Comparisons between naturally aspirated direct injection (DI) and indirect injection (IDI) diesel engines of closely comparable design and size indicate that the DI engine is always more efficient, although the benefit varies with load (Figure 7.18). At full load, differences of up to 20% in brake specific fuel consumption (BSFC) have been noted, especially in engines with larger displacement per cylinder. At part load, the gain is less (circa 10%). Comparisons should be made with equal emission levels, a task that is difficult to accomplish in practice. Emission control with the DI engine is more difficult, so this constraint reduces the performance benefit. Figure 7.18 shows a breakdown of the indicated efficiency differences between the two systems. At full load, the ID1 suffers a penalty of approximately 15 to 17%, due in part to the retarded timing of the ID1 combustion process and its long, late-burning, heat release profile. At light load, approximately 300 kPa BMEP (brake mean effective pressure) (AFR 50: l),
200 400 600
BMEP @Pa)
Figure 7.1 7 AFR versus retard effects.
12' BTDC
Figure 7.18 The eflect of load on a naturally aspi- rated diesel engine.
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132 An Introduction t o Engine Testing and Development
the combustion effects are small, and the indicated efficiency penalty of the ID1 (about 5 to 7%) is due to the higher heat losses associated with the larger surface area, and high- velocity flow through the connecting nozzle of the divided-chamber geometry, as well as due to pumping pressure loss between the main and auxiliary chambers. Figure 7.18 shows the effect of load on NOx emissions on a DI and ID1 (pre-combustion chamber) six-cylinder 5.9-liter power unit.
At fixed speed and constant fuel delivery cycle, the DI engine shows an optimum BSFC and BMEP at a specific start of injection for a given injection duration. The ID1 engine experiments are at a fixed BMEP; here, the BSFC at full load and fueling rate at idle shows a minimum at specific injection timings. Injection timing, which is more advanced than this optimum, results in combustion starting too late.
Figure 7.19 illustrates typical DI and ID1 configurations.
Figure 7.19 Typical DI and IDI conJigurations.
Injection timing is a factor that can improve the indicated efficiency of smoke and particulate emissions of naturally aspirated and turbocharged small DI diesel engines (Figures 7.20 and 7.2 I). The effect of the start of injection timing on diesel engine per- formance and emissions with a medium-swirl DI diesel engine with a deep combustion bowl and a four-hole injection nozzle at 2600 revlmin, fuel delivery 75 mm3/cycle. In this example, the fuellair equivalence ratio is 0.69.
Figures 7.20 and 7.21 refer to smoke (Bosch smoke number) and particulate mass emissions (in grams per kilowatt hour) as a fimction of load and injection timing for a six-cylinder 3.7-liter ID1 swirl chamber diesel engine at 1600 revlmin. Note that there is no exhaust gas recirculation (EGR).
Exhaust gas recirculation, where a controlled mass of exhaust gas is fed into the induction system, is a popular method of reducing NOx levels. Uncontrolled EGR due to the flow reversals across the cylinder head between the induction and exhaust ports is thought to be a primary cause of unstable running in engines. Running at constant speed, note the effect of increasing the levels of EGR, as shown in Table 7.4.
A medium-capacity DI diesel engine running at 1500 revlmin and developing 88 Nm (Figure 7.22) illustrates the effect on brake specific hydrocarbons with differing injec- tion timing and variable EGR. Figures 7.23 and 7.24 show various aspects and effects of EGR.
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Exhaust Gas Emissions and Analvsis 133
B0Sc.h Smoke numbtr
I I ! I I
100 200 300 400 500 600 700 800 BMEP @Pa)
Note the minimal effect on emissions when the point of pilot injection is moved when running at constant speed (Figures 7.25 through 7.27).
Compare Figure 7.26 with Figure 7.28. This is a mirror image and is typical of the relationship between fuel used and BMEPItorque.
Figure 7.20 EIffect of injection timing on smoke.
Figure 7.21 Eflect of timing on particulates.
At part load, EGR can be used to reduce diesel engine NOx emissions. Note that because diesel engines operate with the airflow unthrottled, at part load the C 0 2 and H20 concentrations in the exhaust gas are low. They essentially are proportional to the fuellair ratio. Because of this, high EGR levels are required for significant reductions in NOx emissions.
