CASCADED CHARGE AIR COOLING FOR PC DIESEL ENGINES

Transcription

1 CASCADED CHARGE AIR COOLING FOR PC DIESEL ENGINES Raising specific power output in turbocharged engines with reduced displacement is a common way to achieve low fuel consumption and emission values without sacrificing driving performance. This is primarily achieved by increasing the degree of turbocharging, resulting in increased thermal and mechanical loads. In order to keep the temperature of the charge air at a low level, Mahle has developed a cascaded indirect integrated charge air cooling system and has now investigated its potentials for diesel engines. 46

2 Authors DR. SIMON SCHNEIDER is Project Manager Corporate Advanced Engineering at Mahle International JÜRGEN STEHLIG is Head of the Air Intake Module Product Development Segment at Mahle Filtersysteme DR. ANDREAS EILEMANN is Head of Pre-development Light Vehicle Engine Cooling at Mahle Behr CHARGE AIR COOLING IN GASOLINE AND DIESEL ENGINES Increasingly, greater charge air pressures and the associated rising temperatures, as well as more stringent emission regulations require more efficient charge air cooling systems that allow further reductions in charge air temperature. Motivated by the development and presentation of an integrated cascaded charge air cooling in the air intake module for gasoline engines [1], Mahle investigated the potential of integrated cascaded charge air cooling for a diesel engine as the net step. The resulting increase in air mass flow was observed with respect to reducing soot emissions, increasing performance, and improving responsiveness. The main area of interest for the diesel engine tests, however, was the optimisation of combustion (power output, fuel consumption) at high loads. TEST SET-UP, CONFIGURATION AND DESIGN The test engine was a 2.2 l four-cylinder passenger car diesel engine with turbocharging. It meets Euro V limits and has a diesel particulate filter as well as highpressure (HP) and low-pressure (LP) ehaust gas recirculation (EGR). This engine was selected for its high specific power output of 70 kw/l. An entirely new air intake module with an upstream throttle valve was designed for the tests, 1. The indirect two-stage (cascaded) charge air cooling integrated in the air intake module is realised with a compact high-temperature (HT) stage cooled by the engine cooling circuit and a larger low-temperature (LT) stage. In order to assess the effect on condensate formation, a condensate drain with a collection unit was implemented downstream of the cooler. The HP EGR inlet was placed after the cooler as well and uses interchangeable inlet geometries. Temperature measurement points directly in the cooler, for eample a direct EGR rate measurement for each cylinder, and energy balancing on the air and coolant sides of the cooler provided etensive data collection. The coolant for the charge air coolers was conditioned eternally for test bench operation. RESULTS FOR PARTIAL LOAD Further reduced charge air temperature and respectively increased air mass flow in partial-load operation show particular potential for emission control [2, 3]. The chain of effects for charge air cooling begins with the greater air mass flow, which leads to a better lambda value and better oygen supply. Combustion is accelerated, which promotes burn-off and produces less soot. The more rapid combustion also increases efficiency; however, this does not result in lower fuel consumption since the gas echange work increases proportionally with the air mass flow rate. The improved soot performance in the Throttle body ❶ Schematic of the air intake module with integrated cascaded charge air cooler High temperature charge air cooler Low temperature charge air cooler Condensate duct High pressure EGR Inlet Flange to cylinder head autotechreview September 14 Volume 3 Issue 9 47

