Internal combustion engine supercharging

Cent. Eur. J. Eng. • 4(2) • 2014 • 110-118 DOI: 10.2478/s13531-013-0133-6

Central European Journal of Engineering

Internal combustion engine supercharging: turbocharger vs. pressure wave compressor. Performance comparison Topical Issue: AMMA 2013

Atanasiu Catalin George1∗ , Anghel Chiru1 1 ”Transilvania” University of Brasov, Brasov, Romania

Received 30 May 2013; accepted 24 September 2013

Abstract: This paper aims on comparison between a turbocharged engine and a pressure wave charged engine. The comparison was accomplished using the engine simulation software AVL Boost, version 2010. The grahps were extracted using AVL Impress, version 2010. The performance increase is limited by the mechanical side of the simulated engine. Keywords: Supercharging• Turbocharger• PWS, Engine• Comparison © Versita sp. z o.o.

1.

Introduction

The will for the automotive engine builders to manufacture thermic engines with higher and higher efficiencies, but with lower emissions and lower fuel demanding, has led to research intensifications in terms of combustion improvement and geometrical optimization [1]. By comparing the internal combustion engines, in terms of efficiency, we can clearly see that the maximum efficiency has the diesel engine, or the compression-ignition engine. The spark ignition engine has an overall efficiency slightly lower. It is followed by the Stirling engine, gas turbine and gas machine (Figure 1). Although the electric engine has an efficiency of 85%, this does not compensate the losses of the electrical energy build-up [2, 3]. The efficiency to convert the oil into electrical energy, ∗

E-mail: [email protected]

Figure 1.

Thermic engine efficiency comparison.

including transport, is 29.3%. The same calculation made for coal has an overall efficiency of 31.1%. In this case, the electric engine powered by oil-source energy is 24.9%, and powered by coal-source energy is 26.4% [2]. In order to improve the efficiencies of the internal combustion

110

Unauthenticated Download Date | 9/24/15 11:18 PM

Atanasiu Catalin George, Anghel Chiru

engines, more air is needed into the cylinder, to be able to burn more fuel. To do this, a supercharging or turbocharging equipment is needed [4].

2.

Supercharging solutions

Superchargers and turbochargers are compressors mounted in the intake system and used to raise the pressure of the incoming air. This results in more air and fuel entering each cylinder during each cycle. This added air and fuel creates more power during combustion, and the net power output of the engine is increased [3, 5]. The internal combustion engines (ICE) can be supercharged in various ways, but he most common way to do this is to use a turbocharger [6]. A very rare type of supercharger, but pretty promising, is the pressure wave supercharger. Both of them are treated in this paper as a supercharging unit of the 1.8i gasoline engine. A gasoline unit has be chosen due to low torque at low revs. The aim of this paper is to demonstrate that the pressure wave supercharger is way better than turbocharger, at low rev operation [3, 7].

2.1.

Turbocharging

At turbocharging, a compressor is powered with an exhaust turbine using energy from the exhaust gases, i.e. the engine is only fluidically connected to the turbocharger (Figure 2). The exhaust gas of the internal combustion engine flows through the exhaust manifold to the turbine and spins it. A compressor sitting on the shaft of the turbine, can convert the drive power of the turbine into compressing power (minus bearing friction), itself compresses fresh air, raises its temperature, which must be cooled by an intercooler [8]. Via the manifold, the compressed air enters the engine. In the case of highly unsteady applications (e.g. vehicle operation), the power of the turbine should be controlled. This can be done in the schematic shown here with a wastegate [5, 7, 9].

2.2.

Pressure wave supercharger

The pressure-wave supercharger (PWS) utilizes the energy of the engine exhaust gas to build up the intake air pressure [1, 2, 4, 10] like the more classic turbo charging. However, the operation principle of pressure wave supercharger is some what different. As shown in Figure 3, The PWS is made of a set of tiny and narrow channels, called cells, placed on a rotor. The rotor spins between two casings, the exhaust gas housing and the fresh air housing, with inlet and outlet for the exhaust gas

Figure 2.

4 cylinder engine with turbocharger; 1 - compressor, 2 intake manifold, 3 - exhaust manifold, 4 - turbine.

and the fresh air. The PWS works based on the physical principle that if two fluids having different pressures are brought into contact, equalization of pressure occurs faster than fluid mixing. The actual energy transfer occurs in a single cell joining area of different states. At the beginning of a cycle, the fresh air fills in the rotor cell. When PWS works, the rotor keeps in continuous rotation and the hot exhaust gas flows from cylinder into the cell, so that the fresh air meets the exhaust gas directly during one phase of the cycle. As the rotor makes one revolution, the ends of each cell are alternating either nearly hermetically closed or widely open toward the passages of the casings. These alternative open and close of cells result in serial shock waves, compression waves and expansion waves, which contribute to the energy transfer between the exhaust gas and the air. The shock wave moves much faster than the gases. The compression waves build up the pressure of the charge air. The charge air with increased pressure flows out of the cell into the casing passage and then to the cylinder as the end of cell opens. Simultaneously, the expansion waves cause the exhaust gas pressure to go down. With that, the exhaust gas of low enthalpy finally goes toward the exhaust pipe [4, 7, 10]. This direct contact action inside the PWS results in energy transfer in such short time that the PWS can respond quickly according to the different engine working conditions, hence there is no so-called ’turbo lag’ problem like turbo charger [3, 5]. Furthermore, this unique direct contact may also cause internal exhaust gas recirculation (EGR) usefully for reducing the NOx emission [4, 5, 10]. Meanwhile, there are other attractive advantages, such as large torque at small engine speeds and less soot during 111


