The U.S. Army has acquired several hybrid-electric platforms to assess the technology's applicability for military missions. The 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.
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 and a four-speed automatic transmission, and it has a gross weight of 5216 kg (11,499 lb). The XM1124 is a series-hybrid version of the M1113 with a PGU consisting of a four-cylinder 100-kW (134-hp) diesel engine coupled to a 100-kW PM brushless generator. Electric traction is provided by two 100-kW PM brushless motors. The XM1124 uses a 100-kW lithium-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 were 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 comprised of time vs. 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.
Engine operation efficiency was analyzed by superimposing 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.
Engine torque was not directly measured during the HEVEA tests, since this was not available on the CAN 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 calculated using an inverse table lookup of the 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 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, total regenerative braking energy was computed. Total regenerative braking energy was converted to an equivalent fuel consumption using the minimum brake specific fuel consumption 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.
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. 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 article is based on research by Ashok Nedungadi and Robert Smith of Southwest Research Institute, and Abul Masrur of Army RDECOM-TARDEC.