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Application Of Supercharging To Si Engines

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Published: Tue, 25 Apr 2017

Enhancements in fuel consumption can be achieved through engine downsizing. However, it is essential to provide a corresponding increase in power in order to improve or maintain vehicle performance. This increase can be obtained through supercharging or turbocharging. Supercharging has the most visual impact of any high performance modification. Supercharging or turbocharging may help achieve engine downsizing that reduces relative value of losses at lower engines loads and vehicle weight, thereby decreasing road fuel consumption of a vehicle. By employing the appropriate motor-management, it is also possible to regulate the necessary full-load speed curve [1].

On the other hand, the danger of knocking or mixing auto-ignition restricts the optimum compression ratio established for the best efficiency. Furthermore, the optimum combination of spark advance and combustion ratio needs to be investigated particularly at low engine speed at which knocking often takes part. The driving power demand of a supercharger or the exhaust back pressure exerted by a turbine is likely to compensate for the efficiency benefits from engine downsizing in strong reliance on the system of boost pressure control. In essence, it is important because it allows for the reduction in the engine speed at maximum torque. Thus, such multi-parametric optimization is not generally provided by experiments since the process is too time-consuming and too expensive [2]. The most appropriate way is to simulate a virtual-engine into some basic experiments employed for initial calibration of engine model and eventually for confirming the optimum results by checking the vicinity of simulation-predicted optimum matching. This simulation offers a useful tool to compare between different boost pressure control factors within rapid or quasi-steady change operation parameters [1].

Toward that end, a supercharger is said to be an air compressor that is extensively used for performing forced induction of an internal combustion engine. Moreover, the higher mass flow-rate

The application of supercharging to SI engines

offers more oxygen for supporting combustion than the naturally-aspirated engine that lets more amount of fuel to be supplied and more work to be performed per cycle, thereby increasing the overall power output of the engine. In essence, a supercharger is powered mechanically by a gear, belt, chain or shaft attached to the engine’s crankshaft. A Supercharger can also be powered by an exhaust gas turbine. Furthermore, a turbine-driven supercharger is termed as a turbosupercharger or a turbocharger. And the term supercharging relates to any pump that is driven directly by the engine, as against turbochargers that are driven by the pressure exerted by the exhaust gases [1].

2. Types of superchargers with respect to the method of compression:

Dynamic compressors:

Dynamic compressors depend on accelerating the air to high speed and then interchanging that velocity for pressure by slowing down or diffusing. Main types of dynamic compressors are Centrifugal, pressure wave supercharger and multi-stage axial —flow.

2.2 Positive displacement:

Positive displacement pumps are responsible for delivering a nearly-fixed volume of air per revolution at all speeds. The device separates the air mechanically into distinct parcels to deliver it to the engine mechanically moving the air into the engine gradually. Main kinds of positive-displacement pumps are roots, sliding vane, Wankel engine, scroll-type supercharger, piston, and Lysholm screw [3].

Positive displacement pumps are again categorized into internal and external compression types. Moreover, roots superchargers are mainly external compression only. External compression is for pumps that transfer air at enclosed pressure into the engine. If the engine runs under boost conditions, the pressure at the intake manifold is greater than the one coming from the

The application of supercharging to SI engines

This results in a backflow originating from the engine into the supercharger until both reach equilibrium. And this backflow actually causes compression of the incoming gas [4].

Internal compression on the other hand refers to the compression of air inside the supercharger itself, which is already at boost level, can be transmitted smoothly to the engine without any backflow occurring. Internal compression is much more efficient and effective than backflow compression and allows for greater efficiency to be obtained. Furthermore, internal compression pumps usually employ a fixed internal compression ratio. That is, when the boost pressure equals the compression pressure in the supercharger, the backflow becomes zero. However, if the boost pressure becomes higher than that compression pressure, backflow may still occur similar to the roots blower [4].

3. Supercharger drive types:

Superchargers are also defined according to their method of drive, i.e. whether turbine or mechanical.

3.1 Exhaust gas turbines:

Radial turbine.

Axial turbine.

3.2 Mechanical:

Belts (Flat belt, V-belt, Synchronous belt).

Gear drive.

Direct drive.

Chain drive.

The application of supercharging to SI engines

Practical mechanical supercharging pumps are classified into:

Sliding vane compressors.

