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Supercharging Diesels

Turbochargers vs Superchargers

 

 

 

 

 

Superchargers, a forced induction device, are most commonly found under the hood (or mounted through it) of high performance gas engines, or on race engines using more exotic fuels such as nitro and alcohol. However, superchargers are slowly becoming popular as performance enhancers for diesel engines too. While there are currently a limited number of options available, select versions of the Duramax, Cummins, and Power Stroke engines have complete bolt-on kits readily available. Empire Diesel Performance has been a major pioneer of the adaptation and proved that concept produces an impressive street machine. Surprisingly, supercharging diesels is not as new as you may think; many of the old Detroit 2 stroke diesels were supercharged from the factory.

Supercharger vs Turbocharger

A turbocharger has a compressor wheel connected to a turbine wheel via a common shaft. Exhaust gases directed at the turbine wheel force both the turbine and compressor to rotate. As the compressor wheel rotates, air is drawn in, compressed, and forced into the engine. A turbocharger converts wasted heat energy into usable mechanical energy, increasing engine output. And since turbochargers rely on waste energy, a turbocharger must reach a minimum speed before it converts enough of this wasted energy to produce positive pressure into the engine. This is known as turbocharger lag, the time in which it takes for a turbocharger to reach the point when it builds boost. The pressure that a turbocharger creates is relative to engine load and not engine speed.

 

A turbocharger's characteristics, including responsiveness and maximum airflow, depend on a number of design considerations. Turbocharger compressor and turbine wheel sizes, compressor and turbine housing sizes, and the geometry of the compressor wheel all play a considerable role in manners. This is how turbochargers are produced in such variety - from small, quick spooling turbochargers to large, high flow models, and everything in between. Modifying performance characteristics therefore requires swapping turbos, adding more, or at the very least, changing compressor wheels.

 

Superchargers feature a compressor or screw rotors that are driven off the engine crankshaft via a belt drive - as the crankshaft spins, the compressor spins. The supercharger draws in ambient air, compresses it, and feeds it to the engine. Unlike a turbocharger, a supercharger uses energy from the engine to increase the total engine output. The pressure that a supercharger creates is relative to engine speed - as long as the engine is spinning, the supercharger is compressing air and forcing it into the engine. The pressure increases with engine speed, but peaks well before the engine reaches redline (for most street superchargers, peak pressure is reached shortly off idle).

 

A superchargers characteristics are less about geometry and more about displacement. Getting more pressure (boost) from a supercharger is as simple as changing the size of the drive pulley so that it spins either faster or slower relative to engine speed. A larger supercharger will obviously meet higher airflow demands, while a small one will be more limited. There are also several types of superchargers, the most common of which are the roots type, screw type, and centrifugal. A roots style supercharger, which mounts directly to the intake manifold, has a pair of lobes that rotate relative to one another, compressing the air between them. A screw type supercharger utilizes, literally, a large pair of screws that compress air between each other, and is mounted directly atop the intake manifold. A centrifugal supercharger works much in the same way the compressor side of a turbocharger does, but is powered by the engine via a pulley rather than by exhaust gas. Centrifugal superchargers are common because, aside from other factors, finding room for one under the hood is relatively easy.

 

 

Turbocharger

Supercharger

Advantages:

• Highly efficient, powered by engine waste heat (exhaust).
• Capable of providing high volume and boost pressures.

• Instant boost - no lag since system operates relative to engine RPM. If the engine is spinning, the supercharger is creating boost.

Disadvantages:

• Turbo lag - boost pressures dependent on engine load. Turbochargers do not spool instantly and provide very little boost under light load.

• Requires power from engine to operate, less efficient.
• Limited boost/airflow characteristics.

Supercharged Diesel

By playing on the advantages of both a turbochargers and a supercharger, a system can be created that provides instant throttle response and higher boost in low-load driving situations while meeting the airflow requirements of the engine under heavy load and high rpm. On a supercharged diesel, the supercharger compresses air and feeds it into the intake side of the turbocharger, where it is further compressed and sent through the intercooler and into the engine. The result is a combination which yields instant throttle response at any speed (since the supercharger builds boost off idle with negligible lag times), more horsepower/torque, and significant improvements in fuel economy.

 

 

Supercharged & Turbocharged Diesel (Super-turbocharged)

Advantages:

• Instant throttle response at any engine speed, since the supercharger reaches maximum pressure shortly off idle.
• Maintains airflow at high engine speeds and loads due to the turbochargers superior flow rates.
• Boost available at cruising speeds and low load conditions.
• Overall increased horsepower, torque, & fuel economy.

Disadvantages:

• Supercharger uses engine power to operate, higher parasitic loss compared to compound (twin) turbochargers.

 

While a compound turbocharger setup remains better suited for all-out performance (sled pulling, drag racing engines), a supercharged and turbocharged diesel is an intriguing option for streetable trucks and tow rigs. Additionally, manufacturers offer a range of sizes to meet your performance needs.