Steps of Electrification of Driveline
When we speak about E-mobility we have to distinguish the different steps of E-Mobility from Micro Hybrid up to BEV and ReBev or Rex Vehicles.
A Micro Hybrid uses a starter generator in the vehicle in an intelligent way to generate electricity when the car is breaking. The same starter generator starts the engine. The driveline is optimized as a start stop system. The e-motor is not used to accelerate the vehicle.
You can say that the vehicle is doing some recuperation (the official word for using breaking energy) but most of the fuel saving comes from the start stop system. The limiting factor of the recuperation is the size of the starter generator and the energy storage system.
The Micro Hybrids can save some fuel, depending of the traffic situation. For City buses and Delivery vehicles this System makes sense as it is relatively cheap.
A Mild Hybrid has a bigger e-motor in the driveline which is used together with the engine in driving mode and used for recuperating while braking. The E-motor is not strong enough to move the vehicle from stand still. It just assists the normal engine.
The Mild Hybrids are just a step towards the full hybrid. They don’t give the real hybrid feeling of accelerating quiet from stand still.
The fuel consumption of the Mercedes S 400 Blue-Hybrid is just 7,9l per 100 km and a CO2 emission of 190g/km. It has a 3,5-Liter-V6- petrol engine. The e-motor is a three-phase asynchronous motor with 120 Volt, 20 hp and gives an extra 160 NM of torque, helping the combustion engine during acceleration.
Full Hybrids have an e-motor in the driveline strong enough to start the engine from stand still. The size of the e-motor is optimized for the necessary force to start the vehicle and use the biggest part of the breaking energy in normal conditions.
Full hybrids use in addition to recuperation, load point shifting to generate electricity. Load point shifting means, that the internal combustion engine (ICE) produces a different power than is needs for driving the vehicle. In most driving conditions the ICE is not used in the most fuel efficient way. Though the gearbox is used to assist, there is some room for improvement even for the very energy efficient diesel engine. By using the e-motor and the energy storage system (batteries or super capacitors), the engine produces more energy than needed for driving or less. The on board computer steers the intelligent energy flow, taking under consideration, that there are losses in the e-motor and especially in the storage system when you store energy in the system and use the energy later.
With full hybrid you can drive full electric, driving off completely silent. The E-motor can assist the ICE in normal acceleration, giving it some extra power. In some configurations of the driveline the E-motor can help in the time gap during shifting, making the acceleration or breaking smother. It is possible to switch off the diesel engine in a constant speed situation and using the e-motor to hold the speed, making it possible to drive silently.
In heavy commercial vehicles there are auxiliary systems, which are normally attached to the ICE, like the hydraulic steering pump, which supports the steering of the vehicle, the air compressor, which supplies the air for the pneumatic breaks and the compressor for the air-conditioning system. If these auxiliary systems are still attached to the ICE than the combustion engine cannot be turned off during driving (especially the steering pump is critical), which is reducing the hybrid driving experience.
In full hybrids all the energy you use is produced in the vehicle itself. That means, that it’s just an optimization of the efficiency of the conventional combustion engine driveline. It can offer a very limited local emission free driving experience (Noise, exhaust gas).
Toyota Prius introduced in Japan in 1997 is the first mass-produced full hybrid vehicle. The Prius is sold in almost 80 countries and regions, with its largest markets being those of Japan and the United States. More than 3 Million units were sold until June 2013.
Volvo is the most successful European Hybrid bus manufacturer. All Volvo Euro 6 Busses in Europe are delivered as hybrids. It’s combination of hybrid and light weight design received fuel reduction up to 37 % in optimal conditions. It has a parallel hybrid with batteries as energy storage.
MAN is selling the MAN Lion city hybrid. It has a serial hybrid with ultra-capacitors as energy storage. Ultra capacitors have advantages in lifetime and aging, but are more expensive than batteries. The Serial Hybrid system has advantages against the parallel hybrid in tough inner-city conditions with a lot of start stops.
The Fuso Canter Eco Hybrid (here shown 7,5t Gross vehicle weight truck) has a 150 hp Diesel engine and a 55 hp E-Motor. The combination together with the automated dual clutch gearbox should save up to 23 % fuel.
Plug In Vehicle
Plug-in Vehicles do have an interface with which they can get electric energy from outside the vehicle. These vehicles have still an internal combustion engine on board. The idea is that these vehicles can be operated for a specific time local emission free. (Noise, exhaust gas). The batteries are designed for a typical driving gang of up to 50km with car. For Busses Volvo is preparing a Plug In, that makes it possible to drive local emission free for a limited range. The Ice and the E-motor are both designed to drive the vehicle separately alone.
