air intake for a small net increase in power or torque.

Figure 2.14: Inlet Manifold Design

Figure 2.13: Exhaust pipe Design

2.9) Self Sustaining flame

Combustion normally begins at the spark plug where the molecules in and around the spark discharge is activated to a level where reaction is self sustaining. This level is achieved when the energy released by combustion is slightly greater than the heat loss to the metal & the gas surroundings.

Initially, flame speed is very low as the reaction zone must be established, and heat loss is high as the spark plug is located near the cold walls. During this period, pressure rise is also small because the mass of mixture burned is small. Unburned gas ahead of flame front and the burned gas behind the flame front are raised in temperature by compression, either by a moving piston or by heat conduction from advancing flame. In the final stage, flame slows down as it approaches the walls of the combustion chamber (from heat loss & low turbulence) and is finally extinguished (wall quenching).

a) Fundamental requirements of the ignition source Figure [2.16] below shows a conventional spark plug with its different parts.

A high ignition voltage to break down in the spark-gap A low source impedance or steep voltage rise A high energy capacity to create a spark kernel of sufficient size Sufficient duration of the voltage pulse to ensure ignition

Figure 2.15: Pictures of Combustion process in SI Engine w.r.t crank angle

2.10) Cylinder pressure diagram for 4-Stroke engine

The sequence of events which take place inside the engine cylinder is illustrated in Figure [2.17]. Several variables are plotted against crank angle through the entire four-stroke cycle. Crank angle is a useful independent variable because engine processes occupy almost constant crank angle intervals over a wide range of engine speeds. The figure shows the valve timing and volume relationship for a typical automotive spark-ignition engine. To maintain high mixture flows at high engine speeds (and hence high power outputs) the inlet valve, which opens before TC, closes substantially after BC. During intake, the inducted fuel and air mix in the cylinder with the residual burned gases remaining from the previous cycle. After the intake valve closes, the cylinder contents are compressed to above atmospheric pressure and temperature as the cylinder volume is reduced. Some heat transfer to the piston, cylinder head, and cylinder walls occurs but the effect on unburned gas properties is modest.

Between 10 and 40 crank angle degrees before TC an electrical discharge across the spark plug starts the combustion process. A distributor, a rotating switch driven off the camshaft, interrupts the current from the battery through the primary circuit of the ignition coil. The secondary winding of the ignition coil, connected to the spark plug, produces a high voltage across the plug electrodes as the magnetic field collapses. Traditionally, cam-operated breaker points have been used; in most automotive engines, the switching is now done electronically. A turbulent flame develops from the spark discharge, propagates across the mixture of air, fuel, and residual gas in the cylinder, and extinguishes at the combustion chamber wall. The duration of this burning process varies with .engine design and operation, but is typically 40 to 60 crank angle degrees.

Figure 2.16: Spark plug construction

As fuel-air mixture bums in the flame, the cylinder pressure as shown in Figure [2.17] by (solid line) rises above the level due to compression alone (dashed line). This latter curve-called the motored cylinder pressure (is the pressure trace obtained from a motored or non-firing engine). Note that due to differences in the flow pattern and mixture composition between cylinders, and within each cylinder cycle-by-cycle, the development of each combustion process differs somewhat. As a result, the shape of the pressure versus crank angle curve in each cylinder, and cycle-by-cycle, is not exactly the same.

There is an optimum spark timing which, for a given mass of fuel and air inside the cylinder, gives maximum torque. More advanced (earlier) timing or retarded (later) timing than this optimum gives lower output, Called maximum brake-torque (MBT) timing, this optimum timing is an empirical compromise between starting combustion too early in the compression stroke (when the work transfer is to the cylinder gases) and completing combustion too late in the stroke (and so lowering peak expansion stroke pressures).

About two-thirds of the way through the expansion stroke, the exhaust valve starts to open. The cylinder pressure is greater than the exhaust manifold pressure and a blow down process occurs. The burned gases flow through the valve into the exhaust port and manifold until the cylinder pressure and exhaust pressure equilibrate. The duration of this process depends on the pressure level in the cylinder. The piston then displaces the burned gases from the cylinder into the manifold during the exhaust stroke. The exhaust valve opens before the end of the expansion stroke to ensure that the blow down process does not last too far into the exhaust stroke.

The actual timing is a compromise which balances reduced work transfer to the piston before BC against reduced work transfer to the cylinder contents after BC. The exhaust valve remains open until

Figure 2.17: Pressure Diagram & V/Vmax curves w.r.t Crank Angle

just after TC; the intake opens just before TC. The valves are opened and closed slowly to avoid noise and excessive cam wear. To ensure the valves are fully open when piston velocities are at their highest, the valve open periods often overlap. If the intake flow is throttled to below exhaust manifold pressure, then backflow of burned gases into the intake manifold occurs when the intake valve is first opened.

References http://yanswerz.blogspot.com/2009/12/valve-timing-diagram-of-four-stroke.html

http://www.classle.net/bookpage/valve-timing-diagram

http://www.waybuilder.net/sweethaven/MechTech/Automotive01/default.asp?

unNum=1&lesNum=3&modNum=4

http://thecartech.com/subjects/engine/engine_testing3.htm

http://ispark.computersolutiontrio.com/how-it-works.html

http://moodle.student.cnwl.ac.uk/moodledata_shared/cdx%20etextbook/dswmedia/

engines/comp/vlves/valvetimingdiagram.html

http://www.austincc.edu/wkibbe/vvt.htm (FOR VVT)

http://www.2dix.com/document-pdf/variable-valve-timing-diagram-pdf.php (FOR VVT)

http://www.2carpros.com/articles/how-camshaft-variable-valve-timing-works (VCT)

http://www.enginebuildermag.com/Article/39596/variable_valve_timing.aspx

http://www.2carpros.com/questions/toyota-other-variable-valve-timing-vvt (VVT-I)

http://www.wiringdiagrams21.com/2009/05/27/toyota-vvt-i-variable-valve-timing

intelligent-system-and-schematic-diagram

http://asia.vtec.net/spfeature/vtecimpl/vtec1.html (VTECH)

http://www.usautoparts.net/bmw/technology/valvetronic.htm (VALVE TRONIC)

http://www.freeautoanswers.com/choke.html (Choke operation)

http://teacher.buet.ac.bd/zahurul/ME401/ME401_combustion_SI.pdf (Self sustaining

flame)

Internal Combustion Engine by J.B Heywood (Book)