发动机工程代写 Introducing The Spark Ignition Si
The demand over the years for vehicles to have less weight, and better aerodynamics has paved the road for the customer wanting more power/torque at lower speed or lower engine load. The supercharger, being connected directly to the crankshaft of the engine is the most appropriate solution for a satisfactory power output at low engine speeds (H. Richter page 283book). H.Richter and N.Hemmerlein conducted an experiment on supercharging the Porsche 944 which proved that when maximum power output is required at low speed the supercharger was the best option over its counterpart the exhaust gas turbocharger.
One more recent method of supercharging is the use of Continuously Variable Transmission. This type of transmission can go through the gear ratios without stepping, resulting in a constant velocity from the output shaft. A.T Rose worked on this method using a supercharger driven from the engine crankshaft using a CVT. The CVT arrangement acts as a pre-boost to the traditional turbocharger and has been identified as a possible solution to improving the low-speed engine torque and turbo lagged response of downsized turbocharged engines. The concept was modelled around a diesel engine model with a variable-geometry turbocharger (Rose A.T. 2011) Mechanically driven superchargers present the means to increase the density and charge of the inlet air which in turn increases the BMEP ( K.Banisoleiman page 171 book)
EDS (Electronically driven Supercharger)
The electrically-driven supercharger (EDS) is an increasingly interesting and useful way of charging the air intake for a common engine. Firstly as with any supercharger, it can charge the air with no losses or lag at low speeds and low loads, and secondly, no mechanical connections are required with the engine, which decreases parasitic losses and frictional/inertial losses within the engine. However with recent compressor technology, the required compressor speed for an EDS application can be very high (more than 100 Krpm). These high speeds can lead to problems with the individual components within the supercharger including the bearings, turbine, and casings (Villegas, 2006).
Problems associated with superchargers
The main problem associated with mechanically driven superchargers is that they induce parasitic loss to the engines output power and torque. This is due to the turbine requiring some of the work output of the engine’s crankshaft, to drive itself. This problem was confronted by the development of electrically driven superchargers, however the electrical power required is very high and a drain to the power source therefore an EDS would have to be coupled with a traditional turbocharger to become more efficient than a normal supercharger (Villegas, 2011).
Turbochargers vs. Superchargers, or Both
When an engine’s performance is increased by using higher charge pressure of air, the big advantage of the turbocharger over a mechanically driven supercharger is that it attempts to control the engine’s demand for charged air whereas the supercharger would not as it is driven solely by the engine and has no control. (B.E. Walsham page 39 book). In the case of an EDS being coupled with a turbocharger, the electrical supercharger would only operate at low engine speed where the turbo lag is significant, while at high engine speed only the turbocharger would operate to recover the free exhaust energy (Villegas, 2011). In this case it would be the best option to pair them together.
In an investigation done by the marine engines division by E.T.F Kirkman, a comparison table of supercharger performance against turbocharger performance was produced and is presented below. This is based on the engine being used to power a submarine but is applicable to land vehicles as well;
The use of the Mechanical supercharger system increases the specific fuel consumption of the engine particularly at low loads
The turbocharger exhibited a much lower specific fuel consumption
The work being done by the engine is the power source for the supercharger therefore some power is lost
The energy required to charge the air is taken from the exhaust gases therefore there are no mechanical losses
Supercharger is independent of the energy in and exhaust gases therefore it possesses a very good load acceptance rate
Load acceptance is reduced due to reliance on energy in the waste gas
Sensitivity to fluctuating inlet depression and exhaust back pressure
Not sensitive at all the exhaust pressure and fairly insensitive to inlet depression
Highly sensitive to both inlet depression and exhaust pressure and also to wave induced fluctuations
The exhaust temperature will be very high as no energy is removed from it
The exhaust temperature will be lowered as some of the exhaust energy is removed to power the turbocharger
Low load running
The supercharger is matched to provide sufficient air at full load, providing more than sufficient compression at low load and ensuring good combustion
Low load running of the turbocharger creates problems of low fuel/air ratio and poor combustion
Taken from Turbocharging for submarines – a special case by E.T.F Kirkman and R.A.Hopper, Marine engines Division
It is clear that the supercharger is a better option at low speeds for maximum output, but at high speeds the turbocharger provides the better efficiency. In an investigation by Richter and Hemmerlein they concluded by stating that the decision with which to have came down to cost. However with recent advances in technology, the two can be paired together which satisfies both ends of the speed scale and provides excellent engine efficiency, however can be expensive to implement.
发动机工程代写 Introducing The Spark Ignition Si