Improving alternator efficiency measurably reduces fuel costs By Mike Bradfield lectrical power on a vehicle is not free. It comes as a direct result of consuming fuel within the engine to drive the alternator. With a typical engine effi- ciency of 40 percent, a belt efficiency of 98 percent and an alternator efficiency of 55 percent, this leads to an overall energy conversion efficiency of only 21 percent. Assuming a fuel cost of $4/gal., this leads to an on-board electrical power cost of $0.51/kWh, or roughly four times a typi- cal household utility rate. Consequently, the fuel costs associated with providing electric power can be quite significant and should no longer be ignored. This article addresses the topic of how improving alternator efficiency can com- bat this problem by measurably reducing fuel cost. First, a fundamental overview of how an alternator works is given. Each of the major power related elements of the alternator are reviewed: rotor, stator, rectifier and voltage regulator. Next, an inside look at where the actual power losses occur within the alternator is examined. This includes electrical losses, magnetic losses and mechanical losses. This is followed with a discussion on the topic of alternator efficiency from an over- all machine standpoint. Prepped with the preceding informa- tion, case studies of three different real-world applications are completed for a line haul tractor, city tractor and school bus. Real-time operating data was collected on each of these applications for the overall vehicle and electrical system. This measured data serves as the baseline for each of these three studies. The fol- lowing chart shows the fuel cost savings possibly over a typical life cycle by increasing the alternator efficiency 20 percent over the baseline condition. Average Current Application Fuel Cost with Base Efficiency to feed an energy conversion chain. This chain, as depicted below, starts with chemical energy stored in the fuel and ends with electrical energy from the alternator. Along the way there are power losses associated with every energy con- version process, including the alternator. These losses result in more fuel being consumed to produce a given amount of electrical power. Clearly, as the alternator becomes more efficient in the process of converting mechanical power into electri- cal power, less fuel is consumed. And Fuel Cost with a 20% Improvement in Efficiency Fuel Savings with Higher Efficiency Line Haul Tractor 84 amps $4534 / 500k mi $3778 / 500k mi $756 / 500k mi City Tractor 40 amps $2235 / 350k mi $1863 / 350k mi $372 / 350k mi School Bus 102 amps $9040 / 250k mi $7533 / 250k mi $1507 / 250k mi What may not be obvious from this chart is the fact that an efficient alterna- tor on a commercial application can more than pay for itself in less than one year. Over the life of the vehicle, these savings can reach thousands of dollars. A wide range of efficiencies exists between current production manufactur- ers and even from within a given manu- facturer’s portfolio of products. Peak efficiencies at full output vary from 55 percent to almost 80 percent. With the high price of fuel, fleet and vehicle own- ers should no longer ignore this impor- tant parameter. Increasing alternator efficiency can, and will, measurably reduce fuel costs. The energy conversion chain Electrical power on a vehicle is not free. This power comes as a direct result of fuel being consumed within the engine even though the power demands of the alternator are generally small in compari- son to the overall vehicle, the impact on fuel cost is not trivial. In fact, with today’s high fuel prices, changes in alternator efficiency can meas- Fuel Overall Efficiency 21% 6 kW Power Loss 0.1 kW Power Loss Alternator 1.8kW Power Loss Electric Power Figure 1— Energy conversion chain. urably impact fuel operating cost as we shall see later. (See Figure 1). Engine efficiency To understand how improving alter- nator efficiency reduces fuel usage, we will examine each element of the energy conversion chain starting with the engine and ending with the alternator. The engine combusts fuel to convert the stored chemical energy of the fuel into thermodynamic pressure. This pressure does expansion work within the cylinder of the engine creating linear power. It is converted into rotational power by the connecting rods and crankshaft. The conversion of fuel chemical energy into rotating mechanical power by the engine is accompanied with power losses. These losses, and therefore the efficiency, are dependent upon a number of factors including engine speed and mechanical load. Often engine efficiency is reported as brake thermal efficiency. This is simply engine efficiency as measured on an engine dynamometer (brake). The effi- ciency of the engine may be expressed as: Where n= arene out aati in Pengine out = Pengine in — Prosses n = engine efficiency eeneiretont mechanical power output erence fuel energy input Pisses