3 Operating point: 1500 rpm, 50 Nm Low pressure EGR sweep = 60 C, LP EGR variation = 40 C, LP EGR variation = 30 C, LP EGR variation 0. Specific soot emissions Lambda value Soot spec Reduced soot emissions Lambda [-] Increased air supply Higher EGR rate Higher EGR rate ❷ Soot/NO influence and lambda/ NO influence at 1500 rpm and 50 Nm soot/ NO trade-off, in contrast, is very clear. The test used LP EGR at various partial-load points with the charge air temperature adjusted to 60, 40 and 30 C. The resulting soot/ NO trade-off is shown correlated with the EGR rate in 2, using the eample of the operating point at 1,500 rpm and 50 Nm. A change in temperature, for eample of -30 K, produces 50 % less soot emissions at high EGR rates (on a Euro V emission level). A comparable trend is also described in [4]. There are limits in using a lower miture temperature, however, because combustion becomes unstable when the temperature of the charge air is too cold, producing more residual products. The limits were investigated in order to obtain the minimum target level for the charge air temperature, 3. At charge air temperatures significantly lower than 40 C, the overall temperature level under partial load operation drops and combustion at the edges of the combustion chamber becomes increasingly incomplete. This leads to increased CO and HC emissions. As the charge air temperature drops further, combustion becomes unstable. The preconditions designing of indirect charge air cooling provide opportunities to influence the charge air temperature. As soon as warm coolant is available which is possible even after a long parking period with an enthalpy storage tank, for eample the charge air temperature can be regulated independently of the ambient air. Particularly at low loads and near-idle speeds, there is a significant positive effect on emissions, 4. The benefit of this provision is also shown in [5], for eample. RESULTS FOR FULL LOAD In contrast to partial-load operation, the combustion advantage is predominant under full load. In this case, the increase in air mass flow is fundamentally beneficial in the diesel engine because the available air mass is a limiting factor for the power curve. Charge air cooling can thus be used to reduce fuel consumption by maintaining a greater distance from the soot limit (with the optimal efficiency point in a diesel engine being below full load). Alternatively, by raising the absolute quantity of the cylinder charge, performance can be increased while retaining Operating point 1500 rpm, 50 Nm Variation of LP EGR rate = 60 C, LP EGR variation = 40 C, LP EGR variation = 30 C, LP EGR variation ❸ Disadvantages of CO and HC emissions due to subcooling 12 Specific CO emissions Specific HC emissions CO spec. 8 6 HC spec

4 the same level of efficiency, 5. Even though the basic engine is already very efficient, the design optimised for fuel consumption produces a significant BSFC improvement of 5 g/kwh throughout the power curve. The reserves in cooling capacity provided by integrated cascaded charge air cooling with an appropriately sized main cooler also allow the application of cooled high- and full-load LP EGR. This aspect is relevant particularly with respect to potential RDE (Real Driving Emissions) requirements under high loads. CONDENSATE FORMATION AND CONDENSATE PREVENTION The risk of condensate formation fundamentally increases at high EGR rates, high charge air pressures (due to the lower water-holding capacity of the air), lower cooler outlet temperatures, or high water content in the intake air. Qualitative and quantitative analysis is therefore essential in order to determine the actions to be derived. This means that all parameters that affect condensate formation are identified in order to develop a control strategy for prevention. This is initially done with a projection of condensate quantities from the water balance of mass flows in the charge air cooler. The calculation was performed for the fluid flow downstream of the charge air cooler and the dew point was determined as saturation of charge air. The computation results were then confirmed with engine tests by collecting and weighing the condensate, 6. The controllability of indirect charge air cooling indicated above can also be used to prevent condensate formation. While the eercise of influence for a single-stage charge air cooling configuration is limited to controlling the cooling capacity, the cascaded configuration provides complete fleibility, as it is also possible to heat the charge air. This means that condensate formation can already be largely prevented. The unavoidable residual condensate must be controlled with design features and then introduced into the engine in a reliable manner as evenly distributed tiny droplets. The tests clearly showed that condensate formation reacts very sensitively near the dew point, so that sufficient distance T p CAC, T int. ports [ C] CO spec. Low load operation (1000 rpm, 25 Nm) W/o EGR P. CAC Runner Runner P. CAC Air temp. post charge air cooler Air temp. in intake runners Specific CO emissions HP EGR HP EGR unc. LP EGR 27 % ❹ Heating of charge air in near-idle operation Lambda [-] BSFC Lambda value LP EGR 35 % HC spec. Sigma IMEP [bar] W/o EGR = 40 C = 90 C Specific HC emissions = 60 C, baseline = 30 C, BMEP = BMEP baseline, λ > λ baseline = 30 C, λ = λ baseline, BMEP > BMEP baseline Specific fuel consumption BMEP [bar] Smoke [FSN] Standard deviation of IMEP (average of all cylinders) HP EGR Smoke emissions HP EGR unc. LP EGR 27 % Ma. brake mean effective pressure ΔBMEP = 2.5 bar LP EGR 35 % ΔBMEP = 2.2 bar 16 ΔBMEP = 2.2 bar ❺ Fuel consumption improvements along the power curve and BMEP increase at the run-up line autotechreview September 14 Volume 3 Issue 9 49