Plug In have a big potential for passenger cars. The idea is that the normal daily driving range from home to work and shopping can be done using the charged electricity. When you need the vehicle for a longer trip, like vacation than the normal combustion engine gives you the range you need to reach your destination.
For commercial vehicles is gives you the option to drive emission free in sensitive areas like city centers, hospitals or recreation areas. There is also the option to drive silently in the night for a limited range.
The negative aspect of Plug Ins is the weight as there is a full combustion engine driveline and big battery packs.
BMW i8 is a Plugin with 231 hp internal combustion engine and 131 hp e-motor. The range is 50 km full electric and 550 km additional range with gasoline.
Battery electric Vehicle
Battery electric vehicles are no hybrids, as there is no internal combustion engine any more. Therefore they have a weight and cost advantage against the Plug In vehicles. But they need bigger batteries as they need al longer pure electric range.
The positive aspect of the e-motors is that you have the full torque from standstill. That mean that you can you can start you can start from standstill without a clutch. The torque of the e-motors used in vehicles have a wide range of engine speed in which the torque is stable. Therefore gearboxes with less speeds can be used.
One negative aspect of driving only with batteries is, that the batteries, even the newest Lithium Ion ones, are ageing. That means that the range of vehicle is decreasing over time.
Another negative aspect of Battery electric vehicles is the missing heat from the combustion engine. This causes in winter problems, when you have to do the heating for the driver electrically which leads to a dramatic reduction of driving range.
The most famous full electric vehicle is Tesla Roadster. This in the end is the one, which makes environmentally least sense, as it is a sports car, that means it is the second or third car and only used from time to time. It has a huge range of more than 320 km because of the big 440 kg battery. But the batteries are seldom fully used, so they are dying over the time not because of use but because of time like a cake left over from a party.
In commercial vehicles there a small vans like Smith which build van’s with 7,5t and 12t GCW and a range of up to 160 km, which are mostly used for Mail delivery in the UK. But due to the high price this is just a niche market for some “green fleet” operators.
There are some battery busses, mostly from china in the market, like BYD. There are concerns about the battery lifetime and range of these vehicles over the lifetime. The price is still far from being competitive with normal or hybrid busses even if you take the cheaper electricity into consideration. Another question is the loading infrastructure. For the bus operators and the time needed for charging.
Range extenders do have a full electric driveline and an additional second engine which is producing electricity when the batteries are empty. The second engine is normally an internal combustion engine which is used at defined optimal operating points. This gives the range extender vehicle additional range.
The vehicle has 2 driving modes. It usually drives off with its batteries externally fully loaded and drives its normal daily route. Only when extra mileage is needed the second engine is turned on and generates electricity for the e-motor.
The fuel efficiency of the internal combustion mode is worse than in Plug In’s where the engine is directly attached to the driveline as there are additional energy conversions. From the rotational speed of the combustion engine to electricity and from electricity back to rotational speed in the e-motor. When the produced energy and the used energy are not the same, because of load shifting for example, there are additional losses. The extra energy goes or comes from the battery which is working on direct current. The generator and the e-motor use alternating current. In the transformation process are losses.
The BMW i3 is an optional range extender. The normal range is around 150 km and up to 200 km in a special fuel efficiency mode, where acceleration and the use of electricity for the drivers comfort is reduced. The additional combustion engine is a 2 cylinder engine from the BMW motor cycle division has 34 hp and is extending the range of the vehicle by up to 150 km with 9 liters of fuel. Which shows the bad efficiency while driving the extra miles.
Range Extenders use there battery more efficient than full electric vehicles. Therefore the batteries are cheaper. The additional combustion engine is smaller, lighter and cheaper than in plug-ins but the fuel efficiency is worse.
In Commercial vehicles MAN showed on IAA 2012 a Range Extender Garbage Compactor which is tested at a customer starting 2013. The vehicle can collect 20km in full electric mode. Even the body is electrically driven. Therefore it’s extremely quiet. The story is that it can be used in 2 shifts instead of one because it can be operated in the early morning and an night, when other garbage compactors are not accepted by the cities because of the noise. On the way to the collecting area to the dumping ground it’s using an Audi motor to produces electricity for the e-motor.
Micro, Mild and Full Hybrids are a reasonable way to save fuel in the driveline and can be seen as the next step in optimizing the normal driveline. No investment in infrastructure is needed and there is no restriction in driving range. Therefore in the long run the Hybrids will become standard when the total cost of ownership will pay off.
Starting with Plug Ins the infrastructure question arises and the question which driving range is necessary or accepted for the customer. BEV have the hard range limit which is in addition suffering in winter. Range extenders and Plug Ins can both make sense. When you want to have best flexibility in range and the electric range of around 50 km is the standard range you drive, Plug Ins are the best solution. But only for the people who have a charging option at home and enough money to invest being a “first mover”