Superchargers

Super-Turbocharged Diesels Explained

A supercharger is a forced induction device most commonly found on high performance engines. The concept of supercharging is similar in nature to that of turbocharging, although the operating principles between the two forced induction processes varies considerably. Whereas a turbocharger converts heat energy from the exhaust stream to drive a compressor, a supercharger utilizes engine power in order to pressurize air and increase performance potential. Superchargers and turbochargers each display unique advantages and disadvantages characterized by their respective processes. The use of supercharger-turbocharger combinations (twincharging) in diesel engines arose under the notion that a supercharger could be used to significantly reduce, or in some instances eliminate altogether, turbocharger lag.

Types of Superchargers

Roots Type Superchargers (Blowers) - A Roots type supercharger is best explained as a positive displacement air pump (a fixed volume of air is moved with every rotation of the rotors). In a Roots type supercharger, the compression process is external - the supercharger itself does not compress air. Instead, pressure is built in the intake manifold as the result of the volume of air pumped into the engine at a given time. As the volume of air present in the intake increases relative to the actual volume of the intake manifold, the pressure of the air is increased proportionally. This process is largely inefficient, but the system allows for relatively high airflow in low pressure differentials (the difference between atmospheric and manifold pressure).

Roots types blowers mount directly to the intake manifold and typically must protrude through the hood of a vehicle. They are relatively large and obtrusive, but can provide a substantial rate of airflow. You'll typically find roots type blowers on top-fuel dragsters and funny cars, in addition to hot rods and modified muscle cars. These blowers are actually derived from the blowers developed for General Motors' line of two stroke Detroit diesel engines; 4-71, 6-71, 8-71, were superchargers developed for GM's 71 series two stroke diesels. Blowers currently available on the market are derivatives, not replicas, of this original design.

Screw Type Superchargers - The screw type supercharger is similar in function to a roots type blower, but relies on an internal compression process by which the pressure out of the supercharger is greater than the pressure in. Therefore, the rotors, or "screws" in this system pressurize air as they rotate in the body of the supercharger. They also mount directly to the intake manifold of the engine, but are typically much smaller than a roots type blower. The internal compression process makes these units highly efficient.

Centrifugal Type Superchargers - Centrifugal type superchargers are similar in nature to a turbocharger, but instead of driving a compressor with exhaust energy, the compressor is driven by the engine crankshaft. Centrifugal superchargers also rely on an internal compression process, but under different principles than a screw type. A centrifugal supercharger relies on significantly increasing the velocity in incoming air across the impeller. This relatively high velocity is exchanged for greater pressure as flow restrictions cause the velocity to slow. Centrifugal superchargers are smaller and less invasive than screw and roots types, and therefore offer more flexibility in mounting and plumbing of the system.

Superchargers vs Turbochargers

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 simultaneously. 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 exhaust energy to produce positive pressure in the intake manifold. This is known as turbocharger lag, the time in which it takes for a turbocharger to reach the point when it builds positive pressure. 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 variables. Turbocharger compressor and turbine wheel sizes, compressor and turbine housing sizes, and the geometry of the compressor wheel all play a considerable role in performance characteristics. This is how turbochargers are produced in such variety - from small, quick spooling turbochargers to large, high flow models and everything in between.

Superchargers feature a compressor or rotors that are driven off the engine crankshaft via a belt drive - anytime the crankshaft is spinning, the supercharger is rotating proportional to engine speed. The supercharger draws in ambient air and compresses it, creating positive manifold pressure. Unlike a turbocharger, a supercharger uses energy directly 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. This 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 tend to be less about geometry and more about displacement. Getting more pressure (boost) from a supercharger can be as simple as changing the size of the drive pulley so that it spins either faster or slower relative to engine speed.

 

 

Advantages

Disadvantages

Supercharging

Instant boost - no lag since system operates relative to engine speed. A supercharger will create positive pressure directly off -idle.

 

Requires power from engine to operate and is therefore a less efficient process.

Limited boost/airflow characteristics (the degree of which depends on the type and size of the supercharger).

Turbocharging

Highly efficient process, powered by engine waste heat (exhaust).

Turbo lag - manifold pressure is dependent on engine load, not engine speed, and turbochargers are much slower in creating boost off-idle than superchargers.

High airflow and pressure characteristics.

 

Supercharging in Diesel Applications (Twin Charging)

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, low engine speed situations while meeting the airflow demands of an engine under heavy load. This process of utilizing both a supercharger and a turbocharger is called twincharging. In a typical twincharged diesel, the supercharger outlet feeds into the turbocharger compressor inlet. Air is therefore drawn in through the supercharger, compressed, and then further compressed by the turbocharger. The supercharger ensures quick throttle response and positive boost off idle, while the turbocharger provides greater maximum boost and meets greater airflow requirements of the engine as the engine speed increases.

 

 

Twincharging (Supercharger + Turbocharger)

Advantages:

Instant throttle response at any engine speed, since the supercharger reaches maximum pressure shortly off idle and compensates for turbocharger lag times.

Maintains airflow at high engine speeds and loads due to a turbocharger's superior flow rate.

Boost available at cruising speeds and low load conditions.

Overall increased horsepower, torque, & possibly fuel economy.

Disadvantages:

Supercharger uses engine power to operate, higher parasitic loss compared to turbocharging alone.

 

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.