Transient Behavior Of Multiple Turbine Engineering Essay

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The problems of turbo lag associated with the larger turbochargers and the flow boost up problems of the smaller turbochargers can be addressed using sequential turbo charging. However the steady state and transient behavior of such multiple turbocharger systems necessitates a thorough study of the various processes associated with it. This mandates to develop simulation tools and to characterize experimentally the transient behavior of multiple turbochargers with complex and unique engine exhaust manifolds.



Sequential turbo charging

Sequential turbo charging is a method adopted to overcome,

The flow boost up problem of the smaller turbo chargers when an engine is running at high speeds

The problems of inertia associated with the larger turbo chargers at lower engine speeds.

In this technique, the system uses a small turbo charger to give boost at low RPM (Revolutions per Minute), with an additional larger turbo charger kicking in at higher RPM.

All through low to mid engine rpm, the available spent exhaust energy is minimal; the small high pressure turbocharger alone (the primary turbocharger) is active and all of the engine's exhaust energy is aimed at the primary turbocharger, lowering the boost threshold, and rising the power output at low engine speeds. At the end of this cycle, the bigger secondary turbocharger is partly activated (both compressor and turbine flow) in order to pre-spool the secondary turbocharger prior to its full utilization.

After reaching a preset engine speed or boost pressure, the valves which control the flow through the compressor and turbine of the secondary turbocharger are opened entirely. When this point is reached, the engine functions like a full twin-turbocharger providing maximum power output. The entire process is shown in the fig 1.

In this way the two turbos comes on and produces the boost even at lower RPM and also does not stall at higher flow rates as the big turbo still has the potential due to its superior flow efficiency. These kind of sequential turbo charging systems helps decrease the turbo lag without compromising the boost and engine power. [1]

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Low engine speeds (b) Medium engine speeds (c) High engine speeds

(Small Primary TC alone works) (Large Secondary TC kicks off) (Large Secondary TC controls)

Fig1. Working of a sequential turbocharger

Divided Exhaust Period (DEP) turbo charging

The concept of divided exhaust period (DEP) turbo charging aims at

Improving the performance and emissions of a turbocharged engine by dividing the exhaust flow

Utilizing the exhaust pulse energy in a better way to improve the performance of the turbine and producing quick increase in boost

The need to reduce the very high boost pressures in turbocharged engines results in higher exhaust pressure than inlet pressure at engine speeds when the waste gate is opened. This imbalance has a negative influence on the exhaust scavenging of the engine and results in high levels of residual gas and consequently the engine is more prone to knock.

The divided exhaust period (DEP) turbo charging is achieved by dividing the exhaust flow from the two exhaust valves into two different exhaust manifolds, one connected to the turbocharger and one connected to a close coupled catalyst (or could be vented to the exhaust). By separating the valve opening period of the two valves and keeping the duration of both valve opening events shorter, the disturbance of the exhaust blow down pressure pulse during valve overlap can be completely eliminated [2].

The method also can be made used to best utilize the engine's pulse energy at the exhaust of the cylinders whose cycles interfere with one another which results in enhanced pressure distribution in the exhaust ports and a more efficient delivery of exhaust gas energy to the turbocharger's turbine. For instance the firing order in a four-cylinder engine is 1-3-4-2. At the end of the expansion stroke of cylinder 1, its exhaust valve opens whilst the exhaust valve of cylinder 2 has its open still (its overlap period). In case of an undivided exhaust manifold, during the cylinder 1's expansion stroke the pressure pulse from exhaust blow down event with high pressure exhaust gas is more prone to contaminate cylinder 2. This has two effects:

It hurts the cylinder 2's capability to breathing.

The turbine would have better utilized the pulse energy

The complementary cylinders are to be grouped together properly. Cylinders 1 and 4 & cylinders 2 and 3 are complementary. The consequence of this superior scavenging effect leads to improved pressure distribution in the exhaust ports and a more proficient delivery of exhaust gas energy to the turbocharger's turbine which improves the turbine's performance and boost quickly.

