Turbocharging of Diesel Engines
Turbochargers (also known as turbosuperchargers or turbos) for medium-heavy highway diesel engines, which boost intake air pressure, are now standard practice. The turbo principal involves the harnessing of pressurized hot exhaust gas to spin a radial-inflow turbine (the hot side) that directly drives a centrifugal air compressor (the cold side) on the same axle shaft turning typically over 100,000 RPM at full load. As the exhaust gas lets down its pressure and temperature through the turbine, mechanical work is done on the compressor side. Diesel engines are lean-burning with excess air—they need lots of air to make power and in the very largest diesels, power is limited by the ability of turbochargers to deliver adequate air, especially at higher altitudes. For stock street diesel engines, turbo boost above atmospheric air pressure can range from 30 to 40 psi, well above that for stock street turbo gasoline engines, which are typically 7 to 12 psi, or around 15 psi with temporary overboost.
See an example below of a typical production turbocharger for diesel engines, followed by a cutaway showing the cold compressor wheel and hot turbine wheel, among other components:
The turbocharger was invented by Alfred Buchi of Switzerland, with a first patent in 1905. Early US turbocharger experiments involved aircraft gasoline spark-ignited engines in World War I. Sanford Moss of GE demonstrated turbo-boosting of a Liberty V-12 on top of Colorado’s 14,000 ft. Pikes Peak in 1918. Diesels began to benefit from turbochargers by the 1920s, and the first turbocharged diesel truck appeared in 1938. German and Swiss turbo suppliers were the first to market for diesels, and Elliott in the USA began to offer a Buchi-licensed design to diesel OEMs in 1940. Industry legend Garrett (now Honeywell Turbo Technologies) began to supply the diesel engine community in the early 1950s.
A theoretical alternative to turbocharging is mechanical crankshaft-driven supercharging, a path not preferred for highway 4-stroke diesels, mainly due to packaging and cost, among other considerations. Crankshaft-driven superchargers, which draw lots of power,, are more commonly found on gasoline spark-ignited engines. The mechanical supercharger has superior low-RPM response, which is lacking in turbochargers. Drivers have to wait for spool-up, called turbo lag, which can be annoying. The turbo community improves low-end performance and transient response with variable-geometry designs—moving vanes or sliding nozzles on the exhaust side. Another strategy is e-boosting, which has an electric motor assisting on the axle shaft of the turbine, shortening spool-up time. H High electric power requirements have discouraged e-boost usage. To reduce lag, turbo designers can also use wheels with less inertial mass, closer coupling to the exhaust manifold, twin scrolls (one optimized for lower loads, one for higher), and bearings with less frictional drag.
Unlike gasoline spark-ignition engines with detonation limits, diesels can take all the air (intake manifold absolute pressure--MAP) that engineers can deliver, up to the point where combustion stresses will break up the engine. That said, many highway diesels today use exhaust gas recirculation (EGR) to control NOx emissions, and the exhaust can’t flow back into the intake if the manifold air pressure is too high. So, many diesels add an intake air control valve--ACV (a restrictive throttle!) to allow EGR to temporarily flow with a “positive delta P” when needed.
Turbochargers in road vehicles are often sized to overboost at full-speed/full-load conditions in order to assure a stronger low-to-midrange performance. To mitigate the overboost at higher power settings, a wastegate is often fitted inside the scroll housing under electronic control to open and release some exhaust pressure, thus limiting the RPM and air boost capability of the machine. In lieu of a wastegate, variable-position vanes or nozzles in the exhaust flow can be manipulated to limit boost. Another means to optimize control of air flow is to stage twin turbo so that a smaller turbo handles the low-mid range and a larger turbo kicks in only for higher-power needs. The medium-duty Ford diesel truck engines made by Navistar formerly used a variable-geometry single turbocharger from Honeywell/Garrett (6 liter V-8 engine), but switched to a 2-stage/fixed-geometry BorgWarner twin-turbocharger layout in the next-generation 6.4 liter V-8 engine.
Turbocharging not only improves specific power (hp or kW per liter) of diesels (on the order of 50 percent) as well as the power density (power per unit of mass), but also helps reduce exhaust emissions, by introducing lots of turbulent air mass to the combustion chamber. That turbo performance gain, however, is not exactly free. A complex system is added, involving the basic turbomachine, exhaust gas and intake air plumbing, a heat exchanger (the intercooler, also known as an aftercooler, or charge air cooler) to reduce heat of compression before engine intake, electronic or electro-pneumatic boost controls, engine coolant connections, engine lubricant connections, and mounting hardware.
ome people believe that turbochargers capture free exhaust energy with no penalty. However, all turbochargers present a restriction to the exhaust flow, raising back pressure and limiting power due to incomplete exhaust scavenging. Fortunately for diesel engine designers and owners, the vastly enhanced intake breathing associated with turbochargers more than makes up for the “plugged” exhaust. The higher specific power from turbocharging does add extra stress to diesel engine cooling systems as well as the lubricating oil, which can shorten its useful life.E. Engineers must understand and accommodate for this stress, as with added cooling capacity.
Turbochargers do have failure modes, and they are not guaranteed to last the life of a diesel engine. A common problem in the past was inadequate cooling for the shaft bearings (typically a oil film/plain bushing type, or more recently high-speed ball bearings), so that a hot soak could coke (pyrolize) the hydrocarbon lube oil, freezing the bearing. Turbo shaft bearings need a continuous flow of water/glycol engine coolant to their housings, even after engine shutdown. Lube oil passageways to the bearings may also clog up due to deposits over time. When the bearing freezes up or even slows down, the truck driver knows it immediately: power drops dramatically. By some estimates, a boost pressure drop of 10% can be readily detected by the driver. According to one major diesel turbo manufacturer, a leading life limiter for turbos today is fatigue failure of aluminum impeller (compressor) blades due to cyclic bending, especially with higher pressure ratios (over 3:1). Use of stronger/stiffer/heavier investment cast titanium impellers is one solution. It is essential to have good air filtration, as aluminum turbo impeller blades are subject to long-term deterioration from particle-impact erosion. On the hot side, there are few if any, failures due to creep or oxidation. Nickel-based superalloy turbine materials do a fine job (diesel exhaust gas temperatures are not that extreme), and heat-resistant ferrous alloy cast scroll housings are long-life items. Many experiments have been done with expensive monolithic ceramic hot turbines, but these are not found in production highway diesel turbos.
Highly stressed HD turbodiesel engines have demanding lubrication requirements. Engine manufacturers have recommended more frequent lube oil changes, if turbocharged. Lubrizol additives oil can help mitigate soot-related wear and viscosity increases while allowing the OEMs to meet stringent emissions regulations. Lubrizol advanced dispersant systems keep carbon soot particles separated in suspension, thus limiting abrasive wear. Lubrizol detergents and anti-wear chemistry control wear and deposits in both in the modern and heritage engines.
Selective catalytic reduction (SCR) is a widely popular means to reduce harmful smog-forming oxides of nitrogen (NOx) in diesel engine exhaust. NOx is formed at the high-temperature flame front commonly found in compression ignition engines. Nitrogen and oxygen are natural constituents of the engine intake air, but given sufficient heat and time will transform into unwelcome compounds (such as nitrous oxide (NO), and nitrous dioxide (NO2) the most prevalent). SCR is a well-proven technology known to reduce NOx by more than 90%. Click here to read more about SCR.