PS 111. Professional Studies Unit 10: Diesel Propulsion
Unit 10 / Diesel Propulsion

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PS 111. Professional Studies Unit 10: Diesel Propulsion

Schematic of major cylinder components and dimensions

  • piston cylinder size is given as “bore x stroke” thus a 3 1/2 x 4 in cylinder has a bore of 3 1/2 in. and a stroke of 4 in.
  • In marine use cylinder sizes range from 89 x 89 mm (3 1/2 in) to 1050 x 1750 mm (41 1/3 x 68 7/8 in)

The DIESEL CYLCE is comprised of4 EVENTS

INTAKE (bring in fresh air charge)

COMPRESSION (raise pressure & temperature of charge)

POWER (inject and combust fuel – “exploding” gas expands)

EXHAUST (remove combustion products)

FOUR Stroke/Cycle [4 S/C] Engines

Each event completed in one stroke

Cycle completed in TWO revolutions

Notes:

/ / Rev 1 (TDCBDC)
Intake Stroke
intake valve open, exhaust valve closed, fresh air drawn in (naturally aspirated) or forced in via blower
Rev 1 (BDCTDC)
Compression Stroke
all valves closed, work done on combustion air to raise pressure & temperature. Fuel injection begins just before TDC
/ / Rev 2 (TDCBDC)
Power Stroke
All valves closed, combustion occurs with resultant increase in pressure, piston forced downward. Injection ceases shortly after TDC
Rev 2 (BDCTDC)
Exhaust Stroke
Exhaust valve open, intake valve closed, upward movement of piston forces combustion gas from cylinder

Notes:

The reciprocating motion of the piston is converted to a rotating motion of the crankshaft by a connecting rod

The 4 s/c Diesel Engine is similar to the the typical automobile gasoline engine but there are some crucial differences:

Diesel engines intake and compress only air, gasoline engines intake and compress a fuel-air mixture.

The temperature rise during compression is sufficient to ignite the diesel fuel when it is injected,. Compression alone is insufficient to ignite the gasoline-air mixture; a sparkplug is required.

The combustion in a gasoline engine is very rapid (almost instantaneous) approximating constant volume heating. This is characteristic of a thermodynamic cycle known as the Otto Cycle

The combustion in a diesel is slower, nearly maintaining constant pressure as the volume above the piston increases (constant pressure heating). This is characteristic of the appropriately named Diesel Cycle

Notes:

Crankpin location and Timing

Since piston location is tracked by the location of the crankpin in each revolution (12 o’clock = TDC; 6 o’clock = BDC), events in the Diesel Cycle may be associated with a “timing circle.”

Notes:

TWO Stroke/Cycle [2 S/C] Engines

Each event completed in less than one stroke

•POWER (TDC before BDC)

•EXHAUST (before to after BDC)

•INTAKE (before to after BDC)

•COMPRESSION (after BDC  TDC)

Cycle completed in ONE revolution

Notes:

2 s/c EVENTS

/ / Power
TDC --> 40o/60o Before BDC
Fuel injection, ignition, combustion and expansion of combustion gases
Exhaust
40o/60o Before BDC
Exhaust valve opens (or port uncovered), combustion gas escapes under pressure
/ / Savenging
40/60o Before BDC  40/60o After BDC
Intake port opens before BDC, fresh air charge under pressure from scavenging blower continues to push exhaust gas out
Compression
40/60oafter BDCTDC
Intake port and exhaust valve (port) closed, fresh air charge being compressed. Injection starts just before TDC.

Notes:

DEISEL CYCLE SUMMARY

Comprised of:

INTAKE, COMPRESSION, POWEREXHAUST EVENTS

Fuel injection at beginning of power event)

4 S/C Engines—
•Each event takes one stroke
•Cycle completed in two revolutions
•One power stroke every other revolution
•Must have intake AND exhaust valves / 2 S/C Engines—
•Cycle completed in one revolution
•(Shorter) power stroke every revolution
•Mayhave intake and exhaust valves, or
–Intake port and exhaust valve, or
–Intake and exhaust ports

SLOW SPEED Diesel engines

The type of diesel engine most used for the propulsion of ocean-going ships is the 2-stroke cross-head engine. This engine type is the world’s most economically thermodynamic machine. Moreover, it can be designed with a very low crankshaft RPM which means that high propeller efficiencies can be achieved, even though the crankshaft is coupled directly to the propeller shaft.

Notes:

MAN-B&W and Sulzer cover approximately 90% of the world matket for 2-stroke, crosshead engines.

