mode, the fuel is injected during the compression stroke. in contrast, in a homogeneous charge operating mode, the fuel is injected during the intake stroke to achieve a homogeneous mixture.
7.2 Direct injection spark-ignition (DISI) injector
the spray images of a Siemens swirl injector at 4 ms after start of injection with injection pressure of 2.9 MPa and 8.4 MPa by Zhao et al. (1996).
It can be identified that a hollow-cone structure exists at the lower fuel injection pressure case. Due to its hollow-cone structure, the swirl injector is often referred to as a hollow-cone injector. it can also be seen that there is an air-entrainment zone at the edge of the spray cone. The other feature of the hollow-cone spray is that the spray cone decreases as the ambient pressure increases, as shown in Fig. 7.4. In DISI engine operations, the ambient pressure could go as low as 0.03 MPa at an idle condition with a homogeneous-charge mode. it could also reach as high as 1 MPa for injection late in the compression stroke during stratified-charge operation.
The back pressure sensitivity of the spray shape coincides with the trend of the spray shape requirements for both homogeneous-charge and stratified- charge operations. For homogeneous-charge operation, the fuel is injected during the induction event. A widely dispersed fuel spray (a wide cone angle) is helpful in achieving good fuel–air mixing. For stratified-charge operation, the fuel is injected during the compression stroke. A compact spray (small cone angle) is preferred to achieve a stratified-charge mixture distribution. However, the spray-cone angle sensitivity to ambient pressure also adds further complexity to the combustion system design and optimization.
7.4 Spray photographs of a Siemens swirl injector with gasoline at 2 ms after start of injection at ambient pressure of 0.1 MPa (left) and 1.48 MPa (right) (Zhao et al., 1996); fuel injection duration = 5.0 ms, fuel injection pressure = 5.5 MPa.
Figure 7.5 shows a typical fuel droplet size dependency on the fuel injection pressure reported by Harada et al. (1997). it is evident that as fuel pressure increases, the droplet size decreases. Based on his modeling work, Dodge (1996) recommended that a mean droplet size of 15 mm or smaller is required in DISI engines. The recent development in injector technology to generate such small droplets makes DiSi engines feasible engine design solutions (Anderson et al., 1996).
Air-assisted injection is a technology that produces sprays with well- atomized distributions even operating at relatively low fuel supply pressures.
Shown in Fig. 7.6 is a typical air-assisted injector and its injection sequence developed by Orbital (Cathcart and Zavier, 2000). This fuel system comprises a direct injector, a conventional PFI-type fuel injector, and an interface region that provides the path between the air and the fuel. The conventional PFi injector provides the fuel metering function, operating with a constant differential pressure of normally 0.5 to 1.0 MPa. The metered fuel is combined with air in the interface region, and this charge, comprising fuel and air, is injected by the direct injector at a gauge pressure of normally 0.65 MPa. Fuel droplet shearing forces enable the air-assisted injector to produce smaller droplet sizes as compared with swirl injectors. Figure 7.7 shows the droplet distribution for a typical low load fueling event, with a SMD of approximately 10 mm. it is evident that the injection window of the air-assisted injector is limited by the available air pressure. During late compression stroke injection, the in-cylinder pressure may be too high to have positive differential pressure across the direct injector of this injection system.
The multi-hole injector for a DiSi engine is an extension of gasoline PFi and diesel injectors. The main difference between these injectors concerns the operating fuel pressure. PFI injectors typically operate at around 0.4 MPa, and diesel injectors with rail pressures as high as 200 MPa. Multi- hole DISI injectors operate in the range of 3 to 20 MPa. Shown in Fig. 7.8 is a Bosch six-hole injector and a representative spray image (Stach et al.,
7.5 Spray characteristics of a swirl injector (Harada et al., 1997).
5 10 15
Fuel pressure (MPa) 25
20
15
Mean droplet size (µm) 10
2007). One of the unique features of multi-hole injectors is the wide range of available flow rates. The other important feature is that the spray shape is not significantly dependent on the ambient gas pressure, even though the penetration of each jet decreases as the ambient pressure increases. Another important feature of the multi-hole injector in combustion system development is that the orientation of each individual spray plume (or jet) can be varied
Fuel metering
event
Direct injection
event
*IGN
720° 360° 180° 0°
TDC TDC TDC
Fuel 8 bar
Air 6.5 bar
MPI fuel injector
injectorAir
Direct injected premixed charge
7.6 Schematic of Orbital air-assisted direct fuel injection system (Cathcart and Zavier, 2000).
0 20 40 60
Diameter (µm) OCP DI
Volume (%)
Fuel pressure: 7.2 bar
Fuel injected: 6 mg Air pressure: 6.6 bar Mean droplet size 10.3 µm 6.0
4.0
2.0
0.0
7.7 Air-assisted fuel injection system PDPA particle size data (Cathcart and Zavier, 2000).
Injector sequencing
almost independently relative to the other spray plumes. This provides design engineers with great freedom for optimizing the injector spray patterns.
An outwardly opening piezo injector has a similar spray shape to the swirl injector but is formed through a conical annulus between the body of the injector and an outwardly opening pintle. The outward motion and quick, repeatable spray formation of this spray type lends itself well to being actuated by a piezoelectric stack. Such a piezo injector has a much wider dynamic fuel flow rate range and a quick transient response time. Shown in Fig. 7.9 is an image from a Siemens piezo injector. A characteristic feature described in the caption is the very small injection durations for both first and second injections, of the order of 200 ms. The capability to deliver fuel at short pulse widths with robust spray characteristics is a unique feature of the piezo injector. This feature also enables highly flexible multiple injection pulses per engine cycle.
Slit injectors emit fuel sprays from a rectangularly shaped slit. The resulting spray shape is similar to that of a fan. This injector design has been applied in Toyota DISI engines (Ikoma et al., 2006). More details of this combustion system design will be discussed in Sections 7.3 and 7.4.