new 6- and 4-cylinder petrol engines with high precision

Vol.39, No.3, May 2008.

Material

89

20084533

New 6-and 4-Cylinder Petrol Engines with High Precision Injection and Stratified Combustion *

Erik Schuenemann 1) Peter Langen2) Stephan Missy3) Christian SChwarz4)

According to its powertrain strategy， 8MW introduces the new 6-and 4-cylinder inline petrol engine generation， featuring a stratified DI combustion system with High Precision Injection. The spray-guided combustion system eliminates the disadvantages of the wall-and air-guided 151 generation DI combus-tion systems， providing a stratified mixture at the spark plug and significantly reduced wall wetting. The piezo injector permits an extremely fast actuation， providing high flexibility for the calibration strategy to supply a very efficient combustion with low unburnt hydrocarbon and carbon monoxide emissions The paper presents the realised potentials and technical features of this new engine generation.

Keywords: BMW inline petrol engines， high precision injection， spray-guided combustion③

1. Introduction

In the development of its new engines the 8MW Group is pursuing a course of optimising fuel consumption， dynamics and weight (Fig. 1). At the same time， reliable compliance with the exhaust emission limits of the respective markets as well as the uncompromising realisation of the 8MW characteristic "Sheer Driving Pleasure" are， of course， indisputable development objectives.

Performance

Weight Consumption

Fig. 1: Efficient dynamics

Direct injection with stratified engine operation represents the greatest individual thermodynamic potential for reducing fuel consumption of spark ignition engines. The main requirements in terms of the combustion process are:

Provision of a compact， stable， reproducible and homogenised air/fuel mixture cloud at the point of ignition in synch with the ignition timing

Reliable ignition of the prepared mixture by the ignition spark

* Received July 2，2007. Presented at the JSAE Annual Congress on May 25，2007. 1)・2). 3) . 4) 8MW Group， Munich， Germany

First generation walllair-guided DI combustion systems met these requirements only incompletely and were not used by 8MW in series production as the advantages gained in terms of fuel consumption were achieved to the same extent by VALVETRONIC technology already introduced worldwide by 8MW [1]

The 8MW spray-guided DI combustion system with centrally positioned fuel injector and High Precision Injection effectively avoids the disadvantages of the first generation direct injection processes while enabling maximum utilisation of thermodynamic potentials

The technical data and the specific design features of the new generation of engines are presented in the following.

2. Technical Data and Design Features

2.1 Engine Data

Table 1 lists the technical data of the new 8MW spark ignition engines shown in Fig. 2.

Fig. 2(a): New 3 1 six-cylinder petrol engine with high-precision injection and stratified combustion

自動車技術会論文集90

New 6- and 4-Cylinder Petrol Engines with High Precision Injection and Stratified Combustion

Fig. 2(b): New 2 I four-cylinder petrol engine with high-precision injection and stratified combustion

Table 1: EnQine dat

庁ype Inline 6 Inline 4

Stroke 88mm 90mm

Bore 85mm 84mm

Stroke/bore 1.035 1.071

Oisplacement 2.9961 1.9951

Compression ratio 12

Cylinder spacing 91 mm

Valves per cylinder 4

Combustion system BMW 012 spray-guided

Crankcase Magnesium Aluminium crankcase with crankcase with aluminium insert， cast iron Alusil cylinder liners face

Valve train Roller cam follower. double VANOS

Intake system 3・stageresonancel2-stage intake manifold intake manifold

Engine management Oigital engine management MS080 Mixture preparation High Precision Injection Ignition Individual ignition coils

