Note: This is a guest article sent to us last week.

By Bill Destler

I’m a regular reader at GM-Volt and have owned this astonishing vehicle for about 4 months and 4000 miles. I’m also president of the Rochester Institute of Technology (RIT), and some of our engineers worked on the Volt’s battery development. A lot of people seem to think that the Volt is cheap to operate because electricity is cheaper than gas, but that isn’t the reason and it isn’t true in general anyway.

A gallon of gas contains about 33 kWh of energy, and at the national average price for electricity of about $.11 per kWh, the same amount of electrical energy as is available in a gallon of gas costs $3.63. This is almost exactly the same as the national average price for a gallon of petro. In terms of cost, the two sources of energy are comparable.


A Chevrolet Volt drives past Detroit-Hamtramck assembly plant.

So how can the Volt go 40 miles or so on an 11 kwh charge? That means its equivalent gas mileage in electric only mode is about 120 mpge. The answer is simply that electric drivetrains are much more efficient than internal combustion engine (ICE) drivetrains (about 75 percent vs. 25 percent). In fact, there is little hope that ICE drivetrains could ever compete with electric drivetrains in terms of efficiency.

Why are ICE drivetrains so inefficient? There are many reasons, including heat losses and inertial losses of various kinds, but ICE’s are also thermodynamic systems with efficiencies limited by the heat cycle they operate under. Engineers have done amazing work in improving the efficiency of gas-powered cars, but they are up against fundamental limits.

So in any efficiency comparison between a Volt operating in all-electric mode and an ICE driven car, the Volt wins hands down. It isn’t even close. But even more amazing is the fact that a Volt can get 40 mpg in charge sustaining mode.

Let me explain this statement a bit further by comparing the Volt to a Chevy Cruze. The Cruze uses an ICE connected to a transmission to drive the wheels directly. So the energy cycle goes from potential energy stored in the gas to heat to mechanical energy which drives the wheels. In the Volt when operating in charge sustaining mode, the potential energy in the gasoline goes into heat and then to mechanical energy to drive the generator, and then to electrical energy from the generator which powers the AC motor providing mechanical energy to turn the wheels.


Chevrolet Volt chassis.

Now a Cruze weighs significantly less than a Volt, but it gets about the same mileage as the Volt in charge sustaining mode. The extra weight, along with the additional energy conversion cycles in the Volt when it runs on gasoline, should make the Volt in this mode less efficient than a Cruze, but the overall efficiency of the whole Volt system as engineered by GM has, remarkably, produced about the same operating efficiency.

I think this is primarily because of the high efficiency of the electric parts of the drivetrain and the fact that the gas engine in the Volt can operate closer to its optimum efficiency rpm more of the time than can an ICE powering a mechanical drivetrain, which operates through several gear changes over a much wider rpm range.

So in the future, electric drivetrains will probably dominate whether the power source is a battery, a fuel cell, or even biofuels. There’s just no other way to get this kind of efficiency gain from an ICE drivetrain.