Engine Balance – The Thinking Has Changed
By Charles G. Ely II, PE and Randy L. Anderson
Anderson Consulting, Training & Testing

Introduction

Balanced combustion on reciprocating engines is important for reliable, emission-compliant operation. Through the years, a number of engine balance techniques have been developed and utilized. Unfortunately, no clear-cut standard has been established for engine balance methodology or frequency. This paper will discuss the fundamentals of engine balance, describe the history of balancing, and present the most current thinking on proper engine balance technique, equipment, and frequency.

Fundamentals of Engine Balance

What is engine balance?

In gas compression applications, reciprocating engines are designed to proportionately distribute the compressor and auxiliary loads between the engine’s power cylinders.

Unfortunately, a number of factors introduce variability into the cylinder-to-cylinder and cycle-to-cycle combustion process.

Practical factors affecting cylinder-to-cylinder variability include:

• Mechanical construction - stroke length, head and piston heights, gasket and ring size, camshaft profile, fuel manifold
  wave harmonics, etc.

• Engine and component condition - worn rings, weak lifters, leaking fuel valves, spark plug gap wear, ignition coil
  degradation, etc.

• Combustion controls – air/fuel ratio, ignition timing, pcc pressure, engine cooling, etc.

To help reduce cylinder-to-cylinder variability, engine operators install fuel balancing valves, hereafter referred to as a fuel gas modulator valves, in the fuel manifold piping upstream of the cylinder’s fuel injection valve. Using these valves, the operator affects combustion by adjusting the fuel delivery to a given cylinder (thus affecting the cylinder’s air/fuel ratio and burn characteristics).

By properly adjusting each cylinder’s fuel gas modulator valve, the combustion in each cylinder can be stabilized and controlled for good engine balance.

Why should I balance my engine?

There are five reasons to perform an engine balance.

1. Control peak combustion pressures – assuring safe operation within manufacturer’s specifications

2. Proportionately distribute the horsepower load across the power cylinders – minimizing unbalanced crankshaft torsion forces

3. Reduce misfires and cycle-to-cycle variability - minimizing fuel consumption

4. Minimize excessive stresses on engine components created by high peak firing pressures and detonation – maximizing
   reliability and availability

5. Control combustion temperatures – stabilizing exhaust emissions

Generic Balance Technique

The engine balance process can be divided into four simple steps.

1. Direct pressure readings are taken for each power cylinder.

2. The pressure readings are analyzed and compared to determine which cylinders are firing high and which cylinders are firing low (compared to the average pressure).

3. Using this information, the fuel gas modulator valves are adjusted to make the high pressures lower and/or the low pressures higher (see example below for more details).

4. After the adjustments are made, pressure readings are retaken on all cylinders to assure that the pressures are balanced within the balance criteria. If the balance is not achieved, Steps 2, 3, and 4 are repeated.

Engine Balance Example

Imagine that the fuel system of an engine is a line of sprinklers in a sprinkler system (Figure 1). The system has one control valve (the governor) and six adjustable sprinkler heads (modulator valves).

The yard needs 10 gallons of water per minute (or so many BTU’s of fuel to generate the required horsepower) and the water should be evenly distributed to the grass around each sprinkler head (or the same pressures in every cylinder).

To do this, the individual sprinkler heads (or modulator valves) are adjusted. By pinching valves #2 and #5 on sprinklers (modulator valves) the spray will be lower. If the flow through the control valve (governor) stays constant, then sprinklers 1, 3, 4, & 6 would increase. Why? To get the same flow through a smaller orifice requires a higher pressure.

Alternately, opening #4 sprinkler head would decrease the height of 1, 2, 3, 5 & 6. By continuing to adjust the individual sprinkler heads, the spray on all the sprinklers will be the same height. The same is true of a fuel system on an engine.

History of Balance Equipment and Methodologies

The key to the balance technique is choosing the “best” pressure. There are two thoughts related to choosing the “best” pressure. They are:

Mean Effective Pressure (Horsepower) ?
or
Peak Firing Pressure ?

