SUPERIOR DOWNSIZING
5.2 Fundamentals of wide-open-throttle exhaust gas recirculation (WOT-EGR) operation
5.2.1 Knock suppression effects
Brustle and Hemmerlein, with up to ~10% improvement in fuel economy achieved at moderate load with the geometric compression ratio increased from 10 to 13.
In more recent work, Alger and co-workers (22) studied the benefits of eGr when applied to a modern production turbocharged gasoline Di engine.
This study clearly demonstrated the potential of eGr across the entire excess fuel operating map, achieving reductions in CO2 of up to ~20% that were associated with reduced knocking tendency and hence lower exhaust gas temperatures. reductions in CO of up to ~95% were also recorded at high speed and load. elsewhere, Kapus et al. (23) have insinuated that the eGr technique should perhaps be combined with a water-cooled exhaust manifold to reduce exhaust gas temperatures to a level where diesel-like variable geometry turbochargers could be used for spark ignition applications.
in summary, partially due to strong synergy with the engine downsizing approach, the so-called ‘wide open throttle eGr’ (WOT-eGr) technique is now subject to significant research efforts, including those at MAHLE Powertrain (24, 25).
5.2 Fundamentals of wide-open-throttle exhaust
as discussed further later on. Additional details of the engine assembly have been presented elsewhere (24, 25).
The experiments were specifically concerned with directly comparing the effects of cooled eGr with those of conventional excess fuel or excess air charge dilution at fixed engine speed and load (4000 rpm, 19 bar BMEP) under knock-limited conditions. By increasing the mass of fuel, air or cooled eGr charge dilution at this site, it was possible to demonstrate the thermodynamic effects of each method of dilution on engine combustion, performance and emissions. At this speed and load it was just still possible to
Table 5.1 Details of the research engine used for the fundamental studies
Number of cylinders 4
Bore (mm) 87.5
Stroke (mm) 83.1
Geometric compression ratio 9:1
Variable valve timing Dual independent (35∞c.a. inlet, 55∞c.a. exhaust) Cam duration (end of ramp) Inlet: 234∞c.a., exhaust: 220∞c.a.
Fuel injectors Bosch (6-hole, 25 cc/s) Peak fuel rail pressure 115 bar
Fuel 98 RON unleaded gasoline
Turbocharger Mitsubishi model 16T-6 cm2 Spark plugs NGK HR7 high ignitability Ignition coils Bosch coil-on-plug (100 mJ)
Throttle
Intercooler
Comp.
Turb.
EGR valve
Main cooler
Pre-cooler
5.2 Schematic showing the ‘hybrid’ EGR circuit used to uncover the fundamental effects of EGR boosted operation.
operate the engine under stoichiometric fuelled conditions. Therefore, these experiments do not allow the benefits of EGR on over-fuelling reduction to be directly quantified. Rather, the tests were performed to isolate the effects of increased eGr, excess fuel or excess air on knock-limited combustion when starting under stoichiometric conditions (as often incurred at low to moderate engine speeds).
During these tests, the spark timing was set to borderline detonation minus 1.5° (BlD – 1.5°) at all sites. The pre-turbine gas temperature under such conditions was approaching ~950°C, which is the limit imposed on many current production turbocharged engines. The inlet charge temperature was fixed (40°C) as were the start of fuel injection timing (300° bTDC) and fuel rail pressure (115 bar), previously optimised for homogeneous operation. The inlet and exhaust valve timings were also held constant at 88° crank after top dead centre (88° aTDC) maximum opening position (MOP) and 100°
bTDC MOP respectively.
The corresponding key measurements are shown in Fig. 5.3. Firstly, the meaning of the x-axis on the graphs is dependent on the case being considered.
The hollow squares represent the data set produced using cooled external eGr dilution under stoichiometric fuelled conditions. Here, the x-axis is simply the ratio of CO2 recorded in the inlet to that simultaneously monitored in the exhaust. The second data set (solid circles) was produced using excess air charge dilution, where the x-axis is the increase in mass of air relative to the total exhaust mass flow. Finally, the stars denote data produced using excess fuel, where the x-axis is the additional percentage of fuel relative to the original stoichiometric mass of fuel at the start of the sweep under stoichiometric conditions. Owing to these differences, the x-axes allow only qualitative comparison of each method of dilution.
Observing Fig. 5.3, it can clearly be seen that all three methods of charge dilution reduced the exhaust gas temperature as required. However, to maintain load when using either gaseous form of charge dilution, it was necessary to significantly increase the intake charge boost pressure, with the wastegate duty cycle increased to a maximum of 73% (excess air) or 100%
(eGr) respectively at the end of each sweep. in contrast, the boost pressure marginally decreased when employing fuel dilution, which was the result of species dissociation and hence potentially increased indicated mean effective pressure (iMeP) under such conditions.
