Single cylinder SI engine modelling using GT-Suite

                                                             Introduction to GT Suite

Effective utilization of simulation software with human intelligence is required in order to achieve feasible results. 

GT-Suite is one of the leading system simulation software used worldwide. Industries like ON- Highway vehicles, Off-highway vehicles, Marine, Rail, Industrial machinery, Aerospace, Power Generation, Refrigeration(HVC) etc. 

                                                                    GT -Power

GT - Power is widely used across the globe for modelling internal combustion engines using. Gt Power modelling is classified into two types. 

1. Development

2. Application

Development involves Programming Numerical methods and Models to solve physics (Navier stokes equations, etc). This will be done by the company that developed the software (Gamma Technologies)

Applications mean effective utilization of the tools with very much in-depth knowledge in that particular domain. Therefore human intelligence is required. 

In this project, we mainly focus on the applications of GT Power

Simulation approach refers to Solving mathematical models it is a numerical technique in order to capture real physics- Heat transfer, Chemical kinetics, Fluid mechanics etc. It could be either 1D or 3D

CFD focuses on solving Navier stokes equations (NS-eq). The N-S equations are the mass, momentum and energy conservation expressions for Newtonian fluids. Newtonian fluids are the one that follows a linear relationship between viscous stress and strain. Analytical solutions exist for simple problems while complicated problems can be solved numerically. 

In 1D CFD properties of fluid across the other two directions are constant. The only change of properties takes place in one direction. Here properties are pressure, temperature, density etc. 

 

 

 

For Simulation tools, the given values need to be feasible and effective. Tools just provide numerical results but the user needs to check the result is accurate or a garbage every time.

Garbage in   `rArr`    Analysis tools  `rArr`     Garbage out 

                                                                   GUI (Graphical User Interphase) of GT - Power

 

GT- Power has inbuilt templates. Inside every template, there will be an object family. From the objects, we can make different parts and Every part has attributes.

 

 

 

 In the above End Environment template it has both Env-inlet as well as Env-Outlet as its objects in Object family. If there are more than one objects then they are referred to as parts. Each part has attributes. In the above picture 'Main', 'Options', 'Altitude and Humidity', 'Plots'are referred to as Attributes. Each attribute has a value selector. 

 

Run Setup : 

Time control :

Periodic: This indicates a cyclic process (engine cylinder cycle - for every 720 degrees it repeats 4 strokes)

Continuous: No periodic is continuous. (ex: DOC, DPF, SCR, 3-way Catalytic Converter)

 

ODE Control: Solver - Explicit - Runge Kutta method. Everything is set to default. 

 GT Power solver solves and ensures mass, momentum and energy are conserved at all components level. 

Burn Rate: The Rate at which fuel and air are converted to combustion products. This explains how fuel is burning inside the cylinder

Combustion Models: 

•Non-predictive combustion – The instantaneous burn rate is directly imposed as a simulation input – The total amount of energy released from the fuel depends only on the mass of fuel and air in the cylinder. Everything is predefined. Change in parameters like engine speed, the start of injection doesn't change the burn rate. Non-predictive studies are useful when focusing on particular objects like turbocharger, Valves etc. Non-predictive studies usually ignore emissions. 

• Predictive Combustion – The instantaneous burn rate is predicted by a physical model within GT-POWER based on various simulation
inputs and results. Predictive combustion basically refers to change in one object changes the other objects. Objects like the start of injection by `2^circ`or `4^circ` there will be a change in `NO_x` emissions, BSFC. 

 `NO_x` formation: If the fuel is injected at a faster rate it will burn faster. This leads to a higher rate of heat release inside the combustion chamber. This means the pressure release will be high which leads to high combustion noise and a higher rate of the temperature inside the cylinder. This leads to the `NO_x` formation which needs to be avoided. 

 

                                                                              SI Engine Modelling

 

Gasoline Direct Injection (GDI): Instead of injecting the fuel in the port, fuel is getting injected in the cylinder. In the GDI system, there are no scavenging losses. Because the fuel is getting injected once after the inlet valve gets closed. This avoids overlapping. 