This in turn allows greater valve overlap, resulting in an improved quality and quantity of the air charge entering each cylinder. With more valve overlap, the scavenging effect of the exhaust flow can literally draw more air in on the intake side. At the same time, drawing out the last of the low-pressure exhaust gases help pack each cylinder with a denser and purer air charge. [2][3][4]

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Cast manifold with a divided turbine inlet  Welded tubular manifold with a divided turbine inlet

Turbo technologies:


The demands for emissions together with faster response are the imposing reductions in turbocharger turbine rotor inertia, with cropped tip diameter or mixed flow configurations. Reduced rotor inertia can also be obtained by reducing the blade count, rotor disc contouring and by using materials such as titanium alloys and ceramics. For a given material, reducing the blade count is the most effective way of decreasing the inertia, but must be weighed against the turbine efficiency penalties.

Turbine rotor disc scalloping or blending is additionally required for high temperature operation of radial turbines, both to reduce low cycle fatigue stresses and rotor inertia. The effect of disc scalloping on rotor inertia and efficiency reduction can be significant, in addition to which end gap flow leakage losses between the scalloped blade and back face stationary shroud reduce turbine efficiency. This deficit is dependent upon the depth of the scallop, scallop configuration and end gap.

The turbine wheel disc is commonly scalloped between the vanes towards the outer diameter to reduce stresses at the shaft/wheel junction. There is some latitude to fix the amount of scalloping to minimize thrust loads. A turbine wheel having a deeply scalloped back disc will have a minimum rotational inertia. [5][6]

Lightweight materials:

To facilitate improved acceleration and to trim down the amount of harmful substances in the exhaust gases, quick response is mandatory for the turbochargers. The easiest way of achieving this is by using lightweight materials for the turbine wheel. However, since the turbine wheel is exposed to exhaust gases at higher temperatures normal lightweight metallic materials such as Ni -based super alloys cannot be used. High-performance alloys such as TiAl has been found to answer the above setback and has been confirmed for the successful usage in practical applications after numerous engine tests.

The material can be manufactured using the same processes as that for the conventional metallic materials, and it is positioned as a material which can fill the gaps between heat-resistant metals and ceramics. The properties of TiAl suits the rotating components in general and hence are extremely attractive, and it could be used in the following types of applications.

Where, the performance of the equipment would be directly improved by reducing the weight of the targeted parts.

Where, the performance of the equipment would be improved by the effective utilization of materials such as TiAl alloys with superior material properties. [7]

Variable Geometry Turbine (VGT)

Variable geometry turbines (VGT) are shown a particular interest in regards to the future advanced power trains, as they can significantly improve the system transient response for the sudden changes in speed and load characteristics of the automotive applications. It is also viewed as the key enabler for the application of the EGR system for reduction of emissions in order to meet legislated, current and future, emissions standards. This is due to the fact that VGT systems have the potential to provide accurate control of the pressure difference across the engine, as well as very quick response during engine transients.

It was initially considered as a method to eliminate the turbo lag, to improve the low speed boost and torque. Its ability to lessen the emissions through increased transient air/fuel ratio with improved transient fuel/air mixing was also noticed. The controlling strategy varies from a simple increase of turbine area along with the engine speed, through transient strategies widened from steady-state operation to systematic development of a multivariable controller. However with the advancement of the conventional turbochargers the gains are less tangible. [8, 9, 10, 11]

Waste gate

The waste gate controls the passage of the exhaust gas using either a poppet or a swinging flap type of valve. Either of the valve are normally operated using a diaphragm actuator controlled by either the boost pressure from the volute impeller housing or by the exhaust manifold gas pressure. In case of the waste gate using poppet valve, the diaphragm actuator is connected directly to the long stem of the valve, with the stem generally enclosed in a finned housing to help the heat dissipation process from the valve and the actuator assembly. On the contrary a short external lever operates the swinging-flap type waste gate, which is linked by a long push rod to the diaphragm actuator, so that the actuator is practically insulated from the exhaust gas heat. The bypass passageways and the waste gate can be integral to the turbine-wheel housing, in case of small turbochargers or it can be mounted separately for the larger turbochargers, away from the turbine-wheel housing.

