An Introduction to Efficiency

You’ve probably heard that hybrid vehicles are more efficient than conventional ones, or that a small car is more efficient than a big SUV. That might seem like common sense if you’ve heard it enough times, but what do those people really mean by “more efficient?” If efficiency is a good thing, we’d like to be as efficient as possible, right?

You Get What You Pay For

One of the simplest ways to describe efficiency is the amount of work you get with respect to how much energy was invested in the first place. An old thermodynamics professor of mine once said that efficiency is:

  • What you want divided by what you pay for

To find the efficiency of a conventional car, you would need to figure out how much work your engine is producing and divide it by the amount of energy in the gasoline that was burned. The energy contained in one gallon of gasoline is about 115,000 British Thermal Units (Btu). One Btu is enough energy to raise the temperature of one cubic foot of air 55 degrees F.

If a car’s engine were 100 percent efficient at burning gasoline, then every Btu worth of energy contained inside the gasoline would be used directly to propel the car forward. However, this is far from being true. You only have to burn your hand on a hot engine once to know that most of the energy in your gasoline is actually being turned into heat. The most efficient gasoline engines available in today’s market have a peak efficiency of about 30 – 35 percent (on a good day).

This is similar to the efficiency of your conventional incandescent light bulb, which actually produces more heat than light. Compact fluorescent bulbs are quickly replacing inefficient incandescent bulbs because they last much longer and use less electricity to produce the same amount of light. If you feel the compact fluorescent bulb with your hand, you can tell why it’s so efficient: it produces plenty of light and very little wasted heat.

A Boat That Won’t Float

When someone says, “that car gets poor gas mileage,” it means “that car isn’t very efficient at using gasoline”. Either way, they’re right. Conventional cars are relatively inefficient at utilizing the total energy stored in gasoline, and it isn’t just the engine’s fault. Automatic transmissions are another notoriously inefficient component in a vehicle’s powertrain. Put that inefficient 8-cylinder engine and large automatic transmission into a big SUV with mud tires and a barn door for a front grill and you may as well just sign your whole paycheck over to OPEC!

I think you get the point: conventional cars are inefficient users of gasoline. They’re heavy, generally not very aerodynamic and their engines utilize relatively little of the energy contained in every gallon of gas they burn. So how can hybrid cars get away with using less gasoline than conventional cars? The key is smart design to help increase energy efficiency. Hybrids use electric motors to provide some portion of their driving force. Unlike gasoline engines, electric motors can be greater than 90 percent efficient at using electrical energy. So, if an electric motor uses 100 kW-hrs of energy from the battery, 90 kW-hrs worth of energy will be converted into useful work. Hybrids sometimes use continuously variable transmissions, which can be much more efficient than automatic transmissions and in some cases are competitive with manual transmission efficiency.

Battery-dominant hybrids

So, if electric motors are so much more efficient than gasoline engines, why don’t you see a lot of hybrids with really big electric motors and tiny gasoline engines? Because gasoline engines are a robust, thoroughly developed and currently cost-effective energy conversion device (from a manufacturer’s standpoint). Also, large electric motors require large energy storage systems, such as batteries. Big batteries are currently very expensive—although that could quickly change if battery-dominant hybrids were introduced to the world market, and adopted by consumers as quickly as today’s hybrids.