Powertrain electrification is a key element in strategies to reduce fuel consumption and CO2 emissions and, therefore, a major driver of new technologies in the global automotive industry. FEV has developed a compact and low cost hybrid concept called P-REX, which allows extended usage of electric driving in a range of vehicles from sub-compact up to mid-size. The compact but powerful concept is capable of providing two electric drive modes, two parallel modes and two CVT modes with only one electric motor and the system can also be used as a launch device as well as to start the combustion engine without the need for an additional starter.
The P-REX Plug-In Hybrid concept
The P-Rex transmission consists of two planetary gear sets (PGS1 & PGS2), two clutches (C1 & C2), one brake (B1) and a one-way-clutch (OWC). The single electric motor (EM) is concentrically positioned around the gear set. The first planetary gear set PGS1 acts together with a brake (B1) and a clutch (C1) as a two-speed transmission for the combustion engine. The second planetary gear set (PGS2) acts as a power-split device with the lockup-clutch (C2). A smooth transition between the two electric drive modes is realized by a one-way clutch that is installed in the gear set. This one-way clutch enables the transition by actuating a shift of one clutch against the WC, thus allowing optimum shift quality.
Low costs, high performance
To realize a transmission with lowest possible costs, all of the clutches and the brake use the same disc size and both single pinion type planetary gear sets are identical with equal ratios. Six different operating modes are used to generate a high degree of flexibility: Electric driving is accomplished with two different speeds, allowing the electric motor to be operated at very efficient operating points. For highway driving, two parallel modes can be used over different speed ranges and two CVT modes allow launching of the vehicle in hybrid mode without slipping launch devices. During vehicle launch, an EM Boost can be performed even with an empty hybrid battery.
Determination of hybrid strategy and influence of fuel consumption
The high degree of flexibility of the P-REX transmission with two parallel-, two CVT- and two E-Drive modes requires careful layout of the shift-strategy to optimize fuel consumption. This was realized by a detailed vehicle simulation based analysis. The simulation model that was used incorporates a 4 cylinder, 1.6 L SI engine and an electric machine with a peak power of 82.5 kW and a maximum speed of 7,000 rpm in a standard small size vehicle. For simplification, the inverter efficiency was set to constantly 97 percent.
The project results indicate a total system energy consumption in each combined operating mode. In this mode, the E-Motor can be charging or boosting, dependent upon the system power required. With the resulting data, the optimum operating mode for each vehicle driving condition can be determined.
To determine the parameters, which define the strategy for shifting from electric driving to hybrid driving and vice versa, FEV uses its TOPexpert PROcal toolset in combination with the vehicle simulation. Based on a set of boundary conditions for each parameter, PROcal defines a (Design of Experiments based) model and validation points using design of experiment. At these points, the vehicle simulation then calculates the fuel consumption in one selected driving cycle – NEDC in our case.
With these limits the vehicle achieves a minimum fuel consumption of 4.9 L/100km in the charge sustaining mode in the NEDC. The same vehicle with a conventional powertrain and manual transmission has a NEDC fuel consumption of 6.7 L/100km. Therefore, a reduction in fuel consumption of almost 27 percent has been achieved with the P-REX concept.
The FEV development team is confident that P-REX will add substantial benefit to the process of development and integration of hybrid powertrains into a range from sub-compact up to mid-sized vehicles. A gear set very similar to the P-REX concept has already been realized and tested in a demonstrator vehicle with very promising results.