LEGM4736 00(Arranque)
Transcript of LEGM4736 00(Arranque)
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Marine Engine Application
and Installation Guide
Starting Systems
o General Information
o Starter Types
o Starting Aids
o Starting Smoke
Information contained in this publication may be considered 2004 Caterpillarconfidential. Discretion is recommended when distributing. All rights reserved.Materials and specifications are subject to change without notice. Printed in U.S.A.
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General InformationStartability of a diesel engine is affected primarily by ambient temperature, engine jacket
water temperature, and lubricating oil viscosity. Any parasitic loads (usually associated
with the driven equipment) can greatly influence the startability, as well.
The diesel engine relies on heat of compression to ignite fuel. When the engine is cold,longer cranking periods or higher cranking speeds are necessary to develop adequate
ignition temperatures. The drag due to the cold lube oil imposes a great load on the
cranking motor. Oil type and temperature drastically alter viscosity. SAE 30 oil
approaches the consistency of grease below 0C (32F).
Starter TypesThere are three different types of starting systems normally used for Caterpillar diesel
engines. They differ in the method of storing and recharging the energy required for
restarting the engine.
Electric Starting SystemsElectrical systems use chemical energy stored in batteries, automatically recharged by anengine-driven alternator or by an external source.
Air or Pneumatic Starting SystemsPneumatic systems use compressed air in pressure tanks, automatically recharged by an
electric motor-driven air compressor.
Hydraulic Starting SystemsHydraulic systems use hydraulic oil stored in steel pressure vessels under high pressureautomatically recharged by a small engine-driven hydraulic pump with integral pressure
relief valve.
Which One to Choose
The technology of all three of these systems is well developed. Any of the systems are
easily controlled and applied either manually or automatically. Several of the factors that
influence the choice of systems have been tabulated below.
Electric
Battery-powered electric motors utilize low voltage direct current and provide fast,convenient, pushbutton starting with lightweight, compact, engine-mounted components.
A motor contactor relieves control logic circuits of high cranking currents.
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Electric System Considerations
Lead/Acid storage batteries.
Lowest relative cost. Batteries require considerable maintenance. Good starting reliability
Hydrogen gas, released from the batteries during charging, is very explosive;
compartments containing lead acid batteries must be properly vented..
Batteries
Batteries provide power for engine cranking. Lead-acid types are readily available, havehigh output capabilities, and are relatively inexpensive. Nickel-cadmium batteries are
costly, but have long shelf life and require minimum maintenance. Nickel-cadmium types
are designed for long life and may incorporate thick plates that decrease high dischargecapability. Consult the battery supplier for specific recommendations.
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Ambient temperatures drastically affect battery performance and charging efficiencies.
When operating in cold climates, the use of battery heaters is recommended. Theheaters should be set to maintain battery temperature in the range of 32 to 52C
(90 to 125F) for maximum effectiveness. The significance of colder battery
temperatures is described below.
All battery connections must be kept tight and coated with grease to prevent corrosion.
Battery Location and Hydrogen Venting
Install batteries only in well-ventilated compartments. Visual inspection for terminal
corrosion and damage should be easy. Batteries emit hydrogen gas during the recharging
cycle. Hydrogen gas is highly explosive and very dangerous in even small concentrations.Hydrogen gas is lighter than air and will escape harmlessly to atmosphere, if not trapped
by rising into a chamber from which there is no upward path to atmosphere. Devices
which can discharge electrical sparks or cause open flames must not be allowed in thesame compartment or in the vent path for the escaping hydrogen gas.
Battery Disconnect Switches (Battery Isolating Devices)Solid-state electrical devices will suffer when installed in vessels whose electrical systemincludes battery disconnect switches which can interrupt load-bearing circuits. At the
instant of a circuit disconnect, transient currents and voltages will often cause failure in
any component whose transistors are not otherwise protected.
Use Battery Disconnect Switches (Battery Isolating Devices) that do not cause voltagetransients (spikes).
Battery Chargers
Various chargers are available to replenish a battery. Trickle chargers are designed for
continuous service on unloaded batteries and automatically shut down to milliamperecurrent when batteries are fully charged. Overcharging shortens battery life and is
recognized by excessive water losses.
Conventional lead-acid batteries require less than 59.2 mL (2 oz) of make-up water
during 30 hours of operation.
