4.2.1 Engine and optical access
The optical engine was based on a Volvo D5, which is a passenger car size five cylinder CI engine. The engine was used in three different configurations. Engine data are given inTable 2.
Table 2. Engine specifications, Volvo D5244 in optical configuration
Configuration I II III
Displacement 0.48 L 0.48 L 0.48 L
Valves per cylinder 4 4 4
Bore 81 mm 81 mm 81 mm
Stroke 93.2 mm 93.2 mm 93.2 mm
Combustion chamber Pancake pent-roof Pancake
Compression ratio 12:1 9:1 12:1
Valve train Standard Fully flexible Fully flexible
Injection system PFI / DI PFI PFI / DI
Fuel 50 % iso-octane
50 % n-heptane
40 % ethanol 60 % n-heptane
90 % ethanol 10 % acetone The engine was operating on only one cylinder while the other four were motored.
The motored pistons were drilled through and did neither compression nor expansion work. Additional mass was added to the motored pistons in the form of tungsten alloy weights to compensate for the increased mass of the working piston. The increased mass of the working piston is due to the Bowditch piston extension [32]. Using the piston extension enabled optical access to the combustion chamber from below through a 58 mm in diameter quartz window in the piston crown. In combination with a 45 degree UV enhanced mirror mounted on the cylinder block below the piston crown allowed stationary horizontal mounting of image recording systems in the vicinity of of the engine.
The piston rings in the piston extension had to be run unlubricated. If not, the lubricant would have contaminated the optical surfaces and fluorescence from the lubricant could interfere with measurements. Piston rings made of Rulon® J were used. These have a very low friction and are self lubricant. The downside is that they are temperature sensitive compared to traditional piston rings. An elevated temperature will result in excessive wear of the rings and thereby gradual degradation of the engine behaviour.
4.2.2 Configuration I
The first optical studies (Paper III) were conducted with the standard automotive CI cylinder head and cam shafts, although modified for single cylinder operation with modified cooling water and oil flow as well as deactivation of the valve train on cylinders 2 through 5. The cylinder head had four valves per cylinder with a port design inducing a swirling intake flow pattern. A butterfly valve in one of the intake ports enabled the swirl number of the engine to be moderated from approximately 2 to 2.6. A quartz ring, 25 mm thick, was mounted as an elongation of the cylinder liner for radial optical access to the upper part of the combustion chamber. As the piston quarts glass was flat, there was no bowl-in-piston as in the production engine, instead
4 Experimental apparatus
a pancake shaped combustion chamber fully accessible through the quarts liner close to TDC. The engine in optical configuration is shown in Figure 15. The piston extension and the quarts liner can be seen in front of the camera to the right in the image. The engine had capability of both PFI and DI. The DI system was the original common rail system modified for single cylinder usage and featured a solenoid injector with a 5 hole nozzle and an umbrella angel of 140 degrees. The system was capable of injection pressures up to 1600 bar. In cylinder pressure was monitored by an AVL GU12S piezo electric transducer mounted in the glow plug hole.
Figure 15. Optical Volvo D5 engine with pancake combustion chamber.
4.2.3 Configuration II
To enable spark assisted HCCI combustion a pent-roof 4 valve SI cylinder head replaced the CI head. The cylinder head originates from a Volvo B5254 engine and has been modified for single cylinder operation during an earlier project [33]. To permit a variable NVO the cam shafts were removed and replaced by a Cargine pneumatic fully flexible valve train. The features of the valve train are further described in chapter 4.2.5. The pneumatic actuators were mounted on top of the valves, acting directly on the valve stems, resulting in a compact setup.
Due to optical constraints and the pent-roof combustion chamber the CR was lowered to 9:1. The optical access through the cylinder liner was limited to two Ø 15 mm quartz windows in the pent-roof giving pass through optical access to the spark plug area. The cylinder head with the optical access and the pneumatic valve train is shown in Figure 16.
Since the cylinder head had three access points for measurement, the two windows and the spark plug hole, pressure was monitored with a Kistler 6117BFD16 combined spark plug and pressure sensor. The piezo electric pressure sensor was flush mounted in the spark plug, i.e. no ducts for the cylinder pressure to travel through. The spark plug had heat range 6 and an electrode gap of 0.8 mm. This is the setup used in Papers IV & V.
4 Experimental apparatus
Figure 16. SI cylinder head with pneumatic valve train and optical access.
