Over the years, it has generally been thought that if an engine had severe detonation, the unit had an extra rich fuel-to-air mixture. On start-up of a turbocharged engine, the ignition timing is retarded to force the peak firing pressure later in the combustion stroke. When the burned gases exit the exhaust ports the pressure is higher and hotter. This extra heat is what drives the turbocharger to make the proper air manifold pressure faster. The goal here is to discuss what peak firing pressures look like when the ignition timing is retarded on startup. Relatedly, it will be useful to explain the differences in detonation and pre-ignition, and how pre-detonation can impact engine operating efficiency.

Typical detonation

Detonation usually occurs when the air/fuel ratio is on the rich side of stiochiometric. This can happen when the engine is in an overload condition, or if there is a mechanical problem and all the power cylinders are not firing properly. Just before top dead center (TDC) the spark plug fires to start combustion. With a rich air/fuel mixture this starts burning very rapidly and the peak firing pressure will be higher than normal and also closer to TDC. The very rapid rate of pressure rise of the uncontrolled burning mixture causes detonation.

This excites the natural frequency of the combustion chamber. The cylinder resonates creating pulsations in the pressure trace as the pressure trace bounces back and forth. At times this detonation will create so much heat that the air/fuel mixture can auto-ignite. Auto-Ignition is the temperature at which a flammable mixture of a gas and air will spontaneous combust. Table 1 shows the Lower Heating Value and Auto-Ignition Temperatures of the main components of natural gas. This very rapid explosion of the air/fuel mixture causes very high vibrations which in turn damages all the power cylinder components; i.e., heads,
pistons and cylinders. The vibration trace indicates a very loud auditable noise from the rapid pressure rise just after TDC. Figure 1 shows a classic detonation pressure pattern with high vibration at combustion.

Typical pre-ignition

Pre-ignition is sometimes called the silent killer. As the piston rises in the cylinder, compressing the air/fuel mixture, something in the cylinder lights the mixture early before the spark plug fires. In some cases, the pressure will start rising as soon as the piston covers up the exhaust ports. As the piston continues to rise to TDC the pressure rise in the cylinder is tremendous. The pressure at TDC is normally compression pressure in most turbocharged 2-cycle engines; this can be between 280 to 440 psi. When pre-ignition occurs, this pressure at TDC can reach 1600 to 2000 psig. or even higher. This scenario is usually silent because all the fuel is burned before the piston reaches TDC and even though it is burning early it is a controlled burn with no explosion.

Case study one

One pre-detonation case occurred with an engine that had been installed in 1979, a 4,000-hp, two-cycle, non-clean burn unit, with normal ignition timing set at 3° before top dead center (BTDC). This unit has always had severe detonation, from the time the engine started idling until the by-pass valve closed and the engine went to full load and speed. At this point the engine would run and sound normal.

In trouble-shooting the problem, the following observations were made. Upon engine startup, what appeared to be severe detonation was coming from the power cylinders. The first step was to connect the analyzer up so as to examine the firing pressures and to make sure that the ignition timing was retarding to -22° after to dead center (ATDC). To trigger the analyzer as the unit started, it was connected to the once per turn magnetic pickup and used the 70-ft cable was used so that there would not be a delay waiting on the wireless signal. Figure 2 shows the number #2R power cylinder pressure and ignition timing; this cylinder was firing normal with the ignition timing at -22° ATDC.

On power cylinder #3R, severe detonation was heard, and Figure 3 now shows that the peak firing pressure looks like detonation but it is lighting off 22° before the spark plug fires. Ten pressure traces are shown and three of them are lighting early. We said earlier that if the air/fuel mixture lights before the spark plug ignited it, then the cylinder had pre-ignition. But when we look at Figure 4 with the vibration trace, it shows very heavy vibration as the pressure rises at TDC. With typical pre-ignition, there is no vibration at this point. These traces are showing pre-ignition with all the characteristics of detonation. This type of combustion is what is common on a dual fuel or diesel engine. As the piston compresses the air/fuel mixture, the temperature of compression exceeds the auto-ignition temperature of something in the natural gas mixture.

