Libralato HEV / PHEV Architecture
The following schematic diagrams compare vehicle energy losses between a conventional internal combustion engine (ICE) vehicle and a Libralato powered TtR PHEV. The principal PHEV operating modes and characteristics are:
- EV Mode (Urban) – Under speeds of 40 mph, EV batteries power front wheels via electric motors. Separate electric motors connected by a CV joint to each front wheel allow differential speed and torque control
- ICE Mode (Highway) – Above speeds of 40 mph, ICE drives rear wheels through standard (or possibly CVT) automatic transmission
- In all modes highly efficient regenerative braking via front motors
- Beyond the 16 mile AER, the vehicle reverts to ICE Mode (with optional regenerative charging via a small resistance through the front wheels)
- Four Wheel Drive Mode – combines motive power from EV and ICE powertrains
- Torque characteristics of electric motors allow EV dominant acceleration for short bursts i.e. up to 10 secs, up to speeds of 60 mph, under harsh pedal control
Conventional ICE (Urban)7
Libralato TtR HEV / PHEV – EV (Urban)
Conventional ICE (Highway)
Libralato TtR HEV / PHEV – ICE (Highway)
The key advantages of the Libralato TtR HEV / PHEV powertrain are:
- Electric motors x3 more efficient than conventional ICE
- Libralato engine 30% more efficient than conventional ICE
- Elimination of engine idling
- Engine almost exclusively operates within peak efficiency zone
- Significant gain from regenerative braking
- Accessories not taking power from engine
It can be seen that this approach offers a very simple, low cost, means of combining an internal combustion engine with an electric drive traction system, with the bonus capability of four wheel drive. The advantage is that mechanical complexity is reduced and the existing conventional powertrain can be largely carried over unchanged. The interaction between the internal combustion engine and electric drive is completely done ‘by wire’ and either of the systems can be turned off to drive the vehicle as an electric or ICE system. A through-the-road hybrid relies on the traction of the road to transfer torque. Because of the indirect connection between the electric and IC drive systems, the blending control is somewhat less sensitive and easier to implement. The vehicle would need to retain the conventional starter/ alternator on the engine.
Further consideration of the Peugeot approach leads to a key improvement – two electric motors should be connected to the front wheels and the driveshaft should be retained for the ICE to power the rear wheels. This would significantly increase the regenerative braking capacity of the vehicle. This is also a more cost effective use of two motor / generators in the vehicle. Two small motors are generally less costly than one large motor. Either both motors are fully engaged in powering the vehicle or they are both fully engaged in regenerative braking. There is less redundancy in the system than in either the GM or Toyota approach. From a high level design perspective it can be seen that the Toyota approach prioritizes the ICE. The GM approach prioritizes the electric traction motor. A through-the-road approach balances both prime movers equally.
Cost Effectiveness Comparison
The following table provides a detailed comparison of the cost effectiveness of a standard family sized vehicle (e.g. Ford Focus), converted to operate with a Libralato (TfR) PHEV16 powertrain.
CO2 emissions calculations are based on the average EU electricity generation mix, with emissions of 362 g/kWh and 7% distribution losses. Li-ion battery prices based on US Electrification Roadmap forecasts, Nov 09.
Our analysis shows that a Libralato powered TtR PHEV16 can reduce average NEDC T-T-W CO2 emissions to 52 g/km (W-T-W 88 g/km) for an incremental cost of $2,738 without subsidy, based on 2020 anticipated Li-ion battery costs of $325/ kWh8 and the average EU electricity generation carbon intensity of 362 g/kWh plus 7% distribution losses9).
Libralato PHEV Powertrain Cost Effectiveness Comparison
Vehicle component costs are generally commercial secrets, however the automotive revolution taking place and the vast number of studies being generated, have meant that there is an exceptional openness of information currently available. Cost estimates have been triangulated from numerous studies including: US DOE funded report - Plug-In Hybrid Electric Vehicle Value Proposition Study, 2009, IEA Energy Technology Essentials Report 2007, Ricardo plc Presentations 2009, US Electrification Roadmap 2009, Nippon Steel Technical Report on Induction Motors 2003, ANL Simple Cost Model for EV Traction Motors 1995, US NRC PHEV Cost Analysis 2009 etc.
The most controversial area concerns the forecast of Li-ion battery costs. We have accepted the US Electrification Roadmap’s forecasts, based on the direct involvement of companies such as Nissan and the massive investments being made (particularly in the USA) to scale up production of automotive Li-ion batteries. Forecast costs are illustrated below.
Forecast Li-ion Battery Costs
Just as important as the unit cost of Li-ion batteries, is the capacity specified. This aspect gives PHEVs a key advantage over pure EVs, since with the addition of an ICE powertrain, range anxiety is removed and battery costs can be radically reduced.
"The key question for plug-ins, from a design perspective, is how much of an electric range is really necessary, and what will that cost,"
Tom Stricker, Toyota US, Director of Energy and Environment
For PHEVs, in order to answer the question posed, the most cost effective solution to radically reducing oil dependency and CO2 emissions, is to specify the batteries just large enough to meet the average daily driving requirements. This way, the average driver will drive the vehicle all day in EV mode and by the end of the day, the battery capacity will be reduced to minimum state of charge, needing to be re-charged overnight. If the driver needs to exceed the daily average, the IC engine substitutes. The control electronics of the vehicle ensure that the IC always operates in its peak efficiency zone and regenerative braking is always utilised.
A summary compiled by Eurostat in 2007, found that people in most EU countries make on average three trips per day, totalling between 30 and 40 km across all modes of transport. These passenger kilometers are predominantly satisfied by the use of private cars. In the EU-25, close to 460 million citizens travel a daily average of 27 km (16.8 miles) by car. UK figures below are very similar.
Average daily driving distance and battery capacity required
A study by Imperial College London 200910 , analysed life cycle costs for PHEV powertrains as a function of battery size and concluded that lifecycle costs increase with vehicle size, however optimum battery size doesn’t change dramatically: 4-8 kWh for a small vehicle, 6-14 kWh for a large vehicle.
Powertrain lifecycle cost as a function of battery size
Source: US DOE, 2005
Source: US Electrification Roadmap, Nov 2009
Source: IEA, 2007
Source: Techno-economic and behavioural analysis of battery electric, hydrogen fuel cell and hybrid vehicles in a future sustainable road transport system in the UK, 2009”