Improving the Torque Behaviour of Turbocharged Diesel Engines by Injecting Compressed AirThrough the combination of parallel developments of recent years in the automotive industry, electronic injection, e.g. Common Rail, in the commercial vehicle diesel engine sector, and highly dynamic, electronically controlled compressed air components in the commercial vehicle brakes sector, Knorr Bremse has s ucceeded in developing a system to the point of predevelopment status which is largely based on series components. It can easily be integrated into the charge air pipe, and realises an immense improvement to the response and acceleration behaviour of the engine and the vehicle.

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1 Introduction

Automotive development is caught be-tween the conflicting priorities of in-creasing demands placed by customers and the legislature, which can only be met with technological progress and in-depth knowledge. This challenge is com-pounded by diverging markets with re-gional preferences and increasing energy costs caused by growing demand.

Increasing fuel prices and more strin-gent emission regulations are the most powerful driving force behind the devel-opments. This particularly applies in the commercial vehicle sector, where optimal efficiency is the permanent objective. The commercial vehicle of the future is char-acterised by high cost-effectiveness and meets all applicable emission regulations. Mobility will remain at least at the cur-rent level, but will more likely increase.

The principle known as downsizing is an effective means of achieving future consumption and emission objectives. This requires a further increase to the charging pressure to maintain the en-gines’ performance level. A compact, cost-efficient and proven solution to this is exhaust-gas turbocharging, which, however, reacts with a further undesired delay at transient full load demand con-ditions by increased charging-air pres-sures. Even multi-stage and variable charging systems are only able to lessen this effect. The delay to the charging-air pressure buildup impairs the vehicle’s driveability, particularly when setting off, changing gears, driving up hills or overtaking, and can only be compensated for by changing gear more frequently or increased clutch slipping.

Besides these reasons, there are other important reasons for increasing the charging-air pressure level: the prospec-tive emission regulations (US EPA10 and EU VI), which cannot be met without high-load or full-load exhaust gas recir-culation, or which can only be met with great difficulty. A full-load EGR engine requires a charging-air pressure level that cannot normally be achieved with a single basic compression level.

In general, the technologies mentioned above, with further increasing of the

charging-air pressure, cause a deteriora-tion of the engine’s response behaviour, which is why new, alternative approaches to improving the dynamic driveability are required.

The development of such an approach, the pneumatic booster system (PBS), will be presented in this article. It effectively improves the dynamic properties of tur-bocharged diesel engines, particularly for commercial vehicles and buses. The PBS utilises the compressed-air conditioning available for the brake system in these ve-hicles and assists the turbocharger with additional, very quick air injection in transient situations [1, 2, 5-8].

2 Concept and Mode of Operation

2.1 Concept DeterminationThe main objective of concept develop-ment was to overcome the engine’s low volumetric efficiency, i.e. to briefly assist the charge cycle with compressed air in transient areas where the turbocharger does not supply sufficient charging-air pressure. Thanks to the electronic moni-toring and control of pump nozzle or Common Rail injection systems, the ”in-creased“ charging-air pressure thus brings about an immediate aligned increase to the fuel injection rate which, in turn, causes a rapid build-up of the necessary exhaust gas flow, and thus a spontaneous increase to the turbocharger speed and, ultimately, significantly improved torque build-up.

During development of the system, priority was given to the efficient utilisa-tion of compressed air, high robustness and suitability for use in various en-gines. The possibility of direct com-pressed air injection in the combustion chamber was therefore not considered, due to the complexity of cylinder head changing, although theoretically this would have achieved the best ultilisation of compressed air. When looking for al-ternative concepts, the fundamental idea was that compressed air injection should take place as close as possible to the cylinder, small volumes replenished, and the engine’s torque improvement achieved proactively, rather than reac-

tively, as with a variable-turbine-geome-try turbocharger, thus realising the greatest improvement. Alternative con-cepts were developed as a result, al-though the main difference between these was the point of injection.

The concepts were reviewed using simulations and highly-dynamic engine test benches [3]. For calculation purposes, the alternative versions were implement-ed and analysed as 1D engine models [4]. The concept shown in Figure 1 was cho-sen, as it achieved the greatest transient performance enhancement and lowest air consumption, as well as for its cost-ef-fective design type. The PBS module (4) is integrated directly into the charge air pipe between the intercooler (5) and in-take manifold (8). The PBS module is sup-plied by separate additional air reser-voirs, which are connected to an auxilia-ry consumer circuit of the vehicle pneu-matic system (3). The pressure level in the vehicle pneumatic system is normal-ly between 8 and 12 bar.