 Limiting factor for SI engine power: SI engines are prone to knocking. Knocking is caused due to abnormal combustion. Due to this,  in-cylinder pressure increases and leads to fluctuations. This leads to damage of Piston components.

Reasons for Knocking :

`->` Due to high in-cylinder pressures and temperatures.

`->` Due to high compression ratios

`->` Due to high boosting

`->` Due to the low octane number of fuel used

 

 

 

 

 

 

Numerical simulation results are not just numbers. It is very much necessary to check whether the acquired results are feasible. Relevant inputs give feasible outputs. 

Example: If we increase the compression ratio (CR) to 16 we might get good BSFC in the simulation results. In reality, it is not possible to increase CR by more than 11/12 for SI engines. 

 

                                                                     Octane and Cetane Number of Fuel

 

Fuel	Research Octane number	Motor Octane	Cetane Number	Boiling point
Gasoline	92 - 98	80-90	0-5	37 - 205
Diesel	-25	-	45-55	140 - 360
Methanol	106	92	5	65
Propane (LPG)	112	97	-2	-42.15
Methane (CNG)	120	120	0	-161.5

 

 

If the Octane number is very high it is necessary to use Spark ignition. If the cetane number is very high it is necessary to use compression ignition.

Knocking:

Because of Autoignition, another flame front starts travelling in the opposite direction to the flame front. When the two flame front collides, a severe pressure pulse is generated which produces an abnormal increase in pressure and thus causes knocking

Higher octane number indicates slow-burning. Higher cetane number indicates faster burning. Fuel with a higher octane number (which burns slowly) requires a spark ignition. When the spark is initiated the in-cylinder mixture should be consumed by a flame generates by the spark. When the spark is developed near the spark plug then the generated flame travels throughout the cylinder and pushes it down. 

Diesel fuel with its high cetane number is burned very fast due to its self-ignition temperature after compression stroke. Fuel will be burned anywhere in the cylinder 

 

                                                                           OTTO Cycle

 

 

 

 

SI engines work on otto cycle. The area under the P-V diagram gives us the indicated work. Subtracting frictional losses and other losses gives us the Brake work developed and measured at the flywheel.

Mechanical efficiency is the ratio of Brake work to the Indicated work

`eta_m = [(Brake Power)/(Indicated Power)]`

 

                                                     Thermal Efficiency vs Compression ratio

 

 

 

From the above graph, it can be observed that with an increase in compression ratio the thermal efficiency is also increased. In order to achieve that, more work should be done which is possible when the area under (P-V) diagram increases. Which means overall volume and or pressure need to be increased.

But practically there are certain limitations. Due to higher compression ratios, combustion takes place in different locations which lead to knocking. This is the reason SI engines have very less compression ratio

Diesel cars usually provide higher fuel efficiency because of higher compression ratios. Thermal efficiency can be minimised by controlling the supply of fuel. 

 

If the Bore and Stroke are the same then it is called a 'Square Engine'. Engines can be differentiated easily by just checking the compression ratios. SI engines usually have 9-12 CR, Whereas diesel engines it will be 16-18. 

In SI engines Lambda (`lambda`) plays an important factor in maintaining air-fuel ratios. The Air fuel ratio of SI engines usually will be 14:1. The amount of air required to burn 1 kg of fuel is the stoichiometric air-fuel ratio. This means to burn 1 kg of fuel it needs 14.5 kg of air. Lambda (`lambda`) sensors are used to maintain equality between actual and stoichiometric air-fuel ratios. 

The Intake port design is quite different for both SI engines and CI engines. SI engines usually use a Dumbel port whereas CI engines use a Helical port in order to generate swirl motion. 

 

 Equivalence Ratio

 The air-Fuel ratio and Fuel-Air ratio are the parameters used to describe the mixture ratio.

AF = `M_a/M_f` = `dot(m_f)/dot(m_a)`

FA = `M_f/M_a` = `dot(m_a)/dot(m_f)` = `1/(AF)`

The equivalence ratio is the ratio of stoichiometric air fuel ratio to the actual air fuel ratio. It is denoted by `phi`

                                        `phi` = `(FA)_(actual)/(FA)_(scthio)` =  `(AF)_(actual)/(AF)_(scthio)`
                                            

                                         `phi` < 1 `to` Lean mixture

                                         `phi` > 1 `to` Rich mixture

Gasoline engines are operated at equivalence ratio = 1. Whereas Diesel Engines are lean burned.

