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Proper lubrication prevents intake port carbon formation.

Large integral two-cycle natural gas engines such as those originally manufactured by Cooper Bessemer, Clark and Worthington have been in operation for 40+ years. In recent years, many companies have lost much of the experience in operating and maintaining these units due to workforce reductions and retirement. At the same time, many of the old operating practices which were once "normal" are generally considered unacceptable today. One such practice is "punching" carbon in the ports of these engines. This article will review the factors which affect carbon formation.

This article will only deal with intake port carbon formation. Exhaust port carbon tends to be rare and comes from low exhaust temperatures. Normally, exhaust port temperatures are hot enough to burn any deposit away.

Lubrication Rates

Lube rates to the engine's power cylinders are measured in units of Horsepower Hours per Gallon (HP-HR/GAL). Generally speaking, a rate of 7,500 HP-HR/GAL serves as a good baseline number from which to begin. Note that this number is for power cylinders only and does not include oil required for compressor lubrication. If your lubricator system has not been calibrated for some time, it may be delivering too much or too little oil. Wear in these systems causes misdeliveries.

Because oil should be delivered by the horsepower used and lube systems deliver by engine speed, running an engine at part load (100 percent speed, less torque) automatically causes overlubrication. A better method is to run the engine at full torque with a lower speed, if practical. if your equipment is routinely required to run at part load, it may be worthwhile to recalibrate the lube system to match the lower load. Keep in mind that you will be underlubricating if you run the engines at full load again.

If you choose to change your lube rate, take small steps (5-10 percent) and monitor the cylinders often. Underlubrication causes scoring.

Other Factors Contributing To Overlubrication

* Loss of oil control by the piston rings.

* An oil ring installed upside down will "pump" oil up the cylinder and into the ports.

* Scavenging cylinders which are overlubricated send the excess oil into the intake system and from there into the ports.

* Turbocharger oil seal leaks into the intake system.

Cylinder Scavenging

Two-cycle engines have an optimum load and speed at which cylinder scavenging is most efficient. This point is determined by the amount of intake air available and the port configuration. Ideally the ports are configured so that the exhaust ports are uncovered first on the down stroke. This starts the flow of hot exhaust gases in the right direction -- out the exhaust, The intake port should open fully after the cylinder has "blown down" or let cylinder pressure fall to or below intake air pressure.

Maximum scavenging (and torque) occurs when the intake port opens just as the exhaust gas' momentum has caused a slight vacuum in the cylinder compared to intake pressure. If there is still pressure in the cylinder when the intake ports are uncovered, the hot exhaust gas will backwash or pulse into the intake ports for a brief period until the cylinder pressure falls below intake pressure. This causes intake port carbon formation because the hot exhaust gases cook oil in the ports.

Intake Air

Of course the intake air should be clean. Any bits of debris can settle in the port area and form a "holding area" for oil. This oil is not washed away and will eventually turn to carbon.

The most significant factor with intake air is its temperature. One hundred degrees F or below is highly recommended. At these temperatures detonation is easily controlled and proper peak firing pressure timing can be achieved. Under these conditions the engine should run very well, be less sensitive to load changes and properly scavenge the cylinders. Fuel efficiency is also enhanced.

High scavenging air temperature elevates the overall cylinder temperature, causing oil breakdown. Additionally, ports are not adequately cooled.

Timing

Retarded timing is defined here as timing which is retarded vs. factory settings, as required by high intake air temperatures or high BTU fuel gas. When due to high intake air temperature, it causes the peak firing pressure to occur at a later time in the two-stroke cycle. Since cylinder scavenging is highly dependent on the proper timing of cylinder pressurization and blow down, the later occurring flame tends to leave high pressure exhaust gases in the cylinder when the intake ports open. Again, port carbon is increased.

High BTU gas causes a need for retarded timing due to its lower octane. Lower octane fuel mixtures tend to burn quicker so actual peak cylinder pressure may occur at the proper time and have relatively little affect on cylinder scavenging when spark timing is retarded to make up for the octane difference. However, the higher BTU components of the gas (i.e., propane, butane) may, not burn completely and form fuel soot. This soot can then collect in the intake ports, causing carbon deposits derived from the fuel itself.

