Tuesday 27 November 2012

POWER BALANCING OF ENGINE



POWER BALANCING OF ENGINE

 for economy and efficient running of an engine equal power should be produced from each cylinder this is then said to be a owe balanced and it may be necessary to make some minor adjustment to achieve this

1. power produced is related to the quantity of fuel injected and balancing is carried out by small adjustment to individual fuel pump controls
2 .adjustments are limited to ensure units are not overloaded ,exhaust temperature are not excessive and that pump controls still cut off when brought to stop
3. fuel pump rack positions and exhaust and cooling outlet temperatures from each cylinder should be noted
4. equal exhaust temperatures on their own do not necessary show on accurate balance
5.  power balance can be checked by measurement from indicator diagrams by calculating the area of mean high from each cylinder
6. if an engine operates in an unbalanced condition some bearings and running gear may be become overloaded this may cause overloading and bearing failure
7. overload in cylinder may cause piston blow past with the corresponding dangers of overheated or seized piston
8.unbalance will also set up vibrations which if maintained for prolonged periods will cause fatigue from the fluctuating stress induced this may in turn lead to fatigue  cracking of metal in bearings  fracture in studs or bolts, cracks on crankshaft and bed plates,and slackening or failure of holding down bolts
9.for proper balance engine running condition should be normal by observation of the relevant temperatures and pressures particularly exhaust and cooling outlet temperature lubricating oil and boost air pressure, the exhaust should be clear of smoke and there should be no unusual noise or vibration

Wednesday 21 November 2012

Valve bouncing

VALVE BOUNCING

Photo: VALVE BOUNCING
 
The valves are opened by the rocker arm pushing on the valve stem tip, but they are closed by two or sometimes three coil springs. Multiple springs are used, not only because of the additional force they provide but because of a characteristic of the spring known as resonance. Valve opening is done with a series of impulses, and at a certain engine RPM, these impulses will occur at the resonant frequency of the spring. When this happens, the spring loses its effectiveness and allows the valve to float. To prevent this floating condition, two or more springs having different pitch, diameter, and wire size are used; and because of their different configuration, they have different resonant frequencies, so the engine can operate throughout its full range of RPM without valve float problems. 
Valve bouncing is a phenomenon, which happens due to sudden release of compression energy of the spring at the moment of valve closure due to tappet clearance allowance. The valve would jump up and down on seat before closing shut finally. Spring surge (resonant vibration) at this moment also tends to help generate valve bouncing.
Valve bouncing can be reduced by:
(i) Increasing number of springs.
(ii) Improving spring material.
(iii) Using inner and outer springs, right and left handed.
(iv) Using hydraulic valve actuating gear.
(v) Using air spring.
 VALVE BOUNCING

The valves are opened by the rocker arm pushing on the valve stem tip, but they are closed by two or sometimes three coil springs. Multiple springs are used, not only because of the additional force they provide but because

of a characteristic of the spring known as resonance. Valve opening is done with a series of impulses, and at a certain engine RPM, these impulses will occur at the resonant frequency of the spring. When this happens, the spring loses its effectiveness and allows the valve to float. To prevent this floating condition, two or more springs having different pitch, diameter, and wire size are used; and because of their different configuration, they have different resonant frequencies, so the engine can operate throughout its full range of RPM without valve float problems.
Valve bouncing is a phenomenon, which happens due to sudden release of compression energy of the spring at the moment of valve closure due to tappet clearance allowance. The valve would jump up and down on seat before closing shut finally. Spring surge (resonant vibration) at this moment also tends to help generate valve bouncing.
Valve bouncing can be reduced by:
(i) Increasing number of springs.
(ii) Improving spring material.
(iii) Using inner and outer springs, right and left handed.
(iv) Using hydraulic valve actuating gear.
(v) Using air spring.