Internal combustion engine supercharging: turbocharger vs. pressure wave compressor. Performance comparison

Figure 3.

Engine fitted with pressure wave supercharger.

acceleration. All these good characteristics make the PWS suitable for automobile engines whose loads vary a lot [3, 11, 12].

• s - distance between crank axis and wrist pin • Am valve area Air flow rate:

3. Theoretical approach of the simulation The theoretical approach of the simulation is supplied by [7, 9, 13, 14].

˙= m

C D · AR · p 0 (R · T0 ) 2

1

   1 γ−1 gamma  2 pT pT 2·γ   · · 1−  p0 γ − 1 p0 (3)

Volumetric efficiency:

• CD - discharge coefficient ηv =

˙a 2·m ρa · V h · N

(1)

• p0 - upstream stagnation pressure

˙ a - mass flow inducted into the engine • m

• T0 - upstream stagnation temperature

• ρa - air density

• pT - pressure at the restriction

• N - engine speed

• Am - reference area of the valve

• Vh - engine displacement

• γ - adiabatic coefficient

Flow velocity:

Equivalence ratio: vps =

π · B2 ds 1 dV · = · Am dθ 4 · Am dθ

φ=

• V - cylinder volume • B - cylinder bore

A F actual A F theoretic

(2)

•

A - Air-fuel ratio Z

112


(4)


Burnt fuel quantity:

Intake mass speed:

patm − pc =

X

¯ p2 ∆pj = ρa · S

X

ξj ·

Ap Aj

2

• patm - atmospheric pressure

4. Practical simulation

• pc - cylinder pressure ¯ p - mean piston speed • S • Ap - piston area • Aj - component minimum flow area • ∆pj - total quasy steady pressure loss • ξj resistance coefficient for that component which depends on its geometric details Mass flow rate: dm = Aeff · p0I · dt

•

s

2·ψ R0 · T0I

(6)

dm - mass flow rate dt

• Aeff - effective flow area • p0I - port upstream static pressure • T0I - port upstream static temperature • R0 - gas constant Combustion model: dx a = · (m + 1) · ym · e−a·y·(m+1) da ∆αc dQ dx = Q α − αc y= ∆αc

(7)

• Q - total heat amount received • α - rank angle degree • α0 - corresponding angle for beginning of combustion • ∆αc - combustion duration • m - form coefficient • a = 6.9, Viebe coefficient

x = 1 − e−a·y·(m+1)

(5)

approach

(8)

of

the

The AVL code is a full software bundle used to simulate the processes within an internal combustion engine and includes: Boost, Fire, Cruise and Excite. In this paper, the AVL Boost had been used to simulate a complete engine and its internal processes. After the simulation was completed, AVL Impress was used to read and easily generate the graphs. AVL Boost can simulate a very wide range of parameters: performance evaluation for different operating points in accordance with geometrical and functional optimization, comparison of different concepts of internal combustion engines, etc. The construction of the model in AVL Boost follows a verification and also an analyze of the real-world engine, the Audi 1.8 TFSI. In Figure 4 and Figure 5, the engine model is presented, used for turbocharged (Figure 4) and pressure wave supercharged (Figure 5) engine operation. The model is built with the help of the AVL Boost internal library and is formed by: C1, C2, C3, C4 - engine cylinders; PL1, PL2 - plenums; TH1 - engine throttle; CO1 - air intercooler; CL1 - air filter; TC1 - turbocharger; PWSC1 - pressure wave supercharger; CAT1 - engine catalyst; MP1 to MP5 - key measuring points; R1, R2 - flow restrictions (used to simulate EGR opening and turbocharger bypass); SB1, SB2 - system boundaries; J1 to J14 - joints; 1 to 42 - connecting pipes. The fluid flow within pipes is simulated using unidimensional pipes, with the friction coefficient input required. The pipe bending losses are simulated also with a friction coefficient. The software describes the fluid flow through the pipes using 3 basic equations: the continuity equation, the impulse conservation equation and the energy conservation equation. For each element of the model, the input data is required. For example, in Figure 6, Figure 7 and Figure 8 are presented some of these required inputs. The Vibe function is a very convenient method for describing the heat release characteristics. It is defined by the start and duration of combustion, a shape parameter ’m’ and the parameter ’a’. These values can be specified either as constant values or dependent on engine speed (in rpm) and engine load (expressed as BMEP in bar). In this case, fixed values were chosen [14]. The heat release characteristic of gasoline engines, 113



Figure 4.

