Linear Engine / L2V4bGlicmlzL2R0bC9kM18xL2FwYWNoZV9tZWRpYS83MTc1

linear alternator (PMLA), and electro-magnet linear alternator (EMLA). Of the two possibilities, the permanent magnet linear alternator would provide a higher power density with a less complicated control. Its disadvantage would be that the magnetic field on the translator is not controllable. This might result in undesired voltage spikes when switching from actuator mode to alternator mode. Compared to the permanent magnet linear alternator, the EMLA has several advantages as described by Bromborsky [77]. Its major advantage is that the field flux of the machine is controllable, allowing active regulation of the output voltage. This design allows for easy starting of the engine by controllable electromagnetic force when the alternator operates as an actuator. Disadvantages of the EMLA include the need for connection to an external excitation source and heating effect of the current flowing in the winding. The heating can limit the current density of the field windings, resulting in decreased strength of field. Another disadvantage of the EMLA is the increased weight to magnetic field strength ratio of the moving mass because of the amount of copper required to provide a magnetic field that is comparable to the PMLA.

6.2 Linear engine

The engine is a dual-piston arrangement, two-stoke, compression-ignition linear engine. Figure 43 shows the arrangement of the second-generation linear engine developed at West Virginia University.

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Figure 43. The linear engine developed at West Virginia University [109].

The engine was designed and built by McDaniel [118]. His intention was to build the linear engine from off-the-shelf components to minimize design and manufacturing cost and time. The design was based on Kawasaki cylinders and pistons out of a watercraft engine (JS300). The cylinders were two-stroke, gasoline with a bore of 76 mm (part #: 11005-3714). The effective stroke of the engine was 38 mm and a full stroke 76 mm. The effective stroke indicates the distance between the cylinder head and the upper edge of the exhaust port and the full stroke represents the distance that the piston can physically travel. The cylinders were arranged on a common axis about 800 mm apart. The pistons were connected with a 25.4 mm diameter aluminum shaft that accommodated an iron slug in the place of the magnet of the linear alternator. Scavenging air was supplied to the bottom side of the piston from a 83 kPa pressure source. The air flowed to the topside of the piston through the scavenging ports, which were controlled by the piston itself, just as it would occur in a regular two-stroke gasoline engine.

The engine used a Bosch common rail direct injection system with a highpressure pump (part #: B 445 110 031-01) and Bosch injectors (part #: A611 070 0687). Mercedes-Benz uses these components in many of their diesel engines.

The engine was constructed of off-the-shelf components when it was possible and the rest of the engine was custom made to match the existing components. Each cylinder

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head required an injector, a piezo-electric pressure sensor, manufactured by PCB (part #: PCB JP 145 A01 SN 212), and a sparkplug (part #: Champion C59A). A racing ignition system, manufactured by MSD (part #: MSD6A6200), was used to supply the spark. Ignition was used as an aid to start the engine. The reasons for this are explained in section 6.4

6.3 Control of the engine

The linear engine was digitally controlled with a 40 MHz micro-controller (part #: PIC18F452). The injector control was comprised of a MICRO-EPSILON inductive position sensor (part # VIP-100-ZA-2-SR-1), the micro-controller, two ignition drivers, and two signal shaping, high-pressure injector drivers [119]. The input parameters to the controller were:

-Position of the translator -Injection position -Ignition position

-Reference mean (for calibration) -Fueling rate

The output signals were:

-Injection on, high current -Injection on, low current -Ignition timing

Based on piston position, reference mean, injection position, spark position, and fueling the controller calculated injection timing and duration and ignition timing. A potentiometer was installed to calibrate the reference mean of the system. This was necessary because the engine was disassembled and reassembled frequently, and the location of different components (cylinders, position sensor, wiper of the sensor) had

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slight variation after every time the engine was put together. The reference mean potentiometer was used to compensate for the possible changes in the position sensor calibration. Figure 44 shows the flowchart of the linear engine control.

Figure 44. Flowchart of the linear engine control. Translator position was a feedback input. Mean, Spark threshold, injection threshold, and fueling rate were user defined inputs.

Since the injector solenoids required a higher opening current (20 A) for fast action and lower holding current (12 A) for protection of the injector a simple signalshaping circuit was applied to the output of the micro-controller to send the proper current through the injector solenoids. Figure 45 shows controller operation during testing with an input signal from a signal generator.

