The sharply rising crude oil prices have put a technical challenge to the automotive sector to reduce the fuel consumption. Automotive business is looking towards electric vehicles to reduce the dependence on oil. However, battery technology development has been found deficient to support electric vehicles. Fuel cell powered vehicles are being sought as permanent solution but infrastructural demands of transporting hydrogen currently cloud their progress. Hybrids assimilation of electric vehicles and current IC engine based vehicles have been shown to reduce fuel consumption by Toyota (Prius, Lexus RX- 400h), Honda (Insight, Civic) and Ford (Escape).

The fundamental difference between a hybrid vehicle and an IC engine based (conventional) vehicle lies in their power train. A hybrid uses at least two prime-movers whereas a conventional vehicle uses only one. This implies that the transmission gearbox be able to accept two inputs rather than one that is, it needs to be a two degrees-of-freedom mechanism. Additionally, the gearbox needs to be compact. A planetary gear train (PGT) is the most basic two degrees-of-freedom mechanism. But a single PGT gearbox (as in Toyota Prius) means that the wheels never receive the full torque of the prime-mover, whenever the vehicle is being run by a single prime-mover. A compound PGT with two such basic PGTs interconnected, then becomes a natural solution However, most compound PGT configurations suffer from a serious problem of recirculating power within the gearing, leading to adverse affect on efficiency of the system and overloading of gear teeth, possibly causing failure of the system.

Technology being offered
The technology developed by Prof Bharat Seth and his student in Mechanical Engineering Department relates to one such compound PGT meeting all the desirable features such as: (a) harmonious addition of the power of both the primemovers without recirculating power going to disastrous levels; (b) operation of the engine in favourable region of its torque-speed map; (c) a good torque multiplication at lowend speed a characteristic extremely important considering the increasing vehicular traffic density around the globe; and (d) fail safety that allows the vehicle to be operated by any of the prime-mover (operating alone), should the other energy arm of the power train fail for any reason.

Further the transmission mechanism is implementable in real-time, that is, one devoid of any a prior knowledge of drive cycle, scalable, non-myopic (a myopic operation of vehicle misleads into notions of lower fuel consumption based on one round of a drive cycle) and balanced (such that, none of the two energy paths, electric motor and IC Engine, are overloaded at any time, whereby health of both the energy paths is taken care of, at all times).

The engine is connected to the transmission via an auxiliary gear set that lends it the scalability for various types of engines (gasoline or diesel) or for various categories of vehicles (hatchbacks or SUVs). Additionally, this auxiliary gear set ensures that speed longitude (on torque-speed map) in which the engine operates is narrow and is at the lower end of speeds (1200-3000 RPM), a characteristic extremely important for current (as well as those in future) Lean Burn SI Engines and also Common-Rail Diesels.

The augmenting control architecture is simple and real-time since it decides based on measurable quantities such as battery state of charge (SOC), vehicle speed and change in throttle movement. The decision for gear number in next instance is based on state of these variables in this instance, thereby devoid of any a prior information of drive cycle and hence can be practically implemented. The control architecture exhibits negligible increase in fuel consumed for 25 rounds of the New European Drive Cycle (NEDC) compared to one round of NEDC. This implies that the IITB controller is non-myopic. Further, the controller alters the engine torque contribution based on a "SOC-correction factor" which ensures that the battery is sufficiently charged always.

The result of this unison-act of transmission and control architecture gives (for a Tata Indica class of vehicle of 1010 kg mass, co-efficient of drag of 0.25 frontal area of 1.9 m2 equipped with 67 hp, 1.0 L VTEC Gasoline Engine and 10 kW permanent magnet DC motor) a consumption of 2.7 litres per 100 km over New European Drive Cycle. On the US-EPA drive cycle the fuel consumption is estimated to be 3.0 litres per 100 km and on the Japanese 10-15 mode driving cycle the fuel consumption is only 2.3 litresper 100 km over with a maximum speed of 190.5 kmph.

This performance estimates compare well with performance simulated for Toyota Prius. The invented transmission is a generic two degrees-of-freedom transmission and is not specific to IC Engine plus motor hybrids, hence can be adapted on to a two-motor architecture for fuel-cell vehicles. Patents have been filed in India, US and European countries and currently negotiations are on to find a suitable automotive industry partner for prototyping and commercializing the IIT Bombay technology.

Contact: seth@iitb.ac.in