Coolant Leaks

When heads crack or gaskets leak, engine coolant can leak directly into the combustion chamber. Several things can happen and the results tend to be unpredictable. Problems come from both the water/glycol (if used) phase and the coolant additives.

Water is a natural octane enhancer, causing slower flame speeds and cooler combustion temperatures. If the peak firing pressure is delayed too long, the cylinder will not blow down before the intake ports are open. Also, since water which is turned to steam expands much more than dry air, overall cylinder pressures can be increased.

Coolant additive metals such as sodium and potassium can lay down an ash deposit on the piston and head. This deposit tends to block proper heat transfer out of the cylinder, raising temperatures throughout the combustion cycle and increasing the tendency toward detonation or preignition.

Detonation, Pre-ignition

Detonation occurs when the fuel's octane is not sufficient to produce a single, even flame front. As the initial flame front pressurizes the cylinder, very high pressure causes a second flame to self-ignite. When these flame fronts collide, knock occurs. Pre-ignition is caused by a "hot spot" in the cylinder acting as a second, uncontrolled spark plug. Some sources of pre-ignition are:

* Ash deposits which pick up heat and glow red hot.

* Loose spark plugs or fuel valves which cannot shed their heat, causing a hot spot.

* Any metal sliver (i.e., broken threads on the piston.)

Detonation and pre-ignition cause dramatic upsets in cylinder scavenging as well as raising cylinder temperatures. Often other cylinders must pick up the load, driving them into detonation as well. These conditions are prime for carbon formation, not to mention worn or broken cylinder components.

Lube Oil Quality

Perhaps the most common issue with lube oil is ash containing oils vs. ashless oils. Lube off ash containing additives such as zinc, calcium, magnesium and others are very effective in preventing carbon formation and cylinder wear. However, there are serious side effects.

Ash deposits are an insulator. A 1/16-inch of calcium ash deposit insulates like four inches of steel. Heat flow through the piston crown to the lube oil and from the cylinder head to the cooling system is impaired. Since heat flows through the piston crown to the lube oil, extra insulation on the piston crown can cause lube oil in/out differentials to be decreased while coolant in/out differentials are increased. More heat is left in the cylinder, causing a tendency to detonate. Ash deposits also soak up heat and can glow red hot, causing pre-ignition.

Since the rate of thermal expansion and contraction of ash is different than the metal underneath it, stress cracks develop in the ash causing bits of very abrasive ash material to dislodge. These bits lodge in the rings, causing cylinder and ring damage. Finally, ash coats turbocharger exhaust blades, causing imbalance, eventual failure and efficiency losses. The amount of ash deposit is directly affected by the amount of ash in the lube oil and the engine's oil consumption.

Ashless oils are therefore preferred for all types of two-cycle gas engines. Ashless oils can be made more or less effective by the amount and type of ashless additives used and the base oil quality.

Base Oil Quality

Oils formulated for two-cycle natural gas engine service contain far less additive than a typical passenger car motor oil, by design. Additives themselves tend to be high molecular weight materials and often leave deposits (carbon) of their own when burned. Therefore, the quality of the oil's base stock has a much larger role in two-cycle gas engine lubrication than it does in other types of motor oil.

The base oil should be refined as a 40-weight oil. Many base oil refineries produce only a 30-weight oil. This 30-weight oil must then be blended with a heavier component, such as bright stock or another very heavy neutral oil, to produce the 40-weight oil required for these engines.
COPYRIGHT 1996 Oildom Publishing Company of Texas, Inc.
No portion of this article can be reproduced without the express written permission from the copyright holder.
Copyright 1996 Gale, Cengage Learning. All rights reserved.

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Title Annotation:natural gas engines
Author:Reber, Joel
Publication:Pipeline & Gas Journal
Date:Oct 1, 1996
Words:1499
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