Exhaust system Single-tube manifold with lightweight flange，

2 close-coupled 3・waycatalytic converters

2 NOx storage 1 NOx storage catalytic catalytic converter converters

Cooling Electric coolant pump， electrically controlled thermostat

Fuel RON98-91

Engine weight 163 kg 136 kg according 0lN70020・GZ

Emission standard EU4

Effective power 200kW 125 kW output @6700rpm @6700rpm

E仔'ectivetorque 320Nm 210 Nm @ 2750-3000 rpm @4250rpm

2.2 Fuel System

The 200 bar high-pressure fuel system High Precision Injection， as used in the new BMW six-cylinder engine and iIIustrated in Fig. 3 consists of the high-pressure pump with volume control valve， fuel rail with fuel pressure sensor as well as piezo-injectors all connected by the fuel lines. The low-pressure system operates at a pressure of 5 bar. Controlled by the volume control valve， the 3-piston high-pressure pump delivers the fuel mass flow required by the engine at the specific fuel pressure.

Fig. 3: High-pressure fuel system for high-precision injection

The needle of the outward-opening nozzle of the fuel injector (Fig. 4) is activated directly by the piezo actuator and releases an annular gap， through which the fuel is injected. A thermal compensator is used for the purpose of equalising the differences in thermal expansion between the piezo-stack and housing and therefore to ensure constant needle lift at all injector temperatures. The use of piezo・electric actuation ensures rapid， delay-free and reproducible opening and closing of the needle， thus enabling the shortest injection times and the injection of minimum quantities. At the same time， the piezo・injectorfeatures high volume dynamics， i.e. it can inject large masses of fuel in a very short space of time. Multiple injection with very short pause times and the possibility of injector operation in the partial-needle lift range provide the degree of flexibility required for the stratified combustion process in line with the optimum injection strategy

Calibration stamp

Electrical I Piezo Actuator ccnnectors I Unit (PAU) Sealing

Fig. 4: Sectional view of piezo-injector for high-precision injection

Vol.39, No.3, May 2008. 91


2.3 Exhaust Gas Recirculation System

For the purpose of reducing NOx raw emissions at stratified engine operation， the new six-cylinder and four-cylinder petrol engines are equipped with an external exhaust gas recirculation system. The exhaust gas is taken from the second cylinder bank after the close-coupled catalytic converter and the recirculated exhaust gas is introduced centrally at the throttle valve. The EGR valve is arranged beneath the intake manifold and is water-cooled. It is actuated by means of a rotary magnet that actively controls the EGR mass f1ow. The design of the EGR piping system with expansion bellows e仔ectivelydecouples vibration between the intake manifold and exhaust system.

3. Combustion System

3.1 Components

The BMW spray-guided 01 combustion process of the new six-cylinder and four司 cylinderpetrol engines consists of the following main components (Fig. 5):

-01 cylinder head with squish area combustion chamber， centrally positioned injector， spark plug on exhaust side， filling po吋S

-High Precision Injection with piezo-injector -Ignition system with 01 specific spark plug and high

energy ignition coil -01 piston with central piston bowl -Valve train with double VANOS

Fig. 5: Components of the BMW spray回 guided01 combustion system

The fuel injector is positioned centrally in the combustion chamber and tilted slightly towards the intake side. The spark plug is arranged tilted towards the exhaust side. The intake ports are designed as filling po同swith low tumble to enable high specific power output levels.

The injection creates a stable hollow cone spray with limited penetration depth and a well-defined recirculation zone. The spray cone angle is virtually independent of the injection pressure and cylinder pressure， thus ensuring stable combustion both in homogenous mode as well as in stratified combustion mode. The selected spray layout ensures rapid mixture preparation and e仔ectivehomogenisation within the mixture cloud located at the spark plug， thus ensuring complete combustion. The outward-opening nozzle is fundamentally resistant to the formation of deposits and changes in the spray configuration during operation. The homogenisation is further improved and the penetration depth reduced by multiple injections with up to 3 injections per operating cycle. Injector operation in connection with partial-needle

lifts utilises additional degrees of freedom for the injection process.

The injection system is specifically adapted to the requirements of stratified combustion. The spark plug design includes measures to provide effective robustness， thus improving ignition of both rich as well as lean mixture. The modified ignition coil makes available higher ignition energy levels thus additionally increasing the robustness of the combustion process.