Boost pressure can be controlled from the turbine inlet by a waste gate valve blowing off either the surplus exhaust gas or from the compressor delivery via a blow-off valve sending out the surplus air. The latter case results in higher turbocharger speeds than the former. This is because, although the compressed air load is reduced, there will be very little change in the actual gas energy which passes through the turbine wheel. As a result the rotor assembly spin speed is raised to a higher level with the surplus energy input to the turbine. When the compressed air is blowing off the thermal efficiency of the engine decreases as a portion of the compressed air delivery is discharged back into the atmosphere. Hence the waste gate means has been commonly followed in diverting the exhaust gas away from the turbine. However, in case of the pressure build up rate exceeding the waste gate's ability to divert the energy from the turbine wheel, the blow-off valve method is incorporated as the secondary method of limiting the boost pressure owing to its simplicity. [12]

Engineering Sciences

Pulse Vs steady flow turbo charging

Pulse turbo charging is a phenomenon which happens inside the turbochargers. Engine configuration and exhaust manifold design is the key parameter of it. Pulse turbo charging methods are used in engines in which the turbine inlet is coupled closely with exhaust manifold. Hence a highly pulsating flow field occupies the turbine and synchronizes it with the opening and closing of the valves.

The system is able to transmit a higher value of pressure energy to the turbine as the pressure differences are smaller. Since the gas flow into the turbine is highly unsteady and the turbine operates under variable conditions its efficiency is lower. The net benefit of pulse turbo charging is faster spool up and a steeper boost curve. Once the boost is being controlled by the waste gate (e.g.), the benefit is unrealized. The unsteadiness in the flow of the pulse turbo charging system could be reduced by grouping the cylinders together in a common exhaust pipe, which could be then connected to a pulse converter. Such systems are called the pulse converter turbo charging system.

In case of the steady flow turbo charging system, it damps the flow of the fluctuating gas from the cylinders. This creates conditions which are essentially steady with time at the entry of turbine, providing nearly constant-pressure turbo charging. The turbine attains high efficiencies as the mass flow is relatively constant. The disadvantage of this system is that there exists a significant dissipation caused by the throttling effect of the exhaust valves. The effect is proportional to the pressure difference between the exhaust manifold and the cylinder. Other drawbacks include poor part-load performances and transient responses. Since with pulse turbo charging systems the pressure difference is smaller, they are able to transmit a higher value of pressure energy to the turbine. However, the turbine efficiency is lower because the gas flow into the turbine is highly unsteady and the turbine operates under variable conditions.

Both pulse and constant pressure turbo charging could be combined using a conventional turbocharger in applications that are configured appropriately. Notably smaller in cross section can be found in exhaust manifolds for the applications utilizing the pulse turbo charging than those that are designed as purely constant pressure systems. The small cross section maintains the desired high gas velocity and concomitant kinetic energy to the turbine inlet. [13, 14, 15, 16 and 17]

Turbine-Compressor Characteristics

Operating characteristics of Turbine:

The performance of the turbine increases with the increase in pressure drop between the inlet and outlet, i.e. as more amount of the exhaust gas is dammed upstream of the turbine due to higher engine speeds, or due to a rise in the exhaust gas temperature owing to higher exhaust gas energy.

The characteristic behavior of a turbine is determined largely by the throat cross-section, the specific flow cross-section, the transition area of the inlet channel to the volute. Reducing the throat cross-section increases the turbine performance as a result of higher pressure ratios consequently results in higher boost pressures. It can be easily varied by changing the turbine housing. The exit area at the wheel inlet also influences the turbine's mass flow capacity. A contour enlargement results in a larger flow cross-sectional area of the turbine. For the turbines with variable turbine geometry change it gives an advantage to change the flow cross-section between volute channel and wheel inlet by variable guide vanes or a variable sliding ring.

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The maps of the flow parameters plotted against the pressure ratio of the turbine describe the operating characteristics. The map shows the turbine efficiency and the mass flow curves for various speeds. The individual curves can be simplified by a mean curve.

