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SECTION II DIeSeL eNGINe FUNDaMeNtaLS
the crankshaft. Turbochargers are also on high output engines along with the supercharger to improve volumetric efficiency.
Detroit Diesel General Motors’ Detroit Diesel division originally developed the most popular version of two-stroke diesels in the 1930s for US military patrol boats; millions of these 71 series engines were produced and used around the world. Until the late 1970s, this engine represented astounding, advanced construction and operating concepts, which remained mostly unchanged until it was discontinued in the late 1980s. This engine still has, by far, the best hp-to-weight ratio of any diesel. It included innovative features, like modular engine construction with cylinder kits in standard displacements of 51″ (130 cm), 53″ (135 cm), 71″ (180 cm), and 92″ (234 cm) that were used to make inline or V-block engines in 4, 6, 8, and 12 cylinder configurations. A roots blower, a device used on farms to blow grain into silos, was used to pressurize intake air almost half a century before turbocharging became common place. Other features of the two-stroke Detroit Diesel include: ■ Weight and power: With twice as many power strokes per engine revolution as a four-stroke cycle engine, the Detroit Diesel two-stroke engine produces more power than a four-stroke diesel of the same displacement ■ Response and acceleration: Because each cylinder of a two-stroke engine produces a power stroke every revolution, there is a quick response to load changes. These engines have no turbocharger lag and change speed quickly. The engine is a high-speed diesel, which made it ideal for marine and urban transit bus operation when connected to an automatic transmission. ■ Smoothness: Two-stroke engines run smoother than four-stroke engines. The two-stroke cycle distributes the combustion force each cylinder produces, using two lighter power impulses per two revolutions instead of the single heavy impulse of a four-stroke engine. The lighter, more frequent power impulses mean less torsional dampening (the rapid speeding up and slowing down of the crankshaft) is required from the flywheel or torsional vibration dampener. ■ Lower exhaust temperatures: More air goes through a two-stroke engine than a four-stroke engine for the same amount of fuel consumed. In fact, in a two-stroke engine up to 30% of the engine cooling is performed by pushing air through the cylinders during scavenging. Additional airflow produces lower exhaust temperature and longer exhaust valve life.
ecoMotors Opposed-piston Opposed-cylinder two-Stroke engines New, innovative two-stroke engines developed by EcoMotors use opposed-piston opposed-cylinder (OPOC) technology FIGURE 6-38 . Navistar is currently collaborating in the development of this engine technology. Navistar’s EcoMotors EM100 design uses two cylinders and four pistons. Outboard pistons take the place of the cylinder head, and each piston only travels half the distance needed for a complete stroke FIGURE 6-39 . Short
FIGURE 6-38
A prototype of Navistar’s EcoMotors EM100.
stroke lengths enable higher engine speeds. The engine uses a turbocharger with an electric assist that can supply boost pressure during starting. Using a modular engine design, more cylinders can be added in pairs with a clutch between additional cylinder modules. This configuration allows one cylinder pair to start the engine or provide power at light loads. Using a clutch to engage other cylinders keeps power losses, which are caused by moving gases in and out of the engine, to a minimum. Current test engines have achieved 100 mpg (43 km/L) in automotive applications. This technology is likely to be used in trucks and buses for medium-duty engine applications. The advantages of Navistar’s EcoMotors EM100 include: ■ 50% fewer parts than an equivalent four-stroke cycle engine ■ lower production costs due to few parts ■ the cylinder head and valve train are eliminated, which reduces engine complexity ■ 15‒50% improvement to fuel economy ■ high thermal efficiency for a reduced cooling system capacity ■ ultra-low exhaust emissions, which may eliminate the need for exhaust aftertreatment systems
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Engine Systems
Engine operation also requires other subsystems, which are covered in detail in later chapters. These systems include cooling systems, engine brake systems, fuel systems, lubrication systems, and air induction and exhaust systems.
Cooling Systems Cooling systems are used to remove excess heat (produced by the combustion process) from the engine. It also has two other purposes: heating the passenger compartment and removing excess heat from the airflow into the intake manifold.
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