Centrifugal compressors.

Rotary compressors.

Sliding vane compressors and Rotary compressors are the positive displacement compressors. Centrifugal compressors are aerodynamic compressors [5].

Sliding vane compressors consist of deep slots that are cut into the rotor in order to accommodate thin vanes that are free to move radially. The rotor is placed eccentrically in the entire housing. With every rotation of the rotor, the centrifugal forces exerted on the vanes drive them outward against the housing, and divide the crescent-shape space into various compartments. Furthermore, ambient or enclosed air is drawn through the intake port into every compartment with the volume increasing to reach its maximum. Then, the trapped air is compressed whenever the compartment volume reduces, and is further discharged through the outlet port. Moreover, the flow capacity of the sliding vane compressor relies on the maximum induction volume that is determined by the bore of the housing cylinder, rotor length and diameter, number of vanes, eccentricity, and the dimensions of the intake and outlet ports. Also, the actual pressure and flow rate rise at constant speed will decrease due to leakage, the heat transfer from the vanes and rotors in motion and the stator surfaces will decrease compression efficiency except when cooling is used for removing the thermal energy produced by friction at the vanes, the rotor and the stator [5].

The roots blower is an alternative positive displacement supercharger. It consists of two rotors connected by gears. The working of roots blower follows transmission of air trapped in the recesses between the rotor lobes and the main housing, towards the delivery port without any

The application of supercharging to SI engines

substantial change in volume. When these recesses open to the delivery ports, as the suction side is closed, the trapped air is compressed by the backflow generated at the higher-pressure delivery line. This sporadic and abrupt delivery generates non-uniform torque on rotor as well as pressure pulses at the delivery line. Moreover, the volumetric efficiency relies on the rotor length, rotational speed, pressure ratio and the running clearances [3].

A performance map of a distinctive small roots blower is demonstrated in Figure [1]. It is almost same as that of the sliding vane compressor.

Figure [1]: A performance map of a distinctive small roots blower [3].

Furthermore, the flow rate is dependent on increasing pressure ratio, at constant speed, only via the resulting decrease in volumetric efficiency.

In essence, screw compressor need to be precision machines to obtain close tolerance between stationary and rotating elements for acceptable operation. Moreover, they operate at speeds ranging from 3000 rev/min to 30,000 rev/min. Generally, it is essential to cool the rotors internally, and high values of isentropic and volumetric efficiency are claimed [6].

The application of supercharging to SI engines

A centrifugal compressor is mainly used to boost inlet air or the mixture density that is coupled with an exhaust-driven turbine within a turbocharger. This compressor is single-stage radial flow device, most suitable for the high mass flow rates at the comparatively low pressure ratios needed by the engine. In order to operate effectively, it should rotate at high angular speed. Therefore, it is better suited to direct coupling to the exhaust-driven turbine of the turbocharger rather than to mechanical coupling via a gearbox to the engine for mechanical supercharging. Essentially, the centrifugal compressor comprises of a stationary inlet casing, a stationary diffuser, a rotating bladed impellor, as well as a volute or collector for bringing the compressed air leaving the diffuser to the engine inlet system [6].

4. Turbines:

The turbocharger turbine is motored by the energy produced at the engine exhaust. The ideal energy consists of the blowdown work transfer generated by expansion of the gas within the cylinder at exhaust valve opening to atmospheric pressure and the work done by the piston that displaces the gases remaining in the cylinder after the blowdown occurs [1].

5. Supercharging vs. Turbocharging:

Similar to a supercharger, the purpose of a turbocharger is to enhance the mass of air inbound to the engine in order to create more power. A turbocharger, however, differs in that the compressor is driven by a turbine powered by the engine’s own exhaust gases [5].

Positive displacement superchargers absorb as much as one-third of the total crankshaft power driven from the engine, and is most supplications, are less effective than turbochargers.

The application of supercharging to SI engines

Significantly, in applications for which engine power and response are more crucial than any other factor, like the top-fuel dragsters, positive displacement superchargers are commonly used. The thermal efficiency or fraction of air/fuel energy converted to output power is less in a mechanically-powered supercharger than in a turbocharger, since turbochargers use energy from the exhaust gases that are normally wasted. As a result, both the economy as well as the power of a turbocharged engine is much better than with superchargers [3].


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