Controversy has surrounded engine balance in gas compression service from the beginning. Two issues lead to this controversy.

First, some engine manufacturers recommended against engine balance for years. Concerns about unqualified personnel changing the air/fuel ratio weighed heavily on there minds. Secondly, many of the early instruments used for engine balance came from diesel applications. However, as balancing tools and training improved so did the practice of engine balance.

The history of the engine balance controversy was strongly influenced by technological advances in pressure indicating equipment. The BMEP gaugeTM and Pi meterTM (mechanical devices used to measure Mean Effective Pressure) and the BacarachTM (Figure 2) and MaihakTM (mechanical devices used to measure Peak Pressure) were initially used on diesel engines.

With the invention of electronic pressure indicators a clearer picture was drawn inside the power cylinder. Electronic tools include the Beta-TrapTM (Figure 3) and Enspec 1000TM (peak pressure indicators), GET 1000TM (maintenance analyzer which graphically displays pressure-time and pressure-volume patterns) and Recip-TrapTM, WindrockTM, PFMTM and CarmaTM (performance analyzers with both mean effective calculations and peak firing pressure traces and analysis).

Interestingly, in diesel engine applications, both mean effective and peak firing pressure readings are good balance indicators due to the stability and repeatability of combustion. The diesel is a compression-ignited (CI) internal combustion engine that uses the heat from highly compressed air to ignite a spray of fuel introduced after the start of the compression stroke. Extremely consistent peak firing and mean effective pressures characterize compression ignition. As such, balancing by either pressure
generates similar engine balance results.

Conversely, a spark-ignited engine (SI) propagates the flame from one or two points across the cylinder. The peak firing and mean effective pressures are very susceptible to flame front velocity changes due to poor homogenization. Adjustments to the fuel gas modulators don’t usually result in proportionally similar changes to peak firing and mean effective pressures. As such, balancing by mean effective pressure and peak firing pressure generate different engine balance results. Thus leading to the
question of which pressure is the “best” or “right” one to use for engine balance.

Mean Effective Pressure (Horsepower)

One horsepower is the energy required to lift 33,000 lbs of weight one foot in one minute.

It is numerically expressed using the formula:

Horsepower = P L A N / 33,000

Once installed in an engine, the piston stroke and area are fixed. Leaving horsepower as a function of mean effective pressure (hereafter referred to as MEP) and engine speed only.

MEP is defined as the average (mean) theoretical pressure throughout the power stroke. It is measured primarily and most accurately with some type of performance analyzer. Figures 4 & 5 are pressure-volume traces of 2-stroke and 4-stroke cycle engines.

The area inside each pressure-volume trace determines the MEP of the power cylinder. Imagine that the line representing the pressure volume is a string. To determine the MEP, the odd-shaped power cylinder pressure volume is stretched into the shape of a rectangle from TDC to BDC. The height of that rectangle would approximate the MEP (See Figure 6).

Pressure Volume
MEP is equal to the average
theoretical pressure throughout
the power stroke, represented
by the rectangle stretched from
TDC to BDC on the power stroke.

Engine speed affects horsepower by increasing or decreasing the number of cycles per minute. Higher operating speeds result in
more pressure-volume cycles per minute and consequently more horsepower.

What Controls MEP?

The first rule of thermodynamics generally states:
Net Work Output = Net Heat Input When applied to gas compressor applications and reordered, this rule can generally be restated as:

Brake HP (Engine) =
Indicated HP (Compressor) + Parasitic HP + Mechanical Loss HP

As a rule of thumb, the gas industry roughly approximates the lost mechanical horsepower as 5% of the indicated compressor horsepower. The parasitic load (auxiliary pumps, blowers, fans, etc) is typically fixed at a given engine speed. As such, it can be said that:

Brake HP =
c1 (Indicated Compressor HP) + c2 where c1 = 1/0.95 and c2 = Parasitic HP

Then removing the constant values c1 and c2:
Brake HP (engine) µ Indicated HP (compressor)

Then, substituting the PLAN/33,000 formula for Indicated Engine HP, assuming a fixed piston stroke and piston area, and lumping all constants together, the formula is further simplified to state:

Mean Effective Pressure x Speed (engine) µ Indicated HP (compressor) Indicated horsepower for the compressor is also calculated with the PLAN/33,000 formula (using compressor dimensions and pressures).