The Authors

Dr. Manuel Marx is Project Manager PBS at the Knorr Bremse Systeme für NFZ GmbH in Munich (Germany).

Dr. Huba Németh is Head of Advanced Engineering at the Knorr Bremse R&D Center in Budapest (Hungary).

Dr. Eduard Gerum is Vice-President Global Development at the Knorr Bremse Systeme für NFZ GmbH in Munich (Germany).

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2.2 Structure and Mode of OperationThe most important components and the mode of operation will be explained on the basis of Figure 2. The module fea-tures an electric forcibly actuated non-re-turn valve in the form of a flap (I) and also two individually controllable com-pressed air injection valves (II) actuated by solenoides as its key components (II: for reasons of clarity, only one com-pressed air injection valve is shown in Figure 2 and Figure 3).

In normal operation where there are no high dynamic requirements, the en-gine’s air demand is met by the turbo-charger (IV), during which time the PBS is in the rest position with the flap fully opened and injection inactive (Figure 2, left). In a highly dynamic situation, when

the turbocharger is not able to meet the engine’s air demand, the flap is closed and air injection takes place (figure 2, right). To activate PBS, an ECU integrated into the PBS module, which is connected to the vehicle CAN, checks whether the necessary activation parameters such as accelerator position, engine speed and current charging-air pressure are fulfilled. The activation parameters can be freely selected and applied in the ECU both vari-ably or specific to the engine. Injection normally ends when the pressure built up by the compressor of the turbocharger in front of the closed flap has reached the level of the pressure built up by the PBS behind the flap. Both pressures are meas-ured continuously by the pressure sensors (III). From then onwards, the turbocharg-

er is able to supply the engine with suffi-cient air. The maximum acceptable charg-ing-air pressure and/or a maximum injec-tion time can be defined as a further switch-off criteria (safety function). The concept described is able to optimally uti-lise the compressed air, as the injection time is limited to a few engine cycles. This is due to the rapid response of the flap on the one hand, and the rapid turbocharger run-up on the other. Trials on the engine test bench and in the vehicle have shown that the average injection time, i.e. the time until both pressure sensors show the same pressure value, is between 0.4 and 0.8 s. The quickly operating flap is thus a key component of the system. It allows the injected air to flow only in the direc-tion of the cylinder.

To simplify application, the pneumat-ic booster system is based on the modu-lar design principle. The pneumatic dia-gram for the PBS is shown in Figure 3. The air from the intercooler arrives at port 11 and flows in the direction of the intake manifold at port 21. The flap, which is fully open when inactive, is lo-cated between the two ports. After the activation parameters have been con-firmed, compressed air injection takes place through port 12 and/or 13, depend-ing on how much air and/or what charg-ing-air pressure the engine requires in its current operating state. The integrated control unit (ECU) is supplied with volt-age via the EC connector and connected to the vehicle CAN, for example.

The ECU synchronises the injection valves and the flap balanced via the so-lenoids. The pressure sensors arranged on both sides of the flap are responsible for controlling and monitoring the pro-cesses.

Figure 1: System diagram for PBS integration

Figure 2: PBS inactive (left) / PBS active (right)

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3 Means of Supplying the Additional Air Demand

The compressed air consumption of a city bus in a highly dynamic cycle in-creased by approx. 25 % in vehicle tests. This is representative of an extreme case and is significantly less in distribution transport, for example. Furthermore the injection strategy influenced the air con-sumption significantly, e.g. injection on-ly by acceleration out of standstill.

The simplest means of supplying the additional air demand is to increase the size of the compressor. This solution ought to be possible in most vehicles. However, the advantages of the PBS in re-lation to the achievable reduction in fuel consumption (see section 4.2) are partially lost. Other concepts for generating com-pressed air were developed for this reason. This particularly applies to vehicles for which the solution shown above is not feasible due to limited installation space and/or extreme operational profiles, such as in the case of the city bus.

In one of these concepts, the built-in compressor is operated not by suction, but is instead supplied with turbocharged air after the intercooler. This increases the possible flow rate at the respective speed by a factor of 2, so that the addition-al air demand can be met by PBS. So as not to withdraw charge air from the engine

through the compressor in situations where the engine requires the maximum available charging-air pressure, intelli-gent communication between the engine control unit and the electronic air treat-ment unit is required. If this situation oc-curs, e.g. during full-load acceleration, the engine control unit sends a signal to the electronic air treatment unit. If the pressure available in the vehicle air sys-tem is not below a minimum value, the electronic air treatment unit switches off the compressor which is equipped with a clutch for this purpose.

