|From Ocean Navigator #115 |
Auxiliary engines have been packed with more power than ever, but still need to fit into a small space; as a result increased rpm and turbochargers are now the norm for sailboat diesels.
The very word “turbocharger” conjures up the image of power. Everything from computers to kitchen cleanser has had the prefix “turbo” added to signify more power or speed. In fact, a turbocharger will indeed help an engine deliver more horsepower, torque, and speed (to the vehicle it is propelling), while reducing emissions, fuel consumption, and noise (incidentally, it will also produce better performance at high altitude). Additionally, the horsepower-to-engine weight ratio can be significantly enhanced, effectively squeezing more power out of a smaller, lighter power plant. This may sound like a free lunch; however, it’s not. The quid pro quo is more wear and tear on the engine and greater expense. Turbos are precision-machined pieces of gear, and they are expensive to produce. Additionally, turbocharged engines have greater demands placed upon them and, therefore, require maintenance that is religious in its regularity. The turbocharger also adds more complexity, moving parts, and the opportunity for failure to an engine.
Although turbochargers are a comparatively recent addition for pleasure craft, both power and sail, the first one was actually invented in 1905 by a Swiss engineer named Alfred Buchi. In 1915, he introduced a prototype turbocharged diesel. Diesels in those days were massive, so there was no market other than for large stationary plants, ships and locomotives. In 1925 the first seagoing, turbocharged diesel engines (2,000 hp) were installed in two German ships. This led to additional research in the area of turbocharged gasoline aviation engines, and by WW II most military aircraft were turbocharged, albeit gasoline. The Germans did field several diesel-powered aircraft beginning in the 1920s through the end of the war. The most well-known turbocharged version, a Junkers JUMO 205, enabled the reconnaissance aircraft it powered to operate at an unprecedented 50,000 feet.
By the mid-1950s, many major diesel engine manufacturers began experimenting with, and ultimately offering, turbos for truck and other large power plants. Fast-forward to today, and nearly every diesel engine manufacturer offers a turbocharged engine in a vast range of sizes and horsepower. These turbocharged diesel engines power every type of marine craft, from small sailboats to megayachts.
In order to understand how a turbocharger works, a discussion of basic terminology is in order. It is important to remember that turbochargers have few things in common with most other machines. Bearings, seals, and metallurgy have more in common with gas turbines rather than internal combustion engines. This is a result of high rotational speeds, demanding lubrication needs, and the temperatures at which they operate.
All turbochargers share the same basic components: turbine wheel, compressor wheel, turbine housing, compressor cover, and bearing housing. The turbine wheel is a fan of sorts that is acted upon by exhaust gases. Fast-moving exhaust gases cause it to spin very quickly, in some applications at 150,000+ rpm. The compressor housing contains these gases. As a child, did you ever hold your rubber-band airplane out the car window while your parents weren’t looking? The little red prop spun vigorously, and this is what the exhaust gases do to the turbine wheel, only much faster. In fact, as an illustration of just how much faster, the blade tips actually exceed the speed of sound.
The compressor wheel, which is connected to the turbine wheel by a shaft, now spins as well. This device is a centrifugal air pump, which draws air into its center and slings it out where it is captured and channeled by the compressor cover. The compressed air created by this turbine is sent to the engine’s air-intake manifold. The turbine compressor connecting shaft spins on bearings, which are located within the bearing housing. These are the most basic components of the simplest form of turbocharger. Additional components and terminology include the wastegate, which controls the amount of exhaust gas that is channeled to, or passed by, the compressor wheel. Its function is to prevent smaller turbos from over-revving. The boost pressure is a measure of just how much air the turbo is pumping into the engine, and it’s usually measured in pounds per square inch, or bar (each bar is equivalent to one atmosphere, or 14.7 psi). Typical boost pressures vary depending on the application; however, averages are in the six- to eight-psi range. Turbochargers are simply air compressors powered by exhaust gases.
One of the side-effects of compressing a gas, however, is an increase in its temperature (remember Boyle’s Law from high school science class?). Since hot air is less dense than cool air, and therefore contains fewer molecules, hot air burns less efficiently in the combustion process. An optional piece of equipment designed to rectify this problem is the charge air cooler (CAC), sometimes referred to as an inter-cooler or after-cooler. The compressed heated air, after leaving the turbocharger, passes through the CAC and then on to the engine intake manifold. Also running through the CAC, but separated from the air by tubing, is either seawater or coolant, called salt water after cooling (SWAC) or jacket water after cooling (JWAC), respectively. This intermingling process allows the liquid to absorb some heat from the charged air. The result is cooler, and thus denser, air, which is more advantageous to the combustion process.