In most cases, the compressor speed is equal to the engine speed thus leaving the simplest form of the equation:

Mean Effective Pressure (engine) µ Mean Effective Pressure (compressor)
Answering the question of “How is MEP controlled?” is now quite simple.

The engine’s average MEP is controlled exclusively by the average compressor MEP (or compressor load). With a fixed speed, the average compressor MEP (compressor load) is affected operational by changing clearance volume (bottles or pockets), valve spacers, lifters, or unloaders, or by varying suction and/or discharge pressure. Engine operating variables such as air/fuel ratio, ignition timing and air manifold temperature don’t change compressor load – and subsequently don’t change the engine’s average MEP.

Can Air/Fuel Ratio Affect MEP?

Overall adjustments to the air/fuel ratio controller won’t change the engine’s average MEP. However, these changes will affect the combustion properties within the power cylinder. Assuming a “normal” air/fuel mixture, leaning or richening the mixture will affect the burn rate (flame front velocity), peak firing pressures, and temperatures. In general, the average pressure-volume graph for the power cylinders will change in shape – but remain equal in terms of area (or MEP). The pressure-volume chart and table in Figure 4 show the effects of changing the engine’s overall air/fuel mixture.

If the mixture is further richened, the high peak firing pressures and advanced firing angle will result in unstable combustion and eventually lead to cylinder detonation. On the other hand, as the mixture is further leaned, partial and complete misfires will begin to occur. In both cases, severe detonation and misfires will reduce the cylinder’s MEP.

When spark plugs start to misfire or the air/fuel mixture doesn’t burn properly, engine performance decreases and fuel consumption increases. Most experts agree that a typical two-cycle spark-ignited engine misfires approximately 25% of the time.

A misfire is incomplete combustion as opposed to a dead or laying out cylinder that fails to burn. Some misfires look very similar to a lean mixture (low peaks with high exhaust pressures) but are able to maintain MEP. Some misfires are so severe that peak firing pressure is lower than running compression also resulting in a lower MEP.

Severe detonation or misfires that results in lower MEP for a given cylinder will consequently deliver less horsepower to the crankshaft. Assuming the compressor load has not changed, the engine will begin to slow down. In turn, the governor will open up and delivery more fuel to all cylinders. With the increased fuel, the good cylinders will pick up load (increase MEP) for the
cylinder where MEP decreased. The pressure-volume chart and table in Figure 8 show the effects of detonation and misfires on a cylinder’s MEP.

Can an Individual Cylinder’s MEP Change by Adjusting the Fuel Modulator Valve?

During the engine balance process, adjustments to individual fuel gas modulator valves change the air/fuel ratio within a given power cylinder. Generally MEP is not very responsive to small changes in air/fuel ratio. To significantly change the cylinder’s MEP, the fuel gas modulator valve must be adjusted until the cylinder begins to misfire or detonate. In this case, MEP is changed by adversely impacting the cylinder’s combustion characteristics.

Peak Firing Pressure

Peak Firing Pressure (hereafter referred to as PFP) is much simpler to describe. It is the highest pressure in the power cylinder due to combustion (See Figures 4 & 5). This pressure is easily measured with mechanical and electronic devices.

In addition to PFP, another useful measurement is Peak Firing Angle. Peak firing angle represents the crank position when peak firing pressure is achieved. The location of the peak firing angle has a dramatic impact on the transfer of energy from the piston to the crankshaft. When peak firing pressure occurs too close to TDC, some of the combustion occurs before TDC - resulting in wasted energy and excessive wear on bearing surfaces.

