When it comes to ocean passagemaking under power, engine reliability is utterly essential. In this respect, going to sea in a powerboat is much like aviation; in either arena, the consequences of engine failure are dire to say the least.
Conventional wisdom holds that a straightforward, all-mechanical diesel should be more reliable than a new-generation engine equipped with electronic fuel injection and computerized controls. After all, electricity and salt water are, at best, an uneasy combination, and it’s imperative that the engine keeps on running in even the worst circumstances. Unfortunately, this rather simplistic line of argument ignores the reasons why electronics have been finding their way into so many new diesels. Lower manufacturing cost is not the explanation, because these new fueling and control systems are obviously more complex and expensive than their mechanical equivalents. Neither are tightening emissions standards the whole story, although they have certainly encouraged developments in this area. The bottom line is that electronic engines can be quieter, smoother, more fuel efficient, less polluting and better monitored than their conventional counterparts.
This article aims to provide some background and a general overview of the state of the art. However, it’s only a starting point for the detailed homework that a would-be power voyager should undertake before finalizing the all-important engine selection.
Common-rail fuel systems
The unit injectors found on most present-day diesels are, in essence, compact little piston pumps installed in the cylinder heads. Most conventional injectors are powered by cam action, much like the engine’s intake and exhaust valves. However, in recent years, a new generation of cam-actuated and solenoid-actuated injectors &mdash both equipped with computerized controls &mdash have been gaining popularity. These electronic injectors allow the fuel to be metered with far greater accuracy, and in some cases, allow peak injection pressures to be maintained at low engine revs. Ideally, injection pressures should be at least 1,200 bar (about 18,000 psi), because driving the fuel through a tiny nozzle at extreme pressure results in a finer spray pattern and more complete combustion.
Common-rail diesels are the latest variation on the electronic fuel-injection theme. A precision gear pump delivers fuel at 1,200 to 1,600 bar to a tubular reservoir (the common rail), which supplies the individual injectors atop each cylinder. The injectors are simply fast-acting needle valves, controlled by small solenoids or piezoelectric mechanisms. The common rail houses enough fuel to preclude appreciable pressure fluctuations as the injectors open and close.
With a common-rail system, it becomes feasible to have three or more injection events during each combustion cycle, rather than introducing all the fuel at once. This eliminates percussive ignition &mdash the characteristic “diesel rattle”&mdash and results in a significantly quieter, smoother-running engine. It also minimizes shock-loading of engine components. Fuel efficiency improves, too, because the burn cycle can be initiated by a small pilot injection, which establishes turbulent combustion and ensures complete oxidation of the main fuel charge. And last but not least, common-rail diesels are self-bleeding because air bubbles in the fuel will no longer disable the injectors but will pass freely into the combustion chambers.
In Europe, where fuel prices remain several times higher than in North America, diesel automobiles now make up about 40 percent of new car sales. The great majority of these European diesel autos now feature common-rail fuel systems. Here in the United States, diesel passenger vehicles remain quite rare, but diesel power is an increasingly popular option for full-sized pick-ups and SUVs. Incidentally, GM’s Duramax V-8s and the Cummins diesels used in Dodge trucks both are common-rail engines, as are the new Vecter and Delta automotive diesels recently introduced by Detroit Diesel Corp.
First marine common rails
In the small-marine-diesel sector, common-rail technology barely has a foot in the door, yet it’s clearly on the way. Last summer, Volvo Penta of Sweden introduced three series of common-rail engines: the D3 (2.4 liter), D4 (3.7 liter) and D6 (5.5 liter). In their current high-speed versions, these engines are intended primarily for recreational and light commercial planing craft, but models with both higher and lower power ratings are likely before long.
The most conservatively rated D3 (deemed suitable for big sailboats and smaller trawler yachts) produces 110 hp at 3,000 rpm. This is a relatively inexpensive engine based on a light aluminum, five-cylinder automotive block &mdash probably not the ideal choice for a long-range power voyager. The larger D4 and D6 engines are purpose-built marine units featuring cast-iron construction and replaceable cylinder liners. Both currently top out at 3,500 rpm but might become promising candidates for long-distance voyaging once more conservative, continuous-duty versions are available.
This past fall, Cummins MerCruiser brought out their first common-rail marine diesels: the QSL9, an 8.9-liter, straight-six engine intended primarily for commercial marine applications, and the similar but smaller QSB5.9. Continuous-duty output for the former is 285 hp at 1,800 rpm, making it a promising candidate for long-range trawler-yacht applications. This spring the QSB8.3 (8.3 liter) became the third common-rail engine in the Cummins MerCruiser stable. The manufacturer claims that their common-rail fuel system “vastly improved economy, reduces idle noise by more than 80 percent, and virtually eliminates smoke and odor.”
