DieselShip

Mitsubishi low speed engines

The dominance enjoyed for many years by MAN B&W and Sulzer in the low speed two-stroke crosshead engine sector is challenged by Mitsubishi, the only other contender in the international arena. The Japanese group has widened its UEC portfolio in recent years to offer new and refined models across the full propulsion power spectrum. Mitsubishi has traditionally served its home market but a stronger direct-export thrust has earned installations in ships built for European and other overseas owners in Japan and Europe.

Most Mitsubishi engines, unlike MAN B&W and Sulzer engines, are built by the group itself rather than overseas licensees. The complete UEC programme can be handled by its Kobe factory while engines up to 600 mm bore can be built by its Yokohama works and the Japanese licensees Akasaka and Kobe Diesel.

An experimental UEC model built in 1952 yielded a power output 50 per cent higher than its predecessor, the MS engine, which had been developed from the early 1930s. The first commercial UEC engine, a nine-cylinder UEC 75/150 model developing 8830 kW, was introduced in 1955. The UEC engine now shares a common design philosophy with MAN B&W’s MC and Sulzer’s RTA series, exploiting constant pressure turbocharging and uniflow scavenging through a central hydraulically opened/pneumatically closed exhaust valve in the cylinder cover. (The UEC designation refers to uniflow-scavenged, exhaust gas turbocharged and crosshead type engine.)

The original UEC engines—progressing from A to E type—featured impulse turbocharging, three mechanically-actuated exhaust valves per cylinder and a single centre-injection fuel valve. This arrangement fostered high performance with a smooth exhaust gas outlet and good combustion but exhaust valve lifetime was sometimes short.

A two-stage turbocharging system (unique in production low speed engine practice) was introduced for the UEC-E type in 1977 to boost specific output and achieve a mean effective pressure of 15 kg/cm2. Low pressure and high pressure turbochargers were linked in series (Figure 1). To match this increase in maximum cylinder pressures, the engine frame, bearings and combustion chamber components

Figure 1 Two-stage turbocharging system for UEC-E engines

were made more rigid than those of the UEC-D type predecessor while still retaining the monobloc cast iron construction (three elements held by tie bolts). A new piston crown of specially reinforced molybdenum cast steel was specified with a combination of radial and circular ribs behind the frame plate to withstand the rise in maximum pressure (increased from 90 to 110 kg/cm²).

The UEC-E engine was comparatively short-lived in the programme, however, as the steep rise in bunker prices in the late 1970s dictated a stronger focus on fuel economy rather than sheer output. Mitsubishi therefore introduced the long stroke UEC-H type model in 1979, abandoning impulse turbocharging in favour of constant pressure turbocharging based on the group’s own MET-S turbocharger. The H-type also broke with previous UEC engine practice in using one large diameter exhaust valve in the centre of the cylinder cover and twin side-mounted fuel injection valves (Figure 2). A significant reduction in fuel consumption was yielded. The H-type is detailed at the end of this chapter. The HA-type, which followed in 1982, adopted the higher efficiency non-water-cooled MET-SB turbocharger and an improved combustion system, further enhancing fuel economy. Output was also raised by 11–14 per cent.

Advances in fuel economy were pursued with the development of  longer stroke/lower speed L-type engines from 1983 with successive refinements embodied in the LA-type (1985), the LS-type (1986) and the LSII-type (1987). Stroke–bore ratios were increased to achieve lower direct-coupled propeller speeds and hence higher propulsive efficiency. Efforts to improve specific fuel consumption were reflected in rises in the maximum cylinder pressure.

For most of the 1980s Mitsubishi was content to concentrate on the small-to-medium size ship propulsion sector with just four bores (370 mm, 450 mm, 520 mm and 600 mm). Four- to eight-cylinder models covered outputs up to around 15 000 kW. The UEC-LA models, with a stroke–bore ratio of 3.17:1, were supplemented in 1985 by longer stroke (LS) options for the 520 mm and 600 mm models (3.55 and 3.66:1 ratios, respectively).

A return to the large bore arena, long surrendered to MAN B&W and Sulzer, was signalled in 1987 by the launch of the UEC 75LSII series with a maximum rating of 2940 kW/cylinder. The 750 mm bore/2800 mm stroke design retained the main features of the LS engines but exploited an even longer stroke–bore ratio (3.73:1) and lower running speeds (63–84 rev/min). Mitsubishi stepped up its challenge in the VLCC and large bulk carrier propulsion markets in1990 with the introduction of an 850 mm bore/3150 mm stroke version.

