发动机工程代写 Introducing The Spark Ignition Si
A spark ignition engine is an engine that follows the thermodynamic model of the Otto cycle. Spark-ignition engines have become a huge success since the invention of the first Otto cycle engine. During its first years of production and implementation, manufacturers and engineers focused on how to increase the engine power and how to improve the engine working reliability. However due to recent findings on CO2 emissions, and the depletion the earths fossil fuel reserves, researchers have now turned their attentions towards the development of more advanced combustion systems. Thus power generation with the lowest energy input from fossil resources has become more and more important since then (Xudong Zhen, 2012). This problem has been tackled with the constant improvement and innovation of engine boosting.
13/12/12 15:52 An ideal Otto Cycle plotted on a
Pressure vs. Volume diagram, taken from
(http://theory.phy.umist.ac.uk/~judith/stat_therm/node16.html)
Engine Boosting
The latest trends in automobile engines are mainly focused on the design of economic, efficient and significantly less polluting cars, whilst maintaining power, torque, and driving performance (Khiar, D, 2006). Global warming and increasing fuel price are among the top public concerns and, because of this the automotive industries have had to develop various technologies to achieve this. The most cost-effective solution to this problem is engine downsizing. However, when an engine is downsized, its torque and power output is significantly reduced dues to the reduction in cubic capacity of the cylinders. This must be compensated for to suit the requirements of the vehicle for the customer. The most effective and efficient way of doing this is by utilising a pressure charging device such as a turbocharger, supercharger or EDS. (Villegas, 2011) These are methods of boosting the engine, forcing more air and proportionally more fuel into the combustion chamber of the engine, resulting in a higher magnitude power stroke from the engine.
Turbocharging
Turbocharging is one way to boost an engine and is a very powerful means to improve fuel economy, emissions and engine efficiency (Khiar, D, 2006). A turbocharger is typically a turbine that is driven by the rejected exhaust gases from the cylinders of the engine, and this turbine in turn powers a compressor that compresses the air obtained by the air intake. The turbine is fed by the exhaust manifold, where the exhaust gases enter the turbine housing enclosing the rotor and turn the turbine (N.C.Baines, page 7 of book). The advantages of a turbocharged engine primarily are the significantly higher output of torque and power whilst reducing the specific fuel consumption. These advantages outweigh the disadvantages of this type of boosting an engine (B.E.Walsham page 39 book). 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). Turbochargers were first widely used, and still are, on large heavy trucks. The turbochargers “along with their associated intercoolers could provide the trucks with a pressure ratio of up to 2.6:1, and BMEP of 12 bar at an engine speed of 2000 RPM”. (F.J.Wallace pg 99 book). Usually to obtain a prediction of how a turbocharger is going to behave, the system and its components are usually modelled under steady state conditions due to the very small variation in behaviour of the air at the intake manifold. However at the exhaust manifold there can be significant amounts of turbulence as well as pressure pulses (D.E.Winterbone page 153 book). Benson and Scrimshaw 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) This was then backed up by the work done by Wallace, Adgey and Blair, who used a test rig much like Benson and Scrimshaw’s but used a hydraulic dynamometer to measure the characteristics of the turbocharger turbine under pulsating conditions ( D.E.Winterbone page 154 book).
发动机工程代写 Introducing The Spark Ignition Si
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
http://paultan.org/2006/08/
16/how-does-variable-turbine-geometry-work/
13/12/12
16:32
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.
Turbo Lag
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
发动机工程代写 Introducing The Spark Ignition Si
Superchargers
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;
Feature
Mechanical Supercharger
EGR Turbocharger
Fuel Consumption
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
Power Loss
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
Load Acceptance
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
Exhaust temperature
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
发动机工程代写 Introducing The Spark Ignition Si
Knock
The phenomena of engine knock continues to provide a limit to the compression ratio and the cylinder pressure of spark ignition engines. Diesel engines have a reputation for sometimes operating nosily and this can be due to knock therefore there is substantial investigation in how to reduce the risk of knock. Generally, it is accepted that engine knock originates from a local, rapid pressure rise; for example, in spark ignition engines, such pressure rise is due to a spontaneous ignition of the unburned gas, or the end gas, ahead of the flame front (Kono, Shiga, Kumagia, Iinuma Year?). Knock is a very serious and sometimes destructive occurrence in the engine, resulting in reduction in engine performance and thermal efficiency. Under certain conditions, knock an cause severe permanent damage to some of the engine components controlling the combustion process (Xudhong Zhen, 2012). There are two generally accepted theories of knock, auto-ignition and detonation. Auto-ignition is the process of the so called end gas reaching its auto ignition point in temperature and pressure. This is caused by the compression by the cylinder head, the walls around it and the propagating burned gas. Detonation theory presumes that knock occurs from the propagating flame front. This acceleration causes a shockwave that reflects from one cylinder wall. The impact pressures have a large magnitude but are only in contact for a short but substantial time, long enough to cause knock to occur (Xudhong Zhen, 2012). A large number of studies have been carried out with respect to the conditions of occurrence of knock and its prevention; however none of them focus on knock intensity. In this area, Draper investigated knock phenomena and concluded his findings as follows:
“1. Knock is accompanied by pressure waves within the cylinder. These pressure waves form standing waves, i.e., resonance phenomena within the cylinder charge.
2. For the cases that the cylinder diameter was over three times the chamber height, the pressure record is due to transverse vibrations”(Kono, Shiga, Kumagia, Iinuma Year?).
From the research done to date it is clear that the two main culprits of engine knock are temperature and pressure within the cylinder.
Computer aided modelling of an engine
The computer modelling software that will be used in this project is from Gamma Technologies, in particular their GT-SUITE. GT-SUITE a virtual engine/power train modelling platform and is used by over 500 business customers worldwide and is the leading engine simulation software provider in the market. (http://www.gtisoft.com/ 13/12/12 15:58) From the GT-SUITE the software I will be using is GT-POWER and GT-POST. GT-POWER is essentially used to build and model any type of engine and then simulate the operation of that engine. The results from this simulation are then sent to the GT-POST program for post processing. The post processing presents all the results that were required from the model and then can place these results instantly in graph format.
Example showing a 4
cylinder, spark ignition
engine modelled in
GT-POWER
Taken from Gamma
Technologies Website
(http://www.gtisoft.com/
applications/
a_SiL_HiL_real_time.php)
13/12/12 16:06
An example of the types of output graph from GT-POST using the model built and simulated in GT-POWER.
Taken from Gamma Technologies Website (http://www.gtisoft.com/applications/a_SiL_HiL_real_time.php)
13/12/12 16:06
Put some of my own graphs from GT POWER in here !!!!! And picture of working model engine my own!!!!!
Blow By
The Hydra