Tech Briefs

Hybrid vehicle powertrains show improved fuel economy gains due to optimized engine operation and regenerative braking.

The Army has acquired several hybrid platforms to assess the applicability of hybrid technology for military missions. These hybrid platforms include both series and parallel hybrid topologies. This work compares a conventional HMMWV (High Mobility Multi-purpose Wheeled Vehicle) M1113 with a series hybrid HMMWV XM1124 in terms of fuel economy improvements over three military drive cycles.

A schematic of the XM1124 Series Hybrid Vehicle. It consists of a 4- cylinder, 100-kW Peugeot diesel engine, coupled to a 100-kW PM brushless generator from UQM.
The attributes of the hybrid powertrain that help improve fuel economy of their conventional counterparts are more efficient engine operation and regenerative braking. In a series hybrid topology, the engine operation is decoupled from the vehicle road load. In the XM1124, the battery system is charged by the Power Generation Unit (PGU) (engine-generator) and by regenerative braking. The PGU can potentially be operated at higher efficiency, producing more power than what is required at the wheels, since the battery pack can absorb the difference between PGU power and road load power, within the limits of its allowable state of charge.

Three drive cycles were analyzed for fuel economy comparisons between the conventional HMMWV M1113 and the series hybrid XM1124. The HMMWV M1113 is equipped with a 6.5-L V8 turbo-charged diesel engine, a four-speed automatic transmission, and has a gross weight of 5216 kg. The XM1124 is a series-hybrid version of the M1113 with a PGU consisting of a 4- cylinder 100-kW diesel engine, coupled to a 100-kW PM brushless generator. The electric traction is provided by two 100-kW PM brushless motors. The XM1124 utilizes a 100-kW Li-Ion battery pack.

The fuel economy comparisons are based on HEVEA (Hybrid Electric Vehicle Experimentation and Assessment) data collected for both the XM1124 and the M1113 vehicles over the three drive cycles. In addition, only HEVEA data was analyzed that resulted in SOC (State of Charge) equalization, i.e. the battery SOC is equal at the beginning and end of the test cycle. The HEVEA data collected was comprised of time versus vehicle speed, front and rear motor current, generator current, battery current, battery pack voltage, battery pack SOC, fuel rate, and engine speed. The fuel economy was calculated from the fuel rate and vehicle speed.

The engine operation efficiency was analyzed by superimposing the engine operating speed-torque points over the engine efficiency map (which shows the speed-torque characteristics at different efficiencies of the engine). The XM1124 engine was shown to be more efficient than the older M1113 engine.

The engine torque was not directly measured during the HEVEA tests, since this was not available on the CAN (Controller Area Network) data bus for either the XM1124 or the M1113. As a result, the engine torque was derived from the other available data. In the case of the XM1124, generator electrical current, engine speed, and battery voltage were recorded. In the case of the M1113, the engine torque was known fuel map (engine speed and torque vs. fuel rate) and measured instantaneous fuel rate. The engine speed of the XM1124 is constrained over a narrow speed range, whereas the conventional M1113 engine speed is coupled to the vehicle speed.

The contribution of regenerative braking on overall fuel economy was determined from analyzing the braking events of the HEVEA test data for the XM1124. Once the braking events were identified, the total regenerative braking energy was computed. The total regenerative braking energy was converted to an equivalent fuel consumption using the minimum brake specific fuel consumption (BSFC) of the engine (220 g/kWh). This equivalent fuel consumption was added to the recorded fuel consumption for the cycle, and a new fuel economy was calculated. This new fuel economy represents the estimated fuel economy of the vehicle if regenerative braking was disabled.

The regenerative braking plays a very small role in the fuel economy improvements of the XM1124 over the M1113. The main reason for this is that the regenerative braking of the XM1124 is restricted to 10% of its full potential. As a result, most of the available braking energy is lost in the friction brakes of the XM1124. It can be concluded that the fuel economy benefits of the XM1124 over the M1113 are due to more efficient engine operation of the series hybrid powertrain over the conventional powertrain.

The analysis of the HEVEA data revealed that the hybrid XM1124 does not always produce better fuel economy than the conventional M1113. Factors that adversely affected fuel economy of the XM1124 include low vehicle speeds (<10 mph), resulting in the engine operating at lower efficiency; wet and cold road conditions, which affected fuel economy; and excessive charging of the battery using the PGU, resulting in an overall lower efficiency from fuel tank to wheel.

This work was done by Ashok Nedungadi and Robert Smith of Southwest Research Institute, and Abul Masrur of Army RDECOMTARDEC. ARL-0150

This Brief includes a Technical Support Package (TSP).

Quantitative Analysis of a Hybrid Electric Humvee for Fuel Economy Improvement (reference ARL-0150) is currently available for download from the TSP library.

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