Charged compressors are state-of-the-art and are used particularly in the USA. Production of a Knorr-Bremse clutch com-pressor will commence in 2009. With this, firstly, the additional air demand can be met by the PBS and, secondly, the engine’s requirements can be allowed for by com-bining proven technologies and intelligent

communication between the subsystems involved. Furthermore, the charged com-pressor with clutch allows additional fuel savings to be realised. Besides the concept described, alternative means of providing the additional air demand using the PBS are still in development.

4 Results

4.1 Torque Build-up In the case of spontaneous torque requests at a low speed or when the engine is idling, the turbocharger is not immediately able to supply the engine with sufficient air. To prevent clouds of smoke and high particle emissions, the amount of fuel injected is reduced by a smoke limiting function and is only gradually adjusted to the turbo-charger’s increasing supply of fresh air. This phenomenon, known as ”turbo lag“, is almost entirely eliminated by the PBS. Compressed air is injected by the PBS im-mediately after the torque request, which also means that the injection system is able to inject the corresponding amount of fuel immediately. The high gas mass flow generated as a result of this also ac-celerates the turbocharger, so that it is able to supply the engine after a very short time. Figure 4 shows a torque curve meas-ured on an engine test bench with sponta-neous torque request at idling speed for an 8-l-diesel engine with exhaust gas recircu-lation and two-stage turbocharging.

Despite two-stage turbocharging, the engine without PBS does achieve 90 % of its maximum torque until almost 5 s have passed, whereas the same engine with PBS achieves this value after just 0.7 s.

4.2 Fuel ConsumptionThe quicker torque build-up also has a positive influence on the specific fuel consumption, as the engine reaches a

Figure 3: PBS module circuit diagram

Figure 4: Torque curve of an 8-l-diesel engine with EGR and 2-stage turbocharging with/without PBS

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low-consumption operating point more quickly. Furthermore, the driver is able to change into a higher gear sooner, avoid-ing high-speed phases. Figure 5 shows an engine speed distribution, measured on a solo city bus with manual transmission.

Thanks to the engine’s improved re-sponse behaviour, drivers adapt their manner of driving of their own accord, favouring lower speeds. It can clearly be recognised that the proportion of speeds

between 1200 and 1600 rpm is signifi-cantly higher with PBS than without PBS, whereas the proportion at high speeds between 1700 and 2100 rpm is lower. This brings about a reduction in fuel consumption. The working points need to be optimised accordingly in or-der to tap the full potential in the case of automatic gear boxes.

The fuel consumption was able to be reduced by 2 % in test drives, while the

driving track time was simultaneously reduced by 3 %.

A 2 % improvement to the specific fu-el consumption due to PBS was also measured on the engine test bench in the relevant cycle. Other clear potential is currently being investigated. The main focus here is on adapting the gear shift logic for automatic gearboxes (AMT). The first step was to reduce the connection speed after a up-shifting by 250 rpm. Without PBS, this would lead to an unac-ceptable deterioration of the driving dy-namics. With PBS, this drawback in com-parison with the initial state was able to be compensated for. The result was a 3 % reduction to the fuel consumption in measurements taken in an interurban cycle (test vehicle: Euro4 with EGR; 10 L displacement; 1-stage turbocharging; 40 t total weight), while driving dynam-ics remained the same.

4.3 Particle EmissionsThe high air excess during the injection process significantly reduces the particle exhaust emissions before the catalytic converter phase. The transient peak val-ues are reduced by approximate 40 %. Figure 6 shows the reduction to the parti-cle exhaust emissions before catalytic con-verter phase measured on the test bench over a complete ETC (European Transient Cycle) on a Euro 5 engine with 10L dis-placement, exhaust gas recirculation and two-stage turbocharging. The two curves with PBS reflect various settings of the PBS activation conditions. The black curve

Figure 7: Load step; left: influence on lambda and EGR; right: influence on combustion temperature calculated using 1D simu-lation program

Figure 5: Engine speed distribution for a city bus driving cycle

Figure 6: Particle measurement in the ETC with/without PBS (exhaust emissions before the catalytic converter phase)

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is the basic engine without PBS. A reduc-tion of particle exhaust emissions before the catalytic converter phase in the ETC of up to 25 % is possible with PBS.