What Affects PFP?

Unlike MEP, Peak Firing Pressure is affected by most changes to the engine and compressor. The primary variables affecting PFP include compressor load, engine speed, air/fuel ratio, ignition timing, fuel BTU, and mechanical condition.

How Does Air/Fuel Ratio Affect PFP?

As described in the MEP section, air/fuel ratio affects the burn rate and combustibility of the mixture. In richer air/fuel mixtures, the rate of burn is accelerated causing the burn to occur more quickly. The faster burn occurs closer to TDC and generally results in higher peak firing pressures (Review Figure 7). Conversely, a lean air/fuel mixture slows the burn and results in later and lower peak firing pressures.

Comparison of Balance Techniques

With a basic understanding of Mean Effective Pressure (MEP) and Peak Firing Pressure (PFP), a logical comparison can be made to determine the “best” pressure for balancing. Comparing each balance technique against the desired objectives results in the following table:

Which Pressure is “Best”?

Peak Firing Pressure is clearly the best choice based on the engine balance objectives. For every objective, except horsepower distribution, peak firing pressure balance control is superior. And, for the objective of distributing horsepower along the crankshaft, the peak firing pressure technique errs to the safe side by limiting combustion pressures within the manufacturer’s
safety guidelines. In addition, PFP is much more responsive to modulator valve adjustments than MEP in the stable combustion range.

When is the Engine Considered Balanced?

The most widely accepted technique for measuring engine balance is to compare each individual cylinder’s average peak firing pressure against the average peak firing pressure of the entire engine. The largest individual cylinder pressure difference from the average is divided by the engine average. The resulting number is the “balance percent”. Most companies agree that 5% is a good target for balance criteria.

Interestingly a number of studies have shown that tighter balance standards (i.e. 4%, 3%, etc.) have negligible benefits to engine performance and efficiency.

How Often Should an Engine Be Balanced?

Engines should be balanced every time maintenance is performed, after any change in operating condition (load, weather conditions, etc.), or on a periodic basis (weekly). This is important because engine balance directly impacts reliability, fuel cost, emissions, and maintenance cost.

Note: The balance frequency often varies by engine, but weekly is a good rule of thumb – especially if balance is also used as a maintenance and troubleshooting tool.

When Should Engines Be Balanced?

Anytime of the day is acceptable, but to protect the engine from detonation, balancing in the heat of the day is preferable (except to the person balancing the unit). If the engine is balanced during the heat of the day, when ambient temperatures are the highest, the peaks will be at the highest level. But if the engine is balanced in the cool of the morning, the peaks will rise as ambient temperatures increase – potential creating unacceptably high peaks. .

Basic Engine Balance Procedure Regardless of the instrument used to balance engines, the principles are the same. Below is a basic engine balance procdure.

Note: Refinements to this procedure should be made to conform to the operating instruction of the specific balancing instrument.

Preliminary Tasks

1. Check ignition timing and ensure it is correct. If possible, have someone with an ignition analyzer inspect the condition of the primary and secondary of the ignition system.

2. Check the mechanical condition of the injection, intake, and/or exhaust valves to insure proper clearances.

3. Inspect the rocker arm bushing for wear and repair as needed.

4. Load the engine as near as possible to 100% speed and torque. If an engine begins to detonate, unload the engine.
   NEVER PERMIT AN ENGINE TO DETONATE!!!!! Detonation is usually caused by one or more cylinders not carrying their share of the load. It is also important to remember detonation is the symptom of the problem. The real problem is somewhere else.