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Float-equalize chargers are more expensive than trickle chargers and are used in
applications demanding maximum battery life. These chargers include line and loadregulation, and current limiting devices that permit continuous loads at rated output.
Chargers must be capable of limiting peak currents during cranking cycles or have a
relay to disconnect during cranking cycles. Where engine-driven alternators and battery
chargers are both used, the disconnect relay is usually controlled to disconnect the batterycharger during engine cranking and running.
Engine-driven generators or alternators can be used but have the disadvantage ofcharging batteries only while the engine runs. Where generator sets are subject to long
idle periods or many short stop-start cycles, insufficient battery capacity could threaten
dependability.
Continuous Cranking Time Limit with Electric Starter Motors
To avoid overheating of the starter motors, an engine should not be cranked continuouslyfor more than 30 seconds.
Starter Motor Cooling Period Between Cranking Periods
Allow the starter motor to cool for two minutes before resuming cranking.
Battery Cable Sizing (Maximum Allowable Resistance)
The start circuit between battery and starting motor, and the control circuit between the
battery, battery-switch, and motor solenoid must be within maximum resistance limits
shown below.
Not all this resistance is allowed for cables. Connections and contactors, except the motorsolenoid contactor, are included in the total allowable resistance. Additional fixed
resistance allowances are:Contactors
Relays, Solenoid, Switches0.0002 Ohm each
Connections(series connector)
0.00001 Ohm each
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The fixed resistance of connections and contactors is determined by the cable routing.
Fixed resistance (Rf) subtracted from total resistance (Rt) equals allowable cableresistance (Rc): Rt - Rf = Rc.
With cable length and fixed resistance determined, select cable size using the followingchart. Only full-stranded copper wire should be used. Arc welding cable is much more
flexible and easier to install than full stranded copper wire cable, but welding cable is not
so durable and will be damaged from corrosion in a much shorter time.
To meet cable length and resistance requirements, cable size must be No. 1. To determinefixed resistance in a parallel circuit, only series connections in one leg of the parallel
circuit are counted.
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Connections/Proper Practices
Electrical connections are often a source of problems for shipboard electrical systems.Salt air and water are highly corrosive. Electrical connections are almost always made of
dissimilar metals. Corrosion is more destructive between dissimilar metals.
The following lists good practices for marine electrical systems.
When making electrical connections between wires, connect wires mechanically so
tugging or pulling can be withstood without any other treatment of the joint. Then coat
the joint and the nearest portions of each wire with electrical solder. Do not expect solderto increase joint strength. The solder is for corrosion protection. Do not use crimp-type
connectors for marine service the plastic sleeve tends to hide the corrosion from view
rather than protecting the joint.
Pneumatic (Air Starting)
Air starting, either manual or automatic, is highly reliable. Torque available from airmotors accelerates the engine to twice the cranking speed in about half the time required
by electric starters.
Air is usually compressed to 758 to 1723 kPa (110 to 250 psi) and is stored in storagetanks. Stored air is regulated to 759 kPa (110 psi) and piped to the air motor. A checkvalve between the compressor and the air receiver is good practice, to protect against a
failure of plant air that might deplete the air receivers supply. The air compressors are
driven by external power sources.
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Air starter air supply piping should be short and direct and at least equal in size to the
motor intake opening. Black iron pipe is preferred. The piping requires flexibleconnections at the starter. Deposits of oil and water will accumulate in the air receiver
and at low spots in the piping. The accumulation of oil and water must be removed daily
to prevent damage to the starting motors. Manual or automatic traps should be installed at
the lowest parts of the piping and all piping should slope toward these traps.
Air tanks are required to meet specific characteristics, such as the specifications of the
American Society of Mechanical Engineers (ASME). Compressed air storage tanks mustbe equipped with a maximum pressure valve and a pressure gauge. Check the maximum
pressure valve and pressure gauge often to confirm proper operation.
Air Storage Tank Sizing
Many applications require sizing air storage tanks to provide a specified number of starts
without recharging. This is accomplished using the following formula:
Where:
Vt = Air storage tank capacity (cubic feet or cubic meters)
Vs = Air consumption of the starter motor (m3/sec or ft
3/sec)
See Air Starting Requirements Chart (page 9).
T = Total cranking time required (seconds): If six consecutive starts are
required, use seven seconds for first start (while engine is cold), andtwo seconds each for remaining five starts, or a total cranking time
of seventeen seconds.
