SUPERIOR DOWNSIZING
4.3 The lean boost direct injection (LBDI) concept
The lean boost system is an example of such a downsized engine and combines direct injection, lean operation and pressure charging. a critical factor in the function of pressure-charged gasoline engines is octane requirement.
It is normally necessary to significantly reduce the geometric compression ratio of a boosted engine in order to avoid combustion knock, and this has a negative effect on thermal efficiency. However, direct injection and homogeneous lean operation both reduce octane requirement, allowing a higher compression ratio to be used.
Figure 4.1 illustrates the theoretical basis of the lean boost combustion system. as the intake charge is boosted, the operating band of air/fuel ratio
changes. at the rich limit, combustion stability is hindered by ignition retard due to knock, so the rich limit moves to a higher excess air-factor (l). at the lean limit, combustion stability is limited by spark initiation and flame propagation. The lean limit improves with increasing boost pressure, so the overall operating band shifts to a higher l value. as the air/fuel ratio is increased from the rich limit to the lean limit, the engine output decreases. The objective is to boost to a level at which the indicated mean effective pressure (IMEP) is significantly higher than the naturally aspirated stoichiometric value, to allow downsizing, but with a homogeneous lean mixture (l = 1.4–1.6). To minimise the airflow demand of the engine, the richest mixture that allows stable operation at the desired IMep would be selected. So, the fundamental precept of the lean boost system is the reduction in octane requirement due to the direct injection system and the role of excess air as a knock suppressant.
This allows operation at around 1.5 compression ratios higher than for a conventional direct injection turbocharged engine. Typically a compression ratio of 11.5–12 to 1 with 95 RON fuel is viable. LBDI requires no over- fuelling for component protection and the low peak exhaust temperature allows the use of a comparatively cheap variable nozzle turbine (VnT), currently not suitable for conventional gasoline engines due to its limited temperature capability. This gives advantages in low-speed torque and reduction of turbocharger lag.
Ricardo conducted an extensive testbed programme, followed by the development and calibration of a C-class vehicle showing comparative driveability with a production 1.6 litre stoichiometric equivalent. A VNT turbocharger was used for the reasons outlined above. owing to the lower
4.1 Characteristics of the lean boost DI concept.
Ignition retard stability limit
Increasing MAP
Lean stability limit
1.0 1.1 1.2 1.3 1.4 1.5 1.6 1.7
Excess air factor (l)
IMEP (%)
200 180 160 140 120 100 80 60 40
exhaust temperatures of the LbDI concept it was possible to use a standard production diesel VNT unit over a higher-specification, higher-cost gasoline equivalent. This results in superior transient performance compared with a conventional fixed geometry unit but at a reduced on-cost. The 1.1 litre LbDI unit developed for a european application produced 5% more power and 20% more torque than the conventional 1.6 litre PFI engine (Fig. 4.2).
Installed in the C-class vehicle, LbDI delivers 21% lower Co2 emissions as illustrated in Table 4.1. Validated system simulation (V-Sim) showed that downsizing plus direct injection delivered 9% economy benefit (154.1 vs 169 g/km). Test results show that the addition of lean operation and higher CR gives a further 14% improvement. Lean nox trap regeneration added only 1% to overall cycle fuel consumption.
Transient response has been enhanced through the use of a VnT enabled by low exhaust temperatures and appropriate control strategies and allows LbDI to achieve driveability targets. The LbDI vehicle had a similar driving feel to a diesel engine vehicle, but with an extended engine speed range.
The tractive effort curves in fourth and fifth gears illustrate that LBDI has the ‘fun to drive’ performance feel of a diesel vehicle (Fig. 4.3).
