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August 2022 MMS Newsletter

Balancing Slow Speed Reciprocating Engines

A Note from Steve

We are very pleased to announce our new strategic partnership with Radical Combustion Technologies. We will be working together to incorporate their new technologies into our ProBalance^® and ProBalance^® Plus Systems in order to further improve engine health, reduce engine emissions and improve operating performance for natural gas engines. Our first joint product release will be incorporating their PPR algorithms into our balancing routines. We expect to release this product early in 2023. We will be releasing the additional technologies they are working on as they are fully developed, and field tested. Please read below for a more detailed explanation of the PPR methodology and give us a call if you would like to learn more.

Back to Basics

On July 25th, MMS and RCT jointly presented a paper at this year’s GMRC Engine Analyzer Workshop in San Antonio, TX, the title of which was, Balancing Slow Speed Reciprocating Engines, The Old and the New. The idea of the paper was to introduce, or re-introduce, a method of engine balancing that is rarely used, but may provide significant benefits to the health and longevity of the engines.

First, a bit of history to explain the methodology. In the early days of engine balancing, exhaust temperatures were one of the few aspects of the combustion process that were measurable, so technicians attempted to add or subtract fuel to the individual cylinders such that the exhaust temperatures were equalized. While crude by today’s standards, it was easily measurable and typically relatable to the load of the cylinder. This method is still used on some locations today.

Then came Peak Firing Pressure (PFP) Measurement. This method employs a pressure gauge, much like those used in measuring the compression of an auto engine, that is connected to each individual cylinder and captures the Peak Firing Pressure of the cylinders. The gauge is typically screwed onto a cylinder valve attachment (Kiene Valve), the valve is opened, and the pressure gauge is exposed to several firing cycles (usually 8 to 10). The gauge captures the maximum pressure that it read. That value is documented as the peak pressure for that cylinder and the process is repeated for all the other cylinders. Once the values have been collected for the entire engine, a mean value is calculated and the fuel pressure to each cylinder is modified so that the new peak pressure of each cylinder approaches the mean of the engine. This typically requires many iterations, but that’s how it was done.

Historically, we used the parameters we could measure in the field, but today’s equipment allows us the luxury of many more data points. For PFP balancing, the relationship of cylinder pressure to Top Dead Center (TDC), while useful in diagnostics, is not used in the peak pressure mean calculation, but we can measure it if we choose to. What we find is that at or about TDC, the cylinder pressure, that has been increasing steadily since the exhaust port closed reaches a peak, called the Peak Compression Pressure (Cp). In most slow speed engines, this value will vary from cylinder to cylinder, usually due to unequal airing of the cylinders. This is caused by the individual cylinders’ location, as it relates to the air inlet from the turbocharger or piston scavengers, and the pressure pulses generated by the other power pistons as they allow this air to enter during their individual cycles. There may also be differences in the piston and heads on some of these legacy engines causing differences in Cp’s.

In the combustion world, we ideally want to equalize the Equivalence Ratio. While Air-Fuel Ratio is more frequently used in layman’s terms it is not the same as Equivalence Ratio. Air-Fuel Ratio, or lambda (λ), is a measurement of the mixture as compared to stoichiometric while Equivalence Ratio, or phi (Φ), is the ratio of fuel mass flow rate to air mass flow rate. Further, equalizing the phi’s will result in significantly lower Coefficient of Variations (COV’s) in the engine’s performance.

The cylinder with the lower Cp will not have the same mass or quantity of trapped air as the cylinder with the higher Cp. The two cylinders will therefore require different quantities of fuel to reach an equal phi. The only measurable value to assess the trapped volume of air is the Cp. We know the Cp and we assume the volume is constant between the cylinders, then we can calculate the Peak Pressure Ratio (PPR) as:

PPR = PFP ÷ Cp

Multiplying the Average Engine PPR by the Individual Cylinder Cp’s generates the Target PFP for each cylinder, which is the value to balance to.

Research has proven that utilization of the PPR method reduces NOx, COV’s, and the associated crankshaft stresses induced by the rapid variations in angular velocities imparted by unbalance and misfires.

In the graph to the left, note the differences in Cp’s as well as PFP’s with the PPR’s ranging between 1.13 and 1.20 with an average of 1.18. As you can see, this engine is an ideal candidate for PPR Balancing!

In today’s market, computers and electronic sensors are available to replace the manual methods detailed above. Several vendors and OEM’s offer automatic engine balancing systems, including MMS. No doubt there is a place in the market for these systems, but they’re expensive and frequently remove the technicians from the day-to-day interaction with the engines. Some technicians might say, “That’s the point!”, but the reality is that it’s difficult to replace the eyes and the ears of a conscientious technician, which is why we offer an intermediate system, ProBalance^®.

With ProBalance^®, the cylinder pressures are still constantly monitored and visually depicted on a bar graph on our Balance Control Module typically mounted on the UCP as well as sent to your PLC via MODBUS. This way the engine is balanced when it needs it rather than an arbitrary schedule. The technician carries a HazLoc approved tablet up on the unit, adjusts the fuel valves that need to be adjusted, and watches for changes to the other cylinders as the governor readjusts. What used to be a 1 ½ hour job on a fixed schedule is now a 5-to-10-minute job when it’s needed!

ProBalance Engine Balance Monitor

Training and Development

Kent Petersen will be presenting Engine Balancing during the Resonance User Group Meeting in Pigeon Forge, Tennessee on August 2nd at 2:15pm. Kent will be there to provide balancer education and answer any questions related to balancing or maintaining your compressor health and safety through monitoring.

Contact Us

Fairview Technology Center

11020 Solway School Rd | STE 105

Knoxville, TN 37931