Lean burned refers to more air than fuel in the combustion process. Rich fuel refers to more fuel than standard air fuel mixture.  

 

 

 

 Running a gasoline engine with the lean mixture will give higher `NO_x` emissions due to more presence of nitrogen in air Rich fuel mixture will give lower `NO_x` emissions but it will affect the BSFC.

 

Lambda (`lambda`)

`lambda` = `1/(phi)`

                                        `lambda` < 1 `to` Rich mixture

                                        `lambda` > 1 `to` Lean mixture

 

 

 

 

In some cases, it is very much necessary to run the engine with `lambda` < 1 because in gasoline engines at higher RPM exhaust gases temperature goes very high up to `1400^o`C. This affects the durability of the engine. In order to avoid this situation, it is necessary to put extra fuel for combustion instead of air. This drop-in combustion efficiency will lead to a reduction in exhaust gas temperatures and in-cylinder pressures. 

The compression ratio (CR) is defined as the ratio of total (maximum) cylinder volume to the minimum cylinder volume. Increase in compression ratio increases torque, reduces BSFC, increases in-cylinder pressures, pressure change per degree `(DPM_(x))/(DCA)``[bar/deg] also increases. 

There are so many design constraints in engine design. An effective engineer will make sure that all the assigned values are feasible. 

 

In order to increase the torque in the engine, it is necessary to increase the supply of air into the cylinders. This is not possible in naturally aspirated engines. But using turbochargers/ Superchargers/e-turbo can help to achieve this by utilizing exhaust gas kinetic energy to drive the turbine which in turn drives the compressor and supply more air. Another solution for this problem is to change the dimensions of the geometry of bore and stroke to increase swept volume. 

Emissions are one of the major environmental concerns in engine design. Emissions like CO increases with an increase in fuel in air-fuel ratio. which in turn increases the BSFC and reduction in `NO_x`. Emissions like `NO_x`could increase if more air is entering into the cylinder and due to the presence of `N_2` in the atmosphere and high temperatures. This will eventually Increase BSFC and Reduce CO emissions. 

 Anchor angle [MFB50]: Anchor angle is the number of crank angle degrees between TDC (top dead centre) and typically 50% combustion point of the wiebe curve. Anchor angle defines whether the combustion is fast or slow.  Anchor angle usually ranges in between 8 - 16. Increase in Anchor angle results in an increase in BSFC and decrease in torque due to slow-burning. For good efficiency, it is recommended to maintain [MFB50] between 7 or 8-10.  The decrease in anchor angle results in an increase in NOx and CO emissions and decrease in BSFC due to fast burning. 

Engine calibration: Engine calibration is all about balancing air-fuel mixtures in a given engine with or without changing the engine geometry to maintain optimum values of NOx, CO and HC emissions without losing efficiency. 

Port fuel injected engines get mixed well with air throughout the suction stroke due to the availability of time and get compressed and produce uniform combustion during the power stroke. 

Engine calibration mainly depends on lambda sensors to maintain lambda value less or greater or equal to one under different operating conditions. 

 

 Higher octane number fuel eliminates knocking by avoiding simultaneous combustion at different locations across the cylinder and promotes uniform combustion. 

Brake mean effective pressure (BMEP) is the important deciding factor that will decide whether the engine under consideration should go with either naturally aspirated or Turbocharged. If an engine required more than 8 or 9 BMEP then it is necessary to use a turbocharger. 

 Higher flow coefficients define minimum resistance to flow. It could be in either throttle or valves. This increases volumetric efficiency. Volumetric efficiency is one of the most important factors of the IC engine performance parameters. The volumetric efficiency of an Ic engine indicates the efficiency with which it can move the charge of air in and out of the cylinders. 

Torque and Power: Torque defines the engine's ability to do work. It is the product of the force applied at the top of the piston to its perpendicular distance.  Power defines the rate of doing work.

Torque is measured in N-m/ ft-lb; Power is measured in KW/HP.