The usage of twin-entry turbines in pulse turbocharged engines, allow exhaust gas pulsations to be optimized, as a higher turbine pressure ratio is reached in a shorter time. When a high, more efficient mass flow passes through the turbine, by increasing the pressure ratio, the efficiency can be increased. On account of this improved utilization of the exhaust gas energy, the boost pressure characteristics and, hence, torque behavior of the engine is improved, particularly at low engine speeds.

Operating characteristics of Compressor

Compressor's operating behavior is generally defined by maps which show the relationship between volume flow rate (mass flow rate) and pressure ratio. The useable sections of the map connecting the compressor are limited by the maximum permitted compressor speed and the surge and choke lines.

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Surge line

The surge line limits the width of the map on the left which denotes "stalling" of the flow of air at the compressor inlet. A very small volume flow and a very high pressure ratio interrupt the discharge process because the flow can no longer adhere to the suction side of the blades. The air flow through the compressor reverses until a stable pressure ratio with positive volume flow rate is achieved, the pressure builds up again and the cycle repeats. The flow instability prolongs at a fixed frequency and the resulting noise is called "surging".

Choke line

Cross-section at the compressor inlet limits the maximum volume flow rate through the compressor. Flow at the wheel inlet with sonic velocity chokes the flow with no possibility of added flow rate increase. The deeply descending speed lines at the right of the compressor map marks the choke line. For achieving high overall efficiency if the turbocharger the synchronization of the compressor and turbine wheel diameters is vital. The turbocharger speed is determined by the position of the operating point on the compressor map. For maximizing the turbine efficiency in this operating range the diameter of the turbine wheel has to be appropriate. [18]


Measurement methods for turbo (system) transient response

A comprehensive methodology for measuring and post processing (not discussed) during the transient conditions in a turbocharged engine can be performed and is discussed below. This has been focused on the handling of instantaneous measured variables from the points of view of acquisition, post-processing and synchronization.

Transient operation in an engine is describes the important variation in the running conditions of the engine. One such phenomenon is the time delay which causes the poor performance in these conditions. It can be caused by thermal, mechanical and fluid dynamic processes. The mechanical processes influencing the time delay are friction and inertia of the turbo charger and other rotational elements of the engine. Thermal and fluid-dynamic processes include the mass and transfer of energy from the exhaust valves to turbines and from the compressor outlet to the cylinder. These are also determined by pressure pulses, gas friction, flow inertia and heat transfer. The most critical phenomenon which delays the speed or load increase in a turbocharged engine is the transient response, namely turbo lag.

Engine test bed instrumentation could be connected to a high-speed data acquisition system to conduct a detailed parametric study in both steady and transient conditions by observing the engine response. The engine could be coupled to a hydraulic brake (dynamometer). Strategic measuring point could be used in recording and processing engine and turbocharger variables by connecting to a computer for processing the data.

Firstly the steady state performance could be validated with the standard results to make sure the set up is accurate. This could be followed by the investigation of transient operation. The load changes could be done with a constant setting of the governor. The speed could be varied by changing the load from no load to full load. Factors such as the time taken for applying the load changes should be considered appropriately for getting accurate results.

The measurement of the related data and the post processing of the same is done subsequently during a full-load transient test at a constant engine speed. The variables are measured synchronously with a frequency equivalent to a predefined crank angle degree interval which gives a good understanding of the transient process taking place. Hence it is better to establish a group of control parameters, whose variation has to be recorded throughout the full load transient conditions. Some of the variables such as the engine speed have to be kept into a narrow range in order to ensure the reliability of the signals acquired instantaneously. Apart from the above said variables there are other parameters which need to be kept track of and is given in the table below.

Control parameters

Narrow range parameters

Measured variables

Cooling temperature

Lubricant temperature

Intake temperature

Engine speed

Fuel temperature

Engine throttle position

In-cylinder pressure

Boost pressure

Exhaust pressure

Air charge temperature

Exhaust temperature

Fuel mass flow


Air mass flow

EGR valve position

Non-cooled piezoelectric transducers are used in measuring the in-cylinder pressures. The cooled piezo-resistive transducers are used for boost and exhaust pressure measurement. The principle of piezoelectricity is apt for the measurement of quasi-static and dynamic pressure processes. Hence, for measuring the in-cylinder pressure, the pressure sensors based in this principle are well suited.

