Valve Timing

EXPERIMENT NO.

DATE:

THE VALVE TIMING DIAGRAM OF A 4-STROKE AUTOMOBILE ENGINE OBJECTIVES: After Studying this practical students are able to know. (1) Ideal valve timing diagram. (2) Factor which affects ideal valve timing diagram. (3) Actual valve timing diagram. APPARATUS: 4-Stroke I.C.engine, marking pencil, feeler gauge, a device for measuring crank angles. THEORY: In 4-stroke SI engine the opening and closing of the valves, and the ignition of air-fuel mixture do not take place exactly at the dead centre positions. The valves open slightly earlier and close after their respective dead centre positions. The ignition also occurs prior, to the mixture is fully compressed, and the piston, reaches the dead centre position. A typical valve-timing diagram of a SI engine is shown in fig. Similarly, in a CI engine both valve do not open and close exactly at dead centre positions, rather operate some degrees on either side in terms of crank angles from the dead centre positions. The injection of the fuel (diesel) is also timed to occur earlier. A typical valve timing diagram of a diesel engine is shown in fig. There are two factors, on mechanical and other dynamic, for the actual valve to be different from theoretical valve timing. (A) Mechanical factor: The poppet valve of the reciprocating engines are opened and closed by cam mechanisms. The clearance between cam, tappet and valve must be slowly taken up and valve slowly lifted, at first, if the noise and wear to be avoided. For the same reason valve cannot be closed abruptly, else it will bounce on its seat. Thus the valve opening and closing periods are spread over a considerable number of crank shaft degrees. As a result, the opening of the valve must commence ahead of the time at which it is fully opened (before dead centre). The same reason applies for the closing time and the valve must be closed after the dead centers. (B)Dynamic factor: Besides the mechanical factor of opening and closing of valves, the actual timing is set taking into consideration the dynamic effect of the gas flow. INTAKE VALVE TIMING: Intake Valve timing has bearing on actual quantity of air sucked during the suction stroke i.e. it affects the volumetric efficiency. For both low and high speed engine the intake valve opens 10o before the arrival of the piston at TDC on the exhaust stroke. This is to insure that the valve will be fully open and fresh charge starting to flow into the cylinder as soon as possible after TDC. As the piston moves out in the suction stroke, the fresh charge is drawn in through the intake valve. When the piston reaches charge tends to cause it to continue to move into the cylinder. To

take advantage of this, the intake is closed after TDC so that maximum air is taken in. This called ram effect. However, if the intake valve is to remain open for too long a time beyond BDC, the up moving piston on the compression stroke would tend to force some of the charge, already in the cylinder, back into the intake manifold. The time the intake valve should remain open after TDC is decided by the speed of the engine. At low engine speed the charge speed is low and so the air inertia is low, and hence the intake valve should be close relatively early after BDC. In high-speed engines, the charge speed is high, and consequently the inertia is high and hence to induct maximum quantity of the charge due to ram effect the intake valve should be close relatively late after BDC (up to 60o after BDC). There is limit to the high speed engine for advantage of ram effect. At very high speed the effect of the fluid friction may be more than offset the advantage of ram effect and the charge for cylinder per cycle falls off. EXHAUST VALVE TIMING: The exhaust valve is set to open before BDC (say about 25 o before BDC in low speed engines and 55o before BDC in high speed engines). If the exhaust valve does not start to open until BDC, the pressure in the cylinder would be considerably above the atmospheric pressure during the first portion of the exhaust stroke, increasing the work required to expel the exhaust gases. But opening of the exhaust valve earlier reduces the pressure near end of the power stroke and thus causes some loss of useful work on this stroke. How ever the overall effect of opening the valve prior to the time the position reaches BDC results in overall gain in output. The closing time of the exhaust valve affects the volumetric efficiency. By closing the exhaust valve few degrees after TDC (about 15o in case of the low speed engine and 20o in case of the high speed engines) the inertia of the gases tends to scavenge the cylinder by carrying out a greater mass of the gas left in the clearance volume. This results in increased volumetric efficiency. There may be period when both inlet and exhaust valve are open at the same time. This is called valve over-lap (15o in case of low speed engine and 30o in case of high-speed engine). This overlap should not be excessive otherwise it will allow the burned gases to be sucked into the intake manifold, or the fresh charge to escape through the exhaust valve. PROCEDURE: 1. Fix a plate on the body of the engine touching the flywheel. 2. Mark the positions of both dead center on the flywheel with reference to the fixed plate TDC and BDC in case of vertical engines, and IDC and ODC in case of horizontal engine. 3. Mark on the flywheel when the inlet and exhaust valves open and close as the flywheel is rotated slowly. 4. Measure periphery of flywheel. 5. Measure distances of valve opening closing from BDC or TDC. 6. After calculations plot actual valve timing diagram.

OBSERVATION: Periphery = _______cm. Sr. No.

Valve timings

1. 2. 3. 4.

IVO IVC EVO EVC

Distance from TDC or BDC in cm

Angles from TDC of BDC in degree

PRECAUTIONS: 1. Marking plate should be fixed properly with the flywheel. 2. The positions of the dead centers and locations of opening and closing of valves should be marked carefully. ASSIGNMENT: Answer the following questions: (1) What do you mean by valve timing diagram? (2) What are mechanical factors? (3) Why inlet valve is opened before TDC & closed after BDC? (4) Why exhaust valve is opened before BDC & closed after TDC? (5) Why ignition takes place before TDC? (6) What is valve overlap? Write usefulness of valve overlap. What are adverse effects if we keep valve overlap more than required?

Assessment: Involvement and Participation (03)

Study and Observation (03)

Discipline (02)

Total (08)

Date:

Signature of faculty: