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Introduction To DieselEngine Analysis
ED KELLEHER, DYNALCO CONTROLS
This paper provides a brief introduction andexplanation of diesel engine analysis. Engine analysisconsists of data collection, analysis and trending ofengine cylinder peak firing pressures and cylindervibration and ultrasonic signatures. Engine analysis can beachieved by anyone with a basic understanding of theoperation of an engine and proper analysis training.
BACKGROUND
For many years, combustion analysis has been performedon diesel engines using a mechanical indicator device suchas a Bacarach indicator or by monitoring exhaust tempera-tures to spot potential problems. Engine analysis is muchmore extensive than combustion analysis. In addition tocombustion monitoring, engine analysis also involvesmonitoring or measuring the actual mechanical condition ofengine components, much like a doctor does during a routinephysical.
A typical owner/operator of a slow or medium speed dieselengine will perform periodic maintenance based on the enginemanufacturer’s recommended time-based intervals. Althoughtime-based maintenance schedules may be easy to follow,they are not the most cost effective or reliable methods. Mostmechanics report seeing parts replaced that looked “likenew.” Owners/operators have also seen premature compo-nent failures due to the maintenance activity itself or haveengines that just don’t seem to run “right.”
Computer-based, easy-to-use, portable engine analyzersare available today in many forms. The engine analyzerdiscussed here is a multichannel, simultaneous, real-time,portable system that collects cylinder pressures and cylindervibration and ultrasonic signatures. The DIESEL-TRAP9240™ system is the newest analyzer introduced by acompany that has been performing engine analysis for over30 years.
ENGINE CYLINDER PRESSURE BALANCE
Diesel engine performance is affected by many parameters:air inlet temperatures and pressures, fuel pressure andtemperature, and the condition and characteristics of acylinder. The stack-up of tolerances alone within a cylindercan account for an imbalance of engine pressures. Measuringdynamic power cylinder pressures enables us to evaluatecompression, peak and scavenging pressure. With knowl-edge of engine geometry, indicated horsepower and indicatedmean effective pressure is easily calculated, stored andtrended.
Cylinder peak firing pressure is a key indicator of engineperformance and mechanical condition. Historically, cylinderbalance was measured or evaluated through cylinder exhaust
temperatures. Although this method is very easy, it is not anaccurate method of determining pressure balance. Peakpressure is affected by the quality of combustion and theangle at which the peak pressure occurs. Exhaust temperaturemeasurements cannot accurately reflect these combustioncharacteristics. By measuring the peak pressure, the angle atwhich it occurs and the pressure through out the entire cycle,a more accurate assessment of balance is achieved. Adjust-ments or repairs, such as valve timing or injection duration,are then based on the actual performance of that cylinder.
ENGINE MECHANICAL CONDITION
Monitoring the mechanical condition of cylinder powerpack components is similar to the use of a screwdriver orrubber hose to determine the location of an abnormal noiseon machinery. The vibration and ultrasonic energy of enginecomponents are captured with an accelerometer referenced tothe crankshaft. The resultant waveform discloses informationabout abnormal events and their severity and can be storedand trended for future reference. This ultimately means thatyou can assess the mechanical condition of individualcylinder components, and predict and prevent failures beforethey occur. Additionally, routine maintenance is performedonly on the components that require it. Component conditionand degradation can be identified for items such as:
Pistons Rings InjectorsPorts Bearings ValvesCamshafts Liners CrankshaftsConnecting Rods Rocker Arms Lifters
The DIESEL-TRAP 9240 is the only 4-channelinstrument that collects data simultaneously and inreal-time.
EXAMPLES OF ENGINE ANALYSIS DATA
This is the signature pattern of atwo-stroke engine just prior to anengine overhaul. The signature isnormal with no “unexpected”mechanical noise.
This is the signature pattern of thesame engine just after theoverhaul. On the power strokethere is a clear, unexpectedmechanical pattern. Uponinspection it was discovered thatthe top compression ring wasbroken. Thanks to quickidentification, no liner or otherengine damage occurred.
These two signatures are beforeand after a repair to a two-strokeengine. The signature on the leftclearly indicates blow by on boththe power and compressionstrokes (circled). The liner for thiscylinder was “washed out” in theareas of the blow by. Scheduledmaintenance activities had beenredirected to this engine from anengine that was operatingnormally.
-119.0 Exhaust 103.0 -135.0 Intake Ports 135.0
8.0 Fuel -8.0
0
500
1000
1500
-180 -135 -90 -45 0 45 90 135 180
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- Scale 4.0
P4L VT4
EMD-20 - E Cyl inder P4L 3 /20/97 12:10:55 PM Period 2
Pre
ssure
(psi
g)
Angle (deg)
-119.0 Exhaust 103.0 -135.0 Intake Ports 135.0
8.0 Fuel -8.0
0
500
1000
1500
-180 -135 -90 -45 0 45 90 135 180
P4L VT4
Scale 4.0
EMD-20 - E Cylinder P4L 1/26/96 6:47:18 AM Period 1
Pre
ssure
(psi
g)
Angle (deg)
These vibration signatures arefrom eight cylinders on oneengine. Cylinder #3 shows anabnormal event around bottomdead center (circled). Since thispattern occurred at bottom deadcenter, a bearing problem wassuspected. Upon inspection, thisbearing was found to be out oftolerance. Early detection of thisproblem may have prevented acatastrophic failure of the engine.
This cylinder signature shows thepressure, vibration and ultrasonicpatterns for a normal four-strokeengine pattern. Compare thevibration on this cylinder with theone in the figure below.
This cylinder signature is from thesame engine as above andclearly shows unexpected andabnormal mechanical spikesduring the compression strokeand also at top dead center. Inthis instance the engineperformance has not yet beenaffected. However, the magnitudeand location of these eventsrequire further investigation.
-160.0 Intake 320.0 -320.0 Exhaust 130.0
5.0 Fuel -10.0
0
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100
150
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-360 -315 -270 -225 -180 -135 -90 -45 0 45 90 135 180 225 270 315 360
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- Scale 2.0
- Scale 2.0
- Scale 8.0
- Scale 8.0
4F ULT
4 ULT
4F VT4
4 VT4
PME-F Cylinder 4 3/28/99 9:12:47 PM Period 3
Pre
ssur
e (b
arg)
Angle (deg)
-160.0 Intake 320.0 -320.0 Exhaust 130.0
5.0 Fuel -10.0
0
50
100
150
200
-360 -315 -270 -225 -180 -135 -90 -45 0 45 90 135 180 225 270 315 360
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- Scale 2.0
- Scale 2.0
- Scale 8.0
- Scale 8.0
9F ULT
9 ULT
9F VT4
9 VT4
PME-F Cylinder 9 3/28/99 9:12:47 PM Period 3
Pre
ssur
e (b
arg)
Angle (deg)
SUMMARY AND COST BENEFITS
A properly implemented predictive maintenance programwill result in both immediate and long-term cost savings. Peakfiring pressure balance alone will result in 2–5% fuel savingsby simply optimizing engine performance. Additionally, 60–70% of mechanical degradation can be attributed to cylinderpressure imbalance.
Avoiding one major valve failure, which can run intohundreds of thousands of dollars, can pay for many years ofa condition-based maintenance program. Engine analysis willlimit maintenance to only the components that require itand avoid unnecessary part replacements while maintainingand improving engine reliability and performance.
Engine analysis has been successfully implemented inmany areas to the extent that no periodic or time-basedmaintenance is performed. All maintenance and overhaulsare based solely on engine performance and condition.
FREQUENTLY ASKED QUESTIONS
How easy is the system to use?
Our system is used by mechanics with a high schooldiploma and by engineers with advanced degrees. With theproper training, anyone with basic knowledge of a computerand an engine can operate this equipment .
I have been told that vibration data cannot becollected on a reciprocating engine.
Absolutely not true! In fact, over half the benefits ofengine signature analysis are lost if vibration analysis isn’tused. The people who promulgate this misconception aretalking about spectral analysis. As applied to the engine,spectral analysis is only appropriate for analyzing theturbochargers, pumps and main bearings. Phased markedvibration signature analysis—which is the cornerstone of oursignature analysis program—focuses on timing events withinthe engine. These methods have been developed and provedover a period of twenty years.
What is the cost of an engine analyzer?
A comprehensive engine analyzer will cost about $50,000.Basic hardware and software training would be included.However for an engine analysis program to be successfulimmediately, some level of analysis training is required.Customized integrated maintenance programs and engineer-ing support can range from $10,000 to $50,000.
The DIESEL-TRAP is cost effective. Almost all usersreport a payback period of less than a year. Many report apayback period of a few months. The price reflects the factthat the engine analyzer is a durable and precise instrument,which can survive years of daily use in an industrial environ-ment. The first time the system identifies a major fault—before actual failure—or eliminates a major overhaul, it haspaid for itself.
When this cost is compared to the cost of a failed engine/major engine component or the cost of a typical engineoverhaul or even the loss of a vessel’s ability to producerevenue, the cost of the analyzer is insignificant.
What is the value of analysis to an engineer aboarda vessel or in a power plant?
Engine analysis provides engine fundamental informationthat leads to reduced operating costs and improved enginereliability and availability.
Dynalco Controls also manufactures ABS type-approved instruments and meters that are appropriatefor marine applications. To obtain more information ondiesel engine analysis and engine diagnostic, controland monitoring products, please contact:
Dynalco Controls Sales Department
3690 N.W. 53rd StreetFort Lauderdale, FL 33309 U.S.A.Tel (954) 739-4300 Ext 550Toll Free (800) 368-6666 (U.S.A. only)Fax (954) 486-4968E-mail: [email protected]
The DIESEL-TRAP 9240, 9220 and 9220/Pportable diesel engine performanceand vibration analyzers (in con-junction with Dynalco’s RTwin™analysis software) have receivedType Approval from the AmericanBureau of Shipping (ABS).
