Getting a diesel to run well on heavy fuel or bunker oil is no mean feat. Heavy Fuel Oil (HFO) has almost the consistency of treacle and in some parts of the world, has an excessively high sulphur content.
Burning HFO is about as good for a diesel, as kerosene is for a petrol engine. A minimum operating temperature must be reached before the fuel will burn cleanly in the combustion chamber; for example, with the direct-reversing two-stroke Sulzer in the first ship I served on, we had to start the engine on distillate (diesel) then switch to HFO at about half normal operating revs.
But when you're operating a heavy-duty commercial vessel, where the engine may be running 24 hours a day for weeks at a time during an ocean passage, the savings by using HFO are too good to ignore.
This is where Yanmar's four-stroke direct-injection inline HFO diesels come in. Outputs range from 197hp (147kW) to 4437hp (3310kW). Yanmar supplies its own single-speed hydraulic reverse-reduction gearboxes with ratios from 2.08:1 to 4.15:1 depending on the model, but the 6N260, 6N280/8N280 and 6N330/8N330 series are available with reduction gears only if needed for use with controllable pitch propellers.
Yanmar's HFO models are measured by their cylinder bore diameter and not piston displacement. For example, the smallest model (the S165) has a bore of 165mm, though its stroke is a massive 210mm.
In ship-speak, smaller models in the range are regarded as high-speed diesels, while the larger models are medium-speed diesels.
To give some idea of the bulk of a Yanmar HFO diesel, the S165-ET develops 591hp (441kW) at 1350 revs and complete with gearbox weighs 3780kg, whereas the distillate-only and four-valve head 6LAA-UTE is rated at 523hp (390kW) at 1850 revs and weighs 2390kg. The S165-ET measures 2914mm long by 1070mm wide and 1586mm high, while the 6LAA-UTE is 2123mm x 1013mm x 1414mm. These are b-i-g powerplants.
Yanmar's HFO diesels have single-side component access, such as filters and fuel-injection equipment, and provision for removal and refitting of the main bearings and connecting rods through large inspection hatches on both sides of the engine. The camshafts are mounted high on the block to reduce pushrod length, and all pumps and filters are located above engine mounting level. A PTO may be fitted to the forward end of each model. Wet cylinder liners are used across the range and the individual cylinder-heads have massive cooling water passages.
To comply with strict International Maritime Organisation (IMO) NOx exhaust emission regulations for commercial shipping diesels, Yanmar has introduced an OEN' series, to operate on distillate or HFO. These comprise the high-speed 6N165-EN, a stroked version of the S165-ET which develops 788hp (588kW) at 1400 revs, through to the medium-speed 6N330/8N330 series (2958 to 4437hp/2207 to 3310kW at 620 revs).
Comparing a medium-speed N-series diesel with its distillate-only counterpart, the 6N260-UN (1578hp/1177kW at 750 revs) measures 5614mm x 1490mm x 2648mm and weighs 20,000kg with gearbox, whereas the 16LAK-STE-1 develops 1479hp (1103kW) at 1900 revs, measures 3702mm x 1506mm x 1585mm and weighs 5330kg.
Yanmar states it has reduced NOx emissions without resorting to electronic engine management systems. The steps taken include raising compression ratios, reducing combustion temperature, modifying piston crown design and using high-pressure/short-duration injector sprays.
By deepening the piston crown recess around the central peak and increasing wall angle steepness, both fuel consumption and NOx emissions were reduced due to a more effective air/fuel swirl. The angle of the injector spray was also altered to work in with the new recess design.
Most NOx is produced during the middle of the fuel combustion cycle, but by using a new cam injector, spray output was reduced in this period - lowering NOx emissions by an impressive 50 parts per million to meet the current IMO target of 45g/kWh.
The high-pressure injection pumps deliver fuel at 22,500psi, compared to about 2850-3000psi for naturally-aspirated direct-injection diesels operating in the 2000 rev range.
The pumps have a closed-type plunger barrel with a sealed upper end securely supported within the pump body to minimise the distortion induced by high pumping pressures. A pressure equaliser valve is fitted to the delivery valve to prevent cavitation within the injection system. The pump incorporates a fuel discharge passage to prevent any leaking fuel from escaping, while fuel accumulation at the fuel-level rack and reduced plunger performance from leaking fuel are eliminated.
When the engine is modified to run on HFO, the injection pump has a lubricated pinion sleeve in the lower plunger area. Oil from the main lubricating gallery passes through a pressure adjustment valve, filter and restrictor valve installed in the injection pump body before being dripped onto the sleeve.
The fuel-injection cam tappet has a roller-drive and an injection-duration adjuster bolt which can be easily accessed through a port in the pump mount and tappet bore.
The injectors have lightweight components for rapid needle valve response and are cooled by water from the cylinder cooling jacket coming into direct contact with the protector sleeve around each injector. This eliminates the need for separate oil and water lines and cools the fuel spray for more effective air/fuel combustion.
To reduce maintenance and down time the engines use pipe-less fuel delivery, where drilled fuel passages in the cylinder-head and injection pump housing have replaced external fuel lines. Yanmar states this significantly improves engine reliability by reducing the number of welding operations needed to fabricate small-diameter fuel lines. Short elbow couplings replace high-pressure fuel lines, while compression-type line connections provide reliable sealing and are easily removed or installed.
Yanmar has also made the N-series diesels smoother-running and quieter than their predecessors. The cylinder blocks have an underslung bearing construction with a box-girder dual-wall structure to increase rigidity and reduce deformation, while the gearboxes are also more rigid. Through the use of finite element stress analysis, ribs on strategic engine parts maintain structural rigidity while reducing engine weight.
Noise levels are reduced by locating the pushrods within the cylinder block; using a large capacity exhaust silencer; lagging the cast-iron exhaust manifold; fitting fuel pump chamber and rocker arm covers; and mounting the charge-air and oil coolers on the cylinder block.
The end result of this development is some of the cleanest-running commercial marine diesels ever produced. Yanmar supplied pics of coastal freighters, ferries and tugs powered by its diesels underway at normal service speeds and in none of the shots was the slightest trace of exhaust smoke apparent.
When installed in long-range heavy-displacement motoryachts and ocean-going workboats, the smaller HFO models should outlast light duty-rated diesels many times.
Ed's note: regular Australian Yanmar importer Power Equipment does not distribute Yanmar HFO engines. Readers interested in more information regarding these powerplants should contact:
Yanmar Asia (Singapore) Corp P/L,
4 Tuas Lane, Singapore 638613.
Tel: + 65 861 5077
Fax: + 65 861 1509