Them 200+ mpg hybrid car.

Recently, there has been a rush of announcements of hybrid cars getting more than 200 miles to the gallon.
Toyota's Prius claims to get around 50 mpg. Many people (beleivers) in Internet-land claim they get high mileage (> 60mpg) for the Prius. However, some (skeptics), who record on paper the amount of fuel bought and the odometer readings, are saying that the hybrids are getting smaller mpg than a Ford Fiesta which gets about 32 mpg. What gives?

The truth lies between the beleivers and the skeptics. The purpose of this essay is to give some ballpark figures on the hybrid versus conventional cars so as to make one become a better, more objective judge on carmaker claims.

The skeptics do a lot of highway driving, putting in at least 100 miles daily; for such driving the hybrid loses most of its competitive advantage. The beleivers may do a lot of street driving or they are putting a lot of faith on the mpg readings from the dashboard.

The main purpose of the electric component of the hybrid system is that of a kinetic energy recovery system (KERS), namely recovering the car's kinetic energy upon braking, which happens often in street driving and not so much on the freeway, especially during the off-peak hours. Conventional cars are engineered to be at maximum efficiency at 50 miles/hr. During a long period of highway driving, the battery by itself cannot provide the energy needed and thus the internal combustion engine (ICE) provides for the propulsion and battery recharge.

There are mainly 2 types of hybrids: serial and parallel. The Toyota Prius is a parallel hybrid: both the ICE and battery can provide simultaneously for propulsion, with ICE providing battery recharge at appropriate times. The Chevy Volt is a serial hybrid: the role of the ICE is to recharge the battery which in turn provides all the propulsion energy. During prolonged highway driviIng, for serial hybrids, the battery is between the ICE and propulsion and thus lowering efficiency; for parallel hybrids, the battery can be seen as dead weight.

Still staying with the highway driving case (50 miles/hr), what would be a realistic estimation of the maximum highway mpg for an ICE-only car? The car has to overcome at least 3 types of resistance: wind, rolling (tires), and drivetrain (tranmission). As a rule of thumb, at such speeds, wind resistance is about equal to the sum of rolling and drivetrain resistances. To overcome wind resistance at 50 miles/hr, an average car needs about 7.5 kilowatt of power; after 1 hour the car has travelled 50 miles and used up 7500 watt-Hour which is about the combustion energy content of about 0.2 gallon of gasoline. Assuming an average ICE converts about 30% of the combustion energy into mechanical work, the 0.2 gallon becomes 0.67 gallon. Ignoring rolling and drivetrain resistances, the mpg is 50/0.67 = 75 mpg. Accounting for rolling and drivetrain resistance, the actual mpg is closer to 75/2 = 37.5 mpg. So any claim of highway mpg much greater than 75 brings out the skeptic in me.

The American customer should not be considered a sucker, despite what P.T. Barnum said. For conventional cars, one is given at least 3 measures: city street mpg, highway mpg, and acceleration time from 0 to 60 mile/hr. Giving only 1 measure for hybrid insults the buying public. There has been many comments made in the media and Internet-land on these 100+ mpg claims, so some of what I say now is not new. These large mpg claims are predicated that 1) one drives only 60 miles per day, 2) the first 40 miles are due to the electric motor alone. What makes people contemptuous is the idea that said 40 miles is added to the remaining 20 miles powered by the ICE to calculate the mpg: 60 miles/(gasoline used to go 20 miles). If one assumes only 40 miles driven in a day, then the mpg becomes infinite, an absurd number.

The electric hybrid has an advantage over the conventional vehicule for street driving since they have KERS and the conventional car does not. In an ideal, frictionless world, the minimum energy needed to go from point A to point B, both points at same altitude is zero. However, reality is neither ideal nor frictionless. The carmakers owe the public to give an objective assessment of the hybrids' performance. I would like to see a stress test of the KERS system: disable the ICE, have the battery fully charged, go 100 meters at less than 15 mph, stop for 10 seconds and repeat until the car can go no more. Take the distance and divide by the gasoline equivalent of the battery charge. A fully charged Prius battery pack contains about 10 MegaJoule of energy, equivalent to 0.3 gallon of gasoline. Most likely the electric power is provided by coal or
natural gas plants with a 50% efficiency: said 0.3 gallon becomes 0.6 gallon. If the car is set not to allow the battery pack to below, say, 20% to prolong the battery pack's life, then the 0.6 gallon becomes 0.48 gallon.

As stated by many people, the gasoline equivalent for electric charge needs to be given. Currently electric power is a good bargain: in the US, average electric power costs 10 cents/kWH. Gasoline at $3/gallon amounts to 8.25 cents/kWH; since ICE converts 30% of combustion energy into mechanical energy, the 8.25 becomes 27.5 cents/kWH. However, Cap/Trade/Tax bill, transition from coal (cheap) to wind/solar (expensive), market forces (greater electric use brings higher electric prices) will diminish the hybrid advantage. Also, people will pay a premium for performance; if energy economy is most important then everyone will travel no faster than, say, 10 miles/hr. In future posts I will discuss some potential transportation engineering issues.

The conventional car has its advantages, especially for long distance. The ICE automobile took off after some very publicized transcontinental travels at the turn of the 20th century; there was no need for government subsidies. In 1906, a 6 cylinder, air-cooled car crossed the US in 15 days. In 1968, CalTech and MIT had an electric van race, with both vans crossing the US in 9 days; not really impressive given that there wasn't much in terms of roads and infrastructure in 1906. I would like to see a Winter Canadian transcontinental electric (or electric-hybrid) car race.

Electric Hybrids have a proper niche in the transportation ecology. Do not overhype.