Because of the environment in which turbos operate, already mentioned high speeds and high heat (over 1,200 degrees F), the materials and manufacturing processes used for many of these components are necessarily exotic indeed. The turbine housing, for example, is fabricated from spheroidal graphite iron. This material has good thermal fatigue resistance, and it’s strong enough to contain shrapnel-producing turbine wheel “bursts.” The bearings used in most turbos are not the familiar ball-bearing type. Because of the speeds at which these components turn, the turbine shaft is separated from bronze journal bearings by a high-pressure oil “wedge,” sometimes referred to as hydraulic stabilization, which is thinner than a single human hair. For this reason, when the turbine, compressor, and shaft are properly balanced, they are actually very lightly loaded. However, if a non-running, and therefore oil-wedgeless, turbo blade is turned gently by hand, it may appear to drag. As long as the blade tips are not contacting the turbine or compressor housings, this is normal. Without the oil wedge, the bearing is not functional. Turbine wheels, living the hellish existence they do, must be extremely durable. These are frequently fabricated form high-nickel super alloys, which will withstand high temperatures, resist corrosion and metallic creeping. Compressor wheels, while not subject to extreme heat, must still be durable and creep resistant (creeping is metallurgist terminology for loss of shape). They are fabricated from copper, silicon, aluminum alloys, which are solution treated and aged. For extremely high-pressure applications, such as high-output gensets, cast titanium is the order of the day.
How does all of this exotic, expensive, and arcane gear enable a turbo diesel to produce up to 50% more power than its naturally aspirated cousins? Diesel engines are air pumps, as are all internal-combustion power plants. They draw fresh air in, compress it, mix it with fuel, burn it, and expel the exhaust. In the process, chemical energy is transformed into heat energy, which then produces mechanical energy. The turbo-charger simply improves upon this process by forcing more air into each cylinder on every fresh-air intake stroke. More air added to the equation means more fuel can be added as well, and the result is more power from each given combustion event – i.e., each time a cylinder fires. The result is a more powerful power plant without increasing displacement, size, or weight (except for the turbo itself). The energy, which spins the turbine and thence the compressor wheels, is derived from the high-pressure exhaust gases, which are present whether a turbo is fit or not. The small price to be paid, from a conservation of energy standpoint, is a slight increase in back pressure created by the restriction of the turbo’s turbine wheel. This requires the expenditure of slightly more horsepower to expel the exhaust gases during the exhaust stroke. However, in spite of this deficit, the net gain is appreciable. Turbochargers are able to increase a diesel engine’s volumetric efficiency upwards of 150%.
Frequent oil changes
As mentioned earlier, not all of this derived efficiency is a free lunch. In addition to the previously mentioned issues of increased impact on the engine and cost, there are other turbo demons with which to contend. Because turbochargers require a constant supply of lubrication for the high-speed turbine-compressor shaft, regular oil changes are even more critical than those for naturally aspirated diesels. The oil supplied to the turbo provides not only the bearing wedge, it also acts as a heat sink, assisting in maintaining workable temperatures within the bearing housing. One of the most common causes of turbo bearing failure is a result of the lube oil’s exposure to extremely high heat. When a turbo-charged diesel is run under heavy load for extended periods, it must be allowed to cool off before being shut down. If this turbo cool-off procedure is not observed, a process known as carbonizing occurs. In this process, the lube oil left in the turbo bearing journals literally cooks, leaving behind an abrasive carbon deposit. The next time the engine is started, this gritty substance scores the bearings and clogs oil-supply ports and drains, dramatically shortening the life of the turbocharger and perhaps the engine. When running under heavy loads, these unfortunate circumstances can be forestalled by idling a turbocharged diesel for five minutes before shutting down.
