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Volvo Penta has a remarkable history when it comes to marine power. The company, which began in 1868 as a foundry specializing in cast-iron goods and hardware, made its entry into marine engines in 1907 with a paraffin-fired test engine dubbed the B1. In 1922 it introduced the U2, a two-cylinder, two-stroke outboard, and in 1954, the world’s first series-manufactured turbodiesel joined its growing line of engines, along with the MD1, the world’s smallest direct-injection diesel. Two years later Volvo Penta introduced the first charge-air-cooled marine diesel.

Lately this Swedish industrial giant has concentrated on developing a series of engines incorporating the latest electronic technology that could deliver power and torque where they are needed most. Complicating the project were the all too-familiar concerns of safety, performance, fuel efficiency, and of late, emissions. That program has now culminated the new D-12s, four diesels rated at 615, 650, 675, and 700 hp.

The D-12 series is based on a 12.1-liter, turbocharged and aftercooled in-line six featuring one electronic unit injector for each cylinder. Its induction-hardened crankshaft has seven main bearings for durability and two vibration dampers designed to reduce the roughness inherent in a diesel engine. A freshwater aftercooler increases horsepower while minimizing corrosion, and a large expansion tank is designed to accommodate optional keel cooling.

Because the unit injectors develop very high injection pressures, fuel is well atomized and dispersed when it enters the combustion chamber, creating more complete combustion. Little fuel is returned to the tank, which prevents warming of the tank’s fuel. Cooler fuel results in more horsepower.

Making sure all that horsepower is used efficiently by the propulsion system was a major concern of Volvo Penta engineers. “We designed this engine to develop power and torque at the engine speed where it is most needed,” says Urban Larsson, technical manager of the D-12 project. “No power or fuel is wasted on torque that the propeller cannot handle or power at the wrong rpm.” To understand how the engineers accomplished that, you need to understand something of the nature of horsepower and torque aboard a boat.

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