Two different systems could be used to measure the fuel mass flow.

Gravimetric balance which provides mass flow at a predefined frequency. A measuring vessel filled with fuel that relates weight loss.

A system in the engine control unit (ECU) that provides a value of volumetric fuel mass flow at a predefined frequency.

But, since it is not a direct measurement but a calculation, it needs to be calibrated by reliable experimental information. Hence a process consisting of calibrating the ECU fuel volumetric flow calculation is suggested. The fuel information calculated using ECU is connected with the fuel mass flow measurements obtained by using gravimetric balance, whose data are reliable under these conditions.

The flow of air mass can be measured using a hot-wire anemometer placed slightly away from the intake valves of the engine. The registered signal of the hotwire anemometer should be corrected due to the effect of the pressure waves travelling and the effect of air mass storage inside the intake system. During full-load transient tests the ECU measure the EGR by closing the valves. Concerning the exhaust gas temperature, the difficulties related to its measurement in transient conditions necessitates performing an iterative procedure based on the interaction between a combustion diagnostic code and a 1-D gas dynamic model. [20]

Other factors such as injection timing (if appropriate), time taken for the application of load with the angular acceleration, effect of the number of turbines and compressors and the turbocharger speed of response (when the effect of turbocharger mass moment of inertia is examined) should also be considered in interpreting the results properly. The effect of exhaust manifold design on the engine response is another vital phenomenon. The property of exhaust manifold pressure, configuration such as a single and a twin entry turbine and volume are the parameters which define the engine response. In addition the effect of the turbocharger mass moment of inertia, size of the turbocharger along with its inertial effects towards the engine response is tracked on the response of the engine is also kept track of. The following factors needs to be considered appropriately before starting with any experiments: Engine Model and Type, Speed Range, Bore/Stroke, Compression Ratio, Maximum Power, Maximum Torque, Intake and exhaust Valve Open/Closure timing, Fuel Pump, brake, Total Moment of Inertia, Turbocharger Model & Type, Turbocharger Moment of Inertia and Data Logging System. [19]

Simulation: 1-D gas dynamics modeling

Lotus Engine Simulation (LES)

Events such as wave propagation involved in manifold tuning mechanisms can be simulated using computer programs and are used in the design and development of internal combustion engines.

The governing equations of the 1D gas flow serves as the base for the most comprehensive engine simulation programs as it provides a realistic compromise between modeling accuracy and computational speed. But the pressure wave propagation is essentially a multi-dimensional phenomenon. A major challenge lies in modeling such junctions within a one-dimensional simulation as its geometry cannot be represented fully but may have a key influence on the flow.

Lotus Engine Simulation (LES) is a 1D modern simulation system employed in predicting the engine performance, combining models for unsteady gas flow in manifold components with those differentiating the combustion process. It is a software package developed by Lotus and it is a powerful tool for rapidly developing the engine models. The software allows the user to build a model by inputting a series of known engine parameters and then solving the model to obtain results predicting variable such as power, torque and fuel consumption. Having validated a model against test data it is then possible to change some of the engine parameters and examine the effect this has on different aspects of the engines performance. It uses a combination of 1D flow modeling and combustion and heat transfer models to accurately model the processes occurring within an engine. An iterative process is then applied to these models enabling values to be generated at a specified interval measured in CAD. The software will solve for a given number of engine cycles or until a converged solution is reached, whichever is reached first.

The software utilizes the standard thermodynamic principle of control volumes and considers the conversation of mass, momentum and energy through the volumes to model the flow of gases. A mesh of the pipe is automatically defined and enables the governing equations to be iterated at the specified intervals using numerical methods based on the two-step Laz-Wendroff Scheme. This enables the software to accurately handle the pressure waves and supersonic flows that can occur in engine manifolds. The pipe mesh allows the program to work in a similar manner to FLUENT and is automatically selected to reach a suitable compromise between accuracy and calculation speed.

