Tuesday, December 15, 2009

Plug-in Hybrid Types Compared


One year from today, you should be able to walk on to the lot and buy a Chevy Volt or a Nissan Leaf or one of many other cars that have a plug. There is a lot of confusion about the new world of plug-in cars.

In this Green Car Reports article, the author attempts to clear things up by saying that the Chevy Volt is not a hybrid. He is wrong; it is a hybrid, just not the same type that have been in the market. In the comments, he is corrected and admits that he was trying to simplify things by avoiding "technically accurate esoteric" terms that "the general car-buying public" would not understand.

If you are reading this, I am going to assume you are smart and can handle technically accurate and esoteric.

First, there are a lot of different ways to propel a vehicle including flywheels, fuel cells, compressed air and many many more. This discussion will only focus on internal combustion and battery electric (and the combinations thereof) because these are the car types that are planned to be on the mass market in 2011.

When examining vehicles, we'll look at two things:
  • The Fuel(s): the external energy inputs to the vehicle.
  • The Drivetrain: the engine and/or motors that propel the vehicle.
Fuel(s)
Drivetrain(s)
Name or Example

Gas

Gas
good ol' internal combustion (ICE)
Gas
Electric
Gas
HEV - Toyota Prius

Hybrids

Electric
Gas
Electric
Gas
Plug-in Hybrid Electric Vehicle* (PHEV)
Electric
Gas
Electric

EREV - Chevy Volt
Electric

Electric

Battery Electric Vehicle - Nissan Leaf

Line 1 ICE - Gas fuel powers the drivetrain: this is simply the internal combustion vehicle that dominates our roadways today. No explanation needed.

Line 2 HEV - Fuel = Gas, Drivetrain = Both: This is the Hybrid Gas-Electric Vehicle. The "gas" portion of that description is usually just assumed, leaving it label HEV. These came to the US with the Honda Insight and the Toyota Prius in 2000. At the time this writing, HEVs are still less than 3% of new vehicle sales, but you surely have seen one by now. These vehicles are parallel-series hybrids meaning that the power from the gasoline engine can be directed to either drive the wheels or to run the generator to charge the batteries. Even though this vehicle has batteries, the initial source of all of the energy comes from the gasoline put in the tank. Electricity is generated from regenerative braking and running the engine as a generator.

The two above are mass produced cars that are on the road today. The rest of the list is rare and generally less understood.

Line 3 PHEV: this is the plug-in version of the HEV. To make a point the complete name would be plug-in parallel-series gas-electric hybrid vehicle (PPSGEHV). You are not likely to see that lengthy description used anywhere else, so just stick to PHEV. The simplest way to explain a PHEV is start with an HEV and add more battery capacity and a charger to charger them.

Line 4 EREV: is the Plug-in series hybrid. It is also where the Chevy Volt sits. GM has branded this category the Extended Range Electric vehicle (EREV) and their internal technology as "Voltec". Chrysler refers to this category as Range Extended Electric Vehicle (REEV). This category has only an electric drivetrain. All propulsion is provided by an electric motor. The batteries provide power for an initial range and then a gasoline (or diesel or ethanol) generator provides the electricity for operation. The generator runs at an optimal speed that is independent of the vehicle's demands. Any surplus energy that the generator creates is sent to the batteries.

Although the generator does provide energy to the batteries, it does not charge them up completely. Doing so would use gasoline and reduce the MPG of the vehicle. That defeats the purpose. So, if the batteries become replenished due to the generator and regenerative breaking to a threshold above "customer empty", then operation returns back to battery powered mode. The generator and fuel act as a safety net range extender, not as a complete battery charging system.

These transitions of the generator turning off and on will not be readily noticeable to drivers or passengers. If you have driven an HEV like the Toyota Prius, then you know the engine turns on and off often based on driving speed. The transitions of the EREV generator will be related to the battery charge level, rather than the vehicle speed.

Line 5 BEV: This last line is the battery electric vehicle (BEV). These vehicles are not a hybrid. They are fueled by electricity and propelled by an electric motor. This category includes Tesla's Roadster and Model S and the Nissan Leaf. The vehicles have a fixed range and must be plugged in when the batteries are depleted.

Categories
The vehicles can be categorized in many ways:

Internal Combustion: ICE
Hybrids: HEV, PHEV, & EREV
Plug-in: PHEV, EREV & BEV

Plug-in Vehicles: The last 3 rows include "electric" as a fuel type. Put simply, these are cars with plugs. This is a useful categorization for discussing the electrification of personal transportation. Depending on the design of the PHEV or EREV, they can use electricity for just the few miles, up to most of a days driving. And BEVs are completely propelled by electricity. What all the Plug-in vehicles have in common is that they use grid electricity to displace liquid fuels.

The Plug-in Vehicle category includes Plug-in hybrids such as the BYD F6DM and Hymotion modified Priuses. And it includes EREVs like the Chevy Volt and the (now cancelled) Chrysler ENVI line.

There you have it. If the vehicle uses more than one fuel type OR has more than one propulsion engine type, it is a hybrid. That is a simple definition; the complexity is in all the ways that these can be combined.

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