Stationary parts / Moving parts

Valve & Injector timing

The are many fuel injection arrangements…

•Fuel pumps may be mounted on injector (unit injectors) and activated via a rocker arm from above—or an “overhead cam”

•Fuel pumps may be activated electrically, timed with a microprocessor (no fuel cam—electronic injection)

•Fuel may be delivered a constant high pressure to injectors and the injector activated mechanically or electronically at precisely the right moment

… but ALL fuel injection systems must satisfy the following requirements.

accurate
METERING / the fuel must be accurately metered in response to the engine power output required
proper
TIMING / injection must commence at precisely the correct moment at the end of the compression stroke
suitable
INJECTION RATE / the fuel must be delivered to the cylinder at a constant and predetermined rate throughout the injection period
clean
CUT-OFF / at the end of the injection period, the injection must terminate sharply and completely
adequate
ATOMIZATION / fuel must enter the cylinder as a finely atomized spray of minute droplets throughout the injection period
sufficient
PENETRATION / the droplets must penetrate far enough into the combustion space to ensure they are evenly distributed without impinging on cylinder wall

Fuel Oil System (L.S. Diesel Propulsion)

SETTLING TANKS / filled daily via fill and transfer systems; fitted with heating coils and stripping connections for removal of major contaminants
DAY TANK / holding tank for supply of clean fuel for engine consumption
PURIFIERS / continuously deliver FO to day tank from settlers or storage tanks; rating of purifiers in excess of engine consumption at maximum continuous rating (MCR)

Notes:

MIXING TANK / functions to prevent overheating of day tank; to enable gradual changeover from hot heavy fuel to cold distillate fuel; permits lighter fuel fractions to vent (note: not all ships have mixing tanks)
BOOSTER (SERVICE) PUMPS / deliberately oversized at 2-3 times engine consumption at MCR to limit the cooling of heavy fuel between heaters and engine and to ensure constant pressure at injection pumps
FINAL HEATERS / at least two heaters, each with the capacity to heat the heaviest fuel likely. Steam supply to heaters is regulated by the viscosimeter
FILTERS / remove fine particulate contaminants, non-particulate organic, hydrocarbon contaminants (sludge) and even trace amounts of water

Notes:

Lube Oil System (L.S. Diesel Propulsion)

LO SERVICE (CIRCULATING) SYSTEM / Oil draining from bearings and cooling passages drops to main LO sump; drawn by (usually rotary) motor driven circulating pumps, LO is delivered to engine moving parts, hydraulic governor (if fitted) and turbocharger bearings via filters and a cooler
CYLINDER OIL SYSTEM / an independent system for the lubrication of the piston rings fitted on most cross-head engines. Lubricators, mechanically driven by the engine, inject a metered quantity of oil into the cylinder as the piston ring pack passes. Most of the oil is burned
STORAGE, TRANSFER & PURIFICATION SYSTEM / A LO storage tank with a capacity of at least one charge for each engine it serves is filled from the deck by gravity. The settling tank is normally empty. Should the main engine’s circulating oil become massively contaminated (i.e. by water) it can be transferred to the settling tank. LO purifiers, sized to circulate the contents of the main sump 3-5 times per day, are usually fully automatic in their operation: programmed to shut down and alarm with any malfunction, and self-cleaning types cycle through sludge ejection sequences

Notes:

Cooling Systems (L.S. Diesel Propulsion)

JACKET WATER COOLING / Cylinder liners are protected from overheating by jacket water. Jacket water may be cooled directly be sea water or by lower temperature fresh water. Circulated through the engine block by centrifugal pumps, jacket may be used to recover heat from the engine to produce fresh water. Jacket water is sometimes used as the cooling medium for LO
PISTON COOLING / While all trunk piston engines and some cross-head engines utilize oil to cool pistons, some cross-head engines use a cooling water system separate from the jacket water system. The water reaches and leaves the piston through telescoping tubes enclosed within compartments inside the crankcase
INJECTOR COOLING / Injector cooling may be accomplished simply be conduction to the cylinder heads, water circulated as part of the jacket cooling system or by fuel in a closed loop. On some engines and independent injector cooling system is installed, often without a cooler and with a heating coil in the expansion tank to maintain a sufficiently high temperature at low loads with heavy fuel
SEA WATER SYSTEMS / Sea water systems include cooling services in the engine room and may be cross-connected to ballast and fire main systems. A thermostatic valve may be fitted to recalculate sea water in cold seas

“CENTRALIZED” SYSTEMS--nested loops of cooling media where sea water is circulated to cool low temperature fresh water (LTFW) which in turn is circulated to cool lube oil and high temperature fresh water (HTFW). HTFW is circulate through the engine as jacket water, piston and injector cooling

Notes:

MEDIUM SPEED DIESEL PROPULSION

Multiple sets of smaller, higher RPM 2 s/c or 4 s/c engines

Speed reduction (transmission) gear required to connect to a low speed propeller

Up to 24 cylinders per engine—in line or “V” arrangements

Auxiliary systems (fuel, lubrication, cooling) engine driven (via gears on crankshaft)

Only jacket water cooling supplied by external machinery (and in some cases—emergency generators—jacketed water is cooled by an engine-mounted fan)

STARTING—Diesel engines are not self-starting. They must be turned over at a speed high enough to produce compression pressures & temperatures sufficient for combustion of injected fuel.

Most medium & High Speed engines are started by rotating the crank shaft with a motor.

  • An electric motor (Electric Start)
  • An air motor (Air Motor Start), or
  • A hydraulic motor (Hydraulic Start)

Large, Slow Speed Diesels utilize a Direct Air Start system

Notes:


ELECTRIC START

AIR MOTOR STARTHYDRAULIC START

DIRECT AIR START SYSTEM

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