Torque = `F *D`

Power = `(2*pi*N*T)/60`

The drag force, rolling resistance, vehicle weight etc forces are generated when a vehicle is in motion. In order to overcome these and forces and smooth travel, the power should be generated inside the engine.  Power is a function of speed and Torque. Whereas torque is a function of volumetric efficiency. 

 

 

Rated speed: The speed at which power is maximum in the above torque-power graph. 

Intermediate speed: The speed at which torque is maximum.

Beyond max power friction increases thus power is reduced. 

 

                                                                           Valve Timing Diagrams : 

 The camshaft runs at only half the engine speed because to complete the four strokes of an engine the crankshaft takes 2 revolutions to complete it. For every two revolutions, there is only one cycle. Therefore camshaft needs to run at half the engine speed. 

Theoretical valve timing diagrams:

`0^o`-`180^o` = Suction stroke

`180^o` - `360^o` = compression stroke

`360^o` - `540^o` = Power/expansion stroke

`540^o` - `720^o` = Exhaust stroke

Cam profile decides the opening and closing of valves. Camshaft should be designed according to our requirements. This is one of the reasons the valve operations will not happen according to the theoretical cycle. 

 

Practical valve timing diagrams:

 

 

Valve overlap refers to the condition in which both the valves stay open for some time. In the above figure at `theta` = `0^o`inlet valve is open as well as the exhaust valve doesn't fully close. 

If there is valve overlap there are chances that the incoming air when combines with fuel directly go into the exhaust. This is also called Gas exchange TDC. If both the valves are closed then the combustion process gets started. This is called combustion TDC. special care should be taken while designing valve timings to avoid gas exchange TDC. 

In real-time applications, the inlet valve will close after BDC. By doing this more air can be drawn into the cylinder which increases the efficiency. 

Inlet valve closing is a function of engine speed. In modern engines, variable valve timings (VVT) are used in order to change the valve opening and closing at different angles of rotations instead of a fixed one. Honda (i-VTech), BMW (Valvetronic) are some of the examples of VVT's used. Modern-day engines use ECU for electrically operated valves (VVT).  Valve closing can be delayed at higher speeds using VVT's This increases the duration of valve lift. This helps in increasing more air into the cylinder and thus increases the power of the engine. This also eliminates the backflow

 

 

Continuous Variable Valve Timings: This was developed by FIAT motors, it consists of a hydraulically operated cylinder. This gives infinite variabilities in operating valves. This controls the flow of air into the cylinder which in turn controls torque and power and thus emissions. This system is fully flexible. At higher speeds, there will be higher inertia of incoming air, and to compensate that we can delay the IVC, EVC, IVO and EVO.  In order to increase this air inside the cylinder turbochargers/ superchargers are used. 

 

Two general approaches for Port shape : 

The required swirl level depends on the port shape, port position and combustion chamber shape. 

1. Tangential (Directed) port with Tumble

2. Helical port (Swirl)

SI engines use tangential ports because air and fuel are mixed (homogeneous) before entering the cylinder. CI engines use a Helical port to create the swirl.

Tumble motion is important for SI engines because it is necessary that the air-fuel mixture does not hit the walls of the cylinder and get stuck to it. This leads to engine knocking. 

Swirl coefficients and Tumble coefficients are used in predictive combustion tests. 

Self Ignition : 

If the temperature of the air-fuel mixture is raised high enough then 

        `to` The mixture will self ignite

        `to` WIthout use of external spark plug or other external igniters

        `to`   The temperature above which this occurs is called self-ignition temperature. 

This is desirable for diesel engines. Not suitable for SI engines because it would lead to knocking. 

 

It is very much necessary to open the exhaust valve (EVO) before BDC because there will be a lot of burned gases in the cylinder. It is not possible to expel all the burnt gas during the exhaust stroke. The only power stroke is +ve work and all others are -ve work. This could lead to an increase in pumping losses. To reduce that Exhaust valve is opened before BDC so that exhaust gases will start to leave the engine this is called blowdown pulse. 50% of the charge will be released during the blowdown pulse and the remaining during the exhaust stroke. 

Not all exhaust gases will go out. Some will get trapped in the clearance volume. It can be seen in "Burned Residual Mass" (SOC)% in Gt suite. 