The piston with centrally arranged piston bowl minimises wall wetting during injection in the compression stroke and ensures secondary charge motion via specifically arranged squish areas.

The variability of the valve train made possible by the double VANOS is effectively utilised in the partial load range for internal exhaust gas recirculation and for fine-tuning the charge movement.

3.2 Mixture Formation， Ignition and Thermodynamics

The development of the BMW spray-guided 01 combustion process was made possible by the closely coordinated， combined use of "conventional" test rig measurement technology (e.g. pressure indication， exhaust gas analysis)， high resolution optical processes (high-speed visualisation， endoscopy， LlF， POA)， special measurement procedures (e.g. ignition voltage analysis) as well as by the consistent use of CFO tools for 30 simulation of all relevant physical processes， ranging from internal nozzle flow through spray disintegration， evaporation， mixture formation through to combustion.

Under ambient conditions without the influence of engine-related charge motion， the visualisation of the liquid fuel (Fig.6) shows that a hollow cone shaped， stable spray with a virtually constant cone angle is formed over the injection period. The bottom view shows a streaky spray structure which is created as the result of the design layout of the inner nozzle geometry and has a stabilising e仔'ect.In stratified combustion mode， fuel is injected in the compression stroke so that the penetration depth is significantly reduced while the spray cone angle remains constant so as to form a clearly visible recirculation zone. The streaky structure is less clearly visible in the bottom view due to the high spray density.

Fig. 6: Spray propagation into quiescent air



Analysis of the mixture formation in the optical engine confirms the high stability of the spray cone under the influence of in-cylinder charge motion (Fig. 7). The recirculation zone of the evaporated fuel from the first injection makes available an ignitable mixture directly at the spark while the second injection stabilises the ignition and the burn-out characteristics.

1 st injection --0

Fig. 7: Mixture formation at stratified operation in the optical engine

In addition to the layout of the spray， adaptation of the ignition system to the specific requirements of stratified combustion is a fundamental prerequisite for ensuring reliable ignition. A surface discharge spark plug with 3 ground electrodes is used which exhibits advantageous self-cleaning properties in combination with a multi-spark ignition system. The design layout of the insulator tip and ground electrodes ensures excellent accessibility of the mixture and charge motion to the ignition spark so that the ignition spark from the electrodes is deflected into the ignitable mixture by the spray-induced charge movement thus preventing the occurrence of creepage spark or surface discharge (Fig. 8) .

Fig. 8: Influence of the spark plug geometry on the deflection of the ignition spark in simulation and test

Thermodynamic analysis of the pa附alload operating point n = 2000 rpm; we = 0.2 kJII shows the potential of the BMW spray-guided 01 combustion process with High Precision Injection (Fig. 9). Compared to a basic four-valve engine with intake manifold injection with no variabilities and to the current BMW VALVETRONIC system， lean operation and an increased compression ratio result in substantially higher efficiency of the ideal process and reduce charge cycle losses (Fig. 10). In addition， compared to wall/air-guided 01 combustion systems， the second generation BMW spray-guided 01 combustion system exhibits significantly reduced losses due to incomplete combustion and therefore considerably lower HC raw emission levels due to improved

stratification of the mixture (Fig. 11). These features relate to an effective efficiency of almost 30 % at this operating point.