5. If automatic unloaders are used, lock into manual so load changes don’t occur while balancing.

6. Ensure the engine and compressors are up to normal operating temperatures.

7. On engines with mixture controls, check the set points and calibrate to known performance grids.

8. Ensure the balancing device is in good operating condition and calibrated.

9. Record the following information (where available):

   1. Engine Number
   2. Suction Press
   3. Discharge Press
   4. Pocket Setting
   5. Horsepower
   6. Engine Speed
   7. Eng Oil Press
   8. Eng Oil Temp
   9. Exhaust Temps
   10. Engine Water Temp
   11. Ignition Timing
   12. Air Manifold Temp
   13. Date & Time
   14. Ambient Temp
   15. Name of Person
   16. Air Manifold Press
   17. Fuel Manifold Press
   18. Turbo Bypass Position

Balance Procedure

1. Collect peak firing pressures for each individual cylinder. If a mechanical indicator is being used, collect a minimum of 10 peak firing pressures in a readable manner. The greater the number of peaks collected, the better the sample (one of the advantages of electronic engine balancers).

2. Analyze the pressure data and determine the average peak firing pressure for each cylinder and the overall engine average peak firing pressure.

3. Note the high and low cylinders. Begin adjusting the individual fuel valves by opening low cylinders or closing high cylinders. Determine whether to open or close the fuel gas modulator valve based on governor position. Pinching the modulator valves causes the governor to open; opening the modulator valves causes the governor to close. Governor position should be approximately 75-80% open. This permits enough range for the governor to compensate for load
   changes without giving it enough range to compensate for major problems such as dead cylinders.

4. After making any adjustment wait at least 20 minutes to allow cylinder conditions to stabilize.

5. Retake the peak firing pressure readings on all cylinders (not just the cylinders that were adjusted).

6. Calculate the engine balance percent and compare it against the desired balance criteria. If the balance criteria is not met, steps 3, 4, and 5 should be repeated until the balance is achieved.

7. Evaluate the balance data by looking for high cycle-to-cycle pressure deviations on a given cycle to identify problem cylinders.

8. Clean and put away the balancing instrument.

9. Balance the engine once a week, anytime maintenance is performed, or when significant changes in operations occur.

Engine Balance As a Troubleshooting Tool

The more sophisticated, electronic balance equipment provides local operating and maintenance personnel with an effective troubleshooting and predictive maintenance tool. Problems such as dead cylinders, detonation, excessive misfires, leaky fuel
injectors, worn spark plugs, and failing PCC check valves can be identified by carefully reviewing the engine balance data. The following table identifies some of the common problems that may be identified through engine balance.

Conclusions

This paper covers a lot of ground in an effort to fairly and objectively evaluate the engine balance process. As a result of that discussion, the following conclusions and recommendations were made:

• Engine balance is “best” achieved using Peak Firing Pressure.

• Ideal engine balance equipment would have the following attributes:
  1. Accurate peak firing pressure measurements
  2. Calculated cylinder and engine average peak firing pressures
  3. Calculated pressure deviations for each cylinder
  4. Measured peak firing angle (typically only available on performance analyzers)
  5. Capability to measure performance data over at least 32 cycles per cylinder

• General guidelines for engine balance frequency are:
  1. Each time maintenance is performed,
  2. After any change in operating conditions, and
  3. At least once per week.

• A good rule of thumb for balance criteria is to assure that the average peak firing pressure for each cylinder is within 5% of the engine’s average peak firing pressure.

References

Heywood, J.B.: Internal Combustion Engine Fundamentals, McGraw-Hill, New York, 1988.

Jones, J.B. and Hawkins, G.A.: Engineering Thermodynamics – An Introductory Textbook (2nd Edition), John Wiley & Sons, Inc., New York, 1986.

Wylen, G.J.V. and Sonntag, R.J.: Fundamentals of Classical Thermodynamics (2nd Edition), John Wiley & Sons, Inc., New York, 1973.

Anderson, E.P.: Gas Engine Manual (Revised by Charles G. Fackman), Theodore Audel & Co., Boston, 1985.

John Deere Service Training: Fundamentals of Service: Engines (6th Edition), Deere & Company, Moline, IL, 1986.

Acknowledgements

Special thanks to Ken Gilbert for his critical and objective assistance in the developing and proving the fundamental concepts of
engine balance.

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