For many years turbos in the gasoline/automotive industry suffered from a condition known as turbo lag. This side-effect is a function of the time required for the exhaust gases to overcome the mass of the turbine compressor assembly. It has been largely overcome by making turbocharger assemblies smaller and lighter, which makes them more reactive to speed changes. The addition of a wastegate then prevents the lighter wheel from turning too quickly at high speed. More recently, turbo lag has also been combated by the introduction of electronically controlled diesels (see sidebar).
Because turbochargers are built to withstand such rugged environments, they are comparatively long-lived and reliable. However, they do require service, and they are frequently blamed for problems that originate within the host engine. The most common cause of turbocharger failure is oil lag. Applying a heavy load to a turbocharged diesel immediately upon start-up causes this. As the name implies, oil lag occurs when the oil pressure lags behind turbo loading. Not only will this practice inflict damage upon the turbo, it will also cause substantial wear to all bearing surfaces within the host engine. Oil lag can also occur when the host engine is shut down while turning at high rpm (a delivery truck driver who visits my boatyard on a daily basis regularly races into the driveway and shuts the engine down so quickly that the turbo can be heard spinning at the tailpipe for 30 seconds!). When this occurs, the turbo is effectively running with no lubrication because the oil wedge dissipates as soon as the engine is shut down. Another common problem many turbos face is foreign object damage, also known as FOD. This occurs when an object is injected into the turbo compressor wheel. At such high rotational speeds, severe and irreparable damage occurs instantly, and the turbo must then be removed and rebuilt, assuming it’s salvageable. FOD can only occur if a component within the air filter or its mounting dislodges and is sucked into the intake stream or if the engine is operated with a faulty or removed air filter. Both of these scenarios are nearly 100% preventable. Other failure and maintenance issues include imbalance of rotating components, incorrect manufacturing materials or dimensions for any component, and the previously mentioned carbonizing. Oil seals do occasionally leak, and this is usually apparent by the presence of oil residue on the compressor wheel. However, a malfunctioning host engine crankcase ventilation system could cause the same symptom. In this case, the turbo is not at fault, and if it is replaced the problem will persist. Because turbos are carefully balanced, high-speed machines, defects in manufacturing usually become readily apparent early on in their service lives. As a result, the belief among most turbo manufacturers is, the longer a turbo has been run without any problems, the less likely it is to fail.
In order to ensure maximum life from your turbo and its host engine, regular service checks should be performed, ideally at each oil-change interval. The checklist begins (engine not running) with a careful visual inspection of the turbocharger turbine housing and compressor cover. Look for cracks, heat discoloration (the iron turbine housing may develop light surface rust – this is normal) and loose or missing hardware. Check for oil and exhaust leaks, evidenced by trails of oil or soot. Next, remove the air-intake filter and inspect the compressor wheel blades. The blades should be free of chips, nicks, dents, and oil. The wheel should also spin freely, although some drag, as mentioned earlier, is normal. If oil is discovered on the compressor wheel, and the source is identified as either the compressor itself or the host engine, any repair must include cleaning the interior of the charge air cooler, where equipped. Oil leaking into the air passages within the CAC will clog it, which will ultimately reduce engine power. Next, replace the air-filter element and start the engine. Run it until normal operating temperature is reached (over 160 degrees F). Remember, the turbine housing is hot. Listen for high-pitched noises, which would indicate air leaks. Also, note any exhaust smell (your eyes should not be watering while you are in the presence of a running diesel – if they do, there may be an exhaust leak).
The primary preventive maintenance procedures for turbochargers are to ensure an ample supply of clean air and oil. Take care of the engine, and it will take care of the turbocharger.
One turbocharger manufacturer’s advertising slogan reads, “breathing life into the machine.” This is exactly what turbos do.
Some dos and don’ts
Don’t load a turbocharged engine (or any engine for that matter) immediately upon start up. Wait at least until oil pressure is indicated, applying moderate loads the first 15 minutes.
Don’t idle for extended periods. Low turbine and compressor pressure and slow shaft speed will lead to leaking oil seals. This will foul the turbine and compressor wheels with oil, cause smoke, foul the CAC if equipped, and reduce overall efficiency.
Do practice proper cool-down procedure before shutdown. This primarily applies when running under heavy load. Adopting this practice will avoid heat soak-back, which leads to carbonizing.
Do change oil, oil filter, and air filter at manufacturer-recommended intervals. Change these components more often if you are running under light loads on a regular basis. This is because light loads tend to make a diesel run cool, which leads to oil sludging.