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4. Efficiency and Dynamics

4.1 Fuel Consumption Engine Map and Operating Modes

Compared to wall/air-directed combustion systems， the BMW spray-guided 01 combustion system with High Precision Injection permits a significant expansion of the engine map range calibrated in stratified combustion mode (Fig. 12). As the illustrated driving resistance curve shows， the engine operates with optimum fuel consumption in stratified mode at a constant driving speed of up to approx. 160 kmph. Above this classic stratified

Vol.39, No.3, May 2008. 93


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The dynamics typical of BMW engines is realised not only by stationary full load values but is also predominantly determined by properties such as rotary power and response that characterise the instationary aUributes of the engine. The free-revving characteristics of the new engines are noticeably improved by the expansive torque progressions and the expansion of the effective engine speed range up to 7000 rpm. Since the engine operates virtually unthroUled in the idle and pa吋ialload range due to the lean operation， the intake system is not charged as the engine load increases so that the time required to build up the torque for the load jump from wi = 0.2 kJII to wi = 0.6 kJII at n = 1500 rpm is distinctly shortened as the required torque is already reached in the next operating cycle (Fig. 15). This characteristic therefore makes it possible to achieve outstanding engine response.

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Fig. 12: Fuel consumption and operating modes in the engine map

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Fig. 14: Torque and power output of the new 3 1 six圃 cylinder

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The exhaust gas treatment system must ensure that market-specific emission limits are reliably met over the engine life time. Fig. 16 shows the dual-flow exhaust treatment system used for the new six-cylinder engine. Linear oxygen sensors are positioned ahead of the close-coupled 3・waycatalytic converters while binary oxygen sensors undertake the trim control and catalytic converter diagnosis. An exhaust gas temperature sensor that is used for the purpose of controlling the exhaust gas temperature is arranged upstream of one of the two NOx storage catalytic converters. The exhaust gas temperature of the second cylinder bank is modeled. The NOx sensor that is used for controlling the regeneration process is arranged in the junction downstream of the NOx storage catalytic converters.

5. Emissions

4.2 Full Load and Response

The full load curves of the new six-cylinder and four-cylinder petrol engines are shown in Fig. 14. Both the 3 litre six-cylinder engine as well as the 2 litre four-cylinder engine achieves top values compared to the competition at specific power output and specific torque. This is obtained in particular by the consistent optimisation of all components involved in the charge cycle， the variable intake systems， the double VANOS as well as the high compression ratio.

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With the market introduction of the new six-cylinder and four-cylinder engines with High Precision Injection and stratified combustion 8MW has further consolidated its leading position in terms of efficient dynamics while at the same time making a significant contribution to reducing C02 fleet emissions by using the engines in high-volumes. Efficiently managing the operating modes and exhaust gas treatment both in homogenous as well as in stratified mode in combination with exhaust gas recirculation ensures reliable compliance with emission limits that go beyond the currently applicable EU4 requirements. In the future， additional engine variants will go into series production to complete the product po吋folio. The combination of High Precision Injection in stratified combustion mode with turbocharging could e仔'ectivelydevelop and utilise significant additional consumption and performance potentials [2]. This development substantトates the 8MW strategy of basing future engine concepts on a centrally positioned， fast and precise fuel injector in combination with turbocharging.

7. Summary and Outlook

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8. References

Frohlich， K.; 80rgmann， K.; Liebl， J. Potenziale zukunftiger Verbrauchstechnologien Wiener Motorensymposium 2003

)

1

(

Kluting， M.; Missy， S.; Schwarz， C.: Turbocharging of a spray-guided gasoline direct injection combustion system -a good fit? JSAE Annual Congress 2005

(2)

Despite the great complexity of combustion system and the exhaust treatment system， it has been possible to convert the engine potentials into significant advantages for the customer in terms of performance and fuel consumption. An all-encompassing energy management system that includes measures for reducing engine warm-up， intelligent generation of electrical energy for the system network in the overrun and braking phases with the newly developed 8rake Energy Regeneration as well as an Auto Start Stop Function used in the four-cylinder engine also makes a significant contribution. 8y way of example and as a comparison with the predecessor， Fig.18 shows the fuel consumption and performance figures realised for the 530i with the 3 litre six-cylinder engine as well as for the 120i with the 2 litre four-cylinder engine in manual transmission vehicles.

6. Performance and Fuel Consumption

new 6- and 4-cylinder petrol engines with high precision

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