1 34 An Introduction to Engine Testing and Development
TABLE 7.4
EGR SWING
FROM
0 TO 100% MODULATIONCO CO2 0 2 HC NOS FSN %EGR
-
- l O f r o m h s e-
Base timing---
+ l o from baseFigure 7.22 Eflect of dif 0 5 10 15 20 25 30
fering rates of EGR. FGR rate (%)
- - 1 O from base
-
Base timing --
+ l o from base 6b
46
2 p"
Figure 7.23 Eflect of dif
fering rates of EGR on 1
N&
emissions when run-.
ning at 1500 rev/min and O 0 5 10 15 20 25 30 35
developing 130 Nm. EGR rate (%)
Exhaust Gas Emissions and Analvsis 135
-
-1" from base, ,
Base timing 2721
. . . . . . . + 1 " from base10 15 20 25 30 35 CGR ratc (%)
-
Series 430 25 20 15 10 5
Injection timing (OBTDC)
Figure 7.24 Fuel con- sumption versus EGR and injection timing.
Brake specific gaseous emissions
( g k w h )
30 25 20 15 10 5 0
Injection timing (OBTDC)
Figure 7.25 Medium swirl DI timing swing.
3 0 0 750 700 BMEP
W a )
650 600
550 500
Figure 7.26 liming swing BMEP loop.
136 An Introduction to Engine Testing and Development
Smoke units
units
Figure 7.27 Eflect on vis-
ible smoke when injection 30 25 20 1s
ra
5 oLO 3.5 3.0
Bosch smoke 2S units 2.0
1.5 1.0 0.5 0
timing is changed In~ection timing (DBTEC~
280 270
2m BSFC (g/kWh)
150 240 230 220
Figure 7.28 Fuel loop.
30 25 20 15 10 5 0
Injection timing (OBTDC)
NOx concentrations decrease as the inlet airflow of a DI diesel engine is diluted at a constant fueling rate. The dilution is expressed in terms of oxygen concentration in the mixture after dilution. Table 7.4 shows how EGR affects specific NOx and HC fuel consumption, as well as smoke, for a small high-swirl DI diesel engine at a typical automobile part-load condition. Effective reduction of Bosch smoke NOx is achieved, with a modest reduction in brake specific hydrocarbon (bsHC) and only a small increase in BSFC. However, note that smoke increased as the rate of EGR increased.
Fuel injection timing essentially controls the crank angle at which combustion starts.
Although the state of the air into which the fuel is injected changes as injection tim- ing is varied and thus ignition delay will vary, these effects are predictable. The fuel injection rate, fuel nozzle design (e.g., number of holes), and fuel injection pressure all affect the characteristics of the diesel fuel spray and its mixing with air in the combustion chamber.
Exhaust Gas Emissions and Analvsis 137
Injection timing variations have a strong effect on NOx emissions for D1 engines; the effect is significant but less for ID1 engines. Retarded injection commonly is used to help control NOx emissions. It gives substantial reductions initially, with only mod- est BSFC penalty. For the D1 engine at high load, specific HC emissions are low and vary only modestly with injection timing. At lighter loads, HC emissions are higher and increase as injection becomes significantly retarded from optimum. This trend is especially pronounced at idle. For ID1 diesel engines, HC emissions show the same trends but are much lower in magnitude than DI HC emissions.
Retarding timing generally increases smoke, although trends vary significantly among different types and designs of diesel engines. Mass particulate emissions increase as injection is retarded.
The higher injection rate depends on the fuel-injector nozzle area and injection pres- sure. Higher injection rates result in higher fuellair mixing rates and hence higher heat release rates. For a given amount of fuel injected per cylinder per cycle, as the injection rate is increased, the optimum injection timing moves closer to top dead center (TDC).
The higher heat release rates and shorter overall combustion process result from the increased injection rate and decrease the minimum BSFC at optimum injection timing.
However, a limit to these benefits soon is reached. Increasing the injection rate increases NOx emissions and decreases smoke or particulate emissions. The controlling physical process is the rate of fuellair mixing in the combustion chamber. Thus, at constant fuel injected per cylinder per cycle, both increased injection pressure at the fixed nozzle orifice area (which reduces the injection duration) and reduced nozzle area at the fixed injection duration produce these trends.
In a diesel engine, the mixing time is short because the fuel is injected toward the end of the compression stroke.