Diesel engines are a standard means of marine propulsion. Almost every large commercial vessel is powered by an inboard engine of that type that is linked by a horizontal shaft to an external propeller. But when vessels are so small that they need an outboard motor, then gasoline (petrol) engines are pervasive.

Diesel propulsion offers a number of operational benefits, including durability, extended range, ease of access to supply and—perhaps most importantly—enhanced safety inherent in the reduced flammability of diesel compared with petrol. The latter factor may be particularly pertinent where military vessels might come under hostile fire. Indeed the NATO defense alliance has a stated intention to achieve a “single fuel” capability in which diesel supplants petrol in all circumstances.

A Swedish company is finalizing development of what it claims to be the first “high-performance, outboard marine diesel engine”, which it calls the OXE Diesel. The company, Cimco Marine, is based in Engelholm and was founded in 2012.

(Click to enlarge.) Schematic of the engine. Image source: OXE(Click to enlarge.) Schematic of the engine. Image source: OXEThe OXE unit is based on an off-the-shelf, turbocharged, four-cylinder, two-liter-capacity diesel engine capable of producing 200hp at 4,100rpm that was originally developed for automotive use by General Motors. That initial German-built engine, says international sales manager Vladimir Jigaroff, is a product that complies with the latest “Euro5” emission standard.

The innovative aspect of the concept, however, is in the engine’s drivetrain, which uses a belt drive system known as a self-contained belt propulsor unit (S-BPU). This unit enables the engine to dispense with either bevel gears or solid vertical shafts to transmit the power output to the propeller. It also means that the cylinder block can be aligned horizontally in keeping with its original design intent.

Driving Efficiency

Jigaroff says that inverting an internal combustion engine by 90 degrees as is usually the case with outboard motors “compromises the basic physics” in ways that are detrimental to its efficiency. In operation, he says, the OXE engine would consume a little more than 40 liters of diesel to deliver a level of performance for which a petrol-driven counterpart would require some 70 liters of fuel.

Indeed, Jigaroff says that the engine is in effect “an inboard engine that can act like an outboard.” But this also means that the OXE unit can perform supplementary functions that normally only conventionally mounted engines can achieve. For example, because it has a closed cooling system, waste thermal energy can be drawn off to provide cabin heating. It also can be integrated with an alternator to provide electrical power for the boat; indeed, power output of the engine can be as high as 220 amps.

A third possible function is that the engine can provide a “crashstop” capability by switching directly from full power forwards to full power in reverse operation, of benefit for military applications. Jigaroff says that this is facilitated by the fact that the engine's gearbox is an “oversized, multi-clutch” unit.

But the belt drive principle that the engine uses does more than enable horizontal positioning of the cylinder block with consequent preservation of its energy and environmental efficiencies. Jigaroff says that it also facilitates quiet operation and—in the case of the OXE unit—also offers the potential for upgrading the power output. It can handle “huge torque,” he says, adding the company believes that the existing belt drive design could be made to transmit up to 400hp of power and that further development could push that figure to 500hp.

Above the Waterline

The use of a belt drive also has an implication for the engine’s overall configuration. Specifically, it means that apart from the propeller, all the key elements of the engine that need to be protected against damage that could result from a vessel running aground are above the waterline. A further benefit is that the engine’s underwater profile can be streamlined and therefore produce less drag than otherwise might be the case.

Moreover, the fact that the protected elements include the gearbox facilitates another unique capability. As Jigaroff explains, the gearbox is fixed in place by four screws and is also easily accessible and therefore can be dismounted and replaced in an inverted position. This can be done in an operation lasting only about 10 minutes even if the vessel is at sea. It provides the engine with a completely different gear ratio that in turn enables the torque it generates to be either 397NM or more than 730NM, meaning that a single vessel can effectively be reconfigured for two totally different types of service – either for fast independent operation or slower operation with a heavier load. “It should be able to pull 40-50 tonnes,” he says. The whole engine weighs 295-320kg depending on details of its configuration.

The engine includes two belts, one that runs from the cylinder block to the gearbox and one that runs from the gearbox to the propeller. Both are made from what Jigaroff says is a “carbon fiber multi-composite” material. The precise details, he says, are a company secret that remain confidential because their resilience is crucial to the engine's ability to provide high performance.

The engine’s origins, Jigaroff says, date back nearly a decade to work developing a belt drive system that could operate with conventionally mounted marine diesel power units. A few relatively small examples of outboard diesel engines existed at the time, but with power ratings in the range of 26-37hp. That meant they could only be used in light craft. Since then, however, further development has facilitated the appearance of outboard diesels with roughly double the power output.

The engine is not yet commercially available as a standard product but pre-production units are being sent to distributors for evaluation in several countries, including the U.S., UK and Australia. The UK test will be done by Proteum. The firm says that it will provide a trial unit to a commercial salmon farming operation that will use the engine on a workboat operating north of the Scottish mainland.