The temperatures of intake port is very high because the intake port is a part of the cylinder head and cylinder head is exposed to very high temperatures. Due to conduction the temperature of the port also increases. Intake air temperature will be increased before entering the cylinder. 

 

Task 1 : 

Run the case at 1800 rpm and list down important parameters 

• airflow rate
• BMEP
• BSFC
• In-cylinder pressure

Definitions : 

The mean effective pressure is a quantity relating to the operation of a reciprocating engine and is a valuable measure of an engine's capacity to do work that is independent of engine displacement When quoted as an indicated mean effective pressure or IMEP it may be thought of as the average pressure acting on a piston during the different portions of its cycle.

• Brake mean effective pressure (BMEP) - Mean effective pressure calculated from measured brake torque.
• Friction mean effective pressure (FMEP) - Theoretical mean effective pressure required to overcome engine friction, can be thought of as mean effective pressure lost due to friction. Friction mean effective pressure calculation requires accurate measurement of cylinder pressure and dynamometer brake torque.

          

                                                                  BMEP = IMEP - FMEP

 

SI engine / Gasoline Engine:

The gasoline engine, any of a class of internal combustion engines that generate power by burning a volatile liquid fuel (gasoline or a gasoline mixture such as ethanol) with ignition initiated by an electric spark. Gasoline engines can be built to meet the requirements of practically any conceivable power-plant application.

Gasoline, Methanol, Ethanol, Propane (LPG), Methane (CNG) has high octane numbers. So far fuels with higher octane numbers spark plugs are used. Meanwhile, fuels with higher Cetane number uses compression ignition

 

GT- Suite provides a wide range of libraries of spark ignition engines the one which we will be working is a single-cylinder SI engine. 

 

This example file uses 2 modules namely Flow and Mechanical.

 

Flow and Mechanical modules 

 

 

All these templates in the module together form a library. The brief description of each template is mentioned below.

Components 

1. End Environment: This template describes boundary conditions of pressure, temperature, and fluid properties. There are options to include effects of altitude and humidity. 

2. Pipe Round: These template models a pipe with a round cross-section and an optional bend. Data entered to describe the bend will be used to automatically calculate the pressure loss coefficients that account for the associated head losses.

3. Engine cylinder: This template is used to specify the attributes of engine cylinders.

Connections  

1. InjAF Seq connection: This template is used for the operation of a sequential pulse fuel injector, most commonly in SI engines. The user imposes the fuel-to-air ratio, and the resultant injection pulse width is calculated for each injection event. This injector should be used for all engine simulations for which the fuel is pulse injected with an imposed air-to-fuel ratio.

2. Orifice connection: These template models an orifice, defined by diameter or area and discharge coefficients, which calculates the mass flow rate between the adjacent flow volumes.

3. Valve cam connection: This template defines the characteristics of a cam-driven valve including its geometry, lift profile, and flow characteristics. 

 

 

The tutorial file available for study/analysis:

 

The inlet condition/boundary condition of air entering the engine needs to be set before running the simulation to understand the differences in the pressure and temperature it undergoes during compression and power strokes. Later the fuel is injected into the runner with the amount of air entering it by maintaining the required Air-fuel ratio to generate the required power. The valves lift profiles are also extremely important here in order to open and close at the required times. Error in valve timing could cause scavenging losses which cause the reduction in required power.

 

The pictures shown below explain the required settings and inputs in order to run the simulation and study-specific parameters for the user

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Task-1:

Run the case at 1800 rpm and list down important parameters 

• airflow rate
• BMEP
• BSFC
• In-cylinder pressure

 

 

 Task - 1

 

 

 

 

 

 

 

 

 

 

 Air mass flow rate : 

 

 

 

 

 

BMEP = IMEP - FMEP

         = 10.61 - 1.13

          = 9.48 Bar

 

 Air Flow Rate = 24.6 kg/hr

Maximum Pressure [bar] = 48.89

 BSFC [g/kW-h]  = 239.2

 

 

 Task -2 

 Increase the power output at 3600 rpm by 10%

 

 Step 1. Run the simulation at 3600 rpm and determine the power output

Step 2: Change various parameters like compression ratio, geometry  and air-fuel ratio under constant speed conditions and study/determine the change in power output

 

 Under normal conditions, at a speed of 3600 rpm, the given single-cylinder SI engine develops a Brake power of about 

 16.7 KW which is up to 100% more when the same engine operates at a speed of 1800 Rpm.

 

 The Compression ratio has increased and decreased in 5 different cases to check the change in Brake power at the same 3600 rpm speed

 

Increase in compression ratio caused an increase in cylinder pressure which is not a feasible design.