Pa = Atmospheric pressure (psi or kPa): Normally, atmospheric pressure is14.7 psi or 101 kPa.
Pt = Air storage tank pressure (psi or kPa): This is the storage tank pressureat the start of cranking.
Pmin = Minimum air storage tank pressure required to sustain cranking at 100
rpm (psi or kPa) See Air Starting Requirements Chart (page 9).
Cranking Time Required
The cranking time depends on the engine model, engine condition, ambient air temperature,oil viscosity, fuel type, and design cranking speed. Five to seven seconds is typical for an
engine at 26.7C (80F). Restarting hot engines usually take less than two seconds.
Air Consumption of the Starter MotorThe starter motor air consumption depends on these same variables and also on pressure
regulator setting. Normal pressure regulator setting is 690 kPa (100 psi). Higher pressure
can be used to improve starting under adverse conditions up to a maximum of 1034 kPa(150 psi) to the starting motor. The values shown on the Air Starting Requirements chart
(page 9) assume a bare engine (no parasitic load) at 10C (50F).
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Operation
The supply of compressed air to the starting motor must be shut off as soon as the enginestarts, to prevent wasting starting air pressure and prevent damage to starter motor by
over-speeding.
Hydraulic
Hydraulic starting provides high cranking speeds and fast starts. It is relatively compact.Recharging time, using the small engine-driven recharging pump, is fast. Hydraulic
systems can be recharged using a hand pump provided for this purpose, but hand
recharging is very laborious. The high pressure of the system requires special pipes and
fittings and extremely tight connections. Oil lost through leakage can easily be replaced,but because of high pressures in the accumulators (usually 20,700 kPa [3000 psi] when
fully charged) recharging the accumulator/s requires special equipment.
Hydraulic System Considerations
Repair to the system usually requires special tools.
Hydraulic starting is most often used where the use of electrical connections could
pose a safety hazard.
Hydraulic starting systems are not available from Caterpillar. Contact your localCaterpillar dealer for the nearest available supplier.
The Hydraulic Accumulators, if used, contain large amounts of stored mechanicalenergy. They must be very carefully protected from perforation or breakage.
Starting AidsThe diesel engine depends on the heat of compression of the air in a cylinder to ignite the
fuel. Below some specified temperature, the cranking system will not crank the enginefast enough or long enough to ignite the fuel. One or more commonly used starting aids,
such as jacket water heaters and/or ether may be required.
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Jacket Water HeatersJacket water heaters are electrical heaters that maintain the jacket water at a temperaturehigh enough to allow easy starting of the engine. More heaters of higher ratings may be
required in areas of extremely cold temperature.
EtherEther is a volatile and highly combustible agent. Small quantities of ether fumes added tothe engines intake air during cranking reduce the compression temperature required for
engine starting. When using ether, caution is required to prevent spread of fumes to
atmosphere. A proper ether system will meter the rate of ether consumption. Not morethan 1 cm
3(0.033 oz) of ether should be released per 100 rated hp for each 10 seconds of
cranking. Very low ambient temperatures may require increasing the ether consumption
rate. Under no circumstances should ether be released into an engine while running.
NOTE: The 3126 Marine Engine is equipped with an air inlet heater. Under no
circumstances should ether be used on this engine.
Manifold HeatersHeat added to the intake manifold of an engine during cranking will significantlyimprove startability and reduce any white start-up smoke. Manifold heaters are used on
small engines available from Caterpillar. Caterpillar does not offer manifold heaters on
larger marine engines.
Starting Smoke
High performance engines are prone to have some white start-up smoke. The whitesmoke is composed of unburned fuel.* Caterpillar Engines have been designed to
minimize this problem. Electronically controlled engines have a cold mode strategy built
into the software to reduce start-up smoke.
Operators can do several things to improve the situation:
Use jacket water heaters to raise the engine water temperature to 32 to 49C
(90 to 120F) prior to starting.
Keep warm-up idle speeds (rpm) low.
Warm the air to the air cleaners and intake manifold.
* Diesel engines that are designed to have high output power, yet still be relatively lightweight, generally have lowcompression ratios; i.e., in the range of 12.5 to 16:1. This design factor makes them prone to misfire and run roughuntil the engine reaches normal operating jacket water temperatures of 80 to 93C (175 to 200F).