The Turbo Trade-off

The latest iteration of lightweight turbo diesels packs more horsepower per pound than ever before, they run cleaner and smoother than their predecessors, and they bristle with electronics that measure vital signs, control injector output and can communicate with NEMA 2000 networked instruments anywhere on board. But how “pumped upâ€� can the horsepower of a small, light diesel get before you sacrifice performance for longevity? Some feel that better engineering has opened the door to more efficient, more reliable engines with no noticeable drawdown in longevity, while the 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.

Not so long ago, the heavy, reliable, and distinctly noisy and sooty Detroit Diesel owned the waterfront. Then companies like Yanmar, Volvo and others saw a market emerge for lightweight, high-rpm diesels, and boaters flocked toward these high power: 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 hot rod approach that was reengineered to go to sea.

At the heart of this diesel makeover is the turbo charger and associated intercooler that cured the naturally aspirated diesel engine’s problem of weak lungs. The turbo approach to boosting horsepower is based on getting more oxygen into the combustion chamber to ensure 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 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 two-cycle diesels, and scavenging exhaust gas from the combustion chamber was the name of the game. As the popularity of four-cycle diesels grew, the dedicated exhaust stroke of the combustion cycle, not present in two-cycle technology, lessened the need for an external boost of air pressure. In short, the era of the simplified, naturally aspirated four-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 one 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 displacementmode cruising lifestyle. The desire for better performance, however, marked the end of this diesel era.

In order 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.

A turbo differs from a supercharger or blower in how the increased air pressure fan is driven. In the case of a supercharger, the drive force is from belts or gears that tap into engine-developed rotary energy, while a turbocharger is driven by exhaust gas exiting the exhaust manifold. A turbo is like a water wheel in a stream running a centrifugal water pump supplying water to irrigate a field. With 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 the exhaust system is of large enough diameter and a short enough of a run to meet the scavenging needs of the turbo.

Half of a turbo system operates in an acidic, high-exhaust gas temperature range, constantly buffeted by a stream of fine carbon and sulfur waste particles, while the air compressing side of the system needs to run as cool as possible. This puts significant demands upon metallurgy, lube systems and cooling components, and the durability of turbocharged engines is admittedly not as good as their lower output, naturally aspirated cousins. The jury is still out with regard to actual in-the-field life expectancy, but the era of the DD 6-71 and Cat 3208 is unlikely to be matched by engines that are 50 percent lighter and make twice the horsepower at twice the rpm. Many experts, however, see new, lightweight diesels as proving to be much more reliable than originally thought.

A turbocharger produces boost, or increased intake air pressure, and in doing so elevates the temperature of the compressed air. But since hot intake air is less dense than cooler intake air, there’s less oxygen available in the former. To lessen this negative effect and get as much oxygen into the combustion process as possible, an intercooler, also known as an aftercooler, can be built into the system. This is a heat exchanger downstream of the turbo that allows compressed air to pass through water-filled cooling coils in order to lower the air temperature before it reaches the air intake manifold. Some intercoolers are made of ferrous metal, while others are made of much less corrosion-prone material, but all need to be regularly cleaned and inspected.

Performance boat owners would find that turbo boost pressure would be at its highest at the wrong time in the power curve if it wasn’t for an ingenious device called a wastegate. The purpose of this exhaust gas bypass is to shunt exhaust flow away from the turbine blades when the intake manifold pressure rises above a preset point. This is accomplished via an intake manifold pressure sensor attached to linkage that opens the wastegate when the manifold pressure exceeds about 10 psi in low-pressure turbos, and 20 to 30 psi in high pressure or multi-stage turbos. The result is a decrease in turbo spin near top rpm, and a logical link between maximum boost and more mid-range rpm readings. High-performance engines can have two-stage turbos, and some use titanium fan blades to cope with the high heat and severe nature of the working conditions. Multiple valves, efficient electronic injector timing and fuel volume control, as well as better atomization of the injector spray, complement the turbo’s oxygen-rich boost, causing combustion’s big bang to get even bigger. In less performance-oriented applications wastegates and intercoolers are eliminated.

Another up side to modern turbocharged diesels is their tendency to run much cleaner thanks to more complete combustion. Some engines even link injector output to turbo boost level, eliminating the black smoke associated with rapid acceleration. Better fuel metering and plenty of oxygen for combustion means a well-proportioned fuel/air mixture regardless of the rpm at which an engine is running. The net effect of this turbocharging and intercooling process can easily increase horsepower by 50 percent. Add higher volume injection capacity and more rpm and a naturally aspirated 100-hp diesel can be pushed to develop twice that horsepower.

Those wondering how much power to coax out of a given diesel engine can learn a good lesson in longevity from the hotroding done to the Detroit 6-71, Caterpillar 3208 and other vestiges of the good old days. The former DD 6-71 delivered 250 conservatively calculated horsepower and could run 24/7 with little more than regular oil changes. But by adding a turbo in series with the blower, and plumbing an intercooler to even further increase air density, even this heavy workhorse was off to the races. And by adding higher capacity fuel injectors and a better scavenging exhaust system, the “Jimmyâ€� could end up pumping out much more horsepower. Its workhorse demeanor, as well as its long-haul reliability, were dealt a blow, and the tradeoffs to gain performance took a toll on longevity.

How much to push the power:weight ratio through turbocharging and other factory or speed shop options depends on how you use your boat and how long you want your engine(s) to last. Folks crossing oceans aboard a single-screw trawler tend to favor low-tech rather than the latest high-tech alternatives, favoring naturally aspirated, slow-turning engines like the venerable Gardiner line of diesels. The horsepower: weight ratio of these behemoths may be low, but the reliability and longevity stats are off the chart.

The tips of a turbo’s fan blades spin at speeds that can break the sound barrier, causing harmonic noises that rival fingernails on a chalkboard. Some engines exhibit this tendency much more than others, so before deciding upon a new model for repowering, try and go for a ride aboard a boat with the engine you’re interested in and see how it sounds throughout its rpm range. Sound dampening can do an amazing job of decreasing decibel readings, but some high-pitched turbo whines can remain more than just a minor nuisance.

Many engine manufacturers offer several horsepower variations based upon the same block, with the increase in power linked to better air- and fuel-delivery components. Those willing to sacrifice speed for longevity and reliability should look at the lower output alternatives, while those looking for maximum performance will head for higher horsepower per liter of displacement. More bang per power stroke and higher rpm spell more wear and tear, and despite better engineering practices, it’s hard to subdue the effects of heat and friction. After all, when all is said and done, a diesel engine is simply an elegant means of wrapping metal around a series of controlled explosions that turn potential fossil fuel energy into rotary force. The more violent these explosions become, the more stress and strain on components and the greater the fatigue. The true art in picking the right engine for your boating needs is based on a clear understanding of the hotrodder’s dilemma, and not being too greedy.   

Ralph Naranjo is a freelance writer and photographer living in Annapolis, Md.

By Ocean Navigator