 

In the second case, the geometry of the engine has been changed in 5 different cases to determine the change in Brake power. The given engine is a square engine with the same bore and stroke dimensions. The stroke length was changed in 5 different cases

 

 

 

 

 

 

 

It can be observed that due to the change/increase in stroke length of the engine the Brake power has been increased drastically. Also, the cylinder pressure has also remained almost the same. This is the best possible solution to increase the power output. But this could lead to an increase in the overall cost of the engine and volume.

 

In the third case, the overall air-fuel ratio has been changed in 5 different cases to check the possible change in engine power.

 

 

 

 It can be observed that due to the increase in Air fuel ratio the amount of fuel required for combustion reduces and this eventually cause a reduction in Brake power. 

 

 

References

1. Engine and Tractor Power by Carroll E. Goering (University of Illinois)

2. Automotive engines Eighth edition by CROUSE and ANGLIN

3. Introduction to Internal Combustion engines by Professor John B. Heywood (MIT)

 

 

                                                  Toyota dynamic engine - SI engine 1D modelling using GT - Suite

 

Aim: To perform 1D simulation of Toyota dynamic engine at its intermediate speed (Max torque) and rated speed (Max power) using GT suite and list out the outputs

1.Compare the results and list down important parameters

2.Post-process and take snaps for the following data

• PV and log PV diag
• in-cylinder pressure and burn rate
• valve mass flow and pumping loop

 

 

 

 

Audi A4 engine specifications

 

 

Turbocharged in line 4 cylinder
Displacement :	1984 cc
Power :	140 KW (187 HP) @ 4200 Rpm
Torque :	320 NM @ 1450 Rpm
Bore :	82.5 mm
Stroke :	92.8 mm
Compression :	11.65
Valvetrain :	16 DOHC (4 valves per cylinder)
Fuel used	Petrol / Gasoline

 

                                               Toyota Dynamic Force Engine

Toyota Dynamic Force Engines	Gasoline Direct Injection
Displacement :	2487
Power :	151 kw @ 6600
Torque :	250 NM @ 4800
Bore :	87.5
Stroke :	103.4
Compression ratio :	13
Valvetrain :	16 DOHC (4 valves per cylinder)
Fuel used	Petrol / Gasoline

 

 Toyota developed a new engine for its new Toyota Yaris sedan with a naturally aspirated, gasoline direct injection engine in order to reduce emissions and thus maintain euro 6 / Bs6 emissions without fail. The similar engine is also used for Lexus and other Toyota hybrid vehicles also. The main reason why this engine is different from other engines is because of the geometrical modifications that Toyota has done in order to generate more torque.

Toyota reduced the bore diameter of the engine and increased the stroke length compared to its predecessor. This modification completely eliminates the process of knocking by having uniform combustion throughout the process. One more modification can be observed is the increase in valve diameter with an increase in the angle between the intake port and exhaust port. This modification will increase the tumble motion inside the cylinder and thus helps to achieve high compression ratios. 

 

 

 

Step 1: Change the dimensions of the default engine with Toyota dynamic engine specifications

 

 

 

 

 

 There is an increase in valve diameter by 1 mm to increase the volumetric efficiency.

 

 

 It can be observed that the engine is operating at its maximum throttle position. The analysis is a non-predictive combustion

 Heat Transfer Object :  

  

 Run case setup

  

 

 Simulation Results 

 

 

 

 

 

 

 

 

It can be observed that higher levels of CO hs been obtained due to rich mixtures. 

 

• PV and log PV diagram

 

 

 

 

 

 

 

• in-cylinder pressure and burn rate

 

 

 

 

 

 

 

• valve mass flow and pumping loop