How to evaluate combustion quality and detect faults by the indicator diagram?
09 Sep '18, 09:53
There are some irregularities in the shape of the indicator diagram that illustrate incorrect engine operation. If the same fault appears in all diagrams from all cylinders, the cause might be a common one such as fuel condition or supply, the turbocharger system, exhaust, engine cooling or incorrect camshaft timing.
Provided the fuel is at the correct temperature and the injector operating normally the time between the injection of fuel and ignition (the ignition lag) is almost constant. Consequently, ignition timing faults signify a corresponding timing fault in the engine. They will affect the shape of the power diagram but are far more easily detected in an out phase diagram.
This will cause an abnormally high peak pressure in the cylinder at about the top of the piston stroke. A heavy shock load will be transmitted to the running gear and bearings with a corresponding knocking sound. Although thermal efficiency is high, the exhaust temperature is reduced, the shock load and consequent vibrations may cause damage to the engine. Causes of early ignition may be incorrect fuel pump timing, broken or incorrectly set injector springs, incorrect fuel condition, overheating of parts within the cylinder.
Can be seen from the figure that this causes a low peak pressure, which occurs well after the top center of the piston. Power is lost since the fuel is not burned correctly to transmit power at the most effective part of the stroke. Combustion may continue during the expansion stroke and may be an incomplete giving loss in energy produced, high exhaust temperature and smoke. Late ignition may be due to excessive injector spring settings, poor atomization, high viscosity or poor quality fuel, fuel pump leaking or incorrectly timed, low compression, insufficient supply of combustion air or undercooling of parts within the cylinder.
This refers to the slow or late combustion of fuel during the expansion stroke of the piston and is shown by a rise in the expansion line during the latter part of the stroke. Exhaust temperature and pressure will be high with burning fuel and carbon passing to the exhaust. This may burn exhaust valves and foul the exhaust system with a risk of turbocharger surging or uptake fires. High temperatures within the cylinder cause a deterioration in lubrication and possible damage to the line for surface and piston rings. There may also be burning of the piston crown.
Leaking Fuel Injector
This can be detected by a loss of power in the affected cylinder together with smoke at the exhaust and high exhaust temperature. There may also be a knock or pressure wave in the injection system. An indicator diagram taken from this cylinder will show fluctuations of pressure during the expansion process due to the secondary burning of the fuel leaking from the valve. A higher expansion line at exhaust opening indicates after burning. The loss in power is due to incorrect combustion since the fuel pressure wave from the fuel pump will not fill the space left by the fuel leaking from the injector. This may also cause the knock. Hot gases from the cylinder may blow back into the injector tip forming carbon and choking the atomizer. Carbon may also form on the outside of the atomizer due to the burning of the dribble of fuel. The injector should be changed and tested. To reduce the possibility of leaking injectors, good maintenance of the fuel pumps and injectors should be carried out with a routine changing of the injectors. Fuel should be purified and filtered and must be supplied at the correct temperature. Fuel injectors must also be maintained at the correct temperature.
Partly Choked Injector
This may be due to contamination of the fuel in which debris may choke the small atomiser holes in the injector. Alternatively it may be caused by a leaky injector allowing hot gas to blow back into the injector causing carbon to form and choke the injector. Overheating of the injector nozzle may cause build-up of carbon. There will be a loss of power in the engine, there will probably be hammering in the fuel pipes between the fuel pump and injector and this may lead to rupture of the fuel pipe. A choked injector can be confirmed by the indicator diagram, power and draw cards and by the reduced exhaust temperature. The remedy is to change the fuel injector and clean the fuel system, also ensure correct centrifuging and filtering of the fuel and maintain the correct fuel valve temperature.
Shown by the low pressures on a compression diagram it is possible that the first evidence was from faulty combustion. Low pressures may be due to a lack of air being supplied (this may be caused by turbocharger faults, choked scavenge ports or inlet valve etc) or that the air is escaping past the piston during compression which can usually be due to worn or broken piston rings, worn cylinder liner, etc, but blow past is usually detected during the expansion stroke. ‘Blow past’ is the term used when combustion gases at high temperature leak past the piston between the piston rings and the cylinder liner. This will cause rapid overheating, removing lubrication, causing further damage to the surfaces and possible piston seizure. The burning gas will enter the scavenge spaces in two-stroke engines causing fouling, overheating and even scavenge fires. In four-stroke trunk piston engines these gases will enter the crank-case with severe risk of crank-case explosion. Low compression may be caused by excessive cylinder liner wear, lack of cylinder lubrication, worn, broken, stuck or poorly maintained piston rings, worn piston ring groove landings, allowing the rings to cant and jam or carbon jamming rings and grooves. It may be aggravated if the engine is overloaded. The remedy is to measure the cylinder liner and renew if necessary, overhaul the piston, clean the ring grooves and gauge them, machine or fit new groove inserts as necessary, renew piston rings with the correct clearances, maintain cylinder lubrication and avoid engine overload.
Exhaust Valve Opening
On a slow running engine this can be checked by means of a light spring indicator diagram. This will not give an accurate timing check but by comparison with a normal diagram or one taken during the original engine trials it may be seen if the valve is opening early or late. Early opening of an exhaust valve can cause loss of power since pressure is released too soon it will give a high exhaust temperature with a risk of overheating. Late opening will reduce the scavenge efficiency by reducing blow down it will also affect the supply of energy to the turbocharger.
Choked Exhaust Valve
With this fault, the pressure during valve opening does not fall as rapidly or as far as it should and prevents correct blowdown and recharge of the cylinder with air. It causes loss of power, high exhaust temperature, smoke and a possible surge in the turbocharger and is the result of contamination due to incorrect combustion or excessive cylinder lubricating oil supply.
09 Sep '18, 14:30