Balance – The Thinking Has Changed
By Charles G. Ely II, PE and Randy L. Anderson
Anderson Consulting, Training & Testing
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
Practical factors affecting cylinder-to-cylinder
Mechanical construction - stroke length, head and
piston heights, gasket and ring size, camshaft profile,
wave harmonics, etc.
Engine and component condition - worn rings, weak lifters,
leaking fuel valves, spark plug gap wear, ignition coil
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
Control peak combustion pressures – assuring
safe operation within manufacturer’s specifications
Proportionately distribute the horsepower load across
the power cylinders – minimizing unbalanced crankshaft
Reduce misfires and cycle-to-cycle variability - minimizing
Minimize excessive stresses on engine components created
by high peak firing pressures and detonation –
reliability and availability
Control combustion temperatures – stabilizing
Generic Balance Technique
The engine balance process can be divided into four simple
Direct pressure readings are taken for each power
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).
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
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
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
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) ?
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
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
P = Mean Effective Pressure (MEP)
L = Piston stroke in feet
A = Area of piston in square inches
N = Number of power strokes per minute
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
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
What Controls MEP?
The first rule of thermodynamics generally
Net Work Output = Net Heat Input When applied to gas compressor
applications and reordered, this rule can generally be restated
HP (Engine) =
Indicated HP (Compressor) + Parasitic HP + Mechanical Loss
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:
c1 (Indicated Compressor HP) + c2 where c1 = 1/0.95 and
c2 = Parasitic HP
Then removing the constant values c1 and
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
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
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
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
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
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
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
safely by controlling peak combustion pressures.
– Balancing by MEP does not assure that peak
firing ressures will not exceed the manufacturer’s
rating. This can be avoided by using analyzer equipment
that also measures PFP.
– Balancing by PFP inherently
allows the operator to control peak pressures.
unbalanced torsion forces applied to the crankshaft
by proportionally distributing horsepower
across the power cylinders.
– assuming that mathematical corrections are
made to account for differences in stroke length and
cylinder liner geometry.
– this technique errs to the
safe side by limiting peak firing pressures. Note:
Balanced horsepower distribution is not guaranteed
fuel consumption by
minimizing misfires and cycle-to-cycle variability.
– In fact, in order to reduce a cylinder’s
MEP, fuel gas modulator
adjustment frequently result in increased
– Most PFP adjustments can be
made without creating misfires.
reliability and availability by reducing excessive
stresses on engine
components created by high peak firing pressures and
– Since MEP is constant over a wide range of
excessive peak firing pressures can and do occur.
This can be minimized by also
– By directly controlling PFP, the
maximum forces created are controlled
within the safe operating limits of the
with exhaust emission standards by controlling combustion
– Unless misfires or detonation occur MEP is
relatively independent of
temperature and therefore unable to
– Combustion temperature is
directly related to PFP. As such, NOx
exhaust emissions can be controlled by
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
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
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
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
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
Check the mechanical condition of the injection, intake,
and/or exhaust valves to insure proper clearances.
Inspect the rocker arm bushing for wear and repair
Load the engine as near as possible to 100% speed and
torque. If an engine begins to detonate, unload the
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.
If automatic unloaders are used, lock into manual so
load changes don’t occur while balancing.
Ensure the engine and compressors are up to normal
On engines with mixture controls, check the set points
and calibrate to known performance grids.
Ensure the balancing device is in good operating condition
Record the following information (where available):
Fuel Manifold Press
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
Analyze the pressure data and determine the average
peak firing pressure for each cylinder and the overall
engine average peak firing pressure.
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.
After making any adjustment wait at least 20 minutes
to allow cylinder conditions to stabilize.
Retake the peak firing pressure readings on all cylinders
(not just the cylinders that were adjusted).
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.
Evaluate the balance data by looking for high cycle-to-cycle
pressure deviations on a given cycle to identify problem
Clean and put away the balancing instrument.
Balance the engine once a week, anytime maintenance
is performed, or when significant changes in operations
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,
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.
Potential Equipment Cause
Extremely low peak firing pressures
(similar to running compression
Fuel injector problem, faulty ignition components,
worn spark plugs, collapsed lifters, or fuel modulator
Sporadic low peak firing pressures
Fuel injector problem, faulty ignition components,
worn spark plugs, or collapsed lifters
Extremely high peak firing pressures
Detonation or Pre-Ignition
Typically caused by misfires on other cylinders
spots in the power cylinder
Erratic peak pressure / high pressure
Unstable exhaust emissions
(NOx and CO)
Air/fuel ratio, PCC check valves, leaking fuel
improper balance, or defective ignition drive
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
1. Accurate peak firing pressure measurements
2. Calculated cylinder and engine average peak firing
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
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,
Special thanks to Ken Gilbert for his critical and objective
assistance in the developing and proving the fundamental
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