发动机工程代写 Introducing The Spark Ignition Si
Different arrangements of Turbochargers
There are a few combinations and arrangements that can be used in conjunction with turbo chargers. The first is the two-stage turbocharger. This type of system has both a low pressure turbine and a high pressure turbine that work together. (D.W.H.Tennant book page 141). Two stage turbochargers have been developed recently for Diesel engines in order to improve their performances in terms of power, consumption, emissions and dynamic behaviour (Moulin. p. 2009) Benson and Scrimshaw were among those people to first test the two stage turbocharger arrangement, having a smaller turbocharger to deal with the relatively small loads on the turbine and then a larger one to handle the larger load requirements on the turbine ( D.E.Winterbone page 154 book) However this experiment was conducted under quasi-steady conditions so is therefore can be deemed unreliable when applied to actual vehicle engines. Benson and Scrimshaw then conducted an experiment using unsteady state conditions and found that the quasi steady state method under estimated the results by up to 25% for power and 7% for mass flow rate ( D.E.Winterbone page 154 book)
Variable and fixed Geometry Turbochargers
An important aspect of an engine with a Variable Geometry Turbocharger is the complex interaction between the VGT and the exhaust gas recirculation (EGR) valve and their associated flows. (Glenn B.C 2011). However using a variable-geometry turbocharger (VGT) has an effect on engine backpressure, which further influences the in-cylinder mixture temperature and the knock tendency (Xudhong Zhen, 2012)
A variable geometry turbocharger has movable vanes that can direct the incoming exhaust flow into the turbine. The moveable parts which can direct air flow and vary the size of the intake nozzle are controlled by an air filled actuator. (D.P Hartwell Page 340 Book) This is extremely useful especially at low speeds when the turbocharger is at its most inefficient as the nozzle area can be made smaller to increase the velocity of the air powering the turbine. Starkey and Franklin experimented with variable and fixed geometry turbochargers for use in military vehicles and found that when fitted with a VGT the engine produced 24% more torque at high speed and at low speed the performance substantially improved, with reduced exhaust smoke, temperature and better specific fuel consumption. (J.R.Starkey) (Also some useful graphs comparing the two).
A Variable Geometry Turbocharger system
Problems associated with Turbochargers
Whilst the turbocharger is an exceptional way of improving engine performance, it does have 2 major drawbacks. These are fundamentally, the inability to accept sudden load application when at low speeds which can be referred to as ‘Turbo lag’, and secondly, over a wide range of speeds the turbocharger’s reliability to supply the correct amount of air to the engine, is reduced (B.E.Walsham page 39 book). Another major issue in the innovation and development of future engines (Diesel or gasoline) is dependent on the design of the air intake system as this is one of the critical components that directly affect the engine (Moulin. P. 2011).
Matching a Turbocharger to the engine
The purpose of implementing a Turbocharger within an engine is to achieve low fuel consumption, less emissions and good engine operation. Therefore for the turbocharger to allow the engine to achieve this, the flow characteristics of the engine must be matched to the turbochargers turbine and the engine compressor. This will allow the turbocharger to fully operate to its specification and also provide good efficiency over a large range of engine loads. (K. Banisoleiman page 171 book).
Turbocharger improvements to date
One improvement to the turbocharger which has been implemented over the past 20 years is the process of Bypassing. Bypassing is a method of diverting some of the air from the compressor outlet into the exhaust pipe. This air increases the mass flow through the turbine resulting in the charge air increasing. (M.Appel page 163 Book). Another improvement to the performance of a turbocharger is the application of pre-swirl. Pre-swirl can be induced to the exhaust gasses before hitting the turbine in the charger therefore creating a more effective means to turn the turbine. . (M.Appel page 163 Book). J Bucher suggested the changing of the turbocharger turbine cross section to make it smaller therefore resulting in increased charge air pressure.
When the engine is at low speed and low load, upon application of a sudden load change, the engine needs to allow time for the turbocharger to react therefore resulting in a limited acceleration or limited torque available due to the reduction in the air available in the engine cylinders (B.E.Walsham page 39 book).
发动机工程代写 Introducing The Spark Ignition Si