“The potential for grid-connected vehicles to decimate our demand for liquid hydrocarbon fuels should be clear. Freed from the psychological barriers which hinder widespread market acceptance of pure battery electric vehicles, plug-in hybrids with an all-electric capability of just [30] kilometres would slash liquid fuel consumption, since such a high proportion of journeys undertaken are well within this range.”

"Plugged In: The End of the Oil Age” by WWF
(Dr. G. Kendall, 2008)

HEV / PHEV Powertrain Architecture Comparison
The Libralato engine may be ideally suited as an engine for HEVs and PHEVs, but how best should the engine be integrated in an HEV / PHEV powertrain? This is still a new technical landscape. The figure below compares the main features and fuel economy improvements of hybrid architectures.

The pictures below show examples of a Toyota Prius Gen 3 (Full hybrid - HEV) and a Chevrolet Volt (Plug-in Range Extender EV – EREV)

The following figure explains the key features of HEVs, PHEVs and EVs.

Within PHEVs the most well known systems are series hybrid (e.g. Chevrolet Volt) and parallel hybrid / power-split (e.g. Toyota Prius plug in). The following figure explains the key features.

The different architectures have major implications for the modes of operation of the vehicle, the overall energy efficiency of the vehicle and how much fuel is consumed. The Toyota Prius plug in will be a plug in hybrid with initial EV operation, all electric range (AER) of about 9 miles. The Chevrolet Volt will be an Extended range Electric Vehicle with an AER of 40 miles.

The figure below is a visual representation (produced by Toyota) of the energy flows in the different architectures. The Chevrolet Volt (series) architecture is depicted on the left and the Toyota Prius plug-in (parallel-series) is on the right.

Within the series architecture, the battery tends to be larger (Volt = 16 kWh, 40 mile range, 90% < > 40% state of charge (SOC)) the electric motor tends to be larger (Volt = peak power 140kW, 180HP) and the ICE tends to be smaller (Volt = 1.4L, 53kW, 71 HP). Within the parallel and series-parallel architecture the battery is smaller (Prius gen 3 = 4kWh, 9 mile range, 90% < >30% SOC), the electric motor is smaller (Prius gen 3 = peak power 50kW, 67HP) and the ICE tends to be larger (Prius gen 3 = 1.8L, 70kW, 94HP).

As various commentators and analysts have pointed out, both approaches have advantages depending on the driving cycle. At the point where a series hybrid has exceeded its AER range and is relying on the ICE to turn the generator, to create electricity, which is then converted back into mechanical energy by the motor to turn the driveshaft; there is about a 20 % loss of efficiency. In this scenario a parallel topology is more efficient since there is only about a 5% loss of efficiency from the ICE directly turning the driveshaft via the transmission. A series topology does not require a transmission (i.e. there are no gears) because of the torque characteristics and wide rpm range of electric motors. Due to the larger motor and battery, a series topology can deliver full performance (including improved acceleration) in EV mode alone, whereas a parallel topology requires a combination of the ICE and the electric motor to achieve this. In this respect a series hybrid is ‘cleaner’ and will be likely to produce less CO2 and pollution within urban environments. Over longer distances, in highway traffic a parallel topology is likely to be ‘cleaner’. In terms of the primary objective we have defined: to reduce CO2 and pollution from slow speed, urban traffic, both solutions deliver this. Considering that about 80% of daily driving distances are below 50 (miles US / Km Europe); within the AER range of PHEVs; the simplicity, efficiency and zero urban emissions of the series topology approach is particularly attractive. However the major drawback to this, is the extra cost of larger, advanced batteries and power electronics.

The following excerpts from the ‘Power System Level Impacts of Plug-In Hybrid Vehicles’ report by the US Power Systems Engineering Research Center (PSERC), Oct 09, provide a more mechanically accurate comparison between different hybrid architectures.

GM Volt Extended Range Electric Vehicle (EREV) - Series
“In a series architecture an internal combustion (IC) engine drives an electric generator whose electrical output is connected to an electric bus connected to the battery pack and the electric motor controller.