5 Condensate mass flow [kg/h] Operating point: 1750 rpm / 75 Nm Intake air moisture: 16.5 g/kg, LP EGR rate set for 0.6 g NO /kwh Condensate formation measured [kg/h] Mass flow epected at T coolant CAC outlet [kg/h] Mass flow epected at post CAC [kg/h] Mass flow epected at T coolant CAC inlet [kg/h] compensated for the loading with cascaded charge air cooling, even under full load, without making changes to the cooler design. An additional increase in the thermal load from parallel, full-load EGR requires that the high-temperature cooler is adjusted. Condensate management can be handled by simple means due to the controllability in both variants. Owing to the very good emission results under partial-load operation and fuel savings under full load, for diesel engines an increasing use of indirect integrated charge air cooling as well as of the cascaded solution can be foreseen. Temperature post charge air cooler [ C] ❻ Correlation between projection and measured condensate in the test; the projection is displayed for the measured outflow temperature of the charge air as well as for the maimum and minimum temperature that is eistent in the coolant of the low temperature charge air cooler at each operating point (negative values indicate the buffer to condensation) from the dew point should be maintained. The analysis highlighted that significant risk of condensation in a warm engine only occurs in a charge air temperature range below the temperature level that is recommended from a combustion thermodynamics point of view. Following the provisions described above, just a small amount of condensate occurs during a brief warm-up phase at the beginning of engine operation, or under some circumstances after shutting off the engine. The design features for these small quantities can be easily and thus economically implemented. The comparison with direct charge air cooling, however, demonstrates that condensate can occur continuously and in large quantities in that application, requiring considerably more effort to control. SUMMARY AND FEASIBILITY IN THE VEHICLE The reduction of charge air temperatures fundamentally produces advantages for the diesel engine. Under partial load, emissions were reduced, while under full load the effect can be used either to reduce fuel consumption for a constant power output, or the other way around to increase performance at a constant fuel consumption rate. In addition, the greater mass flow increases the charger speed and thus improves engine responsiveness. 50 The advantages identified by Mahle on the test bench must be classified according to their feasibility and the effort required. Assuming an initial basis of direct charge air cooling, converting to an indirect solution integrated in the air intake module provides tremendous potential due to the higher cooling capacity and the ability to regulate cooling capacity. The pressure losses in the air path for this variant are also reduced significantly. Cascaded charge air cooling, also integrated in the air intake module, provides even greater increases in cooling capacity, in addition to full control fleibility. This approach is necessary if the heat dissipation of the low-temperature circuit is no longer sufficient. All of the advantages from the test bench measurements under full and partial load can also be implemented in the vehicle: The emission benefits under partial load can be obtained using a singlestage, conventional indirect, integrated charge air cooling, while the additional fuel consumption benefits under full load require the cascaded variant. This was confirmed by computation of vehicle circuits for all variants. In both cases, the prerequisite is a fullface low-temperature cooler at the front end. The capacity of the high-temperature cooler must be checked for cascaded charge air cooling. This power reserve was sufficient for the tested engine and REFERENCES [1] Stehlig, J.; Dingelstadt, R.; Ehrmanntraut, J.; Müller, R.; Taylor, J.: Air Intake Modules with Integrated Cascaded Charge Air Coolers. 21st Aachen Colloquium Automobile and Engine Technology, 12 [2] Neußer, H.-J.; Kahrstedt, J.; Jelden, H.; Engler, H.-J.; Dorenkamp, R.; Jauns-Seyfried, S.; Krause, A: Die neue modulare TDI-Generation von Volkswagen. 33rd International Vienna Motor Symposium, 12 [3] Lienig, U.: Tieftemperaturverbrennung durch etreme Ladeluftkühlung. Final report from FVV project 879, Frankfurt am Main, 08 [4] Kahrstedt, J.; Dorenkamp, R.; Kuiken, S.; Greiner, M.; Kühne, I.; Nigro, G.; Düsterdiek, T.; Veldten, B.; Thöm, N.: Der neue 2,0 l TDI zur Erfüllung der amerikanischen Emissionsgesetze in Volkswagens neuem Passat. 32rd International Vienna Motor Symposium, 11 [5] Borges-Alejo, J.; Soukeur, Z.; Luján, J.; Climent, H.; Pla, B.: Potential of intake air heating for reduced fuel consumption and emissions in Diesel engines at low temperatures. 22nd Aachen Colloquium Automobile and Engine Technology, 13. Read this article on