AVL Boost turbocharged engine layout.

Figure 5.

AVL Boost pressure wave supercharged engine layout.

114



Figure 6.

Engine burning model - Vibe function was used. Parameters ”m” and ”a” are shape parameters.

with essentially homogeneous mixture distribution in the cylinder, is mainly determined by the flame propagation speed and the shape of the combustion chamber. A high flame propagation speed can be achieved with high compression ratio and high turbulence levels in the cylinder. In diesel engines on the other hand, the combustion characteristic depends strongly on the capabilities of the fuel injection system, compression ratio and the charge air temperature [14].

increase at low revolution. Test setup: • Engine: 1.8i, gasoline • Operating point: 1500 rpm, full load, • Turbocharging: standard (unknown manufacturer),

5.

Results

Following results represent the benefits of the pressure wave supercharging process. The engine was simulated at only one operating point, to indicate the performance

turbocharger

• Supercharging: Comprex CX-93 solution, • Run setup: turbocharged engine, turbocharged engine with VVT, supercharged engine using pressure wave supercharger.

For accurate engine simulations the actual heat release characteristic of the engine, (which can be obtained by an analysis of the measured cylinder pressure history), should be matched as accurately as possible [14]. In order to simulate the burning process within the engine cylinder, the uni-zonal model is adapted witch assumes that the fuel and the air within the cylinder is always in thermodynamic balance, with no temperature gradients, no pressure waves and no unequilibrate composition.

Audi

Measuring point 2 location: compressed air intake, before plenum PL1, right before entering the engine. It is good to know the intake pressure after all pipe and connection losses.

6.

Conclusions

The pressure wave supercharger has great influence on the performance of the gasoline engine, as showed in this paper, and also of diesel engine. The 1.8i gasoline engine performance increased dramatically at low revs in term of power and torque and also develops perfectly fast 115



Figure 7.

Intercooler configuration.

Figure 8.

Pressure wave supercharger setup.

116



Figure 9.

Figure 11.

Cylinder temperature for the 3 setups.

Figure 12.

Mechanical engine work for the 3 setups.

Figure 13.

Air pressure at measuring point 2 in the simulation for the 3 setups.

P-V diagram of the 3 supercharging solutions.

Figure 10.

Cylinder pressure for the 3 setups.

instant response. The turbocharged gasoline engine fitted with VVT witch comes closer, in terms of performance, to the pressure wave supercharged engine also it is more expensive due to integration of VVT. The pressure wave supercharged engine is a little bit heavier (about 10kg+) than the turbocharged one, but overall has a better weight/power ratio. Some factors can affect the pressure wave supercharged engine. The simulation shows that the intake air mass

117



flow and the exhaust gas temperature are the most important influence factors of the 1.8i PWS gasoline engine. Therefore, larger intake manifold volume and higher exhaust temperature are recommended for better power performance.

Acknowledgements This paper is supported by the Sectoral Operational Programme Human Resources Development (SOP HRD), financed from the European Social Fund and by the Romanian Government under the contract number POSDRU ID76945.

References [1] Despreiries P., L’avenir du petrole. Ingineurs de l’automobile 10, 1979, 545-550 [2] Chiru A., Posibilitati de imbunatatire a performantelor unor agregate energetice pentru autovehicule. Buletinul sesiunii stiinitifice, I.I.S. Pitesti, 4-5 Dec. 1981, III, 169-174 [3] Stone R., Introduction to Inernal Combustion Engines, Third Edition, SAE 1999, ISBN: 0-7680-0495-0

[4] Weber F., Guzzella L., Control oriented modeling of a pressure-wave supercharger (PWS) to gasoline engine [C], SAE paper 2000-01-0567, 2000 [5] Heisler H., Advanced engine technology, 1995, ISBN: 978-1560917342 [6] Haider, G.: Die mechanische Aufladung, 2nd edn. Published by the author, Wien, 2000 [7] Heywood J. B., Internal Combustion Engine Fundamentals [M]. New York, McGraw Hill, 1988 [8] Spinnler, G: Ecodyno®: a new supercharger for passenger car engines. abb Techn. Beschreibung, 1991 [9] Pulkrabek W. W., Engineering Fundamentals of the Internal Combustion Engine [M]. New Jersey: Prentice Hall, 2003 [10] Gyarmathy G., How does the Comprex pressure-wave supercharger work [C], SAE paper 830234, 1983, 91105 [11] Hermann H., Peter P., Charging the internal combustion engine, Springer Wien New York, 2003 [12] Spring P., Onder C. H., Guzzella L., EGR control of pressure-wave supercharged IC engines. Control Engineering Practice [J] 2007, 15, 1520-32 [13] LMS Amesim 10, user guide pdf book [14] AVL Boost 2010, user guide pdf book

118
