Optically Accessible Engine

The optically accessible single cylinder Diesel engine employed in this work has been purpose built to facilitate optical and laser optical measurements. It is based on the Mercedes Benz OM450 heavy duty Diesel engine. The crankcase, crank train, and valvetrain have not been changed from their original design. The production cylinder head, cylinder liner, and piston have been modified to provide optical access into the combustion chamber near top dead center. A custom cylinder housing has been designed to house the production cylinder liner and support the cylinder head and valvetrain. Pertinent geometrical data for the engine are shown in Table 3-1, and a cross sectional view of the engine is shown in Figure 3-1.

Table 3-1: Optically accessible engine data

Valves 4

-Optical accesses 3

-Bore 128 mm

Stroke 142 mm

Connecting rod length 256 mm

Compression ratio 16.1:1

-Maximum engine speed 2000 rpm

Intake valve opening* 1° ATDCe

Intake valve closing* 185° ATDCe Exhaust valve opening* 125° ATDCf

Exhaust valve closing* 5° BTDCe

*Valve events defined as the point of 0.15 mm valve lift

Cylinder head

Coolant inlet

Coolant jacket Recessed

cylinder gasket

Intake Port Exhaust Port

Upper housing

Lower housing

Observation access

Cylinder liner

Piston

Figure 3-1: Cross sectional view of the optically accessible engine

Experimental Setup and Measurement Techniques

The cylinder housing is comprised of an upper and a lower housing. It houses and cools the cylinder liner, provides oil and coolant to the cylinder head through the corresponding inlets in the cylinder head, and is integral to the optical accessibility of the combustion chamber.

Both parts of the housing have been CNC machined from a large piece of 42CrMoS4 round stock. The high strength of this material is well suited for applications involving threaded bores that are cyclically loaded, and the sulfur content improves its machinability.

The production cylinder head has been modified to enable optical access even at top dead center. For this purpose, a precisely milled groove has been machined into the lower surface of the head. This raises the position of the cylinder liner and enables the three optical accesses to be placed directly below the cylinder head.

Some optically accessible Diesel engines are built so that part or all of the cylinder liner is made of quartz glass [90, 91, 92, 93]. However, there are drawbacks associated with the use of a quartz glass cylinder. Such an engine is often limited in terms of both the loads and temperatures that the materials can withstand, although this problem can be avoided with a pressurized double-ring system [94]. The compressive strength of typical technical fused quartz glass is more than 20 times greater than its tensile strength [95]. Therefore, a design that utilizes compressive loading of quartz elements will be inherently stronger than one in which quartz is loaded in tension, i.e. an internally-pressurized quartz cylinder. Sealing a quartz glass cylinder liner is challenging as the thermal coefficient of expansion of steel is approximately 20 times that of quartz glass. The thermal conductivity of quartz glass is less than 1/20^th that of steel, so heat transfer and cylinder wall temperatures would be significantly different with a quartz glass cylinder liner. Consequently, the combustion characteristics in a Diesel engine with a quartz glass cylinder liner would likely be much different than those found in a metal engine. Hence, it is desirable to decrease the size of the optical accesses to minimize these effects on the in-cylinder processes. Passing a vertically-oriented light sheet through a transparent cylinder liner results in portions of the light sheet being internally reflected; this phenomenon manifests itself in the form of bright lines on the resulting images and makes processing of data in these regions very difficult.

If the cylinder liner is an extension of the existing liner or if images are taken through the piston from below, then an extended piston is also necessary. While this design allows for a wide variety of measurements, it has disadvantages at higher engine speeds and often requires a special compression ring design. Often, these pistons are built with a flat quartz window through which the optical measurements are taken. A quartz glass piston top with a steep-sided piston bowl would be prohibitively expensive and optically complex [96].

For these reasons, an engine design was chosen with three large, flat optical accesses, two located diametrically opposed from one another and the remaining one oriented perpendicularly to the first two. Similar concepts have previously been developed and successfully implemented elsewhere [97, 98]. This design enables a vertically-oriented light sheet to be passed through the cylinder, where it can be observed from the remaining optical access via an endoscope or a far field microscope. As explained in [99], this configuration is advantageous because the light sheet can intersect with the axis of an injected fuel jet, rather than crossing it horizontally. This configuration makes RAYLIX measurements (see 3.3.3:

RAYLIX) inside the combustion chamber possible. A further advantage is realized in that the optical accesses can rather quickly be removed, cleaned and replaced without having to remove the cylinder head. Figure 3-2 depicts the combustion chamber of the engine, including the optical accesses.

Experimental Setup and Measurement Techniques

Figure 3-2: Combustion chamber with endoscopic access, slotted piston and laser sheet

Two round quartz glass accesses allow the passage of a laser light sheet (also shown in the figure) into and out of the combustion chamber. The third access, seen at the back of the cylinder allows perpendicular or near-perpendicular observation of the combustion chamber.

Two piston designs are employed in this work, both of which have been modified with an observation cutout. They differ in that one design has additional slots cut into the piston crown for the laser light sheet and the other has none. The observation cutout of the piston without slots is widened slightly to match the compression volume of the slotted piston. CAD images of both pistons are shown in Figure 3-3.

Figure 3-3: Piston designs. Left: observation cutout and slot for laser sheet; right: modified observation cutout and no laser sheet slot

The effects of these modifications on the in-cylinder flow has been examined via 3D-CFD simulations in [6] for the piston design shown on the left. The flow is characterized by a weak swirl motion, which is affected by the presence of the observation cutout in that a

Experimental Setup and Measurement Techniques

recirculation zone builds there. Because there are no spray jets directed into this portion of the cylinder, it is not assumed that the observation cutout has a significant impact on the combustion. The squish flow induced between the piston bowl rim and the cylinder head is impacted by the laser sheet slots, so the second piston is used when a laser sheet is not used to minimize the effects of the piston modifications on the in-cylinder flow when possible.

The fuel injector is a Bosch CRIN3-18 solenoid injector with five holes, each with a hole diameter of 139 µm. The spray pattern is based on a seven-hole injector design and is also shown in Figure 3-2. One jet has been removed so that the laser sheet interacts with only one jet. A second jet was removed to provide an unobstructed view of the imaging plane and the fuel jet of interest. More detailed geometric information about the injector nozzle geometry is shown in Table 3-2.

Table 3-2: Injector nozzle data

Number of holes 5

-Included angle 162.5 °

Hole diameter 139 µm

Nozzle hole L/D 6.8 -Nozzle hole conicity (ks) 1.5