“The mechanical output of the motor is geared to the driven wheels, and only the electric motor provides driving torque in this configuration. The mechanical power out of the IC engine is converted from mechanical to electrical power by the generator and then back to mechanical power by the motor. The disadvantages of this series architecture include the fact that this double energy conversion reduces fuel efficiency. Also two electrical machines are required with the IC engine and generator sized to meet the largest sustained road-load for the vehicle. The electric motor must be sized to meet this sustained load plus have an additional capacity to meet some vehicle acceleration specification. This maximum sustained load could be approximately 80 mph up a 6% grade with the vehicle at its maximum allowable weight with passengers and cargo. The advantages of a series architecture are simplicity in the operational control algorithm, and the fact that the IC engine speed is completely decoupled from the vehicle speed. The control algorithm being proposed by GM does not start the IC engine driving its generator as long as the battery SOC level is above its desired low set point. For example, with this scheme, if the set point at a minimum SOC ˜ 20%, and the battery SOC =100%, the powertrain would run as a pure electric vehicle (ZEV Zero Emission Vehicle), depleting the battery until the SOC ˜ 20% level was reached. The IC engine would then go on and off, with some defined hysteresis characteristic maintaining a range of SOC values about the SOC ˜ 20% level. Regenerative braking can use the motor as a generator thus converting some of the vehicles kinetic energy into electric energy that is stored in the batteries. The normal friction brakes that waste this kinetic energy in the form of friction produced heat are still needed to completely stop the vehicle as regenerative braking diminishes as the vehicle speed goes to a zero value.”

Toyota Hybrid-Synergy Drive II (THS-II) – Power-Split

“In brief, the power-split architecture, as implemented by Toyota, has an IC engine mechanically connected to the carrier gear of a planetary. An electric machine (called the generator) is connected to the sun gear of the same planetary. Finally, the ring gear is connected to the shaft of a second electrical machine (called the motor) with this same shaft mechanically coupled through fixed gearing to the driven wheels. The name power-split comes from the fact that the IC engine power is split between a mechanical path to the driven wheels and an electrical path through the generator and its controller to a bus connected to the motor controller and the battery pack. An advantage of this architecture involves the fact that the IC engine speed is decoupled from the vehicle speed as in the series architecture. However, the price paid for this decoupling involves the fact that some of the IC engine output power goes through an electrical path that involves two energy conversions also as in a series architecture. With some of the engine power going directly to the driven wheels this power-split architecture partially includes the synergism of a parallel architecture, whereby the engine torque and motor torque can be blended to power the vehicle. The control of the two electrical machines for minimum fuel operation is complicated with Toyota not publishing details of the control algorithm being used. The use of the THS-II in a PHEV would require a higher capacity battery pack and a modified control algorithm that would allow for depletion of the battery stored energy to some set lower limit with charge sustaining operation continuing beyond this range.”

Although the analysis above and wider technical opinion tends to favour the Toyota approach in terms of all round efficiency and cost, it can be seen that the Toyota solution is not perfect. As noted, there is still a double conversion of energy in certain modes. The planetary gear arrangement itself is costly and in both series and parallel cases, the motor and generator are not well placed in order to maximise energy recovery from regenerative braking. These are not the only powertrain architectures possible and for example Hyundai has recently launched its own patented parallel transmission.

The figure shows a cutaway of the BMW Vision EfficientDynamics concept car powertrain, presented at the Frankfurt Motor Show, Sep 2009.

The PHEV BMW Vision EfficientDynamics concept car is powered by a 1.5-liter, three-cylinder turbodiesel; two electric motors, one on each axle; and a 10.8 kWh lithium-polymer battery pack. The vehicle has a 50 km all electric range (AER). The diesel engine can be the sole power source for the vehicle or it can work in conjunction with the electric drive, or it can be used to recharge the battery pack. Power is transmitted to the road via an enhanced version of BMW’s seven-speed Double Clutch Transmission. Using an EU domestic power source at 220 V, the batteries would take 2.5 hours to charge.

While this vehicle is hugely impressive, it is purely a technical demonstration model. In comparison to the average mid sized vehicle which requires around 85 kW for adequate acceleration and 50 kW for highway speeds, the BMW Vision EfficientDynamics has a peak power of 266 kW, only two seats and would cost at least twice as much as a conventional sports car.

The 50 km AER claimed is based on a battery depth of discharge of 80%, which is likely to cause battery deterioration and which would probably preclude a battery life of 10 years. The 50 km AER is also based on an electricity consumption of 0.175 kWh / km, which again is relative to sports car performance. Field trials of converted Toyota Prius PHEVs and studies⁶ have shown average electricity consumption rates of about 0.25 kWh / mile (0.155 kWh/ km) for mid sized, segment C vehicles in urban traffic. This is an important point which will be referred to again.

