With all the hype around the development and introduction of the Chevy Volt, I would like to open up a discussion on the differences between series and parallel hybrid powertrains. A series powertrain uses an electric motor power by a battery pack to drive the vehicle. The battery pack is charged by an additional electric motor coupled to an internal combustion engine (ICE). In contrast, a parallel hybrid powertrain uses an electric motor coupled to an ICE to drive the car, either in tandem (both electric and ICE), electric only, or gasoline only.
Proponents of series hybrids typically point to the ability to run the ICE at near optimal output, increasing the efficiency of the ICE. They also point to a more efficient drive line which typically has a simpler transmission.
However, series hybrid powertrains have considerable shortcomings, which is why I believe they are inferior to parallel hybrid powertrains:
1. Series hybrids require at least 3 engines, 2 electric and 1 gasoline. However, only 1 of these engines actually power the vehicles forward motion. The other two engines are used to charge the battery pack only. This obviously increases the cost, weight, and size of the vehicle, a major problem.
2. Energy conversion. In a series hybrid vehicle energy starts out in the form of chemical energy (gasoline) and is turned into mechanical energy by the ICE. The coupled electric motor then converts this mechanical energy into electric potential energy. The batteries receive this energy and turn it into chemical energy which is stored in the battery. Then energy then is turned into electric potential energy by the batteries. From here the electric potential energy travels to the electric motor to be turned into mechanical energy and power the vehicle. This is a lot of energy conversions, with each conversion eating away at the energy due to conversion inefficiencies. The losses sustained in the energy conversion trumps any potential gain from operating the ICE at optimal output.
3. Reliability. With 3 engines onboard the vehicle you now have potential for many more problems. With all of the vehicles energy traveling through the charging circuits, high voltage (HV) wiring, and batteries, you create a lot of wear. This is especially profound with the batteries, which lose their ability to hold charge as time goes on.
4. Poor use of energy. In a series hybrid system you are only using a fraction of the vehicles total power to drive the wheels. If for example you had a 100kW electric motor to drive the car, a 85kW ICE engine to drive the charge electric motor, and a 85kW electric motor to charge the batteries then you have a total of 270kW of power production, but you are only using 37% of it to drive the vehicle!
Just like with series hybrid powertrains, parallel systems have regenerative braking and can utilize the ICE to charge the batteries when necessary. However, a parallel system surpasses the series system because it allows for the gasoline and electric motor to be used in tandom when high levels of power are required. In low speed, low power, city driving the vehicle can use only the highly efficient electric motor to propel the car. Leaving the gas motor off. When the vehicle reaches higher cruising speeds it can turn to the well suiting ICE to propel the vehicle. This system allows for the vehicle to meet the power and efficiency demands of the driver.
Parellel hybrid systems are especially suited for racing, where they will be introduced to the 2009 Formula 1 league. A Formula 1 race car has a very powerful (and very efficient) ICE. The vehicle cannot go to full power when accelerating until approximately 100MPH because it does not have enough traction to keep the tires from spinnings below those speeds. This means that the vehicles powertrain is waisting energy while it accelerates to 100MPH. With a hybrid powertrain the race car can use that excess energy to charge a capacitor bank (or spin up a flywheel). Once the vehicle reaches a high enough speed and can use all of its ICE power to propel the car it can turn to the accumulated energy and "boost" the car even faster.
In another case the race car cannot use its power is turning. It this case the vehicle not only has to slow down (losing energy) but it cannot use its engine at full power. With a parallel hybrid system the vehicle can employ regenerative braking as well as unused engine power to charge the capacitors (or spin the flywheel). As the vehicle comes out of the turn it can now accerate extremely quickly with both the ICE and the electric motor, giving it a rapid "boost" to its maximum speed again.
Thus parallel hybrid systems are extremely well suited for racing and high performance applications allow them to not only achieve a higher peak power output, but also nearly 100% total ICE engine out.
Proponents of series hybrids typically point to the ability to run the ICE at near optimal output, increasing the efficiency of the ICE. They also point to a more efficient drive line which typically has a simpler transmission.
However, series hybrid powertrains have considerable shortcomings, which is why I believe they are inferior to parallel hybrid powertrains:
1. Series hybrids require at least 3 engines, 2 electric and 1 gasoline. However, only 1 of these engines actually power the vehicles forward motion. The other two engines are used to charge the battery pack only. This obviously increases the cost, weight, and size of the vehicle, a major problem.
2. Energy conversion. In a series hybrid vehicle energy starts out in the form of chemical energy (gasoline) and is turned into mechanical energy by the ICE. The coupled electric motor then converts this mechanical energy into electric potential energy. The batteries receive this energy and turn it into chemical energy which is stored in the battery. Then energy then is turned into electric potential energy by the batteries. From here the electric potential energy travels to the electric motor to be turned into mechanical energy and power the vehicle. This is a lot of energy conversions, with each conversion eating away at the energy due to conversion inefficiencies. The losses sustained in the energy conversion trumps any potential gain from operating the ICE at optimal output.
3. Reliability. With 3 engines onboard the vehicle you now have potential for many more problems. With all of the vehicles energy traveling through the charging circuits, high voltage (HV) wiring, and batteries, you create a lot of wear. This is especially profound with the batteries, which lose their ability to hold charge as time goes on.
4. Poor use of energy. In a series hybrid system you are only using a fraction of the vehicles total power to drive the wheels. If for example you had a 100kW electric motor to drive the car, a 85kW ICE engine to drive the charge electric motor, and a 85kW electric motor to charge the batteries then you have a total of 270kW of power production, but you are only using 37% of it to drive the vehicle!
Just like with series hybrid powertrains, parallel systems have regenerative braking and can utilize the ICE to charge the batteries when necessary. However, a parallel system surpasses the series system because it allows for the gasoline and electric motor to be used in tandom when high levels of power are required. In low speed, low power, city driving the vehicle can use only the highly efficient electric motor to propel the car. Leaving the gas motor off. When the vehicle reaches higher cruising speeds it can turn to the well suiting ICE to propel the vehicle. This system allows for the vehicle to meet the power and efficiency demands of the driver.
Parellel hybrid systems are especially suited for racing, where they will be introduced to the 2009 Formula 1 league. A Formula 1 race car has a very powerful (and very efficient) ICE. The vehicle cannot go to full power when accelerating until approximately 100MPH because it does not have enough traction to keep the tires from spinnings below those speeds. This means that the vehicles powertrain is waisting energy while it accelerates to 100MPH. With a hybrid powertrain the race car can use that excess energy to charge a capacitor bank (or spin up a flywheel). Once the vehicle reaches a high enough speed and can use all of its ICE power to propel the car it can turn to the accumulated energy and "boost" the car even faster.
In another case the race car cannot use its power is turning. It this case the vehicle not only has to slow down (losing energy) but it cannot use its engine at full power. With a parallel hybrid system the vehicle can employ regenerative braking as well as unused engine power to charge the capacitors (or spin the flywheel). As the vehicle comes out of the turn it can now accerate extremely quickly with both the ICE and the electric motor, giving it a rapid "boost" to its maximum speed again.
Thus parallel hybrid systems are extremely well suited for racing and high performance applications allow them to not only achieve a higher peak power output, but also nearly 100% total ICE engine out.