4.2.4 Configuration III
For Paper VI the engine was modified to feature PFI as well as DI, variable valve train and a spark plug. The engine with the VVA system mounted is shown in Figure 17. To enable DI, the CI cylinder head at hand was used again. The actuators for the variable valve train were fitted on an elevated plate connected to the valve stems with short push rods. The elevated position of the actuators had the consequence that no machining had to be done on the cylinder head and also allowed space for the CI injector to be mounted. By modifying the glow plug hole an 8 mm thread NGK ER8EH spark plug (Figure 18) could be fitted and fixated using a custom made isolator. The spark plug electrode gap had to be decreased to 0.6 mm to ensure breakthrough in the spark gap also at elevated pressure, rather than in the narrow glow plug tunnel. The ignition system was the same as described in 4.2.3. A Kistler 6053C60 un-cooled piezo-electric pressure transducer was fitted through a cooling channel and emerged between the intake and exhaust valves. Since the cylinder head cap had to be removed when the VVA system was mounted, new separate intake runners were built for the two intake ports, each equipped with a separately controlled PFI system. The DI system was the same as described in 4.2.2. Usage of the CI cylinder head meant that a quarts liner could be mounted again with excellent optical access. The combustion chamber was pancake shaped due to the flat piston crown.
4 Experimental apparatus
Figure 17. Single cylinder optical engine with VVA, DI, PFI and spark ignition
Figure 18. Right: Std. spark plug, Center: 8mm thread spark plug, Left: 8mm spark plug modified to fit in glow plug hole
4.2.5 Variable valve actuation
To allow operation with a variable NVO a VVA system was supplied by Cargine Engineering AB®. The system consists of four actuators controlling one valve each.
The actuators are small enough to be fitted directly on top of the valve stem.
However, in case of the CI cylinder head the actuators had to be lifted to allow room also for the injector. The system is working with pressurized air to achieve valve lift.
Two solenoids in each actuator together jointly control valve timing, duration and lift by regulating the air pressure on the working piston. To achieve a stable and constant valve lift a pressurised hydraulic circuit is deployed. To get a smooth low noise valve seating the hydraulic system is also used to decelerate the valve. A small leak flow of oil through the actuator is sufficient to lubricate the remaining valve mechanism. No other oil supply is needed to the cylinder head since the camshafts are removed.
The solenoids and thus the valve control are managed with FPGA (Field Programmable Gate Array) similar to what was used by Trajkovic et al. in [29]. Since the FPGA runs independent of other activity of the host computer this was found a reliable system with the weakest link being the signal from the engine CAD encoder.
Error of the CAD signal can result in an unexpected phasing between piston position and valve opening. In the lab, this clearly showed the benefit of compressibility of air compared to the stiff cam shaft operated valve train. From The GUI controlling the FPGA parameters, it was possible to control valve lift, duration, timing and deactivation for each valve separately as well as all together.
The lift profiles of the pneumatic valve train differ from that of a traditional cam shaft controlled system. Since the performance of the system is based on the air pressure to the actuators as well as the force of the valve springs, it is not dependent on engine speed. Most experiments were conducted at a speed of 1200 rpm of the engine. A valve lift height of approximately 8 mm was used with air pressure and spring forces resulting in an opening and closing ramp of approximately 25 CAD. The pressure of both the pneumatics and hydraulics was kept at 4 bars gauge (5 bar abs.). At Increased speed the valve lift profiles would then resemble a more traditional valve lift curve.
4 Experimental apparatus
To change the shape of the lift, the air pressure can be moderated, but for the descent it is the spring properties that must be altered. Single cycle lift curves are presented in Figure 19. These lift curves were obtained from image processing of high speed videos of the valve movement when running the engine. Due to limitations in the optical access for the SI cylinder head, the opening and closing ramps are not shown.
The actuators had built in optical sensor for detecting valve lift. However, the signal was highly nonlinear, with high sensitivity for detecting opening and closing events, but less for increased lift. Since the system was always run on an optical engine, valve timings were calibrated each day with the PIV camera described in 5.2.1 and the performance during experiments was monitored by the built in sensors to ensure constant conditions.
−180 −90 0 90 180
0 1 2 3 4 5 6 7 8 9
Crank angle [CAD]
Valve lift [mm]
Figure 19. Valve lift curves for 0 and 160 CAD NVO captured at 1200 rpm with high speed camera. 0 CAD is gas exchange TDC.
4.2.6 Test rig and measurement apparatus
Data acquisition of the fast signals i.e. cylinder pressure and triggering signals to and from optical measurement devices was performed with a National Instruments card in a PC running the same type of in-house program as for the multi-cylinder engine. A 1 Hz logger system monitored and stored various pressures and temperatures and also emissions. The emissions were obtained from a Horiba MEXA 8120. The test bed was equipped with a cooled EGR system making it possible to run with high amounts of EGR. Also a 5kW heater was installed on the intake system allowing combustion timing control when not running with NVO. The engine speed was controlled with a 30 kW AC motor used as dynamometer with the ability to both motor and brake the engine.
5 Diagnostic techniques