The German firm MTU &mdash in recent years a commercial partner with Detroit Diesel Corp. &mdash has just unveiled several new 2000-series common-rail engines suitable primarily for megayachts (1,200 to 2,400 hp).
Other major players in the trawler-yacht market &mdash notably Alaska Diesel Electric (Lugger), John Deere, Caterpillar, Westerbeke and Yanmar have not, so far, announced common-rail engines. However, most offer electronic controls on certain models, and all are no doubt evaluating common-rail technology for future applications.
Control and monitoring
Diesels equipped with electronically controlled unit injectors &mdash either electrically or mechanically actuated &mdash offer the ability to vary injection timing, injection volume and, in some cases, injection pressure independent of engine rpm. This added flexibility allows fueling to be fine-tuned to engine operating conditions, taking variables such as fuel, air and individual cylinder temperatures into account. As well, they open a new realm of advanced engine monitoring and sophisticated diagnostics, making it much easier to pinpoint problems, or better yet, identify potential trouble spots before significant harm occurs.
Obviously, any common-rail engine with its multipulse electronic injectors will require fully computerized controls. In any case, the days of the all-mechanical marine diesel are likely numbered because it will be difficult to meet the tough emissions standards anticipated for 2007 without computerization.
Most diesel makers have now undertaken major R&D programs aimed at developing universal computer control systems capable of managing any electronic engine, whether unit injected or common rail. Detroit Diesel’s well known DDEC was one of the earliest examples, and archrival Caterpillar naturally offers a comparable electronics platform.
Cummins MerCruiser uses the SmartCraft multiplex control system initially developed by Mercury Marine, while Volvo Penta offers their Electronic Vessel Control, or EVC, system. The latter systems &mdash and a growing number of others, such as the MagicBus from marine control specialist Teleflex Morse &mdash are designed along the lines of the CANbus software-based platforms now standard on most new cars and trucks. CAN stands for controller area network. A bus refers to a linked single-cable architecture that allows not only engine-related functions but all onboard electronics, including navigational instrumentation, to be daisy-chained together. These systems greatly simplify the task of wiring up a modern yacht and conceivably reduce the risk of problems by minimizing the number of vulnerable connections.
Some marine operators will no doubt continue to look askance at such heavy dependence on electronics, but diesel manufacturers are going to great lengths to ensure their electronics won’t negatively impact reliability. Systems are torture-tested at extreme temperatures, in salt spray booths, and exposed to severe vibrations or strong magnetic fields. Volvo Penta, for example, reports that prior to the 2004 introduction of their EVC system, they had already met rigorous classification requirements for commercial use and completed extensive offshore testing aboard commercial vessels. It remains to be seen whether this will be enough to mollify the more conservative elements in boating, but it’s a good bet that most will eventually be won over.
Pending requirements for much cleaner-running marine diesels (www.epa.gov/otaq/marine.htm) may well have sounded the death knoll for all but the smallest propulsion engines not equipped with turbocharging and electronic controls. Marpol (International Convention for the Prevention of Pollution from Ships) is simultaneously implementing virtually identical standards, making it easier for manufacturers to develop universally acceptable engines.
Before becoming discouraged by the prospect of being pushed into an electronic diesel, ready or not, prospective engine buyers should weigh both the benefits and the possible risks involved. To summarize the positives, electronic engines offer superior fuel economy; quieter, “more civilized” operating characteristics; plus advanced diagnostic and monitoring capabilities. Improved fuel economy alone is a worthwhile safety benefit for smaller trawler yachts that often confront their range limits on offshore passages. Indeed even 5 percent lower consumption could make all the difference if adverse sea conditions develop unexpectedly after passing the point of no return.
No marine engine manufacturer has so far offered to equip small diesels with dual, fully redundant fuel-control systems, but a similar approach has been common practice since the early days of aviation. Most piston-powered aircraft have two complete ignition systems, including two spark plugs per cylinder. Ordinarily they operate in parallel, but if one system goes down, the other keeps all cylinders firing normally. Even this approach wouldn’t help deal with contaminated fuel &mdash likely the most common threat to marine diesels. However, it would provide insurance against the possible failure of a vital electronic component. In any event, the prudent operator of any oceangoing vessel equipped with an electronic diesel will likely wish to carry spare electronics modules along with the usual spare injectors, pump kits and even engine rebuild parts.
Contributing Editor Sven Donaldson is a freelance marine writer living in Vancouver, British Columbia.