at the time, the LbDI vehicle was developed to meet emissions requirements through conventional lean nox trap technologies. off-cycle nox had been
4.2 Power curves for the baseline engine and LBDI concept.
1.6 litre naturally aspirated PFI – 11.0 CR (MTZ, January 1999) 1.125 litre lean boost DI turbo –11.5 CR
Lean boost 77.4 kW
@ 5000 rev/min
Torque
BSFC
Baseline 73.8 kW
@ 6000 rev/min
500 1000 1500 2000 2500 3000 3500 4000 4500 5000 5500 6000 Engine speed (rev/min)
500 460 420 380 340 300 260 220
BSFC (g/kWh)
Torque (Nm)
180 160 140 120 100 80 60 40 20 0
Table 4.1 NEDC drive-cycle CO2 and fuel consumption results
LBDI 1.125 litre Baseline 1.6 litre NA
CO2 emissions (g/km) 130.7 166.0
Fuel consumption (litre/100 km) 5.52 7.00
limited to 0.1 g/km at 130 kph whilst low exhaust temperatures improve nox trap durability. Very high load nox trap regeneration and desulphation were enabled through new control strategies using post-injection, variable valve timing and eGR.
The estimated on-cost for a 75 kW application in 2004 was 7500–650, resulting in a cost/benefit ratio more attractive than that of a conventional diesel powertrain.
Lean operation – especially at high boost pressures – represents a more demanding environment for stable combustion, and the dual injection strategies developed in the single-cylinder work were further evolved in this programme to improve the combustion stability on the multi-cylinder engine. additional investigations were conducted into both air motion and ignition energy effects (using a production ignition system) and significant improvements were achieved, as shown in Fig. 4.4.
one way of addressing concerns over low-speed torque and transient response is to take advantage of one of the key characteristics of the LbDI system: the cooler exhaust temperatures shown in Fig. 4.5. This made possible the use of a low-cost production VNT unit that was specified using the engine simulation code Ricardo WaVe. Transient performance of the LbDI engine with its VNT is, as shown in Fig. 4.6, directly comparable with that of the stoichiometric engine with its fixed geometry turbocharger.
The I3 engine in LBDI build specification was installed in a C-class passenger car as shown in Fig. 4.7. a full calibration was developed to deliver a LbDI vehicle demonstrator, with lean operation throughout the engine speed and load range. acceleration tests were undertaken for the LbDI vehicle: the
4.3 Vehicle tractive effort.
1.8 TDCi 100 PS 1.125 LBDI
0 20 40 60 80 100 120 140 160 180 200 220 Road speed (km/h)
4th gear
5th gear
Tractive effort (N)
3000
2500
2000
1500
1000
500
0
time taken to accelerate from 1500 to 3000 rev/min was measured in each gear, starting under steady-state conditions at 1500 rev/min. These results are shown in Table 4.2, and are compared to the same model vehicle with a 1.6 litre naturally aspirated engine. Comparable acceleration times were achieved from the two vehicles. Table 4.1 shows the Co2 emissions and fuel consumption results for the LbDI vehicle, compared with those for the baseline 1.6 litre naturally aspirated car. Both vehicles were tested at 1250
0 100 200 300 400 500
Cycle number Current status
COV IMEP = 2.5%
Baseline
COV IMEP = 13.1%
IMEP (bar)IMEP (bar)
16 12 8 4 0 16 12 8 4 0
4.4 Combustion stability characteristics before and after combustion system development – 2000 rev/min, l = 1.4, 50% MFB 26° CA ATDC.
4.5 Turbine-out temperature [°C].
500 1000 1500 2000 2500 3000 3500 4000 4500 5000 5500 6000 Engine speed (rev/min)
BMEP (bar)
20 18 16 14 12 10 8 6 4 2
0 150200250 300350400
450500 550
600650
120 km/h LNT temperature window for fixed geometrty exhaust
system 160 km/h 700
750
192 km/h (Vmax) 5th gear road load
4.6 Transient performance comparison.
0 0.5 1.0 1.5
Time (s)
Base engine (fixed geometry t/c) LBDI with VNT turbocharger Steady-state torque
85% torque demand within 1 s
1750 rev/min
Torque (Nm)
200 180 160 140 120 100 80 60 40 20 0
4.7 Vehicle installation of LBDI engine.
Table 4.2 Vehicle acceleration times for 1500–3000 rev/min (seconds)
Gear LBDI vehicle 1.6L NA equivalent
2nd 3.7 3.1
3rd 7.3 6.6
4th 12.4 12.8
5th 20.2 23.4
kg inertia. The cumulative Co2 emissions over the drive cycle are illustrated in Fig. 4.8. no LnT regeneration events occurred during the drive-cycle tests.