Fixing the problem

Detonation, pre-ignition, and pre-detonation are all different and they have to be treated differently. To fix a detonation problem, typically you get more air into the cylinder and make a leaner mixture. Pre-ignition is usually caused by a hot spot in the cylinder, caused from carbon buildup, a scored cylinder, or an extruding spark plug. Pre-detonation is caused when the auto-ignition temperature of the gas mixture lights off due to the temperature of compression. Now that we know this, we need to take a look at our fuel gas delivery system to the engine. This unit is on a fuel gas system that is separate from the other six engines at the station. Figure 5 shows a schematic of the fuel gas system to this engine.

The fuel gas comes into the building at 860 psi at 60°F. The first cut regulator cuts the pressure to 230 psi. At this point, the temperature of the fuel gas drops approximately 44 degrees to 16°F due to the pressure drop. The gas then goes through an engine jacket water heat exchanger, then through the 2nd and 3rd cut regulators. After fuel measurement and the final regulator, the gas goes to the power cylinders. The jacket water heat exchangers are useless when the engine starts because the jacket water is cold. The only place that hydro-carbons can drop out is in the power cylinder. These hydro-carbons are what caused our pre-ignition. Figure 6 shows the changes made to the fuel gas system. Added equipment included: a Kim Hot Start to the jacket water system (so the gas will start warming up at start-up); a coalescing fuel gas filter (to catch any hydro-carbons); and a 12-in. by 6-ft gas volume bottle (to make sure we had adequate gas volume). Also at this time, we removed the governor and added a fuel control valve. In summary, from the time that this equipment was installed until the present, all start-ups have been very smooth and quiet. Also, in the past four years there has not been a single power cylinder, piston or head failure.

Case study two

The second case involved two units that had been originally installed in 1965. In 1995, these units were converted to clean burn with pre-chambers. For years, these two units experienced severe detonation issues on startup. It was thought that the off-mounted turbochargers were not making enough air. After the engine by-pass valve closed and the unit got to full load and speed, the detonation went away. The problem is that it takes 10 to 15 minutes to get the engines warm enough to load and run properly. These units also have a history of premature parts failures, heads, pistons and cylinders. Here again we were trying to fix the loud detonation of the power cylinders, and were not having any luck.

When we looked at the power cylinder firing pressures on start up, that’s when we found our problem. Figure 7 shows the pressure pattern with the ignition timing set at five degrees ATDC. Power cylinder #6 shows the peak firing pressure is lighting off at 10 degrees BTDC, 15 degrees before the spark plug fires. This would be pre-ignition, but the hacksaw pattern of the firing pressure indicates severe detonation characteristics. When this happens, there is excessive vibration in the cylinder. Figure 8 shows #5 power cylinder lighting off early four out of ten cycles. With firing pressures approaching 900 psig, at this point they should only be around 250 to 300 psig. And, instead of the peak firing pressure angle at 20° degrees BTDC, they should be around 45 degrees ATDC. Again, when the mixture lights before the spark plugs do, this is pre-ignition. But if you have detonation characteristics due to auto-combustion of the fuel hydrocarbons, this is pre-detonation.

Final words

Now that we know that the loud noise on startup is not detonation, we can fix the problem. Following severe/preliminary steps, we added a Kim Hot Start on the jacket water and lube oil. First, this allowed us to load the engines after only five minutes, instead of letting this pre-detonation to go on for 25 to 30 minutes. Second, by having the power cylinders and pistons warm at start-up, the thermal growth of the piston and cylinder changed just enough that the fuel components do not auto-ignite. In conclusion, we found that detonation, pre-ignition, and pre-detonation are all different types of combustion. Each has its own characteristics, and each has its on remedy. n

Acknowledgment

Based on a paper presented at the Gas Machinery Conference held in Albuquerque, New Mexico, October 6-8, 2008.

The author

Keith Schafer is Team Leader, Engineering Services/Compression–Columbia Gas Transmission, a Nisource company, based in Charleston, West Virginia.