Farfrumsparkin

CHARGING SYSTEM

An internal combustion engine (IC engine) is used as a generator to charge the battery pack, which in turn drives the electric motor. Since the IC engine does not drive the wheels, the series configuration offers advantages in fuel economy and emissions. The IC engine maintains a steady-state speed, which removes transient emissions and optimizes the engine performance.

Design improvements for subsequent years of the competition include examining other generation sources such as fuel cells. A variable field alternator would also offer advantages to vehicle performance and fine tune the operation range of the IC engine.

IC ENGINE

The IC engine was sized by examining the steady state power required for the various road load conditions. The team heavily weighted the IC engine selection on emission performance and fuel economy. The alternator was designed to support operation at the minimum brake specific fuel consumption of the IC engine (approximately 1800 rpm). Other considerations were charging capabilities, steady state voltage and cost. The system design was intended to keep the Suburban charge sustaining at standard speeds (50 mph).

The IC engine selected was a 1.9 L Volkswagen Turbo diesel. The engine meets Tier II federal emissions standards and has a peak thermodynamic efficiency of 43%. The unit installed this year was removed from a 1996 Volkswagen Passat along with the engine control computer and engine accessories. Several modifications to the engine system were made to adapt it to generate power for the battery pack including a speed control system, starting logic for the IC engine system, and modifications to the flywheel and bell-housing.

ALTERNATOR

The alternator was manufactured by Fischer Electric and is rated for 35 kW. The manufacturer specifications are shown in figures 9 and 10. System parameters are listed in table 6.

Figure 9 compares the voltage and power output for the alternator at a speed of 1850 rpm. This is close to the expected operation point (1800 rpm) and demonstrates the linear relationship between speed and voltage. Figure 10 shows the power output for the 360 volt operation point, which is nearly constant in the operation range from 1500-2200 rpm. These alternator specifications provide insight into the alternator performance that will be used to fine tune the system for additional charging in high load situations.

SPEED CONTROL

To maintain the IC engine speed at steady state (1800 rpm), a voltage feedback speed control system was designed. Maintaining constant engine speed during a generation cycle reduces emissions and also regulates the out put voltage at the rectifier. While the emissions benefits result from reduced acceleration, this strategy also dampens voltage surges higher than the rated amount to protect the electric motor controller.

A typical engine speed control system utilizes a governor attached to an actuator that changes the position of the throttle body lever. The diesel engine has no such lever, and a computer electronically controls a fuel pump to the injectors. To mechanically couple the electronic governor and throttle control, the actuator connects to the throttle position sensor normally located on a lever on the accelerator pedal of the IC engine. Figure 11 shows the mechanical coupling installed to communicate with the speed sensor. This sensor provides a voltage input to the computer for a certain rotational position of the arm. The voltage control element contrasts this control scheme with a typical control unit for a generator.

Instead of feeding back on the rotational speed output of the diesel, the closed-loop feedback system depends on the output DC voltage from the rectifier. The feedback system utilizes a pre-scaled voltage to sense whether the engine speed reaches the set point. At constant load, the engine speed is proportional to output voltage supplied to the battery pack. The actuator responds to the difference between the reference and actual voltage and moves the throttle position as needed. The system operates off a 12-volt power supply from a battery independent of the main battery pack. The system will operate continuously, but its inputs will only have effect when the engine logic starts the engine, and a feedback signal is provided. Specifications for the APECS control system are listed in table 7.

The last consideration in control of the IC engine is related to performance. The permanent magnet alternator allows the IC engine to operate within a range of acceptable speeds. The tow-haul switch on the stock Suburban will be used as an activator for the auxiliary control strategy. In this mode, the start logic is activated immediately without waiting for a signal from the Cruising Equipment Energy-meter (E-meter). This feature allows the IC engine to start immediately rather than activating in the middle of a high load situation.

STARTING LOGIC

To minimize user interaction a starting control system was designed so the IC engine turns on automatically when the batteries reach a state of 50% discharge and shuts off at 90% charge. An E-meter was used to monitor the state of charge. The E-meter contains logic components that allow the user to program full and empty battery charge levels. Upon reaching an empty state of charge, a switch closes and engages the start sequence.

The first stage of the start circuit generates a variable length pulse, which was used to heat the glow plugs. Also included in the first stage is an engine run signal. Once the engine run signal goes high and the glow plugs have heated, stage two of the circuit is started. This stage generates a three-second pulse used to crank the starter motor. If the engine does not start after three seconds, feedback off the engine is used to reset stage one, and the process begins again. This process of attempting to restart the engine can be repeated three times. If after three tries the engine still does to start, a fail to start indicator illuminates.

EMISSIONS

To determine the performance of the IC engine system at several operating points, the diesel engine was load tested. The engine was coupled to the alternator and the alternator was connected in series with two sets of resistance load banks (10 kW, 35 amps, DC). The purpose of the testing was to understand more precisely the emissions and fuel economy of the engine at several speeds. This data, and existing torque and fuel curves were used to select the optimum steady state operation point for the IC engine.

In the next phase of the FutureTruck, additional modifications will be made to the IC engine exhaust system to improve emissions. Primary concerns are further reduction of greenhouse gas emissions and particulate matter, a possible carcinogen. Options include a particulate matter trap and use of alternative fuels such as University of Idaho mustard seed biodiesel. As the IC engine is studied and improved in future years, additional fuels will be tested to understand the performance of mustard biodiesel, competition and standard diesel fuels.

MODELING RESULTS

The city and highway cycles used to predict the fuel economy and emissions of the University of Idaho Future truck were the Urban Dynomometer Driving Cycle (UDDS) and the Highway Fuel Economy Test (HWFET). Steady state energy use in these cycles at 20 mph and 55 mph is tabulated in table 10 with model predictions on fuel economy and emissions.

The modeling predictions have evolved significantly from the original estimates during the design phase. The data in table 10 reflects consistently updated inputs as weights and components were finalized during the design process. Fuel economy comparison of the model projections to the stock Suburban are summarized in table 11. Discussion of emissions reduction is not appropriate for this publication because the stock performance is considered proprietary.

The modeling predicts a 33% increase in the city and a 40% increase in fuel economy on the highway. The competition expectation of 50% improvement is not satisfied. However, vehicle performance is likely to improve as weight is reduced and powertrain systems are optimized in the second year of competition.