The UEC 85LSII series, available in five- to 12-cylinder models, almost doubled the upper power limit of the portfolio to 46 320 kW at 54–76 rev/min. The design offers a maximum rating of 3860 kW/cylinder.

The propulsion market created by a new generation of large Panamax and post-Panamax containerships requiring service speeds of up to 25 knots was addressed by Mitsubishi in 1992 with its UEC 85LSC series, a shorter stroke (2360 mm) faster running derivative of the UEC 85LSII. The design is the highest rated engine in the group’s programme, with an output of 3900 kW/cylinder at 102 rev/min. The series covers a power band up to 46 800 kW from five- to 12-cylinder models.

Figure 2 The UEC A to E series engines featured three exhaust valves and a centre-injection fuel valve (left). Later series (H to LS) are equipped with one exhaust valve and two side-injection fuel valves (right)

A determination to secure small-ship propulsion business led in 1991 to the introduction of a 330 mm bore/1050 mm stroke LSII design (Figure 3). The series succeeded the 370 mm bore LA models which pioneered the modern ‘mini-bore’ two-stroke engine from the late 1970s for small general cargo tonnage, tankers, colliers, feeder containerships and even large fishing vessels. The longer stroke and more advanced UEC 33LSII engine now develops 566 kW/cylinder at 215 rev/min and covers an output band from 1230 kW to 4530 kW at 162–215 rev/min with four- to eight-cylinder models.

Figure 3 Construction of UEC 33LSII engine

Mitsubishi enhanced its medium power programme in 1992 by upgrading the 500 mm and 600 mm bore models to LSII status. Later power upratings of 3–5 per cent for these UEC50LSII/60LSII models, without major design changes, were introduced to meet the demands of larger ships and higher service speeds. The specific outputs respectively rose to 1445 kW and 2045 kW per cylinder. With a stroke/ bore ratio of 3.9:1, the 50LSII series embraces four-to-eight-cylinder versions covering an output range up to 11 560 kW at 127 rev/min.

The small bore programme was strengthened in 1995 with 370 mm and 430 mm UEC LSII designs to attract business from handy-sized bulk carriers, feeder containerships, small tankers and gas carriers. The engines plugged a gap between the established 330 mm and 500 mm bore LSII series. The UEC 37LSII engine offers an output of 772 kW/cylinder at 186 rev/min from five-to-eight-cylinder versions, while the UEC 43LSII engine delivers 1050 kW/cylinder at 160 rev/min from four-to eight-cylinder models. The first UEC 37LSII engine, a six-cylinder model, completed testing in early 2000, and the debut order for a 43LSII engine, a seven-cylinder model, was logged in 2002.

Development of the LSII design from 1998 resulted in the LSE series, headed by a 520 mm bore model (UEC52LSE) targeting feeder containerships and Panamax bulk carriers. The series was extended by a 680 mm bore model (UEC68LSE) for propelling tonnage including Aframax and Suezmax tankers, Cape-size bulk carriers and containerships. The LSE models share a more compact configuration and a 25 per cent reduction in overall number of components compared with the LSII engines. Benefits were also gained in production costs, reliability and maintainability. The mean effective pressure rating and mean piston speed were respectively raised by five per cent and six per cent to 19 bar and around 8.5 m/s.

UEC-LSE engine data

An extension of the same ‘total economy’ LSE concept later resulted in a 600 mm bore version. The current UEC programme embraces models with bore sizes of 330 mm, 370 mm, 430 mm, 450 mm, 500 mm, 520 mm, 600 mm, 750 mm and 850 mm in LA, LS, LSII and LSE versions, covering an output band from 1120 to 46 800 kW at 215 to 54 rev/min (Figures 4 and 5). Typical performance curves and layout diagrams are shown in Figures 6 and 7.

Development progress since 1955 is underlined by the LS-type whose specific output is three times that of the original UEC engine and whose weight/power and length/power ratios are reduced by onethird. The advances in key design and performance parameters over the years are illustrated graphically in Figures 8 and 9.