29 trunk piston engines

The marine diesel propulsion market is dominated by direct-coupled low speed two-stroke crosshead engines and geared high/medium speed four-stroke trunk piston engines but some Japanese and east Asian regional operators of coastal/shortsea, fishing and small oceangoing vessels appreciate the merits of a 'hybrid' alternative: the low speed four-stroke trunk piston engine.

Low speed four-stroke engine designs, a traditional Japanese speciality, are characterized by simple and rugged construction and comparatively long strokes. Nominal operating speeds, in some cases less than 200 rev/min, allow direct coupling to the propeller. In contrast to the equivalent European/US engine sector, served by high/medium speed machinery with a fixed bore size and variable number of cylinders, the Japanese designs are invariably based on a six-cylinder in-line configuration with a range of bore diameters to meet the desired power output.

As an example, Akasaka Diesels' programme has over the years embraced a dozen bore sizes from 220 mm to 510 mm, all offered in six-cylinder form only. Other company ranges have extended up to 580 mm bore with outputs as high as 735 kW per cylinder. Research and development efforts in the 1970s addressed higher specific ratings, with some designers introducing two-stage turbocharging systems to boost power. Enhanced operating economy, heavy fuel-burning capability and reliability were pursued in the 1980s.

The drawbacks of the direct-coupled low speed four-stroke engine concept compared with medium speed rivals are the fixed propeller speed (though some models are arranged for reduction gearing), higher weight and increased space demands. But these are outweighed, according to proponents, by advantages: improved reliability and reduced maintenance derived from a fewer number of components; less noise and vibration; better fuel and lubricating oil consumption figures; and a greater ability to handle low grade fuels.

With few exceptions, however, the Japanese designs have made little impact in Western markets where the geared medium speed engine maintains its supremacy in the small-ship propulsion arena.

This failure to break through decisively reflects the difference in traditional vessel designs and operating philosophies between the Asian and European/US sectors. And not all the Japanese enginebuilders have made special export efforts to the latter territories, acknowledging that a substantial spares and service support network is required for credibility. The doorstep markets of Indonesia, South Korea, the Philippines and Taiwan offer the best export potential. The domestic market represents the baseload for production, the enginebuilders enjoying longstanding relationships with small-to-medium sized yards.

Hanshin Diesel is perhaps the most well-known designer/builder of the hybrids outside Japan, the Dutch shipping company Spliethoff having standardized on its engines for deepsea cargo ships in the 1970s/1980s. The EL design (illustrated in Figure 29.1), evolved by Hanshin since the late 1970s, indicates the construction and

Trunk Piston Engine

performance characteristics of a contemporary low speed four-stroke design.

The current EL programme is reduced to 380 mm and 400 mm bore models but past portfolios have also included 300 mm, 320 mm, 350 mm and 440 mm versions, all produced in six-cylinder form only and featuring a stroke/bore ratio of 2:1. An output band from 2058 kW to 2430 kW at 240 rev/min is covered by the 6EL38 and 6EL40 engines on a mean effective pressure of around 20 bar.

The bedplate, crankcase and cylinder block are of cast iron and form a highly rigid construction. The exhaust valve is provided with a cage and rotator, and the valve seat faces are stellited. The cylinder liner is supported at its shoulder by an annular piece to prevent it from deforming due to thermal expansion. No separate cylinder lubrication device is necessary.

The piston crown is of forged steel and cooled by the shaker method using the lubricating system oil; the skirt is of cast iron; and the piston ring set consists of two barrel face, chromium-plated inner cut rings, two conventional cast iron compression rings and two oil scraper rings with expanders. The connecting rod is comparatively short to reduce the overall height of the engine, and the integral crankshaft is made of continuous grain flow forged steel. Both main and crankpin bearings are of the precision shell type.

Hanshin also offers its LU and LF series, the latter extending to a 580 mm bore/1050 mm stroke LF58A model with an output per cylinder of up to 772 kW at 190 rev/min. A new LHL series design benefits from experience with the LU, EL and LF engines. The range embraces 280 mm, 300 mm, 320 mm, 340 mm, 360 mm, 410 mm and 460 mm bore models which feature a stroke/bore ratio of around 1.9:1. Again, all are produced in six-cylinder form to cover a power band from 1176 kW to 3309 kW at speeds from 380 rev/min down to 220 rev/ min.

Akasaka Diesels' low speed trunk piston engine equivalent to the Hanshin EL programme, the A series, is produced in six-cylinder 280 mm, 310 mm, 340 mm, 370 mm, 380 mm, 410 mm and 450 mm bore versions with a stroke/bore ratio of around 1.95:1. The A45S model develops 3309 kW at 220 rev/min. The overall Akasaka portfolio embraces designs with bore sizes ranging from 220 mm to 500 mm and offering outputs from 375 kW to 6066 kW. The 500 mm bore/ 620 mm stroke U50 model is available in six, eight and nine-cylinder forms developing 674 kW/cylinder at 380 rev/min.

Among other Japanese designers contesting the market are Makita, Matsui Iron Works and Niigata Engineering.

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