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The hydraulic plug-in is a gas/electric plug-in hybrid with the addition of hydraulic regenerative braking. Prohibitive battery costs and weight are preventing plug-in hybrid vehicles from realizing their potential to improve global fuel economy in the near term. An efficient, low-cost hydro-mechanical transmission allows hydraulic regenerative braking to be added to the gas-electric plug-in hybrid at a reduction in vehicle component cost and results in greatly reduced battery costs and size. Since the hydraulic system absorbs the braking and initial acceleration power surges normally handled by the electric motor/generator and battery in a gas electric hybrid vehicle, the electric motor/generator can be reduced in size and cost and the battery capacity requirement can be reduced and the battery design shifted to favor storage capacity over power delivery.

The frequent stops of the typical postal delivery vehicle make it an attractive application for regenerative braking. The hydro-mechanical automatic transmission described in SAE paper 2014-01-1717 contains all the functions necessary to implement hydraulic regenerative braking including the accumulator and reservoir. This paper describes the substitution of the hydro-mechanical transmission for the present transmission of the postal LLV vehicles and estimates the performance benefits. The result represents a low impact path for the US Postal Service to extend the useful life of the LLV vehicles and increase the mpg by approx. 100%. A cost comparison between a convention ICE mid-sized passenger sedan and a similar size gas/hydraulic hybrid vehicle illustrates the cost advantage of the hydraulic approach using the new transmission design. Besides lower cost, the vehicle will have greater initial acceleration and 25%+ better mpg.