The latest iteration of lightweight turbo diesels packs more horsepower per pound, runs cleaner and smoother than its predecessors and bristles with electronics that measure vital signs, control injector output and can communicate with NEMA 2000 networked instruments.
But among savvy users, there's a thread of conversation about how "pumped up" the horsepower of a small, light diesel can get before a skipper shopping for performance sacrifices engine longevity.
Some feel that better engineering has opened the door to more efficient, more reliable engines, with no noticeable decline in longevity. Skeptics say it's like putting a higher-voltage battery in Grandpa's pacemaker: The initial result may be spectacular, but the long-term prognosis may not be so desirable.
Modern turbocharged diesels run much cleaner thanks to more complete combustion.HOTRODS AT SEA
Making sense of the turbo era starts with a sternward glance. Not so long ago, Detroit Diesel owned the waterfront, Caterpillar was an upstart, and Cummins had yet to move from the highway to the high seas. The name John Deere meant tractors, Yanmar was at war with Westerbeke over the 50-horsepower-and-under sailboat market, and MAN, MTU and other European superstars were still on the other side of the Atlantic. The low-rpm lingo of the day alluded to more iron equaling a longer lifespan, and small- to medium-sized planing powerboats were relegated to converted-big block V-8 gasoline engines rather than diesels, because none were light enough or small enough to be shoehorned into these smaller boats.
Yanmar, Volvo Penta and others saw a market emerge for lightweight, high-rpm diesels, and boaters flocked toward these high power-to-weight-ratio engines. This paradigm shift in engine design boosted boat performance and redefined engine technology, creating a market for diesel-driven speed. One of the key ways engine manufacturers answered the challenge was by improving combustion efficiency and upping the rpm – a hotrod approach that was re-engineered to go to sea.
At the heart of this diesel makeover is the turbocharger and associated intercooler, which cured the naturally aspirated diesel engine's weak lungs. In essence, the turbo approach to boosting horsepower is based on getting more oxygen into the combustion chamber, ensuring more complete oxidation of the fuel-air mixture.
A given volume of air is only about 20 percent combustible oxygen, with the remainder almost totally made up of inert nitrogen. Even in the days of the venerable Detroit Diesel 6-71, arguably one of the most reliable engines ever built, the value of clearing out exhaust gas and introducing new air gave rise to a "blower" or "super charger" to force air under pressure into cylinders. These were long-stroke, 2-cycle diesels, and scavenging exhaust gas from the combustion chamber was the name of the game.
Performance boats depend on a wastegate to shunt exhaust flow away from the turbine blades when the intake manifold pressure rises above a preset point.As the popularity of 4-cycle diesels grew, the dedicated exhaust stroke of the combustion cycle, not present in 2-cycle technology, lessened the need for an external boost of air pressure. In short, the era of the simplified, naturally aspirated, 4-cycle diesel was at hand, and with fewer moving parts, low rpm and less-explosive detonations, it proved to be an era marked by engine longevity and reliability. This was the period when you could disconnect the battery from the engine once it was started, and the engine would keep running. Pumps were belt or gear driven, timing was mechanical or hydraulic, and the days of simplicity and reliability matched the easygoing displacement-mode cruising lifestyle. But the desire for better performance marked the end of the dominance of dinosaur diesels.
ANALYSIS OF A TURBO
To get more power and therefore more speed from the same displacement, an engine can be set up to run faster, and the explosion at the end of each power stroke can be made more efficient. Thus the art of turbocharging evolved as a means to achieving both of these goals. In essence, a turbo differs from a supercharger or blower in how the air-pressure inducing fan is driven. In the case of a supercharger, the drive force is derived from belts or gears that tap into engine-developed rotary energy, while a turbocharger is driven by exhaust gas exiting the exhaust manifold.
In essence, the turbo is like a water wheel in a stream running a centrifugal water pump supplying water to irrigate a field. In the case of the waterwheel, the energy of an untapped stream flow is being harnessed, while in the case of a turbocharged engine, the energy of untapped waste exhaust gas is used to increase air-intake pressure, and therefore increase the volume of oxygen that enters the combustion chamber.
Unfortunately, the facts of life when it comes to internal combustion are not quite so simple. Back pressure in an exhaust system can have a significant negative effect on performance, and when a turbocharged engine replaces a naturally aspirated model, it's vitally important to make sure that the exhaust system is of large enough diameter, and a short enough run, to meet the scavenging needs of the turbo.
