School Buses Use Parallel Hybrid System

IC Corp., a subsidiary of Navistar, recently built and delivered 13 out of 19 hybrid school buses across the U.S. as part of an initiative prompted by Advanced Energy Corp. The buses use a parallel hybrid system where the diesel engine and hybrid electric motor work together in tandem. Because of this, the hybrid aspect of the design can be shut off and the bus can run purely on the diesel engine.

With the parallel hybrid system, “if for some reason the electrical system shuts down, the diesel engine keeps driving the bus, just like it would on a standard non-hybrid bus,” says Keith Kladder, manager of school bus marketing for IC Corp.

The buses use a post-transmission system, where the electric motor is positioned after an Allison Transmission, which is placed after an InternationalMaxxForce diesel engine. The diesel engine is always running, at least at a low idle because certain functions of the bus like heat, air conditioning and the air suspension are not run on the electric motor. The electric motor, which runs off of a battery pack, uses the engine and regenerative braking to recharge the battery pack.

The bus comes with two options for a hybrid system that a user can determine when an order is placed. The first option is a charge sustaining system where the bus will run back and forth off the electric motor and the diesel engine with an approximate 1:1 relationship of engine to motor use. Depending on the route, the bus using a sustaining system has between 20-50 percent fuel economy.

The charge depleting system has more options and runs off of the electric motor most of the time for about 40-45 miles, until it reaches a certain level of battery depletion and switches modes. Under optimal conditions, the bus using a depleting system features up to 70 percent fuel economy.

“With the depleting system, for the first, call it the first 45 miles, you’re going to run heavily off of the battery pack and then once you get that battery charge down to the 25 percent level or so, it switches to a sustaining mode, where it goes 50/50,” says Kladder.

Right now the bus costs about twice as much as a non-hybrid school bus but according to Kladder, IC Corp. expects this price point to drop over time as more buses are placed into the market and there are higher production quantities. Remaining states in line to receive buses include New York, Virginia, Washington, Texas and Washington D.C.

As far as the longevity of the buses is concerned, “you’re getting fuel economy savings, you’re getting the emissions reduction, but you’re also seeing less wear on the engine, less wear on the transmission and because of the regenerative braking you’re getting less wear on your brakes,” he says, “which should extend the life of all three of those items.”