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Figure 45. Signal generator signal as input to the controller and the shaped injector signals as response of the controller during controller testing [119].

Although the control program worked with the input signal from the signal generator, noisy input signal from the position sensor resulted in multiple injections. Application of filters was not possible because the time delay they introduced was too long to control the engine at 50-60 Hz of operation. Applying a smoothing function in the code was also not an option because it slowed down the program and introduced a delay. The application of registers in the program code yielded proper injection control even with the noisy input signal. This introduced an error in the precision of injection timing. Figure 46 shows actual injector solenoid control signal during linear engine operation using the position sensor signal as the control input. Noise is clearly visible on the position signal.

Figure 46. Position of the piston assembly and injection signals vs. time during linear engine testing. The controller provided adequate injection [119].

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Figure 47 shows the control box of the linear engine. Figure 47 shows the panels that accommodated the IGBT drivers, micro-controller, injector drivers, and ignition drivers. The potentiometers in the bottom right corner are mean for calibration, injection position, spark position, and fueling rate.

Figure 47. The control box of the linear engine. The IGBTs are shielded in the left hand side of the box. The panels from left to right are: IGBT drivers, micro-controller, injector drivers, and ignition drivers. The potentiometers in the bottom right corner are mean, injection position, spark position, and fueling rate.

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6.4 Starting the engine

Until the linear alternator was designed and manufactured the moving part of the alternator was replaced with an iron slug and the stator part was replaced with two coils. This solenoid system provided the starting motion of the translator. The iron slug, with the connecting rod and the pistons, was moved by the two coils being turned on alternately at about 20 Hz. The starter solenoids were supplied from automotive batteries via isolated gate bipolar transistors (IGBTs) (part #: CM 200DY-24H). The controlling signal to the IGBTs was provided from a signal generator. This method allowed the engine to have a variable cranking frequency to find the system’s natural frequency that resulted in the highest compression ratio. Using 60 V at 20 Hz cranking frequency, the compression ratio achieved was less than 4:1. This did not prove sufficient to start the engine on compression ignition. In diesel engines 15:1 is typically the required compression ratio to ensure operation. A sparking starting aid was necessary because the “starter motor” was unable to deliver a high enough compression ratio to start the engine. The spark helped combust the fuel injected while starting the engine, igniting the charge at lower compression ratios. It was expected that as part of the starting process, the spark timing would have to be retarded gradually. As a result the engine would continuously build up compression ratio until the charge would autoignite before the spark occurred then spark could be turned off. In reality, there is need for only one spark to start a properly tuned linear engine because the power of the combustion on one side provides energy for proper compression ratio in the other cylinder for the fuel to autoignite and for the engine to sustain operation. The same logic would suggest that only one solenoid would be sufficient to start the engine providing a single dynamic force on the slug.

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However, the solenoids applied at this moment are not strong enough to provide the single stroke necessary to start the engine. Instead, the two solenoids built up energy and amplitude in the system over a period of four to six cycles to provide high enough compression for the spark ignition aid. The highest compression ratio was reached after the fourth cranking cycle then the compression ratio decreased to a steady level. The phenomenon can be explained by the overshooting effect that happened when the system’s state was changed from still to a steady frequency close to its natural frequency. It can be concluded that when starting the linear engine, cranking should be done in short periods. Long cranking time is disadvantageous while starting the engine for another reason. Cranking was done at 20 Hz, while the engine operated at around 50 Hz. Once the engine fired there was a sudden operating frequency change in engine operation while cranking stayed at the original 20 Hz. This caused the cranking motor to work against the engine periodically instead of helping it to start. Figure 48 shows the position and cranking signal during an unsuccessful attempt starting the linear engine.

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Figure 48. Position and cranking signal during cranking of the engine. Engine did not start.

Figure 49 shows the position and the cranking signal for a successful attempt to start the engine. It can be seen from Figure 49 that the engine did not require the maximum compression ratio that the cranking motor was able to deliver. With the help of the sparking aid, it was not unusual for the engine to start after one or two cycles of cranking. It can also be seen from Figure 49, that the last cycle of cranking was actually stopping the engine. The cranking and the engine operation had different operating frequencies and on that particular cycle they were out of synchronization.

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Figure 49. Position and cranking signal during cranking the engine. Engine sustained operation.

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