The other key aspects of the simulation are the combustion and heat transfer models. The combustion model used is the Wiebe combustion model which is a single zone heat release model. The heat transferred to or from the in-cylinder gases are calculated at every iterative point and use value of the cylinder wall area, wall temperature and heat coefficient. The cylinder wall area is calculated using the piston displacement at each iterative step while the cylinder wall temperature is calculated using a simple heat transfer model. The user is able to specify the wall materials when building the model, and therefore the heat transfer coefficient of the wall, while the coolant temperature is always assumed to be 100o C.

The program allows a choice of heat transfer models to be used. By combining these flows, combustion and heat transfer models, an accurate representation of the processes occurring in an engine can be modeled and solved. When the problem has been solved, the program allows the user to view a range of data regarding the engines performance over the test cycle, including power, torque, BMEP, fuel consumption and volumetric efficiency amongst many others.

The vital parameters connected to the operation and performance of the engines can be studied and optimized, providing real time and cost savings. It provides a user friendly method of construction the engine model using its 'drag-and-drop' engine network builder where the components are chosen from a tool box and connected directly to build the model. It uses single and two-part Wiebe functions to model the combustion process in engines. Instead, heat release data can be imported from an external source or from the built-in combustion analysis tool. All the input parameters are examined and a detailed feed-back is provided based on four levels of data integrity by the data checking wizard. Since it can optimize the engine performance, it eliminates the need to evaluate the full test matrix for carrying out parametric studies. Specific target performance can be defined using weighting functions and it provides the criterion within which the optimizer operates also allows comparing results from many tests. To sum up the features:

LES Simulates steady-state or transient engine performance

Gasoline or Diesel, pressure-charged of naturally aspirated engines

Two- or four-stroke engines

Multi-Cylinder Turbocharged Model, Turbocharged Model with a Waste-gate and Charge cooler

Modeling the Transient Response of a Turbocharged Diesel Engine, Modeling a Multi-Cylinder Turbocharged Diesel

Engine State of the art numeric's and robust boundary models

Intuitive user interface and model quality evaluation

Comprehensive built-in post processing [21][22][23]

Research Objectives:

To understand the steady state characteristics of complex and unique engine exhaust manifold arrangements.

To characterize experimentally, for the first time, the transient behavior of multiple turbochargers with complex and unique engine exhaust manifold arrangements (e.g. with separated high and low pressure exhaust manifolds (e.g. DEP)).

To develop simulation tools (maybe using commercial software) to model transient response of conceptual turbocharger systems.

To study and understand the multi-order interactions between the reciprocating engine, valve characteristics, exhaust arrangement and multiple turbocharger characteristics.

Proposed Contribution to Knowledge;

Data and understanding of the steady state and transient behavior of multiple turbocharger systems with complex and unique exhaust manifold arrangements.

New approaches to accurately simulating the pulsed flow performance and response of turbochargers.

Outline Methodology:

~60:40 modeling to experimental ratio

Experiment (for complex and unique exhaust manifold arrangements):

Data and understanding of steady state behavior (Validating the model against experimental values)

Data and understanding of transient behavior (using high speed data acquisition system after checking against steady state, as discussed earlier)

Modeling: Using LES (1-D engine simulation)

Should use all the components of the system as used in the experiments for both steady state and transient

Should match the steady state experimental results as it is a well established simulation tool. (Having validated the model it is possible to add components/change parameters)

Should match the transient conditions experimental results for the multiple turbocharger arrangement

Should predict the best exhaust manifold arrangement.

1st Year Plan:

Description of main focus and key milestones:

Background study: Principle of sequential turbo charging, divided exhaust period (DEP) turbo charging,

Possible solutions: Turbo technologies such as scalloping, light weight materials, VGTs and Waste gates


The experiments discussed earlier for steady state and transient conditions using the appropriate data acquisition systems.

Effect of different parameters to be analyzed

The results to be compared against the simulation are double checked and recorded


1-D gas dynamics modeling - LES

Creating models of the actual engine and comparing with the experimental results of the steady state conditions to validate the model

Creating models of the actual engine (changes included) and comparing with the experimental results of transient conditions to validate the model