While all three methods seemed effective at lowering exhaust gas temperatures, in terms of knock suppression, only increasing amounts of fuel or eGr allowed the angle of peak in-cylinder pressure (APmax) to be further advanced. in turn, this resulted in poor combustion stability in the leanest case, with the maximum coefficient of variation in gross IMEP (COV of iMePg) greater than 4% compared with a constant value of ~2.5% when using the highest mass flow of EGR. The lowest variation in IMEP was
5.3 Key parameters during excess fuel, excess air and cooled EGR sweeps at 4000 rpm and 300 Nm/19 bar BMEP.
Excess fuel Excess air EGR
Excess fuel Excess air EGR
Excess fuel Excess air EGR Excess fuel
Excess air EGR
Excess fuel Excess air EGR
Excess fuel Excess air EGR
Excess fuel Excess air EGR
Excess fuel Excess air EGR 4.5
4.0 3.5 3.0 2.5 2.0 1.5 1.0
1.36 1.34 1.32 1.30 1.28 1.26 1.24
50004500 40003500 30002500 20001500 1000500
290285 280275 270265 260255 250245 240235 950940
930920 910900 890880 870860 850840
1.351.30 1.251.20 1.151.10 1.051.00 0.950.90 0.850.80
200195 190185 180175 170165 160155 150
2524 2322 2120 1918 1716 15
Pre-turbine temperature (°C)Relative AFRIntake plenum press (kPa abs) APmax (°aTDC) COV of IMEPg (%)Specific heat ratioNOx (ppm) BSFC (g/kWh) 0 2 4 6 8 10 12 14 16 18 20
Diluent mass flow (%)
0 2 4 6 8 10 12 14 16 18 20 Diluent mass flow (%)
0 2 4 6 8 10 12 14 16 18 20 Diluent mass flow (%)
0 2 4 6 8 10 12 14 16 18 20 Diluent mass flow (%)
0 2 4 6 8 10 12 14 16 18 20 Diluent mass flow (%) 0 2 4 6 8 10 12 14 16 18 20
Diluent mass flow (%) 0 2 4 6 8 10 12 14 16 18 20
Diluent mass flow (%)
–70 –60 –50 –40 –30 –20 –10 0 10 20 30 Diluent mass flow (%)
observed under rich fuel conditions, associated with the faster rate of mass burning.
To fully understand these observations, a ‘reverse-mode’ thermodynamic analysis of the data was performed using the commercial modelling package GT-Power. The three cases simulated were those at the end of each sweep at a pre-turbine temperature of ~850°C. The GT-Power engine model was constructed to ensure accurate simulation of the air path. Cylinder head port flow coefficients were determined in advance using an in-house port flow rig.
Computed and measured values of volumetric efficiency were maintained within a maximum error of 3%. Combustion was simulated using the well- known Wiebe function and in-cylinder heat transfer was computed using the Woschni correlation.
Observing the corresponding predictions in Fig. 5.3, the lean charge incurred a higher ratio of specific heats (g) throughout the cycle. This is a well-known phenomenon for lean operation and one reason why lean burn engines exhibit higher thermal efficiencies at part-load (26). At the high loads tested here, the higher value of g was associated with higher unburned gas temperatures during compression and flame propagation, which presumably explained the lack of improvement in knock suppression observed when lean.
efforts to expand the lean operating regime by increased in-cylinder charge motion or some other technique would help but may add complexity and cost. A significant factor that must be considered is emissions after-treatment, with a vast increase in engine-out emissions of nOx observed when lean.
it could be argued that lean nOx after-treatment could be used to alleviate these effects. However, the associated on-cost and also the high frequency at which such a nOx trap may have to be purged if used often at high load must be considered. eGr seems to present the best compromise in terms of allowing efficient three-way catalytic conversion of all key engine-out emissions, especially if stoichiometric operation can be maintained across the entire operating map.
Given that the engine could still be operated under stoichiometric conditions with a pre-turbine gas temperature of 950°C, the test results in Fig. 5.3 cannot be used to quantify the real-world improvement in fuel economy when replacing excess fuel with cooled eGr, although this is shown later on. However, the results do still indicate the potential sources of fuel consumption improvement under conditions where stoichiometric operation cannot be maintained. Clearly, a large fuel economy penalty was observed when using excess fuel, with the brake specific fuel consumption (BSFC) deteriorating by 17% during the relative air-to-fuel ratio (l) sweep from l
= 1.0 to l = 0.85. At this particular site, a second small additional benefit in fuel economy could be incurred using eGr to advance APmax further towards the optimum, arguably demonstrated during the eGr tests with BsFC improved by ~1.5% during the eGr sweep at l = 1. However, it was
not always possible to achieve such combustion phasing benefit when using cooled eGr, particularly at higher engine speeds as discussed below.