4.4 NOx EmissionsThe NOx exhaust emissions before the catalytic converter phase depend, in par-ticular, on the combustion temperature, the air excess and the realisable EGR rates. There is a high air excess during the PBS injection process (lambda > 3; Figure 7; left above). Initially, this encourages the for-mation of NOx. This is counteracted by re-ducing the combustion temperature, as cold air is injected (decompression of the compressed air from 12 bar to approxi-mate 1.5 bar; Figure 7 on the right). This positive effect is only partially able to compensate for the initial negative effect, so that higher NOx emissions are pro-duced during the injection period. In ad-dition, no EGR is able to take place during the injection period, as a higher pressure is present in the intake tract than in the exhaust tract (positive scavenging gradi-ent). To prevent air flowing from the in-take tract into the exhaust tract via the EGR line, the EGR valve should be forced closed by the engine control unit during the injection process. The optimum en-gine operating point is reached consider-ably sooner, which has a very positive ef-fect. Exhaust gas recirculation is able to take place significantly sooner with PBS, as can be recognised in Figure 7 below.

This effect was confirmed on the engine test bench, as shown in Figure 8.

It can clearly be recognised that the EGR valve (lambda-controlled) even opens during the PBS injection process, although this should be avoided due to the positive scavenging gradient, and that higher EGR rates can be achieved significantly sooner after the injection process. In the WHTC (World Harmonized Transient Cycle), re-ductions or increases to the NOx emissions of up to 5 % resulted, depending on the PBS activation parameters selected in each case. There is further potential if it can be achieved that no positive scavenging gra-dient occurs during the injection process. This would mean that EGR could also be realised in this time. Considerations and concepts to this end are currently in the development stage.

5 Future Prospects

In addition to the results shown above, which have, for the most part, been achieved purely by integrating the PBS into the charge air pipe, there is further poten-tial both in relation to savings in fuel con-sumption and in relation to cost savings. In relation to fuel consumption, the further optimisation and/or adaptation of the gear shift logic and the engine control unit, as well as increasing the diesel particle filter’s regeneration intervals, in order to reduce particle exhaust emissions before the cata-

lytic converter phase by up to 25 %, are mentioned in particular at this point.

In relation to possible cost savings, measures involving other engine compo-nents are being looked into, as well as measures involving the PBS module it-self, such as the use of plastic compo-nents. In particular, the possibility of producing the compressor wheel from the turbocharger out of steel or a ferrous alloy is currently being looked into. Com-pressor wheels of turbochargers are in-creasingly being made from titanium al-loys instead of aluminium alloys, due to the current charging-air pressure re-quirements. This involves considerable costs. Compression wheels made from ferrous alloys are normally not conceiv-able, due to their high mass/ inertia and the associated response drawbacks. Sim-ulations carried out have shown that these response drawbacks are more than compensated for by the PBS.

References[1] Hitziger, H.; Gerum, E.: Verfahren und Vorrichtung

zum Steigern eines Drehmomentes einer Hubkol-ben-Verbrennungskraftmaschine, ins besonders eines Motors in Diesel-Ausführung; offen- legungsschrift DE 10 2004 047975.5, 2004

[2] Németh, H.; Gerum, E.: Device for supplying fresh air to a turbocharged piston internal combustion engine and method for operating the same; offenlegungsschrift Wo2006089779, 2005

[3] Flierl, R.; Weiske, S.: Bericht 05, Turboaufladung mit Druckluftunterstützung, VKM, TU Kaiserslau-tern, 2005

[4] Németh, H.; Kristóf, G.; Szente, V.; Palkovics, l.: Advanced CFD Simulation of a Compressed Air Injection Module; Conference on Modelling Fluid Flow (CMFF’06), The 13th International Confer-ence on Fluid Flow Technologies Budapest, Hungary, September 6-9, 2006

[5] Németh, H.; Ailer, P.: Turbo lag reduction for improving commercial vehicle dynamics; 10th Mini Conference on Vehicle System Dynamics, Identification and Anomalies, Budapest, November 6-8, 2006

[6] Németh, H.; Ailer, P., Palkovics, l.: Diesel Engine Response Improvement by Compressed Air Charg-ing; 11th EAEC Congress, Budapest, Hungary, May 30 – June 1, 2007

[7] Németh, H.: Aufladung mit luftunterstützung an NFZ-Motoren, 1. MTZ-Konferenz, ladungswechsel im Verbrennungsmotor, Stuttgart, Deutschland, 9.-11. November 2007

[8] Németh, H.; Palkovics, l.; Hitziger, H.; Gerum, E.; Flierl, R.: PBS – Ein neuer Ansatz zur Verbesserung des Drehmomentverhaltens aufgeladener Diesel-motoren durch lufteinblasen. 29. Internationales Wiener Motorensymposium 2008, Wien, Österreich, 24.-25. April, 2008

Figure 8: Influence on EGR

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