The GM Chevrolet Volt’s claimed 40 mile AER, is based on electricity consumption of 0.2 kWh / mile and a 50% depth of discharge of a 16 kWh battery. This later figure is generally regarded as a case of GM engineers erring on the side of considerable caution for fear of negative publicity relating to battery failures, which could ruin the reputation of the much anticipated Chevrolet Volt (due autumn 2010).

Examples of the use of Wankel rotary engines in PHEV architecture have appeared since 2009, with FEV and AVL demonstrating their use as ‘range extender’ engines in series hybrid PHEVs (EREV) demonstration vehicles. The small size, low weight and low vibration of Wankel engines have been recognised as their principal advantages in this context as can be seen in the figure below.

In March 2010, Audi revealed the A1 e-tron concept PHEV (EREV) which appears to incorporate FEV developments.

The Audi A1 e-tron combines a 45KW electric motor and a 15 kW Wankel engine. It can be seen that the same claim for a 50 km AER is delivered by a 12-kWh lithium ion battery pack. This would equal a depth of discharge of about 65%, which seems more realistic. The electric motor provides a standard output of 45 kW or 61 horsepower, though peak power of 75 kW or 102 hp is available in short bursts and keeps the car's 0-62 mile-per-hour time around 10 seconds. The Wankel engine displaces just 254 cc of volume in its single rotor. The whole unit is small enough to sit below the cargo floor of the A1, running at a constant 5,000 rpm,

The principal drawback to this powertrain design is that the Wankel ICE is only 15 kW. This means that beyond the vehicle’s AER, it can only achieve a top speed of about 60 mph / 100 kph and acceleration is likely to be very poor (or the battery will be discharged below the minimum set state of charge). The vehicle would also suffer from the double energy conversion losses discussed above in relation to the GM Volt. Finally, the Wankel engine only achieves peak fuel efficiency levels of about 260 g/kWh (31% efficient). Therefore the Audi A1 e-tron is seriously compromised in terms of competing directly with all round mid sized family vehicles.

The examples above serve to show that the development of HEV and PHEV powertrain architectures is a technical design field which is very new, which is not widely understood and in which there are as yet no clear winners. Engineers around the world are still grappling with complex mechanical / electrical system integration issues in which considerations of efficiency, reliability and costs are paramount. As yet, the GEV vehicles being brought to market are niche vehicles, often with poor performance, requiring substantial government subsidies in order to make them attractive to the consumer.

The UK Royal Academy of Engineering report ‘Electric Vehicles: Charged with Potential’, May 2010, outlines three scenarios for the introduction of electric vehicles:

1. Competition - Small numbers of up-market vehicles with extended range
2. Complementarity - EVs adopted as second cars in 2-car households used for short urban trips
3. Substitution - Fully fledged EV system gradually replaces ICE vehicles

The report highlights various barriers to the widespread introduction of electric vehicles and clearly regards scenario 3 as unlikely in the short and mid term.

“Once these subsidies are reduced, a consumer will choose between an internal combustion engine vehicle and an EV on their merits and unsubsidised price. There is a limited range of scenarios under which an EV would be fully competitive and only a committed group of potential customers would be prepared to purchase an EV that costs significantly more to own and operate than a competing internal combustion vehicle.”

Nancy Gioia, Ford Director of Global Electrification, shares this sentiment.

“In order for electric cars finally to win public acceptance, they'll need to be affordable. This is not about making a small niche. It's about affordable transportation for the masses”.

We believe that the cost savings afforded by the Libralato engine are a key enabling factor for HEVs and PHEVs (allied with its efficiency, power density, low emissions, low vibration and noise). We also believe that we have a powertrain architecture design solution for HEVs and PHEVs which is both efficient and affordable, enabling a far faster rate of adoption of HEVs and PHEVs than is widely forecast, by making HEVs and PHEVs competitive without any subsidies.

Through-the-Road
The PSERC report also comments on an architecture known as ‘Through the Road’. “….Where the IC engine powers one pair of wheels and the electric motor powers the other pair of wheels. The torques of the two drives are added together through the road. The electric machine, when operated as a motor, has its electrical terminals powered by the battery pack. In a charge-sustaining HEV, the electric machine provides the bulk of the torque required for acceleration with the IC engine providing steady-state constant speed cruising torque. When cruising, the electric machine can be controlled to switch from motoring operation to generator operation thus recharging the battery pack with the energy previously drawn for acceleration.”

Peugeot intend to produce a ‘Through-the-Road’ (TtR) architecture, for the 2011 Peugeot 3008 crossover and have dubbed their system “HYbrid4 system”.

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.

⁶e.g. Green Power for Electric Cars, FOE, T&E, Greenpeace Jan 2010