Auxiliary Power Unit


The auxiliary power unit (APU) is the second source of energy that powers our HEV. There are many different types of APUs to choose from. These include: spark-ignition engines, combustion-ignition engines, and gas turbines. Based on preliminary research on all of these types of APUs the HEV team has decided to focus on using a direct-injection diesel engine. Reasons for this choice are listed below:

  • Higher baseline Efficiency (40% Peak)
  • Increased Fuel Efficiency (20-35% increase)
  • Lower CO2 Emissions (10-20% decrease)
  • Near-zero evaporative emissions
  • Market acceptance in Europe

Many direct-injection diesel engines were researched, and 3 main choices were obtained and are listed below.

    1. 3-liter TDI Engine: An 80-MPG turbo-charged direct-injection diesel engine produced by Volkswagen.

       

    2. GSE 4318: A lightweight 4-cylinder aircraft engine developed by Greg Stevenson Engineering.

       
    3. Yanmar D36: A marine outboard direct injection diesel engine produced by Yanmar Inc.

In an effort to determine the most appropriate APU that will meet the required goals for the HEV project, a quality loss function is used based on the Taguchi method. Designated target values for specified parameters are defined. Deviations from these target values result in additional costs to both the producer and the consumer. The Taguchi methodology is based on the precept that the lowest cost to society represents the product with the highest quality. This higher quality is achieved by reducing the variation in product characteristics.

The simplest loss function (nominal is best) is known as the quadratic loss function, which can be approximated by the following equation:

L(Y) = k(Y-m)2

Where L is the loss associated with a particular performance characteristic Y, m is the performance target value, and (Y-m) is the deviation from the target. The loss function can also be used for product characteristics other than those in which the nominal value is best. For the case in which smaller is best, the following loss function can be used:

L(Y) = kY2

For the case in which larger is best, the following loss function can be used:

L(Y) = k(1/Y2)

The value of the loss function parameter k is based on which of these three strategies are used. If nominal is best it is desired then that the average of all the product values be chosen. If smaller is best then it is desired that the smallest of the product values be used. If larger is best it is desired then that the largest of the product values be used.

Now that the method for choosing an appropriate APU has been established, performance characteristics are established. These are:

  1. Availability, A (0-1): Is the engine readily available for shipping. How long will it take? How much will shipping cost? The higher the number the more readily available the engine.
  2. Reliability, R (0-1): Has the engine been used successfully in service for a period of time? Has there been significant data collection regarding performance and durability? Has it been used in an HEV before and if so has it been successful? The higher the number the more reliable the engine.
  3. Fuel Consumption, FC (kg/kW× hr @ operating RPM): The fuel consumption at the recommended operating speed of the engine.
  4. Power, P (kW): The power of the engine at normal operating conditions.
  5. Weight, W (kg): The total weight of the engine.
  6. Volume, V (m3): Measure of the total rectilinear volume divided by the maximum allowable volume based on geometric constraints.
  7. Cost, C ($US): Measure of the total cost of engine including shipping and handling.

Using the Taguchi method and the above performance characteristics, an Excel worksheet has been developed to compare the APU choices.

 

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HEV Team
Department of Mechanical Engineering
San Diego State University
5500 Campanile Dr.
San Diego, CA 92182-1323
Fax: (619) 594-3599
E-mail: hev@kahuna.sdsu.edu