Engineering products that are meant to be sold to consumers is a process that is invariably about compromise. If you are building a one-off product for a single client, you can optimize it directly for their needs. But when it comes mass production for customers with widely divergent requirements, engineers have to make judgment calls.
Case in point is the Chevrolet Volt and its Voltec powertrain. Within moments after Bob Lutz stepped out of the concept on January 7, 2007, there has been an almost constant discussion about the merits of the E-Flex/Voltec architecture. One of the more notable features is that, despite the presence of an engine in the car, it has no mechanical linkage to the wheels. The engine only drives a generator to provide electricity when the battery reaches its depletion point. The problem is that this so-called "charge-sustaining" mode involves multiple energy conversion processes. One of the first things you learn when you start studying engineering is that there is no such thing as a 100-percent efficient process, and that means figuring out ways to make the best out of what you've got. Continue reading about how the Volt should drive its wheels after the jump.
There is a principle in physics known as conservation of energy. Energy can neither be created or destroyed, but it can be transformed from one form into another. For example chemical energy (the bonds that hold atoms and molecules together) can be transformed into thermal (heat energy) through the process of combustion. Kinetic or potential energy can transformed into electrical energy when, for example, a moving river flows through a turbine connected to a generator.
However, each of those conversions also produces some non-useful (most of the time) forms of energy that is lost in some way. For example, a generator will have both mechanical friction and electrical friction (resistance) that causes kinetic energy to be transformed into waste heat. The same phenomenon applies when electricity is converted to kinetic energy in a motor or you shift between direct and alternating current in power electronics.
All of this brings us back to the Volt. In charge-sustaining mode, we have the following energy conversions (all of which are downstream of the combustion process):
- mechanical kinetic energy at the crankshaft to alternating current in the generator
- alternating current to direct current for the battery/electric drive
- direct current from the battery/generator to alternating current for the traction motor
- alternating current to kinetic energy for the wheels
When driving directly through a set of toothed, mechanical gears, the power transfer efficiency can be upwards of 95 percent. Certainly General Motors is well aware of this phenomenon. That's why, in developing the two-mode hybrid system, GM went beyond what Ford and Toyota did by adding a pair of clutches that lock the motors and provide a direct mechanical drive path. This allows GM's hybrid trucks to get improved highway fuel economy and better towing capability than other hybrid drive systems.
So, why not do this with the Volt? It's about compromise and mechanical complexity. There is no technical reason that the Volt could not have direct mechanical drive and better efficiency. However, doing so would require the addition of a power-split transmission system as well as re-tuning the engine. The Volt engine is tuned for optimal operation at certain fixed engine speeds and only produces about 80 horsepower. To provide adequate performance, the engine would either have to put out more power or blend gas and electric drive during acceleration.
A power-split transmission that can do the blending would add weight and cost and also limit the packaging flexibility. Currently, the Volt uses a simple single-speed reduction gearbox for the electric drive. The engine/generator mounting location is also not tied to the position of the driving axle thanks to the lack of a connection. This would change with direct drive.
When the GM engineers devised Voltec/E-Flex, they worked from the assumption that three-quarters of daily driving is less than 40 miles and most of the rest of the trips are not much more than 40 miles. All of that means that the vast majority of the Volt's real-world duty cycle would fall well within the electric driving range and the charge-sustaining operation would be kept to a minimum. Adding direct drive would undoubtedly improve the overall mechanical efficiency. However, given the design assumptions, adding the extra cost and weight of a power-split transmission would not be justified. The compromises made in the Volt's design are an indication of how customers are going to have to be more thoughtful in their automotive buying decisions in the future. The ultimate efficiency of hybrids, PHEVs, BEVs, ER-EVs and even conventional vehicles are highly dependent on the usage patterns and consumers need to look at theirs in choosing the right vehicle. These are not jack of all trades machines.
We still don't know what the Volt's charge-sustaining efficiency will be or whether the GM engineers made the right decisions. Those answers will only come once we get to